m'lr''t,''''A'<'t.-''i''r'A .Hi vmm^jTP^m?. W^^^Sf^M&M. CORNELL UNIVERSITY LIBRARY BOUGHT WITH THE INCOME OF THE SAGE ENDOWMENT FUND GIVEN IN 1 89 1 BY HENRY WILLIAMS SAGE RE 925.D6ri86T""'''''''"'^ O" 'he anomalies of accommodation and 3 1924 012 164 178 Cornell University Library The original of tiiis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924012164178 THE NEW SYDENHAM SOCIETY. INSTITUTED MDCCCLVIII. VOLUME XXII. ON THE ANOMALIES OF ACCOMMODATION AND REFRACTION OF THE ETE. WITH A PRELIMINARY ESSAY ON PHYSIOLOGICAL DIOPTRICS. F. C. PONDERS, M.D., FaOfEBSOB OP FHT8I0L00T AND OPHTHALUOLOGT IN TBB UNITBEBITT OF UTBVCBT, TRANSLATED PKOM THE AUTHOR S MANUSCRIPT BY WILLIAM DANIEL MOORE, M.D., Dub., M.R.I.A., HOHOBART rBLHOtV OF THE SWSDI8B 80CIETT OF FHYBICIANB, OF THB NOR-WEQIAN UBUICAL SOCIETT, AND 07 THR ROYAL MEDICAL SOCIETi^OF COPBNHAQBN. THE NEW SYDENHAM SOCIETY, LONDON. MDCCCLXIY. [THE KIGHT OF TRANSLATION IS RESERVED.] A. ^1^37 LONDON: Printed by J. W. Kochv, fi, JSJxhy Street, Hatton Garden. Ov^ TO WILLIAM BOWMAN, F.R.S., WHOSE MERITS IN THE ADVANCEMENT OF PHYSIOLOGY AlfD OPHTHALMOLOGY ARE EQUALLY RECOGNISED AND HONOURED IN EVERY COUNTRY, THIS WORK ON THE ANOMALIES OE REFRACTION AND ACCOMMODATION IS, IN TESTIMONY OP THE WARMEST FRIENDSHIP AND OP THE HIGHEST ESTEEM, INSCRIBED BY THE AUTHOK. PREFACE. My essay upon Ametro^ie en hare gevolgen, ("Ametropia and its Results"), published ia 1860, was confined to the anomalies of refraction, and treated of these exclusively from the dioptric point of view. In the preface, however, I announced my intention of producing, subsequently to the appearance of that essay, a complete system of the anomalies of refraction and accommodation: the anomalies of refraction, including the subject of astigmatism, were to be treated of also from an anatomical and practical point of view, and the anoma- lies of accommodation were to be developed both in their opposition to, and their connexion with, the anomalies of refraction. "When, later, I was honoured with a request, on the part of the New Sydenham Society, to prepare my essay for an English edition, I felt bound to endeavour to complete my work on the plan alluded to. This I have done to the best of my ability. The experience of many years, the examination of many thousands of eyes, in which I have been zealously assisted by several of my pupils, have been made available. I believe that my work has gained by its enlarge- ment. I cannot, however, fail to regret that its bulk has increased beyond my expectation, and I feel bound to apologise to the Coun- cil both for the delay in its appearance, and for the inconsiderate manner in which I have used the liberty allowed me for its exten- sion. May the intrinsic value of the additions made plead suc- cessfully ia my behalf. One object 1 have kept constantly in view, — to make the book, notwithstanding its great size, useful and, in all its parts, easily accessible to the practical physician. To this end, in the first place, each subject is fully treated of in the body of the text; viii PREFACE. consequently^ only those who wish to penetrate more deeply into the subject, need attend to the details of the investigation, to the mathe- matical demonstration, and to the history, added as an appendix, in smaller type. And, in the second place, to faciUtate reference to the several parts, each chapter is so drawn up, that the reader's knowledge of what has gone before, is not taken for granted. The work forms, in a certain sense, a series of lesser monographs, united in a single volume, which is the emblem of their mutual connexion, For the oculist it is perhaps an additional advantage that I am no mathematician. I freely admit that I am not competent to follow the investigations of Gauss and of Bessel in this department, and even the study of the physiological dioptrics of Helmholtz required an effort on my part. I have, therefore, sought a way of my own, and, as I believe, I have found it. The whole theory of the cardinal points of compound dioptric systems, as it is here put forward, is quite explicit and elementary, depending almost exclusively upon the mutual comparison of similar triangles. If the road has thus become somewhat longer, it presents this advantage, that it lies open to all. To guard agamst the possibility of its leading on any point to error, I have requested my friend Hoek, our Professor of Astronomy, to look over it, and to his kindness I am indebted for many improve- ments in the form of the demonstration. In the doctrme of the anomalies of refraction and accommodation, the connexion between science and practice is more closely drawn together than in any part of medicine. Science here celebrates her triumph; for it is at her hand that this branch has acquired the exact character, which makes it also worthy of the attention of natural philosophers and physiologists. It is, indeed, satisfactory to see, how in the accurate distinction between anomalies of refraction and accommodation, with exclusion of every condition foreign to these anomalies, the system assumed, as if spontaneously, an elegant simplicity, and how the cause and mode of origin of many an obscure type of disease emerged into the clearest hght. PREFACE. ix Practice, in connexion with science, here enjoys the rare, but splendid satisfaction, of not only being able to give infallible pre- cepts based upon fixed rules, but also of being guided by a clear insight into the principles of her actions — advantages the more highly to be estimated as the anomalies in question are of more frequent occurrence, and as they more deeply affect the use and functions of the eyes. Is it then strange that the study and treatment of my subject have been to me a labour of love ? the more so, as I felt proud in having been called upon to elaborate it for a country in which Young, Wells, Ware, Brewster, and Airy have pointed out to us the track which we had only to foUow, and happy in being able to offer my work in this form to my highly esteemed friends and colleagues, whose proofs of kindness and affection have left with me the most agreeable recollection of my visits to England. Among the privileges, which my task has procured me, dear friend Moore, is the agreeable relation into which it has brought me with you. If I have admired your talent, and highly appreciated your unwearied care, I have, above all, to thank you heartily for the interest and the love, with which the dif&cult task of the translation of my work has been accomplished by you. I feel that we have become friends, and friends we shall continue. You will, I am certain, gladly join me in thanking the Eev. Professor Haughton, for the solution of many doubtful points, and for his kind revision of certain portions of the work. P. C. DONDEES. Utrecht, 27th February, 1864. NOTICE BY THE TRANSLATOE. The author having mentioned in his preface the circumstances which have led to the appearance of the present volume^ it .now remains only to state, in addition, that every page of the book has received his careful correction and revision. I should wish also to observe, that the chapter on Astigmatism is not a mere translation of his essay on that subject, published in the year 1862, but that it has received so many amendments and additions, as to represent a second edition of the work in question, brought down to the present day, and enriched with the results of the latest researches bearing upon that remarkable phenomenon. While I have to congratulate myself, that the free and unre- stricted correspondence with Professor Bonders, which it has been my privilege to enjoy during the translation of his work, his courtesy and readiness to solve the difficulties arising from the com- plicated nature of the subject-matter of the book, and his accurate knowledge of the Enghsh language, have enabled me to avoid any serious misconceptions of his meaning; I am only too painfully conscious of many defects in the manner of the translation, some of which my present experience would have enabled me to avoid. These, as the wiser course, I would here have passed over in silence, were it not that I think it absolutely necessary to explain one or two points connected with some of them. For example, when I first met with the words zenuwvlak and gezigtszenuwvlak, I translated them too Hterally (pp. 354 et seq.) by the terms " nerve-surface," " surface of the optic nerve," " plane of the optic nerve," instead of by the shorter phrase " optic disc," by which this part is now designated by several modern ophthalmolo- gical writers, and which I have in the later sheets of the volume exclusively employed. In like manner I have to apologise for having followed the Dutch orthography of the term "kyklitis" (p. 370), instead of that of Enghsh, French, and German writers, in accordance with which the word should have been written " cyclitis." NOTICE BY THE TRANSLATOR. xi But I may, in reference to tMs point, mention, that the word in question is not to be found in any Medical Dictionary (Mayne, Dunglison, Palmer, &c.) which I have had an opportunity of consulting. For this reason, I sometime since requested Professor Bonders to supply me with a footnote, giving the meaning and derivation of the term. But as the note did not reach me in time for insertion in its proper place, I venture to introduce it here, giving a reference in the index, by which it may at once be found. " Kykhtis, the cyclitis of Enghsh and German ophthalmological writers, the cycUte of the French, is a term recently introduced to signify inflammation of the ciliary muscle (or Kgament) and the neighbouring parts of the sclerotic, and ciliary processes. It is derived from the Greek word kukXp^, the circle or orbiculus ciliaris." Having mentioned these, as it seems to me, necessary matters, I shall not dwell longer upon my own deficiencies, nor find fault with the kindness of the author, which has led him to take a far too favourable view of the manner in which I have discharged the humble part assigned to me in the production of the English (and in point of priority of publication, original) version of his important work ; I shall content myself with thanking him heartily for his courtesy and readiness on all occasions to solve every difficulty submitted to him, and for the care with which he has revised at least two proofs of every sheet of the translation. Nor can I sufficiently express my thanks to the Eev. Samuel Haughton, M.D., E.R.S., !Pellow of Trinity College, and Professor of Geology in the University of DubHn, for the trouble he has taken in correcting for the press the mathematical portions of the work. Professor Haughton has also been good enough to furnish me with a few footnotes, to which he has, at my request, attached his initials. To Mr. Bowman, P.R.S., I am deeply indebted for many valuable suggestions, and for much important help, freely and kindly offered, and most willingly afforded. 7, South Anne Street, Dublin, March Uh, 1864. CONTENTS. GENEEA.L PART. Page Introduction. . „i? iv.^ § l.-On the Conditions of Accurate Vision.-Function of the ^ Retina ■ • • ' ' „ Note to § 1 CHAPTER I. § 2.— Proofs of the Existence of Accommodation in the Eye . 6 5 3.— Change of the Dioptric System of the Eye in Accommo- dation Note to § 3 § 4._0n the Mechanism of Accommodation Note to § 4 § 5.— Range of Accommodation . Note to Chapter I. Parti. Dioptrics of the Eye Part 2. Range of Accommodation 10 16 20 27 28 38 72 CHAPTER 11. Defects of Refraction and Accommodation in general. § 6.— Distinction between Defects of Refraction and of Accom- modation . . • • .80 § 7.— Causes of the Defects of Refraction in general . . 86 § 8.— Diagrammatic Representation of the Range of Accommo- dation, and of the Anomalies of Refraction and Accom- modation . . . • .90 § 9.— Clinical Determination of Ametropia in general . 96 Note to Chapter II. . . . -108 CHAPTER III. Fuller Development of the different Meanings of Range of Accommo- dation. § 10. — Relation between Accommodation and Convergence of the Visual Lines ; Meaning of 1 : A, of 1 : A„ and of 1 : Aj 110 § 11. — Difference of the Relative Range of Accommodation 1 : A,, according to the Refractive Condition of the Eye . 119 CONTENTS. CHAPTER IV. Page Spectacles and their Action in general. § 12. — Different Kinds of Spectacles . . .126 § 13. — Direct Influence on Vision, of Glasses with Spherical Surfaces . . . . .143 Note to § 13 . . . . .169 Note to Chapter IV. .... x7o SPECIAL PAET. I. — Anomalies of Refraction . . . .171 CHAPTER V. The Emmetropic Eye. § 14. — Definition of the Emmetropic Eye; the Diagrammatic Eye ; the Simplified Eye . . . .173 Note to § 14 . . . . .177 § 15. — Centre of Motion and Movements. Angle between the Axis of the Cornea and the Visual Line . .179 Note to § 15 . . . . .184 § 16. — Acuteness of Vision, modified by Age . . 188 § 17. — Range of Accommodation, modified by Age. — Presbyopia. — Hypermetropia acquisita . . ' 204 Note to § 17 . . . . .214 § 18. — Treatment of Presbyopia . . . .' 215 Note to § 18 . . . . .232 CHAPTER VI. Hypermetropia : H. § 19. — Dioptric Definition of the Different Degrees and Forms of Hypermetropia .... 236 § 20. — Form, Position, and Movements of the Hypermetropic Eye. — Apparent Strabismus . . . 244 Note to § 20 . . . ■ .250 § 21. — Phenomena. — Diagnosis.— The Vision of Hypermetropes 251 § 22. — Asthenopia . . . ■ ■ 259 § 23. — Treatment of Hypermetropia, with Special Reference to Asthenopia ..... 274 § 24.— Strabismus Convergens, the Jesuit of H. . • 291 § 25.— Aphakia . . • • -309 Note to Chapter VI. . . ... 325 CONTENTS. CHAPTER VII. Page Myopia : M. § 26. — Dioptric Determination, Diagnosis, Degrees, Occurrence, Hereditariness, Development with Advancing Age . 332 § 27. — Results of the Ophthalmoscopic Investigation of the Myopic Eye . . ' . . . 3.53 Note to § 27 . . . .366 § 28.— Anatomy of the Myopic Eye . . . 367 § 29.— The Vision of Myopes .... 387 § 30. — Complaints and Disturbances in Myopia . . 392 § 31. — Insufficiency of the Internal Muscles and Diverging Stra- bismus, both results of M. • . . . 402 § 32. — Hygiene.-^ Treatment. — Spectacles. — Illustrative Cases. — History of Myopia . . , .415 Note to Chapter VII. . . . .444 CHAPTER VIII. Astigmatism : As. § 33. — Definition of Astigmatism.— Regular and Irregular Astig- matism ..... 449 § 34.— Regular Astigmatism in the Normal Eye . .451 § 35. — Disturbances and Phenomena in High Degrees of Astig- matism • • . . . 469 § 36. — Diagnosis of Abnormal Astigmatism, and Determination of its Degree ■ • . . . 479 § 37. — Cause and Seat of Abnormal Astigmatism . . 490 § 38.— Cylindrical Lenses, and General Rules for their Employ- ment . . . _ _ _ gQ^ § 39.— Nosology and Clinical Study of Astigmatism . .511 1. Congenital Astigmatism . . . 512 2. Acquired Regular Astigmatism . . 533 Note.— History of our Knowledgs of Regular Astigmatism 539 § 40.— Irregular Astigmatism . . _ _ 543 CHAPTER IX. Difference of Refraction in the Two Eyes. § 41.— Occurrence, Phenomena, Results . 557 § 42.— Treatment and Optical Remedies in Difference of Re- fraction in the Two Eyes . CONTENTS. XV II. Anomalies of Accommodation . . . 569 Introduction . . , , , 571 CHAPTER X. Influence of the Nerves upon Accommodation and upon the move- ments of the Iris. § 43.— The Movements of the Iris . > . .672 § 44. — The Ciliary System and its Function . . 575 CHAPTER XL Paralysis and Debility of Accommodation, § 45. —Mydriatics and their Action . . . 584 § 46. — Morbid Paralysis of Accommodation . . 591 § 47. — Paresis of Accommodation after Diphtheria faucium, Weakening of Accommodation . . ■ 597 CHAPTER XII. Spasm of Accommodation. § 48. — Myotics and Myosis .... 610 § 49. — Spasm of Accommodation. — Myosis. — Painful Accommo- dation , . . . .622 Index ... ... 628 SIGNS AND ABBREVIATIONS USED IN THIS WOEK. p, radius. p", radius of the cornea in the visual line. n, coefficient of refraction. k, nodal point. h, principal point. k' and k', anterior and posterior nodal points of the same system. h' and h', „ „ principal „ „ 0' and ^", „ „ focal „ „ ky and h^, two nodal points of two different systems. A, and h^, „ principal „ „ , greater than. "/o, per cent. ' (aitec a numeral) foot or feet, as 2', 2 feet. ' ( „ ,, ) inch or inches, as 2', 2 inches. '" ( „ ,, ) line or lines, as 2'', 2 lines. ERRATA. Page 23, line 17 from above— i^or " Moreover," read " We admit that." „ 40, „ 6 from below— jDefe " to." „ 67, „ 7 from below-r-J^oj-" 19-875," rea(f" 14-858." „ 75, „ 21 from above — The words, " the correctness," &c., and the following lines, 22 to 32, are incorrect, and may be entirely omitted. „ 75, „ 4 from below— Jbr " f' V = f h"," read " 0' h' = f' 7c". „ „ „ 4 from below— JFbr " 0" h'," read " p' If." „ „ 3, 4, and 5 — For" ij," read " y." „ 81, line 17 from below — For " measure," read " mean." „ 83, „ 15 from above— Jbr "adjoining," read " nearer." „ 91, „ 7 to 15 from below, " : 18 " should throughout be " 12." „ US— For " § 12," read " § 13." „ 169— Jbr " Note to § 12," read " Note to § 13." „ 170, line 10 from above — For " should," read " would." „ 173, „ 13 from above — For " ca.n^" read " oajmot." 173, „ 13 from above — For " and," read " but." ", 173, „ 16 from above— Jbr " § 13," read " § 14." „ 174, ,, 8 from below — Jor " taxation," rea of the membrana Descemetii and the termination at the choroid (C), both of which are elastic, to ap- proach one another. According to this, the iris (I), which is mediately connected with the anterior part of the ciliary muscle, recedes in ac- * Archivfur Ophthalmologie, B. vii. MUELLER'S THEORY. 27 commodation for near objects; and, on the contrary, the place of insertion in the choroid -will advance a Httle forwards. Now with this the origin of the Zonula Zinnii is connected; and as, there- fore, the latter at the same time advances, its tension ceases, and the equator of the lens becomes smaller, the lens itself becomes thicker in the middle, and its two surfaces are rendered more convex. Helmholtz supposes that to this may be added a pressure of the iris, which may make the equatorial surface of the lens arched an- teriorly, and thus increase the convexity of the anterior surface and diminish that of the posterior. H. Miiller's theory is based upon his anatomical investigations of the ciliary muscle. He distinguishes, as we have seen, a circular muscle capable of exercising pressure on the margin of the lens, and thus of rendering the lens thicker, while it would at the same time draw the periphery of the iris backwards. Moreover, he attaches, with Helmholtz, importance to the relaxation of the Zonula Zinnii. Lastly, he sees in the action of the most external layers of the ciliary muscle a means of augmenting the pressure of the vitreous humour, of pushing the lens forwards, of diminishing the increased convexity of the posterior surface, and, by the resistance of the simultaneous contraction of the iris, of increasing that of the anterior surface. Against these two theories I have diificulties which I shall not further develop. It would, moreover, be easy to bring forward other hypotheses, but from this too I shall refrain. I am afraid of de- priving this work of the character I desice, above all, to see attached to it, — the character of exact science. NOTE TO § 4. One poiat I shall take leave to remark upon is, that in the case of acquired aniridesis with normal range of accommodation, described by von Graefe, in accommodation for near objects, no displacement of the then visible ciliary process was observed ,• that, moreover, nothing is mentioned of the possi- bility of a direct pressure of the cUiary muscle on the margin of the lens ; and, finally, that nothing is said of any diminution of the circumference of the lens, although the increased convexity of its anterior surface is proved from the reflected images. On former occasions 1 have also in vain endeavoured, 28 RANGE OF ACCOMMODATION. after iridectomy, in whioli the margin of the lens became visible, to satisfy myself of the diminution of the circumference of the lens in accommodation for near objects. Thus far it has not been directly observed. It is evident that, if it be wanting, with the increase in thickness in the middle, which has certainly been witnessed, attenuation of the parts situated near the equator must be combined, and thus the curved surfaces would obtain such an irregular form, that it becomes difficult to explain the tolerably accurate power of vision in the case of aniridesis described by von Graefe, in accom- modation also for near objects. What we must, in the first place, therefore, endeavour to clear up is the question, whether the circumference of the lens, in accommodation for near objects, becomes perceptibly smaller ? The answer will have great influence on our further considerations. § 5. Eange of accommodation. In all tlie investigations respecting the cause and mechanism of accommodation, observers appear not to have thought of defining the range of accommodation under various circumstances, and of seeking a simple numerical expression for the same. And yet the necessity for such existed almost stiU more for the oculist than for the physiologist. If it be desired to investigate the accommodation, whether in reference to the changes observed in the eye, either at different periods of hfe, or with respect to myopia, hypermetropia, asthenopia, strabismus, paresis, etc., it is evidently necessary to have an easily comparable standard of its magnitude or range. Had the necessity been felt, it would not have been difficult to have provided for it. The knowledge alone of the distance E from the farthest point of distinct vision, and of the distance P from the nearest, is sufficient. With the knowledge of these distances the range of accommodation r may be found by a very simple formula. The formula is * 1_ 1 _ 1 A P R- The distances P and R may be calculated from the nearest point j», and from the farthest point r of distinct vision to a point situated about 3" behind the anterior surface of the cornea in the eye, called * English writers on optics would say 1 _1 J A R~p' the negative sign of A denoting that the lens is convex.— S. H. THE ANTERIOR AND POSTERIOR NODAL POINTS. 29 Fig. 10. \. the anterior nodal point Je. The latter coincides in the eye nearly with the second nodal point ¥, both of which may therefore here be con- sidered as one point. This point corresponding nearly to what is termed the optical centre, has a very important signification j the mys, which in front of the cornea are directed to the node, in the vitreous humour continue parallel to their primitive direction, and also nearly exactly directed to the same point; these rays may therefore be considered not to have been refracted. This is represented in the sub- joined figure 10. The ray i ¥, proceeding from the point i, continues as k" j, and since all the rays, which proceed from the point i, unite in one point, this must occur where they meet conjointly the ray h" j'. Now if the eye be accommodated, the union takes place in the retina, and then the image of the point i lies in/. In like manner the image of the point i lies in j, both being situated ia the axis A A-. Therefore, / / is the image of i i, and it immediately appears that their recipro- cal magnitudes are as their distances from the point k" where the rays i j and i / cross one another. If we express the distance i Ic by /, and the distance/ W by /, the magnitudes of the object B and the image /3 are to one another, in the accurately accommodated eye, as / to /. B : ^ = / : /. In the normal eye / is about 15 millimetres. Therefore, if an object be accurately seen at 15 metres distance, the retinal image is 1000 times smaller than the object; if the object Hes at 1.5 metres (= 1500 mm.) the retinal image is i^ = 100 times smaller. Hence it clearly ap- 15 pears, how important the posterior nodal point (Knotenpunkt) ^'is. If we connect the correspond- ing points of the object and image by right Hues with one another, these all, just as ij, pass through the point k', and they are therefore caUed hues of direction; the pos- terior nodal point Jc' is consequently the point of decussation of the lines of direction. so RANGE OF ACCOMMODATION. Kg. 11. f. The meaning of the formula for the range of accommodation — i - i _ i A - P E is easily understood. In this formula, A is the focal length of a lens, which gives a direction to the rays from the nearest point of distinct vision j», as if they came from the farthest point /. The sub- joined figure (11) illustrates this. The eye in the condition of rest is accommodated for the distance / ^' = E; in the strongest tension of accommodation for the distance jo ^' = P. In the former case the rays diverging from r are united on the retina, in the latter those diverg- ing from p. In accommodation the eye must therefore be so altered that the rays proceed- ing from jB, in the vitreous humour acquire a direction equal to that of the rays proceeding from r in the non-accommodated eye. This can be effected by placing an auxiliary lens in Ic, and we may thus imagine the eye away, and suppose that the auxiliary lens in Ic is in the air. The lens now represents the accommodation of the eye, and its power the range of accommodation. Its focal distance A is found by the formula mentioned : P E~ A" Consequently A is the focal distance of the auxihary lens, of which the eye avails itself in accommodation, and as the power of a lens is inversely proportional to its focal distance, y or XL 1 : A expresses the range of accommodation. It is convenient to represent the value of A in Parisian inches, especially as the focal distance of lenses is usually stated in the same, and this appHes also more particularly to spectacles.* I may be allowed to illustrate the calcula- tion of the range of accommodation by a couple of examples. * In the boxes of Glasses, prepared by Paetz and Flohr, of Berlin, for ILLUSTRATIVE EXAMPLES. 31 Let the distance P of the nearest point = 4', that of the farthest point of distinct vision E. = 12", then the range of accommodation will be — 1_ 1 ^1 4 12 6 If the farthest point lies at an infinite distance, E = ao, the nearest point at 5°, the range of accommodation will be — 1-1=1 5 CO 5 In the first case, the range of accommodation is represented by an auxiliary lens of six^ in the latter by one of five Parisian inches. The same form of expression I now apply to all lenses. The power may always be regarded as inversely proportional to the focal distance F, and therefore find its expression in 1 : ]?. If the focal distance be negative, it becomes — 1: P. ~ GlassSs of i,, of— J, &c., therefore mean glasses of ten Parisian inches positive, eight Parisian inches negative focal distance, &c. We shall subsequently see that the degrees of anomalies of refraction may be expressed in a similar mode, and that it is thereby at the same time shown, by what glasses they may be neutralized. We have above seen that the range of accommodation is contained in the formula 1 = 1-1. ape' Therefore it is necessary to possess a simple method of determining the points p and r with sufficient accuracy for practical purposes. The determination of r is accomplished with a nearly parallel state of the lines of vision, that is, by fixing with both eyes an object at least 5 metres distant. We know, namely, that when the hues of vision converge, accommodation necessarily takes place, and that conse- quently the true farthest point in total relaxation of accommodation cannot thus be found. As an object we may use groups of vertical black Hues, each line 2^ milhmetres thick and 10 millimetres from one another, and examine whether they can at the distance of five metres be seen with perfect accuracy with the naked eye, or whether the sharpness of the object can be increased by glasses. If no improve- oculists, they are defined in Prussian incies, which are less than Parisian inches. In England, English inches are employed, one English inch being equal to about 0-94 Parisian inch, and differing but little from the Prussian. In practice a reduction will rarely be necessary. 32 RANGE OF ACCOMMODATION. ment is attainable by glasses, r lies at least 5 metres distant, which may here be equally represented by an infinite Fig. 12. distance 'oD. Where nearsightedness exists, con- cave glasses, with negative focal distance, are required to obtain perfect accuracy : in this case we determine what is the weakest glass of this nature with which the sharpest possible vision is obtained. In the determination the distance of the farthest point is ascertained ; thus when parallel rays (Kg. 12), a * and a b', derived from a distant object, fall on a concave lens I, they are, after refraction, divergent, ss c d and c d', and appear now to be derived from the point '. The distance ' I is the negative focal distance P of the lens I. If we now designate the distance I M by x, it is clear that the point ', for which the nearsighted eye is accommo- dated with parallel Hues of vision, is at the dis- tance I' -H a; from the point Ic. Consequently, R = P + «. Let us illustrate this by an ex- ample. A nearsighted eye, to see accurately at a distance, needs a glass of 15 Parisian inches negative focal distance (F = 15), placed at a quarter of an inch in front of the cornea, that is, half an inch before the nodal point U in the eye (a? = \), then R = P + « = 15^ Parisian inches. In place of the above-mentioned black lines we may, in the determination, make use of definite Q^ letters or numbers, whereby, by causing them to be named, we may obtain still more objective cer- tainty with what glass they are accurately seen. A sharp-sighted eye recognises letters such as the subjoined in good hght — (Pig- 13) — at a distance of about 20 feet. In Dr. Snellen's sys- tem of test-types, each number corresponds to the number of feet at which a sharp-sighted eye distinguishes them. The following letters (Kg. 13) correspond, therefore, to No. XX. of Snellen. They are the lowest in the Table appended to this work, in which THE OPTOMETER. 33 \ also XXXj XL, LX, LXXX, C, and CO occur. For practical ^ purposes, these experiments with vertical lines and letters afiford Fig. 13. gaale non isl dia una a sufficiently accurate result. If a very exact determination be re- quiredj we must employ a little point of light, which changes form on the slightest alteration of accommodation. This shall be more fully explained in treating of astigmatism. The determination of the nearest point is effected by means of a wire optometer. This consists of a little frame (Fig. 14 A), of the size represented in the figure, in which some fine black wires are ver- tically extended, and wherewith a measure, B, capable of being rolled up, is connected, the scale commencing at the frame, and the bobbin o being applied to the temple, on a line with the anterior surface of the cornea. This bobbin is, by moving the frame out from the eye, un- wound until the vertical wires ^re seen with perfect accuracy. It is, indeed, possible to determine by means of such wires with sufficient accuracy whether they are exactly seen, as by a slight deviation the mar- gins lose their sharp outlines, and more of these lines appear. The persons examined for the most part state this very easily. The reading of print, capable of being distinguished at given distances in due ac- commodation and by a sharp eye, may be used to control the result. Most optometers are based upon the principle of the well-known experiment of Scheiner : through two openings or slits, placed closer to one another than the diameter of the pupil, the object, for example a wire, is seen, and this appears double if the eye is not accurately adjusted to this, distance. If we now cause any one to 3 34 RANGE OF ACCOMMODATION, look into such an optometer and to deteimiue when he sees the wire Fig. 14. single, we shall in general obtain a distance to which the person easily accommodates his vision, hut this distance wiU not correspond to either the nearest or the farthest point. This is looked upon as the mean distance of distinct vision. But to this we must not attach too much importance, for such a mean distance does not exist, or at least it has a very wide range; this appears when a number of determinations are made, for example with Stampfer's optometer: the same person never obtains, even under the same circumstances, similar results, and where circumstances differ, the results vary very much. Only when a person has learned to control his power of accommodation, and can voluntarily bring it into the condition of the highest action and of perfect relaxation, can we with such optometers successively determine his farthest and his nearest point of distinct vision. But such voluntary control of the power of accommodation is acquired only by great practice. Ordinary individuals accom- modate for their farthest point only when they actually look at a distant object, and for their nearest only when they very distinctly see an object approachiag, whose diminishing distance they meanwhile observe and foUow in their imagination. Then, by the effort actually to INFLUENCE OF CONVERGENCE. 35 Fig. 15. see the object distinctly as long as possible, the greatest possible tension of the power of acconunodation is excited. Such an approaching object is the frame above de- scribed, while in the optometer, the distance of the object not being known, no stimulus to tension is created. If greater accuracy be desired, as in observa- tions intended for the solution of scientific questions, it is advisable to employ another in- strument, which shall be described in treating of the relative range of accommodation. The partial dependence of accommodation on the convergence of the lines of vision has akeady been aUuded to. In the determination of the nearest point this should be borne in mind. Theoretically we should, in order to be able to institute a comparison, always determine the near- est point at the same angle of convergence, as the farthest point is examined with parallel lines of vision. This would, however, be attended with great practical difficulties, and, as I shall hereafter show, would moreover lead to wholly incorrect re- sults. The only thing required in this respect is, that in all the cases where the nearest point hes farther from the eye than 8", the determination should be made with the use of such convex glasses, that the nearest point should be brought to about 8° from the eye. It will then be neces- sary to calculate at what distance the eye should have been brought into this state of accommo- dation without the use of the convex glasses. The calculation is not attended with any diffi- culty : — ^Let X be the distance of the convex lens I (Kg. 15) from Ic : P the focal distance of the convex lens, F' the distance from y to I. The rays proceeding from p when refracted through the lens II, assume a direction as if they came from p. The eye is therefore accommodated not for p' but for p. The distance P' from the point p to the lens is now found by the simple formula : — iPH 86 .RANGE OF ACCOMMODATION. 1 _ 1 _ 1 -p,- p» J,- The distance P of the nearest point is = P' + sr. — ^Ab exam|)le win illustrate this. "With a lens of 12" focal distance (F = 12), re- moved ^' from ik' (x = J) the point p', situated 7^" from the lens, is accurately seen. We therefore find the distance P' from 1 _ 1 = 1 7i 12 20 to be 20" from the eye, and P is therefore = 20*5 inches. Lastly, I would here state a method of expressing the ranges of accommodation by the lengths of lines, which exhibits at the same time the commencement and termination of the range of accommo- dation, that is the nearest and farthest points of distinct vision. Above parallel lines, situated at equal distances from each other (Fig. 16), let numbers be placed, expressing the distances of distinct H 2t gj- Fig. 16. ■4- 4ir e 8 12 Z^t J TL ja vision, and in such an order, that the distance between two lines may everywhere represent an equal range of accommodation, for ex- ample, -^. It is evident that in Pig. 16 the differences of the distances from line to line always amounts to — of the range of accommodation: this is true from oo to 24, from 24 to 12, from 12 to 8, &c. ; for ILLUSTRATIVE CASES. 37 1 1 1 24 ■ «> 24 1 1 1 12 '24 24 1 1 1 8 12 24" etc. A single horizontal line now immediately shows the extent and range of accommodation. In Mg. 16 three snch lines occur, I. represents a person whose farthest point of distinct vision lies at an infinite distance, his nearest point at 4". His range of accom- modation is l_ A. = 1 4 0° 4 and is expressed by six lines distance, each of rr of this range of accommodation, consequently st ~ J" II. has likewise his farthest point at an infinite distance, his nearest at 6 inches. His range of accommodation is III. has his farthest point at 6 inches (he is therefore near-sighted) his uearest at 3 inches. His range of accommodation is 1 _ 1 _1. 3 6 ~ 6' corresiponding to four lines distance _ A- i- ~ 24 6 It need not be demonstrated how easily observations may. in this manner be registered. We shall hereafter have much occasion to employ this method in exhibiting in print the several anomalies of refraction and accommodation. 38 DIOPTRICS OF THE EYE. NOTE TO THE FIRST CHAPTER. PIKST PAET. DlOPTHICS OF THE EtE. I. Literature. In order to give an idea of the necessity of accommodation, the eye has hitherto been regarded as a simple lens, with a positive focus. This has been, so far, sufficient for our purpose. But if we wish to go more deeply into many questions relating to the refraction and accommodation of the eye, we shall require more accurate knowledge of the dioptric system of the organ. This knowledge is a necessity, to enable us to form an idea what range of accommodation is obtained by definite changes of the crystal- line lens, and how each range of accommodation may be expressed by an imaginary lens, applied to the eye. It wiU hereafter be serviceable to us in the right understanding of many other questions. I shall therefore endeavour, in a simple manner, to give a satisfactory description of the dioptric system of the eye. Those who are not deterred by the higher mathematics, may consult Moser (Dove's Repertorium der Physik), who has applied to the eye the theoretical investigations of Bessel (Astronomische Nachrichten, xviii., No. 415), and Listing, who, in his Dioptrik des Auges (Wagner's Sandworterhuch der Physiohgie, Bd. IV., p. 451) has followed the mode adopted by Gauss (IHoptrische Untersuehun- gen, Gottingen, 1841) ; lastly, Helmholtz, who in his Physiohgische Optik (Karsten's Allgemeine EncyhpcEdie der Physik, V^ Lief. Leipzig, 1858), has, together with thorough explanation, made the whole theory more generally accessible. II. Pefracting surfaces in the Eye. In the eye three refracting surfaces are to be distinguished, whose curva^ tures may be assumed to be spherical. 1. The anterior surface of the cornea, approaching to an ellipsoid, with the apex in the middle of the cornea. The radius at the apex, which defines the focal distance, amounts on an average to something less than 8 miUi- mStres. The slight thickness of the cornea, and the almost perfect paral- lelism of the outer and inner surfaces, together with the slight difference in refracting power of the cornea and aqueous humour, justify us in con- sidering the system as if the aqueous humour extended to the anterior surface of the cornea. For the cornea and aqueous humour we have there- fore to assume only one refracting surface of nearly 8 millimetres radius of curvature, and with a refracting proportion of 1-3366, found by Sir David Brewster for the aqueous humour. 2. The anterior surface of the lens, 3-6 millimetres from the anterior sur- face of the cornea, with a radius of about 10 millimetres. In accommoda- tion for near objects, this surface approaches to about 3-2 millimetres from CARDINAL POINTS. 39 the anterior surface of the cornea, and diminishes in radius to about 6 millimStres. 3. The posterior surface of the lens (or the anterior surface of the vitreous humour), 7'2 millimetres from the anterior surface of the cornea, and with a radius of 6 millimetres during accommodation for distance, of 5'5 millimetres when looking at near objects. The lens is, however, no homogeneous mass, but consists of layers of refractive power, increasing towards the centre. In the lens itself, there- fore, innumerable refractions take place from layer to layer, which cannot, nevertheless, be separately traced. Consequently, we cannot regard the lens otherwise than as formed of a homogeneous substance, and the question then is, what index of refraction we ought to ascribe to this substance. For a long time physiologists assumed an index, the mean of that of the nucleus and that of the periphery, notwithstanding that Young {On th'e Mechanism of the Eye, in the Philosophical Transactions for 1801, vol. xcii., and in the Miscellaneous Works of the late Thomas Young, edited by G. Peacock ; London, 1855, vol. i., pp. 28 and 29) had already shown that, in consequence of the laminated structure of the lens, with refractive power increasing towards the centre, an index must be adopted, greater even than that of the nucleus. Subsequently, the same was observed by Senff {see Volkmann's article Sehen in "Wagner's Handworterhuch fiir Physiologic, Bd. III., Abth. l),to whom the honour of having first made the observation is in general incorrectly ascribed. The subject is one of im- portance ; for it is only by taking this higher index of refraction into account that we get rid of the paradoxical result, that in a well-formed eye parallel incident rays should be brought to a focus behind the retina. The co- efficient of refraction is now fixed by Listing at 1*455. As a conclusion from these considerations, we may repeat, that in the dioptric system of the eye three refracting surfaces are to be distinguished. 1. The anterior surface of the cornea. 2. The anterior surface of the lens. 3. The anterior surface of the vitreous humour. The index of refraction of the vitreous humour differs so little from that of the aqueous humour that we may consider it as equal to it. III. Cardinal Points. — Their Object. In a compound dioptric system we can successively follow the refraction in the different surfaces, each time determining from or towards what point originally parallel rays converge. Thus we find, lastly, the situation of the focal point after the last refraction. But in order also to determine the point of union of rays falling upon the first surface under different degrees of convergence or divergence, and to find the magnitude of the dioptric images, a separate calculation would be necessary for each case. For this purpose a particular method has been adopted. Optical mathe- maticians seek, namely, for a given system of refracting surfaces, certain fixed points, called cardinal points ; and the knowledge of these is sufficient to enable us to construct and calculate the situation and size of the images of given objects. The conditions are : — 1st, that the system be centred, that is. 40 DIOPTRICS OF THE EYE. that the centres of curvature of all the refracting surfaces lie in a right line, the axis of the system : 2ndly, that the rays intersect the axis at only small angles. The first condition appears to be amply fulfilled by the structure of the eye, as the second is by direct vision, that is by looking nearly in the direction of the line of vision. The situation and signification of the cardinal points are best understood by studying them : — A in the case of only a single refracting surface ; B for a biconvex lens, with two refracting surfaces ; C for a combination of these two into a compound system, such as the eye is. ■ Pig. 17. A, — REFKACTIOlf BY A SpHEEICAl STEFACE. IV. — Cardinal Points. In Kg. 17, let li be the central point of the spherical surface h, on which, parallel to the axis A A', rays of light fall, a h and a' V, coming from the medium with index n', and passing into the medium with index n". If nf' is > n', the parallel rays unite in the axis nearly in a point, the posterior focal point " s : s c and k i : h b =^ k s : s d, consequently, hbxd," s . , hb xk s '"= ^^<'''^"^= kr we see that s c will he = s d, if ^" s k s ^" h ki ' This is evidently the case, if the point s is removed towards j. The situation of the pointy is therefore' determined by 0"./ : <^" h — kj -.ki. , On the perpendicular from j, we have s c-=.s d, and here, therefore, the ray passing through h crosses that passing through 0". This is the case for every value of A 6 or i i', which term does not occur in the proportion. Every point of the perpendicular jf, therefore, has its image on the perpendicular i i'. Q,. B. D. In the above figure, h f" = F" = G' k 0" = G" = P. If we now call the conjugate focal dis- tances, measured from h, hi=:f and hj^^f, and, measured from k,ki-=g' and kj = g", we obtain, in plabe of, ^" j : ^" h = kj • k i, . . . . A the proportion g" - G" ■.G' = g": g', and hence directly »V^ ....(3a g» '^" ^ sfg" g'g" 9- = & g' G" =G'g" G'g" =g' G" 9"{g'- G') =9' O" f'-Bf =" In place of the proportion A we may equally write fi — pi ; pi ^ p j^f — pi -f J^ pi 44 DIOPTRICS OF THE EYE. Hence follows,' ff" —f> F"Jf-f"F"~ F" F" — /" F' + F' F" = /" F" — F" F" + F' F'- ftf" — fi J?" f" ji" — > (/" - F") = f F' f" if' - F') = f< F" •' f — F" f F' la the same figtrre we find still two pairs of .similar triangles, i i k and hf j h h " and ^"jf. Fig. 20. Henee we have two proportions, expressing the rela^ tion between the magnitudes of object and image, namely, j'j •.ii' = hj:hiorp:B='g":g'....{fia and fj:ii = ^"J: VI. — Application to the Fye. All that has been brought forward is applicable to the refraction of the rays through the cornea. If, there- fore, the crystalline lens be absent (aphakia), no other formulas than those above given need be used. The principal point lies on the anterior surface of the cor- nea, the nodal point 8 mm. behind its apex.' The radius of the cornea being 8 mm. (compare 1 a and 1 6) we find F' = F = 8 X 1-33 66 1-3366- 8 106928 0-3366 8 = 31 -692 millimetres. = 23-692 millimetres. 1-3366 — 1 0-3366 Herewith are also given O' ^ F' and G" = F (Com- pare 2 a and 2 d). The above Figure (20) shows the situation of the cardinal points in such an eye. In the normal eye, refraction through a biconvex lens is combined herewith. We shall now investigate the cardinal points of such a lens. B. — REFaACTiosr thuottgh a biconvex lens. Til. — With the exception of the axis-ray, all rays are refracted in a lens. It has above been shown how, by the determination of four cardinal points, the position, as well as the magnitude of the dioptric images, formed by a single refracting surface. NODAL POINTS OF THE LENS. 45 may be constructed and calculated. The question is, whether we can also, for a system composed of various refracting surfaces, find such points, and whether we can likewise make use of the same in the determination of the size and position of the dioptric images. In the eye there is a biconvex lens, which, just as every lens, has two refracting surfaces. "We shajl therefore examine the question specially with respect to a biconvex lens. Does a lens possess a nodal point in this sense, that all rays directed thereto pass through unrefraoted ? A lens possesses such a point only when the nodal points (the cen- tral points of curvature) of both surfaces' coincide. This takes place only in a lens having the form of Fig, 21 ; the two surfaces of curvature are at h^ and'Aj, described for this lens from the point k, and every ray directed to this point, as a 6 and a' V, therefore coincides on both the anterior and the posterior surface with the radius. Such a lens is not biconvex, but convex-concave,* and with a negative focal distance. In every other form of lens every ray is refracted, except that which coincides with the axis. This is easily demonstrated. In Fig. 22 Aj is the centre of curvature of the anterior surface h^, and h^, that of the pos- terior surface h^ Now if the ray a 6 be directed to A,, it is not refracted on the surface h^ ; but arrived at c, it is bent from the normal c v, and proceeds in the direction c d. The same is true of the ray a' V, which, directed to h^, at h' passes through unrefraoted, but at c' is bent from the normal c' v', and proceeds as c' d'. Every ray, therefore, which is not refracted at the one surface, deviates at least at the other from its direction ; and aU rays which, while they are in the lens, are not directed to h^ or k^, are refracted at both surfaces. In a lens, therefore, no nodal point exists, to be classed with the nodal point of a simple refracting surface, of such a nature that all rays directed thereto should pass through unrefraoted. VIII. Every lens has two nodal points, ¥ and k", to he found both hy construction and by calculation. For every lens two points may be determined, k' and k", which stand to each other in such a relation, that every ray directed before the first refrac- tion to k', appears after the second refraction to proceed from V, being at the same time parallel to its primitive direction. These points are the nodal points of the lens, the first /<;' and the second k" (Fig. 23). The ray a h, before the first refraction directed to k', appears after the second refraction to proceed a.&c d from A", whUe cd'n also parallel U) ah. The points k' and k" are easily found for every lens, both by construction and by calciilation. Let /C] (Fig. 24) be the centre of curvature of the surface A„ k^ of * English writers would say concavo-convex, with a positive focal length.— S. H. 46 DIOPTRICS Of THE EYE. the surface Aj. Let an arbitrary normal k^ v^ now be drawn on the sur- face Aj, and parallel thereto the normal h^ d, on the surface hi. Let a ray of light, moreover, be imagined in the direction 6, h„ proceeding in the Fig. 22. Fig- 21- Fig. 23. *^ >v5l lens, then the angles formed by this ray with the two parallel normals A;, u, and h.^ v^ wiU be equal. If the angles be equal, the deviations which the ray 6, \ undergoes at 5, and h^ will also be equal, and as these deviations take place in opposite directions, h^ c is parallel to a J,. The points k' and k", to which a 6, and c 5j (respectively before and after the refractions) axe directed, are there- fore the nodal.points of the lens. In order to calculate the position of these points, first seek the point o, where the ray 6, b^ cuts the axis ; if the radii ki 6, = pi, and k^ ftj = Pj are parallel to one another, the triangles ki 5, A, and k^ h^ h^ are similar. POSITION OF THE NODAL POINTS. 47 Consequently p^ : h^ 6, =pj : h,^ \. As, moreover, the triangles h^ 6, o and h^ b^ o are similar, A, 6, : Ai = Aj 62 : ^2 ; and, consequently, p^•.h^o — p^:h^o; that is, the distances from A, and h^ to are proportionate to the radii of curvature p; and p^ of the refracting surfaces A, and Aj. If the two radii of curvature are equal, will therefore lie midway between h^ and h^. If the point be determined, we can easily, by calculation, find h' and h" : they are, in fact, the points whereon the rays which in the lens pass through 0, or proceed from 0, are directed without the lens — in other words, they are the ilhages of 0. All the rays proceeding from o (Fig, 25), as o 6 and o c, are, after refrac- Fig. 24. Fig. 25. tion at the anterior surface h^, directed to h', the rays o h', and c' to h". Consequently Id and are the conjugate foci by refraction on the 48 DIOPTRICS OF THE EYE. surface \ ; k" and o those for refraction on the surface h^. Both are cal- culated according to the formula 3c; it is therefore sufficient to apply this formula to one of the two surfaces. For the anterior surface of the lens (Fig. 26) A, ^' is = F', A, f" = F" and Ai =/". For A, k'=f we now find Fig. 28. Fig. 26. ^. Fig. 27. f^' St THE OPTICAL CENTRE. 49 F'f" J — J"' fii ■ In this formula/"— J*' has given place to F"—f", because the point of light lies between hi and 0i" (compare Fig. 26), and h' is therefore a vir- tual image. In like manner, k" is found for the surface \ of Fig. 25. We have called k and Icf the images of o. They are really so. Were there a little point in o in the lens (Fig. 25), it would, seen through the anterior surface of the lens h^, appear to be in V, and, seen through the posterior surface h^, in /c". Wow if the rays proceeding from o (Fig. 27), are, after refraction on the anterior surface A,, at 5 and 6' directed as 5 a and V a' to V, after refraction on the posterior surface Aj, aa cd and c' d' are directed to Icf', the rays a h and a' b', which before the first refraction are directed to t, pass in the lens through o ; and appear, after the second refraction as cd and c' d' to be derived from A", c d being, moreover, parallel to a 6 and c d' to a' V. The points found h' and ¥ therefore correspond to the definition of nodal points above given. 12. — To make use of the nodal points. Of these nodal points we can make the same use as of the single nodal point (centre) of a single refracting surface (compare Fig. 10), with this difierence, that between a point (Fig. 28) i' and its image / we have to distinguish two rays of direction : the first, namely, i' k, and the second Vj'. These are, however, parallel to one another, and the angles a, and ^ are therefore equal. Consequently, the image .// is seen from W under the same angle, under which the object ii exhibits itself from V. For the magnitudes of the object i i' = B and the image _// = /S we therefore find : : B=jh' : ilL If the object lies at a great distance from the lens we may set down i V = i h", and the formula then becomes : p : B :=j Ic" : i If, whence the posterior nodal point results as sole nodal point, and we have to do with only one ray of direction. X. — Optical centre. The point o (Fig. 28, compare also Fig. 25) is known under the name of optical centre. Usually we assume that the rays directed to this optical centre, pass through unrefracted. The formula ;8 : B ^jo : 1 is then applied. Ve have already seen that this is not perfectly correct. For thin lenses, with a long focal distance, this method, it is true, gives rise to no notable error. But for thick lenses, with a short focal distance, the deviation may be considerable, and the two nodal points must be taken into calculation. If either object or image be situated at an infinite, or at least at a very great distance from the lens, we may for these lenses, too, assume 4 50 DIOPTRICS OF THE EYE. one nodal point, namely, that on the side of the shorter conjugate focal distance (Compare IX.), — but in no case the optical centre. XI. — Every lens has two principal points, h' and h". Asa lens has two nodal points, it has also two principal points : If there exists only one refracting surface, the refracted ray is directed to the same point of that surface as the incident, and from this point, where the refraction takes place, we calculate the principal focal distance as well as the conjugate focal distances. The principal point lies then in the surface of curvature. If there he more than one refracting surface, a ray is after the last re- fraction evidently no longer directed to the point, where it underwent the first refraction. But we can, however, find two surfaces, perpendicular to the axis, which stand in such a relation to each other, that the rays before the first and after the last refraction are directed to exactly corre- sponding points of these two surfaces. This is, in fact, attained, if the two surfaces are images of one another, of equal size and like direc- tion (situated on the same side of the axis). The two surfaces which fulfil these conditions, are the principal surfaces, and where they are cut by the axis, lie iihe principal points. From the first principal point h' the anterior, from the second h" the posterior focal distance is calculated. XII. — Mode of finding the principal points. To find the principal surfaces, we must in the system, for example in a lens, determine in what position an object must be, in order to form similar images on both sides, these are then, also, images of one another, and there- fore represent the principal surfaces. We immediately see that in a bicon- vex lens the place sought must be found, between the two refracting sur- faces, in the lens. The magnitude of the virtual images, formed by each of the surfaces of an object situated here, we find by the formula (4 h), ^: £ = F'—f: F".' In this each surface is considered, as if the other were not present. Now in Fig. 29 let \ ^{ = J/ be the posterior focal distance of the surface h^ in glass for rays falling parallel in the direction A' A on Aj, hi f" = i^i", the focal distance of tlie surface hi, likewise in glass. Now if an object be at s, sh^ =// and shi =//. If the images are of equal size, then since ^■. J3 = Fs" — // : F./ and p: B = Fi"'-f:Fi' the formula fz-F,"=f":F,' (b) must hold, which signifies, that in order to find the point s, hi hi must be divided into two parts, proportional to the focal distances h-, 0/ = F,' a.nihi^i' = Fi". The correctness of this result appears immediately from the consti'uc- tion (Compare Fig. 30). MODE OF FINDING THE PRINCIPAL POINTS. 51 Fi?. 30, Fiff. 29. #- 52 DIOPTRICS OF THE EYE. Eaoh surface is here again considered, as if the other were not present. The ray drawn from ^i" to s', becomes, as derived from the second focus of h^, at 6 as J a, parallel to the axis, likewise that drawn from ^j" to s', in V as Va'. The distances from h and V to the axis are therefore the magni- tudes of the images of the line s s', and these, b hi and b' h^ assumed as vertical to the axis, are equally large, when h^ h^ is divided propor- tionally to the focal distances. Then, since ipz" s' and ipi" s' are prolonged to 6' and 5 proportionally, they will therefore in b' and 6 again have come to similar heights above the axis, just as was the case in s*. If we now know the position of the point i, we find the positions of its images by the same formula 3 c, F'f f-F'-f, according to which the position of the nodal points from o is determined (Compare VIII.). XIII. — Signification of the principal surfaces. The signification of the principal surfaces is this, that each ray, for example, abed (Fig. 31), after the second refraction as c d, is directed to a point of h" h', removed as far from the axis as the point of h' h', to which the ray a b before the first refraction was directed. And that this must be the case, appears when we see that the ray in the lens passes through «', and re^ member that aU rays passing through s' at the one side of the lens appear to come from e', and on the other side appear to come from e'. XIY.— Condition for the coincidence of the principal points and nodal points. In a lens bounded on both sides by the same medium, for example, air COINCIDENCE OF PRINCIPAL AND NODAL POINTS. 53 (n' ^ n"), s and o coincide, and therefore also h' coincides with k', and h" with A;" (Fig. 32). Fig. 32. :4^ The point s is, namely, found by diyiding the axis of the lens, into two parts, which are as the focal distances belonging to the principal points ; the point o, by dividing the same axis into two parts, proportiona to the radii of curvature of the two surfaces. Now, since the focal dis-' tances are proportional to the radii of curvature, the division of the axis is in both cases the same, and therefore o and s coincide. The same must be true of their respective images, Jc' and k", and h' and ft". XV. — Condition for the non-coincidence of the principal and nodal points. On the contrary, when the lens is not bounded on both sides by a medium whose refractive coeflB.cient is equal (n' 7 «"), s and o no longer coincide and therefore the images k' and k' (of o) no longer coincide with h' and h' (those of s). For the principal points, also when n' 7 n", the formula still finds application : s lies always at the point where the distance h^ h^ = D, is divided proportionally to the principal focal distances. Then, namely, the two images of s s' continue, as a glance at Fig. 30 immediately shows, of equal size. Now, in proportion as — - becomes smaller, the second focal distance ha ^/ of the surface h^ becomes longer, and s' approaches more and more to h', whose focal distance h^ ' has remained unaltered. Lastly, if ^ = 1, h^ 0/ is infinitely large, and s therefore lies in hi. Then two images of s are no longer formed, but s coincides with its image in the point hi : the lens is changed into a single refracting surface, of which I have in IV. shown how to recognise the cardinal points. While in the case that n" becomes greater than n', the point s is removed forward, the point o deviates in the opposite direction, backwards. This n' appears already from the fact, that, when —r„- has become = 1, and con- -lA- 54 DIOPTRICS OF THE EYE. sequently only one refracting surface has remained, * comes to lie on the surface of curvature A„ and o, on the contrary, in the centre of curvature Ai (Fig. 33) (compare Fig. 17). It must, however, be more accurately Fig. 33. ^' T ' b' h', h' ^' is evidently parallel to a h, and h' f to a h", and, moreover, A" 6 = h' V, the two triangles are equal and similar, and h' 0' = V 0", that is F' = F". S VII. — Seeiproeal dependence of the position of the principal points and nodal points in reference to the foci. In §§ XIV. and XV. it was shown, in what manner, when n" is 7 n', and, consequently, the principal points and nodal points do not coincide, these points may he separately found. This was done in order to exhibit clearly the signification of these points, and to aflEbrd a proof, that in a compound dioptric system points are to be found, having the already explained signification of principal points and nodal points. Now we may proceed from this signification, to show their reciprocal dependence in position, whence it will appear that the foci being known, from the position of the nodal points that of the principal points, or vice versd, may be deduced. "We may remember that for a single refracting surface, where only one principal point and one nodal point exists (Fig- 1 7), k . vE' Sh ^ 3*. POSITION OF THE FOCI IN THE EYE. 65 XXII. — Calculation of the position of the nodal points in the eye. In order to find the positions k' and k" of system C, we should remember (VIII.), that they are the images of the point o, which divides the distance between the nodal points of system A and system B (compare XV. and Kg. 41) into two parts, proportional to the focal distances (?i" and (?/ belonging to the nodal points. The focal distance belonging to h, system A, is ^" k = 0' h' = 23 ■692. The focal distance belonging to h', system B, = 43-707. Moreover, h lies at 8, k' at 5-7073 behind the cornea. Their mutual distance amounts to 2-2927. This is to be divided in the proportion of 23-692 and 43-707. We now find g:^927 x 43-707 _ ^.^ge,. 43-707 + 23-692 So much does a lie behind h' of system B, and consequently, 5-7073 + 1-4867 = 7-194 mm., behind the cornea. The image k', formed of a by the cornea, is found at 23-692 X "7-194 . _,, , , . , ,, 31-692 - 7-194 = ^'^^^ ^^^""^ ^^^ ''°'^°'^- The image k", formed of o by the crystalline lens, lies at Afi.'jM , .4.007 = 1-4376 behind the second principal point of the crystalline lens, and consequently 1-4376 + 5-9356 = 7-3732 behind the cornea. The distance k'k" of system C= 7-373 — 6-957 ='0416 mm. = the dis- tance h! h" of system C. If we first determine the position of the foci, that of the nodal points will follow without separate calculation. k" (j," = h' 0' and k' k" = W h" which has already been proved (XVIII. and XVII.). XXIII. — Calculation of the position of the foci in the eye. Lastly, the focal distances of system Care easily calculated. Parallel rays, refracted by h of system A, converge to the point f, situated at 31-692 behind the cornea. On their way lies system B, and of it they meet h' at 5-7073 behind the cornea, that is, while they converge to a point situated 31-692 — 5-7073 = 25-9847 behind K. Calculated from h", F' of system 5 is = 43-707. — We therefore find (according to form. 3 d) the pos- , . „ « c ^ r, ^26-9847x43-707 -,„„„„ tenor focus of system C at 25-9847 4. 43-707 ~ ^^'^^^ behind //of -B, and consequently 16-296 + 5-936 = 22-23 behind the cornea, that is, 22-231 — 2-3563 = 19-875 mm. behind A3 of system C. The pos- terior principal focal distance F" of the eye therefore amounts to 19-875 mm. 5 66 DIOPTRICS OF THE EYE. To calculate the anterior focal distance, we start from rays parallel in the vitreous humour. Refracted by the lens, these converge at 43-707 be- fore K of system B ; consequently they arrive at the anterior surface of the cornea at 43-707 — 6-7073 = 37-9997, and are there refracted at 23-692 X 37 -9997 _ jg.Qio 31-692 + 37-9997 before the cornea, that is, 12-918 + 1-9403 = 14-8583 before K of Syst. C. Fig. 43. C3 s-. Sn ^■ ft; h c^ -^ -■^; — ^ Sv i-o POSITION OF THE CARDINAL POINTS. 67 The anterior principal focal distance F' of the eye therefore amounts to 14-8583 mm. A similar separate calculation is not, strictly speaking, necessary for F', because we may apply the formula 5 d. XXIV. — Heview of the position of the cardinal points in the eye. We are now enabled to combine in a table the positions of the car- dinal points, first separately of the two component systems of the eye, system A, the cornea, system £, the lens j and subsequently of th? com- pound system C, the eye itself. We have prefixed the computed position and curvature of the refracting surfaces, and represented the values found, in tlieir true magnitude for each system in a diagram (Fig. 43). Above system B is placed a scheme of the form of the lens and of the position of the cardinal points in the eye accommodated for near objects. The line C C represents the position of the cornea ; iVJV that of the retina. The lengths are given in millimetres, and the position is calculated from the anterior surface of 103 the cornea. The refractive coefficients are assumed as -,==- for the aqueous 16 . ' ' and vitreous humours, and :pj for the crystalline lens. We have placed the values which obtain in accommodation for near objects, next those for the eye in a state of rest : Kadius of curvature of the cornea Radius of curvature of the anterior surface of the lens Radius of curvature of the posterior surface of the lens Position of the anterior surface of the lens .... Position of the posterior surface of the lens .... The refracting numbers, represented by the diagram, are the following ; — Anterior focal distance of the cornea Posterior focal distance of the cornea Focal distance of the lens Distance of the anterior principal point of the lens from its anterior surface Distance of the posterior principal point of the lens from its posterior surface Distance of the two principal points of the lens from each other Posterior focal distance of the eye Anterior focal distance of the eye . . .... Place of the anterior focus Place of the first principal point Place of the second principal point Place of the first nodal point Place of the second nodal point Place of the posterior focus Accomiuoda- tioii for Distance. 10 6 3-6 7-2 23-692 31-692 43-707 2-1037 1-2644 0-2283 19-875 19-875 12-918 1-9403 2-3563 e-9Sl 7-373 22-231 8 6 5-5 3-2 7-2 23-692 31-692 33-785 1-9745 1-8100 0-2155 17-756 13-274 11-241 2-0330 2-4919 6-515 6-974 20-248 68 DIOPTRICS OF THE EYE. Hence it appears, that K f = h- f, yV^V h', lif'a, and if they are parallel to the axis, as e d" c", in the point 0'. By the application of these rules we can of each given ray in (n', ascertain its course in (ra'". What deviations it undergoes at the several refracting surfaces, we do not in this mode trace. The lines, therefore, represent only the course of rays, so long as they are in the air (n', or in the vitreous humour («'". To denote this they are dotted between the cornea and the vitreous humour, where the ray leaves the course of the line. APPLICATION OF THE CARDINAL POINTS. 69 Now let a ray, a h (Fig. 45) in (re', be given ; it is desired to ascertain Fig. 46. Fig. 45. a- '•^ / / / ^ 5« \ i '^ j ^ 9- i its course in («'". This object can be obtained in two modes. First, let the ray be produced to the first principal surface, which it cuts in c ; the ray will thus in (n'" be directed to the point of the second principal surface, corre- 70 DIOPTRICS OF THE EYE. spending to c, that is to d (rule 3). Let the ray now out the anterior focal sur- face in 6. All rays, proceeding from 6, are parallel in (»'" (compare 4) ; the direction of one of these rays, h K, is known, for it continues as A" -^ parallel to its primitive direction ; but we thus need only, proceeding from d, to draw a line parallel to hk' oi: k" c: to And the direction of the line d e, which the ray follows in {n'°. — We may equally make use of rules 1 and 3 to find the course. All rays parallel io ah unite in the posterior focal surface 0" into one point. Of one of these rays a' V lif we know the direction, which, namely, continues as K' e' parallel to V K. Where this ray cuts the focal surface ^", the ray a h must also cut the focal surface, and the line d e drawn from d therefore gives the direction of the ray o 6 in {ri°. Let a point i' (Fig. 46) be given: it is desired to ascertain its image/. For this purpose it is necessary only to determine the course in {n" of two rays proceeding from «', and to see where they cut one another. The ray iV passes as h" b through the vitreous humour (compare I), the ray i' a, parallel to the axis, cuts the axis in the posterior focal surface at 0" and intersects k° h in/. In/, therefore, the image of i should lie, if the vitreous humour extends so far : its virtual image lies there. XXVI. — Calculation of the position and magnitude of the dioptric images of the eye. From the foregoing constructions it appears, that the cardinal points of a compound system being known, the position and magnitude of the dioptric images of the eye are to be calculated according to the same for- mulsB, as in the case of a simple refracting surface. h' 0' = k" f" (Fig. 47) is the anterior focal distance F< = G", h' (p" ^ k 0' is the posterior principal focal distance F" = &. Fig. 47. kVi <£;_ -^- ii' Let i now be a point of light, y its image, then A' i —f K i = / h"j = g". According to the formula (3 d or b) we can now calculate the position of/ if / or/ be given, and vice versd (according to the formula 3 a and 3 c) the position of i, if/" or g" be known. Of this formula we shall here make use, in order to caloalate at what distance g^ (reckoned from h), the diagrammatic eye, accommodated for near objects, sees accurately. POSITION AND MAGNITUDE OF THE IMAGES. 71 In this eye (compare XXIV.) f liea 20-248 — 6-974 = 13-274 mm. be- hind k'. Consequently O" = 13-274. The retina lies 22-231 — 6-974 = 15-257 mm. behind A". Henoe/ = 16-257, O' = 6-515 + 11-241 = 17-756 Now we find5'' = -J— ^^—1136-6 mm. = 5-047 Parisian -inches. This is the nearest point of distinct vision ; the farthest (diagrammatic eye, accom- modated for distance) lies at an infinite distance. This range of accommo- dation of 1 : 5-047 corresponds to that of the age of about 20 years. In the eye the two nodal points lie so close to one another, and in ordinary vision ^ is so much larger than /, that, just as in a single refract- ing surface, we may draw the lines of direction of the object immediately through the posterior nodal point, without taking the anterior into con- sideration. The magnitudes of object and image are therefore propor- tional to their distances from the posterior nodal point V. In the diagram- matic eye the last distance amounts to 14-858 mm. Now if the object lies 14-851 mStres from the eye, the image is a thousand times smaller than the object. XX VII. — Comparison of It and P, deduced from jneasuremenfs on the eye and optometrically determined. In XXVI. -we calculated the farthest and nearest points of distinct vision of the diagrammatic eye. Now Dr. Knapp has in four eyes, from the radii of cur- vature and the position of the refracting surfaces, together with the two limits of accommodation, calculated the position of the cardinal points, and thence deduced the nearest and farthest points of distinct vision with the range of accommodation. Now it was extremely important to see how far these results of measurement and calculation agreed with those of the simple optometrical investigation in the same persons. The result is, that in three of the four persons the agreement is as great as could be expected in the difficult and complicated mode of determining the surfaces of curvature. There was found (compare .^rcAi'wy. Ophthalmologie, Bd. VIII. 2. 138) : Accommodation cletermined by Measurements of the S of Curvature. Optometrical Investigation. I. II. III. IV. 1:6-207 l:t!-227 1:3-883 1:3-581 1:3-953 1:4084 1:4-248 1:3-214 If it were necessary, we might in this agreement find a fresh argument in favour of the view, that the aeeommoitition depends exclusively upon 72 RANGE OF ACCOMMODATION. an alteration of the system of the crystalline lens. If no farther argu- ments are necessary to this end, this agreement establishes the correctness of Dr. Knapp's measurements. SECOND PAET. Range of Accommodation. . XXVIII, — Foundation and definition of the range of accommodation. The measure of the range of accommodation must be connected with the change which takes place in accommodation in the eye. If the power of accommodation depended, as was formerly supposed, on a prolongation of the axis of vision, the numerical expression of the range of accommodation should be sought in the amount of this prolongation. The alteration which takes place in the eye in accommodation is, however, of a whoUy different nature, and is confined, as we have seen, to the system of the crystalline lens : the anterior surface increases considerably in convexity, and at the same time advances more forward, the posterior surface becomes a little more convex, without going backward, and with these complicated modifica- tions the position of the cardinal points of the lens, and, at the same time, that of the cardinal points of the whole eye, is altered. The chief point, there- fore, which presents itself is: the shortening of the focal distance of the lens; that is, as if the crystalline received the addition of a positive lens. Now from the focal distance A, of such an auxiliary lens, is the range of accom- modation to be determined, and as the power of a lens is inversely propor- tional to its focal distance, the numerical expression of the range of accom- modation becomes \ : A. "We can by investigation determine the distance JB of the farthest point r, and the distance P of the nearest point p, both reckoned to the eye. From these distances — must be deduced, and if the position and radius of curvature of all refracting surfaces of the eye, in accommodation for r and tor p are not known, we have no other data. Now we have assumed -„- — ^ = . . Rays, namely, proceeding from the nearest point of distinct vision, shaU appear to proceed from the far- thest point, when (P and B being reckoned to the cornea) they are re- fracted by a lens I with focal distance A (compare formula, p. 28). Thus, if such a lens be placed immediately before the cornea, it represents the range of accommodation so far, that without the lens the rays proceeding from r, with the lens, those proceeding from^, come to a point upon the retina. But by this the expression p ~ R — 'A^ ^°* ^^^ justified. The lens, namely. DETERMINATION OF THE RANGE OF ACCOMMODATION. 73 which, is added to the eye in aceominodation forp, lies in and not before the eye, and it thus produces other modifications in the position of the cardinal points than accommodation for near objects. Moreover, with the difference of position a difference of focal distance of the auxiliary lens will be combined, and consequently -j" ~ p ~ B ^^ by no means the lens, which the crystalline of the eye adds to itself in accommodation for p. The question therefore is, whether we may consider this formula as the numerical expression of the range of accommodation. Evidently we here deal only with proportional magnitudes. The formula must serve us, to compare reciprocally the different values of the range of accommodation under various circumstances. Now if the alteration of the crystalline lens, although not altogether equal, be at least sufficiently pro- portional to -^ = \r— -W' ^^^^ formula fulfils all requirements and re- presents also the changes in the eye. It shall be shown that this is actually the case. XXIX.— Determination of the range of accommodation by the focal distance of a lens, with its nodal point situated in one of the nodal points of the eye. In XXVI. the distance cf from the first nodal point to the object, for which the eyes were accommodated, was calculated from the position of the retina and of the cardinal points in given eyes. Vice versd, we may from g' and / deduce the position of the cardinal points of the system. We start from the formula (3 6). jT, J- — . =1, and since, if we have n' G' = n G', g"n^ g'~' G"n _ // ^- g"n+g'' Knowing G' and/, it now immediately foUows, what lens the eye has added to itself, to be accommodated to the distance ^. In this we suppose that the nodal points of the eye have continued unaltered, which, as is evident from what has been above advanced, is the case, when the infinitely thin auxUiaiy lens stands in the vitreous humour and its nodal poi^t coincides with the posterior nodal point of the eye, or stands m the air, 74 RANGE OF ACCOMMODATION. its nodal point coinciding with the anterior nodal point of the eye. We start from a system represented in Fig. 48, system A, whose posterior focus = ^" lies in the retina N N. In this system the foci change, and in- deed so (compare system B) that the image j of the point i now comes to lie in N N, where, in system A the focus (j," lay. The question therefore is, what is the focal distance L" of the auxiliary lens in the vitreous humour which should remove the posterior focus from j to 0" Evidently this lens must make the rays, which, arrived at k" converge to ./, converge to 0" Now h" 0" = G", If j = f. Consequently -^, j, = ^,7. Therefore the auxiliary lens which changes system A into system B is j^,. The question is, further, in what relation does -_ — _ stand to g (= y i). As we have seen .1 + L = _L. g" n ^ Q" n Consequently, \_ , ^ __ 1_ 1 — 1=" G' / / Hence it appears that — = ^-' Consequently, to bring an eye which is L g accommodated for infinite distance, to the distance g', an auxiliary lens of L" -^d. focal distance is required. If R he not infinite, we may suppose that an auxiliary lens = was already present, and that in accommodation for the nearest point an auxiliary lens of -^ is required, in which case the difference between these two auxiliary lenses -^ — ^ expresses the range of accommodation. "We thus obtain, as a general formula, 1 11 n -n 1}~P a ' and if B.z=a> 1 _ n L" I'- The formula J5, in agreement with our formula, makes the range of ac- commodation dependent on the factor _y — L But we have assumed R -7 = p — "S , and find A P Ti' i" ^ P R' consequently L° T=- COMPARISON WITH THE RESULTS OF OBSERVATION. 75 The fooal distance of the lens required, placed in the vitreous humour, is therefore n times shorter than was assumed in the formula for the range of accommodation. "We may, however, also suppose the auxiliary lens to be situated in air, and its nodal point to coincide with K of the eye. It is indeed true that there is no air in k' ; but a system might be constructed, whose cardinal points should have the same position as those of the eye, and wherein air should really be present in K. Moreover, there are lenses (although not in- finitely thin) whose nodal points lie without the mass of the glass, and of such we therefore may suppose, although they are placed before the eye, that the nodal point coincides with the anterior nodal point of the eye. But apart from this, we may imagine the eye away, and the auxiliary lens placed in air, its nodal point coinciding with //. Refracted by this lens, the rays are altered as to their direction. We may now suppose the direc- tion to be prolonged backwards, and that the rays already had this direction when they fell upon the cornea. If, therefore, JJ be the distance from the farthest point to h', P that of the nearest point to ^, L' the required focal distance, then }- = L — }-—l U P M A' and X' = A. The focal distance of the lens may therefore be n times greater, when in air, than when it is present in the vitreous humour. The correctness of this is easily seen. In order to make the focal distances of the eye accommo- dated to the farthest and nearest points equal, the auxiliary lens, standing in air, must displace the focus of parallel rays coming from behind from / 1111 to G': it must be — , = ^^ — — = -r- U O gi A If it be in the vitreous humour, it must displace the focus of rays coming from before from ^ to O". It must therefore be 1 1 _1 _w^ n n 'L"~G" 7~"G' g''~A XXX. — Comparison of the obtained formula _ = _ — with the results A P Jx of observation. In the foregoing consideration (XXIX.) it was assumed that, in accom- modation of the eye for^, while the focal distances become shorter, the nodal points maintain their place unaltered. System A (Fig. 49) should be changed into system JS. In both systems V ^' must be = « A" ^', that is, G' =.n G". In this is included that- the principal points h' h" likewise change their position. In fact K f' must be = k" f" and h" ■* S~ Ss Fig. 50. S. ■^T points, on the contrary, advance pretty considerably, and the anterior focus CHANGE IN THE LENTICULAR SYSTEM. 77 at the same time acquires a different position (compare systems C and D of Fig. 43). It therefore appears doubtful whether the change in focal distance which the crystalline lens undergoes in accommodation is nearly proportional to the focal distance of the supposed auxiliary lens, that is to the value of A in the formula _-:=-— — = This we must therefore investigate. We APR' employ for this purpose the diagrammatic eye assumed by Helmholtz, and the results of the measurements obtained by Enapp in four eyes. Focal Bisbmce of the Crystalline Lens in Accom- modation for \:Fa 1 1 1 1 1 1 1 P li A — P RFa' a\ Distance. Vi oximlty. = Tp~Fr Fr. Fp. 00 I. 38-176 31:971 1:196-7 172-4 1:172-4 0-8763 IL 37-706 29-222 1:129-9 118-6 00 1: 118-6 0-9132 III. 41-449 30-944 1:122-1 109-16 00 1:109-16 0-894 IV. 43-133 30-939 1:112 100-97 00 1:100-97 0-9016 Diagram- ■ matic [43-707 33-785 1:148-8 136-6 00 1:136-6 0918 Eye. . It appears that the value i- : 4' ^* ^^ *'^'^®' '^ ^^ ^^^^ ooeiilcient, that consequently no perfect proportion exists between the calculated auxiliary lens, corresponding to our formula of the range of accommodation, and the actual alteration of the crystalline lens, but that nevertheless we come sufficiently near the truth in assuming that the crystaUine lens in accom- modation has received the addition of a lens, having ^^ of the power of ^, the focal distance of which consequently amounts to ^ A. This result is not without importance. The formula ^ - ^ = 1 thus becomes more than a mathematical fiction, it acquires a physiological signification. § 37 If we observe that the change in the lenticular system consists almost exclusively in the application of a positive meniscus to the anterior surface of the crystaUine lens, we readily arrive at the conclusion that au auxUiary lens, placed before the crystalline, should alter the cardinal points of the eye in such a manner as actually takes place in accommodation, and should therefore most accurately express the range of accommodation. This last we can easily test. Let, namely (Fig. 50), < be the posterior focus of the cornea CC for rays proceeding from the farthest point; i> for rays, proceeding from the nearest point, then the auxiliary lens Z "^-t ma^« [he rays converging to p converge to f , before they reach the crystalline 78 RANGE OF ACCOMMODATION. lens. If d be the distance from the principal point h of the auxiliary lens of the cornea, h f = F", hp/=f" then - = piTZTd ~ f' — d. . For d let us put two millimfetres. It is true, the anterior surface of the crystalline lens lies farther removed frpm the cornea ; but the auxiliary lens /, which in a certain sense places itself before the crystalline, has the form of a positive meniscus, whose principal points are situated in front of the convex surface, and the infinitely thin lens, which is supposed to contain the meniscus, must, in order to have such an action, stand in its second principal point. Now, in the diagrammatic eye, the radius of the cornea = 8 mm., the focal distances of the cornea J" = 23-692 and J"' = 31-692 mm. ;i> lies, in accommodation for near objects, as we have seen, 136-6 mm. from the nodal point, therefore 136-6 — 6-5 = 130-1 mm. (=/') from the cornea. The focus p', of rays proceeding from^ consequently lies at (/" =f^F') /" = 31-692 X 1301 _ 3g.75_ 130-1 — 23-692 The rays proceeding from the farthest point r converge in (p' at 31-692 ; those proceeding from the nearest point p in p' at 38-75 behind the cornea. At 2 mm. behind the cornea, therefore the distance of the points of con- vergence amounts to 29-692 and 36-75 mm. To direct rays converging at .,. , , 29-69 2 X 36-75 ,„ 36-75 to 29-692, an auxiliary lens of 'foQS ~ ™™' necessary. We may make the same calculation for all the four eyes measured by Knapp. In doing so, we should come nearest the truth, by taking into ac- count, in each particular case, the found radius of the cornea and the depth of position of the crystalline lens. But if -we start also from fixed focal distances of the cornea, the anterior surface of the lens being situated at 2 mm. from the cornea, the auxiliary lens found by calculation comes really very near the alteration of the crystalline. This appears from the following numbers : 1 -.Fo F« /" = ' -l-l: Posterior fucal distance P. Focus of the cornea for rays pro- 1 : {F"- -2)-l: (/"- -U) Fp Fr of the comea. ceeding from ji. I. 196-7 31-692 165-9 36-97 196-6 11. 129-9 31-692 112-5 40-15 133-9 III. 122-1 31-692 1033 41-22 1219 IV. 112 31-692 95-06 42-21 113-5 Diagram- ■ matic I 148-8 31-692 130-1 38-75 155 Eve . If, thus appears, that an infinitely thin auxiliary lens, placed 2 mm. be- hind the cornea, in the four eyes measured by Knapp, almost exactly corre- CALCULATION OF THE AUXILIARY LENS. 79 sponds to the alteration of refraction of the lens. If it were now desired to express by snch a lens the range of accommodation, a table may easily be prepared of the value of/" for the different optometrioally-found values of p. But in fact it must be called an accident, that in the assumption of a position of the auxiliary lens, unaltered at 2 mm. behind the cornea, so much agreement is found. The position has, in fact, a very great influence. If, for example, the auxiliary lens lies at 4, instead of at 2 mm. behind the cornea, the numbers change — 196-6 \ / 172-9 133-9 118-3 121-9 1 intd 107-8 113-5 1 100-6 155 , V 136-3 Therefore it is not strange that the diagrammatic eye does not answer when d is assumed = 2 mm. In this case d should amount to about 2|. In the second eye, also measured by Knapp, some deviation appears. In both instances this is to be ascribed principally to the fact, that the anterior surface of the crystalline lens Ues more deeply, and thus the distance of 2 mm. for the auxiliary lens is not quite sxifficient. Hence it appears,^ that if it were desired from r and p to calculate the auxiliary lens at the crystalline, we must likewise determine the place of the anterior surface of the crystalline, and this involves a difB.culty of time and means, without leading to more accurate knowledge than is attainable by the application of the simple formula -> =-p — -j.- "We keep therefore to this formula, which for the determination of the range of accommodation supposes only the optometrical determination of p and r (calculated to the nodal point at about 7 mm. behind the cornea), and thus recommends itself for practical utility. If we* wish to know what auxiliary lens has been applied to the crystalline, we multiply the result obtained -j by the coeffi- cient 0-9. We thus come certainly very near the truth. The influence of ametropia, and of the use of spectacles and eye-glasses on the range of ac- commodation, shall hereafter be examined. CHAPTER II. DEFECTS OF REFRACTION AND ACCOMMODATION IN GENERAL. § 6. — Distinction between defects of e^feaction and of accommodation. Hitherto the defects of refraction and of accommodation have been more or less mixed up one with another. This confusion was an impediment to the clearness of description^ which in this depart- ment particularly, is absolutely necessary to the correct appreciation of the subject. The ideas of refraction and of accommodation must therefore in the first instance be accurately distinguished from one another. It wiU then not be difficult subsequently to recal the con- nexion, so far as may be necessary, between the two. By refraction of the eye, we understand its refraction in the state of rest ; that is, the refraction which the eye possesses in virtue of its form and of that of its component parts, independently of muscular action, independently of accommodation. The term, therefore, applies to the refraction of the eye whose muscles of accommodation are inactive or paralysed (for example under the influence of atropia), to the refraction also of the dead, but as yet otherwise unaltered eye. To begin with refraction, in the condition of relaxation proper, the eye possesses a power of accommodation. The farthest point of dis- tinct vision, therefore, corresponds to the state of rest of accommoda- tion. Now, so soon as the action of accommodation occurs, the eye becomes adjusted to an adjoining point, and it is by diminution of this active operation that it is subsequently capable of seeing a more remote point. Hence, accommodation for an adjoining object alone is an active operation (compare §3). The stronger this action is, the nearer is the accurately seen point. Accommodation is, therefore, the voluntary action whereby the eye becomes adjusted to a nearer point than is the case in the state of rest of accommodation. Hence it appears, that refraction is dependent on the anatomical DEFINITION OF TERMS. 81 condition of the component parts of the eye ; accommodation^ on the contrary^ depends upon the physiological action of muscles. With regard to refraction, we call the structure of the eye normalj when, in the state of rest, it brings the rays derived from infinitely distant objects to a focus exactly on the anterior surface of the layer of rods and bulbs ; in other words, when paraEel incident rays unite on that layer (in 16' Pig. 51). The farthest point of such an eye lies at Fig. 51. an infinite distance. If convergent, rays are also capable of being brought to a focus, the eye possesses something which it does not need : for from all objects proceed divergent or at most parallel rays. If on the contrary, the farthest point lies not at an infinite, but at a finite distance, vision is indistinct throughout a great part of the space. Consequently the refraction of the media of the eye at rest can be called normal in reference to the situation of the retina, only when parallel incident rays unite on the layer of rods and bulbs. Then, in fact, the limit lies precisely at the measure ; then there exists emmetropia (from emierpos, modum tenens, and a^, oculus). Such an eye we term emmetropia. This name expresses perfectly what we mean. The eye cannot be called a normal eye, for it may very easily be abnormal or morbid, and nevertheless it may be emmetropic. Neither is the expression normally constructed eye quite correct, for the structure of an emme- tropic eye may in many respects be abnormal, and emmetropia may exist with difference of structure. Hence the word emmetrojna appears alone to express with precision and accuracy the condition alluded to. Emmetropia then is met with, when the principal focus of the media of the eye at rest falls on the anterior surface of the most ex- ternal layer of the retina (compare Kg. 51). This is the simplest definition. The eye may deviate from the emmetropic condition in two re- spects : the principal focus 0" of the eye at rest may fall in front of 6 82 DEFECTS OF REFRACTION AND ACCOMMODATION. (Kg. 52) or behind (Kg. 53) the most external layer of the retina. Fig. 52. In the former case divergent (dotted in fig. 52), in the latter con- vergent rays (dotted in Eg. 53) come to a focus on the retina. In the first case, therefore, in the condition of rest^ objects are accurately .seen which are situated at a definite finite distance (Fig. h% i); in the second they are at no distance accurately seen, for the rays in faUing upon the cornea must, in order to unite in the retina, already converge towards a point situated behind the eye (Fig. 53 i). In the first case the farthest limit lies within the normal measure : the measure is too short, and the condition might, therefore, be called Irachymetropia. In the second case, the boundary lies beyond the measure, and I have, therefore, called this state hypermetropia. Hence it is perfectly clear, that brachymetropia and hypermetropia are two opposite conditions. The definitions are now extremely simple : the posterior principal focus 'f of the media of the eye at rest falls : — in EMMETROPiA On the most external layer of the retina ; in BKACHYMBTROPiA in front of „ „ ,, in HYPERMETROPIA behind „ „ ^^ In order to express that the eye is not emmetropic, we may use the word ametropia (from aiierpos, extra modum, and &^, oculus). Brachymetropia and hypermetropia are both, therefore, referrible to ametropia. Brachymetropia is evidently nothing else thaii myopia, and it appears preferable to use the word myopia, as being an established MYOPIA AND HYPERMETROPIA. 83 term. The word brachymetropia was formed only in contrast to hjpermetropia, to which expression I thought it right to adhere. Hence it is evident that myopia and hypermetropia are opposite conditions. That myopia is of very frequent occurrence, and is to be considered as an important condition, has long been admitted. Still laore common, however, and more important in its results is hyper- inetropia, which has hitherto been for the most part either over- looked, or confounded with other states. I repeat, what, in consequence of its importance, I have already put prominently forward, that myopia and hypermetropia are the opposite conditions of ametropia. From the definitions given of anomalies of refraction, it has been shown, that the distance R of the farthest point of distinct vision is the foundation on which they rest. The shortening of the focal distance, whereby adjoining points become accurately visible, is the work of the muscles of accommodation. Under the maximum action of these muscles the eye is adapted to the distance P of its nearest point. Now we found as a numerical expression for the range of accommodation -T-= -rf — Ti- The range of accommodation di- A P R minishes, as shall hereafter be more particularly pointed out, with advancing years. At the same time R may remain almost unaltered, and P thus becomes greater. The result of this is, that in the emme- tropic eye the nearest point is at a certain period of life removed so far from the eye, that more minute operations can no longer be well performed with near objects. This condition of the eye is called presbyopia. Presbyopia therefore exists, when, in consequence of the increase of years, with diminution of the range of accommodation, the nearest point has been removed too far from the eye. Pormerly writers were in the habit of contrasting presbyopia with myopia. Apparently this was quite correct. But in myopia only near, and in presbyopia only distant, objects can be distinctly seen. In myopia they found " the mean distance of distinct vision" to be situated too near the eye, in presbyopia too far from it. Thus they would feel obhged, while they either overlooked hypermetropia, or confounded it with presbyopia, to place myopia and presbyopia directly against one another, to regard them as deviations, similar in nature, but opposite in direction. On closer examination it appears, however, that such opposition is illogical. The fact is, that both in an anatomical and in a physio- 84 DEFECTS OF REFRACTION AND ACCOMMODATION. logical point of view, myopia and presbyopia belong to very different categories. Myopia is based upon an abnormal construction of the eye ; presbyopia is the normal condition of the normally constructed eye at a more advanced period of life. In myopia the power of ac- commodation possesses the normal range ; presbyopia on the contrary is based upon diminished range of accommodation, as the natural result of advancing life. Myopia, finally, rests upon an abnormal situation of iht farthest point of distinct vision ; presbyopia, on the other hand, on an altered situation of the nearest point. So little are myopia and presbyopia opposite conditions, that they may both occur simultaneously in the same eye. An eye, for example, which can see accurately only from 20" to 14", is at the same time myopic and presbyopic : the farthest point of distinct vision is situated at too short, the nearest point at too great a distance. Hence we may consider it to be fully proved and demonstrated : — 1. That myopia and hypermetropia are to be regarded as opposite conditions. 2. That it is illogical and unpractical to contrast myopia and presbyopia with one another. With respect to presbyopia, this state is no anomaly, but rather the normal condition of the normally constructed, emmetropic eye, at a more advanced period of life. "Were presbyopia an anomaly, it should not be looked upon as an anomaly of refraction, but of ac- commodation. It should not be classed with myopia and hyperme- tropia, but, on the contrary, with the disturbances of accommodation.. As, however, it is no disturbance, but a diminution of the range of accommodation, it must be treated of in considering the influence of the time of life upon the eye. Accommodation is, as we have seen, based upon a change of form of the lens, produced by contraction of the internal muscles of the eye. Hence it follows, that anomalies of accommodation may be dependent : — a. On disturbance in the lenticular system. b. On disturbance of the internal muscles. Of the disturbances in the lenticular system the condition of total absence of the lens, which I have termed aphakia, comes almost exclusively under observation. The disturbances of the muscles of accommodation are of a very varying nature. Principally we shall have to distinguish : — DISTURBANCES OF MUSCLES OF ACCOMMODATION. 85 1. The -weakness whicli not unfrequently manifests itself by definite phenomena after different exhausting ilkesses. 2. The more or less complete paralysis, which, probably without exception, is connected with a similar condition of the M. sphincter iridis, and often occurs only as a part of the paralysis of the oculo- motor nerve. 3. The spasm, which occurs much more rarely than the paralysis, and, like the latter, is based upon a direct or indirect abnormal ac- tion of the nervous system. Besides these rare forms of spasm, we shall observe, as a very ordi- nary phenomenon in hypermetropia, a persistent increase of contraction of the muscles of accommodation dependent upon habit. This sub- ject shall, therefore, be treated of in speaking of hypermetropia. Moreover, it is here to be noticed in general, that the condition of refraction exercises an important influence on the ordinary use of the range of accommodation, and consequently upon accommodation itself. The modifications so produced cannot be separated from the states of refraction on which they depend, and they therefore come with them under consideration. For this and other reasons it was necessary to give an idea of the subject of accommodation, before passing to the description of the anomalies of refraction. Prom the foregoing, it appears that our principal distinction is hased upon the situation of the farthest point of distinct vision. Thus we obtain a classification of the anomalies of refraction, which of itself excludes a con- fusion of the latter with the anomalies of accommodation. The question naturally arises, whether a classification resting on the nearest point of distinct vision, that is upon P, may not also be observed. On a little reflection it will, however, be seen, that this would lead to con- stant confusion of the anomalies of refraction and of accommodation. In- deed P depends upon both factors, both on the refraction of the eye at rest, and on the range of accommodation. Consequently two eyes, in which P is similar, may, with respect to refraction and accommodation, present great differences : it is only necessary that the differences compensate one another in the two factors. A myopic eye with a small, and a hypermetropic eye with a great, range of accommodation, may have their nearest point at the same distance as an emmetropic'^ye, with an average range of accommoda- tion. Now, if they were classified according to their nearest point, aU these different eyes should be referred to the same category. Moreover, how should we, on this basis, determine the categories ? It should evidently be done quite arbitrarily. We should, for example, distinguish — A category with P less than 2", 4" 86 DEFECTS OF REFRACTION AND ACCOMMODATION. etc., or choose other arbitrary numbers. Lastly, the same eye should, in pro- portion as the power of accommodation diminished, belong each time to a different category. This is enough to prove that a classification of eyes, based upon the shortest distance of distinct vision, is entirely unpractical, and almost leads to the absurd. A classification according to the mean dis- tance of distinct vision, which it has been attempted to make by contrasting myopia and presbyopia, is an illusion ; for a mean distance of distinct vision does not exist, and what does not exist is certainly not to be defined. (Com- pare relative range of aooommodation.) On the contrary, a classification founded on the greatest distance of dis- tinct vision is simple and logical. "With the knowledge of JB we perceive, in the first place, whether an anomaly of refraction exists. Taking the time of life into consideration, we can, moreover, thence nearly determine what P ought to be ; and if P does not actually correspond thereto, we may infer the existence of an anomaly of accommodation. § 7. Causes of the Defects of Eepeaction in General. In defining the anomalies of accommodation, their cause is at the same time assigned. For although very different morbid conditions or morbid processes, may give rise either to paralysis, or to spasm of the muscles of accommodation, — we know that, in the first case the phe- nomena are always dependent on diminished or wholly arrested, in the second on involuntarily exalted action of the muscles of accommodation. On the cause of the anomalies of refraction, on the contrary, the dioptric definition laid down does not throw any light. They are defined simply as disturbances of connexion in the relative position of principal focus and retina. On what anatomical or physiological deviation these disturbances of connexion may depend, is thus left undecided. This would seem to be the place to treat of this subject in general. However, we here state only what is the rule. Deviations, of a pecuhar nature, which occur only sometimes as exceptions, will come under consideration first in speaking of each of the anomalies in detail. The rule is expressed in the annexed three figures. Kg. 54 is an emmetropic, Kg. 55 a myopic, and Pig. 56 a hypermetropic eye. It immediately strikes us, that in the myopic eye the axis of vision is longer, while in the hypermetropic eye it is, on the contrary, shorter, than in the emmetropic. To this almost exclusively it is to be attributed, that parallel incident rays in the myopic eye, come to a focus in front of, in the hypermetropic, behind the retina. Of this DIFFERENT LENGTHS OF THE AXIS OF VISION. 87 difference in length of the axis of vision we can even in Hfe satisfac- Fig. 55. Fig. 54. Fig. 56. torily convince ourselves. Thus if we cause the axis of vision to be as strongly as possible directed outwards, we shall observe the slow alteration of the arching of the oval myopic, and the rapid change of the direction of the arching in the anteriorly situated equator of the hypermetropic eye. Moreover, the other axes of the myopic eye appear to be longer, while those of the hypermetropic are shorter than those of the emmetropic eye. Myopia and hypermetropia might also be supposed to be dependent on many other causes. AnomaUes of refraction might depend upon the curvature of the different refracting surfaces (compare p. 38), as well as on the relative coefficients of the refraction of hght. Theories have not been wanting in reference to this subject. The opinion has in the first place been rather generally enter- tained, that in myopia the cornea is more convex. So far as hyper- metropia was known, it was supposed to be connected mth too great flatness of the cornea, which was positively assumed to exist in presbyopia. And on external inspection it would really appear, as if in myopic individuals the cornea was more convex, while in those who are hypermetropic and presbyopic it is flatter than in emmetropic persons. This appearance proceeds from the fact, that in myopia the iris and the crystalline lens lie far behind the cornea, while in hyperme- tropia and presbyopia they are situated nearer to it. An observer is still further misled to assume a difference in the curvature of the cornea, as in a myopic subject the entire globe of the eye is more prominent, while in the hypermetropic it is more sunk in the orbit, as is often seen. But in truth, the curvature of the cornea in ame- tropia does not essentially differ from that of emmetropia, and the time of life also exercises scarcely any influence. Numerous mea- 88 DEFECTS OF REFRACTION AND ACCOMMODATION. sarements of the curval radius of tlie cornea have satisfied me on this point. They have shown me that^ quite contrary to what it was thoughi should be expected, the cornea at an advanced period of life rather becomes a little more convex, and that in the extreme degrees of myopia, on the contrary, a somewhat flatter cornea is met with. Moreover, the radius of the cornea of both eyes of the same indi- vidual seemed in general to present no difference, or at least a much less difference than usually occurs between cornese of different per- sons ; while, lastly, the radius (as well as the whole eye) is in women somewhat shorter than in men. Though in ordinary myopia the cornea is not more convex, it is evident that, ceteris paribus, a greater convexity of the cornea must give rise to myopia, and we shall hereafter see that in diseases of the cornea myopia is occasionally produced in this way. Moreover, it naturally occurs to us to consider the principal focal distance of the lens as a cause of anomalies of refraction. In con- nexion with it both the curvature of the refracting surfaces and the coefficient of refraction may come under notice. In advancing years the lens becomes externally especially firmer, and thus the coefficient of refraction of the outer layers appears to increase. If this actually takes place, and if the coefficient of the cortical layers thus approaches more to that of the nucleus, the focal distance becomes greater (com- pare p. 39). On this the diminution in advanced Ufe of the refractive condition of the eye appears really to depend. But beyond this no facts exist, which give us a right to assume, that definite changes in the focal distance of the crystalline lens usually occur in definite anomalies of refraction. In some measurements of the surfaces of curvature of the lens from eyes of myopic persons, after death, I found no deviation ; it would rather appear as if, in strongly hyper- metropic individuals, a flatter lens were to be expected. Determina- tions of this kind during life take up a great deal of time, stiU they ought to be made. Of the eyes measured by Helmholtz, as well as among those measured by Knapp, there was by accident one myopic eye. The values found by these observers do not indicate that the lens in myopic subjects should present a shorter focal distance; nor do the results of the removal of the lens in myopic patients lead to this conclusion, as shall be more fully shown in treating of aphakia. Now if the lens in myopic individuals evidently lies (compare Fig. 55 with Kg. 54) in general farther from the cornea than in emme- tropic persons, the focus of the dioptric system must in the former lie MEASUREMENTS OF THE RADIUS OF THE CORNEA. 89 even somewhat deeper than in the latter ; and it is in spite thereof, that in consequence of the elongated axis of vision^ myopia exists. In hypermetropia, the lens being situated more anteriorly (com- pare Kg. 56 with 54), mustj ceteris jparibus,hnng the principal focal distance nearer to the cornea; but, the axis of vision being much shorter, the principal focus stiU lies behind the system. In both cases, therefore, the anomaly of refraction is rather compensated than promoted by the lenticular system. As to modifications in the coefficients of refraction nothing is known. Erom a theoretic point of view we must say, that the index of the cornea and aqueous humour being greater, and that of the vitreous humour being on the contrary less, the principal focus should be removed forwards (compare p. 39). The final result, therefore, remains what we laid down in starting : that myopia usually depends upon an elongation, and hypermetropia upon a shortening, of the axis of vision. The measurements of the radius of the cornea were made with the assist- ance of the ophthalmometer (compare p. 17). They were recorded in the Verslagen en mededeelingen der Koninhlijke academie van Wetenschappen (Reports and Communications of the Royal Academy of Sciences), Afd. Natuurhunde, D. xi. p. 159, and subsequently in the ArcMv f. Ophthalm., B. viii. The principal results are here appended : — 1. The radii of the two eyes of the same individual are in general nearly equal. In the statistics, therefore, when two eyes of the same person were examined, only one mean eye was taken into account. 2. The radius in the line of vision p° amounted in mm. to — Maximum. Minimum. Average. In men 8-396 '7-28 7-858 In women 8-487 7-115 7-799 3. As to the iniluence of time of life — In 79 men, average p° = = 7-858 ,, 20 „ under 20 years, average = 7-932 „ 51 „ „ 40 „ „ = 7-882 „ 28 „ above 40 „ „ = Y-819 „ 11 „ „ 60 „ „ = 7-809 In 38 women, average p° = 7-799 „ 6 „ under 20 years = 7-720 „ 22 „ ,, 40 „ average = 7-799 „ 16 „ above 40 „ „ = 7-799 „ 2 „ „ 60 „ „ = 7-607 4. As to refraction — . ( In 27 emmetropic persons, p° := 7- /So J I „ 25 myopic ,, „ = 7-874 '^ \ „ 26 hypermetropic „ „ =7-96 90 DEFECTS OF REFRACTION AND ACCOMMODATION. Average. rt /• In 11 emmetropic persons, p° = 7'7l9 I „12myopio _ „ „ = J 867 fe V „ 15 hypermetropic „ „ — i loi 5. Influence of the degree of myopia (m) — iM. greater than i, p° = 7930 M. „ „ ^, „ =7-829 M. less „ 4 „ = 7'^^7 (Emmetropic = 7-785) g ( M. greater than |, , p" = V-935 a I M. less „ .V „ = 7-780 ^ ( (Emmetropic = 7-719) 6. Influence of the degree of hypermetropia (h.). ^-B. = iby^, p° =7-935 A \ — ■ to 4, = 8-010 iS 1 — 1 to ' = 7-939 ^ (Emmetropic = 7-78o) g (H =^ to4 P° =7-876 g I „ = 35 or under, „ = 7-692 fe I (Emmetropic = 7-719) § 8. DiAGEAMMATIC REPEESENTATION OF THE EANGE OP ACCOM- MODATION, AND OF THE ANOMALIES OF EEFRACTION AND ACCOM- MODATION. In § 5 we have seen wliat is to he understood by range of accom- modation. We described the faculty of accommodation as tbe power of the eye to add to itself a positive lens, and the strength of this lens was for us the measure of the range of accommodation. We further showed^ that the focal distance of this auxiliary lens was im- mediately found by ascertaining the distances from the nearest and farthest points of distinct vision to the nodal point of the eye. These distances we called P and E, and the range of accommodation was then : — A - P ~ R If -r be the range of accommodation, A is the focal distance of the auxiliary lens, which the eye is capable of adding to itself. All this is very clear. But it was still a desideratum, by means of a -drawing, to make it easier. An attempt in this direction suc- ceeded beyond expectation. Not only can we in a diagram express WllK — «:> iJl-c ■» '* > 'JtVJ j > ! i \ "5 ! [ 1 «ln IN I »< i 1 ^"'^=oS;«5°oS;^<«'« EXPLANATION OF THE DIAGRAM. 91 the range of accommodation, proportional to the length of lines, but the beginning and end of the lines at the same time show^ and r, and thus at once make us acquainted with the degree of myopia and hypermetropia of the eye so represented. A glance at the ap- pended table will demonstrate this. The lengths of the thick hori- zontal lines represent the ranges of accommodation. Above the slighter vertical lines the distances from the eye, at which acute vision takes place, are noted: the numbers exhibit (in Parisian inches) the distances whence rays must diverge, in order to come to a focus on the retina. The explanation of a couple of these lines may serve to elucidate this. The first transverse line represents the boundaries and range of accommodation in a child of twelve years. The latter begins at oo , that is, at an infinite distance, and terminates at 2f ". This indicates that the distance of the farthest point R = oo , while that of the nearest P = 2|". The eye is therefore emmetropic, and has a range of accommodation of — ~ oi- ^^^ ^^^^^ ^® ^^^'^ ^^ ^"^^^ ^'o™ the first line. 3 «> 5 The fifth transverse line represents the boundaries of accommoda- tion, and the range of accommodation of a young man, aged sixteen. The farthest point Kes at 18, the nearest at 3i, inches from the eye (R = 18, P = Zj). He is therefore, in the first place, near-sighted, and his near-sightedness is of that degree, that it may be corrected by glasses of — — (that is of 18" negative focal distance). Such glasses give, to the rays derived from infinitely remote objects, a direction as if they came from a point 18" from the eye. Therefore, the degree of myopia is also expressed by ^o' M = f^. Now if, moreover, the nearest point lies at 2|, we find as the range of 111 accommodation 1^ — T-g = 5- -A-H this is included in the fifth line. The principle involved in this diagrammatic representation is this, that by the mutual distance of two vertical lines a definite range of accommodation is each time expressed ; for this ^^ is here assumed. If we now begin at co , and reckon to the left, we find : — above the first line 1 : 34, corresponding to --^ range of accom- modation. 92 DEFECTS OF REFRACTION AND ACCOMMODATION. above tlie second line 1 : 12 2 x -^ „ third ,,1:8 3 X ^^ „ sixth „ 1 : 4 ^ ^ 24, „ seventh „ 1 : 3? "^ ^ 94, etc. The difference between the range of accommodfition of two ad- joining lines is, moreover, always = ^ ; for example : — 6 8 ~ 24, 2| 2i~ 24'" If we now have the nearest and farthest points united by a trans- verse line, we need only to reckon how many intervening spaces of vertical lines the latter runs through, in order to ascertain how much X — the range of accommodation amounts to. The first 24 ° transverse line passes through nine intervening spaces, and therefore 9 1 represents —— = — ^ range of accommodation; the sixth, runs 8 1 through eight intervals, corresponding to ^ = - range of accommo- dation, etc. From this representation it is now very plain, that as much power of accommodation is necessary to come from an infinite distance to 8", as from 8" to 4" ; as much, to come from 6" to 4s" as from 4" to 3", etc. In a word, the distance of each pair of vertical lines corresponds to the same ( ~ oZ ) ^^^E^ °f accommodation, and therefore in order to come from one to the other, the same action of the power of accom- modation is each time required, that is, the eye must each time add to itself a positive lens of . ■ Moreover, we observe, that also to the right of ao vertical lines occur, above which numbers are placed. These all belong to the domain of hypermetropia. The numbers show, namely, in Pari- sian inches, at what distance behind the eye the incident rays must converge to a point, in order to unite upon the retina. The EXPLANATION OF THE DIAGRAM. 93 eighth transverse line therefore represents the eye of an hyperme- tropic person. In total relaxation of its accommodation, the rays must, in order to unite upon the retina, converge at 24 inches behind the eye. In order to see accurately at an infinite distance, the indi- vidual will therefore require glasses of ^, with which parallel rays acquire the convergence just mentioned. This hypermetropia is con- sequently neutralised by glasses of-^; in his eye a lens of ^j falls too short, and therefore we define the degree as H = ^. With the strongest possible tension of the power of accommodation, the same eye sees accurately at the distance of 4 s", from which point the rays must therefore in this case diverge, in order to come to a focus upon the retina. The range of accommodation of this eye therefore reaches, in the first place, to ^, in order to come to oo, and, moreover, to -r, in order to come to the nearest point of distinct vision. It is therefore 24 ^ 4J 4. In accordance with this result, we see that the transverse hne ex- tends over six intervening spaces, corresponding to ^ = -grange of accommodation. In an hypermetropic condition of the eye the distances are nega- tive, that is, they lie behind the eye. Therefore we also find in the table to the right of <», 1 : 24, etc., marked ; and therefore, too, in the formula for the range of accommodation p —^ =-^, the terms are negative, so far as the distances expressed by P and R lie on the negative side. In the example above adduced of the eighth trans- verse line, this was the case with R. The formula therefore became P \ R) A' and consequently the range of accommodatien must be calculated as _+^-^andnotas^,-2-^. 94 DEFECTS OF REFRACTION AND ACCOMMODATION. The other eyes, represented in the table, need no further explana- tion. The above has, however, fully shown how to deduce from the transverse lines the nature and the degree of the ametropia, as well as the range of accommodation. It need not be remarked, that by the method here described, we can rapidly and easily register a series of eyes, whose accommodation we determine, and that on a definite principle we can easily compare with one another cases classified in this manner. We shall hereafter repeatedly make use of this method. The range of accommodation is expressed by the dioptric power of an in- finitely thin auxiliary lens, which is supposed to be placed in air, and to have its nodal point in the anterior nodal point of the eye. I have chosen the same mode of expression for the different degrees of . myopia and hypermetropia. To this I was led by the following reasoning : — If we could place in the hypermetropic eye a positive, in the myopic eye a negative corrective lens, these might thereby be converted into emmetropic eyes. The optical power of the required lens therefore represents the degree of ametropia. With the knowledge of R the lens is given. In the emme- tropic eye, iJ is = co ; in the myopic, iJ is a finite magnitude ; in the hypermetropic eye this magnitude is negative. In both cases -= fg the dioptric power of the infinitely thin lens, which, placed in aii, and having its nodal point in the anterior nodal point of the eye, should make the ametropio eye emmetropic, without altering the situation of the nodal points (compare p. 73 ei seq.). Consequently, -g is the numerical expression of the ametropia itself. The myopic eye has a lens of — too much, the hypermetropic has a lens of -g too little. We may therefore consider myopia M., in reference to emmetropia, as a positive, hypermetropia H., as a negative condition. There- fore, too, as the negative is included in the word hypermetropia, we need not write if = — _, but for the sake of simplicity we may use the expression -ff = ^, as well as 3f — -L. Against this method of expressing the degree of ametropia by the dioptric power of a lens, the objection may be raised, that in ametropia the dioptric system is, by a corrective lens, by no means made similar to that of the emmetropic eye. Ametropia does not, in fact, depend upon a deviation in the power of the crystalline lens, but rather on a deviation from the normal length of the axis of vision. With a positive lens we therefore obtain, in hypermetropia, a stronger dioptric system with a shorter axis of vision : with DEGREES OF MYOPIA AND HYPERMETROPIA, 95 a negative lens in myopia we obtain a weaker system with a longer axis of vision. This objection is not without some foundation. StUl, what applies to range of accommodation does not hold good for ametropia, — that it is in. the eye actually represented by a lens. Nevertheless, I have not hesitated to use this measure also for ametropia. In the iirst place, it recommends itself by its practical utility : not only is the degree of ametropia thus easily found by the definition of iS, but with its expression is at the same time given the focal distance of the glasses by which it may be neutralised. In the second place, no other measure is possible. Were we, in order to fix the actual deviation, to take the length of the axis of vision as a measure, we should be met with the difficulty, that during life it cannot be directly determined, and, could we determine it, it would not afi'ord an immediately practical in- dication. Besides, nothing is easier than to calculate the length of the axis of vision, which about corresponds to difierent degrees of ametropia, and thence to make tables such as shall be found in the Chapters which treat of Myopia and Hypermetropia. Thus I consider the method I have pursued to be fully justified. It is, indeed, new only in form, not in reality. What I term M = ^ would for- merly, if it were desired to express the degree of myopia, have been described as a degree of myopia, for which glasses of R — x Parisian inches' negative focal distance are required, in order to adjust the eye for parallel rays. The value S — X still needs some explanation. M is the distance from r to the nodal point 7c'. Consequently the corrective lens is supposed, in ametropia, to lie in k', as well as the auxiliary lens, which expresses the range of accommodation. This is done on purpose, in order to admit of the distances of distinct vision of ametropio and emmetropic eyes being compared with one another, and registered in the same diagrams. If it be desired to neutralise the ametropia by an actual corrective lens, that is, by an eye-glass, we must always take into account the distance x between the nodal point of the cor- rective lens and the nodal point of the eye, as shall hereafter be more fully explained. A not unimportant question still remains to be solved. The range of ac- commodation we have set down as = p — _g. We found, however, that the actual change of the crystalline lens is not quite equal thereto (compare f.l5etseq.). Now the query arises, whether the length of the axis of vision has nfluence on the value of p — ^, in other words, whether, on a given change of the crystalline lens, a difierence in range of accommodation shall be found, according as the eye is emmetropic, myopic, or hypermetropic. The question is easily investigated. We take the diagrammatic (schematisch) eye of Helmholtz, in accommodation for distant and near objects, as our basis, and calculate S and P, and thence deduce the range of accommodation for difie- rent supposed lengths of the axis of vision. 96 DEFECTS OF REFRACTION AND ACCOMMODATION. Length of the axis of vision. E P in millimfetres. 1 1 P R' Emmetropic Myopic - - - - Hypermetropic 22-231 25-231 20-231 00 118-31 — 177 136-62 65-056 505-73 1 : 136-62 1 : 144-54 1 ; 131-11 Hence it appears that an equal change of the crystalline lens produces, where the axis of vision is longer (myopia), a less, and where the axis of vision is shorter (hypermetropia), a greater value of p^ — -p. By calculat- ing the eyes determined by Enapp, I obtained the same result. The differ- ence is, however, but slight. With the supposed lengths of the axis of vision, jR was, for the myopic eye = 4" ; for the hypermetropic eye = — 6" ; so that the myopia amounted to J, the hypermetropia to ^ ; and with these high degrees of ametropia the deviation in the values of -p — -^ amounted only, in the case of myopia, to about 6 per cent., and in that of hyperme- tropia to 4 per cent. For practical purposes these differences present no difllculty. In this comparison of ametropio eyes with emmetropic, we started from the supposition that the dioptric system of the former agrees with that of the latter. This is, however, not quite correct. In general the crystalline lens lies, in the hypermetropic eye, closer to the cornea, in the myopic, farther from it. Now a change of form of the crystalline lens wiU have less influ- ence on the distance of distinct vision, in proportion as the lens is situated farther behind the cornea (compare p. 62 et seq.). Consequently this influence will be less in the myopic, and greater in the hypermetropic eye, than in the emmetropic. In this we have therefore a second reason why a definite change of the crystalline lens shall represent in the myopic individual a less, and in the hypermetropic, a greater range of accommodation than in the emmetropic. Now if, notwithstanding, a greater range of accommoda- tion be found in myopic than in hypermetropic individuals, the inference is evident, that the former can produce a much more decided change in their crystalline lens than the latter. § 9. Clinical determination of Ametropia in General. A.S we have already seen, and as shall hereafter more fully appear, both myopia and hypermetropia exercise a great influence upon the function of vision, and both are closely connected with numerous affec- tions of the eyes of a different nature. Hence it is, that the ophthalmic surgeon must make it a rule, in every patient who applies to him, to CLINICAL DETERMINATION OF AMETHOPIA. 97 determine the refractive condition of the* eyes. But in acute inflam- matory affections, it is quite allowable in the first instance to defer the determination; though, when the inflammation gives way, it ought not, even in such cases, to be neglected. I have long been accustomed to note this of all my patients : in the hsts in the Ophthalmic Hospital a special column is provided for the purpose. I have in numerous instances found the great advantage of this rule. The determination itself is effected, after some practice, with rapidity and certainty. Two methods have been employed. The first consists in testing the power of vision with glasses of known focal distance. The second in the determination of the refractive condition by means of the ophthalmoscope. I. Tor the employment of the first method we require, in the first place, the necessary glasses from ^ to - and from — stt to — -; in the second place, the necessary objects for testing. The pairs of glasses are kept loose in a box, with a spectacle-frame in which they can be placed.* It is also convenient to have a black plate of metal of the same size as the glasses, which, placed in the frame, closes one of the eyes ; by closing the eye with the finger, the accuracy of vision is easily lost for some moments, so that we cannot make the examination of this eye follow immediately upon that of the other. The most suitable objects are letters and numbers. Dr. Snellen has drawn up theSe in a regular system, and has thus supplied a want which had long been felt. The principles kept in view by Dr. Snellen are the following : — 1 . Detached, separate letters, black on a white ground, in irregu- lar sequence. 2. The letter, large Eoman, square, the vertical strokes being j, the horizontal ^ of the breadth of the letter. * Paetz and Flohr, opticians, unter den Linden, Berlin, supply suoli boxes with the necessary positive and negative glasses. The boxes contain, more- over, prismatic and coloured glasses, with a spectacle-frame. Jaeger's spectacle-frame, prepared by Kraft und Sohn, mechanicians, Vienna, Stadt, Karrnerstrasse, 1043, im Bilrgerspital, is convenient, in which the rings containing the glasses are movable, admitting of their distance being so regulated that the patient can look nearly through the centre of both glasses. 7 98 DEFECTS OF REFRACTION AND ACCOMMODATION. 3. Exclusion of some letters which are much more diificult to dis- tinguish than others. 4. Ascending magnitudes from I to CO, the magnitude being proportional to the number, so that CO is two hundred times larger than I ; XX ten times larger than 11, etc. 5. The several magnitudes distinguishable by a sharp eye, in good light, at the distance of so many feet as the number amounts to. Thus II at 2 feet, VI at 6 feet, XX at 20 feet, etc., aU seen at simi- lar angles (of 5 minutes), are equally easily distinguishable by the eye exactly accommodated to the distance. By the application of these principles great advantages are ob- tained. In the first place, the existence of ametropia is at once apparent, when, with respect to the power of distinguishing, the pro- portion between distance and magnitude is destroyed : for example, if a person sees I at 1 foot, II at 2 feet, and cannot see XX at 20 feet distance, myopia exists, etc. If he sees XX at 20 feet, and does not see I at the distance of 1 foot, the nearest point lies at more than 1 foot from the eye, etc. In the second place, we can immediately with perfect accuracy determine the sharpness of sight. He who, having his eyes properly accommodated, distinguishes XX only at 10 feet, instead of at 20, has a sharpness of vision S = ^ =-,when he distinguishes III at 1 foot, his vision is S =-g; when he o 20 1 sees C at 20 feet, it is S = ^-^ — ~^, etc. He who^ distinguishes C, LX, XII, III, only at the distance of 1 foot, has his vision equal re- spectivelytojJ-Q,^,l,i etc. In the examination for the determination of ametropia, we have to do only with R, and for this purpose we cause the patient to look at the distance of about 20 feet; while on the card intended for distance (as card 2 appended to this work), even CO stiU occurs, thus it appears applicable as far as S = -y^ If S be stiU less, we bring the card nearer to the eye ; finally, reckoning the fingers may be con- veniently substituted for distinguishing letters. For persons who cannot read, we may substitute reckoning vertical strokes. By this method, however, it is difficult to obtain results, and they are, moreover, not capable of comparison with those ob- CLINICAL DETERMINATION OF AMETROPIA. 99 tained with letters. It is therefore better to teach such patients to re- cognise a couple of letters and a couple of figures, which is easily done. The mode of quickly recognising the ametropic condition, is best learnt by means of practical instruction. We must here, however, endeavour to give some general indications on the subject. Por this purpose let us assume a clinical point of view. Minuter details will be given in treating of the several forms of ametropia and modified accommodation. A PERSON AGED TWENTY PRESENTS HIMSELF. The question is : — does ametropia exist ? We give Mm small print — I to IV of Snellen's test-types to read. A. He reads I without difficulty at a distance of from 6 to 12 inches; II at the distance of 2 feet. We in the first place infer, that his power of vision is sharp, secondly, that he is either emmetropic or at least but slightly ametropic. We show him XX at 20 feet. He reads it likewise. Is he then emmetropic? is stiU the question. 1. With — rn he does not see XX at the distance stated, letter de- ■Rned; he is not myopic. With j^ he sees the letters fainter, less blach, although somewhat larger: he has no manifest hypermetropia. May he, nevertheless, be hypermetropic? Latent hypermetropia might exist, which, so long as accommodation is active, cannot appear. This may manifest itself only after the instillation of sul- phate of atropia (gr. i to dr. ii), paralysing the accommodation; if it exists, the eye should now see much more sharply at a distance with — •> perhaps even with -— or — - 40 ^ ^ 24 16" Must we then, in order to satisfy ourselves of the existence or non-existence of latent hypermetropia, in each of our patients, para- lyse the power of accommodation by means of atropia ? By no means ; this ought to be done only when there is reason to suspect hyperme- tropia, and even then we should warn the patient, that, for some days, impairment of vision, particularly for near objects, with dimness, and probably with intolerance of light, will remain. When, therefore, are we justified in assuming or suspecting in a youthful individual the existence of latent hypermetropia ? We may assume it when mani- fest hypermetropia exists ; a portion is then always latent through 2 100 DEFECTS OF REFRACTION AND ACCOMMODATION. the action of accommodation. We may with great probability suspect it: 1, when convergent strabismus is present; Z, when there are complaints of asthenopia; 3, when P is much too great for the time of life. If, for example, the person examined at the age of 20 years says he cannot read accurately at the distance of 6", we shall in 19 cases out of 20 detect latent hypermetropia. As shall hereafter appear, it may then become desirable to give him glasses. 2. Ifwiik — -Th^^ *^^* '^"'■^ accurately at a distance, he is very slightly myopic. 3. If with — le sees as accurately as wit'ho^lt glasses, there is manifest hypermetropia. Let us take glasses of higher power: s^j ^, etc. So long as he continues to see equally well, the mani- fest hypermetropia is not corrected. The highest glasses, with which he sees accurately, indicate the degree. If he stiU sees accurately with — , his manifest hypermetropia is = ^-. In this case we should also determine the total hypermetropia (manifest + latent), after para- lysis with atropia. B. Ke reads I hest at 6", No. II at 9", loth, indeed, much nearer, hut not farther off. From 6" and 9" reading becomes somewhat more difficult. The dilemma is : either myopia or diminished sharpness of vision. At 20 feet distance he does not see No. XX, nor XL, nor LX, which last are three times larger than XX. Myopia almost certainly exists. We try with — -• Now he sees much more accurately and reads No. XXX or even XX at a distance of 20 feet : the myopia is proven. Its degree is, however, not exactly known. Why did we try glasses of — Q? Because the farthest point, at which tolerably acute vision stiU existed, lay at about 9". By attending to this, we come tole- rably near the degree of M. If he sees with — -, the parallel rays acquire a direction, as if they came from a point situated 9° in front of the glass. By comparison with glasses of — - it appears, that with the latter he sees still more accurately ; with — ^ not better CLINICAL DETERMINATION OF AMETROPIA. 101 than with — gfwith - decidedly less accurately. M therefore exists _ 1 ~ 8' C. He cannot, or at least can only with difficulty, read No. I {or even larger letters), at whatever distance the boofe be held. His tirae of life excludes presbyopia. But three cases are still possible : there exists either diminished accuracy of vision, or H, or paresis of accom- modation. Where the pupil is freely movable, with normal diameter, the last is almost with certainty excluded. The shortest way is, however, immediately to make him read with y^j' Spectacles with these glasses should always lie on the oculist's table. It is in very many cases the first number which he tries in order to arrive quickly at a conclusion. If with ^t; No. I be read at 13 inches, even at 16" r ( = 1 1) be read : we can no longer suspect diminished accuracy of vision, and H has become very probable. At the distance of 20 feet XL is distinguished, also XXX, but XX, on the contrary, is not. But with jryr thc patient sees them more accurately ; with ^ he distin- guishes XX, with y^ he still sees them as well ; with ^s the letters 1.0 J.0 begin to be diffused : the existence of H and indeed of H = y^ is thus established : S is at the same time perfect. If positive glasses pror duce a considerable improvement, but if none can be found, with which XX is distinguishable at 20 feet, H is complicated with diminished sharpness of vision, as often is the case. In either instance, the total H should now, by the artificial production of paralysis, be determined.. Had paresis of accommodation existed, without H, the naked eye should have seen accurately at a distance, and even weak positive glasses should have diminished the accuracy of vision with respect to remote objects. The condition would have been immediately distin- guishable from H from the fact, that with y^ at more than 10" the letters would have become somewhat diffuse, and consequently I^ could not have been read at 16". Where compHcation with diminished accuracy of vision exists, examination of the media and of the fundus oculi with the ophthalmoscope is necessary. In H this investigation 102 DEFECTS OF REFRACTION AND ACCOMMODATION. is frequently negative, although the accuracy of visioijis diminished. Not unfrequently astigmatism is at the same time present, the con- sideration of which I must defer to a subsequent chapter. D. The patient reads II, or at least IV and YI at 3 , 4", or 5" from the eye, hut not at a greater distance. Here either myopia with diminished accuracy of vdsion, or a high degree of hypermetropia exists. If he reads No. VIII at 2 feet, it can scarcely be anything else than hypermetropia. If at a distance he sees only LX, with glasses of ^,1^0. XXX with those of ^lesswell, it is hypermetropia, and indeed H»? = ^ ; a portion is still latent. Had myoj)ia existed, with greatly diminished accuracy of vision, the patient would have seen worse at two feet distance, and, what is decisive, the vision of remote objects would have diminished with positive glasses ; with negative, on the contrary, it would have increased. Why in high degrees of H letters of a definite size are seen better very close to the eye than at a distance of 1 foot, shall be explained in the Chapter upon Hypermetropia. E. B^e says he can see quite well and accurately, particularly at a distance, but that his vision is also good for near oljects. But the eye soon iecomes tired ; close work he cannot keep up. This is astheno- pia, to be treated of, in detail, in a separate chapter. Here I may just observe, that in the great majority of cases H is the ultimate cause of it. We should try whether the patient can read at 6', h", and 4"; whether it is difficult or not. We cause him to look to a distance : weak positive glasses of ^, -^, etc., improve or at least do not diminish the accuracy of vision. Thus the presence of H is demonstrated, and it now remains only to determine its increase by artificial paralysis (the latent H). But sometimes, notwithstanding the existence of asthenopia, the letters at a distance are rendered somewhat diffused by weak positive glasses, for example of jj.- Can we thence infer the absence of H ? By no means, it is almost certain that latent H exists. We must, therefore, in such cases determine P, and afterwards have recourse to artificial paralysis of accommodation. Should it thus appear that no H exists, -»- =^5 — ^ will be found particularly small, and we thus come to the question of paresis of accommodation, which is in&nitely rarer than H. All these cases hang upon the determination of R. "With it the CLINICAL DETERMINATION OF AMETROPIA. 103 existence or non-existence, and at the same time the degree of ame- tropia are given. Moreover, we could in the simple manner already described determine the nearest point ; with it the range of accom- modation r = p ~ p ^^ tnown. With the increase of years it diminishes (compare the Third Chapter, p. 126), and vision is conse- quently considerably modified. Therefore it was necessary in the foregoing examples to suppose a definite time of Hfe, and we chose a young man of 20 years. It will be advantageous to bring forward some persons of more advanced age. A MAN AGED FIFTY PRESENTS HIMSELF. A. In good light he easily recognises No. II at 20, and even at 24 inches, No. ¥ doubtfully at either distance, No. I not at all. At the distance of 16 feet he recognises the letters of XX. The accuracy of vision is therefore practically perfect. With jr he sees less accurately at a distance, but near objects with much greater ease. Our conclu- sion is : there exists only Pr, and for close work he had already been obliged to use spectacles. B. He cannot read without spectacles. Even ten years ago he began to experience difficulty at Ms worJc. At a distance, however, he then saw accurately, hut now he sees less sharply : No. XX he does not recognise, at the distance of 20 feet. No. XXX douitfully, and the letters are not llach. We may be nearly certain, that in this case Pr has been superadded to H. With ,-„ he reads No. I at about 12", closer with greater difficulty : the accuracy of vision is perfect; the existence of H has, properly speaking, been already proved by seeing at 12" with glasses of y^. Let us determine it by looking at a distance ; with 5^ vision is acute as well as with ^tjj ^^^ si it is less good. H exists = ^^. At fifty years of age the latent H is very trifling ; we need not determine it. Glasses of o^, may be constantly worn by this patient ; for reading and writing something stronger is required. a C. "He has always had excellent sight, saw distant and remote 104 DEFECTS OF REFRACTION AND ACCOMMODATION, objects exceedingly well, boasts conceitedly of his eyes, but has for some weeks observed that he no longer sees at a distance so accurately with the right eye." He reads I from 6" to 12", No. II. ai 2,, hut not No. Ill at Zfeet distance. We infer near-sightedness. The patient denies it ; is surprised that he cannot recognise Nos. XX and XXX at a distance, and still more that he accurately distinguishes them with glasses of — -^. The eye with which he could still read, but could see less accurately at a distance, appeared to be affected with a trace of cataract. These examples may suffice to point out in general the mode of looking for ametropia. As important for the first indication, I shall add only, that many hypermetropic persons complain of asthenopia ; myopics for the most part know, that they see com- paratively less accurately at a distance ; that, moreover, the first have usually a shallower, the latter a deeper eye-chamber, while, lastly, the age for presbyopia affords an indication. In the determination of R with the aid of glasses, the distance x from the glass to the nodal point k of the eye is neglected. In using glasses with a long focal distance so has less influence ; but when those with a short focal distance are employed, x must be taken into account. If we have to do with positive glasses, x must be deducted from the focal distance ; if with negative, it must be reckoned with it. This has already been explained (pp. 32 and 35) . Thus, if myopia be neutralized by glasses of — -g> aud if a; = 1", M = - ; if hypermetropiabe cor- rected by glasses of 5, and if a; = I", then H = ^. The influence of x may also help us in the determination of the degree of ametropia. If, namely, an equally accurate or even a more accurate image was obtained with the glass employed, by moving it further from the eye, the negative was too weak or the positive too strong. We thus know what glass we should sub- sequently try. It might, perhaps, be supposed that we should have only to determine with the glass first tried, x, and to take its value into account. This might, however, lead to an incorrect result. My- opic individuals, in fact, wiU often prefer to hold a glass, though it is too strong, close before the eye : the image is then larger, and by some tension of accommodation they prevent its being diffused. Therefore we must, as a final determination, with the myopic always ASCERTAINED BY THE OPHTHALMOSCOPE. 105 try what is the weakest glass, which, held close before the eye, gives a defined image. In the hypermetropic we run less risk in taking a great value of x into account ; but it is better in this case also, to make the final determination with a glass, which, held close to the eye, gives defined images. In high degrees of myopia, and where uncertain answers are given, the investigation is often shortened by ascertaining the influence of weak glasses, for example, of j^ and — — , alternately held before the stronger negative glass placed in the spectacle frame. II. In the second place, we may in a certain sense determine more objectively the refractive condition by means of examination with the ophthalmoscope. The great inventor of the instrument has not only pointed this out, but has also communicated the application of this method. It may be explained in a few words. According to well known laws, the rays proceeding from a point of the retina, re- fracted by the media of the eye, shall have, on entering the air, a direction similar to that of the rays which, falling on the cornea, unite in the same point of the retina. If M exists = ^, the point o r, whose emitted rays unite in the retina, lies 8" in front of the nodal point, and in the same point r will the rays emitted by the retina, converging in front of the eye, unite. If H exists = yrr, there unite upon the retina rays, which, converging to a point r, situated 10" be- hind the nodal point, fall upon the cornea ; and, vice versa, the rays emitted by the retina, having reached the air, are diverging, and appear to have proceeded from the said point. Lastly, the emmetropic eye at rest, which has its focus for parallel rays in the retina, gives to the rays proceeding from the retina, when they reach the air, a parallel direction. Consequently the eye of the observer, in order to see accurately a non-inverted image of the retina of the emmetropic eye, must be adapted for parallel rays ; on the contrary, it must be adapted for converging rays, in order accurately to distinguish that of myopic persons ; and for diverging, in the case of hypermetropic individuals. Therefore, if the observer knows the condition of his eye, with which he sees accurately the retina of another, he can form an opinion as to the refractive condition of the observed eye. It is best to practise one's self in voluntarily seeing with accommodation 106 DEFECTS OF REFRACTION AND ACCOMMODATION. for one's farthest point (ascertained by investigation), and to try what glass we must place before one's eye, so as accurately to see the vessels in the retina of another. In order to be able to bring the most different glasses before the eye, I have had a ring made on the oph- thalmoscope, adapted to hold the glasses of the spectacle box. My eye is emmetropic and is accustomed, in the use of all optical inst;:u- ments, to adapt itself for parallel rays. Now, if I need a glass of — 77! to see a retina accurately, myopia of ^ e^cists ; if for this purpose I require a glass of ^77) H of — is present. Some correc- tion, negative for M, positive for H, is necessary, both for the distance between the observing and the observed eye, and for that between the glass and the observing eye (pp. 3 2 and 35); but if we approach as much as possible, this may be reduced to about 1" : by introducing this cor- rection, therefore, in the above quoted examples, M may have been = — and H = -• Moreover, ia the alteration of the distance between y y the observing and the observed eye we have a means of estimating whether we should try a stronger or a weaker glass. — If the eye of the observer be ametropic, the degree thereof is easily taken into account. If, for example, the same glasses as above had been necessary for an eye with M = irg, the eyes examined should have given M = =-5, H = —-)-— = —. J^ice versa, where the same glasses _ 1 1^^ J. 18 "~ 18' 9 "^ 18 6 were required for an observing eye with H = y^, the M found should have amounted to -x- -h ts = -ttj the H = -—-—= -—. y io o y i» io In observing with the ophthalmoscope in the inverted image the estimation is more difBcult, becaiise the influence of the objective glass to be held before the eye and the position of the image cannot be well defined. High degrees of myopia, however, manifest them- selves immediately, as, without holding a convex glass before the observed eye, we see the inverted retinal image stand before this eye. So far as we can determine the distance from this image to the eye, we know also the degree of myopia. I have thus given the principles of the determination of ametropia with the aid of the ophthalmoscope. Generally speaking, this method COMPARISON OF THE TWO METHODS. 107 is inferior in accuracy to the determination of vision with glasses of known focal distance. 1. It is for many observers difficult, in the use of the ophthalmoscope, entirely to relax their power of accommo- dation : if they are not certain of this, the method is inappHcable to them. He who, on the contrary, has by practice attained so far that he can not only wholly relax his power of accommodation, but also justly estimate the degree of voluntary action, can very often usefully employ it. I know this by my own experience. 2. Without producing paralysis of accommodation, we are never perfectly sure that we determine the refraction in the condition of rest. 3. It is sometimes difficult, at least when strongly negative glasses are required, with a narrow pupil accurately to see the vessels of the retina. 4. The vessels which lie at different depths in the fibrous layer afford no perfectly correctly situated object for estimation. 5. Moreover, such a vessel is not a suitable object to determine with precision whether we see accurately. Consequently, the method in each case requires a great degree of attention. 6. The determination in the line of vision, which it chiefly con- cerns, is for the most part difficult of execution, because the place of the yellow spot is not well seen, or our estimation of the accuracy of seeing it is particularly difficult. If this second method, therefore, is not equal to the first in accuracy of results, it nevertheless deserves our attention, because it is applicable in cases where the first wholly or partly fails us. This is, in the first place, true in all young children, hkewise in the blind, and even in high degrees of amblyopia, where the knowledge of the refractive condition is sometimes of great importance. Further, by this method we can better and more easily ascertain the degree of ametropia for indirect vision than by the first : in many instances I have by it alone succeeded in satisfying myself that the myopia for indirect vision was less than when the patient looked in the line of vision. Besides, the want of fixation of a hypermetropic eye ex- amined with the ophthalmoscope, sometimes gives rise to more complete relaxation of the power of accommodation, whereby hyper- metropia, latent in trials of vision, may manifest itself. Finally, this method may be of great use in simulated ametropia. 108 DEKECTS OF REFRACTION AND ACCOMMODATION. NOTE TO CHAPTER II. In the commenoement of this Chapter much stress was laid upon the neces- sity of drawing an accurate distinction between the anomalies of refraction and those of accommodation. Each eye has a definite refraction ; according to this the first distinction is to be made. Now, whether the eye be emme- tropic or ametropic, in either case it has a power of accommodation, and this may be normal or abnormal. Abnormal accommodation is, therefore, as independent of refraction as any other disease of the eye. In my work upon Ametropia and its results {Ametropie en liaregevolgen), Utrecht, 1860, as well as in my papers in Va.6 Archiv f. Ophthalmologie, B. iv., vi., und yii., I had prominently put forward, as the basis of a correct description and of a scientific explanation, the distinction just alluded to. Stellwag von Carion now thinks (Zeitschrift der k. k. Gesellschaft der Aersste zu Wien, 1862) that I should have mentioned his merits respecting this point. I am quite prepared to do so. In his Essay, entitled die Accommodationsfehler des Auges, to be found in the Sitzungsherichte der Jcaiserlichen Akademie der Wissenschaften, Mathem. - naturwissenschaftliche Klasse, B. xvi., pp. 187-281, he calls natural visual line (natUrliche Sehlinie) the line of accommodation, to which the eye in absolute inactivity of the muscle of accommodation is adapted. His natural visual line is, therefore, the farthest point of dis- tinct vision, considered as a Czermackian line of accommodation. " Inas- much as the degree of the greatest possible accommodation-pressure, which the eye can exercise," he says, " in every case is limited, so must the natu- ral visual line determine the position of the nearest point of distinct vision, that is, of the nearest final point of the shortest line of accommodation" (p. 200). " This degree," he continues, " of the available pressure-exciting power of the muscle of accommodation, on the one side, and the natural visual line on the other, are, therefore, the factors which determine the absolute visual distance of the eye, the length of the line connecting the farthest and the nearest points, as well as the position of the latter in the elongated optic axis. But the length and position of this line constitute the measure according to which alone the form and degree can be determined, wherein the dioptric part of the visual function deviates from the normal propor- tions. It is, therefore, evident that the defects of aceommodsition of the eye, from a scientific point of view, can be divided only into those depending upon anatomical disproportions of the whole eyeball or of the several light- refracting media ; further into those, caused by limitation of the function of the muscle of accommodation ; and, thirdly, into those depending upon both causes." In this is, in fact, contained the first indication of a distinction between the anomalies of refraction and the disturbances of accommodation. Nevertheless, the hint was lost upon Stellwag von Carion just as it was upon others. He immediately adds : " Such a division, however, renders treatment difficult, and prevents a proper view of the subject under consi- deration." Had he tried it, perhaps he would have seen, that his second and HISTORICAL REMARKS. 109 third class (in order not again to mix up accommodatiou and refraction) must be reduced to one, comprising tlie anomalies of accommodation in general, independently of refraction, and perhaps he would then also have strictly adhered to the ideas of myopia and hypermetropia, or would, at least, not have included them among the defects of accommodation. But he adopts a quite different (more practical ?) method. He opposes presbyopia to myopia, and subsequently passing over to hypermetropia (N.B., by him termed hyper- presbyopia), he begins by calling the latter a higher degree of presbyopia. I regret not to find in Stellwag's work the merit to which he thinks he has a claim. Those of my readers who take an interest in the matter will please to consult his treatise. They may pass over the less successful mathematical introduction (compare with reference to it : Zehender, Anleitung z. Studium der Dioptrik des menschlichen Auges, Erlangen, 1856. p. 166), which deterred so many, myself among the number, from the earlier perusal of this essay. The diagrammatic sketch of the anomalies of refraction and accommodation, in which the commencement and termination of the lines represent r and^, and the lengths of the lines the range of accommodation, I first applied in the Nederlandsch tijdschrift voor geneeskunde, D. II., 1858. The idea of ex- pressing the range of accommodation by a lens of definite focal distance, is to be met with so early as in the masterly work of Young {Philosophical Transactions, 1801). CHAPTER III. Fuller Development oe the Diffekent Meanings of Eange OF Accommodation. § 10. Relation between accommodation and convergence of the visual lines; Meaning of \ : A, of 1 : A-, and of\:A^. So far as the range of accommodation for both eyes extends, the state of accommodation of the eye corresponds to a definite convergence of the visual lines. Thus the emmetropic eye, with parallel visual lines, is accommodated for in- finite distance; with a convergence at 8°, for a distance of 8", &c. Unmis- takably, therefore, a connexion exists between convergence of the visual lines and accommodation, to which Porterfield* and John MueUert already directed attention. Both these observers, however, appeared to assume, that this connexion is absolute and causal ; that a definite convergence is necessarily attended with a definite accommodation, and admits of no other ; it was thought that only leyond the limits of ac- commodation a greater or less convergence was possible, to which the accommodation, Respectively for the nearest and farthest point of distinct vision, should then still correspond. Now this is incorrect. Even Volkmann showed, J that also within the limits of the range of accommodation such absolute dependence does not exist, and I § gave further proofs of this by simple experiments, which were capable at the same time of determining the degree of independence. The ex- periments were made partly with convex and concave, partly with weak prismatic glasses. It is easy to convince one's seK that both eyes together, as well without as with slightly concave or convex glasses, can accurately see an object at a definite distance, and that, consequently, without change of convergence, the accommodation can be modified. With equal ease, we observe that, in holding a weak prism before the eye, whether with the refracting angle turned in- * A Treatise on the Eye, Vol. I., pp. 410 et seq. Edinburgh, 1759. t Vergleichende Physiologie des Gesichtssinnes, 1826, p. 216. X Neue Beitrdge zur Physiologic des Gesichtssinnes, 1836, p. 148. § HoUdndische Beitrdge zu den anat. u. physiol. Wissenschafien heraus- gegeben von van Deen, Bonders u. Moleschott, 1846, B. 1, p. 379. RELATION BETWEEN ACCOMMODATION & CONVERGENCE. 11 1 wards or outwardsj an object can be accurately seen with both eyes at the same distance, and that, consequently, the convergence may be altered, without modifying the accommodation. When, therefore, it is required for the sake of distinct vision with both eyes, the con- nexion between convergence and accommodation can be, at least partially, overcome. I early stated the method of determining how far the independence existed. Some time afterwards it was applied with the requisite accuracy.* The question is very simple : it is only necessary to know Ej and Pi with parallel visual lines and with a series of converging degrees (to the maximum), and these we find, by a calculation from the nearest and farthest points, discovered by means of different convex and concave glasses. For accurate determination, however, a special opto- meter is required, which shall be described at the end of this section. The results of the examination of the emmetropic eyes of a person aged 15, are represented in the annexed Eigure (57). Fig. 57. n 4 H 6 8 12 24 CO / / y / ./ / f ^ ^ V y— t ^ y / / y \^ / / y ^ / y / / / ' Y 0° 11021' 22°50' 34''32' 46''38' 59°20' V2"50' At different points of the diagonal h V, intersection takes place between the transverse lines, before which, in Parisian inches, * Conf. Mao Gillavry, OnderzoeUngen over de hoegrootheid der Accommo- date. Diss, inaug., Utrecht, 1858. 112 RANGE OF ACCOMMODATION. the distances are placed; and the longitudinal lines, under wWcli the degrees of convergence of the visual lines corresponding to the dis- tances are noted. The mutual distance of the visual lines of the two eyes in the parallel state amounted to 28^", in which case (compare the figure), at a distance of 12", a convergence of 11° 21', at a dis- tance of 6", a convergence of 22° 50', etc., exists. The \me_p^p^p represents, in the consecutive convergence, the course of the nearest point, the line / r^ that of the farthest point. The dots in these lines are the points determined by investigation. Now the figure shows that the eyes here supposed with parallel visual hues can accommodate from infinite distance up to 1 1 , with 22" 50' convergence from about 13" to 4".16, etc. ktp^, where the line of nearest points cuts the diagonal i U, we attain the shortest binocular distance P^ of distinct vision. "With still stronger con- vergence, for example, 46" 38', the line jo^^ remains under the dia- gonal h li, so that accommodation can no longer take place for the point of convergence which here lies only 3" from the eye : the nearest point with that convergence lies, namely, as appears from the figure, at about 3". 8. The absolute nearest point p lies somewhat closer still, and in fact at 3".69, but for this a convergence of about 70" is required, that is at a distance of about 2". With this maxi- mum of convergence all space for accommodation is lost, and there- fore the lines p^ p^ and r r^ cut one another. From the foregoing it appears, that in every one who has two sufficiently equal and movable eyes, we may distinguish : — 1. The greatest distance of distinct vision R, (in figure 58 as co at r). 2. The shortest binocular distance of distinct vision Pj (in the figure = 3.9 at ^2). 3. The absolute shortest distance of distinct vision P, with the maximum of convergence (in the figure = 3". 69 at jo). 4. Relatively shortest distances of distinct vision Pj, at each given convergence (for example, in the figure at 22° 50', Pj = 4''.16). 5. Relatively greatest distances of distinct vision Rj, at each given convergence (for example, in the figure, with 22° 50' convergence, R: = 12"). By the determination of these several distances, three different meanings with as many values of accommodation are to be obtained. I. The absolute. ABSOLUTE, BINOCULAR, AND RELATIVE. 113 11. Ill, 1 1 A ~ P 1 The binocular, The relative. 1 1 A.~ P, 1 a; 1 1 A,- P, 1 They are all to be deduced from Kg. 57. Now in the foregoing sections we spoke only of the absolute range of accotntnodatioQ, comprising the accommodation from the absolute farthest point r to the absolute nearest point js, for each eye in par- ticular : in the figure this is -r- = -ttttk = tttt^ • ° A 3'69 00 3-69 The binocular comprises the accommodation from the farthest point r^ for both eyes at once, to the nearest point ^^ for both eyes at once. In the emmetropic eye, r^ coincides with r, and the bin- ocular range of accommodation, to be deduced from Pig. 57, is con- sequently, _1_ _ J^ 1^ ]_ A^ ~ 3-9 00 ~ 3-9 ■ Finally, the relative is the range of accommodation over which we have control at a given convergence of the visual Hues. It represents the degree in which accommodation is independent of convergence, and is, for every convergence, measured by the distance between the lines ^, JO, J9 and r r^. On referring to the figure it now appears that, with increasing convergence, the relative range of accommoda- tion becomes at first greater, then less, until at the maximum of con- vergence, where the lines mentioned meet one anotlier imp) it is = 0. Ou consulting the figure in detail we see that, with parallel visual lines, 1- 1_ 1-= i- Ai~ll K, ^^ ii' and that, with a convergence of 11*^ 21", 1 _ JL J = i A, " 5-33 ~ n 5-76 1 already attains the maximum. Throughout some degrees -^^ now continues nearly unchanged, at the convergence of 22° 50' it is 114 RANGE OF ACCOMMODATION. diminished to =--r, and at the binocular nearest point, with a con- 6 "4 vergence of about 38°, still amounts to -^■ It is of importance further to observe, that the relative range of accommodation consists of two parts : a positive part and a negative. The diagonal h Ic represents the convergences of the visual lines : the part situated above this diagonal is the positive, that situated beneath ■ it is the negative. The first represents what, reckoning from the point of convergence, we can accommodate still nearer, the latter what we can accommodate still farther ofl". For example (compare rig. 57) : the emmetropic eye is, under a convergence of 11° 21', that is at 12", in ordinary vision accommodated for this distance of 12"; but the accommodation may, with the same convergence, be made more tense, for a distance, namely, of 5'33", and it may likewise be relaxed to distinct vision at a distance of 72". The first is evident, since with negative, the second, since with positive glasses of definite strength, at the distance of 12", with both eyes at once, accurate vision can be attained. At 11° 21', therefore, p^ k^ is the positive, k^ r^ the negative part of the relative range of accommodation. They are calculated as tbepos&e =jij-lj= Jj, SO that at 11° 21', in the case investigated, we find Ai 9-6 ^ 14-4 5-76' that is, as above. A glance at the figure now shows further, that in the emmetropic eye at oo (parallel visual lines) ^ is wholly positive, that, with in- creasing convergence, the negative part rapidly increases, soon also at the expense of the positive, and that at 36° convergence — has Ai become entirely negative. The distinction here made already acquires practical importance from the fact, that the occommodAtion can he maintamed only for a distance, at which, in reference to the negative, the positive part of the relative range of accommodation is tolerably great. DESCRIPTION OF AN OPTOMETER. 115 It is not in every one that we can satisfactorily determine the ranges of accommodation corresponding to dijQFerent degrees of convergence. For this purpose two freely moveable, accurately seeing eyes of nearly equal refraction, and equal accommodating power, are in the first place required, and, in addition, some talent for observation. Each of these requisites was perfectly met with in the person aged fifteen, who supplied the data for Fig. 57. The determination requires special care. As, object we may take wires, which are to be finer in proportion as the point to be defined is nearer to the eye. Accurate results may also be obtained by the use of little holes (from — to — in diameter) in a black metal plate, with a back- ground of dull glass turned towards the clear daylight. Soon the accom- modation for the holes is no longer perfect, they lose their round form and rapidly emit rays. With different glasses of known positive and negative focal distance, at different degrees of convergence, the greatest and least distances of distinct vision are now to be determined. At the same time, in order to obtain correct results, care must also be taken that the distance of the glasses from the eye shall remain unalterably the same ; lastly, that at each degree of convergence the axis of the glass shall nearly coincide with the axis of vision. In order to fulfil these conditions, an optometer has been constructed (Fig. 58), partly in imitation of that of Hasner, Edlem Fig. 58. -^^ von Artha {Prager Vierteljahrschrift f. prahtische Heilkunde, 1851. B. xxxii., p. 166). Our optometer consists of a horizontal, oblong, quadrangular board B B, placed on a stand S. The board is nearly five feet long, nine Parisian inches in breadth (compare particularly Fig. 59, representing a part of the board) ; it possesses three parallel grooves s s' s", in which, by means of a couple of copper handles {h h), a, well-fitting rod, x, with per- pendicular bar can be inserted, bearing the wire-optometer o, or the plate with fine openings. The mutual distance of the two external grooves amounts to 28J'", and therefore corresponds to that of the parallel visual lines ; if the object moves, as in Figs. 58 and 59, in the middle groove, then both eyes contribute equally to the convergence. The one extremity of the board has a notch N for the nose of the person under examination ; in 2 116 RANGE OF ACCOMMODATION. front of his eyes are two half-rings r r, supporting the glasses. Each of these rings is moveable in an arched groove (a a) (whose centre of curvature Fig. 59. lies in the centre of motion of the eye, while the anterior surface of the cornea coincides with the crossed lines), present in the small microscopes m m, which are applied at both sides. The position of the eyes is fixed by two wooden rods (6 b), which, dravm out at pleasure, are fastened by screws under the board, and against which the cheeks rest. The ring-bearing grooves are in two copper-plates P P, which by means of the screws v v can be brought to one another and are kept separate by springs. The mutual distance of these plates is read off on the scale d. During observation the distance of these plates must correspond to the mutual distance of the two parallel visual lines. (This distance can be determined with an instrument, described under the name of visuometer by Alfred Smee ( The Eye in Health and in Disease. London, 1854), and constructed upon the principle laid down by Hawkins. We make use of a similar instrument: two short cylinders of small diameter are moveable along a divided bar ; the head being fixed, one eye sees a distant object in the centre of one of the cylinders ; the second cylinder is now moved until the other eye sees the same object in its centre. Each eye is SMEE'S VISUOMETER. 117 then alternately closed a few times, in order to make sure that the object stands in the centre of the cylinders, afterwards the person looks once more with both eyes, and the mutual distance of the cylinders is read oflf on the divided bar. This distance is then transferred to the plates P P' of the optometer). If the half-rings in the grooves o a be now placed at 0°, the eyes see remote objects through the axes of the glasses placed in the rings. In this position the absolute farthest point r and the nearest point p, with parallel visual lines, are determined. "With these determinations we begin ; the optometer-object is taken away, and an object some metres distant is employed, whether vertical black lines on a white surface, or an opening of about 1'" diameter, in a black plate turned towards dull glass, according as a wire-optometer, or a plate with fine holes is tho'ught prefer- able for the determination at different degrees of convergence. "We find r with the weakest negative or the strongest positive glass, with which the remote object is accurately seen'; on the contrary p^ with the strongest negative or weakest positive. It is only necessary, in addition, to take into account the distance at which the glasses are from the eye ; in this case, where the object remains fixed in its place, the distance in question may be modified as necessary, by pressing in, or drawing out, the rods h h from the optometer. If, now, r and^, the visual lines being parallel (compare Fig. 57), be known, we place the object on the optometer, arid determine, without glasses, the near- est point of binocular vision ^2 ; only in old people, and in high degrees of hypermetropia are glasses required for the determination of pa- For the further determination of ^i and r^, at difierent degrees of convergence, I formerly — (conf. Mac Gillavry, over de hoegrootheid van het accommodatie-vermogen (on the extent of the power of accommodation), "Utrecht, 1858) — placed the optometer-object at such a distance, that it was seen at a convergence of precisely 10°, 20°, 30°, etc. ; and removing each of the two rings re- spectively 5°, 10°, 15°, etc., we found by experiment the strongest positive and the strongest negative glasses with which the object could be accurately seen at each of these distances. Apparently this method was simple and good ; but it nevertheless gave no accurate results : by the long-con- tinued trial, the muscular system for accommodation becomes fatigued, and along with the sacrifice of much time, we obtain too great a distance. "We acquire a sufficient number of points with much greater rapidity and certainty by successively determining with some suitably chosen positive and negative glasses, the nearest and farthest points, by moving the opto- meter object, according to whose distance the lens-bearing rings must be moved in the arched grooves. The distances thus give directly the conver- gence at which vision took place, and by taking the glasses, wherewith this was possible, into account, pi and Vi are found for the convergence. By this method I mark for each glass first the nearest, and then (if the distance is positive and occurs on the optometer) the farthest point, and afterwards wait a few minutes before passing to the determination with another glass. Lastly, the absolute nearest point p is sought. This is not unfrequently attended with difficulties. In those who voluntarily converge very strongly, it often succeeds best by looking with each eye separately, while the other 118 RANGE OF ACCOMMODATION eye is covered with a disc, at the maximum of convergence ; in doing so we may also use positive glasses. Where there is less mobility of the eyes inwards, and in general in those who are strongly myopic, p coincides with Pi, or the necessary convergence for pi is even not obtainable. After this general description, the mode of calculation may be still more accurately, indicated in a couple of plain selected examples. Having deter- mined r and pi with parallel visual lines, and p^, for which no calculation isnecessary,letusflnd: with — — the binocular nearest point at 6" from the eye, that is with a convergence of 22" 50'; the question now is, what^i at that convergence actually amounts to ? We find : the rays from the accurately seen object diverge from a point situated 6" in front of the eye, 6" — 0".5 = 5".5 in front of the glass. Refracted by the glass of ^-' 1 2t they appear to diverge from a nearer point, namely : /^ _I_ 1_\ ^5i + 12 ■"3.77'' from a point 3'-77 from the glass, and therefore 3-77 + 05 =: 4"-27 from the eye. With a convergence of 22°50' therefore^! = 4"-27. Let this distance be noted on the fourth line, under which 22''50' stands. A farthest point r.^ is not to be determined with ' because the eye at the same time becomes hypermetropic and r thus comes to lie behind the eye. With —I on the contrary, both r^ and^^ are to be found at a certain con- vergence. Let us begin withpi. Let us find with — the binocular nearest point precisely at 3", that is at 2''-5 from the glass. In front of the glass therefore the rays diverged from a point 2".5 distant ; refracted by the glass, on the contrary, they appear to proceed from a point situated \2-o 12 3-16/ 3'.16 before the glass, 3".66 before the nodal point of the eye. With a con- vergence of 3", that is of 46° 38', p^ lies therefore at S'.ee. Now, further let the binocular farthest point be found with the same glasses = Y2 ^* ^"" ^^i^nce r, with a convergence to 8", can be calculated : the rays diverge from a point situated V.b from the glass ; after havino- passed through the glass, they diverge ^74 12 20'' from a point situated 20» in front of the glass, 20"-5 before the nodal point of the eye. We therefore make, under the point where the distance of 8' cuts the diagonal, a dot corresponding to the distance of 20''-5. It repre- sents r, at a convergence to 8". By making similar calculations with some other glasses, we soon have dots enough to deduce p^ p.^ p and r , and thus all questions respecting the range of accommodation in an individual are answered. DIFFERENT IN AMETROPIA AND EMMETROPIA. 119 § 11. Difference of the relative range of accommodation 1 : A^, ac- cording to the refractive condition of the eye. We closed the preceding section with a practical result, namely : that accommodation can be maintained only for a distance, when, in reference to the negative, the positive part of the relative range of accommodation is tolerably great. In connexion with this point it is of special importance to show, that the relative range of accommodation in ametropic eyes is some- thing quite different from that in emmetropic. The difference is of a twofold nature. In the first place, with a given convergence, the relation of the positive to the negative part of 1 : Ai is not the same ; in the second, the lines jo^js^jo and r r^ have another form. We shall first treat of the relation of the positive to the negative part of 1 : Aj. We may also thus express what we have said on this point : the relative range of accommodation has, in reference to the refraction of the eye, a totally different position with reference to the line of the convergences ^ //. Kg, 60 illustrates this in detail. This M H 1:2 2^ 2§ n 3 3f 4 n 6 8 12 24 00 -1:24 12 8 6 Fig. 60. 1 T^ / / J^ ' / /f T't v- ^ / 1 / / ■/ ^ M- — ^ i "/ ^ J •^ y y r TU^ 7 -m / --.aj- Vl -/ i r ^ .'"' V / y 7 jft^ ^ ^ '■■ "If- An. ,'-'■' ^''' n 0° 11«'21' 22''50' 34''32' 46''38' 59°20' 72''50' 120 RANGE OF ACCOMMODATION diagram contains the curves of the nearest and of the farthest points in a myopic eye M, and in a hypermetropic H. Both require further explanation. First as to M. The beginning of the line r r^ shows, that we have to do with a myopia of .-^ ; the farthest bino- cular point r^ is found at 5'. Up to this distance, that is up to a convergence of about 28°, 1 : Ai is altogether positive. Now, how- ever, a negative part is rapidly developed, which even at 34° amounts to half the positive. So far 1 : Aj was always increasing, and here attains as a maximum, —j^, while the total range of accommo- dation amounts to 1 : A = -r-; . Henceforth, however, 1 : Aj di- minishes a little ; but the negative part becomes, meanwhile, greater and greater, and indeed up to about 50°, where, the difB.culties of convergence increasing, the farthest point begins so to approach, that at about 58° it coincides with the point of con- vergence. At this maximum convergence still stronger tension of accommodation is, however, possible, as the perpendicularly as- cending line r^ p, here still representing about 1 : A, =: 1 : 18, shows. Hence we see, that in high degrees of myopia, at the maxi- mum of convergence also, a certain range of accommodation still remains, and the more so in proportion as the convergence itself is more limited. Moreover, this diagram shows, that in myopia in the domain of binocular vision, the negative part of 1 : A^ is very slight. On the contrary, the convergence is often very limited. Hence, therefore, it follows, that in the higher degrees of myopia, the difficulty of maintaining binocular vision does not proceed from tension of accommodation, hut rather from difficulty of convergence. In those degrees of myopia where, as in the above diagram. Fig. 60, j!j lies closer to the eye than the nearest point of convergence of the visual lines, p„_ is wanting and gives place to r^. As binocular range of accommodation nothing else can be assumed than 1- i _1 A, K^ X where R'a represents the distance from the point ri to the nodal point of the eye. We now pass over to the consideration of the hypermetropic eye H of Fig. 60. Here we find, just as in the emmetropic eye (Fig. 57), jOi j»3 p, as the curve of the course of the nearest points. IN THE HYPERMETROPIC EYE. 121 and ^m '•to as that of the course of the farthest points, both in rela- tion to the convergence. But besides these, we find a dotted line n ?-ij ; the latter requires further explanation. Thus, as has already been observed, the hypermetropic individual does not completely relax his power of accommodation. In looking at a distance in the case of a hypermetropic subject, aged 16, registered in the scheme, the preference was given to glasses of ^-^ above stronger ones ; consequently there existed — - manifest hypermetropia H,„, represented by the point r,„. But after artificial paralysis of the power of accommodation (by sulphate of atropia), glasses of -, at— o z from the nodal point, were required in order to see accurately at a distance, so that the total hypermetropia H; amounted to —- . Now this is expressed by the point n- In this state of paralysis the refrac- tion remains, on convergence, unaltered ; we cannot, therefore, in- vestigate where the farthest point, at difl'erent degrees of convergence should lie, if the involuntary spasmodic contraction did not exist, and n r^ij] is consequently only an imaginary line, connecting the absolute farthest point rt with the absolute nearest point. It appears that the total range of accommodation amounts to A~8 1^ 7^) 3-87 As to the relative, we see it is particularly great, diminishes tolerably uniformly as the convergence is increased, and that its positive part, which, when the visual lines are parallel, is not inconsiderable, on convergence to 9" (with jb,) under an angle of from 16° to 17°, be- comes completely negative, and, moreover, remains negative. If we reckon from the absolute farthest point r„ the positive part amounts, the visual lines being parallel, to only half of the negative ; if we reckon from the manifest farthest point r„, the positive part is at first, it is true, greater, but we observe that even .at a convergence of 5° the relation is inverted. In connexion with the practical result expressed above, we thus come to the conclusion, that with this degree of hypermetropia, eyes cannot long consecutively aecommodate them- selves to the point of intersection of their visual lines. With still higher degrees of hypermetropia, as shall hereafter appear, binocular, 122 RANGE OF ACCOMMODATION. and with, the highest degree (absolute hypermetropia) even monocular vision is never acute. T/ie foregoing applied to the position of the range of accommoda- tion, with reference to the convergence of the visual lines. — ^We must now take the form of the curves into closer consideration. Even in Kg. 60, we see that the curves of M are concave upwards, while those of H are convex upwards. If with this we compare Pig. 57, it appears, that the curves for the emmetropic eye keep the mean be- tween M and H. The reason of this lies in the fact that, with slight convergence, a myopic eye can accommodate proportionally less; a hypermetropic, on the contrary, more (but also must so accommodate) than the emmetropic. The diagram Kg. 61, Fig. 61. 11»21' 22°50' 34''32' 46<'38' 59°20' '72''50' will make this plain. It contains the curves of the nearest and farthest points, as function of the convergence, both for the em- metropic eye E (the middle ordinary lines), and for the myopic M (the dotted lines), and for the hypermetropic, H (the striped and dotted lines) ;-x is, in order to facilitate comparison, assumed = i- and 4' IMPORTANCE OF THE DIFFERENCE. 123 the maximum of convergence is taken at 59° 20'. The letters E, M, and H are placed before the farthest pointSj as defining the re- fraction ; H„ (manifest hypermetropia) stands before r the manifest farthest point, Hj (total hypermetropia), before A the absolute farthest point. In other respects, the letters have the same signification as in Mg. 60. Now the study of this diagram shows : — 1. That with parallel visual lines, the emmetropic eye can bring into action about -^, the myopic only -^, the hypermetropic, on 3 the contrary, -^ of its total power of accommodation. 2. That, with slight convergence, the myopic eye can accommo- date much less, the hypermetropic on the contrary, much more (but also must do so) than the emmetropic. Compare particularly the curve r r^, which for the myopic eye at first runs nearly transversely, for the hypermetropic strongly ascends. 3°. That with stronger convergence, the accommodation of the myopic eye can still increase much, that of the hypermetropic only a little. We see, with a convergence of 18'^, the curvejs^ jo^ jo of the hypermetropic eye keeping nearly a transverse direction, while the curves of the myopic now begin decidedly to ascend. The diagram will readily answer any farther question which may suggest itself; it appears therefore to be superfluous to expatiate at greater length upon the subject. The difference ascertained is practically of great importance. Thence it follows, namely, directly, that when the ametropia is neu- tralised by glasses (r brought to oo), the eye by no means becomes equal to an emmetropic eye of similar range of accommodation. This is easily deduced from the above figures. It appears still more dis- tinctly on reference to Fig. 63, where the curves of Eig. 61 are re- peated (as Bjd and Er, as Mjo and Mr, and as Hjo and Hr„ and Hrt) but so that in aU, r is brought to oo.* The diagram shows that the neutralised myopic eye has its binocular nearest point at 16", so that 1 : A^ amounts only to the fourth of 1 : A ; and that vice versa for the hypermetropic eye the binocular nearest point jos is nearly equal to the absolute. Moreover, in the myopic, 1 : A^ even at from 8° to * As shall appear in § 13, the reduction of the ametropia is not entirely without influence on the form of the curves. This influence is, however, so slight, that here it may he altogether disregarded. 124 RANGE OF ACCOMMODATION. 9° of convergence, becomes wholly negative; in the hypermetropic, 1 : A, is at more than 30'? of convergence entirely positive. Hence Fiff. 62. 1 ;2 2i / / / / / > / HP, .•'" ^ '^^ / > P" 7 .■i> /- / ^f A / / / It '■ y Mfi ^ -^ / !<; ri U 6 8 12 24 0° 11"=21' 22°50' 34°32' 46°38' 59<'20' 72°50' proceeds for both eyes difficulty in binocular vision, under moderate convergence; for the reduced myopic, because, under these circum- stances it accommodates too weakly, for the reduced hypermetropic, because it accommodates too strongly. The cause of this difference is at once apparent ; it is the result of practice. The myopic eye has learnt to converge in a certain degree, without bringing its power of accommodation into action in the same proportion as the emmetropic eye. Thereby the binocular farthest point (Eig. 61, Mr^, although seen at a tolerably consider- able convergence, remains almost as far from the eye as the absolute farthest point Mr. But on the other hand, the eye has not prac- tised itself with slight convergence, to bring a relatively great part of its accommodation into action, because it has had no necessity to do so. The hypermetropic eye, on the contrary, found itself obliged, in order to see accurately, even with parallel visual lines, to put its power of accommodation on the stretch, and it has brought itself so far in that respect, that it is no longer in a position to become com- pletely relaxed, that at least on every effort to see, the act of accommodation takes place involuntarily. As further, with increas- ing convergence, a disproportionately great part of the range of EFFECTS OF PRACTICE. 125 accommodation must always come into action, it is not strange, that the relative range of accommodation has been considerably displaced. That practice really may produce the difference just described, we have the following proofs. 1. The use of positive or negative glasses has, even after the lapse of*a few hours, an influence on the range of accommodation of the emmetropic eye. Pig. 63 exhibits the effect of practice in look- Fig. 63. 1:2 2i 4 ^ 6 / / / / / / w i < f / ^ ^ >^ / rl. // r / / y ^ / 1 / ■pi / ^ /^ / / ^ ^ [^ r 12 24 0° Il°21' 22°50' 34''32' 46°38' 59°20' 72°50' ing with negative glasses. It relates to a young emmetropic sub- ject, aged 21 years, who with slight convergence can accommodate somewhat less than usual. The dotted curves represent the course of his nearest and of his farthest points. After having occasionally practised for some days with negative glasses, he obtained, by repeat- ing the investigation, the linear curves, which, at least p'\ p^ p, approach much more closely to those of hypermetropic persons than the dotted ones. Had he continued the practice longer, permanent contraction would, just as in the hypermetropic, have occurred, and r would not have stood at oo . For some minutes after the removal of the negative glasses, this was the case. On the other hand, the use of positive glasses rapidly makes the power of accommodation, with a certain amount of convergence, less : it is well known that, particularly when positive glasses are used too soon, reading much without their aid quickly becomes more difficult than before. 126 RANGE OF ACCOMMODATION. 2. The relative range of accommodation is displaced in ame- tropic individuals when they have for a long time worn correcting glasses. That of myopics^ as well as of "hypermetropics, gradually tends to that of emmetropics. 3. In the diminution of the range of accommodation in advanc- ing age, even before proper presbyopia has begun, the curvel of the emmetropic eye approach to those of the hypermetropic. Pig. 64 is Fig. 64. 1 :2 2§ 2i 3 6 8 12 24 00 — 1:24 / / / / / / / / T' ^/ ^ / ■/ ^ — "^ "a V / 'I "A 4 1* 0° 11°21' 22''50' 34''32' 46038' 59°20' 72°50' the faithful expression of the range of accommodation of an emme- tropic individual aged 44. For vertical lines he has a trace of hyper- metropia, so that r is found beneath. Further, to convince one's self that, with the required practice, the curve jo^ pi, p especially has approached to that of a hypermetropic eye, we need only observe that with a convergence to 9", nearly the maximum of accommoda- tion is attained. In the following Chapter, treating of Spectacles ; and also in speaking of each of the forma of ametropia, I shall revert to the important distinction made in the foregoing section. CHAPTEE IV. SPECTACLES AlfD THEIR ACTION IN GENERAL. § 12. Different kinds ofspeotacles.—lu ophthalmic surgery different kinds of spectacles are in use, which are employed for very different purposes. I. Protecting spectacles. — Of these we distinguish two varieties : a, those which serve only to keep off the mechanical influence of foreign bodies, dust, fragments of metal, stone, coal, etc. They have nothiug to do with the refraction of light in the eye, and call for no further comment here. b. Spectacles for warding off light. — These consist mostly of coloured glass, green and especially blue, which is less hurtful to the eye. In general, however, at least in daylight, the grey, so-called neutral glasses, deserve the preference. The white sunlight, re- flected by different objects in their particular colours, is the natural adequate stimulus to the retina, and by good neutral glasses these rays are tolerably uniformly diminished. The same end opticians formerly attempted to attain (Fischer) by combining two glasses of comple- mentary colours. It is desirable in every case that, so far as possible, the whole field of vision should be uniformly obscured, which is accomplished, with tolerable accuracy by the use of large, round glasses, resembling watch-glasses, but more perfectly by lateral flaps of silk or some other semitransparent substance, or else by glass of the same colour as the spectacles, the objection to the latter being, how- ever, the increase of weight. The light falling in from the side has a doubly unpleasant action, when coloured glasses are applied in front of the eye, for the lateral parts are then seen by contrast in the com- plementary colour of the glasses, and in the remaining part of the field of vision the complementary colour appears still stronger as a secondary image, so soon as the spectacles are taken off. In persons affected with irritation of the retina, or with photophobia in general, we may re- commend the use of light-absorbing glasses, when these patients are obliged to expose themselves for a certain time to a bright light. In 128 SPECTACLES AND THEIR ACTION IN GENERAL. the houscj with moderate light, they ought to lay them aside. We should not forget that these glasses when worn in the sun, become heated in .proportion to the amount of rays they absorb, and that very dark glasses may, therefore, be considered to be specially injurious to the eye. In women, moderating the light, when neces- sary, by means of veils, is to be recommended. II. Stenopmio spectacles, stenopeic lorgnette, stenopaic apparatus. — Slight obscurations of the light-refracting media, especially those of the cornea, often produce great disturbance of the accuracy of vision. The cause of this is to be sought not so much in the reflec- tion or absorption of a portion of the rays, as in the diffusion of the light passing through them. This is easily explained. Trom the entire field of vision rays faU on each local obscuration, and from the latter they spread further in all directions through the whole eye. Consequently in the region of the yellow spot also, an image is not merely formed of the object lying about in the direction of the line of vision; but over this image is spread, with the existence of semi- transpareilt spots, a uniform light, derived from the whole field of vision. This diffused light is very disturbing. Indeed the differences of illumination of the image formed by regular refraction are in con- sequence much more faintly distinguishable. Just as in looking through a real mist, the diffused light is added to the relatively weaker image, and therefore also spots give the impression, as if one were looking through an actual mist : the only difference is, that a mist is more perceptible for more remote objects, and that misty vision, pro- duced by a spot, affects all objects alike, independently of distance. It is well known that obscurations produce much less disturbance, when the eye, turned from the light, contemplates a certain object. If a pic- ture or another object be hung against the pier between two windows, and if it be illuminated through a window behind the observer, the latter will see the object much more accurately, and with more con- trast of light and shade when the two windows are closed than when they are open. The explanation of this fact is to be found in the foregoing. In both cases (supposing that the open windows throw no light upon the object) the object sends, in an equal degree, rays into the eye, which, regularly refracted, form a good image in the region of the yellow spot ; but if Kght falls upon the eyes through the windows placed at the sides of the object, numerous rays proceed, in that case, likewise from the illuminated spots over the image of the yellow spot, which thus becomes covered as with a white crape. Even if STENOPiEIC APPARATUS, 129 there be no spots, some light is always diffused iti the eye, and thus even the normal eye will, especially if the time of life be somewhat advanced, perceive, as it were, light crape, when, in the experiment just described, the windows axe opened. We know further, that where obscurations exist, exclusion of the peripheric hght with the hand, looking through a tube, etc., increases the accuracy of the images. Again, it is the warding off of the laterally incident light diffusing itself from the spot throughout the whole eye, which here acts beneficially. The practical rule hence deducible is this : in order, where obscurations exist, to distinguish with relative accuracy, let the small portion of the field of vision, over which the observation is to ex- tend, be properly illuminated, and let the remaining portion be kept as dark as possible. These reflexions on the injurious effect of obscurations led me to the application of stenopseic remedies. Their object is to cut off the light which should reach the obscurations, and through an opening to give, so far as possible, entrance only to the light which is subjected to a regular refraction on the normal part of the refracting surface. The narrow opening is in them the essential part : hence the denomination stenopseic (from (TTev6s, narrow, and owr), a peephole) . In order not to limit the field of vision more than is necessary, the opening must be as close as possible to the eye, and with a view stUl better to keep off aU lateral Hght, it may be surrounded with a wall widening like a funnel. The opening may in general have the form and nearly the size of the clearest part of the portion of the refracting surface corresponding to the pupil: it may, ac- cording to circumstances, be round, oval, or slit-like. Earely will the diameter be less than a millimetre ; often it wiU amount to two or more millimetres. The stenopseic apparatus, which is an indispensable item in the ophthalmic surgeon's means of investigation, is a very short cylinder, furnished with a handle, open at one end and indented towards the eye, and provided at the other with an opening, in front of which is a diaphragm, containing different smaller openings of various diame- ters, and capable of turning round, so that each of these openings can be seen in turn. There are also stenopseic apparatus provided with a slit capable of being widened and narrowed.* With the aid of this apparatus we can examine whether the stenopseic principle increases the accuracy of vision, which is often of importance in a diagnostic * They are, as well as the stenopseic spectacles and eyeglasses, prepared, among others, by Paetz and Flohr, opticians, unter den Linden, Berlin. y 130 SPECTACLES AND THEIR ACTION IN GENERAL. point of view; moreover, we may acquire an indication whether it may be advantageously appUed, and what size of opening is the most useful. Of the result obtained we may then make use in prescribing a stenopeic spectacle, or eyeglass. To spectacles for use in the streets, the stenopseic principle is in general not applicable, as the field of vision is too limited. On the other hand, such spectacles to which the requisite glasses have been fitted, are sometimes very serviceable for reading. The chief application of the principle, how- ever, is to the stenopseic eyeglass. Many persons suffering from opacity have recourse to it of their own accord, by warding off the peripherically incident light with the hand, by voluntarily narrowing the slit between the eyelids, etc., in order to increase the accuracy of vision, but this object is always much more perfectly attained by means of a stenopseic eyeglass. If the part which has remained clear has an oblong form, a slit- like opening will be most suitable. In general, it is a great advan- tage in reading, when a horizontal slit effects the object ; this should, therefore, always be tried. If the opacity is only on one side, we may obtain a great advantage by making an ordinary spectacle-glass opaque over the obscured part (for example, by applying a black lacker). In general the simplest stenopseic spectacles are those in which the preferable form of opening is left as the only part of the glass not obscured, in which also by opaque matter on the outside the light incident from that point can be warded off. The glass may in ordinary cases be fiat, but otherwise according to the necessities of vision it should have a certain focus. In some cases, among others, after the extraction of cataract, I have found great advantage from partially covering the glass with black. It is not uncommon, when the flap-section was made downwards, for the inferior part of the cornea to become soinewhat turbid, particularly if the iris has attached itself to the wound, or is even slightly prolapsed. The dis- turbance is the greater, because the pupil too is thereby drawn downwards, and probably the curvature of the cornea is somewhat irregular, so that reading with an ordinary convex glass is attended with great difficulty, or is even quite impossible. But it is in the most surprising manner relieved, when the glass is covered to a defi- nite height with an opaque black matter over which the eye sees, while the rays which should reach the inferior non-transparent and irregular part of the cornea, are cut off. This mode of using the glass limits the field of vision only inferiorly, and is therefore THE STENOPiEIC EYEGLASS. 131 attended with no impediment whatever to reading, nor even to vision in general. Lastly, it may be mentioned (what has reference more especially to anomalies of refraction), that the stenopeic eyeglass has also ren- dered me very essential service in the highest degrees of myopia, parti- cularly when the accuracy of vision had at the same time suffered comparatively much. If it were only for this reason, therefore, the stenopaeic apparatus ought not here to be passed over in silence. In such cases vision of near objects, at least with one eye, is attended with no other inconvenience than that the object must be brought very near the eye, to 3" or less. But distant vision is extremely im- perfect, and is comparatively little improved by concave glasses, which correct the myopia. If, with their aid, the images are more accurately seen, they at the same time become so much smaller, that an amblyopic eye still distinguishes little, and is, therefore, by no means satisfied. In such cases then, a stenopseic eyeglass with a small opening yields very good service. It here acts in a weU-known manner, quite different from that treated of above, by diminishing the circles of diffusion. It is manifest that, in imperfect accommoda- tion, the magnitude of the circles of diffusion increases with the magnitude of the base of the cone of light (the surface of the pupil). Now in high degrees of myopia the pupil is usually very wide, and the disturbance in looking at distant objects is, therefore, relatively very great. Precisely for this reason it is that a stenopseic eyeglass produces so great improvement. If a myopic individual looks through an opening of from ^"' to 1'" in diameter, he distinguishes at a distance as accurately as through glasses, which im'perfectly neutralise his myopia, and he has the advantage that the objects ap- pear larger. If an emmetropic person wishes to convince himself of this, let him hold a positive glass before his eye, so that it becomes myopic, and he will, on looking through an opening, obtain the effect described, and can estimate the partial neutralisation of the artificial myopia. In like manner we may also within the nearest distance of distinct vision, by diminution of the circles of diffusion, with the aid of a small opening, distinguish with tolerable accuracy, and thus view small objects much nearer to the eye, that is under a much greater angle. However, in either case we lose both in light and in extent of the field of vision. As to light, we lose the more, the smaller the opening is, and in myopia it is therefore often ad- visable not to have the opening very small, but with the stenopeic 2 132 SPECTACLES AND THEIR ACTION IN GENERAL. eyeglass to combine a glass, which partially corrects the myopia. "With respect to the extent of the field of vision we lose the more, the farther the opening is from the eye. In combining a negative glass with the stenopseic eye-piece, the patient will therefore turn the small opening towards the eye, when his principal object is to increase the field of vision ; on the contrary the negative lens, when he chiefly desires to obtain greater distinctness of vision. III. Prismatic glasses, prismatic spectacles. — Prisms are used, in order by refraction of the light upon two surfaces, to obtain the well- known spectrum. The angle, which the two refracting surfaces make with one another, is the angle of refraction of the prism, For the object mentioned a large angle is taken ; most prisms are triangular, and each of the three angles then amounts to 60°^. They are usually made of fiint glass, in order, with considerable declination, at the same time to obtain a decided dispersion. Tor ophthalmological purposes, on the contrary, only slight declination is required, and we therefore use prisms with a smaller angle of refraction, from 3° to 24° ; moreover, it is necessary to select a kind of glass, which, with reference to its refracting power, presents but slight dispersion, for example, crown glass. Such prisms are given in the boxes for ophthalmological use, usually in sixteen numbers. The numbers from III. to XXIV. indicate the refracting angles, S" to M°. In the position of least declination, the angles of decliuation, for the low numbers, are nearly the half of these refracting angles : for the higher numbers, they are somewhat more. If we wish to know these accurately, it is necessary to determine the deviations for each glass separately. The declination of the light by prismatic glasses is the cause why objects, seen through such glasses, exhibit themselves in another direction. Let i (Kg. 65) be a point of light, i a a ray falling at a Fig. 65. on one of the refracting surfaces of the prism, this approaches the per- pendicular V a, instead of continuing in the same direction a a, and runs as « i in the prism ; arrived at h it again alters its direction, now in PRISMATIC GLASSES. I33 passing fcom glass into air, declining from the normal v' I, and as ^ c reaches the eye 0. Evidently, therefore, the point i is seen, in the direction h c, about mj. In this, however, the more refrangible rays, those on the violet side, undergo a greater, the less refrangible, those on the red side, undergo a less declination. Hence proceeds a disturbance by coloured margins to the objects, which is greater in proportion to the strength of the prism, and which, precisely in powerful prisms, it is hard to remove. In order to obtain a decKnation without dispersion, the prism must be made achromatic by being compounded of two prisms, C (Kg. 66) of crown glass, J of flint glass, which, acting in opposition, com- p- gg pletely remove each other's dispersion, but only partially destroy each other's refraction. Such , , achromatic prisms, however, soon become, when a \ '^ / 1 declination of some degrees is required, too thick and too heavy to be worn. This objection might perhaps be partly met, by having the prisms very small (to have lajge prisms as eye-glasses is of no use), because in order to see rather distinctly, we must not look obliquely through the glass, but about at the angle of least declination. So far, however, the achromatic prisms have not yet come into use. What led to trying prismatic glasses, was a declination of the visual lines. The idea of making it possible to look with both eyes, in spite of this declination, occurred first to my friend Krecke, Doctor of Natural Philosophy, of Utrecht, whose idea I endeavoured to realize and to explain physiologically.* On experimenting with the glasses, three remarkable phenomena immediately presented them- selves. In the first place it appeared, that one feels involuntarily im- pelled, by changing the direction of the visual lines, to remove the double vision which has taken place. If we hold the glass with the refracting angle inwards, then, in order to bring the double images into one, stronger convergence is required, and this is immediately almost involuntarily effected. On removing the glass, double images again exhibit themselves, which, by diminishing the convergence, are once more forthwith thrown together ; only, if the prism has been long held before the eye, a tendency to increased convergence con- tinues for some time. In the second place it seemed, that the * Nederlandsch Lancet, uitgegeven door F. C. Bonders, G. L. H. EUer- man ea J. H. Jansen, 2° Ser. D. III., pp. 227 and 233, 1847. 134 SPECTACLES AND THEIR ACTION IN GENERAL. visual lines are usually capable of only very slight divergence, and that they can scarcely decline upwards and downwards, even under the pressure of the necessity of rendering the vision single. In general the prism, when the refracting angle is turned to the out- side of the eye, ought to be very weak, in order to allow the observer still to see remote objects single ; and the same weak prism, when the refracting angle is turned upwards or downwards, produces double images, which cannot be overcome. If, finally, the observer has, after long-continued efforts, succeeded in throwing into one the double images standing (in the last case) above one another, the double images which now arise on removing the prism, do not immediately run together again. Thirdly and lastly, we can convince ourselves of what has been stated above (p. 110), that under the in- fluence of a prism with the angle turned towards the inside or out- side, the observer can converge more or less strongly, without being able to alter the tension of his accommodation. These results, obtained with the aid of prismatic glasses, are of essential importance for the physiology, and for many points in the pathology of the eye. But beyond this, these glasses serve different useful purposes in ophthalmic surgery, which, partly previously fore- seen, have been, especially by von Graefe, practically tested. Thus they may be applied in the diagnosis of different anomalies of the muscles, and of the degree of these anomalies. Thus they may be used to correct slight incurable dechnations of the visual lines, out- wards, upwards, or downwards, whereby confusing double images are produced, or to remove the muscular asthenopia, depending on in- sufficient power of the musculi recti interni. Thus we may further, in paresis of a, muscle, so far meet the disease by means of a prism, that in order to make the double images which have been brought near one another, run together, the muscles will become powerfully tense, which, for the alleviation of the paresis, appears to be no matter of indifference. Mnally, what deserves to be here particularly men- tioned, these glasses are also of importance in anomalies of refraction. They show, that hypermetropic individuals distinguish accurately with greater ease, when they, looking through a prism with the angle turned inwards, for the sake of single vision, can converge more strongly, a fact by which the origin of strabismus, in conse- quence of hypermetropia, is explained; and it will hereafter appear that we sometimes advantageously apply the principle of the pris- matic glasses, by modifying the mutual distance of either the convex VARIETIES OF LENSES. 135 or concave glasses of spectacles, so that the eyes look through these glasses at the side of the axes, which, just like the use of a prismatic glass, modifies the direction in which an object is seen. Glasses with spherical surfaces, ordinary convex and concave spectacle-glasses.— Glasses, which modify the limits of distinct vision, are called lenses. Of these we have two kinds, both of which are used as spectacle-glasses : converging lenses (Fig. 67 I), which cause Fig. 67 ;, I \ \ c ^~___ % ______ — ' — '^ " a- J c parallel rays a h and «' V to converge as c «? and c' d' ; and diverging lenses (Fig. 68 I), which cause a h and a b' to diverge as c ^ and Fig. 68 I. %. c' d\ A. converging lens has its focal point 0" on the other, a diverging lens on the same side as that whence the rays come. The first unites the rays into a real focus ; the latter does not actually unite them, but causes them to assume a direction as if they had pro- ceeded from the point 0», in which the prolongations oi c d and c d (the dotted lines) cut one another : hence this point is called also a virtual focus. Besides the above hiconvex lenses, we have, as converging lenses, the plano-conveie and the concavo-convex or positive meniscus {with shorter radms of the convex smface^, as well as, in addition to the biconcave, as diverging lenses, the plano-concave and the convex-con- 136 SPECTACLES AND THEIR ACTION IN GENERAL. cave or negative meniscus (with shorterradius of the concave surface). The plano-convex and plano-concave have, for equal degrees of power, the greatest aberration, and they are consequently to be re- jected as spectacle-glasses. The biconvex and biconcave answer better. To the menisci the advantage is attributed, beyond these, that, as WoUaston showed, the images suffer less, when the observer looks obhquely (under an angle with the axis) through them, so that the eyes can move more freely behind these glasses. They are there- fore, also called periscopic (from Ttepia-Koireiv, to look around). How- ever, we can also see satisfactorily in an oblique direction through biconcave and biconvex glasses, provided they are not too strong, and if high numbers are required, the periscopic glasses have again the disadvantage of greater weight. Were it only for this reason, there- fore, the latter do not unconditionally deserve the preference. When we add that under some circumstances the periscopic glasses are more liable to produce disturbance by reflexion on the concave surface turned towards the eye, and that they are, moreover, somewhat more expensive, we shall not be surprised that they have not wholly sup- planted the biconvex and biconcave glasses. Biconvex and biconcave spectacle-glasses are ground with equal radii of the two surfaces. The optical centre o (Figs. 67 and 68) lies then in the middle of the lens in the axis A A^. Now the dis- tance from the focus 0' to this optical centre o is usually called the focal distance P. This is, however, not quite correct ; iP is, ia fact, the distance from focus to principal point. For ordinary, not very thick lenses, with equal radii of the two surfaces, that is, for the ordinary biconvex and biconcave spectacle-glasses, this inaccuracy is of no importance ; but in the case of menisci, we must ascertain the principal points, in order to know the position of the foci, and to be able to take into account the distance from focus to principal point as I". The subjoined figures (69, a biconvex, 70, a biconcave lens; 71, a Fig;. 69. -^ hM ^" VARIETIES OF LENSES. 137 positive 72, a negative meniscus), show, for these different forms of lenses the position of the first and second principal points h, and h„ and ot the first and second focus 0, and K The rays which, parallel ^ ^ Fig. 71. % Pig. 1% to the axis, fall on the surface I, have their focus in 0,j those which, coming from the other side, fall parallel to the axis on the surface 2, unite in f,. For a lens standing in air h, , is always = h„ f„, both = Y. If now, as in Figs. 69 and 70, the two surfaces of curvature be equal, the points A^ ^ and h,, f,, will evidently lie symmetrically in the axis. But if the surfaces of curvature be not equal, if even one of them be concave, the other convex, as in Figs. 71 and 73, the points h h^ may have another position, and may even be external to the mass of the glass, and the foci / / ^ / / / / / -f / / / Tf- / y\ / i"i ^ ,/ / z ^' ^ ^ 0° 11021' 34032' 59020' 1 JHf = _ should by glasses of — (or rather of — =— at — distance from W) , o 8 7'0 2 10 146 SPECTACLES AND THEIR ACTION IN GENERAL. simply be so altered, that the lines ^' p^ p and r r' should fall only j-j main- taining the same form, and should then be changed into Fig. 78 h. We should thus obtain : — J__J__J__J 1 _J_ A~ P ~ R A" ~ l'" R"' This is, however, applicable only in slight degrees of ametropia, and with the use of weak glasses. This may be seen from the following consideration. We wish completely to correct the myopia of -^^ represented in Fig. 78 a, therefore, to make the dioptric system emmetropic, by the addition of a lens. Now, if a; be = f, this lens must be — 7^. Then, in fact, parallel rays, re- fracted by the lens, acquire a direction, as if they had proceeded from a point situated at 71 inches in front of the L, and therefore 8' before k (compare Fig. 76), and JSo becomes = <» . Now what is P", assuming that -j = ^. and that P therefore amounts to 2§", as in Fig. 78 o ? The calculation shows : — 1 _ 1 _ 1 _ 1 _ 1 ^ 1. \ 74 3-05' P -X- P° Po — x 7i J - a; = 3-05 and P° 3-55. Consequently j^ ' 1 3-o5 •50 1 1 While -^ was = ^, and the absolute range of accommodation has therefore become greater. In place of by simple reduction obtaining Fig. 79 a (the same as Fig. 78 6) we obtain Fig. 79 b. Fig. 79. • „ i 2 4 6 8 12 24 00 , z ^ / / / / / ^ / / Y / ^ V / ■^ / / /■ / T, / y^ / r ffi! J r" / / / ^ 0" 11''21' 34°36' 59''20' / / f / / / / ^- T' P / / / / / / / / / y / %L °2 / — \A XA rA What is the effect of the glass on the binocular range of accommodation ? The binocular farthest point is by iheglass brought to infinity, therefore, iJ" = CO . In order to ascertain P», we must know where the line p{p\ p> shall out the line of convergence h W. This should be discoverable only by an experi- MODIFICATION OF THE RANGE OF ACCOMMODATION. 147 mental calculation. But in determining the relative range of accommoda- tion the farthest and the nearest binocular points were also ascertained with -r- — , and thence it appeared that this point {p\ of Fig. 79 6) lies at 14'. The binocular range of accommodation of the reduced myopic eye therefore amounts only to : — J J^ J^ _J^ .4" ~ 14 ~" 00 14' and is therefore much less than -^ = ,p=^ — tT^ ^ JToci ' ^®i°S the bino- cular range of accommodation of the myopic eye, without a negative glass (Fig. 78 a). However, -to is stiU greater than in the simple reduction, expressed by Fig. 79 a (whereby -— ^ —\ ; and this favourable circum- stance makes it at least somewhat easier to many myopic individuals, to see near objects with a reducing glass (whereby It = oo), than would have been inferred from the impossibility of, with slight convergence, bring- ing into action a proportionate accommodation. Besides, the range of binocular accommodation in reduced myopia usually proves considerably more favourable than in the case represented by Figures 78 and 79. The above is applicable in the use of glasses with a negative focus ; those with a positive focus have precisely {he opposite influence. The subject is, in its further application, important enough to justify its illustration by an example. Let us suppose, that in a case of hypermetropia S = -^, p a.t 12', there- fore, ^-= g + 12 = 45' Through a glass of — , placed at half-an-inoh from the eye, iJ" becomes = 00 and P" is found as P" ^ ~ Hi + 8^ ~ 4-88- Consequently P„ = 5 '38, so that, with -^ = z:5> — has become = -„„, and the total range of accommodation is therefore decreased. In hyper- metropia, however, the great advantage is always obtained by reducing glasses, that the useless part of -j, namely, that of the accommodation for converging rays, is removed. Moreover, the increase of the range of accom- modation, by the reduction acts favourably on -— , in so far that thereby the too strong accommodation, peculiar to slight convergence, is partially corrected. The same influence which has here been spoken of is likewise exercised 2 148 SPECTACLES AND THEIE ACTION IN GENERAL. by positive glasses, when they are used by emmetropic individuals against presbyopia, or as magnifying glasses (even by myopic persons). If the emme- tropic eye, with i-= J, uses a magnifier of |, at O-S" from the nodal point, JJ„ becomes = 3^ P" becomes = 22^5 If the magnifying glass has only 1" focal distance, -^ becomes = ^,that is three times less than -7. Hence we see how much the range of accom- modation is reduced by the use of a magnifier. The reduction is greater, the farther the glass is held from the eye. With a magnifier = - and a; = 1" _ becomes = ^r-^„ ; with the same magnifier, x being = 3", we find L, _ A_ With the strong lenses of simple microscopes -j, becomes still less, on account of the stronger system, but continues greater, because the eye is usually held closer to the lens. In the use of the compound microscope we see an aerial image (formed by the object-glass) through a lens (the eye-glass). This aerial image possesses, as a simple calculation shows, an extraordinary depth in relation to the slight depth of the object. Since it, moreover, lies very close to the eye, and is seen through a lens, the accommodation of the eye in relation to the difference in depth, that we can see of the observed object, is reduced nearly to %ero. The great depth of the aerial image possesses, however, this advantage for mi- croscopic observation, that of the object a definite surface is accurately seen, and what lies only a little above or beneath appears very difiused, and therefore has no disturbing influence. In the use of telescopes also, the accommodation of the eye is almost en- tirely removed. For with a telescope which enables us to see an infinitely remote object, with relaxation of accommodation, we can, with the greatest tension, see only at a very great distance ; and this difference requires of the eye without any glass scarcely any change of accommodation. But even the accommodation, of which the telescope itself is capable, by altering the dis- tance between the eye-glass and the object-glass, represents, in the diffe- rence in distance of the objects which are distinguished, only an extremely slight range of accommodation. A simple calculation will easily show, that an eye behind a telescope finds its accommodation almost entirely annihilated. A positive lens, like the object-lens, forms images behind it, at a. distance varying from i^ to » . At the distance F lie those of infinitely remote ob- jects, at an infinite distance lie those of objects situated at the distance F before the object-lens. If a more or less myopic eye be now placed so far behind the object-lens, that accommodating for R, it sees accurately the dioptric image of infinitely remote objects, the objects, whose dioptric images are seen with accommodation for P, will also lie at a tolerably great distance. Consequently the range of aoeomraodation is very much limited, TELESCOPES AND MICROSCOPES. 149 even by the objeet-lena alone. Now, this limitation increases considerably, when the eye looks in addition through an eye-piece, and is thus much nearer to the focus of the object-lens. If the eye-piece consists of positive lenses, as in the proper te- lescope, this is so close behind the focus, that the images formed by the object- glass of even rather remote objects fall with- out the instrument, and thus vision will eztend only from infinite to very remote dis- tances. If the eye-piece be a negative lens, as in the Dutch telescopes, this is immedi- ately in front of the focus of the object-lens, so that the strongly convergent incident rays through this eye-piece acquire a (slightly diverging) direction, whereby they come to a focus on the retina. But of less remoteobjects the image lies so much farther from the object-glass, that the rays reach the eye-piece comparatively less converging, and by this negative lens are, therefore, rendered so highly divergent, that the strongest ac- commodation is no longer capable of bringing them to a focus upon the retina. The reduced range of accommodation - in looking through microscopes and tele- scopes may be calculated, when are given : — the range of accommodation. .A. F^ the focal distance of the object-glass. Fr the focal distance of the eye-piece. X the distance from the object-glass to the eye-piece. y the distance from the eye-piece to k in the eye. It may suffice to explain this further for the Dutch telescopes. If the eye (Fig. SO, 0) be adjusted for in- finite distance, the rays a h and a' V falling parallel on the object-glass 1 1, are, after re- fraction, directed as c (?and c' tlirough glasses of 7^, at V' from the eye, he sees from 00 to not much less than 6", — again without change of the range of accommodation, save only in so far as the useless ^ is o rendered useful range of accommodation. The presbyopic subject, on the contrary, who, with a range of accommodation of ^, has a region of accommodation from co to 24' from the eye, loses, by the use of glasses of ^, a considerable part thereof : the accessory modifi- cations being omitted, his region of accommodation is limited to 24' — la" = 12", although his range of accommodation was scarcely modified by these glasses. The examples here given have certaioly been suf&cient, to deduce the general rule : that glasses increase the region of accommodation, when. they make r approach to 00, and on the contrary diminish it when they remove r from qd. Hence it follows, that if for a certain object a definite distance of distinct and easy vision was not usually required, it would ia general be indicated to give myopic and hypermetropic individuals command 162 SPECTACLES AND THEIR ACTION IN GENERAL. over the greatest possible region of accommodation, by the com- plete neutralisation of the ametropia, and thus to put them on a par with emmetropic persons, Frequently, however, this cannot be done, because for a definite object we must also attend to the distance of the nearest point, as in presbyopic persons. 4°. The magnitude of the retinal images does not continue the same. A comparison of the angles, under which objects, accordingly as they are viewed with or without auxiliary glasses, exhibit themselves to the eye, can without further determination, take place only so far as the object at the distance at which it is, can be acciurately seen both with and without these glasses. To a certain extent this is indeed very possible. An emmetropic person, for example, with sufficient power of accommodation, can accurately see an object situated at 8" from his eye, not only without, but also with glasses, whether of ^ or — -„. A myopic person can do the same with respect to a near, a hypermetropic with respect to a more remote object. In all these cases we can easUy satisfy ourselves, that glasses with negative focus diniimsh the images, while those with positive focus magnify them. The demonstration of this is very simple. The relation between the magnitude of the retinal image Fig. 81. jf or j3 (Fig. 81), and the object i * or B, is dependent on the position of the nodal point h. The more the latter moves forwards, the larger does /3 become, in relation to B ; the more backwards, the smaller it becomes. What the eye can now do by accommodation, that is, by alteration of its crystalline lens, scarcely displaces the nodal point, because the latter lies in the crystalline lens itself. On the contrary, so soon as an auxiliary lens is placed before the eye, h moves forwards, if it be a positive, backwards if it be a negative lens, — and the more so for the same lens, according as INFLUENCE ON THE DETERMINATION OF DISTANCE. 153 the latter is further removed from the eye. The amount of this displacement is easily calculated (compare p. 56 and p. 65). There- fore, the retinal image is larger when the object is accurately seen without tension of accommodation through a positive lens, than when it is accurately seen without this lens by tension of accom- modation of the eye; and on the contrary, it is smaller when, by very strong tension, it still exhibits itself distinctly through negative glasses. With glasses of shorter focal distance (common magnifying glasses), the amount of enlargement cannot be in this manner determined. The object must then be held closer to the eye than it can be accurately seen without the lens, and the magnitudes of the images are consequently no longer comparable. In this case we are, therefore, compelled to start from an accommodation for a definite point, and to calculate how large the retinal image under the cir- cumstance is ; this magnitude we can then compare with that of the accurate retinal image obtained with the aid of the lens, while the object is brought to the distance of distinctness of the eye armed with the lens. In this com- parison the enlargement now proves less, in proportion as the distance of distinct vision of the naked eye is shorter ; to this distance, in fact, the re- tinal image is nearly inversely proportional, while, on the contrary, the magnitude of the image seen with a strong lens increases but little in proportion to the increase of the distance of distinct vision of the unaided eye. 5°. The determination of the distance, magnitude, and form of the objects undergoes a change. In order to demonstrate the influence which positive and negative glasses exercise on our estimation of distance, magnitude and form, it is necessary to trace, in what manner, without the use of glasses, this estimation is estabhshed. With differences in the distances, magnitude, and form of objects, are connected peculiar modifications in the requisite movements of the eyes, in the accommodation and chiefly in the retinal images ; and in the changes, which these undergo by accommodation, and by movements of the eyes or of the head, and of the whole body. It is exclusively from these modifications, that the mind is in a position to form an opinion as to distance, magnitude, and form. This, however, for the most part takes place spontaneously, quite in- voluntarily, or at least without consideration. The rapidity of the judgment, without analysis of the elements on which it is based, is the result of practice, partly of the individual, partly of his parents, and is, in the latter sense, innate. 154 SPECTACLES AND THEIR ACTION IN GENERAL. In the first place, we observe that the estimation of distance and that of magnitude are correlative. Three cases are to be distinguished. 1. We know the true magnitude of the object, and thence form, by the magnitude of the retinal image, our opinion as to the distance ; 2. We know the distance and base thereupon our opinion of the magnitude; 3. Distance and magnitude are both imperfectly known, and through reciprocal influence an idea is developed, which brings both into connexion with one another, and thus at the same time more accurately defines them. The connexion just mentioned between our estimation of distance and of magnitude is particularly striking when we project the ocular spectrum of a flame upon a wall, in which case we suppose the flame larger, in proportion as we withdraw from the wall, and smaller in proportion as we approach it, notwithstanding that the retinal image of course remains unchanged. In order thoroughly to investigate how our judgment is esta- blished, we must examine, what and how we are able to distinguish, first under the simplest, and subsequently under more and more com- plicated conditions of vision, whereby new means are each time added to those already obtained, confirming our opinion with greater cer- tainty, and sometimes modifying it. The following are to be distinguished as conditions of vision : — a. An eye, without motion, seeing figures in a plane, to which the visual line stands perpendicular; h, the same eye, looking freely into space ; c, an eye with movable visual line looking on a surface or into space ; d, an eye, by movement of the head or even of the body, changing its place; e, two eyes at rest; f, two eyes in motion; g, two eyes with movement of the head or of the body. We should, however, be led too much into detail, were we fuUy to follow out this scheme. We must confine ourselves to a succinct and elementary development of the intricate question, and shall also even pass over almost completely in silence the literature of the subject, which has attained to a whoUy disproportionate extent. Here, under 5°, we speak of our judgment in seeing with one eye; under 6°, stereoscopic vision with two eyes comes under consideration. We begin by supposing, that the figures are all si- tuated in one plane, which is viewed only from one point with one eye, whose visual line is perpendicular to this surface. In this manner we have no means of directly judging of the true magnitude. DETERMINATION OF DISTANCE. 155 if this is not otherwise hnown to us. It is only witli respect to the dis- tance that the consciousness of our accommodation gives some idea. Let a person, through a tube perforating a wall, so that he cannot estimate its length, read print placed behind the tube ; in this way many will form a not very incorrect idea of the distance, and conse- quently also of the magnitude. If we now place in the tube a weak positive glass, so that less tension of accommodation shall be required, the priat will appear to the observer to be at a greater distance, and he wiU therefore suppose it to be really larger, even though a print so much smaller has been substituted, that the retinal image has retained the same size. If, on the contrary, we place in the tube a negative glass, whose action can still very well be overcome by tension of accommodation, the observer will suppose that the print is closer to the eye, and the actual magnitude is thus set down as less, even if a type have been taken so much larger, that the retinal images, notwithstanding the diminishing influence of the negative glass, have maintained the same size. But if, on jjie contrary, the magnitude of the object be known, the judgment in general yields from the consciousness of accom- modation of the eye having taken place, and the object, seen through a negative glass, is supposed to be more remote, because the retinal image is smaller, — seen through a positive glass, on the contrary, it is supposed to be nearer, because the retinal image is larger. — ^Exceptionally, however, the consciousness of the required accommodation comes into play, even when we look round in nature, where there is no want of objects of known magnitude, and where numerous other means of deciding are at the service of the mind. I observed this, many years since, in myself.* As a phenomenon, namely, of diminished power of accommodation, in consequence of the instillation of a weak solution of belladonna, I saw aU objects too small, because I supposed them nearer to me, and Warlomont has also communicated an example of the same.f In paresis of* accom- modation too, produced by other causes, the same has once occurred to me. The determination of the distance from the accommodation required, appears to me besides in looking with one eye to be accom- plished chiefly through the fact, that the other (covered) eye, in connexion with the tension of accommodatioa, alters its convergence, * Nederlandsch Lancet, 2' Serie, D. VI , p. 607, 1851. t Aanales d'Oculistique, 1853, T. xxix., p. 277. 156 SPECTACLES AND THEIR ACTION IN GENERAL. in which alteration we, in looking with both eyes, possess so accurate an aid in the comparison of distances. In looking at objects in space, with one eye from one point, the estimation of magnitude and distance for an infinite number of suc- cessive planes can be effected in the same manner as above for one plane, and positive and negative glasses can also in the way described modify our estimation. Thus, then, an idea of the form of a body could be developed as well without as with the use of glasses. Under these circumstances, however, the reasoning is often inverted; we know the form, and thence deduce the relative distances of different points. This must be further explained. Many objects have for us known forms, and on their distance it depends, what the form of the perspective image on the retina shall be. An example may illustrate this : — Let the observer place himself before the middle of a square table A B C D (Fig. 82 I). In order to know how large A B and C D are represented on the retina, we have only to ^raw Hues from the points ABC and D, through the nodal point h of the eye, which we suppose here to be placed alme the horizontal plane of the table. We then find the respective magnitudes a i and cd era. the retina E. While the distance from A B to the eye amounts to the double of D, the image c d \^ nearly double as large as a h. The retinal image has therefore the form of Kg. 82 II. This form leads us to infer, that the nearest edge of the table A B is about as far from us as the table itself is long. If we now move the eye from A B to C D, the accommodation required helps us in our decision, and if we know the height of the table, the angle under which the lines A B and C D are seen beneath the horizontal plane, in which the eye lies, assists in determining our opinion as to the distance. The same table, placed at double the distance from the eye, gives another perspective image, in which a b : c (^=2:3 (compare Pig. 83 I), and in which the angles at a, h, c and d deviate less from the right angles (II). We, however, consider the dimensions to be equally great, and the angles to be right angles, only because such is usually the case with tables, and we infer that the length of the table amounts to about half the distance from C D to the eye. If there be now only some object, whose true magnitude or distance is known, this serves as a standard whence to judge of all other objects, whose relative magnitude and DETERMINATION OF DISTANCE. 157 distance are inferred upon the same principle (the perspective projection). Fig. 83. d J etC In the foregoing we assumed a knowledge of the true form of some objects^ and the direction of some lines or surfaces. This knowledge, indeed, is scarcely ever wanting. In a room we see surfaces, which we may consider to be nearly horizontal or per- pendicular, and numerous objects which present to us vertical and horizontal lines. Prom the angles which these surfaces and lines form on the perspective retinal images, we deduce our judg- ment. In nature the ground we walk upon, the horizontal water surface, ascending trees, houses with their frames and windows, lastly, man himself, are sufficient starting-points. Therefore also we 158 SPECTACLES AND THEIR ACTION IN GENERAL. judge tolerably correctly even what is seen only with one eye from one point. Properly speaking our judgment can fail only through an intentional arrangement. Por example, the table (Fig. 82 a) would have produced the same retinal image, if the edge A B had had only the breadth " Z^, but at the same time had been proportionally raised, so as to mate the edges « C and /3 D coincide perspec- tively with A C and B D. Thus we can on one surface (a drawing) represent a number of objects in perspective projection, of which the retina then receives an image, such as the objects themselves would produce, and if the effect of Hght and shade be added, this image may be very deceptive, when seen with one eye from one point. However, if the surface be not too remote, the consciousness of ten- sion of accommodation will even still teach us, that all lies in one and the same plane. But, moreover, these are artificial circumstances, which do not invalidate the rule, tkat the form of the perspective projection on the retina is sufficient to enable ws to judge of the rela- tive, and even of the absolute magnitude and distance, provided only the magnitude or the distance of some object be known. Now on this judgment, deduced from the perspective projection, the use of positive and negative glasses exercises an influence. The cause of this is easily seen. In fact, in using positive glasses, the objects appear to us not only larger but nearer, and the distance in depth between two objects, and hkewise between two lines of the same object, is thus shortened, there is, therefore, an enlargement of the object with diminution of its depth ; the reverse takes place in the use of negative glasses. We can explain this stiU more fully. With actual difference in distance of objects the angles, as we have seen, change, and the proportions of the perspective images alter (compare Figs. 82 and 83) . But proportions and angles remain the same, when by means of glasses the retinal images are magnified and diminished, and we consequently suppose the objects to be at other distances. The form which our judgment connects with the perspective image, must therefore be different. Most remarkable is the influence of the magnitude of the retinal image on the estimation of the depth, provided that we know the angles of a surface. If, for example, the latter be the horizontal quad- rangular leaf of a table, the relation between depth and breadth will be connected with a definite magnitude of the retinal image; in other words, the estimation of that relation will be different, when the ESTIMATION OF DEPTH. 159 retinal linage^ retaining the same form, becomes larger or smaller. A simple construction will demonstrate this. In Kgs. 84 I, and 851, a b and o d are projected in the same proportion upon the re- tinas E: they stand to one another as 2 : 3, A.s the adjoining retinal images II show, a h lies lower than c d, but still in the same proportion. The retinal images, therefore, do not differ in the least in form, and nevertheless we are compelled to suppose the relation between length and breadth in the objects to be different (compare A B D of Kgs. 84 I and 85 I), and on this relation, different for each magnitude of the image, the distance at which we project it, has no influence (compare I'ig. 85 1. A B C D with A B' C D') . That we do Fig. 85. ca bd not connect similar forms of the objects (unless they lie in the same plane perpendicular to the visual line), with similar forms of the retinal images, but of different magnitudes, cannot surprise us, when we see on the other hand, that objects of similar form, but of different 160 SPECTACLES AND THEIR ACTION IN GENERAL. magnitude, do not give uniform retinal images (unless those, witii difference in distance of the objects, are also equally large). If we place in the middle, or in whatever part we think well, of a quadran- gular leaf of a table, a square sheet of paper, uniform with this leaf of the table, the projected retinal images will be dissimilar : the differ- ence in dimension of the superior and the inferior boundary will, for the table, be relatively greater (for this relation depends on the relative distance from the eye), and with it the four angles differ. Hence, it most clearly appears, how positive and negative glasses, which cause us to estimate the distance differently, and alter only the magnitude, not the form of the retinal images, modify our esti- mation of the depth. Hypermetropics observe this most distinctly, who, armed with positive spectacles, see the objects larger, which they before, through tension of accommodation, also saw distinctly, bat smaller, and most of all persons, who replace the lost crystalline lens with positive spectacles. They suppose all distances to be less, and therefore see objects less deep. Myopic individuals experience the opposite influence, but some- what less distinctly, because without the negative glass they did not see remote objects accurately, and the comparison is therefore less perfect. It becomes more evident when they take a too strong nega- tive glass, which they, however, overcome by accommodation ; and in like manner emmetropic persons may in these experiments make use of a negative glass proportionate to their accommodation. But the phenomena appear much more striking in using the Dutch telescope, for instance, when we look through an ordinary double opera-glass with one eye, and close the other. Let us thus look at a small table as above. The retinal image of course preserves its form : if the glass magnifies n times, the image is only n times larger in all its di- mensions. But we see the table broader and shorter at whatever dis- tance we endeavour to suppose it to be. Often we have difficulty in supposing it to be short enough. Then it is, of course, too broad behind : A B (Fig. 83) appears to us larger than C D. This we sometimes correct by, in our thoughts, making C D rise a little; and those persons also do this, who in aphakia wear a convex glass, so that on flat ground they suppose they are running up a hill. The phenomenon is particularly striking when we look with the opera-glass at an ordinary book, lying at a short distance on the table. The book immediately becomes square, although the retinal image retains the same form, it often remains somewhat broader at the MODIFICATION OF STEREOSCOPIC VISION. 161 upper edge, loses this when in our imagination it acquires an incli- nation, and the letters of the title are now broad and low, while be- fore they were decidedly oblong. If we now turn the opera-glass round : the retinal images, while retaining the same form, have become diminished, and the dimen- sions in depth, contrary to our former observation, are very con- siderably increased ; even the leaf of the table and the book appear to become narrow above. All this takes place in using one eye. The estimation of relative distances, in monocular vision, becomes still more correct and certain when the head, or even the body moves, so that the objects are seen successively from different points. In this manner, in fact, a parallax is created between objects placed at different distances, which apparently change their relative places, whereby the relative distances even of simple free points can be estimated. 6°. Stereoscopic vision with two eyes is modified. — Prom what has been said under 5°, it is evident, that the estimation of distance and of solidity of objects, even using only one eye, is tolerably perfect. This must here be remembered, for the beautiful discovery of Wheatstone has been found so important,* that it finally appeared to make us forget what a single eye can do. Two eyes, however, can certainly do more. At least when near objects are concerned, the solidity of the object can be estimated with more certainty j we cannot, in binocular vision, so easily be deceived by an artificial arrangement, and a peculiar * Not to speak of the older literature, whioh is to be found everywhere, that on binocular vision has of late years been increased by the following great essays and works : — Panum, Physiologische Untersuchungen iiber das Sehen mit ztvei Ai^gen. Kiel, 1858. Recklinghausen, Netzhautfunctionen, Jrchiv f. Ophthalmologie, B, v. S. 2, 1859. Volkmann, Die Stereoscopische Erscheinungen in ihrer Beziehung zur Lehre der identischen Netzhautpunkten, in Archivf. Ophthalmologie, B. V. Abth. 2. Nagel, Das Sehen mit zwei Augen mid die Lehre von den identischen Netz- hautpunkten. Leipzig und Heidelberg, 1861. Wundt, a series of articles reprinted from the Zeitschrift f. rationelle Medicin, collected under the title of BeitrUge zur Theorie der Sin- neswahrnehmung, 1862. Feohner, Ueier einige Ferhdltnisse des binocularen Sehens, from the Ahhand- lungen d. li. s. Gesellschaft der Wissensehaften vii., Leipzig, 1861 ; and numerous short articles, by Dove and others, in Poggendorf's Annalen 11 162 SPECTACLES AND THEIR ACTION IN GENERAL. sensation of the solid is developed in us, which the monocrdar person seems not to be acquainted with. This depends upon two causes. The first is, that in connexion with the distance between the two eyes, in fixing a definite point of an object, in which the two visual lines intersect, the retinal images of the two eyes are not equal and uni- form, in consequence of which most points are seen as double images, whose deviation corresponds, for every direction, to the difference in distance of those points. "With this the idea of the soUd arises in us ; but often, if all movement be avoided, and if no adjoining bodies guide us in the estimation, confounding of the stereoscopic with the pseudo- stereoscopic is possible : that is, the point seen under double images may as well be nearer, as more remote than the fixed point. The second cause is : the successive fixing of the different points of the object (Bruecke). In fact we are perfectly conscious (at least, if the movement does not run through large angles), whether, in passing from one point to another, single vision requires more or less convergence, according as the fixed point is less or more remote than that previously fixed. At the same time, in examining the object, that is, in running through different points in different directions, the two retinal images constantly change, and their dissimilarity every time alters, which necessarily forces upon us a definite bodily form, and almost makes us touch and feel — which is precisely the characteristic of binocular vision. If, in fixing some point, we already have a tolerably accurate idea of the position of the point indirectly seen, which we now wish to fix, we even modify, with the simultaneous movement of the visual Hues, the convergence of the axes. If we miss this idea, we see the double images dis- appear only when the visual lines have about reached the desired point. We shall best satisfy ourselves of this, if by holding a car- mine glass before one eye, it colours one of the double images, and if we now look in succession at points of hght situated at different distances. We then observe also, that with a rapid movement of the visual Imes, the required change of convergence is for a moment ex- ceeded, and is then quickly corrected. Now the use of convex and concave glasses modifies the two factors of stereoscopic binocular vision. As concerns the first factor, we observe, that the glasses modify the magnitude, and consequently the apparent distance, without producing a corresponding modification of the difference between the MODIFICATION OF JUDGMENT BY STEREOSCOPIC VISION. 163 retinal images of the two eyes. This difference decreases, accordingly as the distance of the objects increases. While, namely, the angle, under which an object exhibits itself, is inversely proportional to its distance, the parallax of stereoscopic vision, and therefore also the dif- ference between the two retinal images, are, at great distances, inversely proportional to the square of the distance. If on the line li I (Fig. 86), standing perpendicular to the line h Jc, connecting the nodal points of the two eyes, two points a and a be moved without altering their mutual distance, Fig. 86. then, as a simple calculation shows, at great values of fc a, with respect to k Jc and to a a, the angle a k a \s inversely proportional to the square of k a. Now, this angle is evidently the parallax of the stereoscopic vision. Thus it is proved, that the retinal images of the two eyes differ less, accordingly as the distance from the object is greater. With convex glasses, and especially with an opera-glass, the object is now seen larger and apparently nearer ; but the difference between the two retinal images appears only equal to what it was in looking with the naked eyes. Consequently the object exhibits, in reference to its magnitude, too slight a depth. The reverse takes place in the use of concave glasses, which exhibit the objects smaller, and therefore apparently at a greater distance. Thus the first factor is altered. As to the second factor, it is dependent on the first : the difference in the retinal images is precisely that which requires a difference of convergence, accordingly as we view one or other point of the object, and therefore there is no necessity to treat separately of it. TVom these considerations it appears, that, in the use of glasses, stereoscopic vision with two eyes modifies our judgment in the same sense, as takes place for each eye separately. An apparent magnifying, namely, of the dimensions, perpendicular to the axis of vision, in both cases, causes us to estimate the depth as relatively less, and vice versa; on a diminution of these dimensions, we are led to 164 SPECTACLES AND THEIR ACTION IN GENERAL. infer a relatively greater depth. However, in this the modification of the perspective projection for each eye separately is of much greater importance than the modification in the binocular vision. Attention must still be directed to a few subordinate points, relating to the influence of glasses. I allude, in the first place, to the apparent move- ment of objects, when the observer, by moving his head, passes from one object to another. If we see, namely, through the glass, the objects under a smaller angle than with the naked eye, we must turn the head to a compara- tively great degree, in order, by so doing, to direct the visual line alternately to the one or to the other edge of the object, and thus the latter appears to fly before the movement of the head. On the contrary it comes in this movement to meet the glance of the beholder, when the visual angle is magnifled, and therefore, in reference to the latter, little movement of the head is required. If the same does not take place in moving the eye behind the glass, this is to be ascribed to the fact, that a person, in looking obliquely through the glass, no longer sees the object in the direction in which it actually is, whereby the want of harmony between the magnitude of the retinal images and the movement required to make them pass over a point of the retina, is compensated : without this compensation, we could not have said, on the preceding page, that the second factor of stereoscopic vision lies included in the first. Finally, it deserves here to be mentioned, in a single word, that glasses limit, if not the magnitude, at least the symmetry of the field of vision ; that behind glasses the movements of the eyes are not free in all directions ; and that by reflection on the two sur- faces of refraction some light is lost. In the above (under the 6th head), our views were based upon the doc- trine of the identical or corresponding retinal points. Of late this has been much disputed. It therefore appears necessary here to explain myself upon this subject. In eyes whose visual lines exhibit no morbid deviation, the existence of points which project the impressions received upon each other into space, is not to be denied. In this sense the points in the same meridians equally remote, upwards or downwards, to the right or to the left, from the fovea centralis of the yellow spot, may be considered as sufficiently correspond- ing. The identity, however, is not absolute. The images of two circles, of somewhat difierent magnitude, the one received on the right, the other on the left retina (whether through the stereoscope or by convergence), so that the lines may coincide in the fovea centralis, project a circle, whose magnitude is the mean between the two actually present. On the contrary, no matter how the observer may draw the smaller in the larger, and what point he may now fix, he will, with one eye, still always recognise the two circles, and certainly see none of medium magnitude. So far Wheatstone was un- doubtedly right, when his discovery made him reject the theory of the iden- tical retinal points. So far, also, the statement made above, that every variation in the form of the retinal images gives rise to double vision of the points not falling upon corresponding parts, is to be corrected. But the controversy of the identical points has been carried still further. LAW OF IDENTICAL POINTS. 165 Some began by wholly denying the mental projection of the retinal images into space. This denial would not have been made, if observers had always distinguished between two different things : the projection of the field of vision and the projection of a point in the field of vision (Conf. Ueber die Bewegungen des Auges, von F. C. Bonders, in HolldndischeBeitrUge zu den anat. undphysiol. Wissensohaften, herausgegeben von J. v. Been, F. C. Bonders und Jao. Molesohott; B. 1. pp. 105 et seq., 1848). The projection of the field of vision depends on the position of our eye and the direction of the visual line, which we assume to be present ; and the assumption is tolerably accurate, when the eye, in normal movement, is voluntarily brought into this position, and the visual line has been voluntarily given this direction. In what part of the field of vision thus projected we project a certain point, is, on the con- trary, determined by the place which its image occupies on the retina. In this manner every projection in the normal condition is explained. Thus it is also understood how, with accurately directed eyes, of which direction the observer is conscious, corresponding points of the retinas project the impressions received on each other. This may still continue under abnor- mal circumstances. In recent paralysis, for instance, of a muscle, we estimate the direction of the visual line incorrectly, and we consequently project the field of vision, and with it each point of the field of vision, in a false direction. We suppose that the deviating visual line intersects that normally directed in the point that we wish to see. Consequently we project the image falling on the fovea centralis of the deviating eye on the projected image of the fovea centralis of the properly-directed eye. These two different images thus appear to cover one another. If the deviation is not great, we speedily find the directly-seen object of the properly-directed eye as indi- rectly seen on the deviating one, and thus double vision is the result. But in every case confusion arises also in direct vision, which becomes particu- larly great, if accidentally a strongly illuminated part of the field of vision forms its image on the yellow spot of the deviating eye. However, we occupy ourselves almost always with the more illuminated parts of the field of vision, and the result thereof is, that in general the image on the yellow spot of the properly-adjusted eye excels in clearness that of the deviating eye. This makes it easier to neglect the image of this last eye ; and it is very remarkable that this psychical abstraction, always increasing, is at- tended with physiological torpor, that is, with want of sensibility. So far the law of the identical points holds good. But now it happens, particularly in cases of strabismus divergens, that we become conscious also of the direction of the deviated eye. This is the case when one con- tinues to use this eye in its turn. In these cases, an object is still fixed in general with the eye, whose muscles act normally. If we now hold a second object in the visual line of the deviated eye, and request the person to fix it, the eyes sometimes remain completely at rest. The patient can thus occupy himself alternately with the one or with the other object, which respectively forms its image on the one yellow spot or on the other, and he knows perfectly in what direction each is. In this case, it is in the first place remarkable with what certainty he distinguishes with which eye he observes anything : he who has two good, regularly-moving eyes, is not at 166 SPECTACLES AND THEIR ACTION IN GENERAL. all aware of this ; we can ascertain in. which eye we have muscse volitantes, only by closing one. But, in. the second place, it hence appears most dis- tinctly, that in such a person the corresponding points have lost their mutual relations. Indeed, what impinges on the two yellow spots is projected in very diflferent directions, and likewise what touches on similarly directed meridians in points equally removed from the yellow spot. Now the reason why origi- nally corresponding points are projected in diflferent directions is clearly none other than that the whole field of vision is projected in another direction : the projection of the different points of the same retina has, with respect to one another, continued the same. This relation alters only when the retina is plaited or irregularly extended ; and I should not venture to assert, that if the accuracy of vision were maintained, the projection should not again by observation and trial gradually come to correspond to the actual position of the objects. In treating of progressive myopia I shall revert to this point. But from the foregoing thus much has appeared, that the deviation of the visual line, produced by muscular anomaly, which originally gives rise to false projection, may become known to us by experience, with which each eye begins its independent projection ; and no further proof is needed that, consequently, double images which, according to the theory of the identical points, we should see intersected, may be changed into homolateral ones. An organically necessary similarity of impression of corresponding points of both retinas, which should lead to an equally necessary similarity of direc- tion of projection, is therefore out of the question. But, nevertheless, what we at first put forward remains true, whether it be congenital or the result of practice, that in eyes whose visual lines exhibit no morbid deviation, certain corresponding points of the retinas project the impressions received on each other into space. The assertion, that by merely fixing a point with both eyes, confusion of the stereoscopic with the pseudo-stereoscopic is possible, I have above noted, after having also taken cognizance of the most recent articles of Dove and of Recklinghausen (Poggendorff's ./^nnoien, B. 110, p. 491, andB. 114, p. 170, 1861). This is not the place to enter into the subject at greater length. In the beginning of this section I stated, in investigating the action of glasses, that I provisionally assumed, that the axis of the glasses coincided with the visual axis. This is certainly almost never exactly the case. Firstly, the glasses in a pair of spectacles are not placed precisely so, that the distance of their two axes should be exactly equal to that between parallel visual axes, and, moreover, in being placed before the eyes they easily come to stand somewhat higher or lower than the visual axes; or the glasses, and therefore also the axes, have, when the head is perpendicular, a certain inclina- tion. Secondly, every movement of the eye immediately alters the relation between the visual lines and the axes of the glasses. The question now is, what is the result of this deviation ? In the first place, when the axis of the glass is parallel to the DISPLACEMENT OF OBJECTS. 167 Fig. 87. visual axis, but is displaced in one or other direction, we obtain also a displacement of the object seen through the glass. "We satisfy our- selves of this, by pushing a convex or concave glass before the eye, so as to look always parallel to the axis, but alternately through the centre, and through the edges of the glass. If the glass be convex, the displacement occurs in the opposite direction to that of the glass, if it be concave, in the same direction. The explanation is simple. Let k be the centre of the surface at h, representing the dioptric system of the eye, i a point situated in the elongated axis Ic h, this point will then find its image in the axis, for ex- ample in j. If a lens, whose centres of curvature fall in the line kj, be placed before h, the image of i wOl also remain in the same line. If, on the contrary, a refracting surface as K, with its centre of curvature in o, be placed before h, this is no longer the case. If we imagine, for instance, h K to be a reflectei ray, this will, refracted at li, deviate from the vertical v o, and proceed in the direction K i; consequently a ray must, vice versa, come from a point situated in the hne, for example, from i, in order, after refraction at K, to enter the eye as h! h, and to proceed as k j. Evidently, therefore, a point, situated in i', is seen in i, that is, displacement occurs in the direction opposite to that in which the axis of a convex glass is pushed before the visual axis. Hence it follows, that, if the two convex glasses of a pair of spectacles stand too close to one another, the objects are for both eyes displaced more outwards, and thus less convergence is required ; the reverse is the case, when the glasses stand too far from one another. The opposite, of course, takes place, when concave glasses are in question. In either case the change of convergence required is less, the weaker the glasses are, and the less they are pushed to the side. For many years I have advantageously made use of such eccentric placing of the glasses, where otherwise, in insufficiency of one or other muscle, combination with a weak prism was indicated. Nor will it easily happen, that, in doing so, we exceed the hmits, whereby the acuteness of the images suffers too much. If, however, we 168 SPECTACLES AND THEIR ACTION IN GENERAL. desire to encroacli as little as possible upon the convergence of the visual lineSj corresponding to the distance, we must, as has been cor- rectly shown by Giraud-Teulon and Knapp, regulate the distance of the glasses according to the reciprocal distance of the visual lines. We have then, as Knapp has remarked, to attend particularly to the axes of the glasses, for these do not always correspond to the centre. In order now to find the axis, we have only to ascertain what part of the glass we have to hold before the eye, in order to see a vertical line, even when the glass is made to revolve, unrefracted as well through, as under or above the glass. However, we need not be too careful in regulating the axis. "Whether, in order to see an object, a little more or less convergence must be employed, is often rather a matter of indifference; and if this is not the case, we accordingly involuntarily regulate the distance between eye and object. Against this no difBculty is to be expected from the accom- modation: indeed, its limits, under the influence of the spectacles selected, are not defined so precisely, that we could approve or dis- approve of a slight modification of the convergeBEe under which the accommodation for a certain distance is required. We have only to take particular care, that in spectacles worn out of doors, we have not so short a distance of the axes of the concave, nor so great a distance of those of the convex glasses, that in looking to a great distance a divergence of the visual lines should be required, which might easily cause difficulty. A difference in height of the axes, which should cause a mutual deviation of the visual Hues in a vertical direction, we must above all avoid. In the second place, as to looking through the glass under an angle with the axis, we have already observed that this is unavoidable in the use of spectacles. The deviation thence proceeding is of two kinds. In the first place, the object directly seen exhibits itself in another direction than that in which it actually is. A construction as above (Fig. 87), modified so far, that the axis of the lens makes an angle with the visual axis, shows this directly. This altered direction is, as we have already remarked, of that nature, that the disturbance in harmony between the angle under which we see a dimension through convex or concave glasses, and the turning of the eye required in order to traverse them, is sufficiently compensated : more- over, this deviation causes no difficulty. But, in the second place, the objects are seen less accurately. Besides the ordinary aberration, in fact, a new and very important one occurs. Of this we may con- DIRECTIONS OF THE AXES. 169 vince ourselves, by looking at a point of light through a convex or concave glass held obliquely before the eye, and, better still, by receiving on a screen the dioptric image of a point of light formed by an obliquely-placed convex lens. This image has a clear eccen- trically-situated point, whence the light spreads chiefly to one side in the form of a fan, so that it reminds one of the appearance of a comet. In treating of astigmatism I shall return to this subject. Here it may only be remarked, that the diminished accuracy of the images, especially when strong glasses are in question, renders it imperatively necessary to attend to the direction of the axes. If spectacles be used only for distance, the axes must be placed nearly parallel and horizontally ; on the contrary, in spectacles used only for near objects they require to converge proportionally, and to be directed downwards. This, when strong glasses are required, pro- duces a difficulty of making use of the same frame for every distance, even in these cases, where the range of accommodation still admits of the use of the same glasses. The inclination of the axes can be sufficiently modified by placing the spectacles. But the convergence of the axes cannot be altered, without bending the frame. There- fore convergence must correspond to the mean distance, at which the spectacles are used, whereby, during their use, a certain margin in the convergence of the visual axes is least excluded. NOTE TO § 12. In this section we have spoken only of the immediate effects of convex and concave glasses. What results are mediately produced by them, with respect to refraction, accommodation, movements of the eye, &c., will be better discussed in connexion with the anomalies in which particular spec- tacles are indicated. I have also thought it right here to pass over in silence the action of simple and compound cylindrical glasses. It is only in astigmatism that they are applicable, and they will more properly come under consideration in the chapter on this anomaly. Respecting the different forms of spectacle frames, the desirable distance of the glasses from the eye, the use of spyglasses for one and for two eyes, the employment of reading-glasses, of magnifiers and of opera-glasses, &o., some remarks will be found in speaking of the different anomalies of refraction and accommodation, in connexion with which these instruments come under 170 SPECTACLES AND THEIR ACTION IN GENERAL. observation. In general, on this subject, among other writings, the following deserve to be compared : — Szokalski, in Prayer Vierteljahrschrifi, B. v., \, 1848, and Smee, The Eye ill Health and in Disease, London, 1854, pp. 44, et seq. NOTE TO CHAPTER IV. The chief literature on the subject of spectacles is to be found in Ruete {Lehrbuch der Ophthalmologie fur Aerzte und Studirende. B. I., p. 238, Braunschweig, 1853). Spectacles are among the most indispensable instru- ments for man. For many they extend the power of vision to an infinite distance, and others should, for want of spectacles, at a certain time of life see themselves completely shut out from the occupations to which, in a busy society, they are called. If we add that spectacles laid the foundation for the invention of the microscope and telescope, whose mighty influence is powerfully exemplified in the development of most natural sciences, we shall not view these simple instruments without respect. The history of the ordinary concave and convex glasses, which we here have exclusively in view, is somewhat obscure. Any who take a special interest in the matter will find the most essential points briefly collated in the work on the Micro- scope by my colleague Prof. P. Harting. (See the German translation by Theile, under the title of Das Mikroskop, p. 585. Braunschweig, 1859.) W. Krecke, Ph. D., Vice-Director of the Royal Meteorological Observa- tory, suggested the use of prismatic spectacles in strabismus. To his com- munication I added my investigations of the physiological action of these glasses {Nederlandsch Lancet, D. III., pp. 227 et seq., 1847). Von Graefe (Archiv f. Ophthalmologie, hois diverais) especially showed how still more advantage was to be derived therefrom, both in diagnosis and in treatment. The use of stenopseic apparatus was introduced by me (compare van Wijngaarden, over stenopesische hrillen. Diss, inaug., Utrecht, 1856, and Archivf. Ophthalmologie, B. I., Abth. 2). It is true, that in mydriasis use was sometimes made of small openings ; but it did not occur to any one to remove, by their means, the injurious effect of obscurations. How these by throwing difi'used light into the eye, disturbed the power of vision (com- pare pp. 128, 129), was also first explained by me in Wijngaarden's paper. The explanation given is quite in harmony with the law developed by Fechner for the senses in general (Ueber ein wichtiges psycho-physisches Orund- Oesetz, Leipzig, 1859, and Elemente der Psycho-physik, Leipzig, 1860). Green and blue glasses, for moderating the light, are highly valued. In a work recently published by Professor Dr. Ludwig Boehm, of Berlin, under the title of Die Therapie des Auges mittels des farbigen Lichtes, Berlin 1862 blue glasses, of difierent shades, are particularly recommended in numerous especially functional, disturbances of the retina. SPECIAL PART. I-ANOMALIES OF EEFRACTIOK CHAPTEE Y. THE EMMETROPIC EYE. The emmetropic eye presents, both in its structure and in its func- tionsj the standard by which the anomaHes of refraction must be estimated. As such the knowledge of the emmetropic eye must here occupy a prominent place. But, in other respects, this eye, which as little as any part of the body escapes the influence of age, must also not be passed over in silence. The range of accommodation early diminishes ; soon the accuracy of vision lessens, and lastly the emme- tropia is converted into ametropia, giving way to hypermetropia acqui- sita. The emmetropic eye, in its retrogression, in its yielding to the advance of years, is our task". To the latter we are called, the rather because art can protect the eye against the senile metamorphosis, and by suitable means can help to maintain it in a position to discharge its functions. § 13. Definition of the Emmetropic Eye; the Diageammatic Eye ; the Simplified Eye. The emmetropic eye is that, the principal focus of whose dioptric system is, in rest of accommodation, found in the retina (Eig. 88). rig. 88. Of infinitely remote objects, which send out parallel rays, this retina therefore receives accurate images, to be improved neither by convex nor by concave glasses, and by means of its accommodation it sees equally accurately at relatively short distances. No other refraction of the eye is capable of giving to the region of accommodation so great an extent. 174 THE EMMETROPIC EYE. That this condition is to be regarded as the normal, we have already (page 81) shown. Singularly enough, for a long time the opinion was rather generally entertained, that almost every eye is more or less myopic; that at an infinite distance, apart from the imperfect transparency of the air, it is only exceptionally that objects can be distinguished under the same least angle of vision as at a moderate or short distance. This opinion is an error. By far too often does the eye deviate in the opposite direction from the stan- dard, and can, with hypermetropic structure, bring converging rays to a focus on the retina. If the emmetropic eye is to be considered as the typically normal eye, another question is, whether it is at the same time the ordinary eye, and whether, therefore, ametropia is the exception. In an absolutely mathematical sense no single eye is perhaps to be called emmetropic. In the first place, I have never met with an eye whose focal distance in the different meridians was absolutely the same ; in general, as shall be more fully shown in the Chapter on Astigmatism, the focal distance is shorter in the vertical meridian of the eye than in the horizontal. But, apart from this, here, if any- where, we must allow a certain latitude to the rule. Slight degrees of M, for example M =1^, in which, at the distance of 10 feet (= 120') vision is still perfectly accurate, are almost always un- observed. Slight degrees of H are in youth not even to be proved, much less to be reduced to their numerical value : indeed, whenever a deficiency of refractive power exists in the eye, when in a state of absolute rest, it is suppUed by the accommodation. And even if the eye in paralysis of accommodation should be emmetropic, the tone of the accommodation alone effects a slight degree of M. Consequently, the actually emmetropic vision requires, in a certain sense, a minimum of H, and that minimum is capable of no accurate taxation, because to the tone itself a certain latitude, perhaps from -7777. to TTT, must be allowed. In this sense, and it is practically the only correct one, the majority of eyes of young persons are undoubtedly emmetropic. Pinally, should the question be proposed, whether E is the most desirable condition : as concerns myself, I should give the prefer- ence to a slight degree of M, and I shall subsequently state my reasons for doing so. THE DIAGRAMMATIC EYE. 175 The emmetropic eye is dioptrically to be realized in various modes. Apart from the possible differences of the coefficients of the refrac- tion, a compensating action may take place between a. The radius of the cornea : the less this is, the shorter is the focal distance. h. The form of the crystalline lens : the more convex its surfaces, the shorter is its focal distance. c. The position of the crystalline lens : the more anteriorly it lies, the more, ceteris paribus, has it a shortening influence upon the focal distance of the whole system. d. The length of the visual axis : it needs only to correspond to the condition, resulting from a, h, and e, to make the eye in each case emmetropic. However, each of the factors mentioned, by itself presents in the emmetropic eye, comparatively little difference. With respect to the cornea, this is directly seen from a large number of measure- ments.* For the other factors it may be assumed for reasons, the development of which would here lead me too far. Hence we are completely justified in assuming a diagrammatic eye, and for the sake of different calculations, starting therefrom. The values assumed by Listing were somewhat modified by Helmholtz, who considers the crystalline lens to be a little fiatter, and its position to be rather more anterior. With these modifications I have adopted them at page 67, where they are collated with those of the accommodated eye. Following Listing's example,t we may go still a step further in the-, simplification : it is, in fact, allowable to reduce the compound diop- tric system of the eye to a single refracting surface, bounded anteriorly by air, posteriorly by aqueous or vitreous humour, and this reduced eye, where the greatest accuracy is not required, may be made the basis of a number of considerations and calculations. With this simplifi- cation we can, with the greatest ease, form a satisfactory idea of the magnitude of the retinal images, of the position of the conjugate foci, of the extent of the circles of diffusion in imperfect accom- modation, in astigmatism, &c., and of numerous other points. The right to this simplification we derive from the minuteness of * Conf. Verslagen en Mededeelingen van de Koninkl. Ahademie van WetenscJiappen. 1860. D. xi., page 159. t Dhptrik des Auges, in Wagner's Handworterhuch der Physiologie, B. iv. page 493. Bra,mnschweig, 1833. 176 THE EMMETROPIC EYE. the distance between the two nodal points and between the two principal points of the dioptric system of the eye; this distance amounts to less than one-fourth of a millimetre. It is evident that neglecting this will cause only a very slight difference. We thus ob- tain, besides the two focal points, as cardinal points, only one principal point, 1i, and one nodal point, the latter being the optical centre, h : that is, we retain simply the cardinal points of one simple refracting surface (compare pp. 40-44), whose centre of curvature is Ic, fortui- tously the radius of curvature for the human eye is 5 mm. ; the co- 4 eificient of refraction may further be assumed as ^. Hence results such a simple position of the cardinal points, that we can without difficulty imprint them on our memory, and mate many calculations even without the use of figures. To this I attach great importance, because our ideas thus gain so very much in clearness. Kg. 89 represents the reduced eye in its true dimensions. h is the optical centre. h is the principal point. h k=h mm. is the radius of curvature of the refracting surface. « is the posterior focus, that is, the focus of rays, parallel in the air (compare Kg. 90) . <^' is the anterior focus, that is, the focus of rays, parallel in the vitreous humour (compare Kg. 91). Fig. 90. Fig. 91. Fig. 89. h (j)„=P, is the posterior focal distance = 20 mm. k (f),=P, the anterior focal distance = 15 mm. ft ^ Therefore the coefficient of refraction - = -, as beine = 1' • P = M 3 o „ , 20 : 15. The meaning of the reduction thus made is this : that for the ordinary eye we substitute one with a cornea, whose radius of curva- ture is only 5 mm., while behind this is merely vitreous or aqueous MEANING OF THE REDUCTION. 177 humour, without crystalline lens, and with a length of visual axis of 20 mm. In such an eye retinal images would have the same mag- nitude, the same distinctness, and the same position which they ex- hibit in the emmetropic eye with its cornea of nearly 8 mm, radius of curvature, its crystalline lens of a little more than 43 mm. focal dis- tance, and its visual axis of a little more than 22 mm., and it can, therefore, really be substituted for this last. That in the assumed reduction the system alters but Httle, we can render evident by reasoning. In the iirst place, the cornea is the principal refracting surface, where the rays deviate most : its focal distance is 31"7, while that of the crystaUine lens amounts to 43'7. In the second place, in the crystalline lens the two principal points lie so close to one another, that they may be united into a single optical centre. If we now consider, that this optical centre of the lens lies aboat 16 mm., that of the cornea a little more than 14 mm. in front of the retina, we shall understand that the effect of both is combined in one point, situated 15 mm. from the retina. In order now further to combine in one surface of curvature the dioptric action of lens and cornea, the coefficient of refraction being ^, a radius of o curvature of precisely 5 mm. is required. NOTE TO § 14. The reduction assumed in round numbers scarcely differs from that found by calculation (see the method in Listing, l. c, p. 493) from the diagram- matic eye of Helmholtz ; in place of the calcvdated values, we assumed, F,= 15036 15 F,= 20-113 20 hk = 5-077 5. In connexion herewith we assumed, as coefficient of refraction, in place of -==> -fT- = g' which values stand to another := 308 : 309. Listing's dia- 77 to o grammatio eye differs, it is true, somewhat more from the round numbers ; but still Listing has elsewhere {JBeitrag zur physiologischen Dioptrih. Q-ottingen, 1845) found himself justified in substituting the same round numbers. I now wish to show, by some examples, what use we can make of the numbers ascertained, in order to be able to form a quite satisfactory idea respecting several problems. a. In the first place, a ray, directed to k (compare pp. 42 et seq., Fig. 12 178 THE EMMETROPIC EYE. 19), coincides with the radius of curvature, and ;thus passes through unrefracted : let i i he an ohjeot B, jj its image ,8 ; let A; i, the distance from the object to h, be g' ; kj, the distance from the image to k, be g'. Evidently now •&:^=g':g" "We saw that g'= 15 mm. We have thus only to divide the distance / of the ohjeot, expressed in mm., by 15, in order to find how many times the retinal image is smaller than the object : a mfetre, placed at 15 metres (16,000 mm.) distance, gives a retinal image 1,000 times smaller, and there- fore one millimetre in size. b. If the eye remains accommodated to infinite distance, the imagey of a point i, placed at an infinite distance, falls behind the retina. How far does it lie behind it ? In other words, how large is/, — JF, = y ? Above (p. 44) we found the distance, kj, as If we put f, — F, = C Then f, = C + F, F (t A. F) and we may write f^ = ' ^^ '' — '-'j F F f, = F^ + -^' F F consequently /„ — F,= ^ ' In order, therefore, to find^ — JP, = y, that is the displacement of ^ behind the retina, we have only to divide the flked product F„F, = 20 x 15 = 300 by 0,, that is by the distance of the anterior focus f, to the object. If the point i lies 320 mm. from k, that is 300 from ip„ its image/ will fall precisely 1 mm. (300 : 300 =1) behind the retina. The point *, situated at 1 metre in front of ip, (1020 mm. in front of k), makes j fall only 0-3 mm. (300 : 1000 = 0'3), the point i, situated at 100 mm. from ip, makes/ fall not less than 3 mm. (300: 100 ^ 3) behind the retina. Thus we can easily, without the use of figures, calculate g for each distance of i. c. If we thus know y, we can further easily find the diameter of the circles of diffusion, and so obtain an idea of the degree of distinctness of vision. In the reduced eye the matter is simple. A cone of rays, derived from one point i, unites in j. On the retina, in 0,, this cone has yet a certain section : that is, the circle of diffusion. The length 0„ j = y, we have only to divide by the length of the whole cone, calculated from the pupU, in order to find in what proportion the pupil, as circle of diffusion, is reduced. The distance from the pupil to 0, we may fix at 19 mm. Now if y = 1 mm., the circle of diffusion is (1 : 19 + 1) 1 2 5jr of the diameter of the pupil ; with y=2 mm., we find jr-, with l\j 21 y =Z mm. „„, etc. The diameter of the pupil being taken at 4 mm., the 1 8 diameter x of the circle of diffusion is m the first ease = -, in the second — 5 21' MOVEMENTS OF THE EYE. 179 4 in the third =^ mm., etc. Evidently, with equal deviation of aocommoda- tion, the circles of di&usion become greater, and the accuracy of vision consequently diminishes the more, the larger the pupU is. To he quite correct, we should make the position and the magnitude of the pupil, as they manifest themselves as the image of the orystaUine lens, the basis of the calculation. The influence of the crystalline lens is, however, not great : with a true magnitude of 4 mm. its image of the crystalline lens is 4'23, and a position of 3-6 mm. changes into a position of 3'713 behind the cornea: with this the assumed distance = 19 mm. in front of the retina, agrees. If we wish to make the calculations for Parisian inches and Hues, we may, in place of 5, 15 and 20 mm., put down 2"-2, 6"''6, and 8"'8, which values we can easily remember and use in the calculations. We, can, moreover, in reference to the reduced eye, form a very good idea of the accommodation. In the first place, if we suppose this to occur through an imaginary auxiliary lens situated in air, the latter has only to make the diverging rays parallel : the focal distanee of the auxiliary lens must then be equal to its distance from the point for which we accommodate. Thus understood, the imaginary auxiliary lens is equal to a spectacle-glass correcting according to the distance. In this, however, the position of the cardinal points changes in another mode than actually takes place in accom- modation. If we wish to obtain the actually altered position, we must also reduce the accommodated eye in itself. We may assume : r ^ 4-5 mm. F, = 13-5 -P. = 18. Here, now, ^„ lies at 2 mm. before the retina: the visual axis has, in fact, maintained its length of 20 mm., and this must happen, because the principal point has scarcely changed its position (compare p. 67). On the other hand, h has approached to Ti, as actually takes place in the accommodation. We find the distanee to the object for which this eye is accommodated to be (13-5 X 18 = 243, and 243 : 2 = 121-5) 121-5 mm. from 0„ and consequentiy 139-5 from Ti, that is about 5', so that the reduced eye here assumed repre- , . . 1 1 sents an emmetropic eye, using x ^^ =• The calculation of the altered position of A by the use of glasses is rather complicated. The method results from xv. p. 53, and xxii. p. 65. In the section on aphakia we shall be obliged to make use of it, and therefore to illustrate it with examples. § 15. Centre of Motion and Movements, Angle between THE Axis op the Cornea and the Visual Line. It would be going beyond tbe plan of the present work, here to enter into tbe whole doctrine of the movements of the eye. The 2 180 MOVEMENTS OF THE EYE. great complexity of the subject, especially when taken in connexion with binocular, stereoscopic vision, would alone be sufficient to deter us from so doing.* But, besides, there is little application to be made thereof in reference to ametropia; and only so far as they are modified in ametropia, have we here to deal with the functions of the emmetropic eye. As to the mechanism of the movements, a modifi- cation with respect to two points is to be noted, which is of importance for our object : a. with reference to the position of the centre of motion ; h. with reference to the extent of turning round the vertical axis. Eespecting the position of the centre of motion numerous inves- tigations have been made, among others by Yolkmann, Mile, Burow, and Valentin. These investigations yielded rather discordant results, but as the eye does not differ much from a globe, and is in great part contained in a globular cavity, these observers agreed that the centre of motion should be situated about in the middle of the visual axis. The discrepancy of the results obtained is attributable in part to the methods of investigation employed, but in part, no doubt, also to the difference of the eyes. Since, in fact, it was shown, that ame- tropia depends principally on a difference in length of the visual axis, it must even h priori have been supposed, that the distance at which the centre of motion lies behind the cornea, should, in ametropia undergo a modification, and I therefore thought it necessary to in- vestigate that subject. The investigation took place in concert with my friend Dr. Doyer, according to a method described at the end of this section. The results obtained in emmetropic individuals, are collected in the subjoined table. The subjects were aU men. D. signifies the right, and S. the left eye. • Compare Ruete, Lehrh. der OpMhaVmologie, Bd. i. 1848, S. 8. F. C. Bonders, Zur Lehre der Bewegungen des menschlichen Auges in Solldndiaohen Beitr. z. d. anat. und phydol. Wiss. 1848, Bd. i. von van Been, Donders nnd Molesohott. Von Graefe in Archivf. Ophthalm. Bd. i. Meissner, Die Bewegungen des Auges, ArcMvfur Ophthalm. Bd. iii. Fiok, Zeitschriftf. ration. Medicin von Henle nnd Pfeufer B. iv. und V. Neue Folge. Wundt, Archivf. Ophthalmologie, B. viii., 1862. POSITION OF THE CENTRE OF MOTION. 181 Nos. of the Persons. Age. Eye. Position of the centre of motion beliind the apex ofthecomea. Angle between the axis of the cornea and the visual line. 1 23 D. 13-9 6° )1 »1 S. 13-84 6° 2 23 8. 13-72 5° 3 30 D. 13-03 4" jj J» S. 13-58 6" 4 31 S. 13-49 S^S 5 34 D. 13-27 6" S. 14-04 6° 6 35 S. 13-55 4°5 7 35 s. 13-58 6" 8 40 s. 13-17 7° 9 43 D. 13-99 4»66 10 43 D. 13-32 3''5 a S. 13-19 3°5 11 50 D. 13-38 404 The length of the visual axis in emmetropic individuals is sup- posed to be equal to that in the diagrammatic eye, namely, 22'231 mm. Now by the method adopted we found the distance from the centre of motion to the base of the segment of the cornea; this base being 2*6 mm. from the apex of the cornea, 2'6 mm. must be added to the number obtained, in order to find the position of the centre of motion behind the apex of the cornea. The same determinations were made in ametropia. They will be communicated in detail in the chapters upon H and upon M, where the importance of these results in the production of strabismus divergens and strabismus convergens will be shown. In this place I give in the subjoiaed table only the averages of the results ob- tained for emmetropic, myopic, and hypermetropic subjects. a Length of the visual Position of the Centre of Motion. / Angle Ijetweea the axis of the cornea and the Tifiual line. Behind the cornea. c Before the posteiior surface of the sclerotic. d In per centage pioportion. e Behind the middle of the visual axis. 1. E. 2. M. 3. H. mm. 23-53 25-55 22-10 miQ. mm. t3-54 : 9-99 = 57'32: 42-46 14-52 : 11-03 = 56-83: 43-17 13-22 : 8-88 = 59*8 : 40-2 mm. 1-77 1-75 2-17 5«-082 2° 7''-55 Prom this table it appears : — 182 MOVEMENTS OF THE EYE 1st. That in the emmetropic eye the centre of motion is situatet at a considerable distance (1-77 mm.) behind the middle of th» visual axis. 2nd. That in myopic individuals the centre of motion is situated more, deeply in the eye, but also farther from the posterior surface, and indeed so that in the eyes of such persons the relation between the parts of the visual axis, situated before and behind the centre of motion, is nearly the same as in the emmetropic eye. 3rd. That in hypermetropic eyes the centre of motion is situated not so deeply, but relatively very much closer to the posterior surface of the eye. In the above tables a column,^ is assigned to the angle between the axis of the cornea and the visual liae. The subjoined %ures, — ^Fig. 92, representing an emmetropic ; Kg. 93, a myopic ; and Fig. 94, a hypermetropic eye, — are intended to illustrate the meaning of that angle, and at the same time the position of the centre of motion d. Fig. 92. Fig. 94. All are seen in horizontal sections, carried through the optic nerve, n. I is therefore the innermost, E the outermost part of the eye, The axis of the cornea, g a, cuts the cornea in the middle : to this, in fact, the apex of the ellipsoid of the cornea corresponds. Now IN HYPERMETROPIA AND MYOPIA. 183 this axis is by no means directed to the object fixed, which, as such, has its image in the fovea centraHs of the yellow spot I. A line drawn from the retinal image of the fovea centralis towards its object is the visual line I V, and this may be considered to cut the axis of the cornea in the united nodal point h. The angle, ^ k a, is therefore tjie angle between the axis of the cornea and the visual Hne in the horizontal plane. In the vertical plane this is usually much less, and has no special bearing on our present subject. Now it appears that in the emmetropic eye the visual Hne cuts the cornea to the inside of its axis. This had already been ascertained by Senff, and was confirmed in a small number of eyes by Helmholtz and Knapp.* We found it as a rule in more than fifty eyes. I had, however, previously observed t that in myopic individuals the angle I' h a, is less than in emmetropic persons, and that in the highest degrees of M the cornea may be cut by the visual hne, even on the outside of its axis. The investigation carried on with Dr. Doyer showed further, that, contrary to what was observed in M, the angle V k a is in H particularly large. Hence, now, it follows, that in looking at distant objects, while the visual lines are parallel, the axes of the cornea in emmetropic indi- viduals diverge about 10", still more in those who are hypermetropic, but less in myopic persons, in whom they may even converge. This gives, considering the position of the eye in emmetropic subjects to be normal, in hypermetropics apparent strabismus divergens, in myopics apparent strabismus convergens, which, when once one is aware of it, is very evident, and contributes much to the peculiar physiognomy of myopic and hypermetropic persons. EinaUy, as to mobility around a vertical axis, proceeding from the position in which the axis of the cornea stands perpendicular to a vertical surface carried through the centre of motion of both eyes, the normal emmetropic eye can in youth turn from 42° to 51° in- wards and from 44° to 49° outwards. In myopic persons, as will appear, the movements are often limited. • Knapp, Die Krummung der Hornhaut, Heidelberg, 1859. t Verslagen en meded. der Koninkl. Akademie, 1860, D. xi. p. 159. 184 MOVEMENTS OF THE EYE. NOTE TO § 15. In the investigation of the mechanism of the movements of the eye, the knowledge of the position of the centre of motion is a primary requisite. It may therefore cause reasonable surprise, that in the numerous and elaborate researches carried on of late years respecting this mechanism, the deter- mination of the centre of motion should have attracted so little attention. If observers started on the assumption, that the position of this centre had been determined by previous investigations with sufficient accuracy, they were mistaken. In the first place, as Ludwig, among others, observed, the methods employed left much to be desired ; and in the second place, the influence of the length of the visual axis upon the position of the centre of motion was wholly left out of consideration. Now if the latter was found, as a result of observation, at a distance of from 11-9 to 14-1 mm. from the cornea, it was certainly extremely arbitrary hence to assume it as proved, that the centre of motion must be situated in the middle of the visual axis. Some years ago I thought I had found, in the measurement of the dis- placement of a reflected image on the cornea, a simple and accurate mode of determining the centre of motion. In the flrst place I ascertained, with the aid of Helmholtz' ophthalmo- meter (compare p. 17), the radius of curvature in the middle of the cornea. Subsequently I endeavoured, from the displacement just alluded to of a reflected image, to deduce how far behind the centre of curvature the centre of motion was situated. The reasoning was as follows : —if the centre of motion of the eye should coincide with the centre of curvature of a spherical cornea, an image reflected in the axis of this cornea would, on movement of the eye, undergo no change of place whatever. If, on the contrary, the centre of motion, as was to be expected, should lie behind the centre of curvature, then, on turning the eye, the reflected image would be displaced in the same direction in space as that in which the eye was moved, and this displacement, as a simple construction shows, is the sine of the angle of motion, described from the centre of motion of the eye with a radius, equal to the distance between the centre of motion and the centre of curvature. From this reasoning it followed that we should have to measure only this displacement at a known angle, in order, from the sine thus determined, to find the radius, and with it the distance between the centre of curvature and the centre of motion. Now the angle was ascertained, by looking successively towards two sights {visieren) in a horizontal surface. It was, moreover, easy to measure the displacement of the reflected image. Immediately before the opening, to which the eye, the head being fixed, corresponded, a hair was vertically extended. If, on fixing the first sight, the reflected image coincided with the hair, it appeared, on directing the eye to the second sight, to deviate therefrom, and this deviation was measured by making the double images, seen with the ophthalmometer, POSITION OF THE CENTRE OF MOTION. 185 to separate so far, that the second image of the hair coincided with the first of the reflected image. Further, hy repeatedly looking alternately to each of the two sights, the required distance of the double images could be stiU. more nearly, and, indeed, very accurately, determined, while at the same time, the influence of slight movements of the head was excluded. The measurement was accurate when, on quickly and alternately fixing the two sights, the one image of the flame completely coincided alternately with the two hairs, or deviated therefrom by an equal comparative quantity. If the accuracy of this determination left nothing to be desired, there was another diflB.culty. The cornea is not a spherical surface. Its curvature approaches much more nearly to the ellipsoid, and the eccentricity of the ellipse, obtained as a horizontal section, seemed great enough, to exercise an influence upon the position of the reflected image. Professor van Uees had the goodness to calculate this influence, and it appeared, that in consequence of actually established eccentricity of the elliptical meridians of the cornea, a deviation arises, which, in the calculation, may produce, for the position of the centre of motion, a difference of 2, or even of 3-6 mm. Hence the appU- oation of the method was very limited. Indeed, the ellipse of the horizontal section must always be determined, and this determination requires so much time, that it is diflS.cult to apply it to a great number of eyes. The method is here communicated, because in those cases in which the ellipse is deter- mined, it is not unserviceable in the control of other methods. A similar method had, as I subsequently learned, been previously proposed by Professor Junge, of Petersburg. His results, obtained in Helmholtz' laboratory, were published by him in the Russian language. I became acquainted with them from the manuscript of the German translation of his valuable treatise. Junge's method depends, like that above described, on the displacement of the reflected image of the cornea, in movement of the eye. This displacement, however, he determined, by throwing the reflection of the same flame, both with parallel visual lines, and at a certain con- vergence of these lines, on the two eornese, and by measuring the mutual dis- tance of the reflected images, in the two positions of the eye just mentioned. In order to be able to use the ophthalmometer for this measurement, the images must, by reflection, be brought close to one another, for which pur- pose Junge made use of a sextant. He effected the determination on five eyes with great accuracy. But the numbers obtained have not a correspond- ing value, because he neglected to ascertain the eccentricity of the elliptical section, \and therefore could not apply the requisite correction. Subs^uently I succeeded in discovering a method, in using which the form of the elliptical section of the cornea has not to be taken into consi- deration. In concert with Dr. Doyer, I have applied this method to a great number of eyes. We were, in fact, not satisfied with knowing the position of the centre of motion in the normal emmetropic eye ; we wished to inquire what differences in that respect myopic and hypermetropic eyes exhibit. The method consists in this : — That we determine how great the angles of motion (with equal excursions on both sides) must he, in order to make the two extremities of the measured horizontal diameter of the cornea coincide aiternately with the same point in space. 186 MOVEMENTS OF THE EYE. The horizontal diameter. of the cornea was measured with the aid of the ophthalmometer. For this purpose the flame of a lamp was placed perpen- dicularly immediately above the ophthalmometer. The reflected image of this flame in the cornea was seen through the ophthalmometer. A second lamp, placed near the cornea, was covered towards the side of the ophthal- mometer with a screen and served only to produce a bright illumination of the cornea to be examined. By giving to the eye to be investigated a defi- nite direction, by making the patient look to a sight (we call this, moreover, the primary sight), which was movable along a scale,* it was not difficult to make the reflected image of the flame placed above the ophthalmometer fall precisely, in the middle of the cornea. If this reflected image was really in the middle, the reflected images on both sides in fact, by doubling, reached at the same time the margins of the cornese now half covering^ one another (compare Fig. 95 ; C the cornea,^ the pupU, b the reflected image). The result of this first investigation is pig_ 95. evident. The number of degrees read off on the ophthalmometer, which was required to make the refiected images fall on the margins of the cornese half covering one another, corre- I sponded to half the breadth of the cornea, or I rather to half the chord which subtends the cornea. A second measurement, in which the glass plates were turned in the opposite direction, served to verify the first, and at the same time to avoid the error of eollimation. In this manner we obtained, by reading off the ophthalmometer, above and below, four measurements. Of these four the average was taken. A table expressly prepared now directly gave, from the ascertained number of degrees, the corresponding magnitude, whereby, consequently, the half-breadth of the cornea was known. At the same time, the position of the primary sight on the scale showed what angle the visual line made with the axis of the cornea, assuming that this passes through the centre of the cornea. In order further to determine the arc which the cornea must describe, in order to traverse the length of its own transverse diameter in space, a ring was suspended before the eye to be examined, in which a fine hair was perpendicularly stretched. It was now merely necessary to try how many degrees (starting from the position in which the axis of the cornea was directed on the cross of the ophthalmometer) must be sighted at each side, in order, while the head was immovably fiixed, to make each of the margins of the cornea alternately coincide with the hair. The number of degrees ascertained corresponded to the angle which the eye had described * In front of the eye to be examined a horizontal graduated arc was applied, with an arbitrary radius described from the cenbre of motion of the eye. In the middle of this arc, in the direction in which the eye saw the cross of the ophthalmometer, was the zero of the scale. Right and left of this zero the degrees were numbered. POSITION OF THE CENTRE OF MOTION. 187 from the centre of motion. It very soon appeared, that in normal eyes, this angle amounted to about 56°. We therefore began each time with one sight 28" to the left, to place another at as many degrees to the right of the primary sight. The head was placed so, that on fixing the one sight, the one margin of the cornea coincided with the hair ; and it was tried whether, on fixing the second sight, the opposite margin of the cornea corresponded to the hair. Only rarely was this exactly the case ; but it then, nevertheless, appeared, whether a greater or lesser arc must be described. Accordingly, the two sights were removed from, or approximated to each other, by an eqwil distance, which was repeated until at length the exact coiucidenoe of the margins of the cornea with the hair was obtained. By malting the eye look a few times in rapid succession alternately towards the one and the other sight, the influence of movement of the head was with certainty excluded. The knowledge of the half-breadth of the cornea, and of the angle of motion, whereby that dimension in space was traversed, was sufficient to determine the position of the centre of motion. The subjoined figure illus- trates this. It represents a horizontal section of the eye : o is the centre of the visual axis, or pjg^ gg rather of the axis of the cornea g a ; lis the yel- low spot; II' the visual line, which in the posterior part of the crystalline lens (that is, in the nodal point) cuts the visual axis. If we now draw from the centre of motion x the lines xy and ^ry" to the margins of the cornea, and, moreover, the line yy" as chord of the cornea, we obtain an isosceles triai^le, of which the angle y xy' is known to us. The perpendicular x u divides this triangle into two equal and uniform rectangular triangles, (the acute angle of which, and, moreover, the side of the rectangle y u the half-chord of the cornea) are known by measurement. The second side of the rectangle xu is evidently the distance from the centre of motion to the base of the segment of the cornea. It is found by multiplying the side of the rectangle yuhy the co-tangent of the opposite angle yxu. By adding to this the height ua = 2-6 mm. of the segment of the cornea, we obtain the distance a x, that is, the position of the centre of motion behind the anterior surface of the cornea. In many cases, especially in myopic persons, the mobUity of the eye was too limited to make the cornea traverse the required space. In this case we used a ring, provided with two extended parallel threads, whose mutual distance was accurately determined. This usually amounted to 3-02 mm. The sights were now so placed that the one thread coincided alternately with the inner margin, the other with the outer margin of the cornea. In order to know the space traversed, it was now necessary only to substract the distance of the threads from the previously ascertained breadth of the cornea, and this value was further made the basis of the calculation. The results we have comprised in three tables. The first contains the eyes of emmetropic individuals, the second those of myopic, the third those 188 MODIFICATION OF THE ACUTENESS OF VISION. of hypermetropic persons. This distinction was made in order to exhibit the influence of the length of the visual axis. Above (p. 181), a table was given for emmetropic eyes ; and, moreover, one containing the average of the results obtained in B, M, and H. We do not conceal from ourselves, that after this investigation, much remains to be done with respect to the determination of the centre of motion of the eye. In the first place, we have not yet examined how far the centre of motion noay be regarded as a fixed, unalterable point. Our investi- gations extend only to horizontal motion, and almost always to an equal amount. We can therefore answer only for the accuracy of the direct determination — that of the distance between the base of the segment of the cornea and the centre of motion, in rather extensive movements in the horizontal plane. § 16. ACUTBNESS OF ViSION MODIFIED BY AgE. With thie increase of yearsj the eye undergoes a number of changes of different kinds. Some of these are recognisable on mere external inspection^ as the diminished lustre of the cornea and of the con- junctiva, the smaller pupil, the changes of colour and less transparency of the sclerotic and of the iris, the diminished depth of the anterior chamber of the eye, the arcus senilis, etc. Some appear only on proper anatomical examination : to these belong, among others, the warty granulations of the structureless membranes, with secondary changes of the retina, calcareous deposits in the posterior part of the sclerotic, the peculiar metamorphoses of its anterior part, changes of the choroid, atrophy of the musculus ciliaris, greater firmness and a yellower tint of the lens, followed by turbidity of some layers, and lessened transparency of the vitreous humour. Even before anato- mical investigation can exhibit any trace of turbidity, the comparative ophthalmoscopic examination of sound eyes at different periods of life shows, that with the increase of years the perfect clearness and trans- parency are lost, in virtue of which the fundus oculi of the child is seen with such incomparable clearness. With these anatomical changes, different disturbances of function are combined. The principal of these are diminution of the accuracy of vision and lessening of the range of accommodation. Both are to come under our consideration : the accuracy of vision, because it is not only the measure for the estimation of many morbid deviations, but is also effectively diminished in most anomalies of refraction ; lessening of the range of accommodation, because, although no SNU-LEN'S TEST-TYPES. 189 anomaly^ it requires the interference of the oculist. "We shall speal first of the accuracy of vision. Lessening of the range of accommo- dation shall be the subject of the next section. We have already (p. 97) seen, that the determination of the state of refraction must go hand-in-hand with that of the accuracy of vision. "We there became acquainted also with the test-types of Dr. Snellen, the utility of which has become more and more evident to me. In the place alluded to, I have also illustrated by examples, how, by means of these, the accuracy of vision may be determined. The general formula is very simple. The letters bear as number the distance D, at which they exhibit themselves under an angle of 5 minutes, and are recognised in normal accuracy oi vision. If we now determine the distance d, at which they are seen, we find the accuracy or sharpness of vision S- ^. S - 5' To this work is appended a table of Snellen's, extending from XX to C C. If the room does not admit of making the examination at the distance of twenty feet, lower numbers must be added.* In order to be able to estimate also extraordinarily good accuracy of vision, it is de- sirable even, that the number in the table should go somewhat lower than the number of feet, at which the observation can take place. 8 = 1 was assumed by Snellen as sufficient accuracy of vision. This held good for young subjects- It was to be anticipated, and indeed was already known by experience, that even without extraordinary defects, the accuracy of vision at a certain age begins to diminish. In order to be able to use it as a standard in morbid conditions, it was therefore necessary that the accuracy of vision proper to each period of life should beknown. This subject has been recently investigated by one of my pupils, Dr.Yroesom de Haan.f S was determined in 281 persons from seven to eighty-two years of age, by means of XX of Snellen. * The tables are now published also with English tests, under the title of Test-types for the Determination of the Acuteness of Vision, by Dr. Snellen. These are to be had from Williams and Norgate, in London. Those with French text may be procured from Germer-BailliSre, in Paris ; those with German, from H. Peters, at Berlin; those with Italian, at Turin, and all these, with the Dutch, are to be had together from T. Greven, Bookseller, at Utrecht. The publication was designed for the benefit of the Netherlands Ophthal- mic Hospital, and was therefore made a monopoly. t Onderzoekingen naar den invloed van der leeftijd op de gezigtsscherpte. Diss, inaug. Utrecht, 1862. 190 MODIFICATION OF THE ACUTEHESS OF VISION. Placed at first at too great a distance, the persons under examination approached until they correctly indicated V, A, C, and L. These letters are the most easily recognisable, and de Haan could confine himself to them, because he usually had to mate on each person only one good trial, and therefore needed no great variety. Subse- quently I determined the relation between the distinguishing of all the letters adopted by Snellen and of the four used by de Haan, found that it was as 5 : 6, and reduced accordingly the S found by de Haan. It is to be found thus reduced in Kg. 97. The lengths of the ordi- Fig. 97. 26:20 24:20 22:20 20:20 18:20 16:20 14:20 12:20 10:20 8:20 6:20 4:20 2:20 0:20 10 20 30 40 50 60 70 80 nates represent the acuteness of vision proper to the subjacent times of life : 20 is the mean of from 15 to 25 years ; 30 that of from 25 to 30, etc. The value of S is given at the side as d: 20, d signify- ing the distance in feet at which XX is recognised. Astigmatism > ^ and ametropia were carefully excluded — ^the latter so far that M > ^ and manifest H > j^ are not taken in. 50 60 At an advanced period of life, H = - - was still admitted, because it may in great part be considered as hypermetropia acquisita. In CAUSE OF DIMIfJISHED ACUTENESS OF VISION. 191 each case the ametropia was corrected by a glass held close before the eye. AH eyes were carefully examined externaUyj many also with the ophthalmoscope ; all without exception being so examined in reference to which any suspicion of anomaly existed. If the shghtest defect were found, they were excluded. In the first place it was ascertained, that the individual difference is very great. Cases occur in which, after reduction, 8 = 1'6 to 1'7 ; in youth very few where S is < 0*8. With this great individual difference, it is not to be wondered at, that notwithstanding a rather considerable number of observations, the line drawn for the averages of each year of life makes many jumps. It is not until calculations are made for each deceniual period that the course, as Kg. 97 shows, becomes more regular. Eurther, we see that at the thirtieth year, S is still almost unchanged; thenceforward, however, it diminishes rather regularly, and at the eightieth year has de- scended to about one-half. The degree of illumination is, in the determination, not without influence. In order roughly to estimate this, in his observations made on different days, de Haan each time determined the accuracy of vision in himself, and found the limits to be 19'5 : 20 and 32"5 : 30. As the observations for the different periods of life were divided tolerably uniformly over the differences in illumination, the form of the line of curvature was not perceptibly altered by a reduction to uniform illumination. In general, in the experiments of de Haan, the illumination was something better than it usually is in the ocu- list's study. It therefore appears that Snellen has, for practical use, correctly selected the rule as 8 = 1, which in de Haan's observa- tions is a little exceeded for young eyes. Snellen's idea is this: that when in youth 8 is = 1, we have no reason to suppose the existence of an anomaly. Moreover, the method is quite satisfactory where we have to do only with the determination of the relative accuracy of vision. The cause of the diminution of the accuracy of vision with in- creasing age rests on a double basis ; it is, on the one hand, to be sought in the media, on the other, in the apparatus of the optic nerve. The first gives rise to less accurate images on the perceptive layer of the retina ; the second renders perception and conduction more imperfect. As to the media, even examination with the ophthalmoscope in 192 MODIFICATION OF THE ACUTENESS OF VISION. general shows that with the increase of years they lose the great transparency and homogeneousness in virtue of which the fundus of the young eye exhibits itself with such striking clearness. The cornea indeed changes the least, if we exclude the arcus senilis, which, on account of its eccentric position, throws but little diffused light into the eye, and indeed less in proportion as the pupil becomes narrower in advancing years. The crystalline lens, on the contrary, by degrees reflects much more light, which, thrown anew on the anterior surface of the cornea, returns partly into the eye : on focal illumination, the separation of its sectors becomes more distinct, its irregular astigmatism increases, the polyopia monocularis, in imperfect accommodation, becomes (notwithstanding the diminished diameter of the pupil) stronger and more irregular, the colour of the crystalline lens is yellower, and entoptic investigation exhibits, over the whole, more disturbance of homogeneousness. The vitreous humour also loses some of its perfect clearness, at least at an advanced period of life ; it becomes richer in membranes collecting into folds, in corpuscles and filaments, as both microscopic and entoptic investigations have proved to me : in consequence of these we have an increased number of muscae vohtantes. KnaUy, the diminishing transparency of the retinal layers comes under notice. The very appearance of the optic nerve proves that changes of its fibres are not wanting. However, with respect to direct vision, this has, while the bulbs of the fovea centralis lie almost naked (compare p. 4), but Httle importance. The senile changes of the apparatus of the optic nerve itself have been but little investigated, and it is difficult to estimate their influence. The best known is the formation of vitreous elevations, globules and groups of globules (warty granulation), on the anterior surface of the choroid, in connexion with the limiting structureless membrane of the same. The formation in question was observed by Wedl,* then I accurately described it,tand discovered and represented the intrusion of the globules with pigment into the locally atrophying retina. H. Muellerf considers these formations as simple thickenings of the structureless limiting membrane of the choroid. The function of the retina must suffer from them; they occur, however, only to a small extent in the region of the yellow spot, and scarcely before the sixtieth year, after which they soon become constant, * Pathologische Ristologie. "Wien, 1853, p. ."JSO. t Arehivf. OpUhalmologie, B. i., Abth. 2, 1854, p. 107. X Arehivf. Ophthalmohgie, 1865, B. ii., Abth. 2, p. 1, DIMINUTION BEYOND THE YELLOW SPOT. 193 Undoubtedly other less well-known changes in the retina, in the optic nerve, and in the brain itself, come under observation as causes of diminished acuteness of vision at an adva,nced period of life. All the foregoing has reference to direct vision, that is, to vision of fixed objects. Outside the fovea centralis the acuteness of vision rapidly diminishes. Even the surface, which at the same time is seen with perfect acuteness, appears to be so small, that its image does not occupy the whole fovea centralis. The diminution of the acuteness of vision in the parts of the retina remote from the yellow spot, has been investigated by Aubert and Eorster.* At a certain angle (12°.5 to 20°), they found that acuteness nearly inversely proportional to the angle of deviation; at greater angles S dimi- nished much more rapidly. Eather strange and unexpected is the result, that smaller letters and numbers are indirectly distinguished under lesser angles than larger ones : in other words, that in looking at near objects, S is, in the peripheric parts of the retina, greater than in looking at some distance. In different morbid states the acuteness of vision diminishes by no means in equal proportion, in direct and indirect vision. Not un- frequently the disturbance is confined to the region of the yellow spot, with perfectly normal S in the peripheric parts of the retina ; in other cases indirect vision alone has suffered, and even in the periphery limitation of the visual field may occur, without direct vision having suffered. It is therefore of much importance to have formed an opinion also as to the accuracy of vision in the lateral parts. For practical purposes the observer possesses the power of comparison with his own normal eye. On closing one eye, the observed eye looks to the left and the observing eye to the right, or vice versa, at each other, and taking care that the observed eye does not de- viate, the observer now exhibits, about in the plane which is perpen- dicular to the visual lines, in the middle between the observing and the observed eye, different objects — his fingers, cut-out letters and numbers, &c. — and thus by comparison in corresponding parts, he soon estimates the degree of disturbance in the periphery of the retina. In order to obtain accurate knowledge, we should be able to follow the method of Aubert and Torster (/. c). — Limita- tion of the field of vision is determined by projection on a sheet * Archie f. Ophthahn., B. III., Abth. 2, p. 1. 13 194 MODIFICATION OF THE ACUTENESS OF VISION. of blue or black paper, as shall be more fully described in treating of M. In the foregoing, the method was stated of determining the acuteness of ■vision {or practical purposes. The formula /S = ^ gives, however, neither the ahsolute measure of the distinguishing power of the retina, nor mutually comparable relative values. The ahsolute measure we shall learn imme- diately, when we come to speak of the determination of the acuteness of vision as a physiological value. That the relative values are not comparable, Snellen (l. c.) has already observed. If an image has double the magni- tude, it has not at the same time double distinctness. This we should be allowed to infer only in case the acuteness of vision was equal over the whole retina, in case the larger letter, used for testing diminished S, was at the same moment equally accurately seen by the normal eye in all its parts. This, however, is not so. Therefore, also, when the image must have double the magnitude to be recognised, we cannot say that the acuteness of vision is really reduced to half. Perhaps it would be correct to assume that the acuteness of vision is inversely proportional to the number of percipient retinal elements which are required, in a linear dimension, to the distinguish- ing of the image. But, on the one hand, this prejudges the question, whether the accuracy of vision diminishing towards the periphery depends actually on the greater distance from one another of the several percipient elements ; and, in the second place, the principle could not be practically applied, in conse- quence of the dif5eulty of determining the number of percipient elements. Hence the method adopted appears to me to be the only one available. The objection we made, based upon the unequal value of the parts of the retina, on which a larger image is formed, is in great part got rid of when we consider that this larger image is not seen with immovable eyes fixed upon one point, but is inspected consecutively in its different parts by the fovea centralis : the different impressions thus received by the most acutely seeing part of the retina are then combined, in order thence to deduce an id«a of the form of the whole. The physiological value of the acuteness of vision being an angular distance, it is evident that the values of S are to be calculated as inversely proportional to the linear dimensions of the images re- quired for the distinction, and, consequently, to the squares of their superficies. Iq all times letters and numbers have been preferentially employed by ophthalmologists in the investigation of the power of vision. A regular system was, however, for a long time wanting. Stellwag von Carion* pro- posed very useful letter-tests, on a good principle, Bmee,+ too, in his often- * Die Accommodationsfehler des Auges, in Sitzungsberichte der kaiser- lichen Akademie der Wissenschaften. Mathem. naturw. Elasse. B. xvi., p. 187. \ The Eye in Health and in Disease, p. 70. London, 1854. DETERMINATION OF moves considerably back. This change aflFects all eyes without distinction, as well the myopic (provided it be healthy) as the hypermetropic and the emmetropic eye. In the first place,it mayherebe asked, in general, how and from what LATENESS OF THE DIMINUTION OF REFRACTION. 205 cause it is, that at so early a period of life, while all the functions, and especially that of the muscles, are in a state of progressive develop- ment, the power of accommodation, which depends upon muscular action, abeady loses in extent? As it must be admitted, that the ciliary muscle has continued normal, and is therefore still in full force, we come readily to the inference that, at least in the first instance, the diminution is to be sought exclusively in the - condition of the parts, which in accommodation are passively altered, and by no means in the state of those whereby the change is actively produced. Now the organ which is passively altered is the lens. Is the early diminution of the range of accommodation j- to be explained from this ? We know that at an advanced time of life the lens is firmer than in youth. I think I may even assert that the increase of firmness commences at an early period. Now, it is in consequence of this greater firmness that the same muscular action can no longer produce the same change in the form of the lens. It is therefore very probable that the early diminution of ^ depends thereon. After the power of accommodation has considerably decreased, a sHght diminution of refraction gradually takes place. This appears from the fact that now also r begins to remove from the eye, and that, consequently, the posterior focus is transferred to a greater depth in the organ, or even to behind the retina. But, as I have already remarked, the diminution of refraction is not perceptible until a late period of Hfe. At the fortieth year, it has not at all, or has scarcely commenced, and it is not until the sixtieth or seventieth year that it is distinctly present in an originally emmetropic eye. On account of the simultaneously diminished* range of accommoda- tion, the visual Hnes being parallel, the eye can then frequently not be accommodated, even for remote objects, and a positive glass is therefore required also for distance. The doubt might be raised, whether the diminution in refraction is not only apparent, — whether in aU those cases in which, at a later period of life, H is observable, an equal degree of latent H did not already exist in youth. If this were so, the change should be referrible exclusively to diminution of -r ■ We are, however, justified in de- claring this doubt to be unfounded. Sometimes a certain degree of 206 MODIFICATION OF THE RANGE OF ACCOMMODATION. H is developed in relatively so short a time, especially when traces of obscuration arise, and, as it appears, also in glaucoma, that there is no ground whatever to assume the original existence of the same degree of H; Moreover, I have in myopia also occasionally satis- fied myself of the presence of a diminution of refraction. Finally, and this in itseK is convincing, at an advanced period of life H is much more common than in youth. The question therefore is, on what the diminution of refraction depends. Plattening of the cornea, and lessened circumference of the eyeball, the visual axis of which should thus have become shorter, have been suggested. It seems to me more probable that the cause is to be sought in the lens. It is generally known that, at a more advanced time of life the latter, toge- ther with the iris, moves forward, and this renders the cornea appa- rently flatter. But this displacement of the lens should in itself have precisely the opposite effect, and should move the focus somewhat forward. There must consequently be another cause, which, in spite of this influence, lessens the refraction. This, if I am not mistaken, is to be sought chiefly in a more uniform firmness of the several layers of the lens. Even Thomas Young remarked, and it has been more fully demonstrated by Senff, Listing, and others (compare p. 39), that with the laminated structure, with refractive power diminishing towards the periphery, the lens has a shorter focal distance than a lens of similar form, and composed wholly of a substance of the refractive power proper to the nucleus of the lens, would have. If, consequently, with the advance of years, the outer layers become more solid, a greater focal distance must be the result. Now, the existence of this increase of solidity is evident from the increasing reflection in advanced life on the anterior and posterior surfaces of the lens, a reflection which is proportional to the difference in refractive power between the outer layers of the lens and the aqueous or vitreous humour; and it is also capable of being established by anatomical investigation. But, in addition to this, in advanced life the lens appears to become flatter, on which account the radii of curvature of its surfaces, and its focal distance, are increased. — I have satisfied myself (see p. 89) that the cornea does not become flatter, and I have no reason to assume that the visual axis should become shorter, except in the most extreme old age. I therefore believe that the cause of the state of diminished refraction must be sought in the above-mentioned changes in the lens. In favour of this view is also the circumstance that the 207 CAUSE OF THE DIMINUTION OF REFRACTION. diminution of refraction at last goes hand in hand with the diminu- tion ot the power of accommodation: indeed this points to a com- mon ongm, and we have above seen that the latter depends upon a hardenmg of the lens.-The vitreous humour I have not compared with reference to its refractive power, at different times of hfe. It IS self-evident that, as its anterior surface is concave, an increase of Its refractive power must move the posterior focus of the eye back- As I have above remarked, the changes in accommodation and refraction occur in each form of the eye. We ^ave here to subject to a more accurate examination only those which take place in the emmetropic organ. Kg. 104 represents the course of the nearest jojo, and of the 1:2 2^ 21 2f 3 Fig. 104. 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 6 8 12 24 CO -1:24 12 8 \ p ""^ • , r _ farthest points r r, and consequently that of the power of accommo- dation in the emmetropic eye, at different periods of life. The figure 208 MODIFICATION OF THE RANGE OF ACCOMMODATION. needs but little explanation. The numbers, to the left, indicate, as before, the distance in Parisian inches, for which accommodation can take place j those, which are lower than oo, have, as in the previous figures, a negative signification : they represent the distance at which the converging rays, for which the eye is accommodated, come to a focus behind the nodal point. The numbers placed above the figure, indicate the age, expressed in years. On the lines jojo and r r we can, therefore, for each year of life, read off the nearest and the farthest points of distinct vision, while, at the same time, the distance between these two iines exhibits the range of accommodation; the distance between two transverse lines is again = ^ accommodation. Prom the figure it directly appears, that, even from the tenth year, at which the observation becomes possible, p approaches the eye, and indeed with tolerably uniform rapidity, so that at the thirtieth year — has fallen to about one-half what it was at the tenth year. Prom this time the descent appears to taJie place somewhat more slowly, but nevertheless to proceed incessantly to the most advanced time of life. The course of the farthest point is quite different. Up to the fortieth year it remains at the same height j but from that time an extremely slow descent occurs, the emmetropic eye becoming, at the fiftieth year, some- what hypermetropic, which H at the eightieth year amounts to from — to _. This acquired hypermetropia may, finally, become absolute, that is to say, that not only accommodation for divergent, but even for parallel, rays becomes impossible. I have not un- frequently met with this in persons at sixty years of age, who in their youth probably exhibited no H whatever. This was inferred when they did not before the forty-fifth year of their life, need spec- tacles in the evening for minute work. The course of p in the emmetropic eye was deduced from a great number of observations. In Pig. 105 each observation is indicated by a point, and the position of these points shows at the same time that the deviations of the mean course are not particularly great. And still these must undoubtedly be in part ascribed to error in obser- vation; in some a slight degree of H may also increase the deviations. In the preparation of this table, emmetropic eyes were, for the most part, used j but also eyes, affected with a slight degree of M ( = 1 : 40 PRESBYOPIA. 209 or less), were not excluded. These last even deserve to be pre- ferred. Indeed, in these alone we have, without artificial paralysis Fig. 107 • n 'O .O 0- o 00 n A So. 1 O r, O H^ o o Q o ° o o O OO 0° ' o'o \°o °o oo 3 > o ?4 O o" O Q oo o » .°c° o Q%8 G o ■ ••^*. ••**. • O • o o • • O a o of accommodation, sufficient security that the nearest point is not influenced by a latent H, and we may safely assume that, with these degrees of M, T-is equal to that of the emmetropic eye. In proportion to the M ascertained, p was of course reduced. Lastly, I should observe, that when p appeared to lie at a greater distance than 8", it was always calculated from a determination, made with the aid of positive glasses. It was thus brought to the distance of about 8". If this were not done, then, on account of the absence of convergence, x would, in advanced life, he computed too low. On the diminution of refraction just described, and especially on that of the accommodation, depends a condition which has been termed presbyopia, Pr. Presbyopia has been set down as synonymous with farsightechess. By this we by no means wish to convey, that 14 210 MODIFICATION OF THE RANGE OF ACCOMMODATION. the eye sees accurately at a great distance, for of this the young emmetropic and moderately hypermetropic eye is also capable. We wish only to express the fact, that it cannot see near objects accurately. In like manner we call a person near-sighted, not because he can distin- guish small objects close to the eye, — for this too the young emme- tropic eye enjoys in common with the near-sighted one, but because he does not see well at a great distance. I will not, however, dwell upon the incorrectness of the English expression farsightedness. It is only the idea I would refer to, and this deserves to be more closely examined. In this respect I would first observe, that only that farsightedness is to be considered as Pr, which is dependent on the diminution of -T-, as the result of advanced life. The very etymology of the word, compounded of irpea-^vs, old, and aSi/r, eye, indicates this. Were we to term every impediment to the accurate vision of near objects Pr, paralysis of the power of accommodation should be placed in the same category. Even H, so far as with it vision of distant, is easier than that of near objects, should be comprehended under the term, and we have made it clear enough to what great confusion of ideas we should, by so doing, give rise. (Compare § 6). TAe term presbyopia is, therefore, to he restricted to the condition, in which, as the result of the increase of years, the range of accommo- dation is diminished cmd the vision of near objects is interfered with. From this definition it appears, that Pr is really included in the diminution of the range of accommodation dependent on advancing years. StiU Pr is the normal quality of the normal, emmetropic eye in advanced age. It is, therefore, properly speaking, no more an anomaly than are grey hairs or wrinkling of the skin. Were it an anomaly, it should be much less one of refraction than of accom- modation. But where are we to place the commencement of presbyopia ? If we consult the Hue pp of Pig. 106, representing the emme- tropic eye at different times of hfe, it appears that, from youth up to extreme old age, p removes with tolerable regularity more and more from the eye, and that, consequently, the vision of near objects becomes progressively more and more difacult. A stop in the Hne is nowhere observed. Hence it follows, that in fixing a limit of Pr, we cannot avoid being arbitrary. In the eye itself, no reason is to be found. NECESSITY FOR THE TERM PRESBYOPIA. 211 for making a definite distinction between presbyopic and non-pres- byopic. Now, if the boundary be artificial, it must be conventional. This, however, leads us to the question, whether it is necessary to speak of Pr, and whether we should not rather confine ourselves to fixing -r-, in connexion with the degree of M or of H, where these are found. Undoubtedly this mode would be scientifically satis- factory. Nevertheless we should, in my opinion, meet with but little response, were we to get rid of so generally known and ex- tensively employed a word. I believe also that by so doing we should confer no favour upon practice. In practice, a word is required, which may indicate the condition in which the eye, at an advanced period of life, must, for ordinary close work, use positive spectacles, and this word ia presbyopia. However, with all this the commencement of Pr is not yet de- fined. That this must be done, is evident. Our social condition requires that we should often be engaged in reading, writing, or other close work. It is plain that the average magnitude of the forms employed in such work, is closely connected with the accuracy of the power of vision, and with the distance of distinct vision for the normal eye. The same is true of the pro- ductions of art and of a number of trades. What the human eye, in the full power of life can do, has in general afforded the standard for this. Before the general application of spectacles the standard was undoubtedly different. If these instruments were no longer to be obtained by all, a larger type in general should replace that at present in use. The common employment of spectacles has, therefore, exercised an influence on the limits of distinct \vision, with which we must allow presbyopia to commence. The changeableness of these hmits thus appears most prominently, We have to investigate how long the eye fulfils the requirements of the assumed standard. Even in the thirtieth year the normal eye dislikes the small print, which the near-sighted person prefers and youth does not avoid. Still, in the fortieth year ordinary type presents no difficulty whatever to the emmetropic eye. In the forty-fifth year the notes, printed in smaller characters, are not un- frequently passed over, and the book is in the evening probably somewhat earlier laid aside. We now soon begin to observe, that an object, to be very accurately seen, is removed a little further from the eye ; a clear light is also sought, rather for the purpose of 212 MODIFICATION OF THE RANGE OF ACCOMRlODATION. diminishing the circles of diffusion, in imperfect accommodation, by narrowing the pupil, than of obtaining more brilliantly illumi- nated images. Ordinary occupations are, however, even in the evening, still performed uninterruptedly without remarkable exer- tion. But where minute matters, which now and then occur, are to be accurately seen, comes the complaint, however unwillingly, from the lips, that our eyes are no longer what they were. The binocular nearest point p^ now often lies at about 8" from the eye. At this point I have already placed the commencement of presbyopia. I think too that I must now keep to it. In the following §, however, we shall see, that this does not always, and indeed not even in general, involve the use of spectacles. If we have agreed upon a definite distance as the commencement of Pr, this may serve also to fix the degree of the presbyopia. This is done in a very simple manner. If, that is to say, p^ be situated at n Parisian inches from the eye, then, assuming the above men- tioned limit, Pr = - — — . Thus, if p, lie at 16 inches, Pr = g — jg = jg j \ip^ lie at 24', Pr = g — ^^ = j^. Por this glasses of about -^ are required, and, in the examples given, glasses of =— and -^ to bring p^ to 8", and so to neutralise the presbyopia. I say, about, for with the increased convergence, ja, has somewhat approached the eye. But as, however, in presbyopia, the relative ranges of accommodation have approached much more to those of hypermetropia (Compare p. 126, Pig. 64), we may usually leave this out of the calculation. To be quite accurate, we should be able to determine the degree of Pr from the glass, with which, by means of direct experiment, j», is brought to 8". But, it will be seen still more precisely in the following §, that the determina- tion of the degree of Pr possesses only a subordinate value, on the one hand, because the commencement of Pr is conventional, on the other, because the accommodation complicates the condition, and this, as well as the accuracy of vision, influences the practical application : we should, therefore, take care to attach to the deter- mination of the degree of Pr the great importance, which is con- nected with that of the degree of M and of H. Thus far we have treated exclusively of the Pr of the emmetropic ADVANTAGES OF THE MYOPIC EYE. 213 eye. But tte hypermetropic and the myopic eye are also subject to the same. The first must be called presbyopic, so soon as in the use of glasses, which neutralise the H, p^ lies farther from the eye than 8'. As to myopics, we hold to the definition given of Pr, and therefore let this first commence, when the distance of p^ amounts to more than 8'. Hence it follows that it is only to the slight degrees of M, that Pr in the ordinary sense of the word, can belong, that with M = -Q it is almost impossible, even with total loss of the power of accommodation. To this we must add, that in the slight degrees of M it occurs much later than in the emmetropic eye. Herein the myopic finds a compensation for what he loses, with reference to the vision of remote objects. The advantage is not small. Up to the sixtieth or even the seventieth year of our age, not to need spec- tacles, in order to see accurately whatever comes immediately under our eyes, is a great privilege. This privilege belongs to a M of from Y^ to — , in which degree the eye is not threatened with any special dangers. With slighter degrees of M a good deal of this privilege is still enjoyed. This is a condition which may well be envied by emmetropic eyes. I never found a normal eye which participated in the same advantage. Many persons, however, suppose that they are so highly privileged. Almost daily it occurs that at fifty-five years of age the distance of p^ lies at only from 8" to 10", and spectacles are not yet thought of. Such people consider themselves a lucky exception. They are extremely proud of their sharp sight. The inquiry whether they are near-sighted is answered in the negative with a smile of self-complacency. At a distance of twenty feet hang Snellen^s letter-tests : XX and XXX they do not recognise, XL not at all, or scarcely ; L and LX are the first which are easily recognisable to them. Not until they try glasses of — ^n or — — do they well distinguish XX, or at least XXX, with accurate contours. Eeluct- antly they acknowledge themselves beaten. They are consequently somewhat myopic ! It is true they had always attached a wholly different meaning to the idea of M. For the ocuHst it is, however, important to have proved the existence of this slight degree of M. He learns from it to recognise the unchangeable, the legitimate amount of the range of accommodation attached to each period of life, and he can sometimes also turn this knowledge to his advan- 214 MODIFICATION OF THE RANGE OF ACCOMMODATION. tage. Thus when we inquire into the hereditary nature of M, its existence in the parents is often denied, yet almost in the same breath it is addedj as a proof of their excellent sight, that up to their fiftieth year, nay even longer, they stiU read and wrote in the evening without spectacles, and — we know what inference is to be drawn. If, on the other hand, a person comes to us, who in order to continue his close work, in his thirty-fifth or fortieth year evidently has need of positive spectacles, we shall almost always find, that a slight degree of H lurks in him. If its degree were somewhat greater, the difficulty would have earlier manifested itself more distinctly, under the character of asthenopia. The more I investigate the subject, the more fully I am convinced, that at a given time of life the range of accommodation is an almost law-determined quantity. If there are no favourable excep- tions, the unfavourable are connected with definite defects, the com- mencement of cataract or glaucoma simplex,* exhausting diseases, and paresis of accommodation. Of this we shall treat from a clinical point of view, in the following Section. NOTE TO § 17. As was above remarked, an eye, affected in a higher degree by myopia, can never become presbyopic. It loses with the advance of years in range of accommodation ; the nearest point recedes, and this may be the case even with the farthest point j the physical changes which therewith go hand in hand in the myopic eye, are similar to those of other eyes. But presbyopia never arises ; p^ does not remove to more than 8' from the eye. Hence ap- pears anew the arbitrary, the conventional nature of the idea of presbyopia. At first I was inclined to attach a more extensive meaning to the word pres- . hyopia. I wished to express by it the senile change affecting every eye. " The change,"thus I reasoned, "indicated by definite anatomical properties, occurs in every eye without distinction. It gives rise to disturbance in vision, and, indeed, in each form of the eye, to one and the same distvtrbance, namely, that the eye, whether unaided, or furnished with definite glasses, cannot, at will, distinguish accurately at a great distance and close at hand. This dis- turbance, peculiar to old age, deserves the name oi presbyopia.'''' From the soientifio point of view this reasoning is perfectly correct. • See, on the meaning of Glaucoma Simp/ex, Bowman, in the British Medical Journal, October 11, 1862. TREATMENT OF PRESBYOPIA. 215 Only so long as presbyopia was opposed to myopia, this extensive significa- tion could not be applied to tbe former term; indeed, so long myopia must exclude presbyopia, and in old myopic individuals, whatever degree the senile change may have attained, the term presbyopia could not be ap- plied. But now that this opposition has ceased, we readily see that the myopic eye also may become presbyopic; and the idea of expressing the senile change with diminished range of accommodation in every eye, without distinction, by the word presbyopia, occurs spontaneously to our mind. Moreover, its etymology is in favour of applying a more extensive meaning to the word presbyopia; indeed, since farsightedness occurs generally in old people, it has been called presbyopia ; still more correctly should the latter term be connected with the state which is inseparable from advanced years. On the other hand : verba valent usii. This has, finally, weighed more with me than the demands of logic and of etymology. I considered that practice has need of a word to signify, that without optical assistance ordi- nary close occupations can with difficulty be carried on, and I should be unwilling to propose another word to designate this condition. I have therefore retained the word presbyopia in its ancient signification. Only the idea was refined. Strictly was everything eliminated from it which belongs to hypermetropia or paralysis ; the senile change, with diminution of accommodation, was assumed as an essential requisite, and it was not difficult from this point of view to fix the limits of the application of the word presbyopia also to myopic and hypermetropic eyes. § 18. Treatment op Presbyopia. Diminution of the power of accommodation leadsj as we have seen, to presbyopia. To this, at a certain time of hfe, the emme- tropic eye is iaevitably liable. In youth it sees small objects, by preference and without perceptible tension, at a distance of about six inches. At a later period this distance becomes greater, in spite of powerful effort, in spite of more advantageous management of the stiU existing accommodation, even at relatively less convergence. Thus the time approaches that close reading and working are attended with difficulty. Presbyopia thus announces itself. Seldom do we hear at the same time that work fatigues. The complaint is rather that vision is not accurate : the letters n and w are not easily distinguished ; the numbers 3, 5, and 8 are confounded; a stroke is seen double, a point sometimes multiple, etc. If we place smaU print in the hand of such a presbyopic person, he begins by holding the book too close 216 TREATMENT OF PRESBYOPIA. to his eye, and does not distinguish ; he subsequently very patho- gnomonically moves the book forwards and the head backwards, seeks a bright light, and — reads. The bright light is a principal point, not so much because the retinal images are by it more strongly illuminated, but because the pupil contracts, the circles of diffusion therefore become smaller, and the retinal images less diffused. Therefore, also the individual first perceives some diffi- culty in twilight, unless it be particularly strong. Inconvenience would have arisen even earlier, if the diminution of accommodation had not been accompanied with diminution of the diameter of the pupil. Thus also the small pupil of the old man makes the loss of the power of accommodation lighter to him : to this he is indebted for the fact that, even at distances for which he is not accurately accommodated, he still distinguishes tolerably well. In full day- light, in the open air, a person can often, even in advanced pres- byopia, read ordinary type, and this always succeeds on looking through a small opening. But much earlier, even before the presbyopia manifests any disturbing influence, the small pupil is of importance, because in reading and writing the accommodation even then leaves something to be desired. To this I would expressly direct attention. The fact is significant, because hence it follows that in commencing presbyopia convex glasses are useful less by correcting the accommodation, than by increasing the sharpness of the retinal images. The eye already puts its accommodation rather strongly upon the stretch (still more powerful tension has no pro- portionate effect) without any hinderance or fatigue whatever. Aided by weak glasses, the eye continues the tension almost in the same manner. The result is therefore, that the eye now sees more accurately: the letters become black, confusion ceases, and the person rejoices in a distinctness of vision, of which he had almost lost the idea. The correction, hy means of positive glasses, in the commencement of the effort, of diminished accuracy of vision of near objects, is the characteristic mark of presiyopia. Prom this, it may be stated in one word, the vision of hyper- metropics is evidently distinguished. These obtain perfect accuracy of sight, but only at the cost of so great a tension as they are not able to maintain, and therefore they obtain it only for a short time ; weari- ness of accommodation (asthenopia) ensues. In the hypermetropic individual convex glasses aid the accommodation, in the presbyopic they at first rather increase the sharpness of the retinal images. USE OF CONVEX GLASSES. 217 So soon, as, hy diminution, of accommodation, in ordinary work the required accuracy of vision begiMs to fail, there is need of convex glasses. The test is, that with weak glasses of from ^ to j-„, at the same distance as without glasses, the accuracy of vision is manifestly improved. The opinion is rather general that we should refrain as long as possible from the use of convex glasses. But is it not folly to weary the eyes and the mind together, without any necessity, condemning ourselves to guess with much trouble at the forms, which we could see pretty well with glasses ? We have here to do with a prejudice which perhaps finds some support in vanity. It is asserted : practice of accommodation is desirable. Generally speak- ing, this is perfectly true. To look alternately at distant and at near objects, now to occupy one's self with smaller, now with larger objects, developes and maintains the functions of the eye. But we forget that we were obliged to practise more and more, as years have roUed on, and that by these efforts, increasingly necessitated by the diminishing range of accommodation, the power of using, with moderate convergence, a great part of the latter has already been acquired. The annexed Figures, 108 of a man of four-and-thirty (Dr. Doyer), 109 of a man of four-and-forty (myself), 110 of an emmetropic person aged sixty, compared with that of a person of fifteen (Fig. Ill) demonstrate this most evidently. And is it not a Fig. 108. 1 :2 3 4 44 12 ^ 24 l ^- jSi^-r'— T- — -1 — 7""^ x n / -Jti 7^-^ ~r:::::^^=^ WIV 22050' 34°32' 46''38' 59»20' 72''50' 218 TREATMENT OF PRESBYOPIA. Fig. 109. 1 :2 / 2ft / 2i / 2i / 3 / 3? 4 / / / 6 8 12 jS A ^ y -/ ^ T" ? ^/ / ^ 24 ^/1 ^ 00 f, 1 :24 S^ ll°2r 22''50' 34032' 46''38' 69''20' 72050' Fig. no. 1 : 2 2A 2| 2f 3 .33 / y / / "^7 4 4| 6 8 12 / / / / y 24 i„j25 / -^ ^ — - -1:24 . ■ T, T, 0° 11°21' 2205O' 34°32' 46°38' 59°20' 72O50' priori to be considered absurd, at nearly fifty years of age, to com- PREMATURE USE OF SPECTACLES. Fig. m. 2 1 ■ / 2^ n — / ^ ! -5, I*r is, per se, excluded ; but in slight degrees 8 of M it occurs at a later period. From the foregoing table it appears that in emmetropic persons up to the 55th year glasses of -r^^, up to the 60th year glasses of — - are suffi- cient, etc. Now, if M. of — , of — , etc., exists, the necessity of spectacles will be thought of first in the 55th and 60 th years respectively : indeed the glasses sufficient for these ages com- municate to the eye in emmetropic individuals the degrees of M just mentioned. Now where the myopia does not wholly compensate the more advanced time of Ufe, it must in each case be subtracted. If, for example, at 65 M = — exists, we shall, in place of glasses of — , as in emmetropic persons, find glasses of — — — = — or — sufficient. Generally speaking, I have found that myopics re- quire convex glasses at a still later period than the degree of M should have led me to expect, and that the glasses required for emmetropics admit of a still greater reduction than that indicated by the degree of myopia. In the above example, we shall often have to give glasses, not of — » but only of ^. The reason is. CAUSES OF EARLY PRESBYOPIA. 223 that, on the one hand, in persons looked upon as emmetropic, a trace of latent H easily occurs, while, on the other, the limited distance (for example, 20 feet, representing —^^ ), at which the de- termination of M takes place, may make it appear somewhat too slight. We have now to estimate the circumstances, which, both in ame- tropic and in emmetropic persons, may modify the degree of the glasses required. As such we mentioned a. A range of aocommodation not corresponding to the time of life. Those who are occupied almost the whole day in reading, writing, or other close work, usually accompany their demand for spectacles with the observation that their eyes have certainly suffered much, but that they have also exacted a great deal from them. I hasten to set such people right. Comparative observation has shown me that much dose work does not essentially iajure the eyes, at least those which are emmetropic, and that the range of accommodation diminishes scarcely, if at all, more rapidly under such circumstances, than it does in agriculturists, sailors, and others, who for the most part look to distant objects. It is true that eyes predis- posed to M, are, by much reading and writing, easily rendered more myopic, but these occupations have no influence on the range of accommodation. The same is true of the frequent use of the micro- scope, or of a magnifying glass, as is required in the work of en- gravers and watchmakers : the regular course of -j- is maintained despite of much or little tension. But there are morbid conditions which cause the range of accommodation, and sometimes also the amount of refraction, to diminish more rapidly than usual. In the first place, general debility, the result of exhausting diseases, is to be noted. Premature old age, in general also deserves to be mentioned. Of the influence of glaucoma I have already spoken. If a person has quickly and repeatedly to strengthen his glasses, we should suspect the existence of glaucoma simplex, and accurately examine the tension of the eyeball and other points connected with this affection. The commencement of cataract alse appears to hasten presbyopia, probably through more rapid hardening of the crystal- line lens interfering with its mutability of form. Emmetropic per- sons are then apt to complain that they can no longer accurately distinguish near objects, which is to be attributed partly to dimi- 224 TREATMENT OF PRESBYOPIA. nished S^ partly to more rapidly lessening -— , and they seek the aid A. of spectacles. The morbid condition, which especially interferes prematurely with the vision of near objectSj is paresis and para- lysis of accommodation. This is not the place to enlarge upon this subject, the last chapter of this work shall be devoted to it. Let it at present suffice to observe, that ordinary paralysis may occur at any time of life, but more particularly in youth, that it usually sets in suddenly, and that it is further characterised by a tolerably wide, immovable, or scarcely movable pupil : it is, there- fore, hardly conceivable that it could be confounded with true pres- byopia. Now where, from whatever cause, -r- is abnormally diminished, A. stronger glasses are required than where -r- is normal. This is particularly true of a comparatively early time of life ; but at a more advanced age — is already, independently of special disturbance, so reduced, that its action in the convergence, whereby the spectacles produce accurate vision, has scarcely any influence, and that in no case has binocular vision, with the use of the required convex glasses, a margin of any extent. The knowledge of what glasses are necessary follows in this case, where we have not to take the accommodation into account, directly from the refrac- tion and from the distance, at which we desire to see. In order, for example, to see at 12" (which is sufficient with tolerably good 8), the emmetropic person needs glasses of :j-^; while with M = 2^ glasses of ^^ - ^ = ^ ; with H = 1 glasses oi ^^ + ^ = g- are necessary. It is another question, whether these may be used. To this we can in general only answer, that the sole diffi- culty is when the morbid condition is such as to prevent tension of vision. In incipient senile cataract there is usually no difficulty ; in paresis of accommodation, tension of accommodation is, when the acute period is past, even desirable, and this is very much promoted by glasses, whereby the distance of distinct vision remains somewhat greater than the patient would wish ; but in threatening glaucoma. DIMINISHED ACUTENESS OF VISION. 225 prudence requires us to avoid tension of the eyes, and we are there- fore recommended not to permit, unless exceptionally, the use of spectacles with which the patient can read or work at near objects. h. JDimitiished acuteness of Vision. — The distance of distinct vision at different periods of Ufe, estimated to be necessary, and therefore to be obtained by means of convex glasses, is in close connexion with 8. Consequently with the increase of years, to which diminution of S is related, we find this distance lessen (compare the table) . Where morhidly diminished S is concerned, we can take this also into account in the detarmination of the glasses : we can cause the retinal images to become greater about in the same proportion as the acuteness of vision diminishes. This is to be attained simply by means of stronger glasses. Not only do these bring the distance of distract vision nearer to the eye, but they also make the angle, under which the object is seen, increase in a still greater ratio than the distance diminishes (compare p. 153). They can thus render ordi- nary close work still possible with dimioished S. The question however, is : may this means be employed ? We must admit that its application is liable to great restrictions. In the first place, in acute diseases of the eye, with diminished S, all tension is injurious, and the eyes may, in the hope of improvement, be allowed to rest. And, as to chronic diseases and defects, even lq these diminished S cannot unconditionally be compensated by stronger glasses. In general it is to be considered that the limits, within which compensation is possible, are rapidly attained through too great proximity, and that, in proportion as we approach these limits, the survey of objects becomes less, and the necessity increases, of keeping the same distance unchanged, beyond, or short of which, with the range of accommodation still diminished by glasses and the comparatively wide pupil, objects cannot be properly distinguished. A peculiar weariness, ia consequence thereof, soon makes itself felt. Moreover the bent position acts injuriously, which, if the distance of distinct vision is very short, cannot well be avoided. Where the accommoda- tion and refraction must be aided, convex glasses seldom give any annoyance; but where the acuteness of vision fails, we must, so far as practicable, rather meet the difficulty with larger objects than with larger images, by abnormally diminished distance of the objects. All this, finally, renders the cases rather rare, in which the annoyance of diminished S can be met by the use of stronger glasses. The latter is evidently unadvisable in chronic keratitis or 15 226 TREATMENT OF PRESBYOPIA. iritis, and even in apparent tendency to inflammation ; in deeper- seated congestive affections tension is even to be considered dangerous. It is connected with the least inconvenience in chronic opacity of the corneaj in uncomplicated incipient cataract,* in congenital am- blyopia from unknown causes, and finally in disproportionately rapidly diminishing acuteness of vision in advanced years — senile amblyopia. We should, under these circumstances, recommend the use of large type, and in general, occupation with coarser work ; but ii' fine work .cannot be avoided, the glasses required should be strengthened so far, that the desirable degree of distinct vision should, without too much effort, be obtained. That we must sometimes * Cataract or opacity of the lens is divided into primary and secondary ; the latter is developed secondarily, in consequence of diseases of the fundus ocnli (chorioiditis, retinitis, etc.), extending through the vitreous humour to the lens. The first is considered to arise independently and primarily in the lens. The secondary nature of cataract we may infer, when the pre- ceding disease of the fundus oculi is recognised, or when the field of vision is limited or the power of visiop has evideptly suffered more than is to be explained by the obscuration. To look upon all other eases as primary is, however, an error. "Where the characters, just mentioned, are wanting, the cataract is nevertheless often secondary. I will go so far as to assert, that in comparatively young persona spontaneous primary cataract very rarely occurs. Often we can, in the commencement, establish the pre- sence of slight deviations in the fundus oculi ; not unfrequentlyalso we find turbidity of the vitreous humour, and rarely are the phenomena absent which indicate a congestive condition. The primary disturbance need not therefore interfere much with the power of vision ; especially when it is situated in the anterior part of the chorioidea, little or no inconvenience is felt from it. But that in many cases some disturbance of the function of the retina is connected with it, is shown by the improvement,of jS', which is very often obtained in the commencement of cataract, by therapeutic treatinent — and even by the discharge of the aqueous humour, wittiout the opacity of the crystalline lens (it has been very ingenuously admitted) undergoing the least change. A practical observation may here be made : — Through a certain oharlatanophobia (siV venia verba), which made charlatanism to be seen where it did not exist, many have thought they ought to refrain from all treatment in the commencement of cataract. Now this is certainly not for the interest of the patients. To think of solution or diminution of senile cataract, appears to me, from a physiological point of view, an absurdity. Other morbid changes of the lens give way also only in exceptional cases. But bearing in mind the frequent complications, treatment is here often desirable, and hygienic prescriptions are absolutely necessary. Therefore too, great caution is requisite with reference to allowing the use of convex glasses. FORM OF SPECTACLES. 227 at a very short distance, for the sake of binocular vision^ have to contend with the difficidties of convergence, will immediately more fully appear. c. The natv/re of the work to be performed. — Two points are here to be distinguished. In the first place, the minuteness of the objects, which renders it necessary that the work should be per- formed close to the eye, and therefore with relatively stronger glasses. For some work even the young normal eye is insufficient. Minute drawing, engraving, watch -making, and some anatomical operations; require the constant use of the magnifying glass. In other work the eye must at least, with normal S,be still accommodated for the distance of from 4" to 6". Hence convex glasses are even early neces- sary to render permanent accommodation for that distance possible. But, apart from its minuteness, the nature of the work sometimes requires a definite distance : in writing in large registers, in reading in the pulpit, in the use of certain musical instruments, it is often desirable that spectacles should bring the distance of distinct vision to one and a-half or two feet, and therefore weaker glasses are necessary than are given for writing, and particularly for reading. Guided by sound principles, we very soon find practically what glasses fuMl the object : an insuperable difficulty springs only from diminished S ; with this, as shall hereafter appear, we have to contend most in the case of M. I hava. thus, I think, laid down the principal points which must guide us in the choice of spectacles for presbyopic persons. Some observations on the form of these auxiliary glasses may here not he out of place. In general, oval glasses in a frame, with wings resting on the ears, placed at a certain iachnation, so that in work the axes of the glasses coincide nearly with the axes of vision, are to be pre- ferred. The nasal portion should have such a form, that in looking at a distance, in the horizontal direction, the person wearing the spectacles should be able to look over the glasses ; and if this should be attended with any difficulty, the rings might be flattened above (the pantoscopic spectacles of Smee). Some men, from habit, wish for round 'glasses^ which we may safely allow: they are usually old, quiet men, who, when they desire to look at distant objects, simply take off their spectacles. Others are not at all content with looking, thus unaided, at distant objects over their spectacles ; they prefer, as hypermetropics, wearing spectacles insufficient to read with, and they do not see accurately over their reading-glasses ; or 228 TREATMENT OF PRESBYOPIA. they have at the same time M and Pr, and desire with rapid alterna- tion to read through their spectaclesj and to see distant objects, whether in their office, or as teachers in a school, or ia the theatre, or elsewhere. This wish may be gratified by means of glasses h double foyer. With these some express themselves satisfied; in the case of others, rays fall at the same time through both sur- faces of curvature upon the eye, and these give them up again. I found these glasses most satisfactory for presbyopic painters, who require to look through the upper half, at a certain distance, at persons or at nature, while the lower haK is to bring the distance of distinct vision on the canvass or on the paper. White Cooper* states, that Sir Joshua Eeynolds was much in the habit of using such glasses when painting his inimitable portraits. The stronger the glasses are, the more attention must be paid to their mutual distance. Great accuracy is seldom required either in Pr — where somewhat more or less convergence of the visual lines has so little influence on the accommodation — or in youthful range of accom- modation, which leaves a proportional space for the biaocular. There- fore, it is usually sufiicient to mention to the optician only when either a particularly great or a particularly short distance of the glass-bearing rings is necessary, and in the absence of such direction to let the medium be used; in giving directions, let it be borne in mind that the less the distance for which the glasses are to be used, the closer they must be to one another. But, so soon as insufficiency of the internal or external recti muscles in binocular vision threatens to give rise to muscular asthenopia, it is of impor- tance that the mutual distance of the glasses should not aggravate this, but should rather counteract it. Now, less convergence of the visual lines is required when convex glasses are placed nearer to one another, and concave further from one another, and vice versa. If, therefore, the internal recti muscles are insuflicient, we should take care that the axes of the convex glasses are closer to one another than the visual lines ; in this manner we can, where strongly convex glasses are necessary, very much assist the internal muscles, and the images obtained are not perceptibly worse than those we get with a similar effect through prismatic glasses. Whether spherical glasses alone are sufficient, and how great their mutual distance must be, we can ascertain best with the frame of v. Jaeger (compare p. 97). If they are insufficient, we should give the com- * On Near Sight, etc. London, 1853, p. 201. USE OF READING GLASSES. 229 binationa with prismatic glasses, or perform tenotomy according to the indications laid down by von Graefe.* Nearly always, where strongly convex glasses are required to make binocular vision possible at a short distance, it is desirable, by placing the glasses comparatively near one another, to assist the internal recti muscles of the eye. Where very short distances are concerned, the dissect- ing spectacles, constructed and recommended by Bruecke,t with convex prismatic glasses, come into operation. Besides the spec- tacles, two kinds of lorgnettes are in use. Those ordinarily em- ployed by ladies, where the glasses are at a fixed distance, are attended with no ineonvenience when it is necessary to look for a short time, or to do anything for which both hands are not required, for one is occupied in holding the glass. But, for constant use, we should at the same time give a pair of spectacles. The glasses of the lorgnette may be somewhat stronger : during the short time they are used, no injury is experienced, and the advantage is gained of being able, when necessary, to distinguish smaller things. In the so-called nipping spectacles, used particularly by gentlemen, the distance of the glasses, determined by the thickness of the nose, is usually too short. Therefore, if the glasses are strong, the person wearing them sees with too slight convergence, unless the short distance of the rings be compensated by the axes being placed to the outside of the centre. Eeading-glasses, which magnify the visual angle, and are thus in some cases useful, give at the same time to the rays proceeding from a point a direction, as if they proceeded from a more remote point. Myopics can therefore make use of reading-glasses only when they come nearer to the object than the distance at which their farthest point lies. The recession of the objects increases with the increase of the distance between object and glass. So soon as the distance is equal to the focal distance of the glass, the rays proceed in a parallel direction, and the object appears to be at an infinite distance. Thus far therefore emmetropics can, provided the accommodation be totally relaxed, see through the reading-glass, and thus attain the highest degree of magnifying power ; simple presbyopics, who put their accommodation httle upon the stretch, always keep the glass nearly at this distance, because they soon see less accurately when the glass approaches the object. At what distance the eye is from * ArcMv, B. viii., Abth. 2. t Archiv, B, v., Abth. 2, p. 180. 230 TREATMENT OF PRESBYOPIA. the glass, is of little consequence: only the field of vision becomes less, ia proportion as the eye removes. If the distance between the object and the reading-glass becomes greater than the focal distance of this last, the rays fall convergingly into the eye, and it is in this manner that hypermetropics, and especially hypermetropic pres- byopics, make use vdth much advantage of a reading-glass, and by its means obtain a high magnifying power. In the use of reading-glasses binocular vision is usually sacrificed : the one eye looks through, the other close to the reading-glass, best with nearly parallel visual lines; on account of their indistinctness the images have no disturbing influence on the second eye, and the spectator even fancies that he sees binocularly. If no great magnifying power be desired, we can also see binocu- larly through one glass, for which purpose the latter must then be held closer to the object. In doing so less convergence of the visual lines, and at the same time less tension of accommodation, is required, than in looking at the same object vrithout the intervention of a reading-glass, and the object therefore appears, according to the laws of stereoscopy, to lie further off. However, even in the com- mencement of presbyopia, the tension of accommodation required is, in reference to the necessary convergence, quite : too great, so that binocular vision through the same glass is possible only for young persons, and for older individuals, who are somewhat myopic. It is best attained, even in incipient presbyopia, when we begin by hold- ing the glass at first near the object, and then gradually remove it. In general, however, it must be stated, that binocular vision through a reading-glass is possible to presbyopics only when they are, in ad- dition, aided by weak convex spectacles.* Consequently these glasses in general serve only for monocular vision, and they are especially to be recommended for the purpose of magnifying minute objects of art. For reading their use is seldom advisable. They come, however, under notice where diminished S renders a magnifying power neces- sary, which is obtainable by means of spectacles only for a short dis- * It may, in passing, be observed, that an emmetropic person (the myopic and the hypermetropic, of course, also, when their ametropia is reduced) can combine two figures stereoscopically through a reading-glass. The figures must, however, lie closer to one another than for ordinary stereoscopy, and as, with too strong convergence, we see the image situated to the right with the left eye, and vice versd, the figures must also be changed, the left for the right, and vice versa, if we wish not to see them pseudo-stereosoopicaUy. CONVEX GLASSES NOT INJURIOUS. 231 tance from tlie eye. For the special purpose of reading, broad glasses, ground above and below, are the most suitable, and in par- ticular bicylindrical convex glasses, with intersecting axes, deserve to be recommended: the dioptric action of these glasses (compare Astigmatism) is nearly equal to that of spherical glasses ; but they are distinguished by the fact that they have the greatest field of dis- tinct vision in the direction perpendicular to the axis of the surface turned towards the eye, so that by turning the surface with vertical axis towards the eye, we possess in reading the advantage of having a good image over an extensive field in the horizontal direction. No particular results are observed from the use of convex glasses. It is, however, true, that when a person with still suificient -r' in ordinary work regularly employs them — in other words, renders himself myopic, the relative -r- will change, and be modified in the same direction as in myopic individuals, and that, consequently, although -V- should not subsequently be changed, he will have diifi- culty in accommodating himself permanently for the distance of the point of convergence. Fbr this reason also, I gave my opinion against the so-called conservative spectacles, that is, against the pre- mature use of convex glasses : they then soon become a necessity, and hasten to. emmetropic persons the difficulty of seeing accurately at the distance in the point of convergence. Even after a single trial this may be observed, but of course it then soon disappears again. In using optical instruments with one eye, we generally lay it down as a rule, that in order to avoid injuring the organ, it must be re- laxed to its farthest point. I have for many years kept this in view in employing the microscope and in the use of magnifying glasses. However, the idea of the proximity of the object easily produces a slight convergence ; and if I have, avoiding all tension of accommoda- tion, continued this for some time, I experience difficulty, on discon- tinuing it, in aecommodatingforthe point of convergence : this difficulty continues longer, the longer I had, at a certain convergence, relaxed my power of accommodation as much as possible. I cannot, there- fore, recommend to the emmetropic to totally relax his accommoda- tion in using the microscope, the less so, because by doing this, he wiU soon fiud difficulty in applying, in measuring, the method a double vue, which in so many respects commends itself to our adoption. 232 TREATMENT OF PRESBYOPIA. Essential injuries to sight, which are often, with so much exaggera- tion, predicted, I have never seen arise, even from an undue use of convex glasses. On the contrary, as will appear in the Chapter on M, an inconsiderate use of concave glasses may be very dangerous. NOTE TO § 18. I have thought it well to exemplify the application of the precepts I have laid down respecting the use of convex glasses, by some cases taken from life. The reader will excuse me if this, for him, is quite superfluous. 1. Commencement of simple presbyopia. — D., a clerk, aged 48, asks for spectacles, because he begins gradually to find greater difficulty in distin- guishing small writing in the evening, and sometimes makes mistakes in 20 figures. His work does not otherwise fatigue him. He has S ^ -^=- (he sees No. XX at 20' distance), is emmetropic (sees XX at 20' not so well through either ^ or — finJ ' ^^^ ^^ daylight reads IJ with greater ease at 16' than at 10". He receives glasses of ^, which give a space of from 7" to 5', to use in the evening, and also by day in dark weather, and subse- quently when he experiences difficulty, regularly. When the necessity has become constant, the period will have approached at which the advisability of strengthening the evening glasses must be considered. — After some weeks, he states that he can no longer dispense with the spectacles in the evening. We readily believe him. 2. Moderate Pr, with a desire to see at more than the usual distance. — Mr. 17 1 R., aged 58, is emmetropic, has S = ^ and uses spectacles of -r^, which give him in reading a binocular space of from 16' to 11". With these spectacles he sees well, with the book in his hand ; but his handwriting, which, while he is lecturing, lies at 20", he does not easily distinguish, and his auditors are in a mist. He gets periscopic glasses of 5-, with rings flattened above, which he finds to meet all his requirements. He may use the same glasses in writing and for ordinary reading by day. For reading small print in the evening, his glasses of :^ are to be reeom- 16 mended. 3. Advanced Pr, with H. acq. and diminished 8. — L,, aged 73 has ILLUSTRATIVE EXAMPLES. 233 ■ff = (j^i ^ = ;;7,> is fond of reading (" the only thing -which remained to ut> 2Sj him"), and cannot do so correctly with his spectacles, particularly in the evening ; he is soon tired. He has glasses of -^ which make Pj nearly = -Bj ^ 13'. He gets glasses of =, sees with them at 9', and is quite satisfied. Large print is recommended to him. With the ophthalmoscope we see, when the axes of vision are directed downwards, traces of radiating opacityi which, however, are not the cause of the diminished S. I do not hesitate to permit him the free use of the spectacles, nor shall I hesitate to give him stronger glasses, when, with diminishing S and increasing H, the latter shall be more agreeable to him. 4. Commencement of Presbyopia, mth m,yopia. — Prof. S., aged 56, cannot sufficiently praise the excellence of his sight. " I see admirably at a dis- tance, read, write, and draw without difficulty, even in the evening." " Go on," is my answer. Prof. S., aged 62 : "I still usually see very well, but the work is sometimes difficult to me in the evening. Should I also need spectacles?" He reads IJ by preference at 14", deciphers it stiU at 18", 1*5 and therefore has S = about ^^-r ; on the contrary, without spectacles, only I'O 1 fi 1 1 R — , and with — tt,; ^^ ^^ amazement, — . " Was I then really myopic ? " He gets — -, with a recommendation to use them in the evening. " These glasses magnify, and at a moderate distance, for example 2', I see by no means so well with as without glasses." I answer : " That is unavoidable, at 60 years of age we can with no glasses in the world see both distant and near objects, but set the spectacles somewhat lower, so that at pleasure you can see over them, or use rings flattened above." 5. Sliglit H, requiring convex glasses before the usual age. — Madame v. L. complains : " I have done much work, and spoiled my eyes ; I am only 36 years of age, and I can no longer see anything in the evening." " Not read ? " "0 yes, but I can do no fine work, and reading also tires me ; I get terrible neuralgic headach, for which my doctor, and also Dr. K,, the 20 homoeopath, have given me all sorts of things in vain." She has 8 = ^, ■with—- more easily still S ^ -— , and therefore Hm ^ __ . She wiU de- 36 20 3o rive benefit from glasses of ^. "Must I then use spectacles ? " "It is oo desirable, indeed necessary, even by day, particularly if you wish to be free from your nervous headach. Besides, by the proper use of spectacles, when you are alone, you avoid fatigue, and you will now and then be able in company to work at intervals without glasses." "Is it not the case that I have spoiled my eyes ?" " Not in the least ; the form of your eye is the original cause of your requiring spectacles at a comparatively early age. Perhaps other instances of the same are to be met with in your family?" 234 TREATMENT OF PRESBYOPIA. "It is possible, but I had always such good eyes, and can see so far. Can I do nothing to strengthen them ? " " Often rest is the chief thing. Cool them now and then. Rub, if you choose, a little eau de Cologne over the eyebrows, but put nothing into the eyes, and depend upon it, the spectacles will cure your nervous headach." 6. Pr, with M ; reading at more than the usual distance. — Madame U., aged 65, can in clear daylight still easily read good print. Jf is suspected 118 1 and found = --^ ^__ S]ie lias spectacles of gj, but these tire her, and the letters are not black. iJj is at the same time ^ 13', P, not much less. Somewhat surprised at her statement, I request her to hold the book in her usual way. She lays it flat on the table, holds herself quite straight on her chair, and is thus at a distance of nearly 16". Evi- dently for such use the spectacles were too strong ; she required -^t which oo gave her a space of from 17" to 15'. For very small type she would do well to use the stronger glasses. "I never read small print," was her answer. 7. Hapidly decreasing accommodation, with incipient cataract. — Hefer B., aged 45. His vision has for some time been less accurate than it was, particularly for near obj ects, and he has used spectacles only at his work. I suspect the existence of slight S.~ It appears, however, that the eyes are 14 emmetropic, with ;S ==jr. Without spectacles he cannot read even III, for which, with perfect accommodation for 1' distance, only S ^ - would o be necessary. There is, therefore, evidently a very slight power of accom- modation. The ophthalmoscope and focal illumination' reveal radiating opacity of the lens, particularly inferiorly, with extremely slight granular turbidity. Thus the diminution of S and of -j is explained. The papilla nervi optici is redder (capillary hypereemia) than is proper to the time of life; the fundus is otherwise normal. "With — III is easily read at 13'; »-j lies at the same time at about 18"; p^ at 10". He must make only moderate use of these glasses, avoid too much laterally incident light, be- ware of a bent position, and to this end write at an inclined desk, take care to keep his feet warm, avoid whatever can cause congestion of the head cool his eyes often with cold water, frequently rub a spirituous fluid strongly over his eyebrows, report himself again at the end of six months, or sooner if anything particular should occur. I say nothing of cataract ; " his eyes are in a congested state." 8. Rapidly increasing presh/opia with S acquisita, with glaucoma simplex. —Madame K, aged 54, saw perfectly well until her 47th year ; in her 48th year she began to use weak spectacles for fine work in the evening ; these answered her purpose up to her 52nd year, she then got stronger glasses, but now constantly requires stiU stronger, and even with them she does not see with facility. She complains of weariness and a feeling of pressure. ILLUSTRATIVE EXAMPLES. 235 At present she uses ^j, and with them sees at 14', not much closer. This great distance is explained by S 5^, which, in connexion with her accu- rate vision at 47, is looked upon as aoquisita. In any case, ^ ^^^^^ t° ^^ 21 very limited. However, jS" is i= ^, and is therefore very satisfactory. On handling the eyeball, it feels too hard (2", on the right and T^ on the left eye, according to Bowman). This indicates glaucoma simplex. With the ophthalmoscope commencing excavation of the optic nerve is recognised, and on slight pressure with the little finger arterial pulsation ; there is no re- striction, but in feeble light there is less certain distinguishing of the fingers in the inner part of the field of vision. The iris, the size and mobility of the pupil, the depth of the chamber of the eye, and the sensibility of the organ, are still normal ; there are no coloured rings about the candles ; the subconjunctival vessels are perhaps somewhat dilated. I speak seriously to her : " There is the commencement of a disease of much consequence, which is sometimes rapidly, sometimes slowly developed. Art can, however, prevent it, for this I can answer. I shall expect to see you again in a month. If red- ness or pain come on, come to me immediately, even if you are indisposed, for by neglect, but by neglect alone, irretrievable blindness might be the result. I shall give you a few lines for your medical attendant. Meanwhile you must spare your eyes. E/eading I wiU not absolutely forbid, but use a large print, stop often, and immediately whenever you have any feeling of uneasi- ness." These words are the introduction to the proposal of iridectomy, which at the following visit she has to expect. Humanity urgently requires, that prejudice and ignorance should no longer oppose the use of iridectomy in glaucoma. CHAPTER VI. HTPERMETROPIA. H. § 19. DioPTEic Definition or the diffbeent Degeees and FOEMS OF HyPEEMETEOPIA. The refracting system of the emmetropic eye has, as we have seen, in the state of rest of accommodation, its focus on the layer of rods and bnlbs of the retina : parallel rays, derived from infinitely remote ob- jects, refracted by the media of such an eye, there unite in a focus (Kg. 112 -^, H is also ^, Y^, g, or, more exactly still, j^, ^, -fj> since the glass was removed 1" from the nodal point. H may be divided into acquired and origmal. Of the acquired we have spoken so far (§ 17) as it is developed by the senile changes, in the emmetropic eye. As in the latter, it begins after the 50th year, the original hypermetropia must also, after that period, gradually increase j but only in the same slight degree in which it oc- curs in the emmetropic eye. Under the head of ' acquired H,' we must here provisionally include aphakia, that is, the condition in which the lens has disappeared from the eye, or at least from the plane of the pupil. That with such a state a high degree of H must, in the ordinary form of the eye, be combined, needs no proof. To this subject I must devote a separate section of the present chapter. Original H we divide into mcmifest Hm, and latent HI. In my first investigations respecting H, I encountered the difficulty of accurately determining the degree of this anomaly; Thus an eye- sometimes at first refused every glass stronger than ^, while it soon afterwards gave the preference to ^, and subsequently again chose ^ or even yj. I assumed that hypermetropic eyes, obliged to put their power of accommodation upon the stretch in order to see remote objects, sometimes involuntarily to a certain degree kept up the tension, even when the proper glasses rendered this not only su- perfluous, but undesirable for accurate vision. Therefore, from the strongest glasses, with which the eye had, in different trials, stiU seen accurately at a distance, the degree of H was deduced. These should, as I supposed, completely neutralise the H. But when shortly afterwards, still stronger glasses were sometimes found adapt- ed to the same persons, I discovered my error, and comprehended that those first given had not completely neutralised the H, but that in using them the accommodation to a certain degree continued SUBDIVISIONS OF HYPERMETROPIA. 239 in operation. This led me to inquire, what the refractive condition of such hypermetropic eyes should be, when by the instillation of a solution of sulphate of atropia, the power of accommodation should be paralysed; and to my surprise, it appeared that not unfrequently in the trials with glasses, the greater part of the H had been sup- pressed. In slightly myopic, and equally in truly emmetropic eyes, on the contrary, E continues, after artificial mydriasis, nearly un- altered : the power of accommodation here becomes, when the eye is accommodated for r, actually almost wholly relaxed; at most only j^ remains. Evidently, therefore, it is a peculiarity of H, that with the act of vision tension of accommodation is associated, and thus the H is in part concealed. Hence it appeared that in H a mani- fest and a latent part are often to be distinguished. But it was then also to be suspected, that sUght degrees of H, in youthful accommo- dation, might be wholly suppressed, and in confirmation of this suspicion, experience showed me that where, as in cases of asthe- nopia and of strabismus, H was with some reason suspected, without being capable of immediate demonstration, a not inconsiderable degree almost always in fact appeared, on paralysing the accommo- dation. The conclusion is : that H may be wholly latent, = HI, and that, where it occurs in the manifest form, as Hm, a latent part HI may still be supposed to exist. Therefore H = Hm -|- HI, and if Hm = 0, then HI = H. Now, is it also possible that HI = 0, and that therefore H should be entirely manifest = Hm ? This is actually the case, when in advanced age the power of accommoda- tion is whoUy absent, either through paralysis or artificial mydriasis. But even already, while it diminishes, Hm must increase in reference to HI, and experience actually shows, that even in the 40th year HI, in reference to Hm, is very slight, and that in the 55th, it may be wholly neglected. Hence it follows, that an originally latent H becomes gradually more and more, and finally, just as the higher degrees, altogether manifest. I have seen cases of HI in children of ten or twelve years of age, where, in consequence of paralysis produced by atropia, H appeared = -^ ; others, in which Hm = from ^ to ^, gave place to H = ■^. At twenty years of age about one-half of this, at forty more than threcrfourths will 240 HYPERMETROPIA. have become manifest, and at seventy we have to expect simple Hm, iu a still higher degree (on account of the supervening diminution of refraction, as H acquisita) than H was originally present (com- pare, moreover. Kg. 117). A further division of Hm is that into alsol/wte, relative, and faeuUative. The absolute exists, when even with the strongest convergence of the visual lines, accommodation for parallel or converging rays is not to be attained ; of this Kg. 115 gives an example. It shows 1 :2 2! 3 3? 4 4| 6 8 12 24 00 :24 12 8 6 Fig. [16. / / / y / / / / / / ^i- -^^ % y y /^ — / / B" iU -^ -^ 7 ^ -^ ^ •' ^ / 1 / A / /- L^-^ / / / ^ y X y U-* y / < / / / / ^ 0° ll°2l' 22°50' 34032' 46°38 59O20' 72°50' FACULTATIVE HYPERMETROPIA. 241 us the boundaries of accommodation of Dr. de Haas^* one of the strongest hypermetropics I havemetwith. It appears that Hm amounts to very nearly -x, and, nevertheless, with strong convergence, Dr. de Haas still attains to nearly the accommodation for parallel rays. He has, in fact, the, for his time of life, very considerable range of accommodation of nearly g. Some years ag6, we may o safely infer, while -j- was still greater than it is now, his Hm was not yet absolute. Hence we see that in youth absolute Hm is of rare occurrence. Relative Hm, however, even then rather frequently presents itself. Fig. 115, II., represents a case of it in a girl of 17. The manifest farthest point r„ lies about 7" behind the eye, the abso- lute nearest point jo HeslO" before the eye, so that, calculated from r^ r- is = ..-■„ , — and yet the line jbj jo, jo nowhere passes over the line of convergence K K'. This young person can accommodate for a real point i, but only upon condition of convergence of the visual lines to a point, situated nearer than i to the eye. The H is there- fore not absolute, but still it exists in relation to the convergence. For example, in order to see accurately at a distance of 16", there must be convergence to a distance of 12", that is, under an angle of 11° 21'. But of this the person in question makes no use. She does not do so even by covering one eye ; consequently she now never sees accurately, even monocularUy, except with the aid of spectacles, but has, perhaps, when at ten or twelve years of age -r- was still greater, seen accurately even binocularily. When with increasing years j- shall be reduced to t=, this relative Hm wiE have given place to the absolute form. Facultative Hm I have assumed, when objects can be accurately seen at oc both with and without convex glasses. A case of this kind is represented in Fig. 116. It is that of a man aged 28, in whom r„ lies 30" behind the eye, andjo, with parallel visual lines, 30" before tW/ eye ; he sees still accurately at a distance as well * Author of the Inaugural Dissertation, Over de Sypermetropie en hare gevolgen, Utrecht, 1862. 16 242 HYPERMETROPIA. with glasses of ^ as of - ^. His relative range of accommoda- 1 1 _ 1 tion -with parallel visual lines therefore amounts t° ^q "^ 30 ~ 12- At the distance of 10-5, he can for a short time still see binoculayly. Fig. 116. 1 : 2 2| 3 4 "7 / / / / / / i/ / / f1^ 7\ tA ==-7' M^— J\ -^ -^ / ^ ^i a ^ 12 24 00 24 12 0" 11°21' 22°50' 34°32' 46°38' 59°-20' 72°50' Even before his 38th year this facultative H wiU have given place to relative, and about his 45th year, to absolute H. Does not even the originally emmetropic eye end by becoming absolutely hyper- metropic ? The distinction here made is in all respects justified. The dis- tinctive character of H is : position of the focus ?. behind the retina in rest of accommodation. If, moreover, with the most powerful tension, 0, remains behind the retina, Hm is aisolute ; if ^, can Teach the retina only with convergence of the visual lines, Hm is relative; it is, on the couijieisy, facultative, when also with parallel visual lines 0, can be brought.into the retina. The definitions give accurate boundaries. In reference to vision the distinction has, too. VARIETIES OF HYPERMETROPIA. 243 its important aspect, for with absolute Hm vision can never be acute; with* relative only monocular vision can be so (and indeed exceptionally) ; with facultativcj on the contrary, binocular vision may also be acute. But the distinction loses much of its impor- tance, inasmuch as in fatigue and weakness, and also regularly with the increase of years, the facultative passes into relative, and the latter into absolute H. To this attention was already directed in the description of the forms enumerated. A diagram (Kg. 117) may Fig. 117. 8 13 24 QO 24 12 8 10 15 20 25 30 35 40 45 50 65 60 65 70 75 80 further iUustrate it. In the 10th year, with H = g, Hm is here = — , but with tension there is still, even without convex glasses, acute vision to some distance from the eye (facultative Hm). With the a5th year, Hm has, with unaltered H = g, become = —, and while the absolute nearest point jo lies at 30", a binocular 16 nearest point is no longer present: relative H has supervened. Even in the 31st year p here passes the line x , that of the infinite distance, and therewith absolute H is given, further, while -^ gradually diminishes and finally at 80 years of age becomes = 0, the latent part of H, the distance between r /„ and r, /' becomes less and less, and at length entirely disappears, where the lines \ ? N \ \J ^=3: m X \ ■~-~- ^. ^ \ N ■^ ■^ ^ •^ X. 'T.e T i"" ^ ^ ■^i> ^ 244 HYPERMETROPIA. r r'™ and r, r. meet one another : at the same time the total H has in Tit S » risen from ;; to j-? , 4!'0 § 20. FoKMj Position, and Movements of the Hypekmeteopic Eye. — Appaeent Stkabismus. In the preceding section we defined H, from the dioptric point of view. The question now is, on what anatomical deviation this refractive anomaly depends. A great variety of circumstances may, exceptionally, in this respect, come into play.' In the first place, absence of the crystal- line lens (aphakia), by whatever cause produced — an important condition, to which we shall devote a separate section. Moreover, diseases of the cornea, attended with flattening, whether of the cornea at large, or of its central portion. Thus, 1 have sometimes found, with central ulcer of the cornea, a high degree of H, which gave way to E, or even to M, combined with irregular astigmatism, when by mydriasis the lateral parts of the cornea also came into play in direct vision. In commencing glaucoma, too, the eye appears to incline to H, which may be dependent on flattening of the crystalline lens through tension of the zonula Zinnii, if not on a higher coefficient of refraction of the aqueous, or especially of the vitreous humour: at least the cornea, according to my measure- ments, does not become flatter, as might have been supposed, from the increased pressure, which appeared necessarily to make the whole eyeball more spherical. EinaUy, protrusion of the retina through firm chorioideal exudation, may give rise to some H, and by detachment of the retina it may produce even a high degree of the same, which, in that case, soon gives way to blindness.* The rule is, however, that H depends on a peculiar typical structure of the eye, which may be called the hypermetropic struc- ture. The hypermetropically-formed eye is a small eye; in all its • According to Ed. v. Jaeger ( TJeher die Mmtellungen des dioptr. Appara- tes, etc., Wien, 1861, p. 96) H is very characteristic and important as a diagnostic symptom in many aflfeotions of the central nervous system and of the optic nerve (bluish coloration combined or not with phenomena of irrita^ tion). H is said in such oases often to intermit, not unfrequently simul- taneously with the symptoms of irritation, Increased tension would, under these circumstances, be present. STRUCTURE OF THE HYPERMETROPIC EYE. 245 dimensions less than the emmetropic, but especially in that of the visual axis. Immediately around the cornea, the sclerotic has a flat, slightly- curved appearance : the meridians have here a shght curva- ture ; at the equator, on the contrary, the curvature is much greater in the direction of the meridians than in that of the equator itself. A section through the visual axis has the form of an ellipse, of which the visual axis is the short axis (Pig. 118) j on the contrary, a section perpendicular to the Fig. 118. visual axis, carried through the equator, is almost a circle. The hypermetropic eye is in general an imperfectly developed eye. If the dimensions of all the axes are less, the expansion of the retina also is less, to which, moreover, a slighter optic nerve, and a less number of its fibres correspond. Further, the asymmetry in the several meridians (astigmatism) is, in this eye, on an average, greater than in the emmetropic. Both these circum- stances explain the fact, at least in part, that in high degrees of H the acuteness of vision is usually below the normal. If, in addition, the development of the cornea has been imperfect, which is com- paratively rare, the hypermetropic structure passes into true mi- crophthalmos. — The hypermetropic structure is hereditary: if one of the parents suffers from H, we find the same anomaly usually in one or more of the children j sometimes, too, several brothers and sisters are hypermetropic, without the anomaly being observable in either of the parents. How far the structure in question is also congenital, I have not inquired. According to von Jaeger's ophthalmoscopic investigations,* the eyes of most newly-born chU- ren are, on paralysis of accommodation, moderately myopic; but soon, on further development, these lose M, and in the first years of life mostly become emmetropic. The great difference in the form of the eyes does not occur until a later period. However, von Jaeger did not find even in the first days of life exclusively (78y„) myopic, but in 5% emmetropic, in 17% hypermetropic eyes. It may probably be said, that among these last especially the hyper- metropics of more advanced age are to be found. At least I have in the fifth and sixth, and sometimes even in the fourth year, demonstrated considerable degrees of H, and have never seen these disappear at a later period. * /. c, p. 20. 246 HYPERMETROPIA. The shorter visual axis of the hypermetropically-formed eye, which is demonstrable even in life, satisfactorily explains, the form and position of the refracting surfaces being the same, the existence of H. The question, however, is, whether we are right in assum- ing this similarity in position and form. A priori it might be sup- posed, and it has been not only supposed, but also asserted, that less convexity of the cornea and of the crystalline lens is peculiar to the hypermetropic eye. So far as the cornea is concerned, I am justi- fied by the results of numerous accurate determinations, in deny- ing the assertion. Even in high degrees of H, the radius in the visual line (compare p. 89) is nearly equal to that in the emme- tropic eye, and in the highest degrees, when the circumference of the cornea is somewhat less than usual, I found the radius even less. And if this be true for p°, still more must it hold good for p,, the radius ra the apex of the cornea (which alone is here concerned) ; for the angle a between the axis of the cornea and the visual line is greater in H than in E. However, it has really the appearance as if the cornea of the hypermetropic eye were less convex, which, just as in presbyopia, is to be ascribed solely to the diminished depth of the anterior chamber of the eye, and to the relative smallness of the pupil t a more anterior position of the iris and crystalline lens is part of the peculiarity of the hypermetropic structure. The influence of the cornea is thus excluded. — Whether a flatter crystalline lens belongs to the hypermetropic structure of the eye, is quite unknown. It is trae that in some eyes the lens is thicker, in others thinner; but, even if we admit that a shorter and longer focal distance cor- respond thereto (which is not proved), we have no right to place this difference in connexion with H. We know that the cornea has sometimes a particularly long radius, but this is peculiar rather to the highest degrees of M than to H; and accidentally, too, the lens, which in the measurements of Knapp* had a particularly great focal distance, belongs to a myopic eye. In- deed, it is evident that in order to be justified in connecting a flat lens with the hypermetropic structure, we must distinctly demonstrate the presence of the same ia hypermetropic eyes. I have tried an indirect method. I have endeavoured to measure the length of the visual axis of superficiaUy-situated hypermetropic eyes, turned strongly inwards, and thence, in connexion with the simultaneously deter- * Archivf. Ophthalmologie, B. vii., p. 1. LENGTH OF THE VISUAL AXIS. 247 mined radius of the cornea and the degree of hypermetropia, to calculate the focal distance of the lens. But the measurement of the visual axis appeared to be not sufficiently accurate. Now, in the absence of decisive determinations, we assume that the cardinal points of the dioptric system in the hypermetropic eye have the same position as in the emmetropic. That the crystalline lens in the hypermetropic eye is placed somewhat more anteriorly, we leave out of account, partly for the sake of simpKcity, because the influence thereof is slight, — partly in order so far to meet those who feel bound to contend for a flatter crystalline lens in the hyper- metropic eye. Now, the position of the cardinal points being the same, we can apply those of the reduced eye, in order to calculate what degrees of H are connected with a given diminution in length of the visual axis. Thus we find : — With a diminution in length of 0"5 mm., H = 1 : 21"43. 1 mm., H = 1 : 10-34. 1-5 mm., H = 1 : 6-649. 2 mm., H = 1 : 4-302. 3 mm., H = I : 2-955.* In a similar manner we can from the degree of the hypermetropia calculate the diminution in length of the visual axis, and thus the actual length. This has been done in the subjoined table, in which also the position of the centre of motion (compare the method of the determination pp. 185 et seq.), and the angle a are included. As the basis of the calculation, the values of the reduced eye are taken, and the ascertained diminution in length is deducted from the length of the visual axis (= 22-231 mm., according to the diagrammatic eye of Helmholtz). * At p. 178 we found J —^ — fZTp'' °^ y — ^ ' .1 /._F"F' consequently: Q • If the focus .;■ (the retina). lies in front of f", as is the case in hypermetropia, y and f are both negative, because F" > f" and F' > f: even f becomes negative, so soon as i comes to lie behind h. As f, we find the distance of i behind 0' ; we need that of i behind k, and f — (?' is therefore the distance which we seek. "We thus find n which f — at each aiigle of convergence, increasing his accommodation with the degree df the produced H. But fatigue wiU ensue> and he will easily satisfy himself of the diffi- culty of continuing to see accurately also at a distance. — The young myopic individual needs only to take too strong negative glasses, in order to bring himself into the same state; If the myopic person have somewhat exceeded his fiftieth year, he needs only to neu- trahse his myopia in order to observe in himself the phenomena of presbyopia : for those of asthenopia he has then lost the capacity. It will have been evident to the reader, that the phenomena of asthenopia proceed from nothing else than from fatigue of the muscular system of accommodation. In what this fatigue consists deserves to be more closely examined. In my investigations* respecting the elasticity of muscles, I have distinguished two forms of fatigue. One form proceeds from the actual energy,t produced by the muscle. The work consists in the moving of a load. The load may be the body itself or some part of the body, which is moved, or, in addition thereto, an object external to the body. Distinguished from this is the fatigue, which is the result of the simple extension of an elastic muscle in a state of contraction. * See the preliminary communication in Verslagen en mededeelingen der Koninhlijke Akademie van Wetenschappen, 1859, D. ix., p. 113. ' t I "se the terms of Mr. Rankine, adopted also by Mr John TyndaU (Heat considered as a mode of motion. London, 1863, p. 137.) 266 ASTHENOPIA. This takes place when a burden is heldj without being moved, as, for example, when, with the arm bent at a right angle at the elbow- joint, the hand is loaded with a weight : the arm and the weight remain in the same place, and yet fatigue soon occurs. At the moment when the weight was placed in the hand, some actual energy (chemical action in the muscle) was indeed required to make the arm continue in the same position : the muscles (mm. biceps et brachialis internus) had to contract more strongly, in order in the state of extension induced by the weight, to remain as short as before, and the actual energy (the chemical action) was thus con- verted into a potential energy (elastic tension). Moreover, the mus- cular contraction gradually increases as much as the greater exten- sibility, accompanying the increasing fatigue of the muscles, requires. It has, in fact, been proved that, accordingly as the muscle becomes more fatigued, its extensibihty increases, and this increasing extensi- bility gradually requires augmenting contraction, in order, under the extending action of the same load, to keep the muscle as short as it was. This is evident from the fact, that on the unexpected removal of the load, the arm bends involuntarily, as a result simply of the previous extension of the elastic contracted muscle, — the more strongly, the longer the weight has rested on the hand; and in that involuntary motion of the arm the potential energy of the ex- tended muscle again becomes actual. Finally, also, there was con- tinually some actual energy in the oscillations of the electric currents, composing the negative deflection of the tetanized (contracted) muscles and nerves, and most likely converted into heat. It was therefore under more than one mode that, without external pheno- mena having been visible, some energy was actual, while arm and weight, unaltered, occupied the same place. But that energy, in its different modes, seems to be very small in comparison to the often repeated lifting of the same weight. I therefore think the fatigue proceeding from the performance of labour, must be distin- guished from that arising from simple extension. According to the law of the conservation of energy, we may, in the first case, expect more metamorphosis of matter in the organism. The acceleration of the heart's action appeared to me to be the measure thereof. I found, in fact, that when a weight is held during some minutes resting on the hand of the bent arm, the pul- sation of the heart is much less accelerated than when, dtiring an equal space of time, the weight is alternately taken off by another, and with extended arm placed again upon the hand, and now by THEORIES OF FATIGUE. 267 flexion raised once more to the original height. The feeling of fatigne in the muscle is, howeyer, in the latter case not greater than in the former. In explanation of the fatigue, which is the result of the perform- ance of labour, we may take refuge in an accumulation of products of metamorphosis of matter in the muscular tissue, which really goes hand-in-hand with it. The fatigue, proceeding from extension, under the influence of a load not further moved, may, partly at least, have another source. Thus the extension might give rise to pressure on the nerve-filaments in the muscle : in fact, without the extension by a load, fatigue does not arise in the muscle, though contracted in an equal degree. Probably^ however, it depends partly also on an increase of the products of the metamorphosis of matter in the muscular tissue, produced not so much by an acce- lerated formation, as by retarded elimination. Indeed, in uninter- rupted contraction the vessels are compressed and the circulation is impeded, while in motion from muscular action the latter is excited and accelerated. That accumulation of products of metamorphosis is co-operative, is, moreover, admissible, because in both cases the coefiicient of elasticity of the muscle decreases, — which coefficient may, I think, be connected with the presence of some produpts of metamorphosis in the nutritive fluid of the muscle. But this is not the place to enter more fully into tMs question. It is sufficient to have directed attention to the distinction to be made. Now, to which form of fatigue does, that belong, which arises from persistent accommodation for acciirate vision in the hyperme- tropic eye ? Evidently we have here to do with persistent extension of the muscle in a state of contraction. The extension is the result of the resistance exercised by the parts involved in the accommodatiqn, while their form and position undergo a change. By virtue of elasticity they resume their original form and position, so soon as the contraction of the internal muscular system of accommodation ceases. The latter must, therefore, in order to produce persistent accommodation, be in a state of permanent contraction. This permanent contraction causes fatigue, and the fatigue, promoted by extension, increases, as was above remarked, the extensibility : in consequence of this law, the contraction must be always increasing, in order to keep the muscle equally short and to make it permanently exercise the same force (in equilibrium with the resistance). Sooner or later therefore the fatigue must pass into powerlessness. A moderate contraction. 268 ASTHENOPIA. such as is required in tlie normal eye, may be kept up without fatigue for almost an entire day. A degree of contraction is even possi- ble, at which the recovering power of the muscle removes, pari passu, the fatigue proceeding from contraction; the extensibility in that case does not increasei This applies particularly to involuntary muscles, consisting of fibrc'cells, to which (at least in mammalia) the internal muscles of the eye belong. But where hypermetropia exists, such a degree of contraction is required, that increasing fatigue, at length proceeding to complete loss of energy, cannot long be absent. Thus aU the phenomena of asthenopia are readily explained. It seems to me that there is therefore no reason for resorting in addi- tion either to the condition and the function of the retina, or to pressure of the fluids^ or to obstruction to the circulation. To fatigue through work, as has been above said, the same law is applicable : here, tooj the coefficient of elasticity diminishes ; here, too, the extensibility is therefore increased. Actual work is done by the muscular system of accommodation, when the eye is accommo- dated alternately for different distances. This, however, need scarcely ever take place to such a degree as to give rise to fatigue. What are the results of continued excessive tension of accommo- dation in asthenopia ? In former years, especially, the most fearful consequences were pre- dicted from it. Asthenopia was considered to be the first stage of, or at least to be intimately related to, amblyopia,^ and the latter, it was sup- posed, threatened the destruction of the asthenopic eye, if it was not condemned to almost absolute rest. Precisely the mode of treatment of oculists, in giving asthenopics no spectacles or insufficient spectacles, has placed me fairly in a position to satisfy myself of the unfounded nature of their fears. I have seen hundreds of asthenopic patients, who from youth up to their 30th or 40th year, some to an advanced period of Mfe, had every day anew, without spectacles, or with too weak spectacles, obstinately pushed the tension of accommodation to the uttermost, and I have never seen a diminution of the acute- ness of vision arise from such a course. It appears also, that in H a certain immunity against many kinds of diseases, which threaten the power of vision, actually exists ; it is certain, that by excessive tension of accommodation the retina is not brought into danger. In rare cases, perhaps in one in a thousand, I have observed that on every effort to see near objects, almost immediately violent pain arose in the eye, evidently connected with the contraction of the muscles of accommodation. This painful spasm made it necessary FORMER UNCERTAINTY AS TO ITS NATURE. 269 for the patient directly to refrain from work. Nor did the use of spectacles avail him^ for in each case near objects were seen with convergenccj and the tension of accommodation connected with this convergence, was snificient to excite the pains. These remarkable cases, the cure of which was obtained by the regular instillation of sulphate of atropia, which completely excluded the accommodation, while strong glasses meanwhile rendered work possible, I shall have to describe more fiilly in speaking of the anomaKes of accom- modation. An evident proof, that neither the nature nor the causes of the phenomena of asthenopia had heen fully ascertained, notwithstanding many endeavours to investigate them, is to he found in the host of names hy which this con- dition has been designated hy diflferent writers. They are almost innumerable : debilitas vtsus of Taylor, amblyopia a topica retin^ atonia of Plenck, affaiblisse- ment de la vue of Wenzel, Ge'siehtsschwdche or hebetudo visus of Jungken, dulness of sight of Stevenson, debollezza di vista per stanchezza di nervi of Scarpa, dimness of vision of Middlemore, visus evanidus of Walther, impaired vision of Tyrrel, amaurosis muscularis of J. J. Adams, affection of the retina from excessive employment of Lawrence, lassitude ocularis or disposition a la fatigue des yeux of Bonnet, hopiopie or dphthalmohopie of Pfitreqnin, Schwdehe der Augen of Chelius, amblyopie par presbytie ou presbytique of Siehel, languor oculi of Arlt, slowly adjusting sight of Sniee, impaired vision from owerworh of Cooper, and perhaps many others still. Our knowledge of asthenopia commenced with that of the phenomena of the affection. By degrees the description thereof became more true and more accurate, and what did not belong to it was separated from it. It is evident that in a condition such as this, constituting a part of the floating idea of weakness of vision, occurring with many complications, and present- ing numerous varieties, according to the time of life and the use made of the eyes, there was great difficulty in sketching a typical picture, so long as the cause of the leading feature of the affection, and therefore its nature, were unknown. So it was possible that although even Taylor* had given a good sketch in the following words : — " dantur exempla, ubi statim ab initio lectionis, et post earn, litterse confuse permixtse videantur, et hinc legentes a lectione prohibeantur, quod etiam acu subtili nentibus, vel aliud qnodcunque negotium, ejusmodi longam axeos directionem certum versus objectum quoddam requirens, tractantibus, accidere solet," the well-defined lineaments of the picture were again obliterated by unessential phenomena and mixed up with those of amblyopia. According to this the cause of the affection was now generally sought in the retina or in the chorioidea. That Plenckt was inclined to thisopinion, is atonoe evidentfromthe name he * Taylor, Nova Nosographia Ophthalmica, § 189, p. 151. Hamburgi et Lipsiss, 1766. t Plenk, Boctrina de morbis oculorum, p. 188. Viennse, 1792. 270 ASTHENOPIA. gave it of amblyopia a topica retina atonia. Scarpa* sees in the affection fatigue of the nerves, especially of those which have direct reference to vision ; and if the picture, sketched by Beerf, was to he considered as suflBlciently charac- teristic, we might say also of him, that he ascribes the disturbance to a weak- ness of the retina, or to a change in its structure. Without determining the nature of the affection, Lawrence! also states, that this is to be sought in the retina, perhaps primarily in the chorioidea, but still he clearly shows that, while the acuteness of vision is perfect, it has been incorrectly classed together with amblyopia, and Tyrrel § endeavours, rather by number than by force of arguments, to prove that a preceding congestion of the chorioidea is the primary cause of asthenopia, which congestion might even pass into chorioiditis. At first Siohel || is yet farther from the truth : he looks upon the affection as the beginning of amblyopia, " le premier degr6 d'amblyopie oil le malade voit parfaitement ou presque parf aitement bien, mais oh la vue ne supporte pas la moindre fatigue et se trouble, dSs que le malade applique les yeux pendant quelque temps ou memo pendant quelques minutes." JiingkenH still distinguishes his hebetudo visus, of which he assumes not less than ten varieties, from the proper amblyopia, and remarks very justly, that it is distinguished from the amblyopias by the fact, that in the latter "the power of vision has already suffered, that the patient in general can no longer distinctly recognise objects, which in weakness of sight (asthenopia) is by no means the case;" but he adds: "this latter may, however, pass into amblyopia, and this is usually the case, if its causes are not removed." It is by Bonnet {Gaaette mHicale de Paris, 4th September, 1841), and particularly by Pfitrequin {Ann. d' Oculist., v. p. 250, and vi. p. 72, 1841 and 1842), that we first find the retina excluded, and the primary cause of asthenopia sought in the muscular organs of the eye, especially in those of accommodation. The external muscles were by them put most prominently forward, and being too much preoccupied with the idea of tenotomy, in order to cure the asthenopia, they thought more of an injurious pressure of the muscles than of fatigue. Mackenzie,** too, comes, in his remarkable treatise, to the conclusion, " que oe sent les organes ou I'organe d'ajustement, qui sent affectes dans cette maladie, et qui en sont probablement le sifige principal." Perhaps he would have ventured to exclude the retina; but he thought he had observed, that myopics also are subject to asthenopia, * Scarpa, Trattato delle prineipale Malattie degli Occhi, Vol. ii. p. 241. Pavia, 1816. t J. Cr. Beer, Lehre von den Augenhrankheiten, Theil ii. p. 17. Wien, 1817. \ Lawrence, Treatise on the Diseases of the Bye, p. 566. London, 1841. § Tyrrel, Practical Work on the Diseases of the Eye, Vol, ii. p. 25. London, 1840. II Sichel, Traite de Tophthalmie, la cataracte et Vamaurose p. 646. Paris, 1837. t Jiingken, Die Lehre von den Augenhrankheiten, p. 780. Berlin, 1832. •* Memoire sur Pasthenopie ou affaiblissement de la vue. Annales d'Ocul., Tome X. pp. 97, 155. VIEWS OF MACKENZIE, SICHEL, AND BOHM. 271 and that convex glasses do not protect the wearer against attacks of that affection. Elsewhere {Practical Treatise, 1854, p. 984, 4th edition), we read again : " Were it entirely a disease of the apparatus of accommodation, looking through a small aperture, by making the use of the accommodating power unnecessary for the time, would make vision distinct." It hence appeared perfectly logical, net to seek the seat of the affection simply in the accommodation. The observation was, however,, incorrect. The difficulties which myopics sometimes experience, represent another form of disease, and moreover we have reason to believe that Mackenzie has incorrectly supposed the existence of myopia in some of his patients. As to the attacks during the use of convex glasses, we shall see that Mackenzie was too timid to give glasses of sufficient strength, and asthenopia is, of course, not to be removed by the use of too weak glasses, and lastly, asthenopics really see, when they are tired, comparatively well and easily through a small opening. From this time we find fatigue of accommodation mentioned by different writers, as at least a cooperative cause of asthenopia. Sichel* had not yet a just idea of accommodation, but he had nevertheless arrived at the con- viction that his amaurose preshytique, which he now himself identified with asthenopia, occurred only in presbyopic eyes, and this point he correctly maintains against Mackenzie. But, on the other hand, he keeps up the connexion with amblyopia, and still thinks that this affection may very easily pass into incurable amaurosis. With good result, the implication of the retina in asthenopia was by Bohmt set still further aside. He shows especially, that nerves of motion are much more liable to be affected with fatigue than nerves of sense, and he consequently seeks the primary cause of the affection in the motor nerves of the eye. It is true that Bohm's theory remains somewhat obscure and indefinite, in consequence of the part he ascribes to the external muscles of the eye, and of his not very clear ideas of near- and farsightedness ; but in non-squinting asthenopics he recognises as the cause of the affection, the want of permanent power of accommodation for near objects, and with great satisfaction he mentions that he has, by means of convex glasses, delivered many asthenopio patients from their troubles. Bohm is, in truth, the first who unconditionally recommended the use of convex glasses in asthenopia. As, however, he did not hit upon the principle for the deter- mination of the required strength of these glasses, those he prescribed were in general far too weak ^g^ to r^V from which, indeed, much could not be expected, and, moreover, supposing that he had to do with an anomaly of accommodation, he still adhered to the hope of being able to cure the asthe- nopia. Bohm's theory was almost unconditionally adopted by Ruete.t This sagacious author expressly states that the exciting causes are not known, and * Lecons cliniques des lunettes et des Hats pathologiques, consScutifs & leur usage irrationnel. BruxeUes, 1848. f L. Bohm, Das Sehielen, p. 117. X Ruete, Leerhoeh der ophthalmologie in het Nederduitsch bewerkt en met aanteekeningen voorzien door Professor Bonders, p. 713. Utrecht, 1847. 272 ASTHENOPIA. that the disease may be congenital and sometimes hereditary : thus he appears to he upon the point of asking what may be the organic basis of the affec- tion, but he finally contents himself with the ponolusion, that " the proxi- mate cause, as Bohm has proved, is a weakness of the motor nerves of the eye." Thence also his hope of curing the disease. Others have been less circumspect in the assumption of ejtciting causes. The majority of these observers believed the cause? they assigned to be alone sufficient to produce asthenopia. Among these cause?, e?:cessive ten- sion of accommodation for near objects was especially mentioned. The very names given by some to asthenopia, " ^Section of the Retin^ from exces- sive employment" of Lawrence, 'f Impaired Vision from overwork" of Cooper,* show how much importance was attributed there|». By Carron du Villardst we find asthenopia even described as a, peculiar form of disease, from which the embroiderers of Nancy were said to have particularly suffered, and soon after the same affeption was found among the lacemakers of Brussels. Sichel, too, thinks that particular callings develop this condi- tion. Thus the circumstance under which the existing anomaly might manifest itself by peculiar morbid phejiomen^, was considered to be the cause of the anomaly itself. This cannot pause surprise, p'p to the 16th, the 20th, nay,eventothe 25th year, the power of vision had continued normal; no complaints were made ; but gradually, precisely in persons who ■y^ere almost incessantly occupied with close work, the continuajipe of the latter had be- come more and more difficult, and if the work was for some timp suspended, improvement took place. Could it be otherwise than that the affection should be considered a? a purely acquired condition, and that the cause of it should be sought in excessive tension 1 — In addition, a long train of causes has been drawn up by Mackenzie and others from circum- stances which accidentally coincided with the development of the pheno- mena. If we glance back at what has been above stated, we find that asthenopia at first lay concealed in amblyopia, that it gradually — although still referred to amblyopia, whether as predisposition, whether as the commencement, or, finally, as a peculiar form of that affection — emerged from its obscurity, that afterwards, without the participation of the retina being as yet denied, its seat was sought more and more in the organs of accommodation, until at last the retina was almost completely exf^luded, and the condition was looked upon as a disease of the motor nerves and of the organs of motion of the eye. At this time the source of the power of accommodation had not yet been discovered, much less had its mechanism been demonstrated. There was almost as much reason to assign the principal part in that function to the external muscles of the eye, as to the muscular system situated in the eye. This led to the supposition that asthenopia was to be sought in a spas- modic contraction of some external muscles of the eye, and there were practitioners who had the courage to cut through these muscles. This is a melancholy page in the history of operative ophthalmic surgery. It is • White Cooper, On Near 8ight, etc. p. 124. f Annales d' Oculistique, Tome iii., Supplem. p. 256. OPERATIVE INTERFERENCE. 273 the more sad, because thereby in general ignorance is exposed, so great, that myopia, presbyopia, and asthenopia were not even distinguished, and because, on the other hand, we find results communicated to which, to use no harder expression, we will, with Mackenzie, apply only the words of Scarpa : " Istorie di guarigioni sorprendenti, e poco dissimili dai prodigi." Much may, however, be alleged in exculpation. In the first place, history shows that every discovery, and certainly also every new operation, usually leads to exaggeration. This is the result of an enthusiasm deeply rooted in human nature, and which has also its good side. Without it truth appears to gain no victory in the domain of science. Moreover, in the operation for strabismus, improvement of the power of vision was actually obtained, and many now thought that this improvement was the result of a change of re- fraction : in. the then position of science this was certainly rather to be expected than an even now unexplained improvement of the acuteness of vision. We may also assume that, even when asthenopia existed without strabismus, the rest which the patients had to observe for some time after the operation caused the fatigue, on an endeavour to read, to occur less quickly. Besides, it is easy to understand, that when the musculus rectus internus was cut through, stronger tension of accommodation, with convergence of the visual lines to a certain point, became possible, just as may be effected by means of a prismatic glass with the refracting angle directed inwards. In any case, we will not judge harshly of the operators of that season of rage for operating. It is enough that for a time the cause of asthenopia was sought in the external muscles of the eye, and that the results obtained on division of the latter were supposed to furnish a fresh proof of the ' correctness of the views of those who referred it to them. Asthenopia was in these cases really the subject of discussion. Altliough here Bonnet,* for whom the priority of the application of the division of the muscles in these cases is claimed by Phillips and Guerin,t as well as Cunier,t is speaking of myopia, the oases communicated by these writers leave no doubt respecting the nature of the affection. They supposed that myopia existed, whenever anyone could decipher a, certain print better near the eye than at the distance of a foot, and they thought that the myopia had given way, or was diminished, when such a person could subsequently distinguish the same print at a greater distance, or could continue his work longer. From this error even Ludwig Bohm was not completely free. After the discovery of the principle of accommodation, nothing more was said of abnormal pressure of the muscles of the eye, nor of dividing the latter as a remedy for asthenopia. Stellwag von Carion§ refers asthenopia exclu- sively to a diminution of accommodation, and, indeed, in particular to * Annales d'Ocul., Tome vi., p. 73. t Ibid., Tome v., p. 31. X Ibid., Tome v., pp. 139, 173. § Stellwag von Carion, Die OpMhalmologie vom naturwissenschaftliehen StandpunUe aus. Bd. ii. Erlangen, 1855. 18 274 TREATMENT OF HYPERMETROPIA. presbyopia. " In the majority of cases," — thus he expresses his opinion, — " normal vision passes under the phenomena of asthenopia into presby- opia, and koplopla never comes so remarkably under observation as in the eyes of elderly people. It belongs quite especially to the later epochs of life ; and if it is sometimes observed in youth as the precursor of presbyopia, the attending circumstances are in general of such a nature, that a condition of the muscle analogous to involution is highly probable." He could not say more plainly that he considers asthenopia to be a lesion of accommoda- tion. Von Graefe* puts the question on as broad a basis as possible, by here assigning to asthenopia only a symptomatic signification. Thus he demon- strates the existence of asthenopia muscnlaris, proceeding from insufficiency of the musouli recti interni. Moreover, he brings asthenopia into connexion with slight degrees of presbyopia. And when he further sees asthenopia arise, " where the nearest point is but little removed, but where still the region of permanent accommodation lies considerably farther from the eye than is normally the case," he seems really on the point of thinking of hy- permetropia. In other respects, von Graefe attaches more importance to the influence of the retina than I think ought to be assigned to it. Finally, he observed asthenopia in consequence of " actual paresis of accommoda- tion," and in truth the description of the latter may almost completely apply to asthenopia, the result of hypermetropia. Our knowledge had reached this point, when I-j" discovered the cause of asthenopia in the hypermetropic structure of the eye. The supposed anomaly of accommodation then became an anomaly of refraction, the connexion of asthenopia with the circumstances under which fatigue is manifested was made most clear, the necessity of complete relief by spectacles was proven, while at the same time the hope of a radical cure of asthenopia was extin- guished for ever. § 23. Teeatment oe Hypeemetkopia, with Special Reperence TO Asthenopia. The treatment of asthenopia has always been called " rational." I remember the time when this qualification was considered to be an honour, and when he who in medicine stood upon mere empiri- cal ground, was regarded with disdain. Portunately, a change has taken place in this respect. It has become more and more evident that, even with perfect knowledge of the nature of an anomaly, the final decision must remain with empiricism, and that, with our defective * Archivfur Ophthalm., Bd. ii , Abth. i., p. 169. t Nederlandsch Tijdschrift voor Geneesh. Jaarg., 1858, p. 473. METHODS OF TYRRELL, BOHM AND RUETE. 275 and imperfect notions, science can, with respect to therapeutics, at the most, occasionally suggest what deserves, hy preference, to be submitted to investigation, and that her further duty is to endeavour to explain what has been ascertained. Asthenopia has been looked upon as the result of an enfeebled j)ower of accommodation. Hence its "rational" treatment was directed against the causes of debility, and demanded, above all things, rest of accommodation, With this view, Tyrrell prescribed a systematic treatment. Now, rest was certainly most perfectly at- tainable by avoiding all work in which looking at near objects was necessary, and where the latter could not be wholly dispensed with, it was thought that the same object might be attained by the use of convex glasses, although, on account of the tension of accommo- dation connected with convergence, this plan would necessarily be less successful. This constituted the first period of the treatment, in which the organs of accommodation should by rest be relieved from their morbid state. Again on rational grounds, the second period must follow — that of practice : the spectacles were gradually weakened, and close work was permitted for longer and longer in- tervals, though with the strict injunction to suspend the work on the occurrence of the least fatigue. Thus it was hoped that the asthe- nopia might be permanently overcome. Many also actually as- serted — and in good faith — that they succeeded in this. But were the patients not desired always to take care of themselves ? Did not the prescribed spectacles remain in their hands? — and would not many, desiring to be relieved from a long-continued treatment, have represented their state too favourably, and when they got worse, have simply stayed at home ? I am convinced that the great majority, if they used their eyes as formerly, would be equally affected by the same troublesome symptoms. — On the supposition that the retina was in some measure implicated in the affection, Bohm and Euete thought it " rational " to recommend that the convex glasses should be blue. And, indeed, this might, if, as usual, the glasses allowed were too weak, on account of the greater refrangibility of the blue rays, even if the retina was not over-sensitive, be attended with some advantage, which cannot be said of the London smoked glass recommended by Pronmuller, by the simply light- diminish- ing action of which it is certain that few asthenopics would be benefited. AU ophthalmologists did not, however, boast so much of the ex- 2 276 TREATMENT OF HYPERMETROPIA. cellence of their results. " In many cases," says Mackenzie, treating of the prognosis of asthenopia, "it is our duty to declare the disease incurable. If the patient is a young lad, bound apprentice to a sedentary trade, and the disease, from its duration and its mode of origin, not likely to yield to treatment, we may advise him to turn shopkeeper, to apply himself to country work, or to go to sea : if a female, occupied constantly in sewing, to engage in household affairs, or any other healthy, active employment. Many a poor man have I told to give up his sedentary trade, and drive a horse and cart ; while to those in better circumstances, and not far advanced in life, I have recommended emigration, telling them that though they never could employ their eyes advantageously where much reading or writing was required, they might see sufficiently to follow the pastoral pursuits of an Australian colonist.''^ What is the reason that, before taking so decided a step in the destination for life of a man, it has not been without prejudice inves- tigated, what the effect would be of the constant use of stronger convex glasses ? Is it prejudice, in general, against the use of the latter by young persons ? Or had the old apprehension that as- thenopia might lead to amblyopia found fresh support in the expe- rience that, after the use of sufficient convex glasses, the eyes (as we now know, in consequence of displacement of the relative region of accommodation) soon act in close work still less perfectly than before ? Certainly the prejudice must be deeply rooted ; for there lay a very significant hint in the observation drawn by Mackenzie from life : " A child, the subject of asthenopia, engaged in learning his lesson, complains he cannot see, and repeats the complaint so frequently, especially by candlelight, that his father or grandfather at last says, ' Try my glasses.' The child now sees perfectly, and night after night the loan of the glasses is required before his task can be finished." And yet he adds, " It would have been better had glasses been selected of the longest focusj which would have enabled the child to read, or, better still, he had been put to bed, and the lesson left till daylight." There is no doubt that Mackenzie gave far too weak glasses, and therefore he concludes with the unsatisfactory words : " In some instances the state of asthenopia is so very easily excited, that the patient is never able to apply himself to any trade requiring the ordinary use of sight. These facts are sufficient to show the serious nature of asthenopia. It is an infirmity much more to be dreaded than many disorders of the USE OF GLASSES. 277 eye which, to superficial observation, present a far more formidable appearance." It is a great satisfaction to be able to say that asthenopia need now no longer be an inconvenience to any one. In this we have an example, by what trifling means science sometimes obtains a triumph, blessing thousands in its results. The discovery of the simple fact that asthenopia is dependent on the hypermetropic structure of the eye, pointed out the way in which it was to be obviated. Could it be otherwise than that, with the correction of the hypermetropia by means of convex glasses, its resulting asthenopia must also disappear ? But here, too', science might mislead one. Por it might appear rational to determine the degree of the total hypermetropia, and to neutralise this entirely, and at first I proposed this method. But I had overlooked the fact, as experience soon taught me, that even in the use of completely neutralising convex glasses, the hypermetropia remained in part latent, in consequence of which not only was the vision of distant objects with these glasses indistinct, but also, with moderate convergence, the eye was stiU always accommodated for a too near point. The unpleasant sensation connected with the re- laxation to the minimum of the relative range of accommodation, whereby in this case also vision at an ordinary distance was not yet acute, caused the object to be brought closer and closer to the eye, and thus we saw the patient fall from Scylla into Charybdis — that is, from accommodative into muscular asthenopia. My error lay in supposing a neutralised hypermetropic to be equal to an em- metropic eye, which, as I have above (p. 123) shown, is by no means the case. Thus it is in general : science theorises ; practice tests and rectifies her , hypotheses, and of this rectification science has, in order to restore herself, again to give account. In the establishment of the rules to which experience led me, I must distinguish between two series of cases : a, those which, with normal range of accommodation, are exclusively dependent on H. ; i, those where diminution of the range of accommodation, or want of energy, plays a more or less important part. The great majority of the cases belong to the first category : hy- permetropia is the cause, and, indeed, the only cause, why close vision cannot be maintained. It is self-evident that this form occurs only in youth, when, if the eye were emmetropic, the idea of the existence of presbyopia could not yet be entertained. Indeed, as I have already explained, a certain range of accommodation be- 278 TREATMENT OF HYPERMETROPIA. longs to the conditions of asthenopia. Many announce themselves as asthenopics at from the eighteenth to the twenty-sixth year of life, when - has descended to — or — • The majority of these are A 5 6 females. The cause of this appears to me to consist exclusively in the occupation. He who is not constantly occupied with close work acquires, even with a moderate degree of H, no symptoms of asthenopia. Now, in most female occupations, continued close vision is undoubtedly more required than in those of males ; and in certain ranks of society, where the man is almost wholly exempt from special tension of a:ccommodation, sewing and darning in the evening, and often by bad light, iaU, in addition to her household work, to the lot of the woman. In this alone, and by no means in the more frequent occurrence of H, I think we must seek the cause why the complaint of asthenopia is oftener heard from women than from men. The first thing we have to do in asthenopia is to determine the degree of the manifest hypermetropia — ^in other words, to examine what is the strongest convex glass with which the vision of distant objects becomes acute. This glass is, however, seldom sufficient completely to guard against fatigue in close work. The patient may, nevertheless, be allowed to read with it at our consultation ; but it is only when he can do this without any inconvenience, and when, moreover, with these glasses the binocular nearest point jOj lies but a little farther from the eye than in the emmetropic organ at the same age, we can determine in favour of commencing their use. But we often find immediate indication to give- him somewhat stronger glasses, for example, of ^o ^th Hm = 57;: in very young persons, in whom we may expect much HI, and, moreover, in those who are somewhat more advanced, for example, at thirty years of age, when the range of accommodation has undergone much diminution, glasses which correct only the Hm are scarcely ever sufficient. If the patient can easily return, the glasses thought suitable are given him, without any further direction than never to work without the spectacles, every half hour to interrupt his work for some minutes, to avoid excessive fatigue, and in about eight days to bring a report of how he finds himself. Almost invariably he re, turns with expressions of satisfaction and gratitude. He now ob- USE OF GLASSES. 279 tains permission to use his eyes at his own discretion, and is re- quested to return, if he feels any inconvenience — ^in any case after the lapse of a year or two, when probably the Hm shall have somewhat increased, and the use of stronger spectacles shall have become ad- visable. But it may also appear that the glasses were either too weak or too strong. If too weak, the asthenopia has not been en- tirely removed ; and we now often find, especially on some fatigue, a rather higher degree of Hm than before : we also find that p^ defined with the glasses, is further from the eye than it ought to be ; — of course, we must then go up, often even above the Hm now ascertained, and bring the jOs nearly to the normal distance. If the glasses were too strong, the work had to be held too near the eye ; acute vision was, indeed, thus attained, but a peculiar sensa- tion of fatigue ensued (muscular asthenopia). This complaint, al- though rare, occurs sometimes, even when only the Hm has been neutralised : the explanation of this is found in the too powerful tension of accommodation excited each time, and depending upon habit, precisely at a convergence of from 10" to 14". Under such circumstances, we must really ' begin with weaker glasses, and pass over to stronger ones in proportion as the extraordinary tension ceases. In the other cases, the holding the book too near is com- bined with the complaint, that with the spectacles distant vision is so particularly bad. I may say that of late ye^rs it has seldom oc- curred to me, that the glasses chosen with attention to the above rules have not completely answered their purpose. I very seldom, also, find it necessary in asthenopia to have recourse to the employment of mydriatics for the determination of H. It is only in those cases where the asthenopia justifies us in supposing the existence of H, and where, nevertheless, no Hm is observed, not even on moderate fatigue, — further, when the spectacles fixed on and modified on good principles, do not answer, — ^perhaps, also, when we can see the patient only once, that the employment of a mydiiatic is required in the interest of the patient. But it is, however, perfectly justifiable for the sake of the more accurate study of the connexion between Hm and HI, the more so because, during mydriasis, if necessary, close work may be very well performed with stronger spectacles. If we now know Hm and HI, we give glasses which neutralise Hm and about J of HI : in general they will answer the purpose either immediately, or after a few weeks. 280 TREATMENT OF HYPERMETROPIA. Hitherto, as we have seen, ophthalmologists endeavoured, by the use of progressively weaker glasses to obtain a radical cure of asthenopia. With the knowledge of the cause of asthenopia this endeavour is completely reversed. "We wish gradually to give stronger glasses, and therefore we investigate from time to time in our asthenopia patients, the position of the relative region of accommodation, or perhaps only the Hm, and if the latter appears to be increased, we give according to the rules laid down, other and stronger glasses. It is not untU the H has almost completely given way to Hm, and the relative region of accommodation has on the whole acquired a normal position, that we have, in the use of the glasses so indicated, a decided guarantee against the return of the asthenopia, and in young persons the same glasses are now adapted for distance and for near objects. With this strengthening of the glasses laymen have some- times made known their fears to me that, at last, no spectacles should be found strong enough to accommodate their eyes. To persons in their situation this fear would certainly appear to be justified. We may, however, quite set them at ease on this point. I usually take the trouble to explain to them, that if once the deficient power of the eye shall be entirely made good, the limit for the strength of the glasses is provisionally attained, and that what the senile changes at a more advanced period of life in general demand, can still very easily be added. It is evident that even under the most iinfavourable cir- cumstances, we cannot easily be obliged to rise to such strong glasses as are necessary in aphakia. To these unfavourable circumstances belong amblyopia, and especially deficient range of accommodation. What has already been said on these points- in speaking of Pr, may be here also the guide of our treatment. Eules for the management of the combination of H with Pr are there given. I have in this place only to mention, that in amblyopia of one eye, in consequence of exclu- sion, separate practice of this eye with a reading or magnifying glass several times daily, for some minutes at a time, is indicated. So much for the use of spectacles for near objects. It is now an important question, whether asthenopics should also in ordinary Hfe wear glasses for distant objects. On a superficial view there appears to be no objection to such a plan. Why should a person not remove the Hm, and thus make the acute vision of distant objects, without extraordinary tension, possible ? Undoubtedly, if we could make the glass an integral part of the eye, we should have no reason \ USE OF GLASSES. 281 to hesitate. We should then even be justified in neutralising almost the whole H^ convinced that the relative region of accommo- dation would soon adapt itself to the new refractive condition. But this is not the case : the glasses do not stand in the eye, but before the eye, and are sometimes not even at hand. Hence it is of great ^consequence, that the patient should be able to distinguish tolerably well also without glasses, and it is certain that if the hypermetropic individual accustoms himself to wear correcting spectacles, he will gradually lose the power of distinguishing without glasses. That is the unfavourable side of the question of wearing spectacles. Even in employing spectacles for near objects he loses in part the advantageous use of his accommodation : but in this case an actual necessity is in question, and he has, therefore, no choice. Not so with respect to looking at distant objects. With repeated strong tension of accommodation the power of vision, as we have seen, remains undisturbed in hypermetropic individuals. Prom the tension necessary to see distinctly without spectacles at a distance, which is each time required only as for a moment, no injurious effect is certainly to be expected. Therefore this may safely be required of the accom- modation. Consequently, when H is still wholly facultative, when the persons can even say to us : ■" in ordinary life, I have no incon- venience, and at a distance I see excellently,'"' — we should not press spectacles on them to be worn constantly. At most we may say to them, that, when they have become somewhat older (when the facultative H shall have given place to relatively absolute) they will derive great advantage from the use of spectacles for distance also ; they can then apply to the oculist, so soon as they observe that they no longer distinguish satisfactorily at a distance.* But it is quite ano- * Last year I had the good fortune to meet an English gentleman, eminent in the scientific world. I observed that he saw with spectacles of ^ at 11" distance, and concluded therefore that he was hypermetropic. "You do not see well at a distance," I remarked to him. "O no! "replied the able and vigorous veteran, " I no longer recognise the characters of minerals, as I formerly did, at a great distance, and if I wish to look at them before my feet through my spectacles, I stumble over them." " Go," I rejoined, " and ask the optician for glasses of ^-." On the following day he wrote to me: " I cannot tell you how grateful I am for the new sense you have given me. I now see the eyes of the handsome girls, and the wrinkles of the old ladies as well as when I was a young soldier." 282 TREATMENT OF HYPERMETROPIA. ther matter, when, even in youth, relative or absolute H exists ; then, notwithstanding every effort of accommodation, distant vision is not acute, and we need not now hesitate to assist it with glasses. The best result attainable, under such circumstances, consists in this : that the same spectacles, which are not too strong for distance, should be sufficient for ordinary close work. With the usual range of accom- modation this result is obtained in spite of the disadvantages connected with the use of convex glasses (compare p. 145), so soon as the relative region of accommodation has sufficiently shifted, and ' therefore the indication here is, by causing the patient always to wear spectacles (at first weaker ones) to promote this change of place. With respect to the second category of asthenopia, I have here but little to say. The cases contained in it are characterised by the fact, that the accommodation itself is disturbed or morbidly di- minished, and the proper place to consider them is therefore where in the Second Part I shall have to speak of the anomalies of accom- modation. Here it may be in general remarked, that, when in asthenopia either no, or comparatively very slight, H exists, and where, moreover, muscular asthenopia is excluded, we may suspect the presence of disturbed or morbidly diminished accommodation. To this point our investigation must then be directed. If general weakness is exclusively the cause, recovery from the asthenopia is to be expected, if we succeed in restoring the strength. If paresis of accommodation exists, without H, the asthenopia of course gives way when the paresis is removed. In either case convex glasses are meanwhile useful. Where painful accommodation exists a special treatment is required. What I have to say with respect to the treatment of H in general, apart from asthenopia, is included in the foregoing. As to the slightest degrees of H, which scarcely produce asthenopia, they require merely, as has been mentioned while on the subject of presbyopia, that the use of spectacles be permitted some years earlier than usual. The most extreme degrees are in their symptoms characterised by the fact, that very near the eye, the same type is read comparatively better than at a distance. The expla- nation of this apparent paradox has already been given. Since at a distance also vision is not acute, the suspicion of a complication of myopia with amblyopia readily suggests itself. Moreover, the aggregate of the symptoms of asthenopia does not appear distinct : from the beginning the difficulty is there, and results of fatigue PROGNOSIS. 283 do not so readily present themselves. And yet in this case relief is also above all to be sought specially in the use of convex glasses. They are necessary not only for the easy performance of close work, they are desirable also for seeing at a distance. Here we need not be afraid of obtaining, ,by their means, a less desirable displacement of the relative range of accommodation (compare p. 125). The strength of the requisite glasses is deduced from the degree of Hm and fl. Por ordinary wear the glasses must completely neutralise Hm. In reading the spectacles fall somewhat on the nose, and frequently the same glasses are now also sufficient for that purpose ; if they are not, we give some a little stronger, so that p^ comes to lie within the desired distance of distinct vision. Accordingly as Hm increases, we strengthen the glasses. Where complication with presbyopia exists, the use of two pairs of spectacles, one to wear generally, the other for close wort, is indispensable. When strong glasses are necessary, the preference is to be given, among other reasons, for the more ad- vantageous position of the principal points, to periscopic glasses, whose concave surface must be turned towards the eye. Are we in H to hope for a radical cure ? The answer must be in the negative. A priori, we should think, that, as the emmetropic eye may become myopic, and as myopia may be progressive, H might give place to E and even to M. In fact i^ appears possible that H of the as yet undeveloped eye might disappear during the years of development. But if the development have once taken place, I have never seen H give way, in the healthy state I have never seen it pass into E or M : this occurs only where there is increasing convexity from disease of the cornea. On superficial examination one may be deceived in this respect. A gentleman, aged 54, had been obliged to use convex glasses so early as in his 36th year, and now he preferred working without spectacles. With S = 1 he had M =^r?- Had H in this case not given way to M? 15. I found that his power of accommodation was completely paralysed, and that it had been so from his 36th year, and ophthalmoscopic in- vestigation indicated progressive M. It therefore readily suggested itself that at the age of 36 the paralysis of accommodation, with M = — o'^ Hn li^d rendered convex glasses necessary, which through the progress of M had become superfluous, and indeed 284 TREATMENT OF HYPERMETROPIA. inapplicable. Hence it is evident that the eye^ when of hypermetropic structure, has no tendency to M. And if a modification in this direction does not sometimes spontaneously occur, it is not to be expected that art should in such a case be able to do anything. How could we, without bringing the eye into danger, endeavour with any force to make the cornea more convex or the visual axis longer ? The cure of ordinary asthenopia by division of the muscles belongs to the fables of the period of operative mania, to which I have in the course of my historical remarks more than once alluded. We understand that tenotomy of the mm. recti interni interferes with the convergence, and that therefore with certain degrees of the latter in binociilar vision more tension of accommpdation can be associated. This tenotomy acts in the same manner as prismatic glasses with the refracting angles turned in- wards, from which asthepopic persons in fact derive sojne advan- tage. In two instances, where the action of the recti extemi was evidently very weak, von Graefe has* also actually put in practice tenotomy of the interni, He adds, however, correctly, that this method, contrasted with the simple choice of spectacles, is more interesting than practical. And in truth the hypermetropia cannot be diminished either by this or by any other operative method, and the asthenopia will not be permanently removed. I may add that when in developed strabismus convergens correcting tenotomy is performed, the use of the convex glasses is stiH often necessary, to prevent asthenopia and relapse of the strabismus. It is only where the asthenopia depends on insufficienpy of mijscles, that the operative method can have its triumphs. Some cases, which I shall now in conclusion relate, will afford me the opportunity of introducing a few additional practical hints. H does not always cause disturbance, and correction is then unnecessary. I. An elegant lady, aged 22, is under treatment for slight granulations. At a distance her vision is acute, with liegative glasses it is not so good with -^^ and indeed with ^ it is as acute as without glasses, but un- pleasantly large : " men are like giants." There was consequently Hm = — 1 ^^' and we may probably infer Ht = j^ Nevertheless she had experienced no * Archivf. Ophthalmologie, B. viii. Abth. 2, p. 321. ILLUSTRATIVE CASES. 285 kind of inconvenience. But read "she did not much," and of work she "did nothing." Had she heen obliged to do much close work, asthenopia would not have heen absent. Indeed, she now remembered that she could not see the finest things so accurately as other ladies of her age, and that she had even, in gazing at distant objects, sometimes observed a dimness. She had a horror of spectacles. Perhaps, being weU. able to comprehend this, I ought not to have predicted, that with her 30th year she would have need of them ; the recommendation to call in advice, whenever she should not see near objects well, would have been sufficient. Indeed for the moment there was no indication for spectacles, which would only have produced an, as yet, undesirable displacement of the relative range of accommodation. On account of the existing H, stricter rest had to be enjoined, so long as the granulations lasted, than is otherwise necessary. We learn to distinguish, at first sight, an ordinary case of asthenopia, the result of H. II. Miss H., aged 19, is announced. She has a florid look, has clear eyes, without a trace of disease, blue iris, mobile pupil, not a very deep globe, flat margins to the socket, the visual axes appear to diverge. I suspect asthenopia. I make her read and bring the book to 6' : reading becomes difficult ; at 5" it is impossible. There is either H or diminished -r- My eye falls on those about her, I see a brother with converging strabismus. This was decisive in favour of H. " Tou cannot persevere with your work." She answers : " No." " On exertion you get a feeling of tension over the eyes, press upon the part with the hand, rub over the closed eyes, and then it passes ofi', but only for a short time ? " " Precisely," is the answer. Confidence is gained. " Tou have no pain in the eyes ? " " At a distance you see well ? " " Yes." " After a long rest you can continue your work better ? " " Yes, yes." — With — she distinguishes well at a distance, and moving the glasses at the first moment not so well ; with r^ not so acutely as with =-3, wither not more acutely; between the two eyes there is little difference. Ophthalmoscopically all is well. I learn further, that for some years the inconvenience felt in working has been always increasing ; that formerly when weakened by fever, she could for a time neither read nor sew ; that she once tried a pair of spectacles, but was strongly cautioned against wearing them, &c., &c. She gets spectacles of ^^ to work with, with a recommendation now and then to pause for a little, and at first not to do much in the evening. At the end of a week she has forgotten her ailment. She now works too with less trouble occasionally for a short time without spectacles, which I advise her to do, though with the recommendation to return to the use of the spectacles on the least trace, or rather before the occurrence of fatigue. 286 TREATMENT OF HYPERMETROPIA. Asthenopics have sometimes a sad past, and live in a gloomy fature. III. The Rev. G. D., aged 52, looks dejected. " My good Professor," lie says, " I come to you, for I feel that I am getting blind ! " Por the last twenty years he has thought that within a year he should be blind ; and, singularly enough, although he still sees, he continues to look upon every year as the last ! Such is the man ! His life has been a struggle with his eyes. Even as a child he read with difficulty. When a student, the least exertion fatigued him, and he was compelled to learn more by hearing than by his own study. As a preacher, he has been obliged to write his sermons in a rather large hand, and still to get them off by heart. And, what was the worst part of it, he never read nor worked without the idea that he was thus hastening his blindness — interfering with the concentration of his mind upon any definite object. The same fear of blindness had restrained him from a matrimonial alliance with which he believed his happiness for life to be connected. He trusted in art. He had faith in a person he con- sulted in Germany ; and if the optician had sometimes given him spectacles which had brought him relief, these were mercilessly taken from him again by the oculist on the first consultation, as a treacherous instrument which must, in the end, inflict upon him the total loss of his sight. At last he had inhisfortiethyear, got convex glasses of 27,, and he now uses — . " Do you see with these spectacles at a distance ? " was my first inquiry. " Some- thing better," he replied, " but still very imperfectly." I tried ^: "Much better" was his verdict ; — subsequently I gave him ■^: " Still better." In o a word, there was H = ^ ^^^^ ^ = oq' ^^^' "^^^ ^^® slight range of accommodation, he needed glasses of ^, in order to make reading at the distance of a foot easy. He got. ^ to wear. The man was grateful as a child. He left me as one saved from destruction.— Such victims of the prejudice against the use of convex glasses are not uncommon. Where H exists, paralysis of accommodation may give rise to disquieting symptoms. IV. E K., a boy of ten years son of Dr. K., remarks in the morning that he IS not in a condition to read. His father sees that the pupils are rathei large and are immovable. Paralysis of accommodation occurs to him • but at adistance also, the hoy cannot properly distinguish objects: "there must eonse quently be a lurking affection of the optic nerve or of the brain." He brings his sou to me. I establish the fact of paralysis of the sphincter of the pupU ILLUSTRATIVE CASES. 287 ia both eyes. Neither on convergence, nor on the incidence of strong light, does contraction of the pupil arise : accommodative and reflex movements are both absent. The inference that there is paralysis of accommodation is thus justified. Why cannot the boy see even at a distance ? A glance ■with the ophthalmoscope clears all up : it appears I must accommodate about j2 in. order to see in the uninverted image the fundus oculi ; and, as I am emme- tropic, our boy has therefore H of about ..^ With i^^ he then saw admira- lty. lA blv at a distance ; with 3- he read at the distance of a foot. All fear of D an afifeotion of the system of the optic nerve was gone. In speaking of the anomalies of accommodation, I shall revert to such cases. Here it may suffice to observe that within four weeks the paralysis had given way, the E had again gradually become for the most part latent, and that in what the boy had to do or to read, he now no longer complained even of fatigue. In a few years asthenopia may be expected, and the use of the convex spec- tacles while working will then be indicated. Paresis of accommodation in young persons is scarcely distingui stable from asthenopia through H. T. H. J., a boy aged 14, is brought to me, complaining that for some time he has been unable to read. He looks pale and weakly. I suspect asthenopia, whether in consequence of a slight degree of H, with peculiarly debilitating causes, or in consequence of paresis of accommodation. The pupils move well. " Do you feel weak ? " " Yes, I have not yet recovered my strength after an attack of sore-throat." The articulation of words is imperfect, he speaks through his nose, and the soft explosive consonants (J, d, and ff) are, especially at the end of the words, pronounced as corresponding nasals (m, n, ng) (paresis of the palate). These symptoms are charac- teristic as the result of angina diphtheritica (better, diphfherina). I therefore infer the existence of paresis, notwithstanding the movable pupils. At a distance S is = 1, and neither convex nor concave glasses are borne : consequently we have to deal with E. The nearest point lies, instead of at 3", at 9', and can be maintained there only for a moment, as by spasmodic tension. The statement of the case is more accurately for- mulated as : Reading can be maintained only for a moment, vision at a dis- tance is excellent. From ordinary asthenopia, in consequence of H, the condition is distinguished by its rapid appearance (N.B., about a fortnight after the symptoms of angina had given way), by the easy permanent vision at a distance (this was not observed by the patient, but was found on examination), and by the almost immediate occurrence of fatigue and of absolute impossibility of seeing near objects. Muscular asthenopia may be connected with H. VI. P. C, aged 20 years, comes with the ordinary complaint of asthe- 288 TREATMENT OF HYPERMETROPIA. nopia, wliich. has existed from youth. Hm -^ is established, and H 24 given to read, wherewith p comes to lie at 6". At the end of a week the patient comes again ; he can persevere somewhat, but not much, longer. Paralysis by atropia discloses H = -i . Glasses of ^-s^are permitted, with which, after the paralysis of accommodation has ceased, every distant object is in a mist, and the patient reads by preference at only 8" distance, notwithstanding that 8=1; moreover, the unpleasant tension in the eyes and in the forehead becomes greater than before. InsuflB.cienoy of the muscles now suggests itself. The movements are free in every direction ; the convergence, on the contrary, is, on the approach of the object, maintained only at 5", and behind the hand, the one eye deviates much more rapidly outwards ; on the contrary, there is at a distance single vision, when a prism of 10° is held before the eye with the angle outwards, for which a. divergence of the visual lines of about 5° is required (compare p. 132.) Evidently, therefore, the mm. recti interni are insufficient. Glasses of np*, placed very close to one another, make the convergence required somewhat less, but render no assistance. Permanent help is derived only from spherico-prismatio glasses (j^ with a prism of 5°). In this case we should have been justified in cutting through, in one of the eyes at least, the tendon of the m. rectus externus ; the con- vergence wotild have become easier, and yet even at adistance there would not have been double vision. " Shall I then be able to read without spectacles? " I was obliged to give a negative answer. Indeed, when the convergence became easier, less tension of accommodation could be associated therewith, and the existing H would therefore, still more than before, give rise to asthenopia. Therefore, too, in the combination with a prism, the convex glass must be rather strong (j^V My answer made the patient shrink from the operation. The almost perfectly latent existence of the H was con- nected with the insufficiency of the mm. interni ; the prism alone, without combination with a spherical glass, made a greater portion manifest. Strong H in a child has hitherto been almost invariably regarded as M. VII. A girl, aged six years, is said to have very weak eyes. If she wishes to see anything, she runs to a bright light, and holds the object directly before her eyes. Her anxious parents had taken much advice respecting her ; the child was generally considered to be near-sighted. The fact that she so particularly looks for bright light, in order to see any- thing well, makes me doubt the correctness of this opinion : in that case considerable amblyopia would necessarily be combined with the myopia. ILLUSTRATIVE CASES. 289 From the external appearance, I had scarcely a doubt of the existence of H. At the distance of 3' the child saw, with her head aslant and her eye- lids nearly closed, No. Ill of Snellen ; smaller letters she did not see ; with— on the contrary, she saw II at 8": so the proof was supplied. S was = -. Probably astigmatism exists, to be more fully investigated at a somewhat later period, when the psttieiit shall be able to give a more accurate account of herself. Meanwhile, she may provisionally use - in 6 learning, — if she chooses she inay also wear them habitually. " This she would rather not do.'' Even with a slight degree of H, tension of accommodation may be painful. Tin. Mr. X., a lithographer, aged twentyrone, has, until some months ago, been able without trouble to perform even his work. Now and then, it is true, he got a pain in his eyes, but that he attributed to excessive exertion. Of late, however, the pains are so frequent, and rapidly become so violent, that he is obliged to forego his work. Above the eyes he feels at most a slight pressure ; in the eyes themselves the pain settles, and is at the same time stinging and oppressive. Soon after leaving off work the pain always ceases, and if he refrains from exertion, and from fixing his sight strongly, it does not return. Objectively no morbid change is perceptible in the eyes. The movements, too, are normal; 4ivergence with prisms held before the eyes is impossible. Ejsamination indicates only Hm == oo- After artificial mvdriasis we obtain Hm ^ -— . Glasses of _- and of ,-- are of no use. In- ^ 24 36 24 deed, at the patient's time of life, and with his range of accommodation = 1 : 4-3, he would easily have overcome the existing II. Former cases had taught me, that all medication in this instance would be useless, except daily repeated paralysis by atropia. To this I had in the present case immediate recourse, at the same time prescribing glasses of sg- for distant, and of ^ for near objects. The pain forthwith ceased. — This case belongs properly to the anomalies of accommodatioji, to which I shall revert more at length. I have communicated it here, because I have observed such instances of pain- ful spasm, on every tension of accommodation, only where H exists. It is true they are rare : altogether I have seen but three. We must beware of mistaking apparent for true asthenopia. IX. Mrs. N., aged thirty-three years, a nervous, weak little person, complains that she cannot continue her work. She soon becomes tired and suffers pain, 19 29 TREATMENT OF HYPERMETROPIA. the eye begins to weep, and she cannot resume her occupation during the entire day. In the evening especially she is obliged strictly to avoid all ex- ertion ; occasionally, too, some photophobia is present. I make her read : she holds the book at about 10', and says she can still distinguish accurately when I bring it 5' nearer. Already I suspect that no H exists. At a dis- tance S appeared only = -^ ; but while positive glasses diminish still more the aouteness of vision, S becomes = 1, with the use of — =-»• there is con- o6 sequently M = _-. Closer investigation of her case shows, that it differs in many respects from asthenopia by H. She has pain in the eyes them- selves, which, properly speaking, always continues, and only increases on exertion ; the characteristic tension above the eyebrows is, on the con- trary, absent ; moreover, to the last moment she sees acutely, and it is only the pain which makes her give up work. With these symptom's there is now a slight irritation of the eyes persistently present. Ophthalmoscopic investigation reveals capillary hypersemia of the optic nerve. Nothing more is to be seen. Such oases are not uncommon ; they occur mostly with myopia, but they are also met with in other eyes. It is a not well explained form of hypersesthesia, in connexion with symptoms of congestion. Blue glasses, resting the eyes, stimulating derivatives, &c., are only too often tried in vain. To refer such oases to asthenopia is to call two conditions widely different both in essence and in symptoms, by the same name. Absolute H of the highest degree simulating M with amblyopia. (Case kindly communicated by Mr. Bowman to the author, rtjompare p. 258.) Mr. T., twenty-five years of age, has very small globes — as far as can be judged they would appear of this size on a horizontal section (Fig. 121 a). Fig. 120 represents about the size of cornea and pupil. The antero-posterior diameter is too short, but so also are the transverse and vertical, so that the eye does not look too fiat on a lateral view. The anterior chamber is shallow (the iris near the cornea) — the pupil has this range under varying liglit (Q). As to the presence of the lens, the patient was unwilling to have the pupil expanded by atro- pia ; but I perfectly satisfied myself by the catoptric test that the lenses are present— the reflexion from the front being very distinct. The convex form of the iris and the prominence of the pupil are also evidently due to the iris being thrown forwards by the lens. When I first saw Mr. T., in 1856, he was at College, and had distinguished himself by scholarship, but his sight had become STRABISMUS CONVERGENS. 291 much fatigued, and he accordingly consulted me. He was using for the right eye a convex glass of ^ for distant vision, and when he held this glass at 2 or 3 inches in front of the eye, he could read a book held two feet off. Without a glass he could read the smallest print (diamond = Jaeger No. 1) at about f of an inch from the eye. He had up to this time always used the right eye, and attempts were made to bring the left eye into use, but without success as regards comfort, though the vision of that eye improved under them. In the summer of 1857 he had come to use the right eye in reading, with- out a glass, and holding the print very close. He could thus read with tolerable comfort. In May, 1862, he had been some time in the habit of employing strong convex glasses, viz., — for distant vision, being then able to see Jaeger, No. 18, at eight feet; and — for reading, being then able to see Jaeger, No. 2, ^ 4 at 4", with comfort, unless in a strong light. § 24. Steabismxjs conveegenS; the Eestjlt oe H. Strabismus is a deviation in tlie direction of the ejeSj in conse- quence of which the two yellow spots receive images from different objects. In strabismus the visual lines do not cross one another in the point it is desired to observe ; only one of the two, that of the undeviating eye, is directed to that point. Under this deviation not only does the expression of the face suffer from the want of sym- metry in its most eloquent parts, but the power of vision, at least in one of the eyes, is usually disturbed, and the squinter always loses the advantage of binocular vision. Strabismus is not an independent morbid condition ; as is com- prised in the definition given, it is only a symptom. We may add, that it is a symptom, dependent on very different conditions, and as such connected with other very different phenomena. He who pro- poses to write a manual, and in it to treat systematically of all defects of the eye, will more than once meet with strabismus, as a more or less constant result of definite conditions. It will repeatedly occur as a constituent of a compound anomaly, in which it is con- nected with the cause on which it depended, and with all the results of that cause. But there will be no room for it as an independent 2 292 STRABISMUS CONVERGENS. form of disease. It is only in a book on semeiotics that we have to treat of strabismus in general. If this view has been long received, writers have not been faithful to it. To ascertain this, it is sufficient to consult the manuals. A special chapter is devoted to strabismus. In this everything which has reference to this deviation is treated of; elsewhere strabismus is only incidentally mentioned. Even in the investigation of its causes, all forms, different as they may fundamentally be, are treated of alike. In the monographs the case is sometimes no better. Is it, then, strange that the pathogeny of strabismus is still so very obsqure ? It is an attribute of human nature to suppose for each phenomenon which occurs an external cause^ and readily to assume as such the first which presents itself. Prom this rashness and credulity pathology has not entirely freed itself. With respect to strabismus, the cause is, on the authority of mothers and nurses, often sought in all kinds of accidental circumstances, and thus the source of the abnor- mity, which originally consisted in the form of the eye, is over- looked. We now know that by far the greater number of cases of strabis- mus are connected with anomalies of refraction. According to the direction of the deviation, two forms of oblique vision are specially to be distinguished : strabismus convergens and strabismus divergens. The main result of our investigation may be expressed in these two propositions . — 1. Strabismus convergens almost always depends wpon Jiyper- metropia. 2. Straiismus divergens is nsually the result of myopia. We have here to treat only of strabismus convergens. Experience, in the first place, shows, that strabismus convergens is, in the great majority of cases, combined with H. In 172 cases investigated by us, H was 133 times proved to exist in the undevi- ated eye. In nine cases myopia existed, five times to such a great degree that the form of the distended, but little movable, eyeball, admitted of no other condition ; in thirteen cases difference of refrac- tion of the two eyes was recorded ; five times inflammation was the cause ; at least five times paralysis had gone before ; three times there was complication with congenital cataract, twice with nystagmus. — It is evident how greatly H preponderates : it occurs in about 77 per cent, of the cases. And yet I am convinced, that if we could investigate without distinction all cases of strabismus con- RELATION TO HYPERMETROPIA, 293 vergens which occur in a given population, H would be met with relatively still more frequently. In the first' place, ordinary cases of strabismus convergens are seldom brought to the oculist, and these are precisely the cases in which H is the only cause : if in- flammation, paralysis, or any special complications be present, the patients do not delay to call in help ; and thus, in proportion to the whole, a greater number of these exceptional cases comes to be seen. Moreover, cases are included, which it is not usual to refer to the head of strabismus convergens, as tolerably recent instances of paralysis of the abducent muscle; too strongly con- vergent, nearly immovable myopic eyes, &c. And, finally, some ordinary certainly rather than extraordinary cases, whose pathogeny was not wholly cleared up, have been neglected. — I therefore do not hesitate to declare, that it is exceptional, to find strabismus convergens without hypermetropia. In general, it is not the highest degrees of H with which stra- bismus is combined. Often even, at least in young persons, the hypermetropia is completely latent : it was involuntarily neutralised by tension of the power of accommodation, and appeared first on artificial paralysis of the accommodation. Where it was manifest, it amounted to from -— to ^tij rarely to - or more. The total hy- 30 10 -^ 7 ^ permetropia was, so soon as it was manifest, usually not examined, but of course attained, especially in young subjects, a considerably higher degree. With — manifest H more than — total H may in general be assumed; for where, with complete want of manifest, the total was defined under the infiuence of paralysis of accommodation, the latter was seldom under zr-- 15 Since in strabismus convergens H in general exists, no other con- nexion is conceivable than that H is the cause of the deviation. H is, indeed, the primary anomaly, to be sought in the structure of the eye, and originally proper to the organ; strabismus is the secondary condition, which does not arise until some years after birth. In the first period, in the commencement of the so-called periodical oblique vision, it can be proved that H already exists : unquestionably, therefore, it precedes the squinting. And if we add, that the incipient strabismus again gives way, when the hyper- metropia is neutralised by a convex glass, we readily infer that H 294 STRABISMUS CONVERGENS. may produce strabismus. The only question, tlierefore, is, how it can do this, and the answer is evident. The hypermetropic individual must, in order to see distinctly, accommodate comparatively strongly. This holds good for all distances. Even in looking at remote objects he must endeavour to overcome his hypermetropia by tension of accommo- dation, and in proportion as the object draws hear, he must still add so much accommodation, as the normal emmetropic eye should need on the whole. The vision of near objects therefore especially requires extraordinary tension. Now, as we have seen (compare p. 110), there exists a -certain connexion between accommodation and convergence of the visual lines : the more strongly we converge, the more powerfully can we bring our faculty of accommodation into action. A certain tendency to increased convergence, so soon as a person wishes to put his power of accommodation upon the stretch, is therefore unavoidable. This tendency exists in every hypermetropic person. An emmetropic person may also convince himself of this by holding negative glasses before his eyes, and thus bringing the latter temporarily into a condition of hypermetropia. He will dis- tinctly remark, that on the endeavour to see accurately, double images every time threaten to appear as the result of increased con- vergence, and that he soon has a choice only between indistinct vision and squint. Probably this conflict exists unconsciously in the case of all hypermetropic persons. Hypermetropia is a very widely spread anomaly. I am convinced that it occurs still more frequently than myopia. IJow, if strabismus convergens is in general the result of hypermetropia, the latter evidently is Very often inet with without strabismus; we may even say, that only in a comparatively small number of cases of hyper- metropia is strabismus developed. This, however, need not by any means surprise us. - In general, in fact, the necessity of seeing an object single with both eyes together, is deeply felt. The direction of the visual lines is thereby forcibly determined. Of this I con- vinced myself, many years ago, in my experiments on the action of prismatic glasses.* If we bring a weakly prismatic glass, with the refracting edge turned inwards, before one of the eyes, the fixed object is directly seen double, but increased convergence is imme- diately involuntarily produced, which makes the double images • Nederlandsohe Lancet, 2° Ser., D. iii. p. 233. 1845. COOPERATIVE CAUSES. 295 coalesce ; and if, some moments later, we again remove the glass, double images for an instant reappear, which, however, equally rapidly disappear, in consequence of lessening the convergence. Now, it is as if the double images, of their own accord, again coalesce : the movement made takes place so spontaneously, that the person is not even conscious of it. This abhorrence of double images, or rather the instinctive adherence to binocular vision, preserves most hyperme- tropic individuals from strabismus. They sacrifice the advantage of seeing accurately, rather than to allow that on the two yellow spots different objects should form their images. In this, therefore, we find the reason, why not nearly all hypermetropics squint. If one eye be covered with the hand, while it, as well as the otlier, is open, the visual line will, in most hypermetropics, rapidly deviate inwards. The same thing takes place when an emmetropic person holds a negative glass before the uncovered eye. The question which now suggests itself is : "What circumstances must cooperate to give rise to strabismus in hypermetropic indi- viduals ? These circumstances are of a twofold nature : «, those which di- minish the valtte of the binocular vision ; I, those which render the convergence easier. To the first class belong : 1°. Congenital difference in the accuracy of vision, or in the re- fractive condition of the two eyes. — In hypermetropia the accuracy of vision is often imperfect, whether in one or in both eyes. This is in part attributable to astigmatism, in part to a still unknown im- perfection of the retina. If the diminished accuracy of vision affects only one eye, then, on too great convergence, the image of this eye will not so much disturb vision. The same is the case when the degree of H in the deviating eye is greater, and the image in this eye is therefore less accurate. In either case, consequently, strabismus wiU more easily arise. But the tendency doubly increases when both circumstances, a higher degree of H and diminished accuracy of vision, as is often the case, occur combined in the same eye. If the eye has long been deviated, there arises a secondary diminution of the accuracy of vision, as a result of strabismus, to which I shall subsequently revert. In that case, however, we can, with the aid of the ophthalmoscope, often demonstrate a still higher degree of H of this eye. 296 STRABISMUS CONVERGENS. 2°. Spots on the cornea. — It is often remarked, that in oblique vision the deviated eye, or, indeed, both eyes, exhibit opacity, or spots on the cornea. Pagenstecher and Saemisch have recently called attention to the frequent occurrence of corneal spots in stra- bismus. It does not appear to me, however, that spots on the cornea should, in themselves, be capable of exciting strabismus. Although the image of the second eye is less perfect, experience shows that even then the preference is given to binocular vision ; nor is it explicable that one of the eyes should be inclined to de- viate, merely for the purpose of making a quite different, rather than, it is true, an unequal, but still corresponding, image fall upon the yellow spot. Ruete* has, upon good grounds, in this way decided the contest between Beer and Joh. Mueller. But it is quite a dif- ferent question whether, where hypermetropia exists, specks on the cornea and other obscurities might not increase the tejidency to strabismus ; whether the less accurate image in the visual axis might not make the image less disturbing, and diminish the abhorrence of an accessory second image. I am very much inclined to assume this. At least, I find specks on the cornea much more common in hyper- metropia with strabismus, than in hypermetropia without strabis- mus. It is true, there may be still another connexion between specks on the cornea and strabismus, to which Euetef has already directed attention : an inflammation^ namely, which produces these specks on the cornea, may extend beneath the conjunctiva to some of the muscles or their envelopes, and produce, first, a spasmodic, and afterwards a nutritive, contraction. Such cases I have already above mentioned. They are, however, comparatively rare ; but they may in part explain the preponderance of specks on the cornea in hypermetropia with strabismus. In the second place, as I have remarked, the origin of strabismus is promoted by circumstances which render convergence easier. Under this head are to be noted : 1°. Peculiar structure or innervation of the muscles ; easy mohility of the eyeballs inwards. — Not unfrequently a congenital insufficiency of the musculi recti interni occurs. It may readily be assumed that the opposite also may be the case ; and, in fact, some eyes converge without any particular tension up to 3", nay, even up to 2", and • Lehrh. der Ophthalmologie f. Aerzte und Studirende. B. ii. p. 520. Braunschweig, 1854. t^-c. p. 537. DIVERGENCE OF THE VISUAL AXES. 297 less, from the eye. We may assume tliat form and position of the eye-ball exercise as much influence in this respect as the structure or innervation of the muscles. Now, while in insufficiency of movement inwards we have a guarantee against strabismus convergens, free motion in this direction wiU increase the tendency to this form of oblique vision. By many the latter can, in a high degree^ be easily produced at will — by others, not at all, or only with great difficulty ; and when it is stated that such voluntary squinting, often produced for the sake of imi- tation, or of mockery^ has, with some, given rise to permanent stra- bismus, I readily admit it> but on condition that hypermetropia at the same time existed. Moreover, I have not been able to satisfy my- self that a special tendency to strabismus may be hereditary. Let me be understood. In a very high degree hypermetropia is here- ditary. It is a rare thing, with hypermetropic structure of the eyes in one of the parents, not to find hypermetropia also in some of the chil- dren. But whether this hypermetropia in the parents was combined with strabismus or not, has, if any^ certainly only slight influence in the development of strabismus in the hypermetropic children bom of them. If in a family one or two labour under strabismusconvergens, we may be nearly sure that in some other members hypermetropia will occur ; but that in the same family most of the hypermetropics should be affected with strabismus, has very rarely occurred to me. 2°. Relation between the visual line and the axis of the cornea. — We have above seen (p. 182) that in general in hypermetropic indivi- duals, in order to give a parallel direction to the visual lines, a more than ordinary divergence of the visual axes is required. Thence we have in so many hypermetropic persons apparent strabismus di- vergens. On the other hand, we know that most eyes can with difficulty be brought to a state of divergence: a weak prism, with the refracting edge held outwards before the eye, produces double images, which most people are not able, by divergence of the visual lines, to overcome. Even for the sake of single vision, many do not succeed in diverging some degrees more. It is therefore natural to assume, that when for single vision more than ordinary divergence of the corneal or visual axes is required, the divergence may very easily be insufficient, and that, accordingly, as a matter of course, for seeing at a shorter distance also, there may readily be too great convergence. What was treated of under Y 298 STRABISMUS CONVERGENS. facilitates convergence in an absolute manner. Comparatively, the relation between the visual Kne and the axis of the cornea has, in hypermetropics, the same result. Now, if, in looking at a distance, the requisite divergence of the axes of the cornea easily falls short, the convergence will likewise, under the influence of the hypermetropia, in looking at near objects, be relatively too great. The condition for the development of strabismus is thus given. In fact, it often seemed to me that in squiaters, after tenotomy, a considerable degree of divergence of the axes of the cornea was required to make the visual Hnes assume a parallel position; — often the eyes are appa- rently quite properly directed, and yet when, on fixing a remote point, one and the other eye are alternately covered with the hand, we observe that the eye just opened has each time to make an extensive movement outwards, to fix the remote point. Some- times this is to so great a degree the case, that for binocular vision, at a distance, a deformity by divergence would be re- quired. This leads me to suspect that while in general the great angle a promotes the occurrence of strabismus convergens with H, an extraordinary magnitude of this angle predisposes more particularly to this form of strabismus. In order to test this suspicion, the angle a was measured in ten cases of strabismus convergens. The measurements were in great part made by Mr. Hamer, now house-surgeon in our Ophthalmic Hospital, according to the method already described, with his usual accuracy. The results are presented in the subjoined Table I. MEASUREMENTS OF THE ANGLE a. TABLE I. 299 i Deviation. Refraction. M i Eye. a S. Observations. h bo j ' in tte meridian of maximum of curvature (usually the vertical). ?. ,, value not determined, or not accurately determined, or not to be determined. In No. 8, permanent strabismus' was demonstrated, which, singularly enough, on artificial mydriasis of the sharp-sighted right eye, temporarily 300 STRABISMUS CONVERaENS, disappeared : the left eye was now properly directed, vithout the right deviating in its turn. This Table again shows what has already been seen, that the angle a for both eyes of the same person is in general nearly similar. Therefore for No. 4 Od. and No. 9 Od., which on account of dimi- nished S did not fix, a was, on calculation of the average, assumed = a of the left eye. As an average we now obtained a = 7°.63. This but slightly exceeds a = 7°.3, previously found as the average in non-squinting hypermetropics ; but in order to make the influence on the position of the centre of motion appear strongly, particularly high degrees of H were designedly selected, in consequence of which a also increases. In order to have a better ground of comparison, the angle a was therefore determined also in some cases of H, in degree about equal to H of squinters. The results are comprised in the annexed Table II. TABLE ir. i Sex. Age. Kefractlon. u Eye. . u S. Ohserratlons. £ Hm.i H. 1 H.? 1 m. 19 Od. is 7 T? 4°.5 0.S5 As.? Os. 1 S9 Vs n 3°.6 0.85 As.? 2 f. 50 Od. ik ? t 5° ? Os. ik 'i 5" ? 3 m. 55 Od. Os. I T2 1 n 5° 5° 4 m. 60 Od. Os. 1 1 55 1 30 5°.7S 5°.5 1 0.95 5 f. 21 Od. O.S. TO 1 6''.25 5°.9 1 1 6 m. 9 Od. A f h 6°.5 ? Os. I ' J 6° ? 7 m. 14 Od. ^ h 1 s 7° 0.4 As. Os. ^ i h 6° 0.32 As. • 8 m. 62 Od. 1 5 1 \ 7° 0.25 Os. h 1 7 7° 0.25 9 f. 13 Od. in 8°.5 1 Os. 23 1 8''.75 1 10 m. 36 Od. A 1 TT 8°. 8 1 Os. 57 A 9°.2 1 11 m. 21 Od. i> 9° 0.9 Os. 1 ^ 9° 0.9 Column H ? gives the total Hypermetropia, reduced by calculation to the period of youth. EXTERNAL CAUSES. 301 NoWj in the first place, we find among these non-squinting hyper- metropics a on an average = 6°.56, that is, 1°.07 less than in squin- ters. In the second place, it appears further, that the degree of H has an influence upon a. Comparison with the average a = 7°.3, found in higher degrees of H, at once indicates this. But still more distinctly does it appear in Table II., in which the persons are arranged according to the magnitude of a, and the suspected degree of the total H, at 14 years of age, was calculated under H ? It immediately strikes the eye, that the latter keeps about equal pace with a. In estimating H ?, the ascertained Hm, and sometimes also H, were made use of, the time of life being borne in mind ; the estimate certainly deviates little from the truth. The result, therefore is, that, with equal degrees of H, high values of a especially predispose to strabismus convergens. To this result I attach the more importance, because it in general proves, that the greater angle a, proper to H, is not indifferent in its bearing on the connexion between H and strabismus convergens. In the highest degrees of H, strabismus is rarely observed. This need not surprise us. In such cases the power of accommodation is, even under abnormally increased convergence, not sufiicient to produce accurate images, and such hypermetropics are thus led rather to the practice of forming correct ideas from imperfect retinal images than of, by a maximum of tension, improving the retinal images as much as possible. We have already seen, that strabismus is met with chiefly in mean degrees of H. These belong to facultative and relative hypermetropia : the eye can adapt itself for parallel and even for diverging rays, and can moreover maintain this accommodation for some time, yet often only with convergence of the visual lines to a point, situated closer to the eye than the point whence the rays pro- ceed. The minimum of H, at which strabismus occurs, depends undoubtedly on the angle a, and on the range of accommodation : the less the latter, and the greater a is, the less degree of H will be sufficient. But diminished energy or paresis of accommodation by itself is as little liable to produce strabismus, as is the diminution of the range of accommodation connected with the increase of years. As to external causes, we often find mention made of the fixing of near, and particularly of laterally placed objects, as a feather of the cap, theflame of a candle, a toy or such like. From what hasbeensaid,it will have been seen that I attach but little influence to these things. At least I am convinced that tlie emmetropic eye will not in this way 302 STEA.BISMUS CONVERGENS. be led to squint. But I would not venture so unconditionally to assert, that, for the hypermetropic eye, no cause of strabismus might be found therein. Particularly the fixing of laterally placed objects, might have influence in this way. Por under these circumstances the fixed point can be seen only by one eye, since the field of vision of the other is limited by the nose ; and if only the one eye sees the object, the second eye wants the guide which directs its move- ments, and there is nothing to prevent too strong convergence, for the sake of distinct vision. It seems to me that there is no ground for denying, that in this manner the internal muscles of the eye might acquire a preponderance, which would promote the further development of strabismus. I have above remarked, that obUque vision differs in kind and form, according to its causes and to the nature of the affection, of which it is the result, and with which it occurs in one and the same morbid type. This is quite true of oblique vision, proceeding from hypermetropia. But as this is the most usual, the typical form of strabismus convergens, it is very natural that what is de- scribed as strabismus in general, should be applicable precisely to this form. I may be permitted to give a short sketch of it. In doing so I must advert to some well-known matters, but I shall thus best find opportunity to add what is still deficient respecting the nature, the symptoms, and the pathogeny of the affection. Converging strabismus, in consequence of H, we see to arise mostly about the 5th year, probably because the effort to see accu- rately then begins to be developed j the range of accommodation is now also sufficiently great, by means of somewhat increased con- vergence, easily to overcome the H. To reports of its occurrence at, or shortly after birth, in consequence of convulsions or of other diseases, no credit is in general to be given. Exceptionally it commences after the 7th, extremely rarely so late as the 18th year, unless special accessory causes exist. At first the deviation is transient, connected with fixing, that is with an effort to see ac- curately, sometimes only with the fixing of near objects : it passes off again when the fixing ceases or the eyes are closed. This is the so- called periodical squint, by some described as a distinct period. Even in this period, and when the strabismus is developed first in the 16th or 18th year, we very rarely hear a complaint of double vision. This is explained, in my opinion, by the fact, that the deviation arises PERIOD OF ITS OCCURRENCE. 303 only on the effort to see a given object aceurately. On that object the attention is fastened. To it the one visual line remains directed. Now the double image of it lies in the deviating eye at some distance from the yellow spot, and must therefore appear indistinct, so that beside the direct fixed one, it is not easily seen as a second image. And on the yeUow spot appears the image of a wholly different ob- ject, with which the observer is in general not occupied, and from which it is therefore easy to abstract. But when the deviation, before S is much diminished, occurs involuntarily, without a special effort to see accurately, there is occasionally some transient double vision. The periodical form of squint on looking at near objects, just described, sometimes continues as such. Stoeber* and Artlt have each described a remarkable instance of this nature, the cause of which was unknown to them. In most cases, however, the squint soon be- comes constant. The rule is, that invariably one and the same eye deviates (strabismus simplex) ; this held good even when the squint was still transient : when it occurs under the form of strabismus alternans, with H, other causes are often in operation. The squint is usually concomitant ; the movements are free ; the excursion normal although with excessive mobility inwards, limited outwards, in both eyes, even when the one constantly deviates, the other being steadUy properly directed ; this is found also to be the case when the squint is still periodical. Both the internal muscles of the eye are therefore to be considered as shortened. The shortening, at first dynamical, has in the constant strabismus become organic : it is the result of excessive action, with relaxation of the antagonistic muscles ; morbid structural change does not exist. That both internal muscles are shortened, depends upon the habit of keeping the fixed object to the side of the deviated eye, so that even in the non-deviated eye the musculus rectus internus is brought into relatively strong con- traction. In this position the H of the non-deviated eye is best overcome. Also when the strabismus has become constant, a com- paratively stronger tension of the internal recti muscles is connected with the fixing of an object, whereby the angle of squinting is in- creased — of course in a less degree where a great angle of deviation already exists, because increased tension is then attended with less motion : after tenotomy the increase of the convergence often again appears very well marked in these cases on fixing an object. This increase of convergence on fixing, when a correct position has been • Ann. d'Ocul, 1855, T. xxxiii. p. 177. t I. c. T. iii. p. 312. 304 STRABISMUS CONVERGENS. obtained by tenotomy, is important in a double point of view. In tbe first place, as it relates to adults, who state that they feel this con- vergence distinctly, and at the same time perceive that they produce it, as it were involuntarily, in order to see more accurately. More direct proof that H may lead to strabismus, and how it may do so, could certainly not be given. We have here, in a certain sense, a return to the first period, with this difference, that the deviation can now be observed by the person himself in its cause and in its signification, while, just as in the original first stage the commencement was to be obviated by means of convex glasses, which should neutralise the H, the relapse is now to be prevented on the same principle. — The practical indication, after tenotomy in such cases, to give convex glasses during work, furnishes the second point, in regard to which I 'called the phenomenon important. In strabismus simplex the acuteness of vision suflfers more and more in the deviated eye. At first, on bringing the hand before the fixing eye, the deviated eye directs itself properly to the object; even when the hand is taken away, the originally deviated eye may continue to fix, soon, however, usually when movement is required or even on the first winking of the lids, giving way to the other. The acuteness of vision in the deviated eye has then already di- minished, but it continues still for a considerable time satisfactory, may be recovered by practice, and improves almost always immedi- ately after tenotomy. After some time, however, on closing the fixing eye, the deviated eye usually directs its visual line no longer to the object ; the line passes to the inside, so that the retinal image of the object comes to He also on the inside of the retina. When this takes place, we may infer, that in the visual line and besides in the field of vision common to the two eyes, the acuteness of vision of the deviated eye is much diminished, while, on the contrary, that of the indirect vision, on the innermost part of the retina, in so far as it has its own field of vision and perceives objects, which are not re- presented on the retina of the other eye, has continued undiminished. It is again von Graefe, who has first accurately investigated this loss of physiological sensibility through psychical exclusion. And this is indeed a remarkable phenomenon ! That through attention we can sharpen our senses, is an admitted fact. How rapidly, on the other hand, a nerve may become blunted, from whose impressions we wish mentally to abstract ourselves, the case here described supplies an example important for physiology at large. Although no organic changes of the retina are to be observed, no improvement of any USE OF CONVEX GLASSES. 305 importance is to be obtainedj if fixing no longer occurs under any circumstances, either by practice or by tenotomy. But we must not too rapidly decide this point. There is a period in which the deviated eye will not see large objects otherwise than indirectly, and nevertheless, on using a convex glass, it will quickly look directly, that is, use the yellow spot, to recognise comparatively small objects, for example the letters of No. X and No. XX. In this period we may by practice and by tenotomy sometimes still attain a brilHant success, A word still as to the practical application of what has been said. I have already above stated, that so long as strabismus occurs only intermittingly with fixing of an object, its development may be pre- vented by wearing convex glasses, which neutralise the existing H. This I first observed in a young man, who in his 18th year began for the first time to squint in fixing. He had Hm = ^, After he had worn glasses of ^ for two days, he began to be no longer able, for the sake of seeing accurately, to make one eye deviate. He then saw also at a distance indistinctly, and it was not until half-an-hour after putting off the spectaples that he again succeeded in producing the squint, and thereby distinguishing accurately. By continuing to wear the spectacles, the squint ceased to be produced, and the tendency to it was completely lost. — If the squint sets in very early in life, wearing spectacles is of course attended with difficulties, and particularly when the patients are of the female sex, they are un- willing to be condemned to wear glasses during their whole life. In such cases I generally confine myself to advising them to look twice daily for some minutes> with the deviating eye alone,* which practice is sufficient to prevent the diminution of the acuteness and the limitation of the field of vision : at a later period, when the strabis- mus is confirmed, the operation of tenotomy is performed. Where the patient preferred obviating the strabismus by wearing spectacles, I wilhngly consented to it, and almost invariably the object was thus attained. Mooren,t too, h^s recently stated that, where a tolerably high degree of Hm existed, he has, in the fii'st stage of strabismus, with * Mydriasis, by atropia, of the eye which is ■usually properly directed immediately causes the other to be used, and is therefore sometimes recom- mended in the ease of young squinting children, on whom it is undesirable as yet to operate. t Klinische Monatshldtter f. Augenheilkunde, B. 1, H. 1, 1863. 20 306 STRABISMUS CONVERGENS. good result prescribed the use of convex glasses. In comparatively great degrees of H the prevention of strabismus is, in fact, more particularly desirable, because subsequently, even after full teno- tomy, the tendency to strabismus continues, and, in order to prevent a relapse, the use of convex glasses, at least for close work, is still necessary. Moreover, I have observed, that when hypermetropics with already confirmed strabismus, especiaUy after insufficient tenoto- my, regularly wear convex glasses, the degree of strabismus often diminishes so much, that the deformity is almost entirely removed. If we now inqtdre whether the cause of strabismus was ever before sought in hypermetropia, this question may be answered in the negative. Indeed, this could scarcely be otherwise. It is only a few years since hyperme- tropia was properly understood ; and the forms which are wholly or in great part latent, were overlooked, until I satisfied myself of their existence, and immediately began to perceive their relation to strabismus. But to this conclusion, what had been observed and recorded by my predecessors, in a certain degree contributed. Not to mention some isolated observations,* which after the discovery of hypermetropia, clearly enough demonstrate the existence thereof in strabismus, I must, in the first place, refer to Bohm'sf investigations upon squinting, where it is plainly stated, that squinters can distinguish a certain print with the aid of convex glasses at a greater distance than with the unaided eye. In this observation of Bohm there is an essential value. It might have led to the discovery of hypermetropia, and particularly of hypermetropia with strabismus, if he, with a thorough knowledge of dioptrics, had comprehended and properly explained the fact he bad observed. In place, however, of thinking of a condition, in which the retina lay in front of the focus of the eye, Bohm has recourse to an enigmatical connexion of "physical presbyopia" with "vital myopia." And in any case he was far from seeking therein the cause of strabismus. In investigating the origin and causes of the latter, he falls into the same error as all his predecessors. He tries to find out the causes, not of a definite form of strabismus, i.e. of a true form of disease, but of a symptom : strabismus in general. Consequently, the causes of whoUy different con- ditions were all investigated and studied together in heterogeneous connex- ion. That in this way the pathogeny of strabismus would not disclose its mys- teries, might indeed have been anticipated. But there is more to be said against the reasoning. Where Bohm treats of the "origin of squintingfromthe condition of the eye " (p. 5), and further speaks of "the etiology of strabismus, originating fromtheeye"(p.l6), thestateofthet^eOTafeMjreyeisandremainsfor him the principal point. He speaks here of " shortsightedness of the me eye, with normal behaviour of the other,' ' of " Hebetude of the one eye,' 'of" weakness * Conf. de Haas, Oeschiedkundig onderzoek omtrent de hypermetropie en hare gevolgen, 1862. Diss, inaug., p. 61. •j- Bohm, JDas Schielen, Berlin, 1845. HISTORICAL REMARKS. 307 of vision of one eye," and always makes the eye affected with the anomaly deviate. In this deviation he sees the endeavour to exclude this eye, but by no means an effort to improve the accuracy of the retinal images in the non-deviating eye. "What we have above assumed as one of the oircum- stanoes, under which the eye more easily allows itself to be withdrawn from the binocular vision, is for him the all-decisive cause. Bohm was, therefore, as far as any one else from comprehending the origin of converging strabismus. Subsequently, von Graefe* was certainly on the point of recognising the cause in hypermetropia. He, however, did not treat definitely of the patho- geny of strabismus. He even asks pardon, when, in passing, some observa- tions on the subject fall from his pen. But we value these observations as' so many useful hints, although, as it did not occur to him to include hyper- metropia as an element therein, his efforts necessarily remained in great part fruitless. Von Graefe puts it prominently forward as a well-known fact, that per- sistent strabismus is very often, indeed generally, preceded by an " inter- current or periodical squint." Subsequently he remarks, that all cases of intercurrent strabismus do not run into the persistently concomitant variety. So long as this has not taken place, these cases must, with respect to the question of operative aid, give rise to special observations. Therefore, von Graefe proceeds to speak of them. But evidently he felt that the pheno- mena, peculiar to these cases, had a particular value for the investigation of the pathogeny of the affection ; for from the pathogenetic point of view, he specially considers the three categories, distinguished by him : — 1. Patients who, with a careless glance, do not accurately fix any definite object, whether near or distant. Von Graefe considers that a disturbance of binocular vision might proceed from the squinting eye, and that therefore this image should be voluntarily set aside. That per- manent strabismus might be thus produced is evident. But he does not think that everything is thus explained. " When under particular circum- stances of the act of vision, namely, in acute perception of the retinal images," (thus we read, I. c. p. 281), "a deviation takes place, but not other- wise, an active connecting link must each time be sought between the act of vision and the muscles of the eye." Further : " if it [the link] is not the dis- turbance of the stereoscopic retinal images, the conditions of the accom- modation next present themselves." In reference to the first quotation, von Graefe, however, remarks, that at every distance, even behind the covering hand, on fixing an object the one eye deviates, and while, with respect to the latter quotation, he did not uaderstand that hyp ermetropia might be the cause of it, he could give no other than this somewhat obscure explana- tion : " Every action of the organ tending to the elabo ration of visual per- ception, reflects the stimulus to the irregular contraction upon the affected muscle." 2. Cases in which the visual axes are directed with precision to a definite distance (eight inches, one foot, four feet), but where, at a gre ater distance, * Arehivf. Ophthalmologie, B. I., Abth. I., p. 17. 308 STRABISMUS CONVERGENS. a deviation arises. These are almost invariably connected vrith near- sightedness. 3. Cases of which he says : " The pathological convergence occurs only with accommodation for near objects." The phenomenon appears equally on covering the squinting eye, and must, consequently, says von Graefe, depend on the condition of accommodation, "probably on increase of the muscular resistance with the augmenting refraction of the eye." " The in- crease of the muscular tension," he continues, " arouses in the affected muscle the slumbering impulse to the irregular contraction." He refers further to the singular cases, in which, in looking at both near and distant objects, stra- bismus convergens arises, but where, at a medium distance, the vision is binocular. He explains this partly from myopia, but he adds that often hyperpresbyopics and presbyopics are met with in this group ; sometimes myopia appeared to exist in distance, etc.. etc. He, at last, formulises his views in the following manner : " For all distances of the visual object there exists, according to the natural inclination to tension, a slight degree of pathological convergence. If a higher refractive condition be taken, whether through approximation of the visual object, or by holding a con- cave glass before the eye, the morbidly increased contraction arises; in a medium or low state of accommodation, and with proportionally large retinal images, the prevailing muscular tendency is, in the interest of single vision, counteracted ; for a greater distance, with diminishing mag- nitude of the retinal images, this ain no longer take place, double images arise, which are again removed from one another by a morbid muscular contraction." Finally, Alfred Graefe* proposes to himself the question, in a case of intermittent strabismus, not quite correctly called spasmodic, whether it is the "conditions of accommodation," which "cause the deviation of the right eye ? " And when he answers : " Certainly not, for it has been ex- pressly shown in the commencement of this chapter, that the deviation always occurs so soon as an object is fixed, and that it is therefore com- pletely independent of the actual state of the accommodation," it clearly appears, that he did not think of hypermetropia, which required a tension of accommodation even for distant objects. From all this it will be seen, that in literature hints were not altogether wanting, which might lead us, after the recognition of the slighter degrees of H, to bring strabismus into connexion with the same. Of H, as the cause of strabismus convergens, I have only cursorily treated + but I have long since, on different occasions, described the results I have obtained. The subject I had not lost sight of. But I wished to take it up somewhat more fully, and to inquire in general, with what anomalies of the eye the diff'erent forms of strabismus are connected. It occurred to me, that • Alfred Graefe, Klinische Analyse der MotilitatstSrunqen des Ames Berlin, 1868, p. 214. ^ ' t Ametropie, 1860, p. 45, and Archivf. Ophthalmologie, B. VI. Abth. 1, p. 92. APHAKIA. 309 suoh an investigation might tend to clear up the pathogeny of strabismus. The investigation required the statistical method. In a great number of squinters, therefore, everything was determined for both eyes, which ap- peared possibly to be the cause or the result of this anomaly, or to be capa- ble of in any way explaining its origin : sex, age, and ordinary occupation were noted ; of each eye in particular were determined the refractive condir tion, the range of accommodation, the acuteness of vision, the extent of the movements, these last in connexion with the variable or unvariable angle of squinting ; to these points were added the time and mode of origin, the hereditary causes ; finally, complications of various kinds and peculiar disturbances in vision (limitation of the field of vision, double vision, etc.). In this inquiry, several of my pupils, and in particular Dr. Hafimans, ably and zealously assisted me. The registers relating to this subject embrace 280 cases. It is true that in very many instances not all the determina- tions just mentioned were made, and the accuracy of other cases leaves something to be desired : he who knows by experience, how much time and trouble are necessary, especially in children, or in unintelligent persons, satisfactorily to investigate both eyes with respect to their function, will very easily understand this. But this will not prevent many a question respecting strabismus from finding its answer in the facts collected. Here I have been obliged to confine myself chiefly to the pathogeny, and indeed, in particular to that of strabismus convergens. In the following chapter, treating of myopia, I shall have to speak of strabismus divergens. After my preliminary communication, the fact that H often exists in strabismus convergens, was confirmed in different quarters. Special com- munications .on this subject we have received from Pagenstecher and Saemisch {Klinisehe Beohachtungen aus der Augenheilanstalt zu Wies- baden, Iste Heft, 1861, and 2" Heft, 1862), and from Mooren (Klinisehe Monatsbldtter f. Augenheilkunde, herausgegeben von Dr. W. Zehender. Jahrg. 1863, pp. 37 et seq.), who, however, confined themselves to the deter- mination of the manifest hypermetropia, and therefore found the propor- tion of H in strabismus convergens less than I did. § 25. Aphakia. Tlie absence of the lens in the dioptric system of the eye is in many respects an important condition. It must, therefore, be con- sidered strange, that writers had neglected to give it a name. I have proposed to designate it by the term aphakia, and this word is beginning gradually to find acceptance. Aphakia may be produced by different causes. It occurs most frequently as the result of operation for cataract or of a wound, which has given rise to gradual solution of the lens. "When the 310 APHAKIA. lens has by luxation or depression of cataract disappeared from the plane of the pupil, though it may still be present in the eye, it no longer belongs to the dioptric system, and we are therefore most fully justified, in speaking of anomalies of refraction, in calling the condi- tion, in this case also, by the name aphakia. Luxation of the lens is usually the result of a wound. Very remarkable cases of sponta- neous luxation of the lens are communicated by Bowman.* In the writings of von Graefe I liave seen it mentioned that he has ob- served congenital aphakia in many members of the same family. Such cases have not occurred to me. Partial luxation of the lens, causing the equator of the lens to correspond to the plane of the pupil, is, on the contrary, very frequently met with, and is not un- commonly found in several children of the same parents. This state cannot, however, be considered as aphakia; it belongs rather to irregular astigmatism. In the condition of aphakia the eye is, complicated as it is in its nor- mal state, the simplest imaginable dioptric system. In consequence of the slight thickness, in fact, and the nearly equally curved sur- faces of the cornea, we may safely neglect the slight difference in the coeflOlcient of refraction between the cornea and the aqueous humour, and therefore suppose that the aqueous humour extends to the ante- rior surface of the cornea; and as, moreover, the coefficients of refraction of the vitreous and aqueous humours are equal, we have in the aphakial eye only one refracting surface to take into account, namely, the anterior surface of the cornea. Hence it follows that, in order to find the cardinal points, we need know only the radius of the cornea and the coefficient of refraction of the aqueous humour. Now as the coefficient we have, with Helmholtz, assumed 1'3365; as the radius of curvature, in the apex of the cornea,t we may, accord- ing to our measurements, take as the average 7"7 mm. We thus obtain the subjoined system {Fig. 122. Compare pp. 40 and 44). Y = h4.' ( = 7-7 : (1-3365 — 1 ) = 22-88. P" = h" {= 7-7 X 1-3365 : (1-3365 — 1) = 30-58. A/5;' = r" — r = 7-7. Hence it appears, that the visual axis, with normal curvature of * Lectures on the parts concerned in the operations on the eye. London 1849, pp. 131 et seq. + The cornea is not spherical, but somewhat ellipsoidal, and, in fact with such eccentricity, that the spherical aberration is partially removed. In the calculation the radius in the apex must form the basis. SIMPLICITY OF THE APHAKIAL SYSTEM. 311 the cornea, should have a length A <^" = 30-58 mm,, in order, in the Fig. 122. absence of the lens, to bring parallel rays to a focus on the retina. Now, since this axis is, almost without exception, much shorter, the aphakial eye must in general be in a high degree hypermetropic. In order to find the degree of this H, with a given length of the visual axis, we need only calculate to what point behind the cornea the incidental rays must converge, in order, after refraction by the cornea, to unite on the retina. This is done according to the formula (see p. 44) _ F/" '^"—fii in which /" is the length of the visual axis, and /' the point sought behind the cornea. We find mm. /' mm. Par. inches. H 30-58 00 QO 1 00 29 420 15-5 1 15-5 28 248-3 9-2 1 9-2 27 172-5 6-3 1 6-3 26 129-8 4-75 1 4-75 25 102-5 3-75 1 3"T5 24 83-4 31 1 34 23 69-4 2-6 1 2-6 22 58-6 2-2 1 2-2 Hence it directly follows, what glasses the aphakial eye requires. 312 APHAKIA. with different lengths of the visual axis, for acute vision at a distance. We need, indeed, to the ascertained distance f, to add only the distance x between the glass and the eye (compare p. 144), in order to find the focal distance of the glass required. If we establish X = 0-5", we therefore need, with/' = 29 mm., glasses (15-5 + 0-5 = 16) of ^; with/' = 24 mm. (3-1 + 0-5 = 3-6) oi^, etc. On the other hand, we can further calculate the length of the visual axis, when the focal distance of the required glass is known. If X be assumed = 0"''5, glasses of 1 : 2-5 correspond to a length of the visual axis/" = 21-5 mm. 1:3 „ „ „ „ / = 22-9 „ 1 :3-5 1 :4 1 : 5 1 : 6 1 :10 1 : 00 /' = 23-9 „ /" = 24-6 „ / = 25-7 „ /" = 26-5 „ /' = 28-l „ /' = 30-58,, Experience has shown, that in the majority of cases glasses are required of from 1 : 3 to 1 : 3"5, placed at 6" from the eye. This corresponds to a length of the visual axis of from 22'9 to 23"9. This length coincides nearly with that of the emmetropic eye. But if the eye was myopic before the development of cataract, weaker glasses are, after the operation, sufficient. A. case even occurred to me, in which the accuracy of vision of distant objects was incapable of improvement by either positive or negative glasses. In this in- stance the visual axis of the eye, emmetropic with aphakia, had actually a length of rather more than 30 mm., and we may as- sume that so long as the crystalline lens was still present, myopia of about g- had existed. o In a second case, in a woman, aged 36, glasses of 1 : 133 were sufficient; in a man, aged 73, glasses of -5. o cataract, a myopia of rather more than way to H = 1 : 7'5 In this instance there must have existed, before the occurrence of the ^, which had now given The patient now declares that he can see without spectacles better at a distance than he could in his youth before the occurrence of the cataract ; as the pupil is now smaller than it formerly was, this is not surprising. In the two other cases mentioned, the difference must have been still greater. The PRE-EXISTENCE OF MYOPIA. 313 woman, aged 36, had only one eye (the other was atrophic) ; and in that operated on by me there had remained from youth, after perforation of the cornea, a lateral leucoma with synechia anterior. Notwithstanding, she declared that with this one eye she now saw better for ordinary purposes than had ever before been possible for her with both eyes. But for near work she now required spectacles of — , which formerly she had been able to do without. — It is a very common case, that for seeing at a distance in aphakia, glasses of - or ^ are sufficient, which, when the form of the cornea is D 6 normal, proves that the visual axis is longer than iisual. Now, in all these cases myopia has previously existed, and the connexion between M and length of the visual axis is thus in aphakia most clearly brought to light (compare p. 88). Especially in cases of congenital cataract have I been able to satisfy myself of this. It has appeared to me that this condition is usually connected with a myopic structure of the eye. We know that under these circum- stances not the entire crystalline lens is obscured, or rather, that ordinarily only some laminae are obscured, while the nucleus, and especially the peripheric layers, are transparent. Now, if the trans- parency of these last is tolerably perfect, vision often becomes pretty good on artificial mydriasis. It is then only rarely that positive glasses are required for reading, etc., notwithstanding that the power of accommodation is removed. The degree of the myopia is now also stiU easily determined. In connexion with this M we shall find, that when the operation has become necessary, and the lens is removed, glasses of from 7 to ^ are in these cases usually sufficient for seeing at a distance. Sometimes I have had the opportunity of determining, in the same eye, the degree of M before, and that of H after, the operation. In the three cases which were most accurately investigated I found : Ametropia. Radius of Curvature of the Cornea. Oalculsted length of the Visual Axis. Before the Operation. After the Operation. M = l:6 M = 1 : 8-5 M = l:24 H = l:5-12 H=l:4-5 H = 1 : 3-2 7-6 7-92 8-04 25-96 26-36 25-02 314 APHAKIA. It so happens that the radius of curvature of the cornea is in the first case somewhat less, in the last two somewhat greater than usual. Therefore the visual axis proves, in comparison to the degree of M, in the last two cases particularly long : with M = 1 : 8'5 it is even somewhat longer still than with 1:6. It would therefore be incor- rect to assume, that with M = ^ the visual axis is usually 25 mm. long. That from the above observations the focal distance of the original crystalline lens, in a given position, may be calculated, the reader will, no doubt, have already understood. I hope to do this hereafter for a greater number of cases. Original H, too, makes its influence felt where aphakia has super- vened. In one case even of congenital cataract, H was found after operation in the right eye = 1 : 2'44, in the left 1 : 2-43. These eyes were evidently rather small (approaching to microphthalmos), without particularly marked curvature of the cornea, and the hyper- metropic structure was in them not to be mistaken. Moreover it is in general rare in aphakia to find H > 1 : 2'5. Where I met with H = 1 : 2'4 or 1 = 2'3, I could during life, the eye being strongly turned inwards, very well satisfy myself that the visual axis was shorter than normal, and calculation, after measuring the radius of curvature of the cornea, gave the same result. The existence of aphakia is, at the first glance, not very easily recognised. Often the anterior chamber of the eye is deep, and we find a certain degree of iridodenosis ; but none of these phenomena are characteristic. The search for the re- flected images of Purkinje is decisive (compare p. 11) : the two reflected images of the lens are absent. Moreover, the sectors and the direction of the fibres of the crystalhne lens are easily seen on lateral illumination with light concentrated by a lens, particu- larly with the aid of a magnifying glass, as Helmholtz first remarked : where these are wanting, we may infer the existence of aphakia. Knally, the degree of H, whether established by experiments with convex glasses, or by ophthalmoscopy, in connexion with the form of the eye, cannot deceive us. When after a blow or knock upon the eye the power of vision has suddenly diminished, without very manifest disturbances in the organ, we should specially bear in mind the possibility of the lens having disappeared from the plane of the pupil, in consequence of luxation, and we should satisfy ourselves upon this point in the manner above described. DIAGNOSIS. 315 The acuteness of vision is in aphakia usually imperfect. The cause of this is almost always to be sought in turbidity of the sur- face of the pupil. Even after the most successful operations for cataract, where on inspection the pupil appears completely black, we shall, on examination with the ophthalmoscope, and especially with concentrated incident light, usually find some turbidity, depending chiefly on a slight deposit on the inner surface of the capsule of the lens. In consequence of this a portion of the light becomes diffused, and diminishes the sharpness of the retinal images. Slight as this turbidity may be, it has great influence, as appears from the fact that when even only a small part of the plane of the pupil is per- fectly clear, vision immediately becomes comparatively very good. The clearest pupils I have obtained in some cases of operation for cataract by solution, where moreover, on account of the larger retinal images produced by the use of convex glasses, S was > 1. 2 In other cases we may be content, when S = ^ or even only o g, which is suificient for all practical purposes. A change in the cur- vature of the cornea, which gives rise to tolerable regular astigmatism, is, after extraction, also not unfrequently the cause of diminished S. This may, as will hereafter appear, be in great part corrected by a certain incUnation of the glasses. That, on account of the high degree of H, without convex glasses the power of vision in aphakia leaves much to be desired, needs no proof. Vision will be the more imperfect, the larger the pupil is, to which, for equal degrees of H, the magnitude of the circles of diffu- sion is proportionate. But even with the use of glasses a large pupil has considerable disadvantages. The power of accommodation is, in fact, as I shall more fully show, in aphakia completely removed, and a glass, therefore, gives a distinct image only for a definite dis- tance. All points, which are either more or less remote from the eye, are now of course seen with circles of diffusion which are larger, the larger the pupil is. With a narrow pupil the smallness of the circles of diffusion even of objects, for whose distance the eye is not accommodated, might lead one to suppose, as has often happened, that the power of accommodation in aphakia is not removed. It is on account of the great advantages for vision of a small pupil in aphakia, that we cannot consider the performance of iridectomy. 316 APHAKIA. especially inferiorly, in the operation for cataract, as an indifferent matter. We know that Mooren, Pagenstecher, Jacobson and others, each in his own way, have given their advice in favour of it. That iridectomy, not only when the object is by numerous punctures to cause a soft lens to be absorbed, but also in extraction, much dimi- nishes the dangers of the operation, is fully established. Por many years I have observed this. Por this reason I have, when there is a tendency to prolapse, or when a portion of the iris has suffered much, advised the performance of iridectomy.* Moreover, following the example of von Graefe, I in general make iridectomy precede operation by puncture. But this iridectomy is performed superiorly. In this case the part which has suffered excision is almost completely covered by the upper eyelid, and the circles of diffusion therefore do not become greater. It is quite a different matter when the iri- dectomy is performed inferiorly. Hence I cannot, even where there is less danger of losing the eye, so unconditionally adopt the method of Jacobson, who performs every extraction by flap-section inferiorly, and at the same time cuts out a great piece of the iris. If this method with the flap above gives an equally satisfactory result, I should be rather inclined to follow it. "With respect to vision in aphakia, I have further to mention, that the polyopia, dependent on the crystalline lens, the rays pro- ceeding from points of light, and the streaks of light on entoptic investigation are wanting (compare p. 200). On the contrary, regular astigmatism of the cornea, as sliall be more fully shown in the chapter upon this anomaly, makes its appearance most dis- tinctly. In order to prescribe the most suitable glasses in aphakia, we begin by determining what glasses are required for distance : the best ob- ject for this purpose is a point of light. We now easily calculate from the result obtained, what focal distance is necessary for near objects. In order, for example, distinctly to see a point, situated at the distance y from the lens, the rays proceeding from it must, after having passed through the lens with focal distance P", converge to the same point as the rays proceeding from oo, after having been * See Ametropie en hare gevolgen. Utrecht, 1859, p. 85. SELECTION OF GLASSES. 317 refracted by the lens with focal distance Pj. Oonaequently, 1^ 1 _ 1 y T'. ^: With y the distance from the point of distinct vision to the glass lens is found. Let x be the distance between this lens and the ante- rior surface of the cornea, then in using the lens with the focal distance Pj, the distance from the point of distinct vision to the cornea /" = y + x. A few examples may illustrate this. Let the eye, in order to see distinctly at a great distance, need a glass of 1 : 3'5 at ^" from the eye ; how far from the eye will the point of distinct vision lie, if this lens be replaced by a lens of ^ ? This calculation is : 1_ 1 1 y 3 H Of ^~' 3 X H y — ^- -3 y = 21 f' = 2U // And if the second lens has only 2r: focal distance, i_ 1 1 y~ w H 21 : X 3^ y = 3^- -n y = 81 /- 9i With the lens of 3' focal distance vision wUl therefore- be acute at 214", with that of 24" at 9i". With a lens of 2" this distance lies only at 5 b'. If we wish to know the focal distance F^ required in order to see acutely at a given distance y, this is found from the formula : i'» Pi ^ y But the question is then, whether we shall have at our command a lens of the calculated focal distance. The object may, however, in case of any difficulty upon that j>oint, be attained, as shall hereafter appear, by modifying the distance of the lens from the eye. With respect to the choice of glasses in aphakia, we should not forget, that, especially in old people, completely as the operation 318 APHAKIA. may have succeeded, the power of vision is seldom perfectly acute, and that, consequently, in order to read a smaller kind of type, the point of distinct vision must be brought rather near the eye. Not unfrequently this distance may amount to not more than 6". In young subjects, in possession of acute power of vision, it may be considerably greater, the more so because in aphakia the retinal images far exceed in magnitude those of the same eye, before it was deprived of its lens. The dioptric system is altered : instead of a lens in the eye, a lens is now placed in front (fthe eye, and conse- quently the united nodal point is moved forwards. If the lens be farther removed from the eye, the nodal poiut wiU come to lie even before the cornea. Hence it appears that the retinal images must be larger, and that they increase still more in magnitude with the removal of the glass from the eye. This increase of magnitude, which, in using a weak glass held at a great distance, becomes very considerable, is found by comparing the magnitude of the retinal image ^i of the original eye with p^ the retinal image of the aphakial eye, combined with a convex lens. The subjoined table gives a view of the increase of magnitude, which we obtained on calculation.* Focal Dis- tance of the Lens employed. Distance of tUs Lens ftvmthe Cornea. /3,:1=;8,: 3 0-5 1-322 4 1-5 1-763 S 2-5 2-203 6 3-5 2-644 8 5-5 3-525 10 7-5 4-406 16 13-5 7-050 * We should bear in mind, that for remote objects ^1 G," and P/ having reference to the originally emmetropic eye, /3 , (? ' and 2^j to the aphakial eye, in connexion with a convex lens. In calculating the increase of magnitude, under the use of different con- vex lenses, we assumed : i^/ = 0-5734, in an eye, with a radius of the cornea of 7-7 mm. and a length of the INCREASE OF THE RETINAL IMAGES. 319 Hence we see, that, when in aphakia, with H = 5^ a lens of =-^ is held at 13'5" from the eye, the patient sees objects rather more than seven times larger than he did with the original eye, furnished with a crystalline lens. This vision is about the same as that of the emmetropic eye using glasses of ^ would be, on looking through glasses of ,-^ at 13"'5 distance. The combination is, therefore, equal to that of a Dutch or Galilean telescope. In aphakia we find it under the simple form of a lens of from 10" to 20" focal dis- tance, while the hypermetropic eye plays the part of an eye-piece. Other strongly hypermetropic eyes, without aphakia, can likewise make use of the same. In the above theory and calculation, we have assumed that in aphakia the power of accommodation does not exist. It is an important question whether we are justified in doing so. Por if it can be proved that in aphakia no trace of accommo- dative power remains, the inference would seem to be legitimate that this power depends exclusively upon a change of form in the lens. Hitherto this question has not been rigidly investigated. It is true that Thomas Young* had already in some cases of aphakia examined the eye with reference to its accommodative power; but the eyes at his disposal were not particularly well adapted for the purpose, and he moreover thought the result only tolerably satisfac- tory in proving the absence of the accommodating power. Von visual axis of 22-9 mm., thus having, with aphakia, S^ Now we found, with the use of diflFerent glasses, F,' = ^=;— — = (compare Helmholtz, Bioptrik, p. 58), F being the F -\- Jh — X focal distance of the lens, F' the anterior focal distance of the aphakial eye, and X the distance from the convex lens to the cornea. In this case x must always be = F — 2.5".— We see from the above table, that /8 is proportion- ate to F, which may be at once deduced from the formula : the numerator increases proportionately to J". On the contrary 2^ — a; (in our case = 2-5'), and with it the whole denominator, is an invariable quantity. Conse- quently if -F'j increases proportionately with F, this holds good likewise for /3j, the value of which is proportionate to F^. * loc. cit., pp. 46 et seq. 320 APHAKIA. Graefe^* on the contrary, found that some accommodating power remained. He remarks, however, that those who made the most accurate, and, on repeated investigation, the most uniform, statements, had the least range. Whatever else we find here and there noted respecting the occurrence of considerable range of accommodation in aphakia, proves only that the writers had no idea of the degree of distinctness of vision, even in imperfect accommodation. My investigations have led me to the conviction, that in aphakia not the slightest trace of accommodative power remains. In old people, and with imperfect acuteness of vision, observers sometimes think they are able to prove the existence of a certain amount of range of accom- modation; but in young persons, with perfectly clear pupils and great acuteness of vision, in whom precisely we might still expect to find some accommodative power, it is quite evident that the latter is entirely lost. A young person, of perfect, indeed of extraor- dinary, acuteness of vision, who was himself interested in the investigation, had suffered from congenital cataract, and had been operated on by me in both eyes with the most complete suc- cess. With glasses of g-, placed at 5''' from the eye, he saw, at a great distance, a point of light snfflciently round and perfectly defined. A sight [vizier) was placed in the direction between one of the eyes and the point of light, and when he now looked witli converging visual lines towards the sight, the point of light remained unchanged or became somewhat smaller and sharper, without changing its form. If the lens was removed only i'" more or less from the eye, the distant point of hght had ceased to be a defined, round point, and was elongated in one direction, to the form of a linej now even with the most powerful exertion, and convergence in the point of the sight, the hue of light became only somewhat shorter, without how- ever a point making its appearance. This shortening, as well as the diminution of the acutely seen point, depended upon narrowing of the pupil, which was, indeed, directly observed. The experi- ment was repeated separately for each eye, with a like result. Behind the black plate, which in the trial was placed before the one eye, the turning of this eye, in looking towards the sight and towards the remote point of light, could be observed. The force of the experiment, therefore, leaves nothing to be desired. No accommo- • Archivf. Ophthalm., B. II. Abth. i. p. 188. ABSENCE OF ACCOMMODATIVE POWER. 321 dation whatever existed. Nevertheless in this case also a small margin of distinct vision was, on examination with the optometer, observed, — a proof that we cannot thence infer the existence of accommoda- tive power. In a second similar case, that of an intelligent young man, the total absence of accommodative power was in like manner proved. In this instance it was, moreover, established, that when a point of light was acutely seen at a distance, through a given lens, the addi- tion of a lens of -^„ or — yqc) O^J ^^^ combination of g^r with no or of q^ with — ^ ] produced the well-known change in the 36 1 point of light : the patient constantly stated that by YqTj ^^^ point of light, was extended in the vertical, by — j^ in the horizonal direc- tion to a short line. On the other hand, the convergence of the visual lines, with the effort to see near objects, was not followed by the slightest change of form ; consequently there was no reason to suppose the existence of accommodative power. I must add, that at the moment when he directed his attention entirely to the sight, an actual change in the sharpness of the point of light was still observed immediately on unexpectedly pushing before him a lens of :j-^ or — -^ — Subsequently I have, at different times, tried a similar ex- periment, in which glasses of ^^r^ or of — ^ttt^ (a combination of -^ with — o?) "'" °^ «n ^^^^ — ■Kfj ) produced an evident change of form in the image of light, while on varying the convergence and endeavouring to accommodate, no change whatever took place. The range of accommodation therefore amounts, in aphakia, even in a youthful eye, certainly to less than ^^, and may therefore be con- sidered = 0. It may well surprise us, that v. Jaeger, who is aware of these investigations, still speaks of accommodation in aphakia (I. c, p. 111). The complete want of accommodative power may readily mislead to the idea, that in aphakia glasses of different focus are necessary 21 322 APHAKIA. for each, distance, fortunately this is not the case. An accommoda- tive power remains, the mechanism of which is extremely simple. The drawback is, that the hand must in it perform the active part. The power of accommodation to which I have alluded, consists in the alteration of the distance between the glass and the eye. The lens placed before the eye has been substituted for the crystalKne. It can also take on itself the part of the accommodation. It cannot do this by altering its form, like the lens in the eye, but it holds to the old theory, according to which the power of accommodation was made to depend upon displacement of the lens. I teach all sufferers from aphakia to accommodate in this manner. Let !Pj =:: V 0' (Fig. 123 A) be rig. 123. the focal distance of the lens required for distant vision, and the distance ^^ ^ be ^ « : then the rays a h and c d, which, refracted at h by the cornea (dotted lines), would meet on the retina, converge towards ^^, at F^ — x behind the cornea. If the same lens be now l^ l^ = sC removed farther from the cornea (compare Kg. B), ^i becomes equally removed forwards to ^^ {^^j ^= I^li = oc'). Now we know that rays, in order to be brought by the refraction of the cornea on the retina, must be directed to / (corresponding to i of A) . Consequently they must proceed from a certain point i situated at a finite distance, whence the rays i h and i" d are supposed to diverge, and of which / is the conjugate focus. The distance * ^j = y we therefore find from ]__ J ^ 1 Pi F, + X' y In order to find the distance of distinct vision to the cornea, we must add to y, the distance l.,h = x + x from the lens to the eye. ARTIFICIAL ACCOMMODATION. 323 In the ordinary position the glass is about half-an-inch from the eye, and the spectacles can easily descend an inch farther on the nose. Now if a person in order to see at a distance, requires glasses of 1 : 3^, placed at half-an-inch from the eye, let the spectacles be re- moved only half-an-inch more, and the above mode of calculation shows that accommodation has taken place for the distance of 29"; let them be removed a whole inch more, and the point of distinct vision lies at 17 J". If glasses of 1 : 3, placed at half-an-inch from the eye, are necessary for distant vision, then, on removing the glasses to 1°, the point of distinct vision lies at 22', on removing them to 1^" it lies at 13|", and some people then read exceedingly weU. Notwithstanding this artificial power of accommodation, it is in general advisable in aphakia to give two spectacles, one for distant, the other for close vision. Each pair of spectacles can then take on itseK a part of the desired region of accommodation, and the neces- sary displacement may now be very slight. But in order to read or write anything for a moment, the spectacles usually worn for distant vision are simply displaced as may be necessary. If, with good power of vision, aphakia exists in both eyes, we must carefully attend to the mutual distance of the axes of the two glasses. The greatest care is then required, in order, under different circum- stances, to guard against double vision. Sometimes I have been obliged to have the glasses more or less ground on the outside, espe- cially when the eyes stood particularly close to one another. Once too, for near vision, insufficiency of the musculi recti interni occurred to me, which was completely corrected by modifying the distance of the axes. The knowledge of the general laws must here be our guide in each particular case. PinaUy, in altered curvature of the cornea, which is seen especially after prolapsus iridis in extraction, an oblique position of the glass is required, of which I shall speak more fuUy in treating of astigmatism. A word, in conclusion, on examination with the ophthalmoscope in aphakia. In general a high degree of H exists in this condition. If the rays, in order to come to a focus on the retina, must converge in a point situated 3" behind the cornea, the rays proceeding in a diverging direction from the retina will appear, after having been re- fracted on the anterior surface of the cornea, to proceed from a point situated 3' behind the cornea. With myopia of nearly 1 : 3, therefore, the observer can still examine the retina in the unreversed image. 2 324 APHAKIA. We understand, moreover, that in order to see the fundus accurately in aphakia in an ordinary eye, an emmetropic eye must either remove, or accommodate strongly for near objects, or make use of positive glasses. We may also combine one plan with another. Tor the oculist it is important, that he should be aware of each change in his accommodative power, and that he should, under aU circumstances, be able voluntarily completely to relax his power of accommodation. With this total relaxation the emmetropic eye sees,. with glasses of 1 : 5, held at about 2" from the observed eye, the fundus ocuh in aphakia, with perfect acuteness. The rays then appear to proceed from a point, situated 3" behind the cornea, and the eye investigated therefore re- quires glasses of 1:3^, at ^" from the cornea, in order to be accommodated for parallel rays, that is, for distance. We can thus, from the observation with the ophthalmoscope, deduce the strength of the glasses required. This is not, however, attended with any essen- tial advantage. The patient himself states, more accurately than can be determined with the ophthalmoscope, the glasses which he requires for distant vision, and in general decides with great precision, at what distance, and with what inchnation of the glasses, a remote point of light is most acutely seen. But the observation is im- portant in another point of view. In fact, it is known, that in the emmetropic eye the whole curvature of the retina lies in the focal surface of the dioptric system : with the ophthalmoscope we see, with unaltered accommodation, even the eccentric parts of the retina acutely defined in the unreversed image; and in the eye of the white rabbit, removed from the head, I saw sharp images of remote objects glimmering through the entire sclerotic, so far as the latter bears a retina on its inner siirface. Thomas Young connected the union of the rays on the retinal surface with the laminated structure of the lens. It is certain that this is not without influence upon it. We can, in fact, satisfy ourselves, as might be a priori inferred, that the retina, where aphakia exists, is not visible with equal accuracy in all directions through the same lens, unless the observer alters his accommodation or his distance from the eye. Another question is, whether the forms perceptible in the fundus oculi also undergo a change, when we look into the eye under a tolerably great angle with the visual axis. I have satisfied myself that this is really the case. If we dilate the pupil in cases in which clearly visible, sharply-defined forms occur in the fundus ocuK; for example, where circumscribed deposits of pigment exist in the retina. HYPERMETROPIA. 323 we see the same figures, trader the slight difference of direction, with which they continue visible, becoming elongated and shortened in various directions. It appears to me that thus also the form of an eccentric image must deviate from that which is produced by the same object near the axis of the eye on the retina. The direction of the projection mnst be modified accordingly, while yet the objects are seen also indirectly in their true form. Now it deserves remark, that in a case of pigmentary deposition on the retina, in which the lens was extracted, this change of form, connected with looking in dififerent directions into the eye, was no longer observed, or at least was perceptible in only a very slight degree. We must therefore assume, that indirect vision, in aphakia, has undergone a change, in this sense, that the forms of the bodies are now represented more cor- rectly in the retinal images,- but are even therefore less correctly pro- jected. However, the importance of this ceases with the use of tolerably strong glasses, which is almost invariably required; for when such glasses are employed, no sharply- defined images whatever can occur on the eccentric parts of the retina. NOTE TO CHAPTER VI. In the writings of the eighteenth century I have sought in vain for proofs that H was observed and recognised as such, or that the existence of this anomaly of refraction was even suspected. It is not until 1811 that we find a case communicated by Wells (Philosophical Transactions, vol. ciii. p. 380). It relates to his own eyes. At the age of 55 he remarked, that his presbyopia was attributable to loss (diminution) of accommodation, and that he required even glasses of 36 inches positive focal distance, in order to see acutely at a distance. This observation did not escape the learned Mac- kenzie {A Practical Treatise on the Diseases of the Eye, London, 1830, p. 729). " Although the eye," we read, " after middle life, loses the power of distinguishing near objects with correctness, it generally retains the sight of those that are distant. Instances, however, are not wanting of persons of advanced age requiring the aid of convex glasses to enable them to see distant, as well as near objects." As he, in addition, quotes the case of "Wells, it follows from the above, that he considers the occurrence of the condition in question at a more advanced period of life as not unusual, and "Wells himself was of the same opinion. The cases here spoken of have, however, reference only to hypermetropia acquisita. If no other form occurred, hypermetropia would not be of any great importance. It would then really be coextensive with presbyopia, and we should have been able to concur in the use of the term hyperpres- hyopia, which Stellwag von Carion, starting from such cases, has given in general to those eyes in which the focus lies behind the retina. 326 HYPERMETROPIA. On the 19th November, 1812, James Ware read before the Koyal Society his Observations relative to the near and distant sight of different persons. After having spoken of the senile changes of the eye, and having, in, conclu- sion, quoted Wells' case of relative hypermetropia acqnisita, he adds the following remarkable words : "There are also instances of young persons, who have so disproportionate a convexity of the cornea or crystalline, or of both, to the distance of these parts from the retina, that a glass of considera- ble convexity is required to enable thprnto see distinctly, not only near objects, but also those that are distant ; and it is remarkable, that the same glass will enable many such persons to see both near and distant objects ; thus proving that the defect in their sight is occasioned solely by too small a convexity in one of the parts above-mentioned, and that it does not influence the power by which their eyes are adapted to see at distances variously remote. In this respect such persons differ from those who had the crys- talline humour removed by an operation ; since the latter always require a glass to enable them to discern distant objects, different from that which they use to see those that are near " {Philosophical Transactions of the Soi/al Society of London, for the year 1813. London, 1813, p. 43). In these few wSrds James Ware entitles himself to be called the discoverer of hyper- metropia.* In a youthful eye,' with sufficient accommodation, position of the focus behind the retina and need of convex glasses for seeing distant objects : the portrait is complete. But not perceiving the great bearing of his dis- covery, "Ware confines himself to this brief communication, which was either overlooked or not comprehended. Even Mackenzie {he. cit. 4th edition, p. 923) speaking of it in connexion with presbyopia can say only : " The cases related by Mr. Ware, as occurring in young persons, seem to partake more of the character of asthenopia than of presbyopia.'' Hence his asthenopia might, vice versd, have suggested to him the idea of hypermetropia. With Ware our knowledge of H was lost. We next meet with Sichel {Des lunettes et des Hats pathologiques, consecutifs a leur usage irrationel. Annales d^ocuKstique, Tome xiii. pp. 5, 49, 109, 169. Tome xiv. pp. 14, 193. Bruxelles, 1845), whose doctrines have long exercised a great influence. Cases of an anomaly of such general occurrence could not escape so excel- lent an observer. As " une espece d'amblyopie congfinitale compliquee de presbytie et prise d'ordinaire pour un tres-haut degre de myopie," to which he gives the name of ambh/opie presbytique congenitale, he sketches a distinct picture of high degrees of hypermetropia. The nature * While the above pages are passing through the press, I find that Janin too was acquainted with H. Thus I read, in an article by Muncke, in Gehler's Physik. Worterbuch, B. iv., 1828, p. 1309, the following words: " To the singular phenomena belongs lastly such a condition of the eyes, that neither near nor distant objects can be distinctly seen without convex glasses, as after the operation for cataract. Now Janin {Mem. et observ. sur I'oeil, Paris, 1772, 8vo., p. 429) has observed this defect, which he attributes to too great flatness of the lens." I have not had an opportu- nity of perusing Janin's communication in the original. HISTORICAL REMARKS. 327 of the affection he did not, however, comprehend. He states, in- deed, that negative glasses only incommode, that on the contrary, im- provement is obtained by convex glasses, " aveo lesquels ils n'ont pas besoin de rapprocher beauooup plus les objets, et qui mSme, pour leur servir effloacement, doivent etre d'une certaine force." But he was far from believing that they really required these. " II serait dangereux toutefois," he says, "de les leur aooorder trop tot, ou de permettre qu'ils usent des verres trop puissants: mieux vaut les en priver le plus longtems possible." — If hypermetropics, who had already worn convex spectacles, resorted to him, he did not hesitate to declare that the use of the glasses was the cause why they could not distinguish without such assistance, and already saw a dangerous amblyopia looming in the distance. He therefore unconditionally forbad the use of positive glasses for remote objects. This he did even in the case of old persons. Nay, he blames Mackenzie in this respect, who nevertheless certainly did not go too far. In this singular prejudice against the use of positive glasses is contained the proof, that the nature of the deviation remained a mystery to Sichel. The same prejudice continued for a long time to be tolerably generally entertained. Even so late as 1853, White Cooper (Ore Near Sight, Aged Sight, and Impaired Vision, p. 97) describes the case of a girl, aged eight years, who used a convex glass, and with it was enabled to work at the distance of a foot. He adds that her parents had both been obliged to wear convex glasses at the age of thirty (hereditary H) ; and still he could give the poor child no better advice than to refrain from the use of spectacles. "There is good reason to believe," he says, "that as she grows older, and her eyes are more employed upon near objects, the distance of the point of distinct vision will decrease.''- — ^So little was it suspected, that the structure of the eye caused the focus to fall behind the retina, and that at any distance correction by means of a convex glass was therefore necessary. When, after the discovery of the operation for strabismus, this form of disease had attracted such general attention, and in a short time became the subject of a long series of essays, it followed, as a matter of course, that the power of vision of squinters should be specially attended to. Many of these essays, however, do not bear the stamp of unprejudiced investigation. Almost always a special desire was manifested to show that by the opera- tion not only was the deformity removed, but that the disturbance of vision was also gotten rid of. In general we find anomalies of refraction and of ac- commodation mixed up and confounded with diminution of acuteness of vision, while theories respecting the influence of the external muscles of the eye upon accommodation were made the basis of the explanation of what was observed, or even determined the observation. It is evident that a squinting hypermetropic eye, which moreover usually suffers from diminished acute- ness of vision, amblyopia, can distinguish an ordinary type only when near, although even then with diflB.culty, — better at least than the same type at a greater distance. Meanwhile, this condition was almost universally con- sidered to be myopia, and Baudens even calls a child extremely nearsighted because it could distinguish objects only with strongly convex glasses. Ludwig Bohm {Das Schielen und der Sehnenschnitt in seinen Wirkungen auf 328 HYPERMETROPIA. Stellung und Sehkraft der Augen, Berlin, 1845), as aminprejudiced observer proved, that in strabismus the vision of near was better than that of distant objects (therefore M ?), and that nevertheless the patient could, with convex glasses, read at a greater distance (therefore presbyopia ?). Thus he saw himself brought into a difficult dilemma, from which he knew not how to escape. Evidently he did not consider, that even without accommodation for near objects, letters of definite magnitude, especially when amblyopia exists, are more easily recognised near the eye, than at a greater distance, simply because the retinal images are larger, and that convex glasses, by improving the images, make it possible under those cir- cumstances, to distinguish at a greater distance. But beyond this we seek in vain for any proof from him, whence it might follow, that in any case he had satisfactorily proved the existence of hypermetropia. Indeed he always investigated only, with what convex glasses a distinct type of medium magnitude was best recognised, and nowhere is it stated, that this distance was greater than the focal distance of the glasses employed. Among the cases of strabismus communicated by Kitterich {Das Schielen und seine Heilung, p. 73, Leipzig, 1843), we find one recorded in which the existence of H was demonstrated. " With a convex glass of No. 24," we read, "he saw (the subject was a boy of eleven years) both near and distant objects better." However, this condition was not more accurately determined or appreciated, nor was the ease further observed. A similar case, described by Fronmiiller {Beobachtungen auf dem Gebiete der Augen- heilhunde, p. 54, Fiirth, 1850), in which it is expressly stated, that " in order to be able to see well at a distance," the patient, who was twenty-two years of age, and was affected with strabismus, was obliged to make use of a strong convex glass No. 8 ; this ease, I say, was in some incomprehensible manner confounded with the myopia at a distance of Kerst, which is nothing else than a slight degree of myopia. In order to ascertain whether earlier writers had any idea of H, we had to search under the heading of presbyopia. Now in the works of many we find it stated, that some people early require convex glasses for reading and writing, and occasionally even very strong ones. Thus We read in Mac- kenzie {A Treatise on the Diseases of the Eye, p. 728. London, 1830) " Young men of twenty years sometimes cannot see to read or write without convex glasses of six or eight inches focus, while persons of eighty years and upwards are occasionally met with who are able to read even a small print without assistance These, and similar differences, depend upon the original formation of the eyes, how they have been used, and the general health and constitution of the individual." Thus Smee (TAe Eye in Health and in Disease, p. 33. London, 1854) also says : " Although far sight occurs most commonly as a disease, yet I have been occasionally con- sulted by patients who have suffered from this abnormal state as a result of congenital defect. The patient in this case prefers to sit before a window with the light falling directly upon the pupil, so that by its contraction to a pin's point, only the central rays infringe upon the retina, and thus fair vision may be obtained. Congenital farsight may exist with most perfect power of adjustment." These last words are of great importance. When the power HISTORICAL REMARKS. 329 of accommodation is perfect, and nevertheless near objects cannot be seen, the condition can be none other than H. He adds : " the disease may be determined with great accuracy, instantly, by the optometer." But from the tables, in which the results of optometrio observations obtained by calculation are collected, it does not appear that Smee was actually aware of the existence of H. Thus no progress was as yet made beyond the point attained by Ware. He who knows by experience how commonly H occurs, how necessary a knowledge of it is to the correct diagnosis of the various defects of the eye, and how deeply it affects the whole treatment of the oculist, will come to the sad conviction that an incredible number of patients have been tor- mented with all sorts of remedies, and have been given over to painful anxiety, who would have found immediate relief and deliverance in suitable spectacles. "We may therefore look upon it as truly fortunate, that many had recourse simply to ordinary empirics, so-called opticians, who endea- our to give men those spectacles which render their vision persistently easier. Siohel's lamentation over the number of patients who had got convex spectacles for distant vision, satisfactorily proves that the opticians knew very well, that in some eyes distant vision is improved by convex glasses. And still every day almost the same thing occurs. But what is more, some of these opticians had more or less correct notions of hyperme- tropia. Thus we find, so early as in 1842, concealed under a chaos of confused ideas, a condition separately described and characterised by Mr. J. A. Hess (Theoretisch en praktisch handboek der mechanische oogheel- hunde, p. 216, Zierikzee, 1842) under the name of presmyopia (sic !), of which he says : " the only certainty consists in this, that the vision of these eyes at all distances is improved by the addition of a convex glass," and thus H is evidently defined. Mr. Hess stated that this condition occurs in different degrees, that the power of accommodation (by him called extensibility and impressibility) may at the same time exist, and thought that in these eyes the lens is either wholly absent or must be partially degenerated, and that there is much hope of curing slight degrees of the affection. At length in 1853, we find hypermetropia, for the first time, described in a scientific manual (Ruete. Lehrhuch der Ophthalmologie fiir Aerzte und Studirende, Bd. i. p. 234, Braunschweig) in the following words, under the name of oversightedness (Uehersichtiglceit) : " Oversightedness is the con- dition in which, on account of a peculiar, and as yet not sufiiciently inves- tigated, construction of the refracting media of the eye, neither near nor distant objects are distinctly seen. The eye appears in it to suffer from a, total want of accommodating power, and to possess but a very slight refractive power. This defect of sight is in general congenital, or is at least developed in very early youth. Vision is considerably im- proved by the use of convex spectacles, whose focal distance must, however, vary according to the distance of the objects, so that those suffering from this defect are able even to read." The description still leaves much to be desired ; the state is not accurately characterised ; moreover, it is incor- rectly supposed, that the power of accommodation is in these eyes almost 330 HYPERMETROPIA. wholly wanting. But still in these few words lies the germ of further investigation, and we thus see here also, as in many other points in ophthal- mology, the impulse given by Euete to our knowledge. Two years later, namely on the 12th of April, 1855, Dr. Carl Stellwag von Carion laid before the Imperial Academy of Sciences at Vienna, a detailed essay entitled : die Accommodations/ehler des Auges. {Sitzungs- berzchte der Kaiserlichen Akademie der Wissenschaften, Mathematisch- naturwissenschaftUche Classe, Bd. xvi. pp. 187-281.) In it we find a correct definition in the following words : " the optical essence of over- sightedness lies in this, that the focal range of the dioptric apparatus in perfect rest of the accommodating muscle is greater, than the distance of the layer of rods and bulbs from the optical centre of the refracting media." Stellwag too has the merit of having satisfactorily indicated the method of determining r in cases of H. Slight degrees of H he appears, however, scarcely to have recognised, the latent not at all (compare de Haas, loc. cit.) ; and on the whole, as the very name of hyperpresbyopia used by him. shows, he has not strictly enough distinguished hypermetropia from presbyopia. Only a few months after SteUwag von Carion had brought forward his investigations on defects of accommodation of the eye, we find in the Archw fur Ophfhalmologie (Bd. ii., Abth. i. pp. 158-186) a paper by von Graefe, under the title of " Ueber Myopia in distans, nebst Betraohtungen liber das Sehen jenseits der Grenzen unserer Accommodation," in which occurs a masterly description of the highest degrees of hypermetropia. Stellwag von Carion had observed, that " the oversighted person," in order to recognise an object, brings it very close to the eye, and singularly enough, he seems to imagine (compare p. 269) that it then appears more illuminated. Von Grraefe has remarked the same, but has also, in part at least, indicated the cause, by proving mathematically, that in the hypermetropic eye, on approaching the objects, the angle under which they appear increases more rapidly than the circles of difi'usion, and by imitating the effect on the eye made hypermetropic by negative glasses. He shows further, that this hypermetropic eye is clearly distinguishable in structure from the myopic eye, by the flatness of its anterior chamber and by the narrowness of the pupil ; but he has not, any more than Stellwag von Carion, properly recog- nised and appreciated the slight and moderate degrees of hypermetropia. In my first communication {Nederlandsch Tijdschrift voor Geneeskunde, Jaarg, 1858, pp. 465-476. Archiv fiir OpMlialmologie, Bd. iv., Abth. i., pp. 301-340), and likewise in the dissertation by MacGillavry ( Onderzoekingen over de hoegrootheid der accommodatie, Utrecht, 1858), soon after written under my superintendence, the condition under consideration was indeed accurately distinguished from presbyopia, but the division into anomalies of refraction and of accommodation, and the opposition of myopia and hyper- metropia, were not yet apparent. It was at the meeting held at Heidelberg in 1859, that I first showed, among other things, that presbyopia and the so-called hyperpresbyopia are, both in essence and in symptoms wholly different conditions, that the latter alone is opposed to M, that an eye may be even very hyperpresbyopic without being in the least affected with pres- byopia, etc. I argued that, consequently, the name of hyperpresbyopia must HISTORICAL REMARKS. 331 be set aside, and Helmholtz, -who was at the meeting, immediately proposed the term hyperopia. This coincided with the word " oversightedness" ( Ueher- sichtiffkeit) first used by Ruete, and quickly found acceptance with some. On more fully working out my system, I thought, however, that the term hypermetropia would be more in accordance with the nomenclature I had already employed in the words ametropia and emmetropia, and this name has since that time been most generally adopted. My investigations respecting H are to be found in Ametropie en hare gevolgen, Utrecht, 1860, and in the Archiv fiir Ophthalmologie, Bd. vi., Abth. i., pp. 62-106, Abth. ii., pp. 210-243. Since I published my researches, diflEerent writers have treated of the anomalies of refraction ; among others Hasner (Klinische Vortrage iiber 'Augenheilkunde, Prag. 1860) ; Giraud-Teulon (Physiohgie et pathologie fonctionnelh de la vision hinoculaire, Paris, 1861) ; Happe (2)8e Bestim- mungen des Sehbereichs und dessen Correction, Braunschweig, 1860), and Soelberg "Wells (On long, short, and weak Sight, and their Treatment. London, 1862). Our knowledge of H has not, however, been increased by these works. That of Professor von Jaeger, Junr. ( Ueber die Hinstellungen des dioptrischen Apparates im menschUchen Auge, Wien, 1861) has also been fruitless in this respect. But the last-named writer has the merit of having been the first to make an attempt to trace the changes of the refractive con- dition of the eyes in the normal development of childhood. I think that less importance is to be attached in this respect to his measurements of the eye- ball and of the crystalline lens after death, than to his derminations of the refractive condition of the eyes at different periods 'of life with the aid of the ophthalmoscope, especially when previously to the investigation he para- lysed the accommodative system with atropia. Von Jaeger, following the example of Helmholtz {Beschreibung eines Augen-Spiegels zur Untersuchung der Netzhaut im lebenden Auge, p. 38, Berlin, 1851), early recommended the ophthalmoscope for the determination of the refraction of the eye ( Oester- reichische Zeitschriftfur prahtischeHeilkunde, No. 10, Marz, 1856), and he has undoubtedly acquired great expertness in the use of this method. His observations extend over more than 1600 eyes. The numbers noted by him are the following : — Of 100 eyes of • Hypcrme- ti-opic Emme- tropic Myopic Infants, from 9 to 16 days old, are . Children, from an infant-school, of between 2 and 6 years of age ..... Boys, in the country, between 6 and 1 1 years old . Girls, in the country, between 5 and 11 years of age ..... . Boys, in an orphan house, between 7 and 14 years ...... Pupils, from a boarding-school, of from 9 to 16 years . Soldiers (Italian), from 20 to 25 years 17 8 11 10 12 2 1 5 30 46 34 33 18 57 73 62 43 56 55 80 42 In von Jaeger's table the degrees of ametropia are also stated. CHAPTEE VII. MYOPIA. M. § 36. DiopTKio Determination, Diagnosis, Degkees, Oocue- EBNCB, HeEEDITAEINESS, DEVELOPMENT WITH ADVANCING AgE. Myopia we have already considered as the condition opposed to H. In the latter, the focus of the dioptric system lies behind, in M, on the contrary, it lies in front of the retina; in other words Fig. 124. parallel rays (Fig. 124, a a, hb), derived from infinitely remote objects, unite iu the myopic eye in front of the retina in f, and each infinitely remote point therefore forms upon the retina a circle of diffusion, (a b) of rays, which have already intersected. Hence it follows, that, in order to unite upon the retina in r^ rays must proceed from a point r situated at a finite distance, and must therefore fall diverging upon the cornea (see the dotted lines). The above is true of the eye in a state of rest. By tension of accommodation the emmetropic eye, and usually even the hyperme- tropic, can bring its focus for parallel rays in front of the retina; but it does not therefore become myopic. Even when by spasm of accommodation the focus comes to lie before the retina, and relaxa- tion to E is not possible, the eye cannot be said to be myopic. M depends, just like H, on the structure of the eye, irrespectively of accommodation, or rather with actual relaxation. In general M is easily recognised. The rule is, that near objects are accurately seen, while at a distance vision is, on the contrary, diffuse. So soon as letters of double magnitude are not recognised DIAGNOSIS. 333 at double the distance, we are, in general, justified in assuming the existence of M. The criterion consists further in this, that with con- cave glasses distant vision becomes more acute. However, this is not true of all concave glasses. In slight degrees of M, vision is even worse with strong concave glasses than without glasses, and in the highest degrees of M, the effect of weak concave glasses is scarcely percepti- ble. The question is, therefore, with what glasses must we in a particular case commence the investigation, in order to satisfy our- selves of the existence of M ; and this is answered by the first trials of the sight. iProm these trials we learn, namely, what is about the distance B. of the farthest point of distinct vision. Almost always the myopic presents himself before us with the statement, that he can see near objects well, while he with difRculty distinguishes at a distance, and if we then place in his hand a book with small type, for example I or II of Snellen's Tests, the distance which he chooses itself indicates about the farthest point. We now, however, make him remove the book farther off, until he reads less easily and sees even somewhat larger letters less acutely, and we estimate the distance at which this diminution of acuteness commences. If this be 6", we try at first glasses of — ^ ; if it is 10', glasses of — y^, &c. Al- most invariably vision at a distance is now better : of the table with CO to XX or less, letters wiU. be recognised at a distance, wliich without spectacles are not distinguished, and the existence of M is thus proved. The matter is simple : at 6", at 10", vision is distinct ; and by glasses of — ^, of — y^, the parallel rays derived from remote objects acquire a direction, as if they had proceeded from a point situated at 6", or at 10° from the glass (compare p. 32), and dis- tinguishing at a great distance has thus become possible. We now investigate immediately and more accurately the degree of M ; in other words, we determine R, the distance from r to k. This is found by trying, what the weakest negative glass is, with which vision is as acute as possible. In order quickly to attain this object, we place the glasses supposed to be suitable in a frame, cause the patient to look through them at the table already mentioned, and now hold, while we raise the frame somewhat with the fingers, glasses before the eye, which are somewhat weaker, and ask whether with them vision is as good or even better : if he answers yes, we then place these in the frame, and compare them, in like manner, again 334 MYOPIA. with somewhat weaker glassesj repeating the trial until we receive for answer^ that vision is not so acute with the glasses last tried. But we should even still try yet weaker glasses, and not be satisfied until we are told that vision is with them much less acute : expe- rience has in fact taught me, that we cannot be too cautious ; for with good accommodation the action of too strong glasses is easily enough overcome, and since with slight differences in strength the sight remains nearly the same, this equality may lead to its being said that the vision is less accurate. Thus I have with M = =-5 16 seen — ^ preferred to — ^^ or — ^p^, although — =-^ finally proved to be sufficient. — ^Not unfrequently, however, the glasses first tried are too weak, in which case, on comparison, the weaker are immediately rejected. This we find to occur especially with older myopes, who prefer reading at a comparatively great distance ; while very young, and still in the possession of a great power of accommodation, they readily bring the object within the distance of their farthest point, and so, on the first trial, they cause a higher degree of M to be suspected. If we have now found that, in order to attain the greater acuteness of vision, a stronger glass is necessary, we go on until further strengthening no longer produces an improvement, and we then try once more, whether a somewhat weaker glass is not equally satis- factory. Thus the object is at length attained. — We have already observed, that the distance at which the glass is held from the eye, has an influence upon its action. While a convex glass, by removal from the eye, acts more and more strongly, the reverse obtains with regard to concave glasses. The thing is plain : parallel rays, refracted by a concave glass, appear to proceed from a point placed as far hefore the glass, as the focal distance amounts to, and that point lies still so much farther from the eye, as the distance between the eye and the glass. Thus a glass of — -, held an inch o farther from the eye, is equal to a glass of — ^, but with this differ- ence, that the images are smaller : therefore — q> if held an inch closer to the eye, is chosen. Now, of the influence of distance on the action of glasses, we may often advantageously make use, in order to see quickly whether the glass tried is too strong or too weak. If DIAGNOSIS. 335 it is too strong, removing it a little is no disadvantage ; if it is too weak, the patient will hold it rather directly to the eye. However we must not be too sure on this point. Thus, the myope who has a good accommodating power, will hold even a too strong glass by prefer- ence close to the eye, because the retinal images are thus rendered larger. Therefore when vision is equally good or even better on the removal of the glass, we may infer that the latter is too strong, not vice versa. In any case we should never be satisfied, until we have determined what is the weakest glass which, held close before the eye, is quite sufficient. By this the degree of M, and at the same time the acuteness of vision are known (compare p. 97). — The investigation may in general be undertaken with both eyes at the same time : almost invariably the M is sufficiently nearly equal in the two eyes, and what has been found for both eyes, may in a moment be tested for each eye separately. But if we have reason to suspect, apriori, inequality of the M in the two eyes, or if we obtain confused answers to the first tests of vision, we may first try the one eye and afterwards compare the other. We should always begin with the eye which the patient himself calls the best, the other should be gently closed with the hand.* Many wiU possibly find the above directions tedious and too minute. And yet they are ia many respects incomplete ; so that, at the risk of incurring the displeasure of my readers, I will take leave to point out some additional sources of error. Let it be borne in mind that an incorrect determination of the degree of M may highly endanger the eyes. In the first place some thmk, although they are not nearsighted, that they see distant objects more accurately with concave glasses : the smaller dimensions of letters and of other forms appear to them so flattering and agreeable, that they feel bound to boast of their distinctness. Therefore we should never be content with the * Von &raefe has lately (see Deutsche Klinik, 1863, p. 10) had an optical instrument made, which, by a simple mechanism, can be so modified in its action as to represent lenses of very different focal distances. It may now, held before the eye, be so arranged that the ametropia is precisely corrected, and thus the tedious determination with different glasses may be dispensed with. Yon Graefe praises it very much. I am sorry that I have not yet been able to try it. It appears to me that the influence on the magnitude of the images must give rise to some disturbance in the deter- mination. In using this instrument, each eye is separately investigated. 336 MYOPIA. declaration : " I see better ;" but we should satisfy ourselves of its correctness, by making the patient name the letters. In some cases objects are really better distinguished by using con- cave glasses, although the eye is free from M. We meef with this especially in misty opacities of the cornea, when the narrower pupil, which is the result of the tension of accommodation made necessary by the concave glass, diminishes the diffused light. Particularly when the turbidity is local, and is so situated that it disturbs direct vision only when the pupil is dilated, constricting the pupil by using a concave glass, produces a very considerable improvement of the power of vision. It need not be said that the existence of a sufftcient range of accommodation is here the sine qua non for obtaining im- provement by means of negative glasses. — Further, one should beware of mistaking spasm of the ciliary muscle, with which the accommoda- tion cannot be relaxed to E, although the vision of distant objects is improved by negative glasses, for M. In speaking of the disturbances of accommodation, I shall revert to this subject. Here it may suffice to state, that the sudden occurrence of the disturbance, in connexion with other phenomena (especially myosis), wiU lead us to suspect the existence of spasm, which is easily tested by paralysis by atropia. We shall further see, in the disturbances proceeding from high degrees of M, that a certain measure of spasm is not unfrequently combined therewith, and makes the degree of M be too highly estimated. On the other hand, actually existing M is not always indicated on ordinary investigation. This depends upon different causes. In the first place, diminished S is to be noted. If, as we have already seen (p. 254), without the presence of H, a convex glass is sometimes chosen for distance, because the advantage of magnifying the images may counterbalance the disadvantage of diminished acuteness, a concave glass, which diminishes the size and adds little to the dis- tinctness, will, under similar circumstances, where M exists, be rejected. Always too, in the highest degrees of M, a too weak glass is preferred above a completely compensating one, and often vision is better with it : the cause of this lies partly in the diminished S, partly in the fact, that the circles of diffusion prove particularly small in relation to the degree of the M still remaining with imper- fect correction (compare the vision of myopes). — In the second place, a smaller pupil is to be noted. The smaller the latter is, the less disturbance do the circles of diffusion cause in imperfect accom- DIAaNOSIS WITH THE OPHTHALMOSCOPE. 337 modationj and thus it is explained why, with incomplete atresia pupillse, with very small papilla artiiicialis, in either case with dimi- nished S, nay, even with senile constriction of the pupil, neutralising a certain degree of M in general yields no advantage, and the neutralisation in such cases may therefore the more easily escape us. We should take particular care that in trying the glasses the eyelids be not squeezed together, as myopes are accustomed to do, and the circles of diffusion be so diminished. Next to the examination with glasses is that with the ophthalmo- scope. Ofthis we have already spokenin general (pp. 105e^«ej.). Ifthe pupil be wide, the investigator can determine high degrees of M in the um*eversed image; with a narrower pupil this is more difB.- cult, and the examination in the inverted image answers better, although the degree of M is not by it to be determined with accuracy. The ophthalmoscopic investigation yields essential service in the determination of the degree of myopia: 1°, when the eye is blind, and the nature of the morbid process may be connected with its structure. (Helmholtz* determined the M in such a case ; it has often occurred to me to be able to give a favourable prognosis for the second eye, because the ophthalmoscope showed me, that the lost eye had been highly myopic). 3°, when, with diminished S, the ophthalmoscope is first used, which now directly gives us tolerably accurate information respecting the existing ametropia : f at the first * Besohreibung eines Augenspiegels. Berlin, 1851, p. 38. + It is desirable systematically to carry out the ophthalmoscopic investi- gation for each eye. The emmetropic individual should combine with his ophthalmoscope a glass of -x, should look 10° or 15° to the outside of the visual axis directed on a given point, beginning at the distance of 10", and should gradually approach, so as consecutively to see the cornea, pupil, lens, and vitreous humour ; he should, in the investigation of the lens, especially make the patient look for a moment downwards (where the opacity usually begins) ; in the examination of the vitreous humour he should make him move the eye in different directions, and then suddenly stop at the originally fixed point (so as to see floating flakes) ; he should again remove, in order, with the interposition of a convex lens, to look into the inverted image ; he should then begin again in the original direction, so as to see the entrance of the optic nerve ; subsequently he should make him move slowly in different directions, and should look from different points, in order consecutively to pass through the whole fundus oculi ; above all, he should not neglect for a moment to make him fix the light in the mirror (in order to judge of the macula lutea) ; and finally, he should examine, if there is indication for it or if 22 338 MYOPIA. glance into the eye from a certain distance it is evident in strong M, that the inverted image stands before the eye ; 3°, when we wish to determine the degree of M in indirect vision, or to establish the existence of locally exalted M, through locally increased staphyloma- tous distention : in either case the examination with glasses fails ; — 4°j where simulated or concealed M is suspected ; 5°, in children, from whom correct answers are not to be expected. The degree of M is, as we have already seen (p. 91), expressed as R is the distance from the farthest point / to the nodal point k I" in the eye, situated about j behind the cornea. The position of r is found by detetmining the glass, which neutralises the M : it lies so far in front of this glass as the focal distance amounts to. If, for example, the required glass is = ■^^ y^j *■ li^s 12' in front of the 1" glass, and if the glaSs stands j before the eye, and therefore 1" 1" 1 . 7 + T — a ^ front of k', r lies 12^" from k', and consequently E is = 12*5 and M == 1 : 12'5 (compare p. 32). We now understand distinctly, that the distance r (that from the glass to A') signifies little when the glass is weak : it is, in fact, nearly indifferent, 1 1' whether a glass of — ^^ stands at ^ more or less before the eye, 111" by which it changes at most from — 'oruh *° — oT' ^^^ 9 difference in distance acquires much importance, when strong glasses are in question; for example, glasses of — -, which may then act as glasses of ~ 2^^°^~ 3- Therefore, in the manner above pointed out, use can be made for strong glasses of the difference in distinctness by differ- ence in distance, in order quickly to find the glass required, and therefore, where high degrees are concerned, in the determination of strong magnifying be desired, with a weak objeot-lens in the inverted image and with the requisite glasses in the unreversed, or employ the method of Liebreioh (Compare p. 367j. DETERMINATION OF ITS DEGREE. 339 the M, tlie distance required for the neutralising glass must also be accurately taken into account. In order that it may appear from the note made, that attention has been paid to this distance, we are accustomed to add it separately after a +, and to write, for example, M = a . I , which signifies that a glass of — ^ at ^" from k' was necessary to neutralise the M. The strongest concave glasses to be met with in spectacle boxes, are of — „. With these we can at most neutralise M = 1:2^. Now, not unfrequently, still higber degrees of M occur, and in order to determine these, we must place — -^ as spectacles before the eye, and examine what glass must in addition be held in front of them, in order to produce complete neutralisation. Let this second glass = — K, then both combined give — (- + -)= — 1:1-2; and to this we have still to add the distance a;from the strongest glass to k',* so that, with « = -^^ M is, in the case supposed ^1 : 1*7. M occurs in the most different degrees from E to M = ^p-^, probably stiU higher. The highest degrees are, however, the rarest. Eor many thousand eyes of myopic patients who have consulted me, the degree of M has been noted. The following table thence calcu- lated for one thousand eyes, gives a synoptical view of the relative occurrences of the different degrees. * If great accuracy is desired, we must, in the calculation, also take into account the distance between the two glasses, and reckon, for example, — i, placed at J" before — |, only as 1 : 3 J. We may then further, with sufficient precision, measure the distance to A', that is x, from the middle of the mass of the biconcave glass — ^. 340 MYOPIA. At first, to begin with the highest degrees, for each ^ M less, the num- her of cases increases. That the in- crease subsequently becomes slower, and finally even gives place to dimi- nution, depends solely upon the fact that comparatively few of those affected with the sHght degrees apply to the oculist. Nevertheless, I have endea- voured, in connexion with other ob- servations, approximativeh/ to express the relative occurrence of B and of the different degrees of M and H, among the Dutch population in gene- ral (Fig. 125). Along the figure are given, from 1 : oo , above, the de- grees of M to M = 1 : 1-3, and be- neath those of H to H ^= 1 : 2|, being the highest degrees observed by me. The lengths of the transverse Hues correspond to the frequency of occur- rence in the adjoining degrees, with this exception, however, that the lines a and a' next to — must be supposed 00 to be ten times longer. Since, there- fore, the mutual distance of the Unes represents ^^ the length (multi- plied by ten) of the first line above 1 : oo , being the liae a, repre- sents the number of cases from E to M = ^, the (simple) length of the second line the number of cases between M ^ -zr^ 96 and M = ^, the length of the third line the cases from M =: jQ to M ^ Kn, etc. — Of equal degrees of H, the occur- rence is expressed by the lines following under 1 : oo («' being again Degree of the Camber 16:24 = 1 :1J , 3 15:24 = 1: If 4 14 : 24 = 1 : If 13 : 24 = 1 : li^ 12:24=1:2 11 : 24 = 1 : 2^ 3 5 13 10 10:24=1:2| 24 9:24 = 1 :2f 47 8 : 24 = 1 : 3 7 : 24 = 1 : 3f , 6 : 24 = 1 : 4 5 : 24 = 1 : 4J 4 : 24 = 1 : 6 49 68 83 110 149 3 : 24 = 1 : 8 171 2 : 24 = 1 : 12 - 169 1 : 24 = 1 : 24 85 : 24 = 1 : 00 ^ COMPARATIVE FREQUENCY OF ITS OCCURRENCE. 341 taken as tenfold) . Prom the figure it now appears, that in the slightest Fig. 125. /.-^ I hIZ- 1:0). = A- 3 f degrees H occurs more frequently than M, while the contrary obtains in the higher degrees : M=^is abeady more frequent than H = 4 24. The highest degrees of both are so rare^that they can be expressed only by a point. From this figure it is very clear, that the emmetropic is the normal eye. If we bring M and H in their almost impercepti- ble degrees from ^g to E, E gives 1650 mm. length in lines, while all the other lines, representing H and M, amount to only 350; and 342 MYOPIA. these are reduced to 200 against 1800, when we take M and H at TTT under the E. 48 On the distribution of M, position iu society has a great influence. It is remarkable how much in the registers of my private patients (the more wealthy) the M, — in those of my hospital patients, on the contrary, the H predominates. To be correct, I must say, that among those in easy circumstances not much less H, but much more M occurs. That, moreover, the inhabitants of towns suffer more from M than those of the country is a matter of general observation. Ware*, upwards of 50 years ago, directed his attention to this fact. " I have inquired," he says, " for instance, of the surgeons of the three regiments of Poot-Guards, which consist of nearly 10,000 men; and the result has been that myopia among the privates is almost utterly unknown. Not half-a-dozen men have been discharged, nor have a dozen recruits been rejected, on account of this imperfection, in the space of nearly 20 years." In the Mihtary School at Chelsea, among 1300 children, no complaint was made; three only experienced some inconvenience. On the contrary, in the colleges of Oxford and Cambridge a considerable proportion of myopes was met with, in one college at Oxford, 32 out of 127. In all writers on the subject we find the same thing stated.f I must, however, remark, that in the country and amongst the least civilized ranks, even the highest degrees of M are exceptionally observed in particular famihes, and I wiU here add, that even among sailors, who never strain their eyes in looking at near objects, I found a few cases of progressive M. Further, I beKeve that M is not equally prevalent in aU countries. It is certainly more specially proper to cultivated nations. FurnariJ teUs us that among the Kabyles no myopes occur, and among the States of Europe visited by me, I, both in general life and in the cliniques, nowhere met with relatively so many myopes as in Germany. It would be of great importance, to possess accurate statistics of the ametropia occurring, at a given time, in a particular * Observations relative to near and distant sights of different persons. Eead before the Royal Society, 1812. t Conf. Szokalski, Prager Vierteljahrschrift, B. xvii. ; von Hasner, Klin. Vortrdge ueher Augenheilkunde. Prag. 1860, 1, p. 36. t Annales d' Oculistique, T. X., p. 145. INFLUENCE OF SOCIAL POSITION. 343 category of men, especially, for example, among the students of a university, in order to be able to compare them with the results of repeated investigations at subsequent periods. If it were thus found— and I can scarcely doubt that it would be so, — that the M is progressive in cultivated society, this would be a very serious phenomenon, and we should earnestly think of means of arresting this ' progression. Not only is the myope not in a condition to discharge all civil duties, not only is he limited in the choice of his position in society, but in the higher degrees M leads to disturbance of the power of vision, and threatens its subject with incurable blindness, The distribution of M, chiefly in the cultivated ranks, points directly to its principal cause ; tension of the eyes for near objects, Eespecting this fact there can be no doubt. But the explanation of it is not so evident. In tension of accommodation for near objects the crystalline lens, as we are aware, becomes more convex ; now if myopia also depended on greater convexity of the crystalline iilens, it might be considered as the permanent result of a frequently repeated state, and the M would thus be explained, But the M de- pends upon a prolongation of the visual axis, and this is not ^Itpred in accommodation for near objects. How then is this prolongation to be explained ? Three factors may here come under observation : 1°, pressure of the muscles on the eyeball in strojig convergejiqe of the visual axes ; 3°, increased pressure of the fluids, resulting from accumulation of blood in the eyes in the stooping position ; 3°, con- gestive processes in the fundus oculi, which, leading to softening, even in the normal, but still more under the increased pressure of the fluids of the eye, give rise to extension of the membranes. That in increased pressure the extension occurs priucipally ^.t the posterior pole, is explained by the want of support from the muscles of the eye at that part. With the imperfect elasticity of fibrous membranes we understand, moreover, that of each in itself imperceptible exten- sion above the normal limits a minimum each time remains. Now in connexion with the causes mentioned, the injurious efi'ect of fine work is, by imperfect illumination, still more increased ; for thus it is rendered necessary that the ipork be brought closer to the eyes, and that consequently the convergence be stronger, and the tendency to the stooping position of the head, particularly in reading and writing, is also increased. To this it is to be ascribed, that in schools, especially in boarding-schools, where, by bad light the pupils 344 MYOPIA. read bad print in the evening, or write with pale ink, the foundation of M is mainly laid, which, in fact, is usually developed during these years. On the contrary, in watchmakers, although they sit the whole day with a magnifying-glass iu one eye, we observe no develop- ,ment of M, undoubtedly because they fix their work only with one eye, and therefore converge but little, and because they usually avoid a very stooping position. The same causes, which give rise to M, are still more favourable to its further development. I have always, with great care, watched the course of myopia. I attach to it a special importance. The well- known fact that myopes, with little light, can recognise small ob- jects, and especially the circumstance that at an advanced period of life, they need no glasses to enable them to see near objects, procured almost general acceptance for the prejudice, that near- sighted eyes are to be considered as particularly strong. Many medical men even participate in this error. But the oculist has only too often been convinced by sad experience, of the contrary. I have < no hesitation in saying, that a near-sighted eye is not a sound eye. In it there exists more than a simple anomaly of refraction. The optical characteristic of myopia may consist in this, the anatomical is a prolongation of the visual axis, and the latter depends upon morbid extension of the membranes. If this extension has attained to a certain degree, the membranes are so attenuated, and the re- sistance is so diminished, that the extension cannot remain stationary, the less so, because in the myopic eye the pressure of the fluids is usually increased. In this progressive extension progressive myopia is included, which is a true disease of the eye. Prom what has here been said, it will easily be understood, that high degrees of myopia are less likely to remain stationary than slight degrees are ; at a more advanced time of life they even con- tinue to be developed, with increasing atrophy of the membranes. In youth almost every myopia is progressive ; the increase is then often combined with symptoms of irritation. This is the critical period for the myopic eye : if the myopia does not increase too much, it may become stationary, and may even decrease in advanced age ; if it is developed in a high degree, it is subsequently difficult to set bounds to it. At this period, therefore, the above-mentioned pro- moting causes should be especially avoided. On this point I cannot lay sufficient stress. Every progressive myopia is threatening with ITS PROGRESSIVE NATURE. 345 respect to the future. If it continues progressive, the eye will soon, with troublesome symptoms, become less available, and not unfre- quently at the age of 50 or 60, if not much earlier, the power of vision is irrevocably lost, whether through separation of the retina from the choroid, from effusion of blood, or from atrophy and de- generation of the yellow spot. In a subsequent section I shall have to ilfba.t of these sad results of M. The number of myopes most accurately examined by me, amounts to more than 2500. Each time the degree of the myopia was accurately determined and noted. If after months or years the myope consulted me again, the determination was repeated. I thus came to the con- viction, that almost always the myopia is somewhat progressive, that such is the rule between the 15th and 25th years, and that the highest degrees often exhibit the greatest increase. I have never in the periods of youth or manhood proved diminution of the myopia, except in the rare cases in which spasm of the accommodating sys- tem had temporarily increased it, and where, therefore, anomaly, not simply of refraction, but also of accommodation was present. Even at a more advanced time of life diminution of the degree of the myopia seldom occurs. Undoubtedly in the near-sighted eye the dioptric system undergoes the same change as in the normal (compare 204) ; but when at the same time the visual axis increases in length, as is very usual in near-sighted eyes, this change is wholly or partially compensated, and the myopia may even continue progressive at an advanced time of life. — ^All this is the result of direct experience, which, however, with respect to the same persons, extends only over some few years. In order, therefore, to get a satisfactory idea of the course of the myopia through the whole of life, the recorded ex- perience of many patients must be made use of. I have attached especial importance to this when, by the production of such spec- tacles as were formerly found sufiicient for distant vision, their report was confirmed. The test has in such cases not failed. When in this manner the ordinary course of the farthest point, that is the degree of the myopia through all periods of life, had been ascertained, it was not difficult to infer the course of the nearest point, as has been done in Eigs. 125, 126, and 127. Eor this it was only necessary to know the range of accommodation proper to each time of life. With respect to this, I have arrived at the conclusion, that in myopes it is about equal to that of normal eyes. In very 346 MYOPIA. high degrees only it is less; and here the whole eye, particularly the anterior part, the m. ciUaris includedj is extended, which may be noted as a sufficient cause thereof. These observations are illustrated in the sl^etches of Kgs, 126, 127, and 128, the meaning of which, after what has been said respecting Kg. 104, needs no further explanation. They represent three cate- gories of myopia, in its course of development, as it mostly oc(*irs. Fig, 126 is a stationary, Pig. 127 a tempora/rih/ progressive, Kg. 128 a permanently progressive mytjpia. The course of the myopia is shown by the line r r', which represents the farthest point, with parallel visual lines. The range of accommodation, proper to each time of hfe, is expressed by the distance between r r and p p. I must allow myself to make some observations respecting each of these categories. Fig. 126 is called stationary myopia. Yet we see the myopia f 2 9 2 3 12 24 00 -1:24 Fig. 126. 10 15 20 25 30 35 40 45 5P 55 60 65 70 75 80 L Tr^ ascend from ^ to ^^r 24 16 As I have above remarked, such ascent is, in the years of development, to be considered as the rule. Conse- quently if the ascent be not more remarkable, the myopia may, in contrast to the progressive, be called stationary. In gene- POPULAR ERROR AS TO ITS DIMINUTION. 347 ral the sligM degrees belong to this form. Por this reason, a myopia of only — was chosen for Fig. 126. However, on the one hand, an originally high degree of myopia may continue stationary, and, on the other, the slightest degree may become permanently progressive, and thus, finally, attain a very considerable height. This last we observe especially, when in the parents or other members of the family, a high degree of myopia occurs, while, moreover, the mode of life, especially stooping and strong convergence of the visual lines, promote its further development. In the most favour- able course of the myopia (Fig, 126), it remains quite stationary during the period of manhood, and may, on the approach of old age, even diminish a little, as the figure shows. This, how- ever, seems to occur but very rarely. The generally received opinion, that with the increase of years the degree of myopia usually diminishes, is an error, based partly upon the incorrect idea, that the degree of the myopia is determined by the nearest point, partly on the incontestable fact, that vision at a great distance gradually becomes more distinct, which is, however, to be attributed rather to the in- creasing constriction of the pupil. The temporarily/ progressive myopia is represented in Pig. 137. Fig, 127. 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 9 2 3 ^■.i ' 12 24 -1 :24 n --— • "•*- ^^ ^ ^ \.^ ^ \. ^^ ^ r ^ r 348 MYOPIA. lu this case the progression lies mostly between the 12th and 25th years. It is fortunate when the myopia becomes stationary at least before the 30th year. In Kg 127 it ascends from the 13th to the 35th year from s to ^; from the 18th to the 22nd year the ascent is the most rapid. After that it here becomes stationary. But, in fact, it is only exceptionally that, after having once attained this degree, it becomes perfectly stationary. High degrees of near- sightedness appear never to be congenital, unless we would refer congenital buphthalmos to that head. I cannot even decide whether, when the nearsightedness is hereditary, it is also in a certain degree congenital. I scarcely beheve it is. I have too often seen here- ditary myopia which, in the 12th or 15th year was present in a very slight degree, for example ^a' subsequently become rapidly developed to a high degree of -p or j, to think it impossible, that in the first years of life it was not almost wholly absent. On the other hand, I have extremely rarely seen near-sightedness arise after the 15th and never after the 20th year in eyes, which were previously normal. It is true it is often supposed by the patients that such is the case, but this is only because the primitive slight degree of myopia was overlooked. In this primitive degree, trifling as it was, lay the germ of the aifection; the complaints of various kinds are not made until the myopia becomes progressive. The myopia is most progressive when even in the 15th year it was rather considerable, for example = ^j as is assumed in Fig. 127. The course represented in Kg. 127, is therefore stiU to be considered comparatively favour- able. It now seldom becomes in manhood completely stationary ; stiU. more rarely does it diminish in advanced old age. Often it continues to increase, at least in some degree, and thus approaches to the Permanently progressive myopia, represented in Kg. 128. In the majority of cases belonging to this category, the myopia is con- siderable even at the age of 15 years. Therefore it is here assumed =: ^. It ascends most rapidly to the 25th, indeed even to the 35th year, more slowly at a more advanced period, incessantly, as it appears, but still often in jumps. The line r r gives a view of this. PREDISPOSITION. 349 It may ascend to ^ and more. The worst is then to be feared. It Fig. 128. 10 15 20 25 30 35 40 45 50 55 60 66 70 75 80 ? . I ,^ T 1: 12 24 —1:24 is rare at 60 years of age to find a tolerably useful eye^ with myopia of 1 : 2^ or even of 1 : 3. Diminution of such degrees of myopia at an advanced time of life^ is not to be thought of: the influence of the increasing distention of the eye in the direction of the visual axisj is never overcomcj and is even not compensated by the dimi- nishing refraction of the lens. Prom the progression of the M it follows, that the higher degrees occur proportionately most frequently at a more advanced time of life. How far very young children are afiected by it, has not been accurately investigated. A commencement in this direction was made by Ed. v. Jaeger, who has also stated his intention of follow- ing the course of the refraction in the same persons through their whole Ufe. We wish him for that purpose a long life and faithful patients. He should, however, not have neglected the value of the method, followed by me, of throwing some light on this subject. If the above-mentioned causes are capable of giving rise to M and of further developing it, the predisposition thereto is very different. I have already stated that I have never seen an hypermetropically constructed eye become near-sighted. Even on many emmetropic eyes, simple tension of accommodation for near objects has but little 350 MYOPIA. effect. In fact the predisposition is almost invariably congenital, and in that case it is, moreover, nearly always hereditary. Beer, Jiingken, Bohm, von Hasner, and many others, have referred to its hereditary nature; I believe, even, that from time immemorial the conviction thereof has been general among the people. At least, at present the patients are accustomed at once to state, that their father or mother was near-sighted, and that the same condition was found among brothers and sisters. I cannot, with any accuracy, give the pro- portion in which hereditary cases occur ; but this I may say, that where I found near'sightedness in one or more of the children, and had an opportunity of examining both parents, I only exceptionally saw M wholly wanting in both, while, on the other hand, when one and equally when both parents were myopic, the predisposition almost always passed on, at least to some of the children, perhaps more specially to the younger (v. Artha). Experience shows further, that where only a trace of M is present in youth, it inevitably becomes further developed, and that the greatest care leads to nothing more than limitation of the degree. In statements of its diminution in youth, I have never met with accurate determinations of the degree of M, and we know how lightly assertions are in general made as to the increase and diminution of M. A consideration of all these facts leads us to the conclusion, that fundamentally emmetropic eyes seldom, fundamentally hypermetropic eyes perhaps never, become myopic ; but that, having once occurred, the M is often transmitted as pre- disposition to posterity, and under fresh exciting causes is developed to its higher degrees. Thus the hereditary principle accumulates in the posterity the effect of the causes repeated in every generation. In some families the M therefore has attained a high degree, and the danger is the greater, because, according to experience, the hereditary tendency manifests itself the more certainly, the more the myopia has already been transmitted through a number of generations, and has assumed a typical character. A double distinction has been made in M, in wUeh I cannot agree. Two classes have been formed with respect to the degree and to the congenital or non-congenital nature of the affection. As to the degree, since observers had based their idea of M upon the fact, that in it acute vision could exist only close to the eye, they were puzzled by those cases in which there was distinct vision at a distance of 2, 3, and 4 feet, and nevertheless at the dis- tance of from 1 5 to 20 feet letters of the magnitude of an inch could no longer be easily distinguished. They did not perceive that in such cases MYOPIA AT A DISTANCE. 351 they had to do with slight degrees of myopia, with degrees of —., oo, toj in which the farthest point lay at about 24, 36, or 48 inches from the eye. At the distance of 15 feet the circles of diflfuaion are then already pretty considerable, at least when the pupil is tolerably wide. The confusion produced by an incorrect Comprehension of these cases is incredibly great. Dr. Kerst met among the young men who presented themselves as pupils at the school of military surgery at Utrecht, some who easily read ordinary print at the distance of from 1 5 to 20 inches, and never- theless at the distance of from 12 to 20 feet could not distinguish letters of the size of 3 or 4 inches. He wrote on the subject to Cuhier, and concluded by asking, whether this condition was not a sort of myopia, although no mention was made of it in the various works on diseases of the eye, under the head of myopia. Cunier communicated this letter to Sichel {Lemons cUniques sur les lunettes, etc. Bruxelles, 1848, p. 99), and the latter most properly answered Dr. Kerst s question in the affirmative. However, as it appears from the lengthy reasoning of the writer, it was not clear to Sichel that in such instances we have to do simply with a degree of myopia, in which the farthest point of distinct vision lies at from 15 to 20 inches. Some years later Fronmiiller treats, in reference to this point, of " a va- riety of short-sightedness," and gives to it the name of " myopia at a distance." Sichel and Kerst, he says, first drew attention to it. However, Fronmiiller diesariii6s{BeohacMungen auf dem Gebiete der Augenheilkunde, Tiirth, 1850, p. 54), as an example of his myopia at a distance, a case of hypermetropia — as appears from the circumstance, that with a convex glass distant vision was distinct and easy, while concave glasses made vision at any distance more indistinct. Fronmiiller has, therefore, confounded a moderate degree of hypermetropia, with which ordinary print could still be read at a distance of 10' (as is not unusual), with the oases of slight myopia, to which Kerst had directed attention, and has given it the name of myopia at a distance. Nevertheless, even Kerst himself subsequently applied the name of " myopia at a distance" to slight degrees of myopia. Finally, von Graefe {Arehivf. Ophthalmotogie, B. ii. Abth. 1, p. 158), not knowing at the time, as he acknowledges, where and by whom the name of M at a distance was introduced into science, makes a rational use of it to characterise those cases in which, with rtferenee to the degree of myopia, the distinguishing of remote objects is very defective. He investigated and ana- lysed a case of the kind with great accuracy, whence it appears that this condition may depend on an involuntary action of the muscles of accommo- dation, which is spasmodically combined with every effort to see farther than the naturally farthest point. Such oases, however, are certainly of extremely rare occurrence. Among more than a thousand myopes I have not met with a single instance of the kind. The too great indistinctness of remote objects, with reference to the degree of myopia, was always to he explained by a more than ordinary size of the pupil. This does not prevent vision being perfectly acute at the distance for which the eye is accommodated ; but on account of the magni- tude of the circles of diffusion increasing with the diameter of the pupil, 352 MYOPIA. it makes the observation beyond the boundaries of accommodation very im- perfect. In this, too, as we have seen, lies one of the causes, why many think, that with the advance of years their myopia has diminished, even when this is not the case : their pupil has become smaller, and they consequently see better at a distance. If we cause the so-called myopes at a distance to look through an opening with a diameter of 5 mm., the disproportion has disappeared. Let us beware, therefore, of explaining every disproportion between the degree of myopia and the observation at a distance, by spasm of the muscles of accom- modation. The size of the pupil will almost always account for it. There- fore, too, in my opinion, the term myopia at a distance, which has already given rise to so much confusion, may very well be dispensed with in science. Neither the slight degrees of myopia, which are as well defined by ^ as the high, nor hypermetropia, which is the opposite of myopia, deserve this name, and the rare form of disease, distinguished by von Graefe, might be denoted as spasm of the muscles of accommodation, produced by an effort at relaxation. With respect, in the second place, to the origin of M, I have above stated the result of my experience. It amounts to this, that M is almost always hereditary, and is then further, at least as predisposition, congenital ; but that it is also, exceptionally, without special predisposition developed in the emmetropic eye as the result of excessive tension of accommodation. This influence of extraordinary accommodation necessarily suggested the idea of change of form of the lens. Eighteen years ago {Nederl. Lancet, 1845) my argument was as follows : " M is the result of accommodation for near objects ; examine what M permanently is, and you will know the change which the accommodation each time excites." By the discovery of the cause of M in a longer visual axis, and of the principle of accommodation in the greater convexity of the lens, my expectation was disappointed. But I never lost sight of the possibility, that in myopia, especially in the acquired variety, the lens should be really more convex. It would certainly not be strange that, especially during the periods of development, the influence of much accommodation for near objects should permanently affect the form of the lens, and that, vice versa, want of this accommodation might lead to unusual flatness of the lens (compare p. 246). I have, however, in no way been able to satisfy myself of this.— Moreover, in the fact that M, almost without exception, is either connected with a peculiar posterior atrophy, or at least before the fortieth year becomes connected therewith, lies the proof, that at all events nearly without exception, another cause is in operation : this posterior atrophy is, namely, connected with a disten- tion of the posterior part of the eyeball, which distention it either accom- panies or rapidly follows. And in the rare cases in which this posterior atrophy is wanting, I have mostly found, by direct measurement with the ophthalmo- meter, a morbidly distended cornea, or I established the existence of morbid distention of the anterior part of the sclerotic, with prolonged visual axis, so that finally, no oases remained in which I was obliged to take refuge in extraordinary convexity of the crystalline lens. (See further the Section upon the anatomical changes in M.) Moreover, in adults, and even in young people, provided they are abso- NON-IMPLICATION OF THE LENS. 353 lutely free from M, simple tension of accommodation is not sufficient to excite M. Even ■wearing concave glasses does not give rise to it. Only in the artificial H produced by concave glasses, as well as in the natural, a part of this H finally becomes latent, that is, the eye accustoms itself in distant vision also to tension of the muscle of accommodation; but that no or- ganic change of form of the lens is in operation appears from the fact that atropia, by paralysing the ciliary muscle, immediately suppresses the apparent M. The displacement of the relative range of accommodation in the use of glasses, which modify the connexion between convergence and accom- modation, temporarily gives rise to some difficulty. This is true also even of positive glasses. Thus, after working with a magnifying glass, in which, perhaps, without wholly parallel lines, I am accustomed to relax my power of accommodation as much as possible, it is at first difficult for me again satis- factorily to accommodate at a convergence, for which I otherwise find no diffi- culty, without, however, the absolute points^ and r being at all altered. The same is soon perceived, when, holding a weak prism with the angle inwards before the eye, we wish to see singly at a distance, and thus, with convergence, to accommodate for a distant object. The opposite is produced by concave glasses or by a prism with the angle turned outwards. Now, it is easily under- stood that at least prisms, which do not at all modify the accommodation, will exercise no permanent influence on the crystalline lens, and the efiect of convex and concave glasses, within certain limits, completely agrees with that of prisms. All the foregoing is most strongly opposed to the importance of the crystalline lens in the production of M. But although there was no argu- ment against the latter, so long as it did not appear, from direct investi- gation, that by tension for near objects the lens permanently acquires- greater convexity, it seems an ignoring of the requirements of science to admit a form of M based upon mere supposition, and to oppose this (writers have even given it the special name of plesiopia) as acquired M, to the almost invariably congenital myopic structure of the eye. Ed. von Jaeger {I. o. p. 28) says : " that in such individuals as are constantly occu- pied with tension of accommodation, fewer eyes do not in general occur with normal length of axis, than under opposite conditions, and that in those classes of the population who are not usually accustomed to fatigue their eyes by effiarts of accommodation, at least as many, nay, even more individuals occur, who, in consequence of prolongation of the axis of the bulb (Staphyloma posticnm), are short-sighted, than in the u{)per ranks of society." So far as the Netherlands are concerned, I can expressly contra- dict this assertion, and with all respect for the zeal of observation of von Jaeger, I cannot admit its truth even for Austria. § 37, Results op the Ophthalmoscopic Investigation of the Myopic Eye. Since the ophthalmoscope has made the fundus oculi accessible to 23 354 MYOPIA. examination during life, our idea of the anatomical basis of M lias undergone a complete modification. Ophthalmoscopic investigation has shown that, almost without exception, even in moderate degrees of M, changes, especially in the chorioidea, are to be observed ; and it has now further been found, that these changes are the expres- sion of atrophy of the chorioidea, which, combined with atrophy of the sclerotic, is, as well as the latter, dependent on a distention of the posterior part of the eyeball. Miopia and Staphyloma ^posticum have thus become nearly synonymous. Thousands of myopic eyes have been examined by me ; of not less than 700 do I possess more or less detailed drawings or sketches, of some eyes more than one, with an interval of some years ; and in every case sex, age, degree of M, and in many instances accommo- dation, movements of the eye, acuteness of vision, hereditariness and accessory disturbances have been noted. Prom these observa- tions, which, communicated in extenso, would form a volume, the following description, and also the conclusions drawn, are in great part borrowed : much of the same is, however, abeady to be found in the writings of my predecessors. The principal changes are : atro- phy of the chorioidea, on the outside of the entrance of the optic nerve, when highly developed, combined with change of form of the nerve- surface, a straightened course of the vessels of the retina, vncom/plete diffuse atrophy of the chorioidea in other places, and morbid changes in the region of the yellow spot. I commence with a description of the changes in general, and shall afterwards sketch their develop- ment, in connexion with the degree of M and of the time of life. 1°. Atrophy of the chorioidea, principally to the outside of the entroMcd of the optic nerve. The surface of the optic nerve of the normal emmetropic eye exhibits itself as a nearly round, rather strongly reflecting, slightly reddish plane, from which the retinal vessels set out ; often an impression is to be seen on it. This plane is distinctly bounded by the commencement of the pigment of the chorioidea, to the inside of which we sometimes observe a thin, white, strongly reflecting line (the so-called sclerotic-boundary of Liebreich), to which the faintly-defined nerve-substance then succeeds. To this part of the fundus oculi we usually first direct the eye. At the first glance we recognise in it with tolerably great certainty the M (compare Pig. 129), distinguished by a crescentic, strongly re- flecting surface (c) between the outside of the nerve {n) and the boundary of the pigment of the chorioidea. That surface is OPHTHALMOSCOPIC APPEARANCES. 355 always poor in pigment. Still it may, if it be slender, when there is Fi&. 129. normal or even diminished fulness of the vessels, be proportionately red, but the colour is then also brighter than that of the rest of the fundus and approaches sometimes to orange ; almost always, however, it soon acquires a whiter shade, on which at first the larger chorioideal vessels, extended in a horizontal or radiating direction, are visible, often even more distinctly than on the adjoining parts of the chorio- idea abounding in pigment. Between the straightened vessels the remaining pigment of the stroma is recognisable as oblong brownish- grey Uttle spots. The chorio-capiUaris in this place now seems no longer to carry blood. At length all blood-carrying vessels may disappear in the atrophied place, which now appears still grey or marbled and, finally, perfectly white, reflficting still more strongly than the nerve-surface itself, although the latter has also increased in whiteness. However, even now some darker pigment-spots (pig- ment-epithelium) occasionally stiU remain in the atrophied place, espe- cially near the margin, similar to those in the neighbouring red tissue. The atrophied surface passes sometimes without defined boundaries into diffuse atrophy, and through this into normal tissue ; in general it is, however, limited by a sharp, tolerably regularly curved boundary hne (circumscribed atrophy). This line is distinguished, at least here and there, by an abundance of dark pigment, as usually occurs on the boundaries of morbid changes of the chorioidea, as well as on 2 356 MYOPIA. its normal limits. Not imfrequentlj there is, at a certain distance, a second dark line, parallel to the boundary line, together with in- creased vasctdarity of the adjoining chorioideal tissue, or on the atrophied part traces of an innermost concentric hne of pigment are to be seen. Theybrwi* of the circumscribed atrophy is almost always that of a crescent whose concave side encompasses the outside of the nerve- surface (crescentic atrophy). In slight degrees scarcely to be recognised, sometimes only locally through a projecting dark point (Kg. 130), it extends usually the farther around the nerve-substance, the broader it is, that is, the longer the axis (a a! Fig. 133) of the crescent is (compare Pigs. 130 to 132). On further development the * I append some sketches merely for the determination of the forms of the atrophic place. The above figures are taken from particular eyes. No. Bex. Age. Eye. Degree of M. 130 P. 12 Left 1:7 131 F. 32 Eight 12 132 M. 42 R. 6 133 M. 51 K. 9-6 134 M. 24 E. 4-75 135 o F. 37 E. 2-5 h F. .16 L. 3-8 c M. 25 E. 2-7 136 a M. 32 E. 2-14 h M. 57 E. 4 137 M. 20 E. 3-5 138 M. 66 E. 3-1 139 F. 46 E. 2-3 140 F. 66 L. 3 141 M. 13 E. 5-6 142 M. 54 E. 7 . 143 F. 22 L. 3 Excellent representations of the fundus oouli of the myopic eye are given by many, among others by Dr. Ed. Jaeger {Beitrage mr Pathohgie des Auges, Wien, 1855, PL 17 and 18 ; and Ueber die JEimtellungen des dioptr) Appar., Wien, 1861, Tab. 11) ; and by Liebreich {Atlas d'ophthalmoseopUt Paris, 1863, Tab. 111). VARIOUS FORMS OF ATROPHY. 357 atrophy assumes very different forms. If the axis is longerj without proportionate extension around the papilla, the crescentic form passes into the semi-elliptical, of which various modifications occur (Eig. 135 a, h, o) ; if, on the contrary, the atrophy extends more around the nerve-surface, without proportionate prolongation of the axis, the semi- annular form arises (Fig. 133), and further, the annular (Pig. 134), which, when more extended, deserves the name of elliptical (Rg. 136 a) or circular (Kg. 136 h). In all these forms the ring is almost con- stantly broader on the outside tlian on the inside, where it is often still slenderer than above and beneath. Finally, the atrophy may iS6a ■tSeh {S7 be very irregularly bounded, for example, it may have an angu- 358 MYOPIA. larly curved form (Fig. 137), may present ramifications (Kg. 138), sometimes in the form of a leaf of clover (Kg. 139), and may even have completely separated atrophic spots around it (Kg. 140). Thus the atrophy attains a considerable extent of three or even six mm. and more in different directions, in which cases still other parts of the chorioidea are in various modes morbidly affected. In the emmetropic eye the yeUow spot lies to the outside of the nerve (from the middle of its surface to a distance of about 4 mm.), almost always, however, somewhat lower. The axis of the atrophic crescent has a similar, but usually somewhat yet more descending direction, and is therefore turned towards the yellow spot. (The figures 129-140 are aU drawn with the image inverted) . Extremely rarely, however, even with the greatest develop- ment of the atrophy, does it reach directly to the yellow spot, which removes more and more from the nerve-surface ; but it is very usual, as we shall see, in high degrees of atrophy, to find the yellow spot independently affected. Deviations from the direction described are, however, on the whole, not uncommon ; the axis may descend much more directly (Kg. 141) and even be directed completely down- wards (Kg. 142), or it may be horizontal, and may even ascend con- siderably (Kg. 143), but it is never directed straight upwards. In rare cases the atrojphic part is particularly strongly excavated, as may appear on its outer margin from a certain curvature of the retinal vessels ; in general, however, the curvature is continued tolera- bly uniformly in the non-atrophied part. 3°. The nerve-mrface has, in high degrees of M, undergone a partly apparent, partly 1/rue change in form. — As to the apparent, the horizontal dimension often appears comparatively smaller. The reason of this is evident : in the emmetropic eye the nerve-surface lies only a Kttle to the inside of the axis of the cornea, and we therefore look through the pupil nearly directly on it ; on the con- trary, in the highly-distended myopic eye (compare Fig. 55), in which the nerve-sTirface is displaced more to the inside, the perpen- dicular to this surface is directed more strongly outwards, and this causing us to look obliquely on the nerve through the pupil, it appears shortened in the horizontal direction (Artl). But besides this apparent, there occurs a true change in form, which everyone had indeed seen, but to which, if I am not mistaken, Liebreich* was the first to direct * Archiv f. OpMhalmologie, vii. 2, 124. Atlas d'ophthalmoscopie, Paris, 1863, p. 6. CHANGES OF THE YELLOW SPOT. 359 attention : the nerve-swrfaee usually has, in extensive atrophy, the greatest diameter in a direction perpendicular to the axis of the atrophy. Yon Jaeger endeavours to attribute this also in great part to appearance; but the fact is true (compare the above figures). To these, other changes supervene. In young myopes the nerve-surface is particularly red (capillary hypersemia), otherwise it is not altered ; subsequently, in high degrees, on the contrary, it is almost always, wholly or partially, strongly reflecting, not unfrequently it is locally excavated, with a more distinct appearance of the (more superficially situated) lamina cribrosa ; also, perhaps, somewhat excavated over the whole surface, apparently atrophic and sometimes passing, without distinct boundaries, into the strongly reflecting atrophy. I have seen cases in which a retinal vessel appeared to come from an atrophied place, which we were therefore again inclined to refer rather to the nerve-surface. — Of the true dimensions of the nerve-surface it is diffi- cult to judge, because the myopic structare has a very complicated influence on the magnitude of the inverted image. In the note to this section some further remarks wiH be found upon this subject. 3°. The retinal vessels, which are depicted with incomparable clearness on the atrophic surface, are distinguished in high degrees of M by their straight and little tortuous course ; this is particu- larly true of the most atrophied places. This straightened course is evidently the result of the extension, which the retina also has under- gone. In the calibre of the vessels there is seldom any great change. 4°. Much farther than the strongly reflecting and completely atrophied part we find, in high degrees of M, proofs of extension and attenuation of the chorioidea, both in the stronger reflexion and in the straighter course, with greater mutual distance of its vessels. Not unfrequently, too, the stroma-pigment, which occupies the intervascular spaces, is in these parts evidently diminished, and the epithelium of pigment-ceUs is unequally distributed. In the intervals the surface is now sometimes dotted with yeUow or white, and reflects strongly. We often find this diffuse atrophy at the inside of the nerve-surface, but especially at the outside, and continuous with the complete atrophy, which is now less sharply defined. The region of the yellow spot has now also often passed into atrophy. 5°. Circumscribed changes of the yellow spot and of the fovea centralis. — These interfere with direct vision, and deserve all our attention. In high degrees of M we should therefore never neglect, after the investigation of the nerve and the parts around it, also to 360 MYOPIA. examine the yellow spot, wliich we have in sight, when the observed eye fixes the flame reflected by the mirror. The changes are con- nected with a continuation of the atrophy from the ontside of the optic nerve, or they exist independently and separately. They appear as scattered irregular granular pigment, lying or not on one or more oval or angular bright-red spots, surrounded by diffuse pigment, or as some larger dark groups of pigment on a white ground or alternating with white spots, or finally as one single, sharply- defined, sometimes bluish and elevated spot, which may attain even the mag- nitude of the nerve-surface : this last form is probably connected with extravasation of blood, which I have sometimes seen limited to a part of the yellow spot. 6°. The further changes in the fundus oculi, which, without be- longing to the essence of myopia, are apt to supervene in the highest degrees of M, are chorioiditis disseminata, distinguished by white and yellow spots of various sizes, with irregular deposition of pig- ment, scattered here and there j moreover, spots of extravasated blood in the retina, whether one or more larger, or numerous smaller ones, which, finally, pass over into pigment-spots, — and detachment of the retina by blood, and much more frequently by exudation. A movable, grey, slender, but tolerably long flake also sometimes extends from the nerve-surface to which it is adherent, into the vitreous humowr. Lastly, we sometimes see, in the highly myopic eye, a particular form of glaucoma arise with the changes peculiar to the same. 7°. In the more fluid vitreous humour, movable flakes are, in the high degrees of M, among the most ordinary phenomena. They are in part connected with the changes described under 6°. 8°. In high degrees of M we see more than ordinarily a com- mencement of obscuration of the lens. Where there is detachment of the retina, whether with or without secondary iritis, obscuration is often rapidly developed even in youth. Of the changes just described the atrophic crescent on the out- side of the nerve-surface is the most common and the most charac- teristic. This is, the time of life being equal, in general more fully developed the higher the degree of M is, and for equal degrees of M it is more developed the more advanced the age of the individual is. The subjoined table exhibits the length of the axis (Fig. 132 a, h) of the atrophied part, in relation to the age and the degree of M, deduced from observations on 1400 eyes. DEVELOPMENT OF ATROPHY. 361 Length of the Axis* of the Athophic Ckescent in Millimetres with Age in years. M = JLtol 00 12 «=A«J ^=l-\ M = ltol M = J.. 10 to 30 30 to 50 50 to 80 0-1987 mm. 0-2975 0-7059 0-4255 0-7035 1-046 0-5563 0-9679 1-183 0-7431 1-356 1-795 1-25 1-68 2-127 The table exhibits most clearly the influence on the extent of the atrophy, both of age and of the degree of the myopia. Even when for less difference in degree and less difference in age the average of a certain number of cases is taken, the same regularity is stiU. apparent. Nevertheless the individual observations differ very much, so that as minimum, in tolerably high degrees, only traces of atrophy occurred, and the maximum may amount to 6 mm. and more. The increase of the atrophy with age appears to be placed beyond all doubt. Taking into account that the degree of the myopia also, at least in most cases, increases, we may even infer that the atrophy in- creases with age still more rapidly than the numbers belonging to the same column indicate. Probably we approach the truth by reading off in the diagonal direction, and thus assuming that, when the axis of the atrophy at twenty years amounts to 0"1987 mm., this at 40 years increases to 0"7035, and at 65 to 1"183. After the simple announcement of the facts, I shaU now endea- vour to give a sketch of the development of the atrophy and of its concomitant phenomena, as the ophthalmoscope reveals them in the myopic eye. In a very young individual atrophy is, in moderate degrees of M, very rarely to be observed. Hasnerf declares, however, that he has seen it remarkably developed at four or five years of age, and Ed. v. Jaeger has, from the examination of children in schools, even come * The axis of the atrophic part was determined by measuring the sketches made of 700 cases of myopia. For actual magnitude the nerve-surface was used as a scale, whose diameter was assumed =: 1-9 mm. In the annular and in the circular atrophy, the extent on both sides of the nerve-surface was assumed as axis. Imperfect and undefined atrophy are not in- cluded. ■j- Klinische Vortrage iiber Augenheilkunde. Prag, 1860. 362 MYOPIA. to the conclusion, that a particular form of atrophy, which some- times led him to recognise one child as brother or sister of the other, differing ia each of the two eyes, may be hereditary. Even in nurslings and new-born infants of myopic mothers, he saw the tendency to atrophy of the same form. I too have seen, without particularly looking for them, some cases of distinctly developed atrophy at the ages of five and seven years, though in simple M the intervention of the oculist is seldom invoked at so early an age. The rule is, however, that even in those, in whom we may subsequently expect a high degree of M, during childhood only sKght traces of atrophy are at most to be seen. There first appears either an irregular, small projection, rich in pigment (compare Kg. 139), to the outside of the optic nerve, or a little crescent, distin- guished from the adjoining fundus by a brighter red colour, its boundary being somewhat darker at the outside than the contiguous fundus. In the first case the atrophy extends directly from the margin of the nerve-surface, while the pigment, in a certain sense, removes outwards, and a white strongly reflecting Hne, which soon assumes the crescentic form, appears along the nerve-surface. In the second case the atrophy is, in a certain sense, directly terminated, evidently from diminution of pigment, and its further development consists at first rather in the becoming more perfect, than in the extension of the atrophy. Often, however, we now see a second con- vex line of pigment arise at the outside of the original crescent. While in adolescence myopia is gradually more and more developed, there not unfrequently set in troublesome symptoms of irritation, and on ophthalmoscopic examination we now often find the most external boundary of the atrophy, as well as the nerve-surface itself, in a congestive condition. If the myopia seems likely to attain only a slight degree, there occur in adolescence but the first traces of atrophy, and where this is the case, a great development of the same is to be apprehended only from an improper use of the eyes. But where myopia of ^ or ^ exists from the sixteenth to the twentieth year, we find, almost without exception, a very decided, almost perfect atrophy accurately defined under the form of a crescent, whence it sometimes appears that precisely this part is more highly concave than the rest of the fundus. The pigment-epithelium has disappeared upon this surface, the chorio-capillaris seems to be no longer present the greater chorioideal vessels are slender, extended, sometimes they have PROGRESS OF THE ATROPHY. 363 almost wholly disappeared, while only brownish-grey spots still indi- cate the intervaseular space. — From a combination of numerous observations we may very well deduce the development of the atrophy j but I have in addition, had the opportunity of examining repeatedly in consecutive years many eyes, and this has shown me that the atrophy sometimes advances more rapidly at intervals, but that it very rarely continues long, for example, for one or two years, unaltered. Especially in youth, when the adjoining tissue of the chorioidea is particularly rich in blood, and long-continued tension of the eyes is attended with inconvenience, the development proceeds regularly. Apparently, the boundary of the pigment is thus displaced outwards. In so far as the membranes at the outside of the nerve- surface are extended, this is also really the case. But in addition we must, in explanation of the process, assume that in the place where, at the inner boundary of the hypersemia, in consequence of extension the atrophy begins, the pigment is absorbed, and at the same time is more strongly formed on the hyperaemic line extending outwards. In general the stronger formation of pigment is due to hypersemia of the chorioidea.* Now, if on the one hand, probably through a more rapid course of the atrophy, the absorption of the pigment is imperfect, and if black spots therefore remain in the atrophic part, the hypersemia on the other, extends in some points more outwards, and here gives rise to increase of pigment. Thus it would appear too, that the spots of pigment existing at both sides of the boundary of the atrophy may be explained. A real displacement of the pig- ment over the surface of the chorioidea, as it occurs in exudative chorioiditis, seems here not to be admissible. — The direction, in which the atrophy at first extends, is at once conclusive as to the form to be expected. This is indeed included in what has already been said. Here we may remark only, that in slight degrees the form is never annular, and that the annular and circular forms are to be expected when the crescent rapidly extends far around the nerve-surface, most of all, when also at the opposite inside of the nerve- surface a second smaller crescent is early formed. The atrophied part is for a long time regular in form, and beyond it scarcely any change is to be seen in the fundus oculi : at most it may be observed, when a strong pigment-epithelium does not prevent it, that the chorioideal vessels are more separated, and are in part more * Compare Coocius. Ueher Olaueom, Entzundung und Autoshopie mit dem Augenspiegel, 18.59, p. 36. 364 MYOPIA. slender than usual. But after the five-and-thirtieth, and particularly after the fortieth year, in high degrees of M, the changes above described begin here and there to occur (compare Kgs. 132, 135, and 139). Now also the boundary line of the atrophy sometimes describes a more irregular form, or atrophic spots have formed next the original ones, which by the extension of both may subsequently coalesce. The most important change beyond the seat of the original atrophy is the degeneration in the region of the yellow spot, especially when it occupies the fovea centralis. It is true this may occur at any age even in non-myopic eyes, and on localised disturbance of the function in the same place, with or without perceptible organic change, a number of cases of ordinary amblyopia depend. But a particular morbid change, based upon extension and atrophy of the sclerotic and chorioidea, whereby the retina here also becomes secondarily disturbed in its function, is decidedly peculiar to high degrees of myopia. As we know that the yellow spot often corre- sponds to the apex of the staphyloma posticum, and that in high degrees the atrophy of the sclerotic and chorioidea is here the strongest, this can by no means surprise us. In the first place, there often arises, in M = 5 or more, in advanced life, imperfect diffuse atrophy of the chorioidea, in a belt passing from the outside of the atrophy through the yellow spot, and recognisable by a dispropor- tionately white or yeUow-dotted grey, sometimes strongly reflecting almost glittering appearance, with interspersed pigment.; and where this occurs, the region of the yellow spot never altogether escapes ; but in addition, at a comparatively less advanced age, the above-de- scribed local degeneration of the yeUow spot not unfrequently super- venes on the original atrophic crescent, while the part situated between this and the yellow spot still deviates but Kttle from the normal. On one occasion I observed this in a patient aged fifteen ; under thirty years of age it is, however, even in the highest degrees of M, stiU rare j after the thirtieth year it recurs with comparative frequency in myopia of i or more, and at sixty the yellow spot, and even the fovea centralis, have in M of J, often, and in myopia of 2^ and i, they have, I might almost say, always suffered. This occurs both when the original atrophy is annular or circular, and when it extends only to one side, sometimes even when the axis of the atro- phy is still short, and is not directed to the yellow spot. The above-described changes of the nerve-surface are distinctly seen only in comparatively high degrees of myopia, for example, higher RELATION TO ATROPHY. 363 than g, and in these degrees the straightened direction of the retinal ves- sels, especially at the side of the atrophy, is not to be mistaken. Parti- cularly remarkable in this respect are those cases where, in the same person only the one eye is strongly myopic, or at least is much more strongly myopic than the other. On the whole the connexion be- tween the degree of the myopia and of the different changes in the fundus oculi, independently of the time of life, is in these cases most strikingly brought to light. Other deviations, to which highly myopic eyes are liable, as effu- sion of blood, detachment of the retina, glaucomatous degeneration of the nerve-surface and opacity of the lens, are not constant and not sufficiently peculiar to M, to justify their description here as the simple results of a further development of M. More correctly might chorioiditis disseminata, to be treated of in the following section, be referred to M. Movable flocculi in the vitreous humour are, on the contrary^ as I have remarked, but seldom absent in high degrees of M. We have still, in conclusion, two important questions to treat of : first, how far the atrophy, above described, is constant in myopia ; secondly, whether it occurs only in M, and is, therefore, characteris- tic of the latter ? Von Graefe established that in ten cases of high degrees of M, the atrophy alluded to (by him attributed to sclerotico-chorioiditis) occnjs at least nine times. Ed. von Jaeger had perhaps earlier arrived at a similar result, and all other observers agree therein. According to my experience we may go still further. As I have already remarked, slight degrees of myopia may exist in young persons without atrophy, but whether the myopia have remained stationary, or have been further developed, in the fortieth year traces of atrophy are usually no longer absent. Even after the thirtieth year I found with M > =-^ the atrophy only three times wholly absent, and still with M ^ — r it was present in every case but five. We are therefore justified in saying that myopia depends on a condition in which the development of atrophy is included. In the following section it will more fully appear that M may also ex- ceptionally be produced by some other changes of form of the eyeball; but, singularly enough, in those cases in which morbidly 366 MYOPIA. developed greater convexity of the cornea contributed to the M^ the atrophy peculiar to staphyloma poaticum is found only extremely rarely entirely absent. As to the second question, we cannot consider the atrophy de- scribed to be completely characteristic of M. We often see, at least in mature age, slight traces of atrophy at the outer margin of the nerve-surface, sometimes even an annular or circular atrophy, with- out M being present ; twice I have observed it even with certain degrees of H. We cannot assume that in all of these cases M has existed at an earKer period, and has disappeared in consequence of senile changes in the eye (compare p. 205). In the second place, a peculiar form of atrophy not unfrequently occurs around the nerve- surface in glaucoma. This is annular, and attains only a moderate magnitude; evidently the chorioidea is here also atrophic, which atrophy appears to me to be connected with the excavation of the nerve-surface : probably in this case inflammation is also in opera- tion. NOTE TO SECTION 27. The ophthalmoscopic investigation of strongly myopic individuals in the non-inverted image, for which the use of a highly negative eye-piece is necessary, requires a great deal of practice. Ed. von Jaeger thinks, how- ever, that there is some advantage in this method, and in any case practice is desirable, heeause, as we have already seen, we may in the glass required find an indication of the degree of the myopia. But for ordinary cases, examination in the inverted image is quite sufficient. Without the use of an object-glass, the inverted image of the fundus oculi stands hefore the myopic eye, at the distance for which the latter is accommodated. With M = —, — , — , it will, therefore, stand about 2, 3, n inches before the eye, so that, in high degrees of M, removing somewhat more than usual, we can, without the intervention of any glasses whatever, looking directly through the opening of the mirror, see the image. But it is better to add a convex object-glass to the observed eye, whereby the field of vision becomes larger and, by movement of the glass in the vertical plane, extends alternately in different directions. In the choice of this object-glass we find ourselves, however, in a difficult dilemma : if we select one too strong, the image becomes quite too small ; if we choose one too weak, we obtain too small a field of vision. The too small image we can partially counteract, by combining a pretty strong eye-piece with the ophthalmoscope, whereby the observer can, without any effort, see the little inverted image very near, and magni- fied : in general, when the investigator is not myopic, such a convex glass is to be recommended. Therefore a pretty strong object-glass, with which ANATOMY OF THE MYOPIC EYE. 367 the magnifying is less, but the field of vision is greater, is advantageous, and this is to be combined with a tolerably strong eye-piece. — Liebreioh {Archiv fur Ojphth., Bd. vii., 2, p. 130) has proposed a particular method of examining very highly myopic eyes. He uses no object-lens, but only a strong eye-piece, and with this approaches the eye so much that the iris lies nearly in the focus of this lens, so that the iris, in a certain sense, dis- appears and a very large field of vision is obtained. At the same time, the inverted image must come to lie between the eye observed and the lens, in order that it may be seen through the latter. This is to be attained in M ^ - with a lens of nearly — -— . Then we have really the advantage of see- ing a large field of vision under a strong magnifying power. In ophthalmosopic investigation in the inverted image, we look as through a compound microscope, whereof the dioptric system of the observed eye, with the lens held before it, is the object-glass, while the lens held before the open- ing "of the mirror is the eye-piece, through which the eye of the observer views the inverted image of the fundus oculi formed by the object-glass. The weaker the object-glass is, the shorter is the distance g" from the fundus oculi B to the resulting nodal point A', and the greater the distance g" from the image ^ to the resulting nodal point k°. — Now, as ;8 ; B := / : g", it appears that the magnifying & of the inverted image is greater, the weaker the object-glass is. But in this it is further included, that the increased length of the visual axis in M, as determining especially the magnitude of g', is co-operating cause of the slight magnifying of the inverted image. If, indeed, a shorter radius of curvature of the surface of the cornea or of the lens, without any change in the length of the visual axis, were the cause of the M, g" would be less ; and if now, while the inverted image was at the same distance from the eye, g" continued unchanged, the magnifying ^, would prove greater. If there was any object of known and constantly equal magnitude in the fundus oculi, it would be a problem from the size and position of its inverted image to deduce the length of the visual axis in life. § 28. Anatomy of the Myopic Eye. In the preceding paragraphs we have seen, that M depends almost exclusively upon the, prolongation of the visual axis, connected with staphyloma posticum. We have therefore here little more to do than, in the first place, to describe the changes, which, under these circumstances, occur in the eye, and in the second place to trace their development. This wiU therefore form the subject-matter of this paragraph. But if we comprise all together, which may lead to 368 MYOPIA. M in the dioptric sense of the word, that is, to a condition of the eye in which the focus lies in front of the retina, we may distinguish the following deviations : — 1. More than ordiMary convexity of the cornea. It is evident that, when, all parts of the eye being equal, the cornea is more convex, M must be the result. Now, until the most recent period M was therefore by many attributed to a greater convexity of the cornea. Our measurements have, however, led to the unexpected result, that myopes have, on an average, a less convex cornea than emmetropes (compare p. 89), and we may add, that in the most highly myopic persons, the cornea is the flattest. If, for example, we divide the 34 eyes measured by us into three classes : the first of M^ 1 : 1"6 to 1 :4 ; the second from 1 : 4 to 1 to sn, we find ; 10 ; the third from yr in the first class, the radius in the visual line p „ second „ ,, „ „ „ p ,, third ,, ., ,, ,, „ n = 7-93 = 7-829 = 7-867 IS cases In emmetropes I found on an average p° = 7-785. In the most highly myopic individuals a long radius of the cornea in fact, the rule. We had formerly measured the following WithM 1-648 2-625 2-66 2-83 2-875 = 8-21 =-7-885 = 8-06 '=7-68 ' = 7-67 = 1:3-5 p°=7-84 = 1.3-75 p° = 8-07 = 1:3-75 P° = 7-97 = 1:3-75 /)°==8-02 = 1:4 p°=7-96, and subsequent observations have confirmed our result. This par- ticularly great radius with very high M is connected with the distended form of the eye ; in ordinary cases we may say, that in myopes similar differences occur in the radius of the cornea as in emmetropes. — Nevertheless a morbid condition of the cornea may make the focus of an otherwise normal eye fall in front of the retina. In the first place, the increased convexity of the cornea occurs as a CAUSES ON WHICH IT MAY DEPEND. 369 result of inflammation, whereby the transparency is rendered imperfect, and the cuiTature is usually so irregular that under the astigmatism thence derived the acuteaess of vision suffers. In the second place, there is the peculiar morbid process known under the name of conical cornea, of which I shall have to speak in treating of irregular astigmatism. In the commencement this simulates an ordinary M with amblyopia. 2. Short focal distance of the crystalline lens. — Determinations during life, with the aid of the ophthalmometer, and direct mea- surements after death, prove that, with respect to the crystalline lens, as well as to the cornea, individual differences occur ; but if it has appeared that the cornea, except In its morbid changes, plays no part in M, — of the crystalline lens, it is at least not proved, that it should have a shorter focal distance in myopic, than in emmetropic eyes. So far as measurements exist, rather the contrary has appeared to be the case. Percy and Reveille-Parise {Hygiene oculaire, p. 32) state expressly, that in myopes the crystalline lens is not more con- vex. The distinguishing a particular form of M, as has been assumed by Stellwag von Carion and Ed. von Jaeger, as the result of partial accommodation for near objects, we have formerly, as I think upon good grounds, opposed (compare p. 353). 3. Dislocation of the crystalline lens. — Two cases have occurred to me in which partial laceration of the zonula Zinnii, the one the ■result of concussion, the other of actual injury, gave rise to a slight degree of M. The one case, in which the crystalline lens acquired a somewhat oblique position, is described in the following Chapter, treating of Astigmatism. These cases are in favour of Helmholtz' view that, under the influence of tension of the zonula Zinnii, the crystalline lens is flatter. 4. Displacement or more anterior position of the crystalline lens. — This might really give rise to a certain degree of M. But we know that in the hypermetropic eye the lens lies closer to the cornea, while in the myopic it is, on the contrary, more remote from it. Hence it is sufficiently evident, how little right we have to bring M into connexion with it. Two cases may here be briefly related. One is, that of luxation of the crystalline lens, in consequence of a violent concussion of the eye, whereby it was forced through the pupil into the anterior chamber, and placed itself very regularly im- mediately behind the cornea. In this case M had actually arisen, in part probably in consequence of increased convexity. The man 24 370 MYOPIA. did not comply with my request to place himself at once under treatment, and when he, not knowing what to do, appeared a couple of months later, the eye was painful, hard as a stone, and had be- come blind in consequence of the supervention of glaucoma. The lens was removed, but the eye continued, of course, blind. — The second case was that of a considerable degree of myopia, with rather soft eyeball and extraordinarily deep chamber, with the iris highly concave forwards. After this state had been long observed, without any known cause, in the course of twenty-four hours an altered position came on : the concave surface of the iris became convex anteriorly, whereby it came to he, together with the lens, very close to the cornea. The myopia must therefore have somewhat in- creased j but in consequence of the slight accuracy of vision it was not possible to determine this with certainty. In the last position the eyeball had become firmer, and evidently the vitreous humour must now have been again secreted under higher pressure than the aqueous humour : the reverse had previously abnormally existed and the iris with the lens had been pushed backwards. 5. Modifications in the coefficient of refraction of the different media are indeed, rather hypothetically, assumed as causes of M. 6. Inflammation of the anterior part of the sclerotic, often com- bined with so-called kyklitis, may have given rise to extension of the anterior part of the sclerotic, and thus to prolongation of the visual axis, of which M is the result. 7. Spasm of accommodation, and therefore the several organic causes of this spasm, render the emmetropic eye myopic. To this head belong the cases of so-called intermitting M. Here an ano- maly of accommodation, not of refraction, is ia question. After this enumeration of unusual deviations, which may form the basis of M, I now pass over to the consideration of the typical form of M, the eye with staphyloma posticum. It occurs under two varieties. There are, namely, cases, in which the eye is uniformly enlarged in almost all its axes, and in which its condition thus approaches to the congenital buphthalmos. In true buphthal- mos we found a flat cornea and extensive atrophy in the fundus oculi, in a case with still tolerable power of vision, but with an extra- ordinary degree of M, so that the person who had buphthalmos on one side, had considered the eye so affected to be blind, until I showed him that at the distance of 1^" he was able to read with it. In by far the great majority of cases, however, the visual axis has LENGTHS OF THE DIFFERENT AXES. 371 increased mucli more in length than the other axesj and the eye exhibits a tolerably regular ellipsoidal form, as has already been represented by Scarpa.* I here append the ascertained lengths of the different axes in some myopic eyes. Visual axis. Horizontal axis. Vertical axis. 33-0 26-8 25-6 31-7 26-0 24-7 31-0 26-5 26-0 30-0 ... . 27-5 . . . 25-4 28-5 24-3 24-0 Von Jaegert has also measured a great number, in which, too, the prolongation in the visual axis was always the most remarkable. In most cases the apex of the sclerotic ellipsoid corresponds about to the axis of the cornea,J and from the relaxation of the membranes in this place it appears, that the atrophy has here attained the highest degree ; sometimes along with the original distention a second is here present, where apparently the membranes have offered stiU less resist- ance to the pressure. W hile, therefore, the apex corresponds more or less perfectly to the region of the yellow spot, which has approached the axis of the cornea, the optic nerve has removed more than ordinarily to- wards the inside (Compare Kg. 144) . In other cases the apex Hes farther from the axis of the '^' cornea, and indeed in different directions, but especially inwards, whereby it may coincide about with the optic nerve. Sometimes the latter is said to be here implanted as on a second dis- tention. I have never seen this ; but there rather occurs at the base of the optic nerve, from yielding of its external fibrous sheath, a thick- ening to 8 mm. and more, which suggests the idea of a second distention of the sclerotic (compare Kg. 146). Von Jaeger says he has fonnd the apex of the enlargement even to the inside of the optic nerve. In high degrees of staphyloma posticum the eye when taken ont rapidly becomes soft and flaccid ; and near the posterior pole the membranes are so thin and transparent, that there the well-known * Traite pratique des maladies des yeux. Traduit par Leveille. Paris, chez Bertrand, 1807, Tome II., p. 190. + /. c, p. 262. J Compare also Dr. Josef Ritter von Haaner, Klinische Vortrdge iiber Augenheilhunde, Prag, 1860, p 19. 2 372 MYOPIA. bluish appearance occurs which is peculiar to staphyloma scleroticse anticum. Turning the distended part to the hght, we see through the pupil the fundus oculi quite clearly illuminated. Even in life, when the cornea is strongly turned inwards, the bluish colour of the sta- phyloma posticum is sometimes visible ; and a case occurred to me, in which the whole visible sclerotic had, through general attenuation, acquired a very disagreeable uniformly blue appearance. In order now to examine the eye further, we make, either in the fresh state or after hardening in a solution of chromic acid, a section in a plane passing through the middle of the cornea and of the optic nerve, taking care that it does not pass the apex of the staphyloma. It now appears that the sclerotic has everywhere become thinner, and indeed increasingly so towards the posterior pole; that it is in general more attenuated at the outside of the globe of the eye than at the inside, and that near the apex of, the ellipsoid it is, in the highest degrees, not much thicker than a sheet of paper, nay that in some points its fibres are even separated from one another. In the second place, we observe that the iris and the lerks are moved more backwards. The same is true of the processus ciliaris, and more than once I have seen that the musculms ciliaris also, with prolongation and attenuation of the vitreous fibres, which, derived from the membrana Descemetii, serve as origin to the muscle in question, commenced farther from the cornea than in the normal eye, and was at the same time longer, flatter, and more or less atrophic. This we find decidedly when the anterior part of the sclerotic also is considerably attenuated, which is, however, by no means the rule. In like man- ner, in ordinary cases, the anterior part of the chorioidea is almost quite normal, and is only towards the back part progressively atte- nuated and discoloured. In general the chorioidea can with tolera- ble ease be separated from the sclerotic, but in proportion as we approach the atrophic parts, it must be removed with the greater care, because the membrane, while it increases in thinness and fine- ness, becomes at the same time more homogeneous and more easily torn. On all sides we can, however, recognise and isolate the chorioidea, atrophic as it may be. Von Jaeger has found this to be the case in numerous eyes, and even in 1856 I saw it, in the collection of Stellwag von Carion, illustrated by excellent preparations. The chorioidea usually exhibits the greatest attenuation immediately at the optic nerve, from which it is now easily separated ; the sclerotic, on the con- MICROSCOPIC APPEARANCES. 373 trary, is thinnest at the posterior pole. According to von Hasner, the retina is more intimately connected with the chorioidea at the margin of the atrophy. — ^We view the chorioidea on its inner surface, after having carefuUy removed the vitreous humour and the crystal- line lens together with the retina. It is seldom that in the eye hardened anteriorly any pigment continues adherent to it ; but, on the other hand, in two eyes examined by me, a part of the outer layer of the retina remained connected with the pigment, especially in the region of the equator of the eye : they were taken from a woman aged 66 ; in other eyes examined by me, I have not seen this. In this woman occurred also the well-known elevations (compare p. 192) on the vitreous membrane of the chorioidea, which, where the cho- rioidea is not yet very atrophic, can always be easily isolated. If we now bring under the microscope, a long slender portion of the chorioidea, extending from the ora serrata to the optic nerve, iso- lated from the sclerotic and carefully treated, so that no pigment is lost, with the inner surface upwards, we can study the transitions from the normal condition to the most perfect atrophy. In the first place it appears, that the pigment-epitheHum is less uniformly coloured, and that the cells are larger, and perhaps also flatter than in the normal eyej outside the highly reflecting atrophic crescent, where they are actually wanting, they form, however, a perfect layer. If the atrophy be diffuse, transition forms occur, and we observe that the cells gradually disappear, leaving only pigment-granules more or less collected in groups. The dark black spots, which we observe with the naked eye or with a magnifying glass, particularly in this diffuse form, scattered here and there, exhibit no regular cells, but perfectly black, irregular, angular, mutually coherent masses, between which, and also around which, we sometimes find colourless atrophic chorioideal tissue. Grey spots, on the contrary, surrounded by similar black pigment, I saw resting on normal cho- rioideal tissue, which glimmered more feebly through them. — If the cell membranes disappear from the pigment-epithelium, while the pigment-granules remain, the reverse holds good with respect to the pigment, belonging to the stroma of the chorioidea. This is observed most distinctly when the pigment-epithelium has been washed away with a pencil. In the first place it then appears that, even in cases of circumscribed crescent, the stroma has already got remarkably pale, where it was still sufG.ciently covered with pigment-epithelium ; and in these decolorized parts the ramified pigment-cells of the intervas- 374 MYOPIA. cular spaces are equally distinctly visible, but are poor in pigment- granules, wbich towards tbe atrophied side progressively diminisb and finally disappear, while the ramified cells themselves, nevertheless, continue present. In the darker places, on the contrary, where the chorioidea also has maintained about its regular thickness, the pig- ment-cells appear perfectly normal.. — The large vessels of the outer layer of the chorioidea I found in the normal and Kttle atrophied parts very highly congested with blood, more so than is usually the case under ordinary circumstances in normal eyes. In consequence of this the membrane had a red, striped appearance. The same sanguineous congestion was exhibited in the same place in the most remarkable manner by the chorio-capiUaris ; but in proportion as we approached the atrophied part, the blood diminished in the vessels, and at length wholly disappeared. Here the thin membrane which remained from the chorioidea gradually acquires such homogeneousness, that it is difficult to suppose that the vessels should have retained any calibre, although, with a strong magnifying power, we can still very well per- ceive the large vessels, and sometimes even the chorio-capiUaris, surrounded with granular matter. Besides, the larger chorioideal vessels long continue to carry blood, when the chorio-capillaris is already closed. I am sure that on the white reflecting crescent, on which we often still see some larger chorioideal vessels running, the chorio-capillaris is wanting, not only because it was not filled on a very successful injection, but also because we do not see it in life. If there be a red or dark background, it is invisible when magnified by the ophthalmoscope on account of the transparency and the slight reflecting power of the thin layer of blood ; but if the background be white, it ought to manifest itself by a reddish tint, and even to be visible as a network. The foregoing description has, in general, reference to the highest degrees of staphyloma posticum, in which the defined atrophy, per- ceptible with the ophthalmoscope, has already visibly connected itself with a diffuse atrophy. It has, however, seemed to me, that difiuse atrophy occurs also where, on account of the tolerably perfect pigment-epithelium, it appears during life only from the great mu- tual distance of the still blood-carrying larger vessels of the chorioidea. In staphyloma posticum the place where the optic nerve enters the eye is important. Some years ago I investigated this place in the normal eye.* It then appeared to me, that the trunk of the optic • Archivfur Ophthah, Bd. i. h. 2, pp. 82 et seq. 1855. ENTRANCE OF THE OPTIC NERVE. 375 nerve is already divided by fibrous tissue into the numerous small bundles into wbicli it at once separates on issuing as retina, and thus differs essentially in its structure from other gradually ramifying nerve-trunks ; that it possesses a double fibrous sheath, an external and thicker one, a (Fig. 145), which is continued at a into the Fig. 145. most external part of the sclerotic, and an internal one b, which en- velopes the trunk as far as the chorioidea d, is connected with the latter, and bends close to it into the there pigment-containing sclerotic V, while at the same time the lamina cribrosa g for the most part proceeds therefrom, and is only to a small extent formed by the cho- rioidea. Between the two fibrous sheaths, a and b, is a thin layer of loose connective tissue, the connective-tissue-sheath c, consisting of a network of sharply-defined fasciculi, which ascends to c' close to the lamina cribrosa. Even beneath the lamina cribrosa at h the nerve-fibres, as Bowman* first stated, lose their medullary sheath, whereby the nerve becomes thinner and at the same time transparent. * Lectures on the parts concerned in the operations on the Eye. London, 1849, p. 82. 376 MYOPIA. These thinner fasciculi now pass through the so-called lamina cri- brosa, which may be regarded as a strengthening of the neuiilemma of the separate bundles, and of which a fine continuation can be traced between the bundles of the fibrous layer of the retina. The chorioidea appears to terminate abruptly at the margin of the nerve, because the pigment, which is here largely accumulated, actually ceases, nor are the chorioideal vessels continued further; but its tissue nevertheless surrounds the fasciculi of the optic nerve, and thus really contributes to the formation of the lamma cribrosa. So the chorioidea, both through its connexion with the internal sheath b' of the optic nerve, and by its prolongation among the nerve-tissue, is here firmly attached. Almost directly at the place, where its pig- ment commences, the elements of the deeper retiaal layers begin to show themselves, over which the nerve-layer now expands. — If with this we compare the section of the entrance of the optic nerve in highly-developed staphyloma posticum (Fig, 146) it will appear Fig 146. that the external sheath, a, escapes near the sclerotic from the nerve, that a small part, a, here turns inwards, but that the greatest part, a", APPEARANCE OF THE RETINA. 377 inclines outwards towards the sclerotic ; that, on the contrary, the inner sheath, ^, continues closely to surround the nerve ; near the nerve-sur- face/ forms the lamina cribrosa, and now passes at 5' outwards into the sclerotic, running to meet the fasciculus, «', of the outer sheath turn- ing inwards. This thin fasciculus, a 6', bounds the loose connective tissue, c, which here, c, has acquired a great breadth, and is therefore evidently very much extended. The sclerotic consequently here consists almost exclusively of a thin fasciculus, a b', derived from the two nerve-sheaths, and this is covered anteriorly by the completely atro- phic pigmentless chorioidea, d'. It appears, that the nerve, which here exhibits superiorly a perhaps accidental thickening, after its fibres have lost the medullary sheath, is still thinner than in ordinary cases, and thus goes inwards, through a smaller opening in the chorioidea, sometimes in an oblique direction, in order immediately as retina, n n, to spread itself over the anterior surface of the atrophic chorioidea. In the instructive drawings given by von Jaeger,* one of which I have here copied (Pig. 147), the sheath of connective tissue has for the most part not only extended upwards, but an ex- tremity of it also stretches between the layers of the sclerotic. Von Jaeger further remarks, that the internal extremity of the nerve, with early bending and spreading of its fibres, appears as it were drawn within the eye : in Kg. 147 (after Jaeger) the lamina cribrosa and Fig. 147. the place where the nerve-fibres lose their medullary fibres, really appear to have approached nearer to the retina. The retina has in general a normal appearance. It is hard to say whether it is more or less attenuated in one place or another ; but thus much is certain, that the atrophy in this membrane cannot be * Ueber die Einstellungen, etc. Wien, 1861, Tab. i. and ii. 378 MYOPIA. compared with that in the sclerotic and chorioidea. The extended course of the vessels, already ophthalmoscopically seen, can be easily proved : how far the proper tissue is altered, it is difficult to say. Coccius,* who often examined the retina in staphyloma scleroticse, in consequence of chorioiditis postica, found, with the exception of some empty places on the posterior layer (the layer of rods and bulbs), no changes in it. H. Muellerf thinks that in one case he found the tissue looser ; but he himself adds, that it is necessary to be particularly careful in one's judgment on this point. In my notes I find, that in isolating the retina, the nerve-substance on the nerve-surface very easily separated from the lamina cribrosa and remained connected with the retina ; that in the tolerably fresh eye the yellow spot was sometimes scarcely visible ; that the pUca was formed as usual ; that the fibrous layer appeared normal ; that on sections of hardened retinas the different layers, with the exception of that of the rods, were admirably to be seen ; that at the edges radiating fibres with adherent granules from the two granular layers occurred already isolated ; that in the region of the yeUow spot the different cell-layers were distinctly visible, and that viewing the recently removed retina on the outer surface, a tolerably good image of the layer of rods and bulbs, which could be isolated in the ordinary way, was usually obtained. But, what here has particidar bearing : how far morbid changes occurred in these percipient elements, how far they were more separated than in the normal eye, how the bulbs in the yellow spot, and particularly in the fovea centralis, are circumstanced in high degrees of atrophy — on these points I cannot speak with certainty. Eespecting the vitreous humour I have only to state, that in high degrees of staphyloma posticum it is fluid and not perfectly clear. The flocculi present in it have a fibrous, granular appearance without any distinct structure. If we now endeavour, from the facts which have been observed, to form an idea of the development of staphyloma posticum, the optic nerve with the membranes of the eye and the nerve-surface itself, will, in the first place, attract our attention. According to ophthalmoscopic examination, we have in these parts to 'seek the * Ueber Olaucom, etc. Leipzig, 1859, p. 40. t Verhandlungen der phys.-med. Gesellschaft. Wurzburg, B. ix. S. liii. 1859. DEVELOPMENT OF STAPHYLOMA POSTICUM. 379 commencement of the affection, and in many instances anatomy at least does not oppose this view. We have then to suppose that in these cases the disposition consists in this, that the sclerotic near the optic nerve is more extensible, particularly to its outside, and that by this extension the outer sheath of the optic nerve is soon a little removed from the latter, and the sheath of connective tissue is so extended (com- pare Kgs. 145 c' and 146 c') . So soon as this happens, the thin layer of sclerotic tissue, b', situated in front of the sheath mentioned, must be very much attenuated, and the same is true of the chorioidea, d', connected therewith, and forming, with the fibrous layer mentioned, the so-called lamina cribrosa, between the nerve-fasciculi. At the side of the nerve-surface, therefore, the retina now rests on an extended and attenuated chorioidea and sclerotic, which posteriorly further lose all support. It is not until we come somewhat more outward that we find the external sheath of the optic nerve, a, strengthening the sclerotic. With this extension, in the immediate neighbourhood of the optic nerve, the origin of the atrophic crescent seems to be really connected : the chorioidea, d', is here united with the nerve, both immediately through the fibres, which are continued between the fasciculi of the nerve, and mediately through their con- nexion with the inner sheath of the nerve. At the borders of this attachment its vessels cease; on any extension, therefore, obstruction to the circulation in the extreme terminations of the chorio-capillaris must easily arise, and thus the condition for incipient atrophy is supplied. We find something similar in the origin of emphysema of the lungs from atrophy of the most distended pul- monary vesicles, which thereby also finally lose both their capillary network and their pigment. Now, if the excessive extension has here once begun, and the resistance is thus diminished, it is more uidikely that the condition should become stationary, than that it should be progressively developed ; and it actually is the rule that the atrophy of the chorioidea advances more and more, without the visual axis, in slight degrees of staphyloma, necessarily becoming persistently longer and longer.—- The extension at the margin of the optic nerve takes place often only at one side, and indeed principally at the outside, with which the atrophic crescent keeps pace at the same side. The number of my observations is not sufliciently great to enable me to form a positive opinion, — but it appears to me very likely that, if the apex of the staphyloma falls nearly in the optic nerve, the outer sheath has given way at all sides of the nerve. 380 MYOPIA. and that to this the annular and circular atrophy of the chorioidea then also correspond : in the eye^ from which Fig. 146 was taken, the atrophy was reaUy circular. If, as is usually the case, the apex of the staphyloma is situated at the outside of the optic nerve, atrophy of the chorioidea is also found especially at this side, and even the fact, that the yellow spot, to which the apex of the staphy- loma often corresponds, is situated at the outside of and a little beneath the nerve-surface, and that the axis of the atrophic crescent usually extends in this same direction, indicates most distinctly, that there is a connexion between the apex in question and the direction in which the atrophy of the chorioidea advances. Whether the sharply-defined atrophy of the chorioidea does not reach further than over the extended connective tissue, h, I cannot decide. Von Jaeger does not admit even any such intimate connexion, and he asserts that the direction of the atrophy does not at all correspond to the side where the sheath of cellular tissue has given way. As to the narrowing of the nerve-surface in a direction perpendicular to the axis of the crescent, observed with the ophthalmoscope, I can give no satisfactory explanation. — Now, in the ordinary cases, where the optic nerve does not lie in the apex of the staphyloma, it may be a question, whether the extension does not commence rather in the region of the yellow spot, and only secondarily become communicated to the outside of the optic nerve. This view is the more admissible, not only because by it the position of the optic nerve at the inside of the ellipsoid would be explained, but also because in very young persons, with comparatively high degrees of M, the crescentic atrophy of the chorioidea is stiU often absent, or at least is very slight. We can also very easily imagine, how extension in the region of the yellow spot gives rise to the atrophy. In the yellow spot the chorioidea, which here abounds in pigment, is more intimately con- nected with the sclerotic. If the membranes here give way back- wards, the extension will be equal on all sides, and the chorioidea wUl, moreover, the more easily maintain its connexion with the scle- rotic, because precisely here it is more intimately connected with it. If now the sclerotic too gradually extends in the direction of the optic nerve, the chorioidea becomes very tense over this part, for we may assume the yellow spot and the outer margin of the optic nerve to be two fixed points : at the attachment, therefore, to the outside of the nerve-surface the chorioidea is particularly strongly drawn, and considering that the chorioideal vessels terminate pre- DEVELOPMENT OF STAPHYLOMA POSTICUM. 381 cisely at this boundary, the production of atrophy by extension can- not surprise us; according to the direction of the tension which takes place it is even easy to understand why the atrophy should assume the crescentic form. The atrophy at this place wiU now certainly be still promoted, so soon as also in the immediate neigh- bourhdtod of the nerve the sclerotic is extended, and thus the outer sheath of the optic nerve gives way outwardly. This giving way can almost without exception be demonstrated, and it is attended with the necessary consequence, that the arterial circle, known even to Zinn, occurring in the sclerotic around the optic nerve, as Jaeger* has shown, is removed from the nerve. On the whole, the outer half of the sclerotic is extended in a higher degree than the part situated at the inside of the nerve : there it is consequently also in general thinner. With this is connected the fact that, as we have already seen (compare p. 182), the yellow spot removes towards the inside, so that it may even go beyond the axis of the cornea, whereby the visual line cuts the cornea to the outside of its axis. That in this extension atrophy does not occur at the outside of the yellow spot in the anterior part of the chorioidea so readily as at the inside, is easily explained from the great extensibility of the chorioidea in this direction. It appears, moreover, gradually to move a little over the surface of the sclerotic: thus, at least, I think the re- trogression of the iris, of the ciliary process and of the chorioidea itself, with the ciliary muscle must be explained, in those cases in which the anterior part of the sclerotic has retained almost its normal thickness. However, there exists everywhere more or less extension of the chorioidea, and that this gives rise to diffuse atrophy and to diminution of the elastic resistance, needs no proof. To the elastic resistance of the chorioidea, I think essential importance must be attached. If the membranes of the normal eye be all cut through together, the chorioidea is seen to retract, leaving the sclerotic bare at the edge. This proves that the chorioidea may be displaced, and moreover, proves its tension. During life, in consequence of the vessels being full, of the tone of the blood-vessels, and of muscular fibres t scattered here and there, the tension of the chorioidea is un- * I. V. p. 62. f H. Miiller ( Verhandl. der phys.-med. Gesellschaft, Wiirzburg, B. x. p. 179) has pointed out these muscular fibres, and also ganglionic cells in the chorioidea, of the presence of which Kolliker satisfied himself, in Miiller's 382 MYOPIA. doubtedly still greater. By virtue of this tension it bears a part of the pressure of the fluids of the eye, which now does not act entirely upon the sclerotic, and it is evident that where an atrophic condition has set in, and the chorioidea has given place to a thin, brittle mem- brane, the full pressure comes to bear upon the already extended sclerotic. Consequently a tolerably advanced staphyloma does not easily become stationary, the less so because the pressure of the fluids in the myopic eye is in general somewhat increased. Therefore, also, in staphyloma the blood-vessels and the inequalities of the chorioidea, as well as the elongated, but not easily atrophied, ciliary nerves,* make deep impressions on the inner surface of the atrophic sclerotic. More than once this has attracted my attention. Various circumstances may, moreover, contribute to cause that, in one instance in one part, in another in a different part, the extension, and with it the atrophy, is particularly great. Especially in the region of the yellow spot it fre- quently attains a maximum, because, from whatever cause it maybe, the apex of the ellipsoid is usually often found here. On this excessive extension the above-described local changes mainly depend, whereby the function of the retina comes to suffer. It is easy to see, although it has not been proved by accurate microscopic investigation, that under such extension the outermost layer, which consists of radiatingly placed very small bulbs, must suffer ; that these bulbs at least must be separated, irregularly distributed and made oblique, and that they must easily be actually destroyed. In other parts, too, of the retina, the rods and bulbs, as we have seen, appeared to be more separated. That, on the contrary, in the fibrous layer little or no disturbance occurs, will not appear strange, when we consider how well the structure and function of nerves in general resist extension by morbid tumours or from other causes. Where there is vital metamorphosis of matter, change of form much more readily takes place under the molecular change, without disturbance, than in solid fibrous parts, and in this respect the retina has a great advantage over the sclerotic. The changes in the vitreous humour may have a different origin. They may depend upon extravasations of blood, which, perfect as the absorption may be, never leave behind them an absolutely clear vitreous humour. Solution of the as yet so badly explained con- nexion of the vitreous humour, gives rise to turbidity, as is seen in preparations. Sohweigger's preparations also appeared to me to be conclu- sive. • Compare also Heymann, in Archivf. Ophthal., ii. 1, 131. CONNEXION OF STAPHYLOMA WITH INFLAMMATION. 383 traumatic injuries, and where foreign bodies are present : a slight thread of india-rubber carried with a needle in the region of the equator transversely through the eye of a rabbit, strongly extended by pulling the ends, and while in the extended state cut off at both sides close to the eye, contracted (in an experiment by my assistant. Dr. Brondgeest) to a small body, visible with the ophthalmoscope in the centre of the vitreous humour, and around it a considerable turbidity was developed, while the little wound of the membranes regularly cicatrised. Now the considerable distention of the vitreous humour in staphyloma posticum probably also causes laceration, at least synchysis, and some turbidity is consequently to be expected.* Finally, in some cases, an irritative or inflammatory condition of the retina and chorioidea is also certainly in operation. We have now approached an important point. The question is, how far staphyloma posticum is connected with inflammation. It is known that von Graefe at first thought the practical idea of sclerotico- chorioiditis posterior applicable to it ; but that he in vain sought, in two eyes which he had the opportunity of examining anatomically, the proofs of preceding inflammation. Von Graefe's opinion was disputed in various quarters. Stellwag von Carion adhered from the beginning to another opinion, and Ed. von Jaeger soon followed him in the same. According to them staphyloma posticum is only the result of a congenital condition, and the atrophy connected with it increases, according to von Jaeger, even with the eyeball itself, until at a certain point it often becomes stationary. There is, indeed, some truth in this statement. Thus much is certain, that the staphyloma posticum cannot be considered as a simple result of sclerotico-chorioiditis, and that without the disposition, and that usually of an hereditary nature, it is not easily developed. Moreover, it must be admitted that without symptoms of irritation or inflammation staphyloma posticum may attain a tolerably high degree. But, on the other hand, there is no doubt that the morbid distention very often leads to irritation and inflammation, and finds therein, with softening of the membranes, a condition of rapid development. It appears to me to be an error not to consider the eye with staphyloma posticum, combined with simple atrophy, as morbid, and a still greater error to overlook the fact, that when highly developed, this condition almost without exception induces symptoms of irritation, which still further threaten the integrity of the power of vision. Von Jaeger^s doubt, whether chorioiditis dissemi- * Compare, oa this subject, also von Hasner, I. c. p. 21. 384 MYOPIA. nata occurs more frequently in highly myopic eyes than in others, indeed surprises me. In high degrees of M this inflammation is at a certain time of life the rule. In the course of the morbid distention inflammation is readily developed, which cannot be considered as a new condition, but usually makes itself known, at an early period, as more or less distinct irritation. Even in youth, as Hasner also observes, symptoms of irritation are usually present even in slight degrees of M, and ophthalmoscopic investigation now reveals to us capillary redness of the nerve-surface, and, what is remarkable, sometimes only over the half of its extent. Probably there is in this case, in consequence of tension of the chorioideal ring and of the lamina cribrosa, some mechanical irritation, perhaps even pressure on the small fasciculi of the optic nerve. Subsequently, as the extent be- comes greater, and the disproportionate tension is augmented, the phenomena of irritation increase, and at the boundaries of the cir- cumscribed atrophy hypersemia exists, showing that the chorioidea before atrophying, here also passes through a period of irritation. Therefore I cannot admit the existence of well-defined limits between the further development of the crescent, and the occurrence of remote atrophied spots, which last exclusively von Jaeger considers as pro- ducts of inflammation. Even the abundant deposition of pigment on the boundaries of the crescent, again absorbed in the progress of the atrophy, in order to be anew deposited more externally, is in favour of a state of irritation. At a later period the subjective phe- nomena, also, become more characteristic, and in different parts of the fundus oculi atrophic spots are formed, preceded by inflamma- tory action : chorioiditis disseminata, I repeat it, is in the highest degrees of M at a more advanced age, connected with its normal development. Our conclusion is, therefore, briefly as follows : — that almost without exception the predisposition to the development of staphyloma posticum exists at birth ; that it is developed with sym- ptoms of irritation, which in moderate degrees of staphyloma do not attain any great clinical importance; but that in the higher degrees an inflammatory state almost always occurs, at least at a somewhat more advanced time of life, as a result and as a co-operative cause of the further development of the distention and of the atrophy. This conclusion is certainly not inconsistent with the fact, that after death so few products of inflammation are to be met with. Por these products pass, as well as the normal tissue, into a state of atrophy. In this way false membranes, as we see in the synechia of the PREDISPOSING CAUSE OF STAPHYLOMA POSTICUM. 385 iris, in adhesions between the pleurae, and in other places, become atrophied and absorbed when extended. Even on the capsule of the lens sometimes nothing else remains of the inflammatory products of iritis than some spots of pigment. In staphyloma posticum the in- flammation exists almost as the transition period to atrophy. That in general no adhesion of the membranes is met with is also easily reconcileable to this view. Adhesions are, however, sometimes found, and it is questionable, whether, in general, the separation would be so easily obtained if the chorioidea before passing into the atrophic form, had not been imbibed with parenchymatous exudation. Moreover, as H. Mueller* states, the tissue of the chorioidea in the in- tervascular spaces exhibits in staphyloma posticum, groups of cellular bodies, which he considers to be also in favour of the pre-existence of inflammation. Finally, the analogy with staphyloma anticum, which is evidently the result of inflammation, is not to be denied. Another important question is, in what the predisposition to sta- phyloma posticum may consist. It appears to be as yet not satisfac- torily solved ; at least I have no decided opinion on the subject. The relative number of adults in whom we can ophthalmoscopically per- ceive more or less atrophy, is much greater than that in children. Von Jaeger is certainly quite right in asserting that something of atrophy is occasionally to be seen in young children ; I even readily admit, that he has himself found defined crescentic atrophy in newly-born infants, both during life and iu the dead body ; but the rule is, that we see nothing of the kind, although atrophy at a later period makes its appearance, and though in all probability there was an hereditary ten- dency to it. This leads us, I think, to the conclusion, that in these cases, the resistance at the outside of the optic nerve was originally less than in the unpredisposed eye. Now, this predisposition has been brought into connexion with the history of the development of the eye. If we look at the drawing given in the Archiv fur OpMAalmologie (Bd. IV. I. 39) by von Ammon, of the protu- berantia scleraUs, described by him thirty-five years previously, as existing at a certain period of the foetal eye, which is here reproduced as Fig. 148, the form of staphyloma posticum is at once suggested to the mind. Von Ammon states, that in an early period of foetal existence the sclera is stiU open at the cerebral side; * Verhandl. der phys. med. GeseUsehafi, Bd. ix. a. liii. 1859. 25 386 MYOPIA. that it then has the form of a cup, and is connected by an oval opening with the anterior cerebral cell. This opening is closed by a tissue proceeding from the margins, and as the eye becomes larger, the supplementary tissue forms the protuberance in question, which is, according to von Ammon, directed at first downwards, but subsequently backwards and outwards, as it is here repre- sented. Von Ammon adds, that the different membranes close the said opening nearly at the same time. In the first place, there is brought into connexion with this mode of development the coloboma chorioidese, which as a defect of the chorioidea, pro- ceeding from the nerve-surface, extends iuferiorly, whether over only a small extent, or over the whole chorioidea, and may now unite even with coloboma iridis.* We have here, therefore, an ordinary arrest of development. And as such Stellwag von Carion and Ed. von Jaeger also consider the disposition to staphyloma posticum, and in a certain sense staphyloma posticum itself. The arrest of develop- ment is here only much less, and might be Kmited even to a slight remnant of the protuberantia scleralis. The direction of the atro- phy, indeed, corresponds to that of the already fnUy-closed protu- berantia scleralis. As I was already about to observe, I am unable, for want of per- sonal investigation, to give any opinion upon the subject. I, how- ever, feel myself called on to remark, that it appears to be chiefly this idea which has led Stellwag von Carion and von Jaeger to con- sider acquired myopia as another condition, and to start from the supposition that congenital myopia is in no way connected with the use of the eyes, and that it should be equally distributed among all classes of society. Against this view I have already raised diffi- culties (p. 352), and I shall again revert to them. In this place I cannot avoid observing, that if the connexion with the protuberantia scleralis reaUy exists, and if congenital staphyloma posticum, as well as the predisposition to the same, may be considered as an arrest of development, this view does not invalidate my statement, that the undue use of the eyes promotes the occurrence of staphyloma posti- cum. Indeed, it is certain that if, with slight predisposition, the eyes are much used for close work, the staphyloma becomes more fully developed, and direct experience as well as analogy shows, that * Compare Liebreich, Arehixi fur Ophthalmologie, Bd. v. ii. 241, and Atlas d'OpUhalmoscopie, Tab. xii. Fig. 6; and Nagel, Archiv fiir Ophthal- tnologie, Bd. vi. i. 170. ITS AUVANTAGES AND DISADVANTAGES. 387 in such a case the probability of transmission to posterity, even in a higher degree, is greater than when the staphyloma was not brought under promoting circumstances to a state of further develop- ment. §29. — The Vision of Mtopes. The vision of myopes, without the aid of spectacles (of which alone I am here speaking), is characterised by comparatively less acute- ness for remote than for near objects. We are already aware that this depends upon the circles of diffusion formed upon the retina by the rays proceeding from remote points. Myopes have a farthest point of distinct vision, and the rays proceeding from a point situated there, can unite upon the retina into one point j with respect to more remote points, the circles of diffusion become greater, and vision consequently becomes more indistinct, the greater the distance is. In slight degrees of M this indistinctness is so trifling that the persons affected with it have not observed it, and it is not until they employ a weak concave glass that it appears that they are thus enabled to see more distinctly at a distance, and are consequently myopic. In these slight degrees myopia is attended with no inconvenience. Against the disadvantage, that at a great distance vision is not perfectly accurate, is the advantage that, the range of accommodation being equal, very small objects brought closer to the eye, and therefore, under a greater visual angle, can be better distiuguished, and that when at a more advanced period of life the range of accommodation is diminished, ordinary work can be still longer performed without spectacles. Even with a myopia of =r^, in which, at the distance of eighteen inches, accurate images are stiM obtained, the anomaly is, in most kinds of work, attended with no inconvenience. In somewhat higher degrees myopes have, at least in youth, with perfect accommodation, a tendency to approach closer than is necessary to the objects, and thus, particularly at sedentary work, to assume a stooping position. If the myopia amounts to — to ^, or more, this occurs without exception : of course, io a greater extent, the higher the degree of the myopia is. With increasing myopia, too, the fancy increases of occupying one's- self with small objects : persons so affected read by preference small 2 388 MYOPIA. print ; they accustom themselves to small handwriting, and as much as possible avoid long Knes. It is evident that in doing so they are able to see more at once, and, for example, in reading, they need not move the eyes and head so much as when they have broad pages and large letters before them. It is not until the objects diminish in magnitude in proportion as the distance becomes less, that myopic are on a par with non-myopic individuals, and \^■ith equal acuteness of vision distinguish those objects equally well. However, as we shall hereafter see more fully, in the highest degrees of myopia the acute- ness of vision is imperfect, and although those labouring under them can distinguish very small objects still better than non-myopes, these must in reference to the distance be really somewhat larger ,• it is, moreover, a very great inconvenience, that a bent position, and, if binocular vision is not sacrificed, particularly great convergence, are not to be avoided, an inconvenience which is attended with actual injury, inasmuch as the further development of the myopia is thereby promoted. Much greater still, in these high degrees of M, is the incon- venience of indistinct vision at a distance. The emmetrope can form an idea of it by holding before his eye a strongly convex glass, and he will be surprised how quietly myopes usually submit to their fate. But at the same time, many peculiarities developed in myopes, Tinless they early accustom themselves to the use of spectacles, wiU no longer be an enigma to him. The circles of diffusion in vision at a distance are the greater, because in myopes the diameter of the pupU, as even Porterfield and Jurin were aware, is in general larger than in non-myopic persons, and to this magnitude the circles of diffusion are proportionate. Therefore myopes under mydriasis see very badly at a distance. "With a bright light they see remote objects much more distinctly, and therefore also at a more advanced time of hfe, the degree of M being unchanged, they distinguish better at a distance, which often makes them incorrectly infer that the degree of the myopia is diminished. Myopes now also distinguish much more accu- rately beyond their farthest point, when they look through a small opening, and in the highest degrees of myopia, as wiU hereafter be seen, even practical use may be made of this fact. The Calabar bean, too, which, dropped in solution into the eye, has the remark- able property of constricting the pupil, is thus serviceable to them : on very shght action of this agent the pupil becomes narrow, without PECULIARITIES ATTENDING MYOPIC VISION. 389 increasing the myopia, and when the remedy acts more powerfully, tlie farthest point, which had at first approached the eye, after an hour returns to its normal condition. Among the early scholars many myopes were to be found. In this number was Dechales,* a Jesuit of the seventeenth century, who has with wonderful accuracy described many peculiarities of myopic vision. In church he saw very remote little flames as round circles of diffusion, and thereupon studied his entoptic spectrum, which he perfectly understood. He was also aware, and explained the fact very correctly by the diminution of the circles of diffusion, that myopes distinguish more accurately through a small opening, and with respect to the influence of the approxima- tion of the eyelids he communicates some particulars, which are not to be met with even in later writers. Thus, regarding this point, he observes, that, properly speaking, only the vertical dimension of the circles of diffusion thereby disappears, so that horizontal lines are better seen, but that alsothe horizontal extension of vertical lines is somewhat divided by the cilia, so that a number of images are seen close to one another, of which one is usually distinguished by particular clearness. Many myopes have so very much accustomed themselves to approxi- mate the eyelids that it actually belongs to their physiognomy, and that they persevere in doing so, even when the use of concave specta- cles makes it superfluous: this reminds us of the involuntary ten- sion of accommodation, sometimes even combined with strabismus, of young persons whose H has been corrected by a convex glass. In strong myopia the imperfect vision has, even at a short distance, peculiar results. Those affected with it seldom fix the persons to whom they speak, because they imperfectly distinguish their features; they have in general no correct idea of the impression which their person and their words make upon another, and, accord- ing to their original disposition, a peculiar freeness and too great self-confidence, or else, what is rarer, a more than ordinary bashfulness, are thence not unfrequently developed. In their bearing, and often also in their gait, a certain awkwardness is frequently manifested, by which some are recognised at a distance. Knally, much more of what passes in the world escapes them than they themselves are aware of, and with respect to a number of things their knowledge is less cor- rect, because they fill up what is deficient through the operation of a brisk imagination. Cardanus asserts that myopes are particularly amorous, because that not observing defects, they look upon human * Cursus s. mundus maihematicus, T. III. p. 393, 390 MYOPIA. beings as angels. The starry heavens they also see in extraordinary splendour. In point of acuteness of vision myopes are in general inferior to emmetropes. In low degrees of M the difference is extremely slight, but in the higher degrees it is, at least in advancing years, consider- able, even without the myopia being combined with unusual morbid changes. Where M is > ^, S is often imperfect, unless the myopia is congenital, and the individual is stiU very young. In myopia > ^ imperfection is the rule; in myopia > ^ it is the rule probably with- out exception:. Especially in high degrees of myopia S decreases with the increase of years much more rapidly than in E. Where M. = j^oT ^, S at sixty years of age usually amounts to not more than s- The cause of diminished S in myopes is evident. In M the retinal images, indeed, are larger for equal angles, under which the objects are seen, because the distance from the nodal point to the retina is greater ; but, on the other hand, the surface of the retina is also larger, and therefore in a given plane comprises fewer percipient elements. Where there is perfect compensation of these two factors an equal number of percipient elements could now, the visual angles being equal, be impinged upon, and S may thus remain equal. But the extension affects especially the posterior pole, also occupies principally the region of the yellow spot, and for direct vision, therefore, perfect compensation is not to be expected, even when the extension pro- duces no further disturbance in the function of the different ele- ments. We had here in view the determination of the acuteness of vision applied to objects situated within the farthest point of dis- tinct vision. The determination with the aid of objects situated at a greater distance, and seen with correcting glasses, proves still more disadvantageous, because k" thereby approaches the retina. With the increase of the myopia, the same objects at an equal dis- tance are undoubtedly seen by degrees with a smaller portion of the percipient elements. As this has occurred gradually, the eye has, during its occurrence, accustomed itself to it, and the magnitude of an object is now also, even on the fixing of a single point, invariably correctly estimated, and the finger, when it is wished to touch it. POWER OF ACCOMMODATION. 391 goes right to its bowndaries. The extension has, moreover, had this result, that the original connexion between the amount of the mus- cular action required and the number of percipient elements, which thereby consecutively receive the image of a certain point, is modified, and nevertheless the harmony has been preserved, so that on move- ment of the eye no apparent movement of the object is seen. We cannot therefore, properly say, that the myope, who sees the same object at an equal distance with fewer percipient elements, actually ^ro- jects it smaller in appearance outwards ; he undoubtedly sees it less acuteh/, but the idea which he forms of the magnitude is not deter- mined simply by the number of affected percipient elements, but is the result of a very complicated psychical process, to which a number of factors contribute. In any case we are justified in coming to the remarkable conclusion, that in consequence of slowly progres- sive displacement by extension, a point of the retina is projected outwards in a direction different from that originally belonging to it, and we hence infer that the direction of projection is not absolutely congenital, but that it has been developed in connexion with other means of observation. That myopes are possessed of a normal power of accommodation could be overlooked only by confounding the region and the range of accommodation. Some early writers have fully comprehended this, and even in Smith * we find it expressly stated. It is also satisfac- torily seen when we neutralise the myopia, whereupon young my- opes see near objects perfectly well, distinguish acutely at a few inches from the eye, and are at the same time in a condition to see remote objects accurately. We have, too, abeady remarked that the range of accommodation of myopes appears to be equal to that of emme- tropes ; but in the highest degrees, in which the musculus ciliaris and sometimes also the nervi cUiares are atrophied, the power of accom- modation diminishes and is at last wholly annihilated : this, however, happens at an age when, moreover, this power has been almost quite lost. We have, too, already calculated what range of accommodaition, with difference in length of the visual axis, corresponds to a certain change of the crystalline lens, and we found that myopes, on account of theirlonger visual axis, are in this respect somewhat in the background (Compare pp. 95, 96). On the other hand, the use of concave glasses has an advantageous influence, and therefore has a compensating effect. Such small quantities are here, however, in question, that this cannot * Complete %s 57^ necessarily present : always indeed (cases of strabismus convergens excepted), when there is acute vision without glasses, the one eye has deviated out- wards. In the foregoing it is included that relative diverging strabis- mus may arise: on the one hand, where there is considerable insufficiency of the musculi recti intemi, without any myopia ; on the other, in high degrees of myopia, without any insufficiency. In fact, it occurs in its most important forms, when M and insufficiency are combined in a moderate degree. Myopia must here be our starting-point. If the myopia be absent, the insufficiency usually produces only muscular asthenopia, and is seldom developed to stra- bismus divergens. If M be present, a number of causes coincide, to produce strabismus divergens, at least relative, and precisely thus to prevent asthenopia muscularis.* The formula is simple, and has already been given above in refer- ence to insufficiency : myopia requires more convergence of the visual lines, because vision takes place closer to the eye, and precisely in M the convergence is for two reasons more difficult, first, on account of the impeded movement, and secondly, on account of the altered direction of the visual lines (the smaller angle a). • Thus we read also in von Graefe (Archiv f. Oph. B. viii. p. 343) :— " It has been already mentioned that myopia suppUes an important, but not absolutely preponderating contingent (in muscular asthenopia). The latter would indeed be the case, if those who are highly myopic did not through the period of asthenopia into strabismus divergens, much rapidly than hypermetropes and emmetropes." EFFORT TO MAINTAIN BINOCULAR VISION. 407 That relative diverging strabismus occurs preferentially in M, follows from these facts as a matter of course. And this is stiU more the case because the need of binocular vision and the aversion to double images, here afford no important counterbalancing power. It is almost always a small object that the myope wishes to see acutely : he approximates it to the eye that he wishes to use, and the other is meanwhile directed to remote objects which, on account of the myopia, supply very diffuse, and therefore but very slightly disturbing images. If now vision once takes place with deviation, there can be little tendency to make the tension required for convergence ; — the less so, because at the same time the distance E (that of the farthest point of distinct vision) would become smaller, and the object should therefore be held still closer to the eye. Precisely when diEBlculty in convergence begins to be experienced, does the associated ten- sion of accommodation become particularly great. In progressive myopia we often observe how binocular vision attempts to maintain itself against the relative diverging strabismus. It is, however, usually obliged speedily to give way to the fatigue arising from the tension. Eeading, for example, is at first binocular, but after some time one eye yields, involuntarily and unconsciously, so that the complaint is made that one page moves over the other. We can now establish numerous transitions. If an object be ap- proximated more and more to the eyes, the convergence increases to nearly its maximum. If the object remains here, one eye yields the more speedily, the nearer the object had approached to the maxi- mum of convergence. It immediately gives way when the object is brought to within the maximum of convergence. This happens likewise directly when in strong convergence the one eye is covered with the hand. If the covering hand is now removed, the yielding of the convergence nevertheless continues. Even when the object was approximated to the eye, while before the other open eye the hand is held, the convergence is seldom sufficient : the endeavour to main- tain the binocular vision commenced at a greater distance, was the absolute condition under which the convergence was developed ; in perfectly established relative diverging strabismus, even under that condition, it is absent. The greatest difficulty is always expe- rienced when the eyes are directed upwards. As a transition we observe, that in fatigue there is no convergence, while the latter appears after rest has been enjoyed. On the boundaries between alternating and confirmed relative 408 MYOPIA. diverging strabismus lies a practically important condition, to which I formerly directed attention.* The condition is this : there is still tendency to convergence, this is seen on approximating an object : but even before the distance of distinct vision is attained, or at least soon after, the one eye deviates- If we now give concave spectacles, which bring the binocular farthest point to 8", 10', or 12", vision again takes place with both eyes. Often, however, com- plaints are now heard of the occurrence of fatigue, and examination shows, that not the tension of accommodation, but the convergence required, slight as it is, is the cause thereof. Consequently, asthenopia muscularis is in operation, so that, in order to make binocular vision possible, a combination of the concave with a pris- matic glass is required. In these cases it is especially evident, that the cause of the relative diverging strabismus is to be sought solely in the impeded movement inwards, while the tendency to co-operation of the two retinas for the sake of binocular vision may continue un- disturbed. It is only in absolute diverging strabismus that that tendency is, as shall hereafter appear, not unfrequently removed. We have above seen, how in. progressive M binocular vision for near objects usually in vain endeavours to maintain itself. To this there are, however, exceptions. " A powerful simultaneous action of the recti interni" «ven belongs, according to von Graefe,t to a " relative normal shortsightedness." He goes so far as to assert, that it is to be considered as a pathological condition, " when the increase in the power of tension of the internal muscles of the eye does not continue in harmonic development with the increase of the refractive condition (of the myopia)." Indeed, even in high degrees of M, whether on account of a favourable size of the eyeball, or of an original or acquired preponderance of the internal recti muscles, the visual lines may sometimes be properly directed in close vision, and may without effort be kept in that direction. But this in many cases is obtained only at the expense of the mobility outwards. Limitation of the latter is, in such instances, rarely absent, and it may now attain to that degree, that the visual lines cannot in distant vision be brought to parallelism, which constitutes relative converging strabismus. And if with increasing M in these cases the conver- gence for near objects also becomes insufficient, we have the singu- lar combination of relative diverging strabismus, in vision for near objects, with relative converging strabismus in distant vision, while * AreUvfur Ophthalm. B. vii. Abth. 1, p. 83. t Archiv, B. iii., 1, p. 309. ABSOLUTE DIVERGING STRABISMUS. 409 at a medium distance a certain margin has remained for binocular vision. It reminds us of the combination of M with presbyopia. — However, as I have observed, this is all exceptional. The rule is, that the facility of convergence does not keep pace with the develop- ment of the myopia, and that the tendency to relative diverging stra- bismus rapidly becomes remarkable. I satisfied myself by investiga- tion, that, proceeding from the visual lines, in M the mobility inwards is in a great number of cases very soon limited,* while that outwards is in no way impeded, nay, that under the influence of a prism, the visual lines can generally be brought even under greater divergence than in non-myopes. It appeared, indeed, to be in favour of the more ready convergence of myopes, that, as my determinations of the relative range of accommodation showed, certain degrees of convergence are possible, without proportionate tension of accom- modation. This, however, furnishes no absolute proof. Prom this we learn only that, by practice, the association of accommodative tension may, up to a certain degree, become independent of the effort at conver- gence, without preventing its reappearance when the effort is stronger. The absolute diverging strabismus is distinguished by divergence of the visual lines in distant vision. In close vision the divergence sometimes remains unchanged ; generally it diminishes, or even gives place to a certain degree of convergence, but not being sufficient, binocular vision is still excluded. In a few cases, however, I observed, that in looking to a great distance divergence existed, but that, on looking to the distance of some feet or some inches, this gave way to sufficient convergence, which was then, however, not to be maintained. The fact is remarkable. The explanation may be, that binocular vision is much more important for the estimation of near, than for that of remote, objects. — At first, diverging strabismus exists usually in a slight degree, and only slowly increases. Sometimes it continues in but a slight degree during the whole of life. I have, indeed, found that precisely the highest degrees of diverging stra- bismus not unfrequently have another origin than simple myopia. It is commonly the custom to apply the term squint exclusively to the absolute form. In this sense it is less frequent than strabismus convergens. And if, nevertheless, a certain number * On this subject still further investigations have been made by one of our students, Mr. Sehuerman : the mobility of the eyes, the maximum of conver- gence, the relation to prismatic glasses, &c., have been, in connexion with the axis of the cornea and the visual line, with determination, at the same time, of the position of the centre of motion, investigated in eyes of different refraction. 410 MYOPIA. of cases, about equal to that in strabismus convergens, is to be explained by primary disturbance of the muscles (paralysis, inflam- mation, spasm, complicated congenital anomalies, &c., a blind eye also often deviates outwards), myopia cannot occupy the same pro- minent position as an etiological element, that hypermetropia does in reference to strabismus convergens ; nevertheless, in about two- thirds of the cases of absolute diverging strabismus, myopia was met with. But if, on the other hand, we take relative diverging stra- bismus also into account, the diverging form is as frequent, if not more frequent than the converging; and now, moreover, the extra- ordinary causes, originally proceeding from the muscles or from blind- ness in one eye, fall into the background : therefore in at least 90 per cent, of the cases of relative diverging strabismus we find M. — It is often remarked that while strabismus convergens usually occurs in childhood, strabismus divergens is most frequently not developed until a later period. The observation is correct. The fact is con- nected with the cause of its occurrence : progressive myopia. Now if, in general, absolute diverging strabismus proceeds from relative, it is far from being the case, as appears even from the relations described, that the relative should always be followed by the absolute form. This appears to be rather the exception. We here meet with a like relation to the cause as in converging strabis- mus. Thus, as most hypermetropes are spared from the latter affection, we certainly find many myopes with relative diverging strabismus, without the absolute form being developed from it. Here, therefore, the question also arises : What accessory circum- stance causes the true absolute diverging strabismus to occur ? Perhaps we may be able to invert this question, if we first con- sider why, in general, the relative deviation predisposes to the abso- lute. The result of this consideration may be thus formulised : — Eelative diverging strabismus gives dissimilar images on the two yellow spots, particularly in close vision. The need of equality of impressions, the effort at simple binocular vision must, in general, be consequently weakened. A commencement of deviation, having arisen where much advanced convergence was required, immediately attains a tolerably high degree, by simply yielding to the natural ten- sion of the muscles, — partly also, perhaps, although unconsciously, in order to make the double images more remote from each other, or to exclude the tension of accommodation associated with diffi- ORIGIN OF ABSOLUTE DIVERGING STRABISMUS. 411 cult convergence, and thus to remove the farthest point of distinct vision from the e}e. In general, when, for example, with hUnd- ness of one eye, the internal recti muscles are no longer for the sake of binocular vision of near objects stimulated to contraction, they soon, through diminished energy, become ineffectual, and strabismus divergens is the usual result thereof. Relative diverging strabismus now leads to similar inaction, likewise followed by diminished energy. Thus two important factors coincide : slight resistance to double images, and diminished force of the internal muscles. It can, there- fore, not be a matter of surprise that, in distant vision also, the action of the latter soon fails. And this must occur the sooner in myopes, because the angle a is particularly small, and therefore distant vision requires a slighter divergence of the corneal axes than in em- metropes. Now, if the action of the internal muscles be once weakened, the effort to overcome the tendency to divergence will easily make the farthest point of distinct vision approach the eye, will thus render the retinal images of remote objects still more diffuse, and instinctively this effort will therefore not occur, or it wiU be removed. Thus, undoubtedly, the origin of absolute diverging strabismus is satisfactorily explained. If I am not mistaken, we must now really, as I foresaw, invert the foregoing question. We ask no longer: What accessory circumstance, where relative diverging strabismus exists, leads to the development of the absolute variety? We rather inquire : What is the reason that not every relative diverging strabismus is followed by the absolute form ? In the first place, I remark that absolute diverging strabismus is, as I find more and more every day, in high degrees of myopia very general, much more general than is supposed. Slight .degrees pass unobserved, because, although the visual lines diverge, the corneal axes exhibit no particular divergence, often certainly still less than in non-squinting hypermetropes : it is not until the properly directed eye is covered, that it appears that the visual Kne of the other was directed too much outwards. But I repeat the question : What is the reason why not ever^ relative diverging strabismus is followed by the absolute form ? The cause of this lies partly in the maintenance of binocular vision. Although, in consequence of relative diverging strabismus, the attempt to obtain equal impressions on the two yellow spots and on further corresponding points is weakened, it is not extinguished. 412 MYOPIA. This attempt alone sometimes restrains the deviation. In many the one eye actually turns outwards behind the hand, to reassume a proper direction when uncovered. And where this giving way does not take place, it is sufficient to hold a weak prismatic glass, with the refracting angle towards the nose, before the eye, to satisfy our- selves of the attempt at binocular vision : we immediately see a con- vergence set in, correcting the action of the prism. Only in the highest degrees of myopia, where even no strongly-marked object forms comparable images, is the convergence absent in this experi- ment. It thus also appears that acute sight is not an absolute condition for, if possible, maintaining single vision. Further, we seek the cause of the absence of absolute strabismus in limited mobility of the eyes. Turning the great ellipsoidal eye- ball of myopes is impeded not only inwards, but sometimes also outwards. Tliis obstruction may go so far that, as I have above remarked, relative converging strabismus in distant vision may be combined with relative diverging strabismus in near vision. But even if it does not attain this degree, it really hinders an excessive deviation outwards, especially when it is allied to the need of bino- cular vision. Thus then, just as in converging strabismus, different impelling and resisting forces contend with one another, and it is in fact diffi- cult to say under what conditions the first acquire the preponderance. Experience did not at least immediately reveal them. Undoubtedly, however, there come under observation : a, circum- stances which promote movement outwards ; l, such as deprive bino- cular vision of its value. Among the first we include an original preponderance of the external recti muscles, more than ordinary dis- placement of the visual lines, in consequence of myopia (extraordi- narily slight or even negative value of the angle a), further a favourable form for outward movement, and superficial position of the eyeball. Among the latter may be reckoned, diminished acuteness of vision in the one eye, and especially a difference of refraction in the two eyes. This last occurs as an influential factor. If the difference of refraction is great, the one eye highly myopic, the other scarcely so or even emmetropic, the rule is perhaps ttat in distant vision the myopic eye has deviated outwards. These cases fur- nish a peculiar form of strabismus divergens, which certainly deserves to be thoroughly investigated and separately described. Sometimes, especially at first, the squint is intermittent, and manifests itself only VIEWS OF BUFFON. 413 either on fatigue or under certain states of mind ; in other cases it may, even when highly developed, be overcome by the will, particu- larly for near objects, for a short interval, although not without rapidly supervening fatigue, and at all events without essential advantage to the sight. Not unfrequently, too, the one eye is used in distant, the other in close vision. Usually each eye projects and judges correctly, while it sees independently, and notwithstanding it is declared that the same object seen with the one eye appears larger, with the other smaller. More that is remarkable still remains to be mentioned, and especially to be investigated, on this subject. As to its pathogeny, with which alone we are, properly speaking, at present concerned, — it is easy to see that, in the first place, binocular vision in these cases has not much value ; secondly, that, particularly in distant vision, the double images of ordinary objects are scarcely observed, and the patient thus easily abstracts from the impression of the strongly myopic eye ; thirdly, that the limited mobihty in this case affects only the one eye, and a relative deviation outwards must therefore meet with less difficulty ; and finally, that so soon as any tension of the muscuH recti interni is required, in order to prevent divergence of the visual linesj this tension will therefore be absent, because the weakly myopic or emmetropic eye seeing tolerably acutely at a distance, in consequence of the accompanying tension of accommodation, immediately perceives less perfectly. It is well known that Buflfon {Sur la cause du strabisme ou des yeux louches, in Memoires de VAcademie, 1743, — to be found also in Buffon, Sistoire, &o., SupplSm. iv., p. 416, Paris, 1777) sougtt the principal cause of squinting in a difference between the two eyes. That difference he calls rather indefinitely, "une inSgaUte de force dans les yeux." Evidently he had in view in his definition a difference in refraction ; but in his exami- nation of squinters this is sometimes confounded with a difference in acute- ness of vision. Buffon especially endeavours to show, that unequal impres- sions of the same objects on corresponding parts of the retina are more dis- turbing than those of wholly different objects. The one eye would there- fore instinctively deviate where there is a great difference between the eyes. In this he has had in view chiefly, I should almost say exclusively, strabismus convergens ; but at the close of his essay he speaks also of some cases, "where one eye is used in looking at distant, the other at near objects, while the unused eye deviates either inwards or outwards." Besides, Buffon thinks that, so far as the regions of accommodation for the two eyes coincide, even when the boundaries of those regions differ, both eyes may receive sharp images of the same object, and that thus the tension of accommodation in each eye, independently of the other, can 414 MYOPIA. regulate itself according to the distance of the object. On this error a great deal of his demonstration rests. Joh. Mueller ( Vergl. Physiologie, etc., p. 228), admitting the fact, is not satisfied with Buffon's explanation. He gives us another, remarkable chiefly because in it a disturbed connexion between convergence and accommodation is assumed. We miss in Mueller the distinction between presbyopia and hypermetropia ; nor does he ask whether strabismus con- vergens or divergens is to be explained, and a clear insight into the cause of the affection could not thus be acquired. But we find the experiment stated in which, by holding a concave glass before one of the eyes, strabis- mus convergens is excited, so soon as this eye is used for acute vision, — an experiment which explains the exceptional cases of strabismus convergens, where the properly-directed eye is hypermetropic, the deviating eye less hypermetropic, or even emmetropic) but originally amblyopic. Had Joh. Mueller held a negative glass before each eye, it would not have escaped him, that thus also a deviation inwards is readily produced, and probably his clear glance would at once have penetrated the character of hyper- metropia and its connexion with strabismus. Some (compare Bohm, J)as Schielen, I. v. ; Artl, Die Kranhheiten des Auges, B. iii., pp. 306 et seq., Prag, 1856) attach too much, others (com- pare Euete, Lehrhuch der Ophthalmohgie, B. ii., p. 624) attach too little value to difference between the two eyes, whether in acuteness of vision or refraction, in reference to the origin of strabismus. I think I have shown that the difference mentioned does not occur as the immediate cause of strabismus, but that it may be the reason why, with certain determining conditions, to be sought in the non-deviating eye, strabismus is produced. The connexion, too, between myopia of the two eyes, and strabismus divergens was formerly not completely overlooked. Joh. Mueller {Ver- gleichende Physiologie, p. 237) even describes a strabismus myopwm. " It is known," thus he commences the explanation of the mode of develop- ment, " that shortsighted people look at very near objects only with one eye, while the other eye, which is also nearsighted, completely tvirned away and directed to a distance, sees indistinctly or not at all." This is the state which we have called relative diverging strahismus. It has been described by Buffon, as occurring in his own eyes. In his own case he in- troduces the difference of the images in the two eyes into the explanation ; but in general he finds in the extraordinary convergence required by myopes, the reason, " that the sight is fatigued and less distinct than in looking with a single eye." Mueller assigns the first place to the same cause, but in addition points to the increasing refraction produced by the convergence. However, in explaining why the visual axis subsequently deviates permanently also more or less from the normal direction, he thinks only of the neglect of one eye resulting from the deviation, nor do we then read that this deviation takes place positively outwards. — Ruete, too {I. o'., B. i. p. 226), speaks of the connexion between myopia and stra- bismus. "We saw, that, while in general relative diverging strabismus and a tendency to absolute are connected with high degrees of progressive myo- pia, the convergence in close vision is exceptionally maintained, though at TREATMENT. 415 the expense of parallelism in distant vision. Now this exception, whereby converging strabismus — relative, if we will — is combined with progressive myopia, was recognised by Ruete, not the diverging strabismus, which is the rule. Even of the existence of relative diverging strabismus he could not satisfy himself, undoubtedly because he sought it in too slight degrees of myopia, where it is usually wanting. On the whole, little satisfaction is obtained by consulting the more recent copious literature on strabismus, with reference to its causes. Strabismus divergens, in particular, is very imperfectly treated of. A distinction of the causes, according to the different forms, is not to be met with, and where the causes of strabismus in general are spoken ofi the writers have evidently been filled with the idea of strabismus convergens. I have yet only to refer to the writings of von Graefe, with respect to the insufficiency of the internal recti muscles, in the numerous modifications of which the gradual transition to strabismus divergens is to be sought, and where it certainly has been sought for by von Graefe. " We may in general" — thus we read in his latest essay (Archiv f. Ophth., B. viii., Abth. 2) — " define the insufficiency as a dynamic outward squint, varying according to the distances of the object, which is at the time overcome by the attempt at single vision." If we reflect that, even according to von Graefe, this effort must in myopia often give way, it is as if we, under certain conditions, already see before us the development of absolute diverging strabismus. Our investigations have thus led us to a striking result, which may be expressed in the following antithesis : — JSypermetropia causes accommodative asthenopia, to he actively overcome by strabismus convergens. Myopia leads to muscular asthenopia, passively yielding to strabismus § 32. Hygiene. — Tbeatment. — Spectacles. — Illtjsteative Cases. — Histoey op Myopia. The cure of myopia belongs to the pia vota. The more our know- ledge of the basis of this anomaly has been established, the more certainly does any expectation in that direction appear to be destroyed, even with respect to the future. So long as it was thought that the cause of myopia might be found in increased con- vexity of the cornea, the endeavour to restore the latter, by pressure, to its normal curvature (Purkinje* and Euete) appeared perhaps not altogether to be rejected j but the idea, that the extended, attenuated, * Neue Beitrdge %. Physiologic des Sehens in subjectiver Hinsicht, p. 147. 416 MYOPIA. atrophic membranes in myopia, might be brought back to their natural condition is simply absurd. We should not even be able to approve of the practice of those, who, in order to compensate for the excessive length of the visual axis, endeavoured to bring the arching of the cornea below the normal. Systematic pressure is an excellent auxiliary in preventing a staphylomatous prominence of the cornea in morbid softening during the process of recovery; but, leaving out of the question, whether a healthy cornea would be affected by it, we should remember, that no more is to be expected from a compensating flattening than from a negative glass : the peculiar morbid process, on which the myopia depends, and which in high degrees threatens the destruction of the eye, would remain unchanged. Treatment is, alas, partly a matter of fashion. Thus dis- charging the aqueous humour from the anterior chamber of the eye, is now the order of the day. Some have even spoken of applying this method in myopia. If it be intended thereby to make the cornea flatter, the object will not be attained in this way, which is the less to be regretted, because, as I have already remarked, the myope would not be benefited by it : besides the paracentesis referred to is not always itself equaUy harmless. We formerly lived under the rule of the my oto- mists. Eendered rash by ignorance, some have actually employed their operation for the relief of myopia, and have even persuaded them- selves that they had thus accomplished a cure. The truth is, that they in general subjected not myopic, but rather hypermetropic, eyes, which they mistook for myopic, to the operation, and that even in these the latter was ineffective. However, in another point of view, where myopia really exists, tenotomy may sometimes be applicable. If the muscles are, as a result of extension, permanently too much upon the stretch, and if the bulb is consequently harder, the displacement backwards of the insertions by tenotomy might diminish the pressure, and thus one of the conditions for further development might be removed. Division of the tendon of the rectus externus, is, as we saw in the preceding section, not unfrequently indicated for promoting convergence in high degrees of myopia, and by that division the bulb also acquires less tension j now it has been proposed to divide the tendons of the rectus internus and rectus externus, in order to diminish the existing tension, and in some cases this has actually been done. Further experience must decide as to the value of the plan. — Lastly, the removal of the crystalline lens has also been suggested. When in a case of highly myopic structure of the eye, QUESTION OF ITS CURABILITY. 417 a lens affected witli cataract has been successfully extracted, and a nearly emmetropic condition has been obtained^ the operator has been exposed to the temptation of endeavouring, by the abstraction of a normal lens, to remove the myopia. A patient, who was an amateur in dioptrics, endeavoured to induce me to perform this operation ! But I need not say, that such a momentous undertaking, doubly dangerous where a myopic eye and a transparent lens are concerned, without that, even in the most favourable case, any real advantage is to be expected, would exhibit culpable rashness. Not only would the staphyloma posticum continue equally threatening, but we should also have sacrificed the accommodation — an advantage which that of somewhat larger images than would be obtainable by neutralising glasses, could by no means counterbalance. From the above it follows, that the idea of curing high degrees of myopia with developed staphyloma posticum must be abandoned. The question is, whether slight degrees can really be cured. That, through senile metamorphosis at an advanced period of life, they may give way, we have already seen. In young persons, on the contrary, I have never estabhshed the fact of any diminution of the myopia. Where the latter appeared, on superficial observation, to have taken place, spasm of accommodation had been in operation. It has, how- ever, often been asserted that slight degrees of myopia have yielded to the employment of suitable measures. Some years ago Berthold* proposed the use of a certain desk, called myopodiorthoticon (.') by which the myope was compelled to remain at a great distance from what he was reading, while this distance was systematically increased. Burowt, however, shows clearly, that no diminution of the myopia is to be expected from this plan, as the accommodation for the farthest point is only a passive, and not an active operation, and he supplies the proof from the experience of his own eyes. Berthold's desk was also tried in vain at Koningsberg ; and according to von Hasner the attempt at Praguewas attended with no better result. How- ever, the last-named writer expressly states, that myopia depending upon slight degrees of staphyloma scleroticee, may, particularly in the commencement, be again diminished by restraining the eye from looking at near objects. Eespect for Hasner's accuracy cannot pre- vent me from doubting the correctness of this assertion. I have, under all circumstances, found steady increase of M in young indi- * Das Myopodiorthoticon, 1840. f- ;. c. p. 49. 27 418 MYOPIA. vidualsj its diminution I have never observed. Many cases quoted here and there lose all value as proofs, because it does not appear that the farthest point of distinct vision, on which our judgment in this instance depends, was determined with the requisite accuracy. Even in presence of the case quoted by von Hasner, where the myopia is said to have given way after typhus, I cannot abandon my scep- ticism. Had spasm, perhaps, previously existed, which was now removed? Had myosis followed, which now made reading at a greater distance possible? — However this may be, we may safely doubt the diminution of the staphyloma posticum, and require con- vincing observations from those who assert the contrary. The task of the ocuhst in myopia resolves itself into the follow- ing :— 1. To prevent the further development of the myopia, and the occurrence of secondary disturbances. 2. By means of suitable glasses to render the use of the myopic eye easier and safer. 3. To remove the asthenopia muscularis by the use of glasses or by tenotomy. 4. To combat the secondary disturbances of the M. I. In the exposition already given lies, I think, the proof, that where predisposition exists, continued accommodation for near objects promotes the development of staphyloma posticum. Where this predisposition. occurs we must be aware of it even from youth. We have remarked that accommodation, as such, is not in this case in operation, for in it only the form of the crystalline lens is changedj and in M the latter has undergone no change. In a mediate way, therefore, through accessory circumstances, it must be, that accommo- dation for near objects promotes staphyloma posticum. Of such cir- cumstances two especially come under observation : strong convergence and a stooping position. As to the first, in order to see acutely, myopes must bring the object within the region of accommodation, and where M is somewhat advanced, binocular vision under these circumstances requires a strong convergence. Children and young myopes, with great power of accommodation, are even accustomed, particularly in bad light, to bring the objects much nearer to the eyes than the degree of the myopia, properly speaking, requires. This strong convergence increases the tension of the eyeball by pressure of the muscles, perhaps also by pressure against the surrounding EVILS OF A STOOPING POSITION. 419 tissues, and increased pressure promotes the staphylomatous disten- tion. Especially when commencing insufficiency of the mm. recti interni renders the convergence diiRcult, the latter is combined with great tension of the eyeball. Now strong convergence may be avoided in various ways. In the first place we cause the patient to look much at a distance. But we cannot absolutely forbid looking at near objects, and we therefore give spectacles which bring the farthest point, r, to a sufficient distance; for example, to from 16 to 18 inches. At the same time the patient is to be strongly recom- mended not to look at a shorter distance than 16" or 14!", to which young people have the greatest tendency ; a ruler of this length may serve as a measure to parents and masters as well as to the myope himself. Moreover, it is desirable that often (for example, every half- hour) work should be discontinued for a couple of minutes. — In very high degrees of M, only one eye is usually employed in vision, and thus convergence is excluded. This appears to me to be often a de- sirable condition : in strong M binocular vision loses its value, and the tension which would be required for it cannot be otherwise than inju- rious. Now, in such cases, for reading no spectacles are given ; in the first place, because the acuteness of vision has usually somewhat decreased, and the diminution of concave glasses is now troublesome ; in the second place, because with the retrocession of r injurious efforts at convergence and at binocular vision might be excited. In any case the spectacles should be so weak as to avoid these results. A stooping position was also mentioned as a prom'oting cause of M. This position of necessity leads to accumulation of blood in the eye : under the influence of gravitation, the afflux of blood takes placej in fact, under higher pressure, and until the efflux, too, more pressure remains ia the veins ; and with the augmented pressure of blood the tension of the fluids in the eye increases. The symptoms of irritation connected with hypersemia, which in young people usually accom- pany progressive myopia are, I think, for the most part also to be ascribed to the cause just mentioned. Even iii non-myopes an un- pleasant feeling of pressure in the eyes speedily occurs when the face is held horizontally. Now the increased tension of the fluids certainly promotes, as such, the development of staphyloma. But in yet an- other way the accumulation of blood is still more injurious, namely, by promoting, perhaps even by exciting, the inflammatory affections under whose influence the staphyloma is so rapidly developed. In the hygiene of myopia, therefore, the very first point is to guard 2 420 MYOPIA. against working in a stooping position. While most work takes place in a horizontal plane, myopes are only too much inclined to it. Such a habit is usually opposed on the supposition that the thoracic organs suffer from it. Without denying this, I think, however, that it must be forbidden chiefly for the sake of the eyes. In general everything will be useful for this purpose which was recommended for the avoidance of strong convergence : to keep objects removed as far as the degree of myopia permits ; to intermit work frequently ; by suitable glasses to bring »■ to a sufficient dis- tance. But we may also add : to read with the book in the hand, and in writing to use a high and sloping desk. To this last I attach much importance. Eectilinear drawing on a horizontal sur- face is decidedly very injurious to myopes. Again, with respect to the thoracic organs, many think that in writing a standing position is to be recommended. For that I see no particular reason. It is sufficient that the height of the desk suit the height of the head, and that the inclination be as great as circumstances permit : in writing the limits are when the ink no longer flows from the pen, but even then a pencil may be used. — Eurther, those who are highly myopic must be earnestly dissuaded from everything which gives rise to in- creased action of the heart, and to tendency of blood to the head, with a view both to limit the progress of the myopia, and to prevent the occurrence of secondary affections. II. The prescribing of spectacles for myopes is a matter of great importance. While emmetropic and hypermetropic eyes do not readily experience any injury from the use of unsuitable glasses, this may in myopes, particularly on account of the morbidly distended condition of the eyeball, and of the tendency to get worse, be very dangerous. There exists in general a dread of the use of too strong glasses. It is laid down as a rule : rather too weak, or no glasses, than too strong. In this rule the necessary distinction is lost sight of. Too strong glasses make hypermetropic eyes myopic, and myopic eyes hypermetropic. The rule, therefore, cannot be equally true for both. In fact, it is in general much less injurious to produce a certain degree of myopia than of hypermetropia, in which last particularly much is required of the accommodative power. The rule would therefore be more correctly stated thus : in hypermetropia we must beware of giving too weak, in myopia of giving too strong, glasses ; a INDICATIONS FOR NEUTRALISATION. 421 rule the second part of which we should especially insist upon. But even by this little is gained. Not using glasses^ or using too weak glasses, may also be injurious to myopes. All the circumstances must therefore be studied, which can exercise an influence on the choice of glasses. It is difficult to reduce these to definite rules. But to attempt to do so is our task. On a superficial view, we should suppose that we have only com- pletely to neutraUse each degree of myopia, in order to obtain all the advantages connected with the emmetropic eye. The case is, however, quite different. If in neutralised myopia the eye is equal in its farthest point to the emmetropic, with respect to the relative limits of accommodation for each convergence there is a great difl'erence, and in acuteness of vision, too, it usually fails. These differences alone would in themselves be sufficient very much to limit the indication for perfect neutralisation, and we shall become acquainted with other circumstances still, which positively forbid it. The indication exists only : 1°. When the glasses are used exclusively for distant vision, for example, in a double eye-glass, which is only at intervals held be- fore the eyes. Evidently, in looking to a great distance with such a glass the accommodation is in a state of rest, and its use can there- fore never cause any injury. But so soon as the same glasses are used for shorter distances in looking at drawings, plates, &c., the exceptions, of which I shall hereafter speak, come under observation. 2°. When the myopia is slight in reference to the range of accom- modation, and the eye is otherwise healthy. In this case, neutralis- ing glasses may be worn as spectacles, and may be used even in reading and writing. I think it is even desirable that this should be done. When persons with moderate degrees of myopia have in youth accustomed themselves to the use of neutrahsing spectacles, the eyes are in all respects similar to emmetropic eyes, and the myopia is, under such circumstances, remarkably little progressive. I am acquainted with numerous examples of this even among those of my friends, who have passed their lives in study. Glasses of — Yh> adopted at seventeen years of age, are often still sufficient at forty-five years, both for seeing acutely at a distance and for ordinary close work. Not until the age at which emmetropes need convex spec- tacles, and often even some years later, do the neutralising spectacles become rather too strong for close work, and it is desirable to procure 422 MYOPIA. somewhat weaker ones, which, with the narrower pupil peculiar to that time of life, are now nearly sufficient for distance also. At a still more ad¥anced age, according to the degree of the myopia, suffi- sciently correcting spectacles may be worn, and laid aside for work.' — In order to obtain all the advantages of concave glasses, the myope must begin early with them. If the myopia amounts only to a fourth or a third of the range of accommodation, we may immedi- ately wholly neutralise it. If it amounts to more, we must usually begin with weaker glasses, and replace the latter at the end of six months with stronger. If we have, without the necessary transitions, given too strong spectacles, this may be immediately evident from too great a distance of the binocular nearest point, but in any case it will be manifested by the occurrence of fatigue (asthenopia) in close work. In this respect great individual differ- ences exist, chiefly dependent on the position of the relative range of accommodation. If this be unfavourable, we must more slowly increase the strength of the glasses. The effect of wearing glasses is, in fact, that the relative range of accommodation is displaced, becoming gradually the same as the position proper to emme- tropic eyes, and therefore the binocular farthest point approaches the eye, while the absolute farthest point r by no means does so. — ^The myopia thus neutralised is less progressive, because both too strong convergence and a stooping position are avoided. But if the tendency to these is so great that they still occur in neutralised myopia, the use of glasses is dangerous, and must be discontinued, so soon as it appears that the myopia is par- ticularly progressive. In this case it is necessary for a time to forbid aU close work. — Besides, if in the cases mentioned the use of concave glasses is desirable, it is not so necessary that we should always be compelled to adopt them. Women particularly have a right to be allowed some liberty in the matter. Many circumstances forbid the complete neutralisation of the rm/opia. They are connected with : a. The degree of the myopia. — In very slight degrees, from — to — , we may leave the myope to himself; in higher degrees the neutralisation, as I explained under 2°, is desirable. In the highest degrees, from ^ upwards, perfect neutralisation is not pleasant for CONTRA-INDICATIONS TO NEUTRALISATION. 423 close work, because, with regard to the iisiial diminution of the acute- ness of vision, the images become too small. We should then rather bring r to 12 or 16 inches,* and let the patient wear these spectacles, with which a lorgnette with glasses of — v^,— -Tr' respectively, is also given, which may for distant vision be held before the spectacles. The idea that there is anything injurious in this combination is an un- founded prejudice. We may also, with weaker spectacles for work- ing, give stronger to be worn ; but completely neutraHsing glasses are not pleasant for a constancy, because the myopia is usually less in indirect than in direct vision. If in general too strong glasses are to be carefully avoided in myopia, the danger to be apprehended from their use is greater the higher the degree of the myopia is. h. The range of accommodation. — If the range of accommodation be, as is usually the case, proportionate to the time of life, the proper mode of proceeding is included in what has been said under 2°. In saying this, however, we take for granted that nearly neutralising glasses, have been used from youth. If this has not been the case, the peculiar position of the relative range of accommodation is at- tended with difficulties. Moderate degrees of myopia, for example of -^, we can no longer completely neutralise at thirty-five years of age. So long as only distant objects are viewed with neutralising spectacles, all is tolerably well j but even in speaking with people we hear complaints of the tension required in order to see the eyes and face acutely, and work is, on the whole, not to be maintained with them. We should then confine ourselves to glasses, which bring r to about 24", giving, if necessary, still weaker ones for working, and at the end of six months or more, we should examine whether we can increase a little, without causing asthenopia.^ — Of the optical aids in actual disturbances of accommodation, I shall treat in speaking of the anomalies of accommodation. c. Acuteness of vision. — This has a great influence in the choice of glasses. We know that in the highest degrees, the acuteness of vision is usually diminished, and therefore we must be very careful. » To bring r, instead of to oo, to n inches, we give glasses which are - weaker than those required for complete neutralisation of the myopia. If strong glasses are in question, the distance from the eye is also to be taken into account in the calculation. 424 MYOPIA. Von Graefe tas forcibly insisted on the great danger of giving strong glasses for near objectSj where the acuteness of vision is diminished. The objects, especially letters in reading, then appear smaller, and from the need of seeing them under a greater visual angle, such shortsighted amblyopes, in order to be able to bring them nearer, put their accommodative power with all their might upon the stretch, and both thus, and by the accompanying convergence, promote the existing staphyloma. But, on the other hand, without glasses, the convergence is still stronger, and the tendency to the bent position is still greater. We thus find ourselves in a sad dilemma, which is to be avoided only by, in great part, forbidding close work. The most favourable circumstance is when in near vision convergence does not take place, and thus only one eye is used. Less injury is then to be expected from reading without spectacles, with the book in the hand, but writing must be avoided. In the same cases we may permit older persons to wear partially neutraHsing spectacles, and with these spectacles, aided by a reading-glass, occasionally to peruse some- thing, which may in this way be done at a greater distance (com- pare pp. 229 et seq.). Smee advises that his amphfier should, under these circumstances, be used. In any case we should insist upon moderation in reading, and on the selection of large print, although the latter is also attended with particular difficulties. — For the pur- pose of distinguishing pretty well at a distance in the highest degrees of myopia with diminished acuteness of vision, there is no other means than the use of a double eye-glass, or of the very portable glass-conus of Steinheil* for a single eye. In such cases, a wholly neutralising concave glass makes the distant images appear so small, and the objects so remote, that vision is by no means satisfactory. Imperfectly neutraHsing glasses in a stenopseic frame, which limits the remaining circles of diffusion, answer still better. d. Age. — The influence of age is, for the most part, comprised in the diminution of the range of accommodation, and of the acuteness of vision. At a very advanced time of life, we need attend. less to the future than to the present. We may therefore, in order with diminished acuteness of vision to make reading still possible, * A simple solid oonus of glass, atout one inch long, the base convex, the opposite surface concave, with a smaller radius than the convex. It acts as a Galilean telescope : parallel rays, refracted on the convex surface, are converging in the glass, and, refracted again on the concave surface, obtain a diverging direction, and can also unite on the retina of a corresponding myopic eye. The magnifying power increases for the glass-ooni, required in high degrees of myopia. Such coni are to be had of Steinheil, Munich. CONTRA-TNDICATIONS TO NEUTRALISATION. 425 in the slighter degrees of myopia, even with convex glasses, bring r ' to 6 , 5'j and even closer to the eye. "We should give only such a form of glasses as the patient can easily see over. Old people seldom know how to derive much benefit from glasses a double foyer, e. The nature of the work, and the distance at which it is to he performed. — He who in youth neutralises his myopia, can, if his sight be otherwise good, also, later in life, perform ordinary work at any distance required. He who, on the contrary, either could not or would not wear spectacles, retains the relative range of accommo- dation peculiar to myopes, and now, when for any particular object vision is required at a somewhat greater distance, he cannot use com- pletely neutralising spectacles. The rule is with the glasses, therefore, to bring the farthest point precisely to the distance at which acute vision is needed. The necessity for this is felt most in ladies, for the reading of music, when r must be brought to from 18" to 24". Meanwhile, they can, for distant vision, use a neutralising eyeglass. It is also in general desirable in writing (in reading it is less neces- sary), in order to prevent a stooping position and strong conver- gence, to bring r to from l^" to 16", sometimes even to 18", to make ledger- work possible. Lastly, in order to see the writing in lectures, especially in the pulpit, the farthest point must be brought precisely to the requisite distance. Particular attention is necessary — even though spectacles were always worn — in the case of elderly people, who have lost their power of accommodation, and have not perfect accuracy of vision, — in general in those whose myopia cannot be neutralised. More especially where there is diminished acuteness of vision we meet with difficulties, which by diminishing the distance as much as possible, and giving proportionately weaker glasses, we cannot always overcome. Persons so circumstanced are not satisfied. They read the smallest writing at 3" or 4' distance, and are surprised that they cannot with spectacles see much larger writing at 18". They do not consider that the distance is from four to six times greater, and that by the concave glasses the images are still further diminished. Consequently there is nothing to be done, but to write particularly large. We have, in a separate section, treated of the vision of myopes. We have also examined the direct influence of glasses on observation, and on the estimation of what is observed (pp. 143 et seq.), and we need the less to return to the subject, because many points bearing upon it came again under our notice in speaking of the indications and 426 MYOPIA. contra-indications of concave glasses. As to the indirect influence (the result of longer use), we became acquainted with the displace- ment of the relative range of accommodation, and the approxima- tion of the binocular nearest point. We may still add that the at first incorrect estimation of magnitude, distance, and form, is tolerably quickly lost. It is remarkable how myopes, when using spectacles, immediately begin, unconsciously, to write larger than before, and how, after some time, they involuntarily resume their smaller hand- writing, unless with considerable energy they resist the tendency to do so. Another result of wearing spectacles is, that by being accus- tomed to look nearly through the axes of the glasses, the eyes gradually Hmit their movements. After the removal of the spec- tacles, too, this limitation continues, and the movements of the head especially provide for the necessity just mentioned, giving a peculiar bearing to myopes who are accustomed to wear glasses. III. — We have already seen that in high degrees of myopia the intern d recti muscles are often insufficient, and that this insufficiency may be developed through different degrees to relative, and even to absolute diverging strabismus (§ 31). The insufficiency first makes if-self known by asthenopia muscularis in binocular vision for near objects. Sometimes we find (at least temporarily) the proper remedy for this state of things in concave glasses, which bring ra to from 12" to 14", and we may try these, if there is no contraindi- cation for other reasons against them. In using them we may, if it seems necessary, somewhat diminish the mutual distance of the glasses,-by which less is required of the internal muscles (compare p. 167). In other cases the use of spectacles may itself give rise to asthenopia muscularis. Thus, in very high degrees of myopia rela- tive diverging strabismus is a very common occurrence : for the short distance, at which vision is distinct, the convergence is, in fact, in- sufficient, and therefore only one eye is used, while the other deviates outwards. When, in such cases, r is by concave glasses brought to a greater distance, the effort at binocular vision sometimes returns ; and the musculi recti interni are then obliged to exert themselves so power- fully, that asthenopia muscularis is unavoidable. Now this is not only troublesome and fatiguing, but it is also injurious, as it affects the further development of the myopia. If the one eye, so soon as it is covered, perceptibly deviates outwards, and on removing the hand again turns inwards, in order to resume its former direction. INSUFFICIENCY OF THE INTERNAL MUSCLES. 427 we may expect the occurrence of asthenopia muscularis. It is often diiEcult enough to decide what is to be done in such cases. ]?or the rules applicable to insufficiency of the internal muscles in non- myopes, by no means hold good in the asthenopia muscularis of myopic individuals. In the former the condition referred to is, in the first place, free from danger, and it is even allowable to try by systematic practice with prismatic glasses to excite the energy of the internal muscles. In myopia, on the contrary, cure of the insufficiency of the internal recti muscles is not to be thought of. Once begun, the insufficiency develops itself more and more, in double proportion, when, as is usual, the myopia is progressive. Often no other result is possible, than the exclusion of the one eye, with diverging strabismus. In the worst cases the mobility is even so limited, that it is insuffi- cient both inwardly and outwardly. Both these conditions have been distinctly enough put forward in the preceding paragraph. Seeing the danger connected with strong action of the musculi recti intemi, and the prospect of not being able to prevent the progress of the insufficiency, I have often asked myself, whether we must not simply submit to the tendency to outward deviation, which removes the asthenopia muscularis by the intervention of relative strabismus divergens. Von Graefe* is of opinion that we should decide upon this sacrifice only in excessive myopia, in order to avoid asthenopia. A middle course may, however, also be adopted: we may allow reading without glasses, that is with the exclasion of one eye ; but for writing and other work, which, to prevent a stooping position, must be performed at a somewhat greater distance, we should give prismatic- concave glasses. The concavity of these should be such as to bring the farthest point to from 12" to 16", and the angle of the prism should be so great that in looking to a distance of from 12" to 16", covering one eye should no longer be followed by any outward movement. Von Graefe is a great advocate of this com- bination, which, if it is arranged with strict care for individual cases, really renders binocular vision, without asthenopia, possible. If we are particularly successful we may allow these glasses to be used also in reading ; if the difficulties are not altogether removed, their use must be limited to writing, but even in this moderation must be ob- served ; and if, after repeated efforts, the patient continues to com- plain, we should not hesitate to sacrifice the one eye. " The proper principal remedy," says von Graefe, " is above aU * ArcMvfiir Ophthahnohgie, B. viii. Abth. 2, p. 314. 428 MYOPIA. tenotomy of the musculus rectus externus." Even where myopia is the cause of the affection it may sometimes yield excellent results. In such cases it is, however, comparatively rarely adopted, so long as there is merely simple insuiiiciency of the recti muscles, with- out strabismus divergens. The cause of this is the Hmited mobility of the eyes in the high degrees of myopia, the basis of which we have already indicated. It will, in fact, in general be admissible only when, after the operation, no permanent converging strabismus in dis- tant vision, even in looking somewhat towards the side operated on, is to be expected. Von Graefe has from this point of view established the indications with great accuracy. The condition, sine qua non, for tenotomy is this : that under the attempt at single vision, a sufficient divergence of the visual lines should appear to be possible. This should be tried (after neutralisation of the myopia by concave glasses placed at a proper distance from each other) with prismatic glasses; we should investigate, vdth what prismatic glasses, held with the refracting angle outwards before the eyes, single distant vision is stiU attainable. The strongest glasses then, which can still be overcome, give the measure of the possible divergence. It is allowable now so to perform tenotomy that this possible divergence shall be completely removed. Even conver- gence of from 1 to \\ mm. may at first, in looking straight forward, be obtained, which, after cicatrisation, again gives way. When, with- out the employment of prismatic glasses, in distant vision divergence of the visual lines already exists, there is not the shghtest difficulty in the way of tenotomy, although we should still even then endeavour to determine the effect to be obtained by the maximum of the prisms to be overcome. But we must not suppose that with the removal of the possible divergence, we have always made binocular vision for near objects easy : when deviation of the eye behind the hand, at the dis- tance at which binocular vision is desired, is much greater than the utmost divergence which we are allowed to correct by tenotomy, we shall, after this correction, retain an insufficiency for this distance, which, with binocular vision, if it is not whoUy absent, will still give rise to asthenopia. We must, therefore, in such cases bear in mind that after tenotomy, the correction must be supplied by prismatic-concave glasses. Hence, therefore, it also follows, that when in distant vision only comparatively very weak prisms can be overcome, the operation certainly effects almost nothing, and where it appears that no divergence is possible, tenotomy is absolutely contra-indicated. Certain rules for regulating the effect of tenotomy according to the degree of the deviation, are laid down by von Graefe. Personal experience is. TREATMENT OF COMPLICATIONS. 429 however, also necessary. In general, I found in strongly myopic individuals the effect of tenotomy to be less. IV. Therapeutic Treatment. — Por myopia, as such, there is no therapeutic treatment. Myopia consists in an anomaly of form capable of no improvement, and of which only hygienic measures must, if possible, prevent the further development. But it is not un- frequently complicated with symptoms of irritation and inflammation, and with other pathological deviations of different kinds, which partly proceed from it, partly promote its further development ; and with respect to these it is the duty of the therapeutist, to the best of his ability, to interfere. It is not part of the plan of this work to enter into detailed discussions respecting therapeutic questions. While it treats of anomalies of refraction, it must make known the dioptric remedies which counteract them; but it cannot treat in extenso of complications, which neither necessarily belong to the essence of these anomalies, nor are characteristic of them — much less still of their treatment. Consequently, only brief indications are to be ex- pected here, respecting treatment, in connexion with some prognostic hints. In the foreground appear symptoms of irritation at the period of puberty, characterised by capillary hyperemia of the nerve-surface (and of the retina), and by fatigue and pain in the eyes, especially on exertion in the evening. Under such circumstances we should be particularly strict with respect to the hygienic directions (com- pare p. 408), which are in themselves in many cases sufficient. It is also of great importance to keep the feet warm. Often, too, a douche on the closed eyelids is agreeable. If the symptoms do not give way, we may, with avoidance of a stimulating diet, combine some derivation on the intestinal canal, and we may in addition recommend the application, to the frontal and temporal regions, of a stimulating embrocation, composed, if there be any coexisting external irritation of the eyes, rather of non-volatile ingredients. At the same time, especially when it appears that the myopia is rapidly pro- gressive, aU tension must be avoided. So far as work is permitted, this must be accomplished in slight degrees of myopia without spec- tacles, and in higher degrees r must be brought accurately to twelve inches. If fatigue or pain should occur, work must in either case be suspended ; and if the use of spectacles appears more rapidly to cause fatigue, we should not lay stress upon their adoption, but take care only that the patient maintains a proper position in whatever 430 MYOPIA. work he still performs. If it be suspected that the symptoms of irritation have excited spasm of accommodation, as often occurs in high degrees of myopia in youth, we should employ sulphate of atropia, partly to test the truth of the supposition, partly to remove the spasm and to prevent it returning on each effort to see. We may then even continue this application for some days, whereby the myope becomes accustomed to look at the greatest distance of distinct vision ; unnecessary convergence is thus prevented, and from this plan no injury is to be apprehended, provided we cause the patient to avoid strong light, or to moderate it by means of grey glasses. In case of a relapse of the symptoms of irritation, with spasm of accommodation, the application of Heurteloup's artificial leeches to the temples, followed by twenty-four hours' stay in the dark, with gradual transition to light, has been found very useful. — In spite of all efforts, however, these symptoms of irritation in some constantly recur. If, moreover, the myopia is rapidly progressive, the patient's state is serious enough to make it necessary to warn him against choosing an occupation in which close work would be con- stantly required ; above all, such myopes should not be office-clerks. But such cases are rare : with few exceptions the inconveniences dis- appear before the twentieth year. At a later period of life, the acuteness of vision sometimes diminishes in high degrees of myopia in a few months in a manner to cause considerable uneasiness. In these cases the hypersemia at the borders of the atrophy often leads us to suspect the existence of progressive myopia, while to this state of things other signs of irritation are usually added. If, in such instances, no organic changes are ophthalmoscopically to be observed in the region of the yellow spot, we almost always obtain, within a few weeks, a con- siderable improvement of the acuteness of vision, by the weekly ab- straction of blood after Heurteloup's method, by keeping the patient in a moderate light, and by making him avoid tension of the eyes, combining this plan, according to circumstances, with the use of the douche and of a stimulating embrocation, with derivation by the intestinal canal and stimulating pediluvia. Even when there are perceptible morbid changes in the yellow spot we need not despair, so long as subjectively a defined scotoma does not remove direct vision. In persons of 60 years and upwards, with myopia of ^ and even of -r, 1 have, by following the above directions, seen the acute- MOTES IN THE VITREOUS HUMOUR. 431 ness of vision rise from ;r^ or -zr-,., to 7 or -s, and thus become quite 30 20 4 3 sufficient for writing and reading. It is quite a different thing when a circumscribed scotoma, ophthalmoscopically perceptible in the yellow spot, is also perceptible to the patient. This indicates a pro- found disturbance in the seat of direct vision. Blindness is in general not particularly threatened thereby ; but improvement of direct vision is not to be expected, and if both eyes are equally affected, the patient must prepare himself for the impossibility of reading, writing, or performing minute work. — In cases of accessory chorio- iditis disseminata the same directions are to be observed. In such we must expect repeated improvement and aggravation of the affec- tion. After many years, however, the result usually becomes so unfavourable, that ordinary work can no longer be performed. Motes are at the same time often present in the vitreous humour ; of the cause of these I have already treated. Especially under these circumstances it is usual to prescribe a long course of small doses of preparations of iodine or mercury. I too, have repeatedly done this, but I would not venture absolutely to assert, that I have seen favour- able results from it. Many patients give themselves more trouble about these motes than they deserve. If no definite morbid changes threaten the yellow spot, we may give a comparatively favourable prognosis ; we should advise that the attention should be as much as possible withdrawn from them, and the attempt to do this should be seconded by causing the patient to wear nearly neutral- ising glasses, made so as at the same time to moderate the light, and thus to make the shadows of the motes appear less defined.*— Complaints of persistent photopsia are yet louder, but are fortunately rarer. It occurs chiefly in diffuse atrophy, and indicates a state of irritation of the optic nerve. I have, in addition to the above- described treatment, tried numerous remedies against it, among others, narcotics, but, so far as I recollect, always in vain. The complaints were, in some cases, especially in nervous women, lamentable, and it has often surprised me, that, with such signs of con- tinual irritation, the acuteness of vision was, in the course even of some years, but slightly diminished. — Against the most melancholy compli- cations of myopia, effusion of blood, and detachment of the retina, * Opaque membranes in the vitreous humour cause more disturbance. The most recent experience of von Graefe raises the question whether some results are not in these oases to be obtained by operation. 432 MYOPIA. treatment is almost powerless. In cases of effusion of blood in the vitreous humour, we may expect absorption, leaving behind it some opaque motes and membranes. The metamorphosis, under which the absorption takes place, is a spontaneous process, which treatment can- not promote, and the physician has therefore to confine himseK to hygienic rules, and to such derivative' or constitutional treatment as may appear to be adapted to each individual case. Pressure, by means of a bandage applied at intervals, might probably favour absorption, but when the bandage is taken off the tension of the fluids is diminished, and, as appears on ophthalmoscopic investigation, the vessels are distended, whereby the danger of fresh effusion must ne- cessarily be increased. After repeated relapses, the vitreous humour remains opaque, and the fundus oculi is sometimes wholly invisible. — Occasionally, after repeated effusion of blood in the vitreous humour, local detachment of the retina occurs, in some cases certainly in consequence of blood accumulated between the retina and the chorioidea. Partial absorption is here also to be expected, but the detached portion of the retina never again resumes its functions. The prognosis in detachment of the retina by a serous fluid, such as often occurs in high degrees of myopia, is somewhat less unfavourable. Irritation of the chorioidea and diminished connexion by displace- ment of the retina over the disproportionately extending atrophy of the chorioidea, must tend to promote this. In very rare cases ab- sorption may occur, which many endeavour to promote by means of all kinds of remedies (mercurials, preparations of iodine, derivants, sudorifics), very problematical in their action ; but in general im- provement of the sight depends upon the fluid sinking to beneath the seat of direct vision, or upon a diminished morbid condition of the parts of the retina bordering upon the detachment. Eupture of the retina* is so far advantageous, as the danger of further detach- ment appears thereby to be lessened. This fact it was which chiefly suggested the idea of dividing the detached part by incision. Sichel had already at an earlier period advised the discharge externally of the effused fluid, by puncturing the sclerotic in the seat of the detachment. This is attended with no difficulty whatever ; but it does not appear that any advantage is obtained by it. The incision of the detached part was performed chiefly by AdoM Weber and by von Graefe,t with a two- * Liebreioh gives a good representation of it, Atlas d' Opkthalmoscome, Tab. vii. Fig. 1. t Compare Archivf. Ophthalmologie, B. is. p. 85. ILLUSTRATIVE CASES. 433 edged needle^ carried from the inside through the vitreous humour. In this manner a communication was established between the fluid accumulated behind the retina and the vitreous humour, with which it mingled ; and the difference of pressure, which plays a part in the origin and further development of the affection, was thus removed. No injurious effect resulted from the operation ; in some cases, at least at first, some improvement was observed ; but experience has as yet by no means decided, whether, and in what cases, permanent benefit is to be obtained by this method : hitherto it has been em- ployed almost exclusively in old and nearly hopeless cases. It has, in- deed, appeared, that in staphyloma posticum the danger of extension of incipient detachment, and consequently of increasing destruction of sight, is greatest. This therefore justifies the practitioner in more boldly attacking precisely these cases in the commencement. — In all cases of recent detachment of the retina, in addition to the ordinary hygiene of the eyej jolting, vibration, &c. (in carriages and on railways), as well as violent exertion in fatiguing work, are to be strictly forbidden. For the sate of illustration I will, in conclusion, endeavour to sketch, in a few lines, the most frequently-occurring types of myopia. Slight degrees of myopia escape the observation of the patient himself. I. Mr. S. brings me Ms son, a boy of 15 years, who has been rejected for nearsightedness at the Military Academy. " The lad is not nearsighted, he reads the smallest writing still farther than he can reach." At twenty feet he sees only No. 60, with — ^ No. 20, with—— no better, with — _ and particularly with — — less acutely. He has therefore M= — , and the rejec- tion is legitimate. Ophthalmosoopioally scarcely a trace of atrophy is found, but the papillae are red. He has latterly worked very hard, in the evening, too, with moderate light, in order to prepare himself for his examination. In doing so he found no inconvenience. He had indeed last year observed, that he could not recognise people so far off as before, but he did not attribute this to nearsightedness, because he could see the finest object sharply at the distance of two feet. My opinion is : that the myopia will increase a little. To prevent it in- creasing much, let him work by good light (by preference by daylight), holding up his head, at the distance of from 14" to 16", on an inclined plane, with intervals of some minutes every half-hour. I add, " Be of good 28 434 MYOPIA. cheer, my young friend, you can be whatever you wish, except a soldier ;* towards your eighteenth year (with M about = j^.), you will get spectacles to wear and to work with, and you will put these off for work about the age at which others begin to wear glasses." In higher degrees young people must forthwith accustom them- selves to the use of spectacles. II. H. comes in his eighteenth year to the University. He is nearsighted, and would now, if allowed, gladly wear spectacles, in order to see better 1 22 at a distance. On examination, I find M = vrr, S = ht,) a slender cres- cent at the optic nerves, good range of accommodation, and also in other respects, sound eyes. With — =^ he sees acutely at 4", and reads, under my inspection, for a quarter of an hour without trouble. In drder, however, with certainty to avoid asthenopia, when reading for a longer time, he gets for the present only — — , with which he is quite content, and a year later, — =^ • After three years he thought he did not see at a distance so acutely with these glasses. I now found M. = j^, and did not hesitate to give him glasses of — :rjr. Under no circumstances does he, in using these glasses, experience any inconvenience to his eyes, and I venture to pro- gnosticate that his myopia will not increase much more. With diminished range of accommodation the myopia can no longer be neutralised. III. Dr. L., aged 37, with myopia = -g- and otherwise sound eyes, has for the last twelve years, now and then, worn spectacles, which corrected about from J to f of his myopia, but he has always read and written with- out glasses. The degree of the myopia has at this time somewhat, but cer- tainly not much increased. Not long ago he consulted an optician, who gave him — "g *° wear, with the advice to continue to work withoiit spectacles. Dr. L. immediately found that these glasses were disagreeable to him ; walking and looking at a distance, he found all very well, but at the distance of two feet he could not see acutely without an effort, which inconvenienced him at table, in speaking to people whom he wished to look at, and under many other circumstances. He asked me what he was to do. My answer was : " If you had commenced in your twenty-fifth year with neu- tralising spectacles, or if you had, at least, used your weaker glasses also at your work, you would now find no trouble with neutralising spectacles with * In lingland, M does not disqualify au officer. ILLUSTRATIVE CASES. 435 convergence at two feet, and even at one foot, in accommodating for that dis- tance. Tou are now obliged to keep to weaker glasses, for example, — ys with which you still see perfectly sharply at three feet, and also suffi- ciently well at a great distance. Tou are tall, and are therefore in- clined to stoop forward, and it is consequently desirable that at work you should accustom yourself to spectacles, which, however, must certainly for the present not be stronger than — on' ^7 degrees their strength may be somewhat increasedj and perhaps you may succeed, within a couple of years, in accustoming yourself to — ^oi without their use being attended with symptoms of asthenopia, in which case you may, without any injury, easily keep the same glasses of — ^-^j, under all circumstances, for half a score of years,'' When symptoms of irritation occur, myopic eyes have need of rest. IV. Miss V. D., aged 18, is much addicted to reading, and not less so to working, and has in both respects fully indulged her inclination, even when her eyes were tender and rather intolerant of light. She now, parti- culai'ly in the evening, on exertion, quickly gets pain in the eyes, and she 1 18 has applied for advice. I establish the existence of M = ^, of S = — , of capillary hyperemia of the optic nerves, of a small, doubly-contoured, half- atrophic crescent, with very red edges, and, externally, of some injection of the subconjunctival vessels ; the lateral movements of the eyes, as well as the convergence, are ample and free. Her mother has M = _. The young lady remembers to have formerly had much more acute vision for distant objects, and particularly during the last two years to have lost ground in this respect. — She must for the present neither read nor work ; she must avoid strong light ; she must get spectacles with round, blue glasses, and must, three times a-day, for five minutes on each occasion, apply a douche upon the closed eyelids ; she must sit up straight, take care to keep her feet warm, go to bed early, and avoid all stimulants. A month later the sym- ptoms of irritation seem to have yielded. Upon her own authority she lays aside the spectacles and again begins to work, with the result, that in a couple of days the pain reappears. After a fresh period of rest she gets leave, on condition of steadily observing the other direetions, and of using glasses of — — , with which she sees acutely at 1 8", to play the piano for two hours dail}', the music being placed at precisely 18", — but on the further condition that every quarter of an hour she rests for some minutes. After the lapse of a month, she may gradually begin to read a little. Fine work is per- manently forbidden, and writing is very much restricted, the more so be- 2 436 MYOPIA. cause she will not use spectacles, and therefore must always strongly con- verge, and be inclined to stoop forward. She was strictly enjoined to hold every object at the greatest distance of distinct vision. — A year subse- quently she again presents herself. The myopia is increased to -x > ^"w and then some sensibility had stimulated her to a stricter observance of the directions, and more severe symptoms were thus prevented. — Moderate reading, with the book in the hand, and piano-playing, with the use of the spectacles, are the only indulgences at present permitted ; for distant vision a glass of — -is given her. — With great care, the inconvenience wiU gra- dually lessen, but it is to be expected that at fifty years of age the myopia will have increased to -, and that at a more advanced period the acuteness of vision will have more than ordinarily diminished, while her eyes will be liable also to other accidents. Spasm of accommodation may supervene upon the symptoms of irritation. V. T. W., aged 17, was nearsighted from childhood, and has for some time been unable to continue working, in consequence of rapidly-increasing pain in the eyes, which are also permanently sensitive. I found myopia of „-^' somewhat stronger still in the left eye ; moreover, capillary hyperoemia of the optic disc ; at its outside a perfectly atrophic, white, sharply- defined crescent, 0.8 mm. broad in the transverse direction (the axis), straightened retinal vessels, no trace of chorioiditis, rather small pupil, easy convergence of the visual lines up to 2J" (he then also read binocu- larly), scarcely any subconjunctival injection, no prominence of the eyes, eyelids healthy, but a tendency to approximate them, not merely in order to see better, but also on account of some intolerance of light. He showed me spectacles of — —, which he had worn, and had also used at work, but by the advice of others, he had afterwards laid them aside, and he had since 13 not got on better. The acuteness of vision amounted to only -=-• This latter circumstance chiefly induced me to prescribe a Heurteloupian abstrac- tion of blood * from both temples, and to make him remain for twenty-four hours afterwards in the dark. After gradual transition to the light, the degree of myopia was again, with the aid of glasses, determined in the most accurate manner, and was found = 5^; at the same time the acuteness of • Heurteloupian abstraction of blood is accomplished by means of an arti- ficial leech making a circular wound, and yielding much blood on exhaus- tion by means of a glass cylinder. (The instrument is to be had at Luer's, Paris. Its price is fifty francs.) ILLUSTRATIVE CASES. 437 17 20' 17 vision had increased to g-. Instillation of sulphate of atropia now brought the myopia to — . The patient was still kept some days in a half-darkened apartment ; subsequently the double abstraction of blood was again had recourse to, and the hygienic rules which have already been repeatedly mentioned, were strictly enj oined. Thus at the end of a month the acute- 18 1 ness of vision had risen to --;, and the myopia was established as — . He 20' •' '^ 3-6 now got spectacles of — -, both to wear and also in order occasionally to read and write for a quarter of an hour, which eould thus be done at the distance of about 10", and therefore must be done. From time to time slight attacks of pain still occurred, which disappeared on strict rest. The spectacles were subsequently strengthened to — — . These glasses, held close to the eye, brought the farthest point to 14", which was quite sufficient. — The strong convergence and bad position, not completely corrected by the glasses of • — j- previously used, had, with original highly myopic structure, been followed by congestion of the optic nerve, with diminished aouteness of vision, spasm of accommodation and other symptoms. They yielded to the rather energetic treatment, and by avoiding strong convergence, &o., with the use of suitable spectacles, they did not return with equal violence. It is, however, to be anticipated that the myopia will continue permanently progressive, and that at a more advanced age, the acuteness of vision will suffer. The eyes must therefore always be watched, and a position in life is to bo preferred which will not require much close work. Even in youth a very high degree of myopia may produce pro- found disturbance. VI. Miss 8., a little deformed personage, aged 27, has prominent eyes, with considerable enlargement of all axes (buphthalmio form), appa- rently small cornea, somewhat bluish sclerotic, with widely-open eye- lids, with myopia = r^ ; and, on correction of the myopia, acuteness of 9 vision at a distance of only j^. Even in her twelfth year the nearsighted- ness was inconvenient, and it constantly increased until now she cannot attempt writing, and can read only comparatively large print, held close to the eye, of course only with one eye, without convergence ; she complains also of motes floating before her eyes, and of flashes of light, particularly in the dark. Ophthalmoscopically we find an apparently small optic disc, of a white colour, with irregularly bound edhalf-elliptioal atrophj-, straightened retiiial vessels, a light-red fundus, here and there some- what spotted with yellow and grey, with separate chorioideal vessels. The employment of Heurteloup's abstractions of blood and strict hygiene of the 438 MYOPIA. eyes produced scarcely any improvement. The patient does not wish to wear spectacles, which, in comparison to the diminution, add but little distinctness to the objects ; she also iinds it equally fatiguing when she endeavours to read large print with spectacles, and she finally consents to confine herself to coarse work, which she can perform by feeling. Some words she reads without apeotaoles, with one eye. For distant objects she finds a stenopaeie eyeglass of — ^ the most satisfactory. In this unfortunate condition nothing but impairment is to be expected, the progress of which may, by the known rules of ocular hygiene, be indeed retarded, but not prevented. If the symptoms of irritation increase, Heurteloup's method of abstracting blood may still occasionally be useful. The occurrence of fatigue during work indicates the existence of insuf&ciency of the musculi recti interni. VII. Miss V. R., aged 18, has for some years been constantly obliged to suspend her work, in consequence of a feeling of fatigue and tension. She has in vain tried various spectacles. I establish M = -, S = 1, with the ordinary crescentic atrophy ; the eyes are otherwise sound and the eyelids healthy. I suspect insufB.cienoy of the recti interni. While she reads at 6" with both eyes, I bring a small screen before the left, and observe it deviate outwards behind the screen. On removing the screen the eye again converges sufficiently. Having made some inquiries, I find that, after having read for half-an-hour, the left eye is always drawn outwards, that this gives her an unpleasant sensation, and that the eifort to overcome the deviation fatigues her still more. Thus, she had herself already remarked the insufficiency, without having been able clearly to account for it, and with- out having spoken of it. On approximating an object, she converges up to 4", after which the left eye rapidly and strongly deviates. However, on lateral movements the excursions appear normal ; towards distant objects the eyes are also properly directed, and they maintain this direction when one eye is covered ; a prism, too, placed before one of the eyes, with the angle upwards, gives double images of a remote light, without lateral deviation, one of the images being almost directly above the other ; while a prism of 6°, with the angle outwards, gives intersected double images, which do not dis- appear. Thus it is evident that our patient can diverge little more than normally, and yet that she has comparatively much too great difficulty in converging. With glasses of from — To *" — q ^^® ''^^t looking at from 13' to 16", maintain the convergence much longer, but yet not sufficiently : the covered eye now always deviates also, still directly outwards. By combining — — with a prism of 8° the difficulties appear to be removed, and the now covered eye actually scarcely any longer alters its direction. Besides such glasses, she receives a double eye-glass with simple glasses of ILLUSTRATIVE CASES. 439 — -=. With this she is exceedingly pleased, but the prismatic combina- tion wearies her. She would willingly undergo tenotomy, which was spoken of, but as, undoubtedly, even in looking straight, and particularly in looking to the left, double images at a distance would remain, I do not think the operation indicated. Finally, I give her glasses of — => with the axes closer to one another than the axes of the two eyes ; and if she has to work only one hour, she gives the preference to these, above the pris- matic combination. She has to be particularly careful not to use these spectacles in looking at remote objects : on account of the position of the glasses she would in doing so be compelled to diverge somewhat, which could not act otherwise than injuriously upon the power of the recti interni. The insufficiency may sometimes be completely corrected by tenotomy. Till. Mr. C, a mechanic, with M = -^, has, except the ordinary crescent, sound, sharpseeing eyes. Yet his work fatigues him after a short time, and he thinks it not well to continue using his sight. His looking at a distance did not exhibit the ordinary apparent convergence, but rather gave .the impression of divergence. This immediately suggested to me the idea of insufficiency of the recti interni. On examination, it appeared that in unconscious distant vision he had the tendency to divergence, which was absent when he saw acutely through glasses of — -=-; that he, however, aided with 5-, for the sake of single vision, could overcome a prism of o 12° with the angle outwards, and consequently could considerably diverge. When, in reading at a distance of 8", a screen was brought before the one eye, this evidently deviated outwards, to turn again inwards on the removal of the screen, and it appeared that this movement was scarcely greater than that which was observed after the removal of the prism of 12°, in looking at a distance. Consequently the insufficiency with a certain convergence, made itself not much more strongly felt than in distant vision. Tenotomy was therefore indicated. It was immediately performed, first, on the left eye, the effect of which was still unsatisfactory ; then also on the right eye, when it was at first too great, so that remote objects appeared single only in the direction to the right of the middle line ; but forthwith convergence to 8" took place, with the greatest ease, and after some weeks the double images ceased also at a distance. The success was complete. Fatigue was no longer mentioned, particularly not after our patient, by my advice, had begun to wear glasses of — ^^, and to use them at his work. 440 MYOPIA. Even in elderly people myopic amblyopia may, in a great measure, yield to suitable treatment. IX. Mr. M., a banker, aged 63, has spent his life writing, " has,'' as he says, " carefully refrained from the use of spectacles, and has, nevertheless, by de- grees nearly lost his sight." For a long time he had still been able to read with the left eye, but some time ago this became worse than the right. I establish in the left eye, M = y with S = j^; in the right, M = ^, with S = ^ The latter eye had also so much opacity of the lens, that without it the aouteness of vision would still have been pretty good; in the left eye, on the contrary, rather well-marked diffuse atrophy, with an atrophic belt through the region of the yellow spot, was to be seen. I stated my regret, that both defects did not exist in the same eye ; hut, at the same time, my hope that in the left eye, in which within some weeks the sight had so very much diminished (without effusion of blood, without detachment of the retina, and without local scotoma of the yellow spot), improvement was still to be obtained. I gave a stimulating liniment, to be applied above the eyes, pediluvia with mineral acids, rhamnus to keep the bowels open, ordered rest of the eyes, avoidance of strong light, prescribed grey glasses, and had a Heurteloup's leech applied to the left temple every week. At the end of three weeks the acuteness of vision had risen again to -7-, and it subse- quently attained, under a continuance of the treatment, with abstraction of blood every fortnight, nearly to -„-, so that he could again very well read ordinary print. He was forbidden to write. He read without spectacles, with the book in the hand, at the distance of 5", while the right eye deviated outwards. For distance he got a double eyeglass, with a glass of -, to 5 be used sparingly. He had lived so far without using spectacles, and I found no indication to give them to him in his sixty-third year ; otherwise he could, without inconvenience, have worn glasses of =- ; but with the diminished acuteness of vision he ought not to read except without spec- tacles, and fortunately convergence did not then take place. Scotomata in the yellow spot make reading and writing impossible, but do not threaten blindness. X. The Consul M., aged 47, presents in his whole bearing the characters of a highly myopic person : stooping neck, closely-knit eyelids, large eyes, of the long axes of which we can easily satisfy ourselves. His myopia has existed from youth, and is of hereditary origin. He has always read and written much, usually with comparatively weak spectacles. In the right eye the myopia ILLUSTRATIVE CASES. 441 I 3 now amounta to — 5-5 with -^ aeuteness of vision ; in the left the myopia is O'O o not less ; hut it cannot be accurately determined on account of a scotoma in the yellow spot. In the space of two months this had developed itself, so that he could not see what he fixed. He was in a sad state of mind, and put the question to me, " Must I become blind ? " Ophthalmoscopically I found in the left eye a tolerably extensive circumscribed crescent, here and there traces of diffuse atrophy, and in the yellow spot a pale granular fold, furnished with, and partly bounded by, larger, irregular, sharply-defined, dark pig- ment-spots. In the right eye, too, the crescent was about equal to that in the left ; and in the yellow spot the pigment was slightly granular, and was irre- gularly distributed upon a paler ground. I could now answer him : " Do not fear blindness, but abstain from all exertion of the eyes. Confine your reading and writing to what is absolutely necessary, and let them be continued only for a few minutes occasionally. I cannot conceal from you that the left eye is for ever useless for fine work, the right eye would become so by ex- ertion." I am accustomed unreservedly to state the danger of the patient's state, only when he has to act accordingly. This was the case in this instance. But nevertheless,. I did not attain my object, " his circum- stances did not permit it." There was no reason to add that, in my opin- ion, even if he left off all work, the further development of the scotoma in the right eye could not be permanently arrested. Dissatisfied with my opinion, the patient repaired to a foreign oculist, who promised him, and also made him believe, that he would cure even the left eye. A full year the patient spent with him, always in hope and confidence. But when the second eye commenced to refuse its service for reading, his confidence began to waver. Two years later he paid me a second visit. On the right eye, also, a scotoma was now so far developed, that he could scarcely decipher a single word. He appeared dissatisfied with himself for not having followed my advice. I gave him the ordinary hygienic directions, and I was able to comfort him with the hope that his condition would not become much worse. It is, in fact my opinion, that he will permanently continue in a state to move about freely and to distinguish large objects . He receives spectacles of — ^■ In loosening of the retina the prospect is very gloomy. XI. Mr. S., aged 43, was, as well as his father, extremely nearsighted from youth. In his thirty-second year he lost in a great measure the power of vision in his right eye, in which cataract became developed. Extraction was followed by atrophy of the eye. Sight now suddenly became obscure in the left eye also. The following day he called upon me. I ascertained the existence of detachment of the inner part of the retina, extending into the neighbourhood of the yellow spot. Large letters he could still read, but at one moment much better than at another. I perceived that loosening of the retina must, at an earlier period, have existed in the right eye also, followed by 442 MYOPIA. secondary cataract, upon which, misled by the remaining perception of light, his medical advisers had operated — with what result, has already been seen. Blood was immediately abstracted by Heurteloup's method, rest in the hori- zontal position was enjoined, and derivation by the intestinal canal and lower extremities was prescribed. The abstraction of blood was again twice repeated, and by these means the patient was able, after the lapse of a week, to read satisfactorily ; the limitation, however, still continued. By degrees he resumed his place in society ; blood is still from time to time abstracted ; he leads a very regular life, avoids fatigue, over exertion, riding in jolting carriages, and travels, if necessary, in the canal-boat ; but he continues to read his price-current and other more necessary matters, and prefers not to go out with grey glasses, " because with them he cannot see by the way." There really exists some torpor and more limitation in weak light. At the end of six months the detachment has, by gravitation, removed farther downwards ; and at the same time the limitation has altered its place ; it still, however, at the inside directly adjoins the fovea centralis. — After the lapse of a year reading is more difficult, and is but little improved by the employment of artificial abstraction of blood. Now, nearly two years after the occurrence of detachment of the retina, distinct traces of. cataract exist ; motes in the vitreous humour had already been seen at an earlier period, without lacera- tion of the retina having been perceptible. The progress of the cataract is certainly not to be prevented. When it is fully developed our duty wiU be to try extraction (by preference with Waldau's spoon), at least if it does not appear that the detachment has extended much farther. We must, how- ever, at the same time, give an unfavourable prognosis. We can scarcely hope that absolute blindness can be permanently averted. A defined detachment of the retina may for a long time remain stationary. XII. Mr. V. d. W., aged 23, has M = i in both eyes, with small, sharply- defined, completely atrophic crescent. For years work has been difficult to him, and he did not like his spectacles. Some months ago his right eye became much worse, and he can scarcely read with it. I here find a red optic disc and a small, roundish, circumscribed, plaited and movable projection of the retina of a bluish colour, with a corresponding limitation. By arti- ficial leeching to the temple, and strict ocular hygiene, the acuteness of vision increases within a month from = to ^. In the other eye it amounts 2 to rather more than q. I earnestly represent to him that it is only by a regular life and a sparing use of his eyes, that he can retain his sight. I permit him at most to read four times a-day for half-an-hour each time, which he does, binocularly, with glasses of — — . The same glasses he may wear permanently, superadding for a moment for distant vision a double eye- ILLUSTRATIVE CASES. 443 glass with glasses of — =^. At the end of two years he returns : his condi- tion is not altered. Shortly afterwards the question is put to me by parents whose daughter he had asked in marriage, whether there is danger of blind- ness. My answer, which I stated that I would give only with the patient's consent, amounted to this : " Mr. v. d. W. is rather highly myopic. In con- nection with this nearsightedness a defect has arisen in the right eye, which, if further developed, might destroy the power of vision. It has, however, been stationary for two years, and we may hope that it will very long, per- haps to old age, remain stationary. The left eye presents no defect except the nearsightedness ; but experience shows that with detachment of the retina in one eye, the other, when it is equallj^ nearsighted, is not unfre- quently attacked by the same defect. Considering the existing degree of the nearsightedness, and its stationary character, it is, however, not probable that, if the directions given be faithfully followed, the left eye will in this case become affected. " — Six months later Mr. v. d. W. presented to me his young wife. I once more impressed upon him how necessary it was that he should in every respect take care of himself. Bather more than seven years have elapsed since that time, and his state is in general statio9ary ; the myopia, too, having but very slightly increased. He who, with moderate myopia, treats his eyes fairly, has nothing to fear at a more advanced time of Hfe. XIII. Mr. M., aged 52, who reads and writes much, has for many years used in doing so spectacles of — ^, with which his myopia is exactly neutralised. For distant objects his acuteness of vision is at the same time = 1. Hitherto he has also seen every near object he required with his spectacles ; it was only when he wanted to distinguish anything when it was half-dark, or for which others would need a magnifying glass, that he put off his spectacles for a few minutes. Latterly, he observes that he in- voluntarily raises his glasses tojook sharply at smaller objects when near. " Do you still work in the evening without trouble with your spectacles?" As I expected, his answer was, " Only in good light, and when the print is not too small." " Then the time is approaching," I replied, "for diminishing the concavity of your glasses in proportion to the degree of the convex glasses which, at your age, you would require, if you were not myopic. Take, therefore, in the evening — yo> ^""i ^"""^ glasses somewhat weaker, and you will be content perhaps also to wear the same glasses habitually, though you will not see distant objects quite so acutely as with — 5. When you are an old man, you will use two spectacles, — ^, to work with, and — -- to wear habitually, and, finally, you wiU read without spectacles, and you will probably, when the acuteness of vision- diminishes, come even to convex 44 MYOPIA. glasses : your myopia will, in fact, diminish somewhat, perhaps to — . Then may you, as a very old, but strong and healthy man, feel yourself still quite satisfied in wearing spectacles with glasses of — th <"" — ys- NOTE TO CHAPTER VII. We are indebted to Kepler for the earliest knowledge of the nature o myopia. He laid the foundations of dioptrics in general, and in particular of physiological dioptrics. The influence which spectacle-glasses have exer- cised in this direction is extremely remarkable. We have already alluded to these glasses as among the most indispensable instruments for man (p. 170). Their great importance in the history of science was demonstrated in the observation that they had led to the invention both of the microscope and of the telescope. After perusing the works of Kepler, I go still further, and think I may maintain that the development of physiological dioptrics has proceeded from spectacle-glasses. Alhazen showed about 1100 {Opticts thesaurus : Basilese, 1572), that the eye is not the source of light, but that the light proceeds from visible objects and enters the eye. With respect to the formation of pictures in the eye, he conceived wholly in- correct ideas. It is still stranger that we must say the same of Johannes Baptista Porta, who, although he compared the eye with the camera obscura invented by him, was of opinion that the pictures were formed on the an- terior surface of the lens (see his Magia Naturalis, 1558 (?), and his work, de Refractione, optices parte, libri novem. Neap. 1593). His contemporary, Maurolyous {PJiotismi de lumine et umbra, Ven. 1575) arrived at more cor- rect conclusions. This writer comprehends that the crystalline lens is to be compared with an ordinary convex lens, looks upon it as more convex in myopes, flatter in presbyopes, explains the action of convex and concave glasses, but nevertheless gives, in order ^o avoid the inversion of the images upon the retina, a totally confused idea of the action of the lens, and makes the images fall on the plane of the optic nerve, without men- tioning the retina. It was in 1601, Kepler himself informs us, that D. Ludovicus L. B. a Dietrichstein, put the question to him, why farsighted people distinguished near objects better through convex glasses, while nearsighted people saw distant objects more distinctly with the aid of concave glasses. Kepler was not acquainted with the work of Maurolycus. The only answer which he was at first able to give was, that convex glasses magnified near objects. But von Dietrichstein, not content with that, rejoined, that the question did not relate to magnitude, but to distinctness : for that concave glasses, which make objects smaller for all eyes, could otherwise assist no one. When, after three years' study, Kepler is at length in a posi- tion to give an answer : " responsum," he says, " si non satis olarum et HISTORICAL REMARKS. 445 indubium, satis oerte tardum," his thankful tone testifies of the stimulus which he had received from the question of the man whom he calls, " Msecenatum meorum prsecipuus." That answer we find in his Paralipo- mena ad VitelUonem quibus Astronomi• ui to Oi Oi en 1 S- -^ / ■-^-~. — -- ,^ . \ — v.\ / a ■o'S Is il 8 S CO 1 1 (N (M *0 1 O C> Cr O O lO lO o OO t^ t— lO rfi. rt.. — \ •^ -^ \ ^ •^ / ^ — .^\ Fh i E 1 1 g CO r-1 00 CO t^ 00 3 CO Tf* CO 00 00 eo OS r^ OS CO t-l ^ CO CO .-J (N 0» 00 oj t^ t-l i— CD OS T— t ■**< ■^; f-J P rH p CO CO QO CO CO CO •■^ rH OS 00 t-l «3 00 c« Xr^ CO CO o 00 OO CD OS O t-^ 00 «3 CO 00 00 t^ t-l S 3 §8 CO 00 i>. t-l g 3 § § S; CO GO 00 CO t— CO CO O OS CO t^ CM 1 00 t-l C>1 O 1-- i-H CD rt O O <-t OS CO 00 CO CO t-l g§ od 00 o CO Oi 00 o !>. 00 3 00 t-l g 3 t-l 00 r-H CO ■^ P t-H 00 00 OO t-l t-l CO i-H OS i-H ^ O OS OS CO* CO l-l t-l. S§3 00 t-1^ 1 s § t-l 00 o 00 03 OO t-l t-^ CO o 1>1 CO t^ ^ CO O 1-1 t-. 00 00 t>l IN CD I>1 3 S S g od b^ tol t-^ t-l t-l OS J>1 CO CO CO 00 r- oi O 00 t-l t-l GO I>1 (M 00 ^ O .-' OO 00 CO I>1 ^ t-l CO •* CO OS -* p CO t^ 00 OS 00 r-1 t-l 1-1. t-l s § g 00 I>1 00 CO r-H I>1 00 en o CO t^ OS t-l 00 00 t-^ Oi -^ Oi ta O QO OS OO CO »>1 t^ t>. CD O 00 i-j J>1 CO 1 ?3 00 CO t-l I— 1 r-H O OS 00 t^ s s? s CO 00 t^ t-l 8 §8 g S - CO I>1 00 t^ l-l 00 CD CD CO a> CO r— ( CO t-y t-l 1— 1 OS CO t-l s g g CO 00 I>1 CO t-l (M 00 t>- O 1-H QO t— 00 00 t^ t-i rC 00 o 1>1. 00 g CO OS CD 00 00 t-l o 00 OS CO CM CO CO «3 00 CO t-l t>- I-H OS GO OO OS 00 t-l t-l l-l J>1 00 OS t^ I>1 8= T— ( CD (7i[ Ui o t-l CO 55 00 SS3 CO t-l 00 ?? S §g 00 00 t< OS I>1 ^ § g §g g 00 00 t^ t-l. t-l O 00 OS OS. t-l I>1 & ■^ CS) 00 OS > 6 6 . Pl< CM "^ d A f^ ^ te CO aj l' H- r \ Fig. 161: y»> r U"' ^ image is then == 0-3808, cortesponding in this meridian to a visual angle of l°25'-7. Finally, when the retina lies at 22-018 mm. behind the apex of the cornea, the linear diffusion-images in v and H are equal, amounting to 0'18222 mm. In this position, therefore, the retina receives that part of the focal interval where the difiusion-image is nearly a circle. The diameter thereof exhibits itself inthe horizontal meridian under an angle of 41'-8, in the vertical under an angle of 43'-4. If we bear in mind that in perfect acuteness of vision 4(0 ASTIGMATISM. letters are recognised under an angle of 5', we can form an idea to what an extent the circles of difiiision mentioned must interfere with correct sight. We shall obtain a still better point of comparison by calculating the ac- commodation required to make 0" of H fall where, in the eye at rest, (p is found in V i in other words, to see with acuteness alternately vertical and horizontal lines. Let Us suppose the eye emmetropic in V (the retina lying at 21-2623 mm. behind the apex of the cornea) and consequently hypermetropic in 5, and let us find that, to neutralise this degree of hypermetropia, a lens (standing in the air, its nodal point coinciding with S' of h) of 176'8 mm. = 6-63" focal distance is required, consequently that there is hypermetropia of -^7^ . capable of being coimteracted by an accommodation of =7^5- But now the same accommodation necessarily takes place in v, whereby for this plane F' becomes = 12-857 mm. and F" ;= IY'183 mm., and horizontal lines appear distinct at the distance of 176-8 mm. = 6-53". While, therefore, by accommodation of - — - , H became emmetropic, v assumed a myopia of 5-55. 6-53 6-53 Hence it may be deduced, that in an accommodation of about — the re- tina corresponds to the middle of the focal interval, the diffiision-image being a circle of difFusion of about the same magnitude as a symmetrical eye, accommodated for 00, perceives of an object placed 13' from the eye, or, when accommodated for the distance of 13', sees of infinitely remote objects. By looking at distant objects, while the eye is by glasses rendered _ myopic or hypermetropic (and in the latter case refraining from tension LO of accommodation), we can observe in ourselves the disturbance proceeding from the circles of difiiision already mentioned. Obtained in this manner, it appears, however, to be somewhat greater than in astigmatics, which is partly to be ascribed to this, that the latter by some play of accommodation can make the diflusion-images alter their form and combine the vertical and horizontal lines thus alternately more distinctly perceived. Moreover, the light is less imiformly distributed in the difiusion-images of astigmatic individuals, and, in fact, more advantageously in the posterior part of the focal interval. In general, too, the difiusion-images, on account of their dis- continuity (the result of the irregular astigmatism of the crystalline lens), may cause less disturbance than should be the case if they were homogeneous. As has been above remarked, the retinal images are, for like dimen- sions of the objects, not of equal magnitude in a horizontal and vertical direction. If the projection corresponds in all meridians to the magnitudes of the retinal-images (which is doubtful), like dimensions of the objects do not appear equally large in the opposite meridians. The magnitude of the retinal images is, now that we have become acquainted -vpith the posi- tion of the cardinal points, easily compared. As the distance of the objects is very considerable, relatively to the reciprocal distances of the nodal points and to the distance A' h „ V, M = \i then there exists simple myopic astigmatism A 111 Am = -^ = -. 6 CO 6 b. Compmnd myopic astigmatism, or myopia with astigmatism, M + Am, M existing in both principal meridians. Thus let there be : In the principal meridian H, M = ^, » >i ,i T, M = ^i we then have M = ^r^, 20* And moreover, Am = t\, — ^ ii ^, to be written as : II. — Hypermetropic astigmatism, Kkewise to be distinguished as : a. Simple Ah, with H in the one^ E in the other principal meridian. In V let there be E. In H let there be H = 5> then there exists simple hypermetropic astigmatism Ah = ^ — - == -. 8 CO 8 b. Compoimd, being H with astigmatism, H + Ah, H existing in the two principal meridians* III H let H = J. In V let H = T^. We thus find H = ^. and moreover. Ah = j ^ ^ = j, and therefore write, H^l+Ahl. III. — Mixed astigmatism, with M in the one, H in the other meri- dian. Of this we may distinguish : a. Mixed astigmatism, with predommant myopia, Amh. DIFFERENT FORMS OF THE DEFECT. 483 In V let M = iv In H let H = Thus we find. ^-^ = ^^+^M = -8- b. Mixed astigmatism, witA predommant hypermetropia, Ahm. InvletM=^; In H let H = ^. Therefore, The above is in general suiBcient for the diagnosis and deter- mination of the degree of astigmatism. The method recommends itself by its simplicity and facility of application. In general it deserves to be preferred to any of the following modes. Only the control described under 8 must not be omitted. This is, properly speaking, nothing more than trying whether the glasses employed in the investigation, described under 4, are really suitable. If the control proves accurate, the investigation in the condition of artifi- cial paralysis may, even where hypermetropia exists, for the most part be omitted. The methods still to be described, come under consideration in particular cases. They cannot be passed over in silence, least of all that of Stokes, which, for its ingenuity, deserves to be knoven, and also sometimes yields good service. Employed as a control, it cer- tainly affords the most accurate indication. 6. Modified method of Young. — -Young determined the distance at which the double images of the wire of his optometer, in accom- modation for the farthest point, held alternately vertically and hori- zontally, appeared to intersect. The method may be applied in myopic individuals, but gives too high a result (conf. p. 451). Moreover, the directions of the principal meridians must first be found, according to the method described under 3, in order to admit of the determination of the inclinations of the optometer, at which the observation is to be made. 7. Method of Airy. — This is applicable only where a tolerably high degree of myopia exists in the two principal meridians, which was the case with Airy. As point of light a small opening in an opaque disc is employed, turned towards the Kght of the sky, to- wards a dull glass or the globe of a lamp, and this is moved along a 484 ASTIGMATISM. graduated scalej for eXample> that of the optometer. We then find a greatest distancej at which the point of light appears as the most slender line, and a shortest distance at which it again becomes a thin line, perpendicular to the fij^st. The distances then give about the degrees of myopia in the principal ineridiansi If it be desired to apply this and the preceding methods to non- myopic subjects, the eye must be rendered myopic by a suitable con- vex glass. In this, however, the difficulty presents itself, that, if the axis of the lens does not accurately coincide with the visual axis, the astigmatism undfergoes a rnodificatioiii Moreover, in both cases the accommodation mtiSt remain at rest. This Caiij however, scarcely ever be accomplished, and therefore in the majority of instances, this method leads to incorrect results. 81 Modified method of Airy; — In order to meet the last difficulty, the accommodation may be paralysed by means of a mydriatic. In strong myopia Airy's method then affords tolerably fair results. But if noj or if only slight myopia^ exists, a remote point of Hght deservfes the preference^ Thereby we avoid the trouble connected with the uSe of strong lenses. To obtain a more accurate result, I made use df a very small point of light; produced by the reflexion of an illuminated little round opening upon a convex mirror. In some eases it was then satisfactorily ascertained, with what spherical glasses the point of light appeaf ed as' the slenderest streak} successively in two opposite directions'. In the nlajofity of instances, on the contrary^ this femainfed undeCJided-; The csiuse of this hes in the irregular astigmatism, which excludes defined lines as diffusion- imagesi Usually, sefcondary lines rapidly shot Out, even before the principal line had beCome slender, in different directions, preventing an accurate determination of the glass required. Only ill absence of the crystalline lensj whereby the irregular astighiatism was removed, did the results attain perfect accuracy. Instead of a very small reflected point df light, w^ niay employ an opening of from 1 to 2 mm. in diameter, such as is to be obtained by means of the boardj described at page 45. Cases of aphakia ex- cepted, the results thus obtained are not inferior to those where the reflected point of light is Used; 9. Investigation with cylindridal lenses. — WhUe at a distance letters without or with the best chosen spherical glass^ dre being seen as distinctly as possible, we take a positive cylindrical glaSs Of about OT, and turn it round before the eye. If astigmatism exists, it is METHOD OF STOKES. 485 observed that, in a definite position of the glass (while the curvature of the cylindrical ^lass cqincides with the meridian of st^iongest cur- vature), the acuteness of vision greatly diminishes^ but that in a posi- tion perpendicular theretQ_, it^ on ' the cqntrary^ increases, The acuteness qf vision now often becomes s,till greater on approximating the object : the cyHndrical glass may, in correcting the astigmatism, have rendered the eye myopic, "V^e niay now further try, with what strength of cylindrical glass, always held in the mqst advantageous position^ the greatest acuteness of vision is obtained, 'Vi'hicl^ must always \>e tested by difference of distancp qf the letters, ox by combination with spherical glasses. We then, however, obt^iii at kgt, with the sacrifice of much time, only a moderate result. The metho^, although thu^ in itself objectionable, is. very ■^ell adapted to control the results obtained by that described under 4. The latter shows from what combination of spherical and cylindrical glagses the ^eatest 9.cuteness of vision is to be expected, and we shoiild never neglpct to try tHs, nor pmit a comparison with slight mqdificatipn of the lenses. We shall thus always be able to congratulate our- selves qn a more coraplete improvement of the acuteness of vision, than ^as obtained by the use of the slit, which, if it be too narrqw, takes away much light and is obstructive by diffraction, and if it be too wide, very imperfectly corrects the astigmatispi, 10. Method, of Stohes. — The distinguished Secretary of the Royal Society had yery well seen that 4-i''y's method could lead to satis. factory results onlv when, together with the successive deteripination of the ferthest points of distinct vision in the two principal merir dians, the condition qf accommodation of the pye underwent nq change. By his method this difficulty is removed. IJe proposes to define the degree of astigmatism, by means of a,n astigmatic lens, the action of which can be regulated in a nianner as simple as it is ingenious, so as to make it assume precisely the degree b,y which tbe astigmatism of the eye is corrected. I have had such lenses prepared, and give thq description qf the instrument, with the arrangeipent which appeared tq me mqst advantageous : the principle is precisely that of the astigmatic lens of Stokes, which name may also be given to the instrument, It consists (|'ig. 1.62b, exhibiting a section) of two cylindrical lenses, the one plano-convex I of tV, the other plano-concave I' of — -Aj, The first is fastened into a broad copper ring, K, the last into K', which rings at 9; are fitted to one 486 ASTIGMATISM. Fig. 162, A, Ki I 51 'is: another, and can turn past one another around their axis. At the same time, therefore, the lenses I V also rotate past one another ; they are turned with their flat surfaces towards each other, leaving a very smaU interspace. Kg. 163 A, represents the instrument, seen on the outer surface. It will be observed that on K an index i occurs, on K' a graduated scale. If the index points to 0° or to 180°, the axes of the two cylindrical lenses are parallel ; the section of the lenses appears then as in B, so that when united, they may be regarded as a concavo-convex cylindrical lens, with equal radius of curvature of the two planes, whose action is about = 0. If the index points to 90° or to 370°, the axes of the cylindrical glasses stand perpendicular to one another. At the same time the system has its maximum m of astigmatic action ; a plane of parallel rays of light, coinciding with the axis of I, will undergo no deviation through I, but through U' will be made convergent to its focus, situ- ated at 10"; on the contrary, a plane of parallel rays, coinciding with the axis of V, are made divergent through I, as if they came from a point, situated 10' in front of the lens, and through l! do not devi- ate further from this course. In the one meridian we thus obtain an astigmatism of ^, in the opposite of — ^, and the astigmatism m of rays, refracted in this position of the lenses, therefore amounts to \. It thus appears, that by turning round from 0° to 90° the astigmatism ascends from to | , and by a simple formula. As = m sin a, we can calculate the astigmatism for each angle a, which the axes of the lenses make with one another. For the sake of convenience definite degrees of astigmatism are directly given upon the instrument, ren- dering the calculation unnecessary. 43T1GM4TIC I,ENS OF STOKES. 487 It is easy to see the use which may be made of this instrument. If any one fails to obtain^ with the most satisfactory accommodation or reduction for distance, the normal acuteness of vision, and if we suspect the existence of astigmatism, we set the instrument about at the degree of astigmatism^ which the disturbance of vision leads us to suspect (rather spniewhat too weak than too strong), and cause it, while the eye is steadily fixed upon the distant letters, to turn round befqre the eye. If improvement be npw observed in a particular position, the action of the astigmatic lens can be increased or dimi- nished in the manner above described, until the maximum of distinct- ness is qbtained ; but this change requires again another position. We should not, however^ imagine tliat our pbject has been thus altogether attained. The eye is now seldom properly adjusted for the distance at which the letters are. The astigmatic lens makes the eye in the meridian of maximum of curvature incline as much to hypermetropia, as it dops in the meridian pf minimum of curvature to myopia, and emmetropia (distinct vision at a distance) will there- fore be pbtained only when the eye, without the astigmatic lens, had selected a glass whereby it was reduced jn its two prinpipal meridians to an equal degree of ametropia (either myopia or hypermetropia). Sometimes this is completely accomplished, and the object is then immediately attained. But experience shows that this is the excep- tion. In gener4> in correcting the astigmatism a slight degree of ametropia remains, and this agq,in suggests the question^ whether the astigmatism has been corrected as perfectly as possible, or not. If any myopia remains, this can immediately be demonstrated by approximating the object; and if the latter be now more acutely seen, the action pf the astigmatic lens can be more accurately set and arranged : however, when in this manner, after a long search, an accurate result is obtained, a tolerably detailed calculation is still necessary to deduce,, from the spherical glass used, from the astigmatic action of the lens and from the greatest distance at which, with this system, acute vision is obtained, the anietropia in the two principal meridians — a knowledge which we need. But if hypermetropia re- mains, there is, unless the accommodation provide for it, no distance discoverable, at which sufficiently acute vision is obtained, perfectly to regulate the astigmatic action of the lens, so that in that case the addition of a second spherical (positive) lens is required, to bring acuteness of vision at a distance to its maximum. !Prom all the foregoing it appears, that the method is not very 488 ASTIGMATISM. applicable in practice. It answers best when the eye is by spherical glasses reduced to a certain degree of myopiaj and it is then tried for near objects, with what action of the astigmatic lens the person can best read. In this, however, it is more difficult to take care that the lenses be held perfectly centred before the eye 5 moreover, the opinion as to the acuteness of vision is not quite certain, and at aU events we have learned only the degree of astigmatism, but by no means the refraction in each of the principal meridians.. For all these reasons the method described under 4. deserves the preference, and the astigmatic lens of Stokes is principally available only as a means of control. If, for example, we have deduced from the results obtaingd, by what spherical glass the refraction in the two principal meridians is reduced to equal degrees of ametropia (either myopia or hypermetropia), we can, with the aid of the astig- matic lens, with great accuracy determine the degree of the astig- matism, and at the same time the instrument presents the advantage of enabling us in a simple manner to regulate it in its action. Its precision will even enable us to discover and counteract little in- accuracies in the result obtained by the above-mentioned methods. This is the place to remind the reader, that above (p. 468) use has already been made of the astigmatic lens of Stokes, in the con- struction of an instrument designed to make the phenomena of astigmatism in very different degrees visible upon a screen. What was there said will have found its explanation in the statement here given, B. — We have now to treat briefly of the objeetive signs of astigmatism. They are so far inferior to the subjective, that they usually do not exist with eqpal certainty, and never show accurately the degree of the asymmetry. But they derive a special value from the connexion in which they stand to the cause of the affection. They have partly reference to the form of the eyeball. Examination with the oph- thalmoscope supplies a second series of objective signs. 1. Astigmatism occurs mostly in hypermetropic individuals. If diminished acuteness of vision exists in such persons, asymmetry is generally in operation. Hence the objective signs of hypermetropia are already not without value (compare p. 252). But the cornea often affords more decisive signs. Sometimes its asymmetry is immediately recognised : it is either shorter than usual in the vertical measurement, or it extends farther backwards (as the result OBJECTIVE SIGNS. 489 of greater curvature), so that the section between the cornea and the sclerotic does not lie in one plane. In other cases, the difference in magnitude of reflected images in the vertical and in the horizontal direction attracts attention. A square, for example the board, above mentioned (p. 480), is represented with a greater transverse dimension. The asymmetry of the cornea is then thus proved, and that of the entire system usually corresponds to it. Even in the form of the sclerotic we again find this difference : we shall often be able to convince ourselves even in the living subject, at least in hypermetropic individuals, that the vertical axis of the eyeb^lJ. is ponsiderably shorter than the horizontal. 3. Examination 'with the ophthaliposcope affords likewise in hyper- metropic individuals the most certain indication of the existence of astigmatism. Iij. a normal eye we see (unless the observer be Ijim- self astigmatic) the vessels, proceeding in different directions from the optic disc ecfually distinct with equal effort of pur accommodation. In an astigmatic eye this is no longer the case. We then observe that, in order to see accxp-ately in succession the vessels running in different directions near the optic disc, we must alter the state of accommodation of our eye. The rule is, that tlj.e emmetrppic individual, in relaxation of his accommodation, observes accurately vessels running horizontally ; on tlie contrary, tp see vertical vessels distinctly, he must induce tensjon of accommodation, The explana- tion of this difference is evident. The vessels running yertically are not acjitely seen until the rays tljence diverging jn a hprizontal plane are brought to a focus in the eye of the observer, and if the . observed eye bfi hypermetropic in the hprizontal meridian, the rays \belonging to this plane maintain putside the eye a diverging direc- tion, so that tension of accommodation is required on the part of the observer to bring them to a focus, On the cpntrary, the rays pro- ceeding from horizontal vessels in the vertical meridian will, jn emme- tropia in this plane, putside the observed eye be parallel, and these vessels will, therefore, without tension of accommodation, appear djstinct.^^In the inverted image of the fi^ndus oculi the difference is also inverted, but, for ttiore reasons than one, is less perceptible : omitting the slighter difference of required accommodation, it is too much influenced by the direction of the axis of the lens held before the eye, which even may correct the difference. At the meeting held at Heidelberg in 1861, Dr. Knapp called attention to a second phenomenon in the fundus oculi in astigmatic 49Q ASTIGMATISM, persons. I refei to the variable form of the optic disc. In the direction of the meridian of greatest curvature the dimension, in examining the non-inverted image appears mpre, in tl^at of the meridiaij of shghtest curvature it appears less, magnified; the reverse obtains in examining the inverted image. If, therefore, in exami- nation by these two methods, the optic disc is elongated in opposite directions, the existence of As is, as Schweigger* remarked, prqved.t § 37. Cause and Seat of Abnoemal A^tigmatisji. Abnormal astigmatism is to be considered as a higher degree of fte same asymmetry, which belongs to normal eyeg : similarity pf geat and conformity of directioii of the principal meridians, in the two cases, afford the proof thereof. As to the normal, the cau^e is in general for the most part to be sought in the cornea ; and the direptipn of the prinpipal meridians, for the whole dioptric system, as well as for the cornea in particular, is of that nature, that the raeridian of maximum of curvature usually approaches to the vertical, that of ininimum, to the horizontal. For q,bnormal degrees of asymmetry the same rules obtain. What is more, they here present still less of exception. If in normal astigmatism it is nothing unusual for the meridian of the maximum of curvature to make a smaller angle with the horizontal than with the vertical plaiie, in abnorraal degrees I have found only a few examples thereof. And, as to the seat-r-if we leave out of considera- tion a few cases of evident ectopia of the lens, to which I shall revert — each disturbing degree of astigmatism was combined with an extraordinary asymmetry of the cornea, Precisely the high degree of this asymmetry explains, ^^hy it preponderates over the influence of the lens. The subjoined Table contains our first results of observation, » Archivf. Ophthalmologie, B. ix. p. 178. t My friend Bowman recently informs me, that " lie has been sometimes led to the discovery of regular astigmatism of the cornea, and the direction of the chief meridians, hy using the mirror of the ophthalmoscope much in the same way as for slight degrees of conioal*bornea. The observation is more easy if the optic disc is in the line of sight and the pupil large. The mirror is to be held at 2 feet distance, and its inclination rapidly varied, so as to throw the light on the eye at small angles to the perpendicular, and from opposite sides in succession, in successive meridians. The area of the pupil then exhibits a somewhat linear shadow in some meridians rather than in others." ITS CAUSE AND gEAT. 491 No. Name. Sex. Eye. I. rad. rad. III. IV. V. As hor. vert. r"horiz. F' vertic. In Parisian incliGS. 1 VI. f- E. 8"^0() ^29 M737 1:0695 10-78 2 »» ft L. 7-80 7-48 1.14t4 i-0975 2004 3 Vo. M. U. 8-29 7-56 1-2163 1-109 9-43 4 f» )) L. 8-14 7-67 1-1943 112S 14-51 5 Rr. M. R. 8-32 7-30 1'221 1071 6-374 6 )} )» l: 8-38 7-38 1-2295 1-083 6-800 7 Er. Jr. M. L, 8'44 7-69 1-2383 1-1283 9-504 8 Er. M. R. 8-72 7-13 1-2794 10461 4-293 9 9} )} L. 8-40 7-25 1-2325 i-0637 5-811 10 Pg. M. R. 7-93 7-50 1-1635 1-1004 15 18 11 Rm. M. L. 8-74 8-04 1-2814 1-1797 11-02 12 Im, M, ?" 7-96 7'34 1-1679 1-077P 10-35 13 )9 k L. 8-2^ 7-33 1-2149 1-0755 7-013 14 Vg. L. 8-29 7-69 1-2163 i-1283 11-67 15 Dt. M, R. 7-69 7-2^ 1-1283 1-0637 13-90 16 1} »» L. 7-84 7-26 1-1503 10652 10-77 17 And. M. R. 8-]9 7-50 1-2017 1-1004 9-767 18 )) 1) L. 8-lG 7'43 1-1973 i-0902 9-118 19 Ben. M. R. 8-11 7-23 1-1899 1-0607 7-310 20 Soh. M. R. 8-9i 7-82 1-3073 1-1474 7-019 21 >» «) I^. 8'81 7-96 1-2927 1-1679 p-051 It is made of like data, and calculated in the same manner as the Table occurring at p. ^60, and referring to normal astigpaatism. We find here 21 cases collected, in which diminished aputeness of vision existed, as the result of abnormal astigmatism.'^ The Table requires httle ejcplanation. Of the five columns of figures, I. Contains in ipillim^tres the radius in a horizontal plane, carried through the visual hne. II. In millimetres the radius in a vertical plane, carried through the visual Une. * In the majority of these caaea the measurements which are required for the calculation of the elements of the ellipse, were made hoth in the vertical and in the horizontal section. I pass them over here as heing less pertinent to the matter in hand. I shall observe only that the eccen- tricity of the elliptical section in the vertical meridian usually proved particularly small. It also deserves to be mentioned that, especially when hypermetropia was in play, the visual line almost always made a great angle (7' to 9°) with the axis of the cornea, which must appear the less strange because, as numerous measurements, made in connexion with Dr. Doyer, have shown ( Verslagen en Mededeelingen van de Koninhl. Akademie van WetSnschappen, 1862), the angle between the visual line and the axis of the cornea is in general, in hypermetropic subjects, con- siderable (compare p. 299). 492 ASTIGMATISM, III. In Parisian inches, the pqstepior focal distance pf tl^e cornea in I. IV. In Parisian inches, the posterioi focal distance of tlje ppmea in II. V. In Parisian inches, the focal distance of the cylindrical Ipns, which, in the requisite direction, placed immediately before the cornea would make the' fpcal distances III, and IV, equal. The dpgrpe of astigmatism, proceeding from the ascertained asymmetry of the cornea is therefore 1 : 10-78, 1 : ^0-04, &c.— E. signifies the right, I^. the left pyp. In some persons both eyes, ii; otherg only one, are meas]u;ed, M. standf for the ma,ie, F. for the female sex. Qn the Tyhoje, I have found the asymmetry more in men than in women ; pf the latter, howe\rer, comparatively fewer were submitted tq measurement. The eye discovers at a glance that in all the cases tjie radius pf the cornea in the vertical plane is considerably less than that in the hprigpntal, that therefore the form of the cornea, without exception, accoiint^ not only for a high degree of astigmatism, but also ^peci^Jly for an astigniatism with shprter focal distance in the yertical meridian, — quite in accprdance with what, likewise wjthout exception, was observed with respect tp the whole dioptric system. The great importance of the asymmetry of the cornea is particu- larly striking on comparing theTable to be found at p.460, containing the results of observation pf normal eyes, with sufficient acuteness of vision ; the maximum pf asymmetry occurring here is stiU below the minimum mentioned in the Table, in cases of abnormal astigmatism, if we leave put of view No. 2 of the abnormal, \vhich had a relatively slight disturbance (S = §), and No. 14 of the normal, which, on closer examination, yielded no perfect acutenpss of vision (8 = 5). Another question is, how far the crystalline lens also hag influence. In my original Essay upon Astigmatism, I was not in a position to give a satisfactory answer to this query. The investigations recently carried out with Dr. Middelburg, according to the method above described (see p. 462), have supplied me with the proof, Uat with a Ugh degree of asymmetry of the cornea asymmetry of the crystalline lens exists, acting in such a direction, that the astigmatism for the whole eye is nearly always less than that proceeding from the cornea. The subjoined Table (compare that at p. 493) contains the results of observation and of calculation, obtained for fifteen eyes, some of which were determined by more than one observsr. The observa- tions show : CT f^ aoca^-'O gooo -^ oao ttj^ oabg'-* No. B E"eSBB_ «,B B B 05003 00 CJt -J O CC 2s i= ^— St tr" td t-i ssi t^ ft) f tc wt -<^ rpr (3 p.^ to H-i I— ' OS t-i to M 00 O O Q Cft "p O. ■ tsJ td-- td- MINI ,r- M OS GO I-" „ irt hfs.Oi7JOC»l»<»7^ OD-a-JMOopoOOOO Sl^ bo ^ CO O o o tf^ bb ta i-j" cjiCOHJ Eye. CD O CO 00 CO CO 00 cb ■^ ■i to ba -^ c T Moeos ■• I r— ' H-t p^ t— ' UP yj i^w ff- ^ !.-» >j- ■—- ; — :_ _^^^^-j-^-sIQDOO GO ^OOCOCopoGOCO \^j — ^ v^pp h,fkj u^ ^^p* 1W1 n — tO~-JOOSW>f^OSCO -^-^^-^-iT^fapooopoco 5 00 bJ >f=^ CO OS -si-^-^-^l^4-MiJDUUuuw*j --^QOOOCOGopO'^ ^bobobsio^ll^lfccooi ^h^t^SSSS cnps o OS Mi-^o a o5"rf^ go =g cm o p t?j en _cDb^cnbDbs^-'^--'t^ot<)COC5jo^-'rfi;^^^7J -3 Oa ^ fo ->J O OQDt>Pt>aOOt^r'>::;PJife ■rf^ci5Dioi>obs*-bsb3Co^ososa;roo^o;2 QO lj( I— I ""J ijl (Ji '.*- ' ^^ L"-/ >-^' i^-* ^ .-J ~< ■■— •.— — .. : bibsbotci-'Cntj.^bsbics ^^^'^^.S-S'Slo t0mi^00*-J0^OO'^O<^ ^Jt>30 3>-"-"OStO ^ bi CO 00 to "in OS '-' '(^ '^^^ *-'■=» -^ ?!? ^ "=5 S rt^ S ..q..4-..4-vi'.oJOCnCl6c Q^»'?OSOi'Ot>S ; .»4-^^4^-j— l^^-ij*J--J-<| 00 --5 —J 00 CO CO 00 bibsbibs^KJoosDoscocoobsbstMCooto Oi 00 -^co to cfc CD OS o aacD ost-'-^^JOsftp'D* ^-— CJOJQOOOCnif*-CP^"~4--J--lrf>-':n \,jy UKi J ".J^^ l>^ ^~ ^^ ^v 'h^^ '.^^ ^.i^ rr~ ^*i^ — ^ , "^ •vl«-CO'-' OBo-j-j's3CT'gao?ro^30gaO*-JOsos^JH=*- ° a> pi / — ,r- / to rf^ CD CO u:i OD M^ OS 00 00 to COk-i 000 t-^cfe ' CO o o 000 CD CO -a OS <— ' to 00 03 p O O^ 3 bobs V3 If>. -^ -a -J -^ -a bs M i^I CD CO b» 3 to \ ^lOioo^cp-^-^io OtOOOCO^^OiOCy. o CO 00000 0« CD O to- ^« . O rf»- 0O-* O O -C o fcOtObOCOtChj!».i-ih-* bo bo **" w ' cain b'N fr-p- jwtf*- / / / \ ' I / / / / s •Q K^<1 bOOS Cn ^^ tc h-j OS 5^ OS o to O o o o __o M bs fj <1 H-i SI >3 •-r' CO 00 ^a rj I— ' ( 494 ASTIGMATISM. 1°: That the clegl?ee of astigmatism for the whole eye (column G Aso) was in generd not so high as had previously been found by Dr. Knapp and niyself. It was On the present occasion ascertained by determination with the stenopeeic slit, with cylindrical glasses, and with Stokes' lens, the average of the results obtained by these three methods being taken. These results, when the acuteness of vision was not particularly slight, usually differed but little from one another. 2°. That almost invariably (column D) hypermetropia existed in both meridians (the sign minus, plabed before the numerical values of the refraction, signifies H). 3°. That the directidn of the meridian of maximum of curvature (column G) is far from iiniform, but yet in twelve cases deviates less than 20° from the perpendicular^ and only in two cases is nearer to 0° than to 90°. (The determination was effected from the direction of the correcting glass required; according to the method described at p. 455, which is more accurate than that formerly used). i°. That, in connexion with the most commonly occurring direc- tion of Moi with the exception only of No. 13, the radius of curva- ture of the cornea in 90° is less than in 0° (column E), and even in five cases the maximum of curvature is found very near 90°. 5°. That the average of the measurements^ in each of the meridians obtained through intervals of 15 degrees (column E), No. 14 excepted, exhibits a very regular sequence in the ascertained values of the radius of curvature. (The lesser regularity found for the cornea with normal astigmatism (compare Table, p. 466), is undoubtedly to be ascribed to errors of observation, which, with the shght differ- ence of the radius of curvature in the measured meridians, must of course be much more evident in the relation of the numbers.) Erom the observations of the radius of curvature in the different meridians, it was now in the first place calculated, to what meridians maximum and minimum correspond and what are the values of maximum and minimum; whence, further, in a simple manner (compare p. 461) the degree of astigmatism caused by the cornea (see column E, under Asc) was then found. — In the second place, it was now calculated what the degree and the direction of the asymmetry of the crystalline lens must be, in order, in connexion with the values ascertained for the cornea, to elicit the direction and the degree of the astigmatism for the whole eye. In column H the results of this calculation for the lens are inserted. They will be found to be in accordance with the general result formulised at p. 492. DEDUCTIONS FROM THE TABLE. 495 In particular, we still remarkj that only iii two caSfes dbes the meridian of maximum of curvature of the crystalline lens, Mj, approach more to the vertical than to the horizontal direction. In eleven cases it deviates even less than 10° from the horizontal direction; It thus appears that the maximum of burvature of the lens is still more con- stantly governed by the horizontal, than that of the cornea is by the vertical directioui Now with this is further connected the fact, that the astigmatism of the cornea is almost invariably greater than that of the whole eye. But at the same time it appears] that we should be far from the truth, if we made the compensating action of the crystalline lens equal to its actual astigmatism, and therefore assumed Asi = -V— — T— . The directions of the axes have too great an As„ ASo influence, as a single glance dt the liiies, by which they are indicated, at once makes evident. — We can therefore attach no particular value to the Table given by Enapp,* the less so, because the deter* minations of the astigmatism for the whole dioptric system must by his method come out too high (see p. 451).i I have called the astigmatism of the cornea greater than that of the crystalline lens : but, in fabt; the astigmatism of the ci^stalline lens is greater than we have here found it. The calculation was made as if the crystalline lens were a single refracting surfacej placed at an infinitely short distance from the anterior surface of the cornea ; and we can easily understand, that the deeper position of the crystalline lens must diminish its influence in astigmatism. I have thought a more accurate calculation on this point super'- fluous. Lastly, the question still arisesj whether, when asymmetry of the cornea exists> the radius in the horizontal meridian is greater, or that in the vertical is smaller, than that of the normal symmetrical eye. In the first place, I can to this answer, that that in the horizontal meridian is usually considerably greater. In measuring 120 eyes of men with perfect accuracy of vision, I found p° in the horizontal meridian on an average 7.858 mm., the maximum being 8.396 and the minimum 7.291. Among these were many myopic and hyper- metropic eyes, up to the highest degrees ; but they exhibited no difi'erence of importance. The 21 asymmetrical eyes of men, col- lected in the Table (p. 491) give, on the contrary, p° on an average 8"291 that is, nearly equal to the maximum found in symmetrical * Archwf. Ophthalmologie, B. viii. Abth. ii. p, 22.5. 496 ASTIGMATISM. eyeSj and among them occur not fewer than five, which even exceed that maximum, namely, p° = 8-44, p° = 8-72, p° = 8-74, p° = 8"81, and p° i= 8.91. As to p° in the vertical meridian, this is in the asymmetrical eyes shorter than in the symmetrical, but the difi'ereiice is here less considerable. On an average we found for the first (Table p. 491) p° vertici = 7'439. Of p° vertic. in symmetrical eyeS I pdsSess no determinations except those given at p. 460, which exhibit an average of 7'695. It therefore appears that, in asym- metry of the feye, p° in the horizontal meridian usually aScends mdre above the normal, than p° in the vertical descends below it. The saitie is to be deduced from the Table giVen at p. 493. Besides congenital malformation of the cornea, various acquired ddnditidiis may give rise to abnormal astigmatismi These deserve separate consideration, and shall be spoken of in § 39, with the clinical forms, undef Which astigmatism occurs; I here append the nlDde of calculating, tlie part played hj the Crystalline lens in astigmatism, as it Kas beeii established, after consultation With my frieilds Professors Soek and Buy^ Ballot. In the first plabe it is necessary to establish for aill the determinations made in twelve meridians a relative influence on the direction of M and »ic, as well as on the radius in M^ and m-^. From the observations must be cal- culated : — 1°. a. the dngle, which the horizontal plane ihakes with the plane m of the gi'eatest radius of curvature. 2°. R, the leslst radius of diirvature and r the gted,te8t radius of curva- ture. For this pui-pose, let the twelve observations be divided into three groups, and of the a, r and R, calculated from each group, let the mean be taken. Let Pa be the radius of curvatute in the horizontal meridian, therefore making an angle a-\-6 with m, thus fi^ makes with m the angle a.-^f. We thus obtain ; 11 1 — i=^ cos* ti -) — sin' a, Po K r to which another form is easily given. The difference is The difference of the two cosines, expressed by the product of two sines, we obtain INFLUENCE OF THE CRYSTALLINE LENS. 497 and likewise Now the quotient of these proportions is : Pi P0 -l 1 — = tan (2 a + 0) (4) Pi5 P + 45 and if we assume f = 90", the simple formulas come = (b- ) cos 2 a (2*) = — (n- — i)sin2a (3*) 1 2_ and ^ ^ = - cot 2 a (4*) Pi5 P'P + 45 If we apply this formula to the fourth line of the tahle, we find : For the first combination of pa pso and p^ pug J L — 0-0107 801 8-76 — .r— = = — cot 2C8 J: i_ — 0-0007 8-37 8-42 2a = — 3° 44' a = — 1° 52'. For the second combination of pij pioj and pg^ pijo, JL. L _ 0-0092 ^•Q^ ^'"^2 = = — cot 2 (a + 15°) _L _ ^ + 0-0056 8-14 8-47 2 (a +15") = 31° 20' = 40'. For the third combination pso p^o and p,5 p^j, A Jl- — 0-0063 8-60 816 , _ , I „„, ^j = — = = — cot 2 (a + 30) _!^ _ + 0-0079 8-02 8-56 ^ 2 (a + 30) = 51° 26' a = — 4° 17'. As the mean of the three combinations, we now obtain : „ = -1° 52' + 40' -4° 17' ^ _ lo 53-^ which signifies that m lies at 1 " 53', M at 91 ° 53'. In the second place we find R and r from the determinations of 32 498 ASTIGMATISM. For the first combination we find : _1_ I 1 V Pa pSO ' ^45 Pl35 ' __ 0-2387 R r ~ 2 For the second combination : -i + i = 1^5 PJS^ Veo Pjd. = 0-2402. For the third combination : 1 , 1 _ Vao Pi80 '' Vts Pi65 '' _ 0-2402 r And w — - is found as (2 *), t J_ J 1^ po pj. R r cos 2 » This gives for the first combination, 1 1 0-0107 „„,„, R - r = ^5^3^' = «'010^- For the second combination, 1 1 0-0092 R-?= cos31-'40' = °-"^«g- For the third combination, 1 1 00063 R-r = cos51°26- ="-0^"'- By adding the values obtained for ^ to the correspoudiDg values of 11. 2 = -\ — , we obtain the value =-, and by subtracting them from the same 11 2 value ^ +- the value of— In the first combination JtC. r r i = 0-2387 + 00107 = 0-2494. H second combination = 0-2510 third „ = 0-2503 I =0-7507 i = 0a251 R = 7.99 INFLUENCE OF THE CRYSTALLINE LENS. 499 ^^^ ? = 02387 - 0'0107 = 0'2280 r second combination = 0-2294 third combination = 0'2301 ~ — 0-6875 r - — 0-1146 r r = 8-73. In this manner now both M^ and the radius in Mc and in m^ are found, and entered in the Table. The ascertained values of the radii in Mc and Die are reduced to Parisian inches, and thence the posterior focal distances F" in the maximum of curvature and f ° in the minimum of curvature are calculated, whence, further, f was found (see p. 461). We thus obtain : a 1 ■o-Sc p. Now, if the direction of Mo be known by direct determination, that of Mc by calculation, and if the values of Aso and Asc have also been found, these may be considered as the focal distances of two positive cylindrical lenses, the direction of whose axes is perpendicular to Mo and to Mc- Hence, therefore, Mj can be found as the direction of the axis, and Asi as the strength of the cylindrical lens, which, added to Mc and Asc, gives for the resulting lens M„ and As„. The following is the question : — If of two infinitely thin cylindrical lenses, I and III, are given the focal distances, or the radii ij, and rs and the directions of the axes, — what is then the focal distance or the radius r^ and the direction of the axis of a lens II, which, added to I, has III as resultiog lens ? Let po = B, of the cylindrical lens be oo (therefore in the direction of the axis), pgo = r the least radius of curvature, a, 13 the azimuths of the plane ' of the axes for each of the cylindrical glasses. y the azimuth of the axis of the lens to be added, then, as in an angle ^ with the axis 1 j^ ^-^ ^ 1 cos" ^ + i sin' 0, po Ji r for the cylinder i-=- sin^ ^ and — = - cos" ^ po r Po r and therefore in an arbitrary azimuth 8, that makes the angles a — S with the axis of the first, (3—8 with the axis of the second, y—d with the plane of the axis of the third lens, sinV"— 8) , si n» (/3-8) sin^y— 8) , 1 ... i-' + ? = T'' + K ^ ' cos= (a— 8) , cos" (13 -8) cos' (y—8) 1 ,„. p \- i' = 7~' f" R ^ ' R always the radius of the sphere, which must be added to the lens y, in order wholly to comprise the system a and p. (2) minus (1) gives, on account of cos' ij> — sin' = cos 2 f, 2 500 ASTIGMATISM. cos 2 (g — 8) COS 2 (13 — g) _ ens 2 (y—S) ,3^ r' r" r" The azimuth 6 is arbitrary, for ia each azimuth it must be quite the same, whether we take a or /3, or y into the sphere. This we express in the following manner : — cos 2 (a — 8) := cos 2 a cos 2 8 + sin 2 a sin 2 8. If we do thus with the other terms, and write the one under the other what is multiplied with cos 2 6, and also the one under the other what is multiplied with sin 2 8, there results (3) cos 2 a o c I sin 2a. „ ^ ; — cos 2 6 ^ sm 2 6 r r + £2?_|_^cos2 8 + !i5_ilsin2 8 j (4) cos 2y „„ sin27.„„ „ pr-^ cos 2 8 ^„-^ sm 2 8 = J In order to express the arbitrariness of S, the coefficient of cos 2 8 and that of sin 2 6 must each separately be equal to zero. Thus (4) falls into (5) and (6). cos 2 a . cos 2 p cos 2 y r- 1 J., — J..' sin 2 a . sin 2/3 sin 2 y r' "■ r' — r" (6) divided by (5) is :— sin 2 a sin 2 /3 (5) (6) r' ^ r* tan 2 y = — ^„ i„- ' cos 2 a , cos 2 /3 r' ' r' and (5)» + (6)2 gives 1 1 . ] , 2cos2 (a-B) (7) !•'„ r^"'"rv+ r'r' Thvis r" and y are found. If r' or r' and a or /3 to be found, while r" and y, and moreover r* or r* and l3 or a are given, we write (5) and (6) thus : cos 2 a cos 2 y cos 2 /3 ,,,, ~1^ = ""^^^ F- ^'^^ sin 2 g _ sin 2 y sin 2 /3 , , „v r' — r" r" — ^ ^ 10 ■ . - gives : sin 2 y sin 2 /3 . „ r" ? r' sin 2 y — r"' sin 2 /3 ,,, tan 2 g ^ = -. ^ ', r-C; n 1 \ cos 2 7 cos 2 (3 r" cos 2 y — r' cos 2 /3 *■ ' of J- = -.^^ 1" [ ^\ + ^„-2 r, r„ cos 2 (^-y) ] of r, = '" ^^ ^ i-". + r-„ — 2 r, r„ cos 2 (/3— y)- USE OF CYLINDRICAL LENSES. SOl According to these formulae Mi (^ a) and r' (and with them Asi, as pro- portionate to them), are now found and filled into the Table. § 38. Cylindmcal Lenses,* and General Rules for their Employment. Eegular astigmatism may, as has above been remarked, be pro- duced by adding a cylindrical to a spherical lens. The action of a cylindrical lens can, in its turn, be counteracted by a second such lens of equal focal distance. If these cylindrical lenses are both either positive or negative, the axes of the cylindrical surfaces must, in order to neutralise each other, stand perpendicular to one another ; if, on the contrary, one be positive, and the other negative, the effect is attained with a parallel state of the axes. Of the latter case the astigmatic lens of Stokes furnishes an example (conf. p. 486) : its action is, in a parallel state of the axes, = 0. The first we find represented in the so-called watchmakers' lenses, which have two convex cylindrical surfaces of equal focal distance, whose action, by the intersection of the axes of these surfaces, nearly coincides with that of spherical biconvex lenses. If the two cylindrical sur- faces be similar and concave, they give the action of a negative spherical lens, with removal of the astigmatism, when their axes intersect. Now as the action of one cylindrical lens may be destroyed by that of another, regular astigmatism may be corrected by means of a cylindrical lens. In order to form a good idea of the correction, experiments should be tried with one's own eye. A. cylindrical lens, for example of to, produces astigmatism, and gives rise to peculiar disturbances in vision already described. A second cylindrical lens of — -fe, with a similarly directed axis, completely neutralises the action of the first, so that the presence of glasses before the eye is now scarcely observed. If, on the contrary, the second cylin- drical lens, as well as the first, is a positive lens of to, the astigma- tism is corrected, if the axes of the cylindrical surfaces be directed perpendicularly to each other ; but the eye has then at the same time become myopic, and that to such a degree, that the farthest * To be had, among others, from Nachet and Son, Paris, and Paetz and Flohr, Berlin, on giving the formulae to be hereafter communicated. 502 ASTIGMATISM. point of a previously ametropic eye (the distance from tlie glass to the eye not being taken into account) comes to lie at 10" (M = n). The glasses required for the correction of the different forms of astigmatism, may be reduced to three kinds. I. Simple ct/Undrical glasses (Kg. 163). Just as the spherical these have either a positive (A, B, C), or a negative focal distance (D, E, P) ; the first we call simply positive, the second negative. If both the surfaces are cylindrical, their axes are parallel. To give a correct idea of their form, they are represented both in a section per- pendicular to the axis (Pig. 163, 1), and in a section, carried through the axis (Fig. 163, II), the surfaces being distinguished as a, the anterior, andjo, the posterior. a. To the joositi/ve belong : — 1. The bi-convex (Fig. 163 A). 2. The plano-convex (B). 3. The concavo-convex or positive meniscus (C). b. To the negative belong : — 1. The bi-concave (D). 2. The plano-concave (E). 3. The convex-concave or negative meniscus (E). Fig. 163. Practically the same is true of the cylindrical as of the spherical glasses : the plano-convex and plano-concave produce the greatest aberration, the bi-convex (provided it be not too powerful) and the VARIOUS FORMS OF GLASSES. 503 bi-concave are in general very satisfactory, and the menisci have the advantage of being periscopic. Of the simple cylindrical, glasses from mc tola, and from — 35 c to — 5 c are .necessary, that is glasses of from 50 to 5 Parisian inches both negative and positive focal distance. That this focal distance obtains only for the surface perpendicular to the axis of the cylin- drical curvature, and that, in a surface carried through the axes the focal distance is infinite, need scarcely be remarked here : they there- fore leave in this last direction also, the focal distance of a dioptric system, wherewith they are combined, unaltered. To express their nature and power we make use, as will have been seen, of the same formula as for the spherical glasses, with the addition of c. II. Bi-cylmdncal glasses (Fig. 164). — These have two cyHndrical surfaces of curvature, whose axes are directed perpendicularly to one another (I a and II j»). If the two surfaces are cylindricaUy ground, but their axes are parallel, they belong to the simple cylindrical, whether to the bi-convex, or bi-concave, or to the menisci, which have been already described above (Fig. 163, A, C, D and F). Of the bi-cylindrical one surface is in general convex, the other concave, as the two sections, taken in each of the two axes (Fig. 164, 1 a, II p), Fig. 164. Fig. 165. ID « show. Such bi-cylindrical glasses, therefore, make parallel incident rays of light, after refraction, converge in the plane of the one axis. 504 ASTIGMATISM. and diverge in that of the other. Their action may be expressed by the formula for each of the two planes, connected by the sign of a right angle f • -A. bi-cylindrical glass of 12" positive focal distance in a plane, perpendicular to the axis of the convex cylindrical surface, and of 24" negative focal distance in a plane, perpendicu- lar to the axis of the concave cyhndrical surface, is therefore ex- pressed as : 1 P 1 12 "^ ' ~ 24 ''■ III. 8phenco-ei/Undrical glasses. — Of these glasses the one sur- face has a spherical (Kg. 165, I and TL a a), the other a cyhndrical curvature (I and IIjo^?). Only those are used whose two surfaces are either convex (A) or concave (B). These lenses may be considered as the combination of a piano-cylindrical with a plano-sphfifical lens, and we actually obtain both by cutting a spherico-cylindrical lens in a plane, perpendicular to the axis of the spherical surface. Now the action of a spherico-cylindrical lens is similar to that of the combination mentioned, and it may be expressed by the formula for each of the refracting surfaces, united by the sign of combination O. If the spherical curvature, as a plano-convex lens, gives a focal distance of 12', the cylindrical curvature, as a plano-convex lens, a focal distance of 24", we write : 12 * ^ 24 ''^ signifying that the positive focal distance, in a section through the axis of the cylindrical surface, amounts to 12', in a section per- pendicular to this surface, to (Vj + ii = I) 8". If the spherical surface, as a plano-convex lens, represents — =r-Q> the cyhndrical, as a plano-cyHndrical lens, — -, the combined spherico-cylindrical lens gives ; 1 ^ 1 ~ T8 * -^ - 9 "' in which lens the negative focal distance, in the axis of the cylindri- cal surface, amounts to 18", perpendicular to this axis to {h + h = i) 6". It is now easy to see what cylindrical glasses remove the different forms of astigmatism (compare p. 482). Suppose our object to be to correct the ametropia at the same time with the astigmatism, that ITS CORRECTION BY GLASSES. 505 is, to bring the farthest point of distinct vision to an infinite distance (E = od). We then find: 1. Simple myopic astigmatism is corrected by a simple negative cylindrical lens (Eig. 163, D, E, F) of a focal distance, correspond- ing to the degree of astigmatism : Am=| by glasses of — -^ c, placed at ^" from the nodal point. Of 3. Compound myopic astigmatism requires a negative spherico- cylindrical lens. Thus, not taking the distance from glass to nodal point into account, is corrected by »ro + ^»a, 3. Simple hypermetropic astigmatism. Ah, is corrected by simple positive cylindrical glasses (Fig. 163, A, B, C), corresponding to the degree of the astigmatism. For Ah = 1' a glass of 1 is therefore required, placed at i" from the nodal point. 4. Compound hypermetropic astigmatism requires positive spherico- cylindrical glasses : H 1 + Ah A 18 ^ 9 is corrected (the distance from the glass to the nodal point not being taken into account) by : — « O — c. 18 9 5. Mixed astigmatism, lastly, yields to bi-cylindrical glasses: Amh = Q-j composed of 506 ASTIGMATISM. and composed of by: 24 ' 12 Ahm = o'' Jl_ H + i- M, 12 24 ' 12 '^ I 24 "' These short examples will form a guide in the choice of glasses, when, with the astigmatism, ametropia is at the same time to be corrected. But it is not always desirable to attain this double object. Whilst by correction of astigmatism the power of vision is under all circum- stances benefited, and we almost unconditionally can attain to it, — to reduce the eye at the same time to the state of emmetropia, is often not indicated. As to this reduction, the same rules apply in complication with astigmatism, as are appKcable to ametropia in general, and which we have stated in detail when treating of hyper- metropia (§ 23) and of myopia (§ 32). It therefore only remains for us to show, how to find by calcula- tion the necessary glass, in complication with astigmatism, when it is established to what distance R must be brought. In the determination of the astigmatism, we proceeded from inves- tigating the refraction in the two principal meridians. Hence the ame- tropia common to both meridians was deduced, and the degree of astig- matism was added as a separate value. Thus we found the formula for the compound astigmatism both hypermetropic and myopic. If we now revert to the two principal meridians, the method of finding the glasses which, neutralising the astigmatism, give the desired value to E in all meridians, is extremely simple. Giving to E a value of 40", 20", 12° signifies nothing else than communicating to the eye a myopia of, or reducing the existing myopia td, — , — , — . "We have therefore from the ascertained refraction in the two principal meridians only to deduct the desired degree of myopia, and if we give the glasses which completely correct the then remaining ame- ILLUSTRATIVE EXAMPLES. 507 tropia in each of the meridians, precisely this degree of myopia remains. The following examples may serve to illustrate the foregoing : — ■ 1. Let there be found : In the principal meridian h, emmetropia, » » ^> ^^ =- OJ and let us desire to bring R to 18", then we obtain by deduction : in H, emmetropia — M — = H — inv,M|--M,^=Ml; and to correct this a bi-cyhndrical glass is required of : %. Let there be in the principal meridian h^ M = -^3 T, M = j^; and if we desire to bring R to 20", we find, by deducting M m, in H, M ^ — M 20 ~ e™™etropia, so that the object ia attained by a simple cylindrical glass of — 25 c. 3. In H let H = g, in V let H = jq ; if we desire, for reading, writing, &c., to bring E to 18', we find by deduction : ^,h1-mA-h- \, m V, u ^ - - j8 - - 9. which (compare p. 505, under 4"^) is corrected by: 1 _ 1 inv,Mf^-M^^ = M^, 508 ASTIGMATISM 4. In a case of astigmatism let there be : in T, M = ^, m H, H - ^. And if we desire E at 24", we obtain by deduction, so that to obtain the proposed object, a bi-cyKndrical glass is required of i- c- - 1- 12 '^ ' 24 ''• The rather complicated method here described may in all cases be adopted, but this is not always necessary. Por when, together with compound myopic astigmatism, a high degree of myopia exists, which we wish partly to maintain, the myopia need only be reduced to the desired degree : for example, if in M b + Am n, we wish to retain M ts, this will be attained by sub- tracting M T^, and thus correcting M b + Am n hy — h s 3 — T^ c. In like manner we have in compound hypermetropic as- tigmatism only to increase the hypermetropia which is to be corrected by so much as corresponds to the desired value of E. Thus, in H^ + Ah^ R is to be brought to 18° by 1 _ 1 In all these calculations we have, for the sake of simplicity, omitted the correction proceeding from the distance between the, glass and the nodal point : indeed, if particularly strong glasses are not required, its influence is so slight as to be in practice scarcely perceptible. In the employment of cylindrical glasses, it is of the greatest im- portance that the axes of the surfaces of curvature should be situated in the principal meridians of the dioptric system of the eye. Even a slight deviation causes, especially when strong glasses are used, a very perceptible disturbance. The proposed end is now best attained by INCAPABLE OF ABSOLUTK CORRECTION. 509 setting circular shaped glasses in a framework with rings, so that by turning round the glasses, the proper direction can easily be given to the axis of the cylindrical surface. By slightly moving the whole arrangement, we soon observe in what direction the glass is to be turned ; and the proof that it has acquired precisely the right direc- tion consists in the correction becoming less perfect, the power of vision less acute, on gently inclining it to one side or the other. When once the correct direction has been found for the round glasses, we can, if it be preferred, still maintaining the directions of the axes, have them ground into oval glasses and placed in another framework. That in the use of cylindrical glasses, it is above all things necessary to take care to have a well-adjusted and but slightly movable framework, is included in what has above been said. The correction of regular astigmatism by means of cylindrical glasses is incapable of absolute perfection. Apart from tbe amblyopia, which, inde- pendently of the light-refracting system, complicates many cases of astig- matism, the acuteness of vision must, even with the most accurate correction, leave something to be desired, because the asymmetry of the astigmatic eye cannot be completely counteracted by the presence of a cylindrical lens. Moreover, the correction is only of that nature that the posterior focal points for the different meridians are brought together without the same being true of the other cardinal points. The absolute coincidence of the nodal points in the different meridians is scarcely attainable. If they lie in the principal meridian of slightest curvature more posteriorly, correc- tion with a bi-convex cylindrical lens.brings them more forward, than those in the meridian of greatest curvature, and vice versd if they be situated more anteriorly, on correction by a bi-concave cylindrical lens, they are moved more backward. In this is implied that the form of bodies, on cor- rection of astigmatism, is elongated in a direction opposite to that in which, before correction, elongation existed. This too great displacement of the nodal points becomes less, the closer the cylindrical glasses are to the cornea, and for this reason also it is desirable, in the use of spherico-cylindrical glasses, to turn that surface towards the eye, whereby the nodal point of the cylin- drical surface lies closest to the organ. If both be convex or concave, the one of least curvature should be turned towards the eye ; if one be convex, the other concave, the concave one should be turned towards it. — The change of form, which is the result of cylindrical glasses, is also the cause, why, in looking for glasses of the required strength, we can less advan- tageously make use of the change in acuteness of vision on altering the distance between the glass and the eye, than when spherical glasses are in question : almost always the glass, both the convex, though it be too weak, and the concave, though it be too strong, is desired close to the eye. Moreover it is easily seen that, especially in using bi-cylindrical glasses, the distance from the eye ought to be short : indeed, in proportion as the dis- 510 ASTIGMATISM. tance increases, the images become in one direction smaller and smaller, in the opposite larger and larger, and under this double influence the change of form must make itself strongly felt. Furthermore, it is to be observed, that in some movements of the eye, which are connected with an obliquity of the vei-tioal meridian, the direction of the axes of the cylin- drical surfaces no longer coincides perfectly with the principal meridians, and the correction is therefore imperfect ; whence it follows that in using cylindrical glasses the eye, in order to maintain the acuteness of vision, is somewhat limited in its movements. However, every spectacle-glass, as such, necessarily causes limitation, and, in fact, experience shows that in the use of cylindrical glasses this is productive of no particular inconvenience. Finally, to omit nothing, I shall briefly state, that the accommodative changes in the astigmatic eye, especially after correction of the asymmetry, do not represent absolutely coincident ranges of accommodation in both principal meridians, so that the correction by definite glasses cannot be equally perfect in all states of accommodation. This difference is however so slight, that neither can it give rise to any practical difficulty. As I have already stated, Airy was the first to discover abnormal astig- matism, in fact in his own left eye. At the same time he conceived that a cylindrical glass might correct the asymmetry, which he actually found to be the case, the disturbance of vision being corrected by such a glass. The formof his astigmatism was the compound myopic. Airy conceived that if he had two concave cylindrical surfaces ground, with axes directed per- pendicularly to each other, each corresponding respectivel3' to the degree of myopia to be corrected in the principal meridians, the object should be attained. But he correctly gave the preference to a negative spherico- cyHndrical glass, whereof the concave spherical surface was to correct the common myopia of the two principal meridians, the concave cylindrical the still remaining simple astigmatism. And, in truth, bi-cylindrioal glasses (with intersection of the axes), of which both surfaces should be either convex or concave, are never necessary : they may always be advantageously replaced by spherico-cylindrical glasses. The bi-cylindrical lenses used in Switzerland, and generally to be found with the watchmakers, whose convex surfaces, together with intersection of the axes, have a similar curvature, in their action closely resemble bi-convex spherical glasses. The watch- makers assert that they have a more extensive field : the truth is, that the field is wider in the direction of the axis of the surface turned towards the eye, but narrower in the opposite direction, so that the form of the distinct field of vision is an oval, which follows the movements of the lens in rotation. These lenses can the less vie with periscopic spherical glasses, as with them the forms of objects seen laterally appear in a pecu- liar manner distorted. That astigmatism is, to a certain extent, capable of correction by convex and concave spherical glasses, when their axis is held before the eye at a certain angle with the visual line, was already known to Young and Gary. (See the quotation at p. 456.) It appears that others, especially in EMPLOYMENT OF IRIDDESIS. 611 high degrees of myopia, have also made use of them. Thus we read in White Cooper (on Near Sight, etc., Londonj.1853, p. 199), that cases some- times occur, in -which, in consequence of a peculiarity in the form of the refracting media, or in the sensibility of the retina (?) the improvement caused by glasses is considerably increased " by sloping them or holding them obliquely," and that he had seen a striking example of this in a myopic patient. He adds, that in order to give the desired position to the glass. Carpenter and Westley made it turn in a second ring, contained in the ring of the spectacle frame. This means of correcting astigmatism is, however, capable of application only when relatively strong spherical glasses are required to neutralise the ametropia, and then, too, a more per- fect correction will be attainable by cylindrical curvature of one of the sur- faces. Only in aphakia can we advantageously, in my opinion, in order to correct a certain degree of astigmatism, make use of an oblique position of the glasses. Almost always it appears that when we give a certain incli- nation to the strongly convex glass, the acuteness of vision is improved, and the necessity of attending strictly to this point in every case of aphakia, is generally recognised. Lastly, we must not omit to observe, that regular astigmatism might also be corrected by operation. Iriddesis, an operation brought into vogue chiefly by Critchett, would be serviceable in such cases, especially double irid- desis in opposite directions, as practised by Bowman and others in kerato- conus (compare Critchett and Bowman, in Reports of the London Ophthalmic Hospital, 1859 and 1860). The pupil is thereby, in fact, changed into a nar- row slit ; and if tl;e direction of this slit corresponds to one of the principal meridians, the aberration dependent on asymmetry would certainly be almost entirely excluded. The indication of this double iriddesis in keratoconus I cannot admit : in this case the cause of the loss of acuteness of vision is not the difference of curvature in the various meridians, but rather -the conical curvature in each meridian ; and, in connexion vyith that. Bowman has proposed some modifications of iriddesis (comp. the treatment of irre- gular astigmatism). But where there is only difference of curvature of the several meridians, that is in regular astigmatism, the narrow slit obtained by double iriddesis would undoubtedly very much promote acuteness of vision. However, considering that we have it in our power to obtain the desired correction by means of cylindrical glasses, I am, looking to the greater or less danger of the operation, and not less to the deformity which is the result of it, far from recommending its application. § 39. Nosology and Clinical Study of Astigmatism. — Histoey OF OTJE Knowledge oi? the Subject. Astigmatism is either congenital or acquired. In the great ma- jority of cases it is congpnital. If it be acquired, it is to be looked 512 ASTIGMATISM. upon clinically as another form of disease, whose practical importance has become more and more evident to me, and of which I shall separately treat, after having first considered : — 1. Congenital astigmatism. — This anomaly is of frequent occur- rence. I am as yet without satisfactory statistics ; but I certainly do not exaggerate when I assert that in forty or fifty eyes, one is, in consequence of astigmatism, disturbed in its function. Boundaries between normal and abnormal astigmatism do not exist. When it attains the degree of to, I have called it abnormal, because the disturbance of vision is then of 'that nature, that cylin- drical glasses are desirable for its improvement. But otherwise it is evident, that the limit I have fixed upon is rather arbitrary. With much slighter degrees the acuteness of vision is no longer perfect. Thus I was formerly of opinion, that an astigmatism of about -m, such as exists in both my eyes, under no circumstances interfered with the distinctness of the images, and was therefore not capable of correction ; and yet I found that m e (the weakest glasses I possess) held with the axis perpendicular before my eye, unmistakably in- crease the distinctness of the images, while, vice versa, with the axis in a horizontal position, the same glasses produce a pretty considerable disturbance. The improvement which is produced, by the addition of TO c, with a vertical axis is now most striking. Hence, we see again, that too high a degree has been considered as normal by Dr. Knapp. The cases which have occurred to me, presented an astigmatism of from Tff to jj" In the majority it amounted to more than n, not unfrequently to more than 15. Astigmatism is often hereditary. Not unfrequently one of the parents labours under the same defect. But nipre frequently still it happens, that difierent children, born of the same parents, exhibit this anomaly, and mostly in the same form ; in this case we are equally justified in calling the condition hereditary, as when it occurs in one of the parents. In the majority of instances both eyes are affected. Often, however, one is completely or almost completely free. Mr. E. had in both eyes Ah between 5 and \ ; in his brother. Ah existed = s's of precisely the same form, only in one eye : his left eye was almost perfectly free from astigmatism. It is remarkable that, vrith such a (hfference ITS EFFECTS ON VISION. 513 between the two eyes, the upper part of the face likewise Is usually asymmetric. Also, when a high degree of ametropia occurs only on one side, asymmetry of the bones bounding the orbit is a very common phenomenon. This is connected with the peculiarities in the form of the face in myopic, and especially in hypermetropic, in- dividuals, — an important subject, which has already long engaged my attention, but on which J. cannot here further dilate. Thus far I have met with many more cases of abnormal astigma- tism in men than in women. I do not, however, feel justified in assuming that this is not partly accidental. This point must remain for future decision. As to time of life, it is evident that this can have no influence. So long as the power of accommodation remains active, the dis- turbance of vision from a moderate degree of astigmatism is, how- ever, productive of less inconvenience. Slight cases, therefore, usually first present themselves, when the range of accommodation (about the thirtieth year) is already perceptibly diminished, while in high de- grees of astigmatism the disturbance is early remarked, and the ophthalmic surgeon is not unfrequently consulted even before the patient has attained his seventh year. On the other hand, at an advanced period of life, in consequence of the diminution of the pupil, a certain degree of astigmatism produces less disturbance. With all this the astigmatism certainly maintains about its original degree. The disturbance of vision, connected with this anomaly, is very peculiar. It is neither to be compared with that proceeding from defects of the retina (amblyopia), nor with that from obscuration of the media, nor even with that which is the result of ametropia. In the ordinary forms of amblyopia, the projection into the field of vision is uncertain and not accurately circumscribed ; in obscurations the diffuse light scattered over the retina produces a mist before the eve, which diminishes the contrast between hght and shade of the objects; in a state of refraction not corresponding to the distance at which the objects are, each point is represented by a circle of dif- fusion, and by the mutual overlying of these numberless circles, the contours of the objects are, as it were, effaced. In astigmatism, on the other hand, in contrast to amblyopia, the projection in the field of vision is as yet perfectly accurately defined, and is correctly described : thus the astigmatic individual will state, to the minutest particulars, under what partly black, partly grey lines, a figure, for example the 33 514 ASTIGMATISM. compound reman letter W, appears. But the retinal image itself, placed in an unaltered skape before the eyes, by the projection, deviates in form and in distribution of light so much from the object, that he is not in a position to recognise the latter from it, at least when the images of different adjoining objects cover one another, and the component lines, in all directions and with different degrees of distinctness, cross one another. Clearly also irregular astigmatism here plays a great part : it causes, that in the meridian wherein the refraction deviates most from the required condition of accommodation, double images arise, which may exceedingly magnify the confusion. It is now easy to understand how, in the endeavour to guess at the form of objects, from the alternating images which appear in the agitation of accom- modation, psychical fatigue is soon created, with which, under some circumstances, as the result of the excessive tension of accommoda- tion, phenomena of asthenopia are combined. It is therefore no wonder that astigmatic persons should feel so exceedingly pleased at- the correction of their anomaly, and should manifest their pleasure in a more lively manner than ordinary ametropic individuals. It has already been shown, that regular congenital astigmatism generally depends upon asymmetry of the cornea, while the cases in which the lens plays the principal part, are among the rarest exceptions. A. — Of congenital regular astigmatism of the cornea we have, from the point of view of refraction in the two principal meridians, distinguished three forms (compare p. 482) : the myopic, the hyper- metropic, and the mixed. Each of these has its peculiarities, which will best appear by describing, after a short introduction, one or more cases of each form. 1. Myopic astigmatism. — Of this we have two forms : a, simple myopic astigmatism. Am, when emmetropia in the one principal meridian is combined with myopia in the other j h, compound myopic astigmatism, M -f- Am, when myopia exists in both the principal meri- dians, but in a different degree. To this latter form belongs, among others, the case of Airy. At first I supposed that myopic astigmatism occurs only excep- tionally. The first cases which I saw, all belonged to the hyperme- tropic form, a few to the mixed. Subsequently, the proportion changed, and now I think it may be established, that in eight cases of astigmatism nearly one belongs to the myopic form. In general, however, it is only, ILLUSTRATIVE CASES. 515 a. Simple myopic astigmatism. — Absolute emmetropia in one of the principal meridians is a condition not easily met -with. Strictly speaking, therefore^ cases of simple myopic astigmatism scarcely occur. But shall wCj where the slightest trace of myopia exists in the second meridian, consider the anomaly to be compound myopic, or when combined with the least trace of hypermetropia, to be compound hypermetropic astigmatism ? This would, it appears to me, not be practical. To the idea of simple astigmatism a certain latitude must be allowed. M == ^ in general requires no correction, nor does H = ro in young persons : a simple cylindrical glass is then quite sufficient. Only it will appear, that in proportion as correction by a simple cylindrical glass leaves a little H or M, as accommodation diminishes, respectively earlier or later than usual, combination with a convex spherical surface will be required for near objects. Case i. Simple myopic astigmatism. — Mr. O., student in divinity, now twenty-one years of age, consulted me three years since. I diagnosed my- opia about = y'g, complicated with amblyopia. The degree of myopia was, bowever, not strictly definable, on account of the diminisbed acuteness of vision. Tbe latter, in fact, amounted to scarcely \, so tbat the patient could distinguisb ordinary print only at a sbort distance, in wbicb tbe ex- isting myopia was very serviceable to him. He bad large prominent eyes, clear media, only a trace of atropby of tbe membranes, on tbe outside of tbe optic disc,— moreover, tbis disc was redder tban is normally the case, without being redder than it usually ap- pears in young myopic persons, who read and write much. He thought that bis power of vision bad latterly diminisbed; but he bad never had easy sight, and had never been able, particularly in the evening, to continue long at a time engaged in close work. Such a disturbance is very common in myopes. But in them it has usually developed itself at a definite time, and as the result of continued work in a stooping position, while the power of vision bad previously been quite sufficient. I have therefore doubted, whether in tbis case tbe cause of tbe amblyopia was to be sought therein, and moreover, derivative means, artificial leeches, cold douches to tbe eye, &c., were employed in vain. Finding no improvement, tbe patient ceased bis visits. Some weeks ago be again presented himself. His power of vision, he stated, was so defective that be feared he should not be able to continue bis studies. He was anxious, before finally making up bis mind, to consult me once more. My former notes were referred to. I immediately sus- pected that astigmatism, previously overlooked by me, must exist in tbis case. 2 616 ASTIGMATISM. In favourable light he saw -vrith both eyes, and with each eye separately, No. I at the distance of from 2i" to 3'. At a great distance the acuteness of vision was, for the right eye, = \, for the left eye, = J. Negative glasses improved the sight, but comparatively little : and yet he preferred a strong glass, about ^, with which the acuteness of vision rose to f. The existence of astigmatism had thus become probable. We therefore proceeded to the trial vrith the point of light (compare p. 454). The right eye saw the point of light as a line /, inclining nearly 30° outwards, and to it, armed with ^ the line appeared stiU longer and slighter ; with ^ and with ^ it was, on the other hand, at the same time broader and complicated with secondary lines. With from — ^ to — ^, the image was nearly round, with — | it was spread out in a direction perpendicular to its primary one. It was, however, rather changeable, so that the glass with which the most slender horizontal image was obtained was difficult to define, the more so, as this image was not a pure line, but a rather compound figure extended in a recumbent position. The eye was now armed with ^ and a negative glass was alternately placed before the positive one, on which the opposite directions formed a cross, which, when the negative glass was —- consisted of the slenderest lines, bg, The direction of both principal meridians was thus ascertained. Through a slit, If mm. wide , held in the direction of the horizontal prin- cipal meridian, the acuteness of vision rose to J ; positive and negative glasses held ia front of the slit produced no further improvement. The same slit, held before the vertical principal meridian, produced no essential advantage ; on the addition of — ^ the acuteness of vision rose to J. As to reading at a short distance, this was only slightly improved by looking through the ' horizontal slit, on the contrary it was very considerably im- proved by looking through the vertical slit. A glass of ^'j c, the axis of the surface being held in the direction of the vertical meridian, caused No. I to be easily read at nearly double the dis- tance, ^c, and J c at more than double the distance ; on the contrary, when the axis of the cylindrical surfaces coincided with the horizontal principal meridian, even No. IX could no longer be deciphered. At a distance — ^c, the axis coinciding with the horizontal principal meridian, was found excellent : with it the acuteness of vision rose, under favourable circum- 3 stances, nearly to 2- The patient had never before had an idea what acute vision was, and felt uncommonly happy. Black lines, inclining 30° outwards, were pretty accurately seen at a distance without glasses, while lines, perpendicular to these, were scarcely recognised as lines. When a glass of — yr- was held before the eye, the greatest sharpness of the latter was obtained, and the first could only with tension of accommodation be tolerably satisfactorily seen. Time did not admit of the radii of curvature of the cornea being measured. ILLUSTRATIVE CASES. 517 That asymmetry existed in this coat could, however, be inferred from the form of the reflected image of a square surface inclining 30". The atrophy of the membranes, observable on the outside of the optic disc, had increased since the first examination. The retinal vessels, running in the direction of the vertical principal meridian, were in the non-inverted image accurately recogniaed by an emmetropic eye ; in order to see those running in the direction of the horizontal meridian in the non-inverted image, a negative glass of about — ^7) "^^^ required. In the inverted image it was difiBcult to establish the difference in distance of the variously directed blood-vessels. The left eye presented a remarkable correspondence with the right. Here, too, Am existed about ^ yjr, and the vertical principal meridian in- clined about 30° outwards \ . A more accurate description appears therefore to be quite superfluous. Epicrisis, or critical remarks.* — The case here described is one of the thousands, in which astigmatism has been looked upon as amblyopia, and has been treated as such. If the aimless and energetic treatment was only vexatious to the patient, his joy, when he found his sight at all distances improved by suitable glasses, was indescribably great. He was accustomed always to hold even large print at a very short distance from the eye, partly in order by looking under a greater angle to make amends for his diminished acuteness of vision, partly, as an associated movement, with the convergence and tension of accommodation, to contract his pupil, and thereby to diminish the difiiision-images. On this excessive tension of ac- commodation required for more accurate definition, depend the phenomena of asthenopia, usually observed in astigmatic persons. Probably the am- blyopia remaining after correction of the astigmatism, is likewise a result of the excessive strain connected with the strongly stooping position so injurious to the eye. — It will have been observed that the patient saw better at a distance with tolerably strongly-negative spherical glasses : this, too, appears attributable to the fact, that the tension of accommodation required in the use of these glasses, gave rise to constriction of the pupil, and thus to diminution of the difiiision-images. I prescribed for the patient provisionally, only glasses of — —-c, with which he saw both distant and near objects satisfactorily. His range of accom- modation amounted to nearly \, and as he had accustomed himself to strain his power of accommodation, it cannot appear strange, that he at first re- quired no other spectacles for reading and writing. However, it is to be expected, that the excessive tension of accommodation will soon cease, and * This term, employed in the original, ought to be introduced into our medical vocabulary; it has the sanction of Galen for its use — ^ rai/ Sia Tns nelpas evpicTKOjiivaiv PorjBrnidTav iirUpiaii. — S. H. 518 ASTIGMATISM. should contimied work then cause some inconvenience, I propose to recom- mend him the use of bi-cylindrical glasses of whereby the astigmatism shall be corrected, and a slight degree of myopia in all meridians shall be produced. Should the acuteness of vision become perfect, of which (now that excessive tension in a stooping position is no longer necessary) there is some prospect, he will not require these latter glasses until a more advanced period of life. h. Compound myopic astigmatism, M + Am. — Of compound myopic astigmatism I have seen only a few cases. It occurs under the form of myopia, compHcated with amblyopia. It is easily recognised according to Airy's method. This accounts for the fact that this form, notwithstanding its rarity, was the first to be discovered. Eyes so affected see near comparatively better than distant objects ; and by spherical negative glasses, which correct the myopia in the principal meridian of greatest curvature, the acuteness of vision of distant objects is, particularly in young persons, improved. Per- fect acuteness of vision at a distance is, however, attainable only by means of negative spherico-cylindrical glasses, while for near objects, when, as occurred in most of the cases observed by n>e, the myopia is slight in the principal meridian of weakest curvature, simple negative cylindrical glasses are in general the best. Case it. Compound myopio astigmatism. — While these sheets are passing through the press, a case occurs to me, remarkable enough to justify its insertion here. Mrs. F., nearsighted from youth, and having in neither eye perfect acuteness of vision, complained some years ago, of occasionally recurring flickerings before the right eye, followed by a very transitory disturbance of vision. About six months since these flickerings returned, but on this Occasion the disturbance of vision was permanent. At first there was a considerable scotoma, in which the yellow spot was included, while, moreover, the superior half of the field of vision in general appeared fainter. After numerous slight modifications in form, during the first weeks after it set in, a small, circumscribed scotoma finally remained, in part absolutely, in part relatively, insensible, with a defined boundary, projecting almbst immediately above the point of direct vision; in the hi-gher parts also of the field of vision a slight degree of torpor remained. The eye reads, however, but not without trouble, while the line above what is read is in great part invisible. The ophthalmoscopic investigation is negative. Certainly nothing ab- normal is to be seen in the portion of the retina corresponding to the sco- toma (bordering inferiorly upon the yellow spot), and whether the smooth, glittering appearance of a part of the optic disc is to be considered as due to atrophy, is at least very doubtful. The left eye gives M about = ,\, but at the same time possesses acuteness ILLUSTRATIVE CASES. 519 of vision of only ^. Ophthalmoscopically, no abnormity was found. Sus- pecting astigmatism, I turned, while the eye was armed with — J, j, c round before the eye, and thereupon, the axis being horizontal, the acuteness of vision immediately rose to f, to descend, when it was perpendicular, below ■^. Trial with the point of Hght gave, with the addition of a weakly positive glass, an extension in breadth ; with a negative glass of — J or more, scarcely any extension in length. Much irregular astigmatism exists. With the aid of the stenopseic slit I found : inv, M = T^.3; in H, M = I, indicating an astigmatism of (g — ^p.^) about ^. At a distance, horizontal strokes are seen better than vertical ; with ^ the horizontal are perfectly defined, and at the same time the vertical ones have become more distinct, attaining perfect distinctness, however, first with — J. "Without glasses No. I is reeogliisBd at the distance 'of aboTit 5". With ^ c, the axis being horizontal, whereby in all meridians M = -J, No. I is read at 8" : the greater acuteness is very striking f the patient emphatically distinguishes between simple recognition and acute vision. The right eye is found to be affected by the same form, and by almost the same degree of As as the left. — -For both eyes I prescribed, (1.) ■^- ^s3 — -^c, asa double eye-glass for distant objects, whereby R = 00, (2.) ^c, whereby R is brought to 9", — very well adajrted for seeing minute objects accurately, and (3.) _ ^ s 3 — jij c, whereby R is brought to lie at 4^ feet ; these glasses, specially destined to look at the model, which is painted, might, at plea- sui'e, be worn as spectacles. I shall, moreover, have a glass ground, in -vyhich tlie cylindrical surface shall represent — ^, and the spherical surface sliall have two foci '{verves a double foyer), acting, namely, in its upper part as — j'j, in its lower as J. Qj, as — ^, By this the astigmatism will be removed, and the farthest point, seen through the upper part of the glass, will be brought to 4^ feA, through the inferior, to l5" or 20" respectively. BemarJcs. — ^I have already (p. 60) stated, that in almost all the cases of abnormal astigmatism observed by me, without exception, the principal meri- dian of maximum of curvature approximated to the vertical position. In the case here described we find the confirmation of the rule : that there is no rule without an exception. Here, in fact, just as with Young, the maximum of curvature coincides neariy with the horizontal meridian, the minimum of curvature with the vertical. The patient had not been aware that she saw less acutely than other people. I therefore think it probable that Yomig also whose astigmatism was of about the same degree, incorrectly ascribed perfect acuteness of vision to himself ; and should many of his accurate observations be adduced as proofs of the contrary, I would state, on the other hand, that my patient drew and painted very respectably. This ap- 520 ASTIGMATISM. parent enigma is easily solved. Myopic persons are accustomed, in order to see more acutely, to squeeze their eyelids close together : the narrow slit thus produced diminishes the circles of difiiision in the vertical dimen- sion. If astigmatism coexist, a narrow slit has a doubly advantageous eflfect, as it allows only the rays to enter, which fall in meridians of nearly equal radius of curvature. Hence it follows that, while ordinary myopes, from narrovsdng the slit, derive advantage only in looking at remote objects, situated beyond the distance of distinct vision, astigmatios find their power of vision improved also for near objects. A very slender slit almost entirely removes the disturbance dependent on astigmatism, and the acute- ness of vision would thereby become nearly perfect, if the disadvantages of diffraction and diminution of light did not intervene. My patient un- doubtedly also made use of narrowing the slit, even in looking at near ob- jects, and the question is whether Young did not also do so. Meanwhile she was loud in her commendation of the advantage obtained from a cylindrical glass : little pictures especially were with it seen much more acutely. And by holding a cylindrical glass before the eye, non-astigmatic persons can easily convince themselves, how rapidly the definition of a picture suffers from that defect. NaiTowing the slit between the eyelids, so universal in myopia, does not belong to astigmatism in general. Especially in hypermetropic astigmatism it is almost always absent. This would appear to have its explanation in the fact, that in this instance, in the horizontal meridian a high dfegree of hypermetropia exists, which, not being overcome by the accommodation, leaves great diffusion in this direction, and therefore affords no advantage. Could the eyelids admit of a vertical slit, hypermetropic astigmatics would undoubtedly have made use of it. The scotoma which exists in the right eye of my patient, is in no way connected with the astigmatism. Respecting the glasses prescribed, little remains to be said : only a word in reference to the glasses a double fo^er. In old people, with acquired hypermetropia, I have often applied them with much advantage, for ex- ample, positive glasses of ^ in the upper, of ^ to J in the lower part. Moreover, in painters, who in the upper part desired and also really needed R ^ 00, in the lower part = 24" to 12'. My patient used — -n, in order to see at a distance ; and in painting, looked over her spectacles. At the same time narrowing the slit, she had her farthest point at only 9". This distance, in one sense too short, made good on the other hand the want of acuteness of vision. I thought, the habit once formed of painting at so short a distance, that the farthest point might be brought to not more than 15" or 20". — So far I had never yet combined a cylindrical surface with a spherical one, a double fo^er. Such a combination cannot, however, in my opinion, produce any inconvenience. 2. Hypermetropic astigmatism. — Most cases of abnormal astigma- tism belong to the hypermetropic form. In general experience has HYPERMETROPIC ASTIGMATISM. 521 shown, that in hypermetropic eyes, even when the astigmatism con- tinues within the hmits of the normal, the asymmetry is greater than in myopic and emmetropic eyes ; and it is therefore not surprising that precisely those eyes are also most subject to abnormal, and indeed to the highest degrees of astigmatism. We shall cer- tainly not be far from the truth in assuming, that of six hyper- metropic eyes, one suffers from abnormal astigmatism, and that the imperfect acuteness of vision so often combined with hyperme- tropia is, in one-half of the cases, in a great measure attributable to astigmatism. The majority of cases belong to simple hypermetropic astigmatism. While, namely, in the horizontal meridian the hypermetropia is con- siderable (from ro to J and more), we find in the vertical if not emmetropia, at least such slight degrees of myopia or of hyperme- tropia, that the limits of simple hypermetropic astigmatism are not surpassed. However, I have also seen numerous cases of compound hypermetropic astigmatism, rising even to H 7 in the principal meridian of strongest curvature with H 5 in that of weakest. — In young persons hypermetropia in the meridian of maximum of curva- ture appears first on artificial paralysis of the accommodation. As to the form of the eye, I would remind the reader, that in hypermetropic astigmatism the radius p,, of the cornea in the hori- zontal meridian is often unusually great. Moreover, we can, in extensive movements of the eye, often satisfy ourselves, that the visual axis is short, and that the vertical axis of the globe is less than the horizontal. The function of the eye characterises the condition as H, with di- minished acuteness of sight. With this a great degree of asthenopia is connected. Por a short while large print can still be read, but soon fatigue, and sometimes even pain, occurs. One of the patients (with ^ _ ^) aged 26, recorded the following : — " My occupation is that of a clerk. The first effort to work was the most painful. There- upon dazzling soon followed, obliging me to shut my eyes, and to keep them closed for some time. After that my work went on some- what better, but I found it impossible to work all the forenoon ; I was constantly obliged to leave off. At the end my eyes were pain- ful, and I felt best when I walked for a considerable time in the open air, out of the sun. In the evening, by gaslight, my work went on at first pretty well, but soon red dazzling came on. I was then 522 ASTIGMATISM. obliged every time to leave off; and with fatigued and painful eyes I returned home." He got A c to work with and to wear. There- upon he communicated to me : — " On using the spectacles I found, even on the first day, an incredible improvement (his acuteness of vision was, in fact brought from ? to ,). Next da,y I expe- rienced no painful affection, and I found it easy to work uninter- ruptedly the whole morning. I saw everything infinitely sharper. In the evening I experienced not the slightest inconvenience from the light. — In the open air, too, when I walk without the spec- tacles, I am free from pain. Spectacles which I had before tried (ordinary spherical glasses), had been of no use to me." In hypermetropic astigmatism the vision of distant objects is improved by positive glasses, particularly in the compound form ; but even when the accommodation is removed by a mydriatic, it appears to be impossible to determine with precision the degree of H. This circumstance in itself leads us to suspect astigmatism. Horizontal stripes are, moreover, seen at a distance more distinctly than vertical ones. Some have of themselves observed this ; some even mention it unasked. In this respect we have here, therefore, precisely the opposite of what we observe in myopic astigmatism : in the latter, vertical lines alone are distinct at a distance. Espe- cially in the simple forms Am and Ah, this contrast is strongly marked. By means of a spherical negative glass we can change Am into Ah ; and by a positive. Ah into Am, the direction of dis- tinctly seen stripes being at the same time inverted. a. Simple hypermetrapic astigmatism. - — Of this I observed a case. Case hi. Am in the right, with Ah in the left eye. — Mr. R. M., burgomaster at O., aged 38, could in earlier years, at intervals, read and write, though only with great exertion. Latterly this has become almost impossible to him. In vain he endeavoured by means of spectacles to remedy this state of things. Very strong light stUl avails him most. The left eye has, at a distance, acuteness of vision of {, the right of ^. He expressly states that the left eye at a distance sees double, and that the hnes of adjoining letters cover one another. I imme- diately suspected astigmatism. "With -,'5 c, turned round before this eye, the acuteness of vision was, in the horizontal position of the axis, diminished to ^, in the vertical it was brought to perfection = 1. The same glass pro- duces no improvement whatever before the right eye : however the gkss may be turned, everything at a distance remains indistinct. And yet it ILLUSTRATIVE CASES. 523 was witli this eye he could read better. That in the left eye about -^ Ah existed, was ascertained by the above experiment ; what was deficient in. the right eye was not yet evident. Strokes having been fixed at a distance, he stated that with the left eye he saw the horizontal ones distinctly ; but he now at the same time discovered, that with the right eye he saw the vertical ones almost quite acutely, while, on the contrary, he could dis- tinguish little or nothing of the horizontal. I now perceived that in the right eye Am must exist, and with — ^ c, the axis being held horizontally, he saw with this eye at a distance almost quite acutely. Hence it fol- lowed at the same time, that with ^ c, in a vertical position of the axis, he should read admirably, and, in fact, with it No. I was seen at the distance of a foot : the acuteness of vision was perfect. He now stated to me, that objects appeared smaller to the unaided left eye than to the right, and on strict inspection it was not to be mistaken, that the left eye was less pro- minent. Thus, by an unusual mode, I had immediately ascertained, tolerably clearly, the condition of both eyes. The systematic investigation, was, however, not neglected. The right eye saw the point of light as a vertical, the left saw it as a horizontal stroke ; by — ^ the first was changed into a horizontal, by -}^ the second was changed into a verti- cal stroke : the principal meridians did not perceptibly deviate from the horizontal and vertical planes. Through a slit If mm. wide, held in a ver- tical direction, the left eye had, at a distance, S = J, with glasses of ^ some- thing still to be improved ; the right only J, with — ^ rising to §. Near at hand, on the contrary, the right eye read very easily through the slit held vertically, the left, under similar circumstances, read with difficulty. Through a horizontal slit the right eye had, at a distance, acuteness of vision of §, which was not to be improved by glasses ; while the left eye saw at a distance very imperfectly, and required glasses of from -^ to ■^. That through the horizontal slit the left eye can scarcely read at all, the right eye only imperfectly, is in accordance with all the foregoing. From the above experiments we now deduced : Sight eye, in v, M = ^ ; in H, E ; Left eye, in v, H = 3^ ; in H, H = ^. In the right eye there was therefore Am -^ without complication. In the left, properly Ah = ^ — j", = ■,!,. With ,!f c on the left, and — .^ c on the right, there was good vision at a distance, and spectacles furnished with these glasses were worn with much satisfaction. The same spectacles were quite insufficient for reading. It appeared that there was only slight accommodation. Consequently it was thought necessary, for near work, to bring the farthest point R to from 18 to 20"- This was accomplished by placing before the right eye a simple cylindrical glass of -^ c, before the left a spherico-cylindrioal, 524 ASTIGMATISM. Remarks. — The above case serves to show how astigmatism may some- times be discovered by a more direct method than the systematic investi- gation . Examination with the ophthalmoscope might also have led directly to the same result : on relaxation of accommodation I saw in the left eye only the horizontal, in the right only the vertical vessels well-defined. How- ever, to save time, it is better for those who have not examined many astigmatic patients, to keep to the rules laid down. But in cases where perfect emmetropia exists in one of the meridians, we may attain our object more quickly by the trial with cylindrical glasses ; such cases are, however, exceptional. On a subsequent visit of this patient, I caused him to look through dark violet glasses, towards a square illuminated opening (compare p. 473 ). What I expected, took place : with the right eye he saw the upper margins blue, with the left the outer margins red ; and on holding before his eyes glasses of — j^, the upper margins appeared to the right eye blue, the outer margins red, while for the left eye both were reversed. It was above remarked, that objects appeared smaller to the left eye than to the right. This does not depend upon astigmatism. Astigmatism caused the vertical dimensions of objects to appear greater in both eyes (compare p. 471), and this, too, was actually the case. On the other hand, that objects were apparently greater to the right eye than to the left, was the result of the longer visual axis in the first eye. In both the cornea had the same form, most probably the same was true of the lens, and the only difference between the two eyes would therefore be the difference in the length of the visual axis. Now where this difference exists, the second nodal point lies in the right eye farther from the retina than in the left, and the retinal images ai-e therefore greater in the first. If no extension of the retina be combined therewith (atrophy of the mem- branes was not seen with the ophthalmoscope), the greater retinal image is also projected greater. By this the difference in magnitude, exhibited by the objects in the two eyes, is explained. On correction with glasses, the magnitude of the ob- jects is altered : with positive glasses the nodal point moves forward, with negative, backward. Nearly always we shall find, that where the eyes are unequal, the placing of R at an equal distance for both, produces an inversion in the magnitude of the images : the eye, which required the strongest negative or the weakest positive glasses, now sees objects small- est. If the difference be great, it may cause disturbance, and the latter must often be avoided at the sacrifice of distinctness of vision in one eye. By placing the glasses very close before the eye, and by correct calculation of the curvature of the surfaces of the glasses, whereby their nodal points, at a similar focal distance, acquire another position, we can paitly counter- act this defect. In the case under consideration, the difference in magnitude of the ob- jects before both eyes was nearly corrected by the cylindrical glasses, pro- vided they were held close in front of the eye. At least no inconvenieifte proceeded from it. Only when the glasses were not close to the cornea, did ILLUSTRATIVE CASES. 525 tte difference in magnitude of the images become rather confusing. The cause is to be sought in the fact, that the hypermetropia of the left eye was not completely corrected by the glasses, and on this side, with the glasses intended for reading, the farthest point continued more remote from the eye. Certainly a slight degree of latent hypermetropia was also present, and, moreover, I gave ^^ s in place of ^ s. In reading, the left eye saw less acutely with the glasses than the right. b. Compound hypermetropic astigmatism has, with a high degree of hypermetropia in the horizontal meridian, usually a relatively slight degree of it in the vertical. In the latter I have rarely found the hypermetropia greater than m ; several times about 4 vi^hile in the horizontal meridian the hypermetropia may amount to \ and more. It thus appears that the compound hypermetropic astig- matism often keeps very close to the simple. It is remarkable, that in general the disturbance of vision was less than should have been expected from the degree of the astigmatism. Usually we find the disturbance of vision not proportionate to the degree of the astigmatism : the size of the pupil, its position in reference to the axis of the cornea, its constriction in accommo- dation, the form, too, of the curvature in the different meridians, the complication, finally, with irregular astigmatism, exercise, all com- bined, a great influence. Case iv. Compound hypermetropic astiffmatism. — Mr. R., aged 18 years, has never seen acutely ; he states that he observes a shadow along the margins of objects ; at his work he has always sought for strong light, and has nevertheless soon perceived symptoms of asthenopia. Notwithstanding this, he has applied himself pretty closely to study. Some years ago he had consulted an oculist, who had referred the disturbance to congenital amblyopiei, and had therefore considered it to be incurable. This opinion found the more acceptance, as his brother (with a normal condition of the right eye) had in the left a disturbance, corresponding to that in the two eyes of our patient. In his parents, and the other children, the acuteness of vision is satisfactory in both eyes. Having been attacked by a slight conjunctivitis, the patient called upon me. This soon gave way ; but it appeared that the acuteness of vision of the left eye was only ?; of the right, for near objects, it was §, for distant, J. Ophthalmoscopic examination showed, in the first place, that the optic discs were as red as the fundus oculi in general, — which capillary hvper^mis-) in consequence of great strain, with the head in a stoop- ing position, is frequently developed, especially in young people. But 526 ASTIGMATISM. at tlie same time I observed that, while the horizontal retinal vessels, in slight tension of accommodation, weve. easUy seen, the vertical exhibited themselves most distinctly mth a lens of |. This held good for both eyes. Consequently there -was no doubt of the existence of astigmatism. On being asked whether he saw horizontal and vertical stripes at a distance equally well, the patient immediately answered, that he had formerly re- marked, that he could distinguish the vertical lines on a target accurately, only when he inclined his head quite horizontally. The radii of curvature of the two corneae were measured, in the first place, in the horizontal plane (h), exactly in the visual line p°, 10° to the nasal side pn', 20° to the nasal side pn", 10" and 20° to the temporal side, pf and pt" ; and in like manner in the vertical plane (v), first in the vi- sual line p°, and subsequently 10° and 20° upwards ps', ps", and 10° an^ 20° downwards pi' and pi". In each place the mean of six observations was taken. The calculated results are as follow : In H In V Right eye, pn" = 9'80 pn' = 8-76 p° = 8-32 pif = 8-24 pt" = 8-61 Zeji eye, pn" = 10-38 pn' — 8-58 p° = 8-38 pf = 8-30 pt" = 8-57 Hence it appears in the first place^ that the cornea exhibits a high degree of astigmatism (the difference of the radii of curvature in the visual line gives for the right eye, As = 1 : 6-374, for the left eye, As = 1 : 6-8) ; secondly, that the eccentricity of the vertical elliptical section, especially in the left eye, is very slight ; thirdly, that the form of the curvature deviates considerably from the ellipse ; fourthly, that the visual line cuts the cornea in a point, which is situated very much to the inside of, and at the same time very much beneath, the apex of the cornea. In the right eye, the accommodation was paralysed by sulphate of atro- pia. Thereupon the acuteness of vision diminished considerably, and im- proved comparatively little by positive glasses = ^ to ^. The diffusion- images of a point of light showed, that the vertical principal meridian in- clined about seven degrees outwards, the horizontal an equal number of degrees outwards and downwards. With ^ *o n the most slender hori- zontal line was seen, from which, however, numerous accessory lines pro- ceeded; with ^ the difiusion-image was a horizontal rhomb <>, with | it was a vertical rhomb y , with J it was a very irregular square figure, bent in at both sides, which, with ^ changed into a vertical line, thickened in pi" = 8-04 pi' = 7-47 P° = 7-30 ps' = 7-08 ps" = 7-82 pi" = 7-59 pi' = 7-43 P" = 7-38 ps' = 7-21 ps" = 7-56 ILLUSTRATIVE CASES. 527 the centre and with ramified extremities, and was slightest with J, held at I' from the eye. Evidently, therefore, there was much irregular astig- matism ; the regular appeared to amount to ,;j — ^= 1 : 5-36. However, we found on examination with the ophthalmoscope, and on trying the cylin- drical glasses, a slighter degree, namely ahout J. Manifestly no accurate result was to he obtained with the linear diffusion-images. The application of the method of determining the refractive condition in the two principal meridians, with the aid of a slit, I had not yet learned. Moreover, I still possessed only one cylindrical glass, and that of J. This glass, held at most 1" from the eye, improved the sharpness of vision from | to §. The ques- tion is, whether with ample choice of glasses no greater acuteness of vision would have been attainable. Probably, in orderwhoUy to exclude asthenopia, this eye would, for proximity, require a spherico- cylindrical glass, while pro- visionally, a simple cylindrical glass is sufficient for distance : for in the range of accommodation of youth, the then remaining total hypermetropia of ^ is easily enough overcome. Of the left eye it is noted only, that it likewise is hypermetropic in the two principal meridians, that the acuteness of vision at a distance amounts to §, with ^ increases to 'j, moreover, that glasses of J c produce a con- siderable improvement. On other phenomena I think it unnecessary to dwell. From what has preceded we are in a position to predict them without fear of being put to shame by the result. A recapitulation and analysis would be only repeti- tion. I prefer recording some points from Case v. — relating to the brother of the above patient, Mr. R., Jr. In the first place, as to the radii of curvature of the two corneas, we found : — In H In v Sight eye, pn" — 9-10 pi" = 8-42 pn' — 8-38 pi' = 8-10 p° = 8-11 p° — 8-10 pif = 8 10 ps' = 8-27 pt" = 8-10 ps' — 8-04 Left eye, pn" = 9-74 pi" = 8-06 pn' = 8-78 pi' = 7-98 p" =8-44 p° =:7-69 pt' =8-61 ps' = 7-85 pf =8-77 ps" = 7-63 A glance at these numbers shows, that the left cornea has a much smaller radius of curvature in the centre of the vertical section, than in the centre of the horizontal section, while for the right cornea the radii of curvature are equal in both directions. In accordance herewith, the acuteness of vision in the right eye is perfect, and for the left amoxmts to only ^, and at the same time the large letters of the same line which are still recog- 528 ASTIGMATISM. nised do not appear to stand regularly in a straight line. Further, the cur- vature of the cornea is in both eyes very irregular : in the first place the radius 20° above the visual line is still shorter than in that line — an anomaly which I had never before met with, either in sound or in astigmatic eyes, and, secondly, in the horizontal meridian of the right eye, the radius of curvature towards the temporal side appears to remain unal- tered throughout a great extent. It is evident, that from the ascertained value, therefore, no ellipses are to be calculated. Further, it deserves attention, that the form of the astigmatism of the left eye quite agrees with that observed in both eyes of the patient's brother. An hypermetropic astigmatism, probably with hypermetropia in both meridians, prevails in this instance. If without glasses a point of light be seen as the most slender horizontal stroke, and with J as the most slender vertical stroke, it is to be expected that with artificial mydriasis a weakly positive glass should be reqtured to see the most slender horizontal stroke, and a glass stronger than J to see the most slender vertical stroke. The direction, too, of the principal meridians, nay, even the peculiar form of the diflTusion- images agrees in both cases, so that neither did the in-egular astigmatism present any remarkable difference. The nearest points at which vertical and horizontal vrires were acutely seen, were determined comparatively for the right and left eye of this per- son : these distances lay, for the right eye, respectively at 9^" and 6J', with glasses of J at 3f' and 3^' ; for the left eye, with glasses of ^ at 8 and at 3i inches. The accuracy with which these distances were obtained, left nothing to be desired ; but evidently the difference of asymmetry was also reflected in these numbers, and it even appears from them, that in the ver- tical meridian the two eyes did not differ much from one another. That the right eye, even in the vertical meridian, has some latent hypermetropia, is, with the great distance of the nearest point of distinct vision, scarcely to be doubted. With a cylindrical glass of -^ c- the disturbance of vision of the left eye was almost entirely corrected, and vice versa, a glass of — ^ c (with opposite direction of the axis) produced a condition in the right eye, nearly agreeing with that of the left. The experiments tried in this respect were remarkable. In particular, in alternate production and correction of the astigmatism in each of the two eyes the difference in form of the objects and in distinctness of the strokes of different direction, were as satisfactory to us as they were surprising to the patient. What has been above said respecting the apparent magnitude of the different dimensions of a square, before and after the crroection of the astigmatism, was in this instance also fully confirmed. I had recently an opportunity of again examining Mr. R., and noted the following : In the perpendicular image the emmetropic individual sees, with relaxa- tion of his accommodation, the horizontal retinal vessels of the left eye ; ILLUSTRATIVE CASES. , 529 with strong tension, or with glasses of ^, he sees the vertical. The optic disc, seen in the inverted image, is extended in hreadth, which is par- ticularly striking on comparison with the optic disc of the right eye. Examination with the stenopaeio slit gives for the vertical meridian H = OT> f°r tlie horizontal H = ^. With the vertical slit, the acuteness of vision rises at a distance from ^ to J, and a remote point of light is with it seen as a point. The horizontal slit produces, as such, no improve- ment, hut, with the addition of ^, it causes the acuteness of vision to rise to i. — The patient prefers holding the slit at from 1" to 1 J" from the eye. — "With ^ o O Vb c the acuteness of vision hecomes ahout |, with i^ c it becomes about ^. 3. Misled Astigmatism.— Amh and Aim. We have remarked, that most cases of myopic, and especially of hypermetropic, astig- matism, differ little from the simple ; the same may he predicated of the mixed, whether we find a high degree of H in the horizontal principal meridian, combined with a slight degree of M in the vertical, or a high degree of M in the latter, with shght hypermetropia in the horizontal. The occurrence of nearly equal degrees of the two forms of ametropia in the opposite principal meridians is an excep- tion. To this the subjoined case most nearly approaches, in which, however, the degree of asymmetry was not considerable. Case vi. — Amh in the left eye. — Mr. V., aged 59 years, has in the right eye S ^ -J, in the left 8 = ^5. The right eye is nearly emmetropic : im- provement of vision at a distance by 55 is doubtful ; — ^ acts injuriously. Experiments with the point of light afford no proof of abnormal regular astigmatism, but indicate a highly-developed irregular astigmatism. From youth, the patient has been unable to use his left eye ; however, there exists neither obscurity nor organic change in the fundus oculi. Positive and negative spherical glasses produce no improvement. The re- flected images of the cornea had suggested the idea of asymmetry. Examina- tion with the ophthalmoscope afforded the proof of it : in the non-inverted image I, as an emmetrope, saw, with some tension of my accommodation, vertical vessels of the retina perfectly acutely; horizontal vessels, on the contrary, appeared, on tension of accommodation, very faint, and on perfect relaxation were not well defined. I hence inferred the existence of myopia in the vertical, and of hypermetropia in the horizontal meridian. On exami- nation with the point of light the principal meridians seemed to deviate little from the vertical and horizontal planes ; the most slender vertical line was seen with ^, the most slender horizontal with — 3',,. The diagnosis was : mixed astigmatism = ^, composed of The cornea more than fully accounted for this : the radius of curvature in 34 530 ASTIGMATISM. the visual line ai&ouiited, in the horizontal plane, to 8'29 mm., in the vertical = 7-69,— indicating ah astigmatism of 1 : 11-67. While (at least by the method with the point of light) onljr Amh -^ was found, the crystalline lens appeared to bompenSate in pstrt for the astigmatism of the cornea. ftuite ih afebotdftnoe with the ametropia in both principal meridians," the left eye sees at a distance vertical lines a little better than horizontal. "With ^ horiiohtal lines are still more indistinctly visible, while vertical lines are acutely seen. Fice verid, with -^ ^ horizpntal lines are very well seen, vertical lines, on the contrary, are only faintly perceptible. The astigmatic lens of Stokes, brought to the action of 2 x si~ri< makes the acuteness of vision at once four times greater, by bringing it from ^ to |. With -fg c, combined with — ^1, it rose above J, and was therefore still better than in the right eye. For distance a flat glass was presiiribed foi- the right eyfe ; for the left a bi-cylindrical glass of ^ c C — w"- F"'" close work, I was anxious, the acuteness of visioh not being perfect, to bring R to 12"i This was effected by means of a spherito-bylindrical glass of j'j s O |^ c : with ^, in fact, E. in the vertical meridian (^ + ^ — n) becomes = 12", and with ^ c JEMn the horizontal is made equal to R in the vertical. Hereby the right eye now acquired simply yi s. The images were of nearly equal magnitude, and the vision Was with both eyes at the Same time vety pleasant; Vision with the left eye was more abute thdn with the right. Semarks: — This base was, notwithstanding thb slight degree of H in the horizontal meridian, referred to mixed astigmatism. I felt justified in this because, with the very slight range of accommodation belonging to the patient's time of life, the hypermetropic could by no means be overcome. The latter must, therefore, necessarily be borrefated, and this would have been serviceable also even in a young person, although the latter should, with — .j'j c, have seen acutely at a distance, and even in reading should have experienced but little ihconvehienbe. The aCuteniBss of vision of the Ifeft eye was, in comparison with the degree of astigmatism, very imperfect. One might have been inclined to connect the important disturbance with the singular circumstance, that the ascer- tained degree of astigmatism was the resultant of a double asymmetry, namelys of a greater asymmetry of the cornea, and of an opposite and slighter asytameti'y of the crystalline lens ; but, were this supposition correct we bould tiot, in my opinion, have expected the considerable im- provement from cylindribal lenses, which we have indicated. Raising the acuteness of vision from ^^ to above I may be considered extraordinary. The diminished acuteness of vision of the right eye also appears to me somewhat enigmatical. It is indeed true that at 59 years of age it is only exceptionally still perfect j but it is equally rare to find it fallen to I, without any perceptible anatomical change. Moreover, it is the rule, that, in astigmatism of the one eye, notwithstanding the most perfect correction, the acuteness of vision of the astigmatic eye continues inferior to that of the other. In this instance the opposite was found. I am therefore strongly inclined to assume, that in the right eye also astigmatism was present in a ILLUSTRATIVE CASES. 531 degree which interfered with the aouteness of vision. No accurate investi- gation was made on this point j nor were the radii of curvature of the cornea measured. The spectacles intended for seeing at a distance could afford the patient no great advantage, commensurate with the trouble of wearing them ; but neither was there any objection to allow their use, if desired. Spectacles for near olgeots were much more important. Leaving out of consideration even the advantage of stereoscopic vision, reading with two eyes is much pleasanter, and, if the acuteness of vision of both eyes is imperfect (provided there be no obscuration), also much easier than with one eye : the acuteness of vision is even perceptibly increased by it. In order to distinguish small objects, still stronger spectacles might be allowed : in that ease, in the com- bination of the spherioo-cylindrical glass, -^ u remains constant and ^ s needs only to be increased, B. We have seen that when the cornea, by itself, produces an abnormal degree of congenital astigmatism, the lens may increase, but commonly diminishes the same. In the latter case, the astig- matism of the cornea continues to predominate ; in the former the action of the lens is weaker, and is therefore to be considered only as accessory. But in a few cases it occurs, on the other hand, that the existing abnormal degree of astigmatism may be said to be dependent on the crystalline lens j and a very remarkable case of this nature is described by Dr. Knapp,* in which the pecuHar form of the crystalline lens was the cause of an astigmatism, in great part regular. Less rarely, an abnormal position is the cause. This condition may, in the first place, be congenital. Numerous cases have been known in which the lens was situated so eccentrically, that the equator passed through the plane of the pupil, and thus a portion of the plane of the pupil remained without a lens. In this case, then, astigmatism exists in a manner to produce great dis- turbance, but it is of an irregular nature, and cylindrical glasses are incapable, in this instance, of producing any improvement (see Irre- gular Astigmatism, in §40). Some cases, however, occur, where the displacement of the crystalline lens is so slight that the latter still occupies the whole plane of the pupil, but at the same time has so oblique a position, as to give rise to a considerable degree of tolerably regular astigmatism. Some years ago, before I was in the habit of investigating the disturbance of function in asymmetry with the requisite accuracy, a case of this nature occurred to me. I shall here merely communicate what was noted at the time. * Archivf. Ophthalmologie, B. viii. Abth. 2, p. 229. 532 ASTIGMATISM. Case yn.— Astigmatism, from congenital eccentricity of the crystalline lens.— Jacoh D., aged 20, called on me on the 24tli of April, 1860 : he had myopia = i in both eyes, S = J in the left, S = i in the right eye. By holding the negative glass obliquely before the right eye, the aouteness of vision could be brought nearly to i. Long visual axis ; on the whole large eyes. In neither, however, was there atrophy of the ohorioidea, but in the left was a white irregularly circumscribed spot, narrower than the optic disc, beneath which it was, and concealing the vessels of the retina. The chambers of the eyes were shallow ; at the same time there was a very marked iridodenosis, particularly at the inferior portion of the iris ; there was good reflecting, but little accommodating, movement of the pupils. The total range of accommodation of the left eye was ^j ; nevertheless, when the myopia was neutralised by ^j, placed at \" from the nodal point, the nearest point in the left eye lay, K being ^ 00, at 17". Through the action of sulphate of atropia, the pupils acquire an appar rent diameter of 8 J mm. The iridodenosis at the same time continues. Now it appears, that a certain space exists between the iris (displaced, it is true, very much forwards) and the crystalline lens, and at the same time that this last lies eccentrically. On examination with the ophthalmo- scope, we see on the outside, a narrow, scythe-shaped red margin around the equator of the crystalline lens ; this bright margin becomes broader when we look a little from within outwards into the eye, but if we pass still more inwards, it rapidly becomes again narrower, and in the right eye even altogether disappears. Evidently, therefore, the outer margin of the lens displaced inwards and upwards lies more forward than the upper and inner margin, particularly in the right eye. The reflected image of the ante- rior surface of the lens is faint in both eyes, difficult to see, situated very near the reflected image of the cornea, and on movement of the flame moving more than the last. The reflected image formed by the posterior surface of the crystalline lens stands at a tolerably great distance, and consider- ably higher than the corneal image. With the phacoidoseope* (conf. the method in Nederl. Lancet, 3rd Series, Part III., p. 242), looking from the outer side into the left eye under an angle of 30° with the visual line, while the flame stood in the line, which on the other side made an angle of 30° with the visual line, the distance between the reflected corneal image and the posterior lenticular image amounted to 3J mm., and the line uniting these images made an angle of 35° with the horizontal plane, in which the visual line, the flame, and the eye of the observer were. On examination with the ophthalmoscope in the inverted image the vessels exhibit in and near the optic disc, with the ordinary movements of the objective lens, in the left eye scarcely any, in the right eye a very considerable parallactic movement (the direction of which, in connexion with the movement of the lens, is not noted). The corncEe, measured in a plane, carried horizontally through the * The author's instrument for measuring the change of shape of the lens in accommodation (see p. 16). — Teanslatoe. ILLUSTRATIVE CASES. 533 visual line, gave, as p°, in the visual line, as pn' and pn', 11" 23' and 22° 46' on the nasal side, and as pf and pt", 11° 23' and 22° 46' on the temporal side of the visual line, the following results (each number being the mean of four measurements) : Might eye. Left eye. pn' = 8-70 8-87 pn' = 8-16 8-16 p° = 8-14 8-10 pf = 8-21 8-17 pf = 8-61 8-50. Hence it appears, that the corneEB have a great radius, that the ellip- soidal curvature in the horizontal plane is very regular, and has a slight eccentricity, and finally, that the axis of the cornea and the visual line nearly coincide. Bemarhs. — I wish only to establish more fuUy, that the crystalline lens, particularly in the right eye, had an oblique position, causing its axis to deviate very much from that of the cornea, and that at the same time a diminished acuteness of vision existed, admitting of improvement by means of an oblique position of the negative lens of — — , whereby the myopia was corrected. Astigmatism therefore existed. And although we must regret, that neither the direction, nor the degree of the requisite inclination of the axis of tbc negative lens were noted, and that therefore the direction and degree of the existent astigmatism thereby corrected are unknown, and although we should, moreover, have been glad to know the curvature of the cornea in the vertical plane, in order to exclude the cornea as a cause of the astigmatism, I nevertheless feel justified in ascribing the observed as- tigmatism to the oblique position of the crystalline lens. The existence of this category of astigmatism is thus proved, and this may for the present suffice. — The different results of measurement, which I have communicated, may subsequently, when it shall be thought desirable for the comparison of more of such oases with one another, be employed in calculation. It also deserves mention, that three elder brothers and one sister of the patient have normal eyes, but that a younger brother, and perhaps also the mother, suffer from the same defect. II. ACQTJIUED EbGULAR ASTIGMATISM. A. Bependmg on the Cornea. — In all the foregoing, acquired astigmatism has scarcely been mentioned. I must acknowledge that, untU a short time ago, I thought it of little importance. Very seldom does it depend on an oblique position of the crystalline lens caused by partial luxation ; and if disturbances of the cornea be its cause, irregular astigmatism is almost without exception to be expected. I therefore supposed, ct priori, that cylindrical glasses would in this case but little or not at all remedy the disturbance of 534 ASTIGMATISM. vision. The result has, however, in many instances proved the con- trary. In a case of a central speck npon the cornea, I performed iridectomy, and obtained a weU-formed pupil, only in the centre admitting some diffuse, but otherwise regularly refracted, light through the cornea. Nevertheless, the acuteness of vision was very imperfect : while the eye had ^ hypermetropia. No. VI could not even with glasses of =-k, be read, The letters had a strange form ; in an oblique direction they exhibited an irregular prolongation. On ophthalmoscopic investigation, the movement of the objective lens appeared to produce a considerable parallax, I tried the combination of a convex with a cyliadrical glass, and the acuteness of vision was nearly doubled. Ordinary print could now be read. — The matter is, a posteriori, evident enough. The existing astigmatism may be resolved into a regular and irregular astigmatism, and after correction of the regular, the irregular causes less disturbance. / Aave found that in many cases in which, on account of opacity of the cornea, iridectomy, or iriddesis is performed, great advantage may be obtained by a cylindrical glass. Let it be tried only, whethir a cyKndrical glass of, for example, -^ c, turned round before the eye, will not produce alternately increased and diminished acuteness of vision; and when the required direction is thus ascertained, it remains only to find out in these cases, to what strength of cylindrical glasses the preference is given. By the more indirect methods above described, this object is, in general, less easily attained. After the extraction of cataract, too, the cornea often acquires a form, which gives great value to the combination with a cylindrical glass. Cylindrical glasses are also often very useful in acquired modifica- tions in the form of the cornea, without the necessity of performing any operation upon the iris. Case vii. — M. Kr., a girl aged 14, has, some years ago, lost the left eye, in consequence of perforating ulcers of the cornea, with subsequent atrophy. On the lower and inner part of the right eye, too, remains a cicatrix: from destruction of tissue and prolapse of the iris. The pupil is thereby drawn downwards and inwards, but it is otherwise unaltered, and only little diffused light enters the eye. Nevertheless, the acuteness of vision leaves much to be desired, and is scarcely improved by the total cutting off of the diffused light. Moreover, there exists a tolerably great degree of myopia, with which, therefore, amblyopia seems to be combined. Supposing that the ILLUSTRATIVE PASES. 535 form of the cornea ini|;ht be the p^use of the diminished aputeness of vision, I made an examination, and found, in fact, that a point of light was geen with -t: ^ as an obliquely vertical, with — J, removed somewhat fupthey from the eye, as an obliquely horizontal line. By the use of the slit, held in one of the two directions, the aputeness of vision was very considerably imr prQve^. With — ^ f mijiute woyk could now be performed close j;o thp eye, which withoyit cylindrical glasses was altogether impossible. I sl;a4 communicate, spmewh^t nfore ii). 6^pta.ilf a case which is cerjiaiiily an extremely rare one. Case virr. — Acquired regulqr astigmatism of the cornea. — J. P., briga- dier in the army, aged 31, complains that for the last two years his sight has become worse and wprse. Perfect acuteness of vision he never h^d, The corneae exhibit, particiilarly on focal illumination, a slight general opacity, stated tp have remained as the result of purulent inflammation whiph set in three days after birth. In the right eye, moreover, the boundary between the cornea and the sclerotic is, in consequence of peripheral spots, not to be determined ; in the centre of the anterior surface of the lens there i^ also a small, sharply-defined, pot elevated white speck. Neither had formerly prevented his entering the service. Now, however, the acuteness of vision had fallen in the right eye to ^, in the left to |, and the patient was no longer in a condition properly to discharge hi§ duties. The ejfisting opapity of the cornea did not fully explain the disturbance of vision. Moreover, without any fresh inflammatory attao]j, the power of vision had become more and more disturbed. It appeared to me thstt the curvature of the corneae was abnormal, — a supposition which was fully con- firmed by ophtljalmpmetric investigation. Tlie result? of the latter were as follow : — ' In H. In T. Might eye, pnf -^ 9-64 pi- ?= 9-69 p° = 8-73 p° = 7-13 pf =? 7-77 ps" =;= 7-38 Left eye, pn" =? 10-97 p^' = 7-59 p" = 8-40 p° =7-25 pf == 8-45 p/ ?=? 7-17 Evidently the radius of curvature is much longer in the horizontal plane than in the vertical. Therefore, although in the form of the curvature much irregularity was observable, arid the experiment with the point of light afibrded no result, improvement was in this instance to be expected from cylindrical glasses : and in fact with a glass of \ c, the only one which I had at the time at my disposal, the acuteness of vision was raised to i. With a more extensive choice of glasses still further improvement would doubtless have been obtained. I regret that an opportunity did not later present itself of examining the patient more particularly by different methods. 536 ASTIGMATISM. Remarks. — The cause of the abnormal curvature of the cornea existing in this ease was not very clearly ascertained. We may, however, with tole- rable probability assume, that the ophthalmia neonatorum had left behind it some change of form, and that this, combined with unequal resistance, whether in consequence of modified intra-ocular pressure, or under the influ- ence of attendant, almost imperceptible, changes of nutrition, had gradually increased. Manifestly the curvature was very irregular. In both eyes the curvature was particularly great in the horizontal plane towards the temporal side, and in the vertical plane at the upper portion, in part it was still greater at 20° from the visual line than in the line of vision itself, although the latter, as direct determination showed, deviated in the horizontal plane only 3i° from the centre. But, notwithstanding this irregularity, the great differ- ence of curvature for the vertical and horizontal meridian was most striking ; and therefore also much advantage was to be expected from cylindrical glasses. In connexion with this subject, it is remarkable that the cornea, if it originally possessed an average curvature, had both in the horizontal me- ridian acquired a longer, and in the vertical a shorter radius. I have said that the case is rare. Usually, in fact, in acquired altera- tions of form of the cornea, whether on account of conical curvature, or of absolute irregularity or inequality of the surface, examination with the ophthalmometer is unattended with any satisfactory result, and it can only empirically be determined, whether cylindrical glasses are of any use or not. B. Acquired regular astigmatism, seated in the lens. — ^Either the acijuired or the congenital displacement of the lens (of which I have spoken at p. 531) may be the cause of regular astigmatism. Often the lens is thereby so much displaced, that it no longer corresponds to the entire plane of the pupil, with which a high degree of irregular astigmatism is then combined. But if the lens assumes an oblique direction in the plane of the pupil, regular astigmatism must be the consequence, and the use of cylindrical glasses wiU then produce improvement. A not unimportant example of this is afforded by — Case ix. — J. S., aged 42 was, four years ago, successfully operated on for cataract in the left eye. Rather more than a year later, he received a blow in the right eye from a bent branch. Up to this time he had had acute vision at a distance with this eye. Now everything appeared to him misty. On ex- amination, I found iridodenosis in a high degree, quivering movements of the lens (established on every strong movement, both in the reflected images and in the lens itself, by lateral focal illumination), and, moreover, a slight degree of myopia. When a glass of — ^^ was held before the eye, the pa- tient declared that he saw as well as formerly. I could not satisfy myself as to the existence of an oblique position of the lens. I therefore regarded his condition as myopia, in consequence of laceration of the zonula Zinnii, ILLUSTRATIVE CASES. 537 and saw in the myopia a reason for Helmholtz's explanation of the me- chanism of accommodation. To the quivering movements of the lens a quivering of the objects, after each strong movement of the eye, corresponded. These quite agreed with those which occurred on little shaking movements of the neutralising positive glass before his lensless eye, and were thus ex- plained. A few months ago the patient applied to me again. The aouteness of vision in the right eye was then diminished. Even with the aid of his spectacles, he could no longer see acutely at a distance. I supposed that the some- what luxated lens was passing into a state of opacity. This was found not to be the case : the lens had continued quite transparent. But I had at once perceived that, since the first examination, the pupil had deviated to the nasal side, so that on this side only a slender margin of iris remained. This slender margin lies deeper than the outer margin, and has a convex curve forwards : the pupillary margin is directed backwards ; thence the curve is formed, and the marginal portion deviates again in such a manner backwards, that it is most probably somewhat prolapsed through the lace- rated zonula. To this relation of the iris an oblique position of the lens corresponds. On the temporal side, the latter is almost in contact with the iris ; on the nasal side it must therefore lie much deeper. After instillation of sulphate of atropia a considerable dilatation of the pupil occurs upwards, downwards, and to the temporal side, so that the lens comes to lie more in the centre. Looking in an oblique direction into the eye, we can, however, nowhere suc- ceed in seeing the equator of the lens. The centre of the vortices also appears to correspond nearly to the centre of the cornea. With the oblique position therefore there is no, or only slight, lateral displacement of the lens. The acuteness of vision is only about = f . This diminution is ascribed to the oblique position of the lens, and consequently to astigmatism. Nume- rous experiments were performed, which also established this point. I shall mention only some of them. With — -j^ the patient sees, at a distance, ver- tical, with — ^ horizontal lines most accurately. Even with — ^ the horizontal lines are tolerably well defined, but the vertical exhibit a shadow, which nearly disappears when the nasal portion of the pupil is covered. — The point of light under no circumstances appears as a line of light, but is, on the contrary, always double. With — ^, each of the double images is smallest. With — ^ they lie above one another, with — ^, close to one another. The astigmatism is, on account of these experiments, estimated at ^ — 1 — 1 Memarks. — In the observation of this case no cylindrical glasses availed me. It may, however, he inferred, that with a spherioo - cylindrical o-lass of M s 3 — ab '^ (M^^ "■^'^ °^ ^^^ cylinder having a vertical direc- tion) K in all meridians should be about = no . Such a glass might be useful as a spyglass. For near objects it would not answer, because the range of accommodation, which at the time of the former examination was still tolerably considerable, was now reduced almost to nothing. For reading, 338 4STIGMATISM, B ipust thprefore be brought to about 12?: ttj? ^ould ])e effepted by plaping 3^ c, with the axis directed horizontally, liefore the eye. The cause of the astigmatism in this instance lay clearly exclusively in the lens, the measurenieiits of the cornea even rpvealin^ an unusual sym- metry. In H. ifn V, pn" = 8-64 pi! = 8-30 pn = 7-94 pi' = 7-98 p° = 7-74 p° :*= 7-74 'pf = 7-74 ps' = 7.76 pif = 8:09 ps" = 8-09 The visual lipe deviated 5" inwards from the apex of j;he cornea. It is reniarkable, that the astigmatism ^^penjient on an oblique ppsition of the lens gave rise to diplopia, which in ordinary cases depending on asymmetry of the cornea, is not so expressly indicated. The diplopia ap- peared most distinctly in Ipoking at a point of light. We have to suppose that the sectors qf the lejis formed four images clearly di§tii)guishable, which, in looking with the naked eyp, had already decussate^. With — ^ they were situated over one another, with ir^ -r those adjoining onp another were brought to a fopus. In the first case the acuteness of vision was im- proved, when the inner or outer half, in the latter when the superior or inferior half of the pupil was covered. In former years it often occurred to me to observe, particularly jn myopes, that of a point of light two or three imageg were seen, which with too weak and with too strong glasses, stood in opposite directions in one line. In these cases no other stripe? ^vere acutely seen, than those corresponding to one of the two directions. Evidently, as agrees also with the above observa- tions, the manifold images must cover one another in the stripe, if the latter is to be acutely seen. Whether in these cases the astigmatism depended on a congenital oblique positipn of the lens, I did not at the time investigate. I hope to find opportunity to do so. On many remarkable points, peculiar to this case, I will not dilate. Only with respect to the myopia I would observe, that this seems to have arisen as the result of laceration of the zonula Zinrui, notwithstanding that the falling backward, as such, of the crystalline lens, must have given rise to the opposite, that is to hypermetropia. On the first examination, when no oblique position of the crystalline lens had as yet occurred, I had not been able to discover any atrophy of the ohorioidea ; and the patient stated that with the aid of a weakly negative glass, just as before, he could again see quite acutely at a distance. Now, however, a narrow atrophic meniscus had become visible at the outside of the optic disc, which in any case is strongly in favour of the view, that a slight degree of myopia had originally been present. HISTORICAL IlEMARKS, ^39 JifOTE. HisTOEY OB Dxra KupwiEBeE of fiEapLAE AsiiejiAiisM;. In Mackenzie's justly-celebrated book (A Practical Treatise on the Diseases of th3 IJye. Jjondon, 1854), and still more completely in the excellent IFrenph edition by Warlomont and Testelin (Traite pratique deg Maladies de rqsil, par Mackenzie. Paris, 1856), we really find almost everything comprised, which science, up to the dates of the publication of thjase \yorks, possessed upon the subject under consideration. From them I h^ve for the most part become aoq^uainted with its literature, and whenever I had no opportunity of consulting in the original the works mentioned in them, my friend Mr, Halke, of London, with great readiness ^nd iii the most obliging manner, consulted them for me, and sent me accurate p^tracts fronj them. It is remarkable that y?6 find the subject treated of almost exclusively in English literature. In the first place we meet with two men, of whom England m^y well boast ; Thomas Young, who discovered normal astig- matism, and the Eoyal Astronomer Airy, who first recognised and described the asymmetry of his own eye as a defect. Eespeoting Young's observation, I have above (compare p. 4§6), already stated what is necessary, in connexion with other investigations relating to the subject of normal astigmatism. Airy's case (Transactions of the Cambridge Philosophical Society, 1827, vol. ii. p. 267), on fhe contrary, described in a manner worthy of the great master, must here occupy us more fully. It relates to a high degree of compound myopic astigmatism. Thus, apcording to his method. Airy could determine the farthest point of distinct vision in the two principal meridians, and ^t the same time the direction of the letter: in the- vertical (with an inclination of 35") B. was == 3-5'', in the horizontal, R = 6". Hence he calculated the glass required for correction, and also stated the reasons, why a negative spherico-cylindrical glass is to be preferred to a negative bi-cylindrical one. Many years later he again described his state (Id., 1849, vol. viii. p. 361). At this time, the farthest point lay in the vertical meridian at 4"7" in the horizontal at 8-9". His myopia had therefore decreased in both meridians, and at the same time the astigmatism appeared to have undergone some diminution, — from --^ to — . But Airy himself supposes, that the farthest point in the vertical meridian might lie somewhat nearer than 4'7", and he is inclined to infer that his astigmatism had continued unaltered. With a man like Airy we may assume, that he observed for his farthest point with unalterd accommodation ; otherwise we should venture the supposition that, in his earliest observations, in consequence of accom- modation on the approach of the point of light, the myopia in the vertical meridian had turned out too great, whereby the recession of the farthest point with the increase of years (which certainly with the existing degree of myopia is extremely rare), should only apparently have taken place. 540 ASTIGMATISM. Airy's observation seems at first to have attracted attention only at Cam- bridge : to Stokes ( The Report of the British Association for the Advance- ment of Science for 1849, p. 10), namely, we are indebted for the astigmatic lens for determining the degree of astigmatism, and Dr. &oode {Monthly Journal of Medical Science, Edinb. 1848, p. 711, — and Transactions of the Camb. Philosoph. Society, vol. viii. p. 493), who studied at Cambridge, first communicated some fresh cases of this anomaly. Just like Airy, he had astigmatism in one of his eyes, to which his attention was directed by the observations of the Astronomer Royal on the subject. From the compli- cated changes of form, which, according to his accurate description, a point of light underwent at different distances from his eye, it may be inferred that the asymmetry was combined with a high degree of irregular astigmatism. As to the regular, the distances of distinctness in the two principal meridians lay about at 6-13 and 25 English inches. Chamblant, the optician at Paris, prepared for him a plano-eylindrical lens, the cylindrical surface of which was ground with a radius of 9' concave. Goode states that, with the aid of this glass he saw both near and distant objects acutely. In a second case a point of light appeared as a horizontal line at 37 centi- metres (about 14J English inches), without at a greater distance being replaced by a vertical one. Horizontal stripes were not seen distinctly at 37 cent, or upwards ; vertical stripes at no distance whatever. A plano- oylindrical glass, with 2^" radius of the convex cylindrical surface, was too strong, with 3" radius it was too weak. A bi-cylindrical concavo-convex glass, with intersected axes, the concave surface having 74', the convex 44" radius of curvature, would probably have answered the purpose. In a third case a point of light appeared at 35 centim. as a trans- verse line, and at a greater distance was indistinct. At a similar distance horizontal stripes were acutely seen, and a little further off a vertical stripe. A piano-cylindrical concave lens of 16" radius produced a consider- able improvement. Goode found three other gentlemen in the University of Cambridge, whose astigmatism in one eye was improved by a piano-cylindrical lens of 12" radius. These cases, much as we value the communication of them, show that Airy's method was insufficient, accurately to determine the farthest point in the two principal meridians, and in general the degree of astigmatism. The seat, too, of the latter was unknown to Goode. Even in the highest degrees of astigmatism he could not satisfy himself as to the asymmetry of the cornea, and he was therefore inclined to seek the cause in the crystalline lens. A case of abnormal astigmatism was almost at the same time communi- cated by Hamilton in the same journal (Monthly Journal, 1847, p. 891). In this instance, torpor of the retina existed as a complication, as it appears, without limitation of the field of vision. As to the astigmatism, it was characterised by distinctness of horizontal, and indistinctness of vertical lines. If I understand the case aright, vertical lines were acutely seen at a shorter, horizontal lines at a greater, distance, and a plano-concave cylindrical lens, placed before the eye with a vertical position of the axis. HISTORICAL REMARKS. 541 produced an improvement. Dr. Thompson found the vertical diameter of the cornea somewhat greater than the horizontal, and he thought, moreover, that the horizontal meridian was somewhat more highly curved. Further, the cases are known which are appended by Hays to the American edition of Lawrence's work (Lawrence On Diseases of the Eye, edited by J. Hays, Philadelphia, 1854, p. 669). The first is that of a clergyman, whose description affords an excellent picture of simple myopic astigmatism. With the naked eye he saw vertical, with a concave glass horizontal lines distinctly. That he did not perceive both lines equally acutely, escaped him, until, by the use of negative glasses, the distinctness was inverted. After an able analysis of his case, the patient came to the conclusion, that he should need a spheroidal or cylindrical glass for correction, but he did not venture to decide whether it should be convex or concave. Hays' note states merely that M'Allister, the optician, ground for him a piano-cylindrical (positive or negative ?) glass, and that vision was remarkably improved by it. " We have,'' continues Hays, "within the past year seen two cases in which this defect of vision existed. " The subject of the first was a lady, sixteen years of age, who consulted me in consequence of her vision being so defective as to materially interfere with her education. I accompanied her to Mr. M'AUister's, and found that, with the assistance of a double concave lens of high power, she could read sufficiently well with her left eye ; but none of the ordinary glasses, either concave or convex, would enable her to distinguish ordinary-sized letters with her right eye. I then instituted some experiments to ascertain, if possible, the cause of this defective vision. Having drawn two bold dark lines of equal length, crossing each other at their centres, at right angles, and shown them to the patient, she was able to see them sufficiently well to state that the perpendicular line appeared to her longer than the horizontal. Mr. M'Allister furnished me with some mathematical diagrams, which, being shown to the patient, she stated that circles appeared to be ovals, the circles appearing elongated perpendicularly. Various other trials were made, all, however, tending to show that objects seemed to her to be elon- gated in their perpendicular, and shortened in their transverse diameters. Mr. M'Allister, having fortunately some lenses, plane on one side, and with a concave and cylindrical surface on the other, I soon found one which corrected the distortion. I had prepared for her spectacles with a double concave lens of the proper number for her left eye, with a plano-concave cylindrical lens for the right eye, with which she can read ordinary print with either eye, and still better when using both eyes. " The second case occurred in a gentleman, about fifty-five years of age, who consulted me for an inflamed eye, about which he was very anxious, as he stated it was his best eye, the other having been always so defective as to be nearly useless. On examining into the nature of this defect, I found that it was similar to that in the preceding case, except that objects were elongated in their horizontal diameter." Besides the above cases, only one more has been recorded, which was observed on the Continent of Europe. It was described by Pastor Schnyder, 642 ASTIGMATISM. bf Menzbutg (Switzerland, canton of Lucerne), *ho discovered ttiS d,nomaly in his own person {yin7i. d'OcuUstigue, t. xxi. p. 222. Bruxelles, 1849 — taken from the Verhdndluiigen der Schweizerlichen NaturforicheHderi Gesellschafl — to which I have not an opportunity of referring). He was nearsighted for vertical, farsighted for horizontal lines. For correction he used bi-oonvex cylindrical glassesj combined with bi-concave spherical; I find no record of what the fofcal distance of the glasses was. Mr. Schny der had no other means of discovering the defect than the fact that horizontal artd vertical wires were not accurately distinguished ki the same distance. In order to determine what glasses were required, he seems to have tried which he needed, in order to see acutely horizontal and vertical wirei, placed at the same distance. I might here close the history of our knowledge of aitigmatism. At least, no (jther fact^ have come under my cognisance. It; however, occurs to me that something further should be stated as to what has been assumed or sus' peeted by diflferent writers respecting the seat of the anomaly. As might he expected from Airy,-^in the absenije of satisfactory reasons, he has wisely refrained from Expressing any opinion as to the seat of the asymmetry. He appears also to have used no eifort, to attain to certainty respecting this point. Goode, on the contrary states, that in a case of highly- developed astigmatism, he in vain endeavoured to satisfy himself, from the fbrni of a reflected image, of the existence of peculiar asymmetry of the Cornea, and he justly addS that he was therefore inclined to look for the seat of the defect in the crystalline lens. In the case described by Hamilton, Dr. Thoinpson examined the cornea, whose vertical measurement he found somewhat greater than the horizontal, " being shaped somewhat irregularly, and the diameter projecting slightly upwards and inwards." Hamilton adds, "Dr. Thompson thought he per- ceived a somewhat more marked curvature of the cornea in the transverse diameter." According to what method the examination was made, is not stated. The result; however, renders it probable that the cornea was con- cerned in the asymmetry. Wharton Jones (Manual of Ophthalmic Medicine and Surgery^ 2nd Edi- tion, London, 1855, p. 362), and Wilde (Dublin Journal of Medical Science, 1st Series, vol. xxviii. p. 105) go still further. Without more accurate inves- tigation, they assume, that the foundation of astigmatism is really to be sought in the cornea. Both put it prominently forward as an established" fact, that the cornea, in its vertical meridian, has a shorter radius of curvature than in the horizontal, and they explain Airy's case (they themselves observed nocases) by a peculiar development of that difference. What W. Jones (Cyolopsedia of Practical Surgery, Art. Cornea, p. 832) describes as "a case of cylindrical de- formation of the cornea, produced by injury," does not apply here. Whence they derive the proof of the fact put forward by them, is a puzzle to me. So far as I know, previously to the date of the communications of Jones and Wilde, the radius of curvature of only one cornea was determined in the ver- tical and in the horizontal meridian, — viz., by Senff (conf. Volkmann, Art. Sehen, p. 271, in Wagner's Handworterhuch der Physiologie, 1846). In this instance the radius was found scarcely shorter in the vertical meridian. How little importance, moreover, was to be attached to one observation, appears iRtlEGtJLAii Asf tbMAti^M; Ui from tte investigition of Enapp, wKo found in the majority 6i his measure- ments the radiu^ longer in the vertical mferidisln. It was; therefore, not until after the riunieroiis mea^ut'ements made by lis, that we ventured to assert, that the hcirizontal meridian usually has the longest radius. — A little work, how- ever, which I foiind quoted in more thdn one place; I have been unable td consult; I mean Grerson {Def&rma coi-nece, Gottiiigen, 1810). Mackenzie (l. b. p; 926) borrowed from it the fact, that Fischer could hot at the same time distinctly see horizontal and vertical lines; Here, therefore, astigma- tito is in question; This circumstance, taken in connexion with the title of the work; leads us to siippose, that Q-erson sought the seat of the defect in the corilea. Might we perhapS find in it reasons foi' the assertion of Whartoii Jones and Wilde ? It seem^ to me that it is not to be supposed that the foi-m 6i the cornea was satisfactorily determined by Gl-erson; — aiid still less, that subaequeiit writers would not have mentioned it. Wharton JTones gave hi§ explanation only as a supposition. Wilde, on the contrary, says ex- pressly, " It is well known that the cornea is not a correct surface of revo- lution, bilt that the curvature of Its horizontal plane is less than that of it^ vertibdl. When this exceeds the normal extent, it gives rise to irregular refradtion; causing a circle to appear an oval," &c., &c. Wilde was so con- vinced of the correbtness of this opinion, that he did not hesitate to replace the name Of cylindrical eye; chosen by Wharton Jones, by that of ' cylin- drical cornea.' That, with some exceptions, Wharton Jones and Wilde approached th^ truth, has appeared from our investigation; But do they deserve credit fOr this ? In my opinion; Young's observation on his own eye should rather havd led them to look for the seat of the defect iii the crystalline letis, and so long as the asyrdmetry of the cornea was not satisfactorily ascertained b5^ measurement, it Appeared more philosophical to keep to that view. Thei^ assertion was therefore a very bold one. We see that in science also thS quotation is sometimes applicable; that " 'audace'sJbrtuhaJui>At.'' § 40. liREGuilAR A§TiGMATis]si:. Irregular astigmatism iaa.j; as well as the regular, te divided intd normal and abnormal. The noimal fofm is connected with the structure of the lens ; the cornea does not participate iti producing it. The abnormal degrees, on the contrary, which considerably disturb the power of vision, may depend upon irregularities of the cornea as well as upon those of the lens. We commence with normal wregular asiigmaiism. The principal phenomenon attending this irregularity is known under the name of polyopia monocularis. With some attention any one can observe this polyopia in himself. Eyes in which, the lens being present, it should be entirely wanting, are undoubtedly rare exceptions. The 544 IRREGULAR ASTIGMATISM. following experiments will be sufficient to satisfy us of its existence and of its importance. 1°. Let a small black spot, on a grey or white ground (as .), be gradually brought nearer to the eye than the distance of distinct vision : most people will then observe that the black spot passes into a circle of greyish spots, which, when the spot is removed from the eye, again approach one another, and, at the distance of distinct vision, coalesce to form a black spot. It is desirable in this and in the following experiment to keep our eye, without alteration, ac- commodated for the farthest point, in order that the magnitude of the pupil may continue the same ; we must therefore, if not myopic, arm the eye with a positive glass, say of 5 — -ns, in order thus, while the eye continues relaxed, to be able to bring the point to either side of the distance of distinct vision. If we subsequently carry the spot beyond the distance of distinct vision, and for the greater correctness of comparison we may for the greater distance, take a proportionately larger point (as ,), several spots usually again appear ; but in this case a central darker spot remains, around which the other paler spots are more or less regularly grouped. This central spot was absent when the black spot was nearer than the distance of distinct vision : on this account with equal deviation of accommodation we distinguish better (the diameter of the pupil being assumed to be unchanged), when the eye is accommodated for a too near, than for a too distant, object. %°. We may repeat this experiment with a white spot on a hlach ground. As white spots, we may make use of small granules of whitelead, got by scraping an ordinary visiting card, and spread upon black velvet. Among these granules we find a great variety of sizes. If we take one of the largest, of about \ mm. in diameter, the experiment will yield nearly the same results as were obtained with the black spot. It will then, however, more distinctly appear, that each spot is radiatingly elongated, and exhibits dispersion, — with the blue turned towards the centre, when the spot is nearer than the distance of distinct vision, with the red towards the centre, if the point Kes beyond it. 3°. Let the experiment be repeated with one of the smallest granules. The radiatingly elongated spots have now given way to slender rays, which, when the granule lies nearer than the distance of distinctness, do not run together in the middle, and which, on the contrary, have a white spot in the centre, when the object is beyond the distance of distinct vision. CAUSE OF POLYOPIA UNIOCULARIS. 545 4°. Let the observer look at a little point of ligMj for example, at a small opening turned towards the light, but still better (in order to avoid diffraction), at a small image of hght formed by re- flexion. The phenomena are then observed, in proportion to the magnitude, precisely as they have been described under 2° and 3°. By these experiments we have now learned that polyopia, in looking at a small object, is the same phenomenon as that of the rays, under which at a distance a bright star or light appears, for which the eye is not accommodated. To each principal ray corresponds one of the marginal spots, ander which the black dot appears. There- fore, too, those have the most distinct polyopia, who in a point of light perceive a comparatively smaU number of distiactly sepa- rated rays. 5°. Let a small point of light, a little reflected image or an opening of g mm. (0-00492 of an English inch) in a metal plate, turned towards the sky, be gradually brought near the eye. Having arrived nearer than the distance of distinct vision, the point of light divides into a certain number of bright rays, and even, when it has reached the anterior focus, the circle of diffusion in the retina being as large as the pupU, the rays are still visible : they are the lines of light of the well-known entoptic image (compare Kg. 103, p. 200), which occurs in most eyes under this form. The transition of the bright rays into the lines of light of the entoptic image is very easily observed. While the light in the entoptic luminous circle, attain- ing on the retina the magnitude of the pupil, has been more uniformly divided, the few very bright rays, of which the image of a star almost exclusively consists, are therein only faintly repre- sented by the said lines of light. The number and direction, how- ever, remain precisely the same. Prom these experiments it appears, that polyopia uniocularis, rays of stars, and radiating lines of light in the entoptic spectrum are dependent on the same cause, rest upon the same pecuharity in the structure of the eye. If, therefore, we have ascertained the source of one of these phenomena, we know that of all. Now we have already, in treating of the entoptic phenomena, seen that the lines of light of the entoptic spectrum (compare Kg. 103) are to be sought in the crystaUine lens : on moving, in fact, the eye behind the point of light, no parallax is perceptible, and their cause therefore lies nearly in the plane of the pupil, and consequently in 35 546 IRREGULAR ASTIGMATISM. the lens. Hence it follows^ that both the rays emerging from points of light and the polyopia, have their origin in the lens. This is more decisively proved by the circumstance, that all these phenomena are wanting when the lens is absent from the eye (aphakia) . Moreover we can now further show that the cornea has no essential part therein. In the first place, in examining the reflected images of the cornea, if such irregularities were here present, as are required for the explanation of the phenomena in question, they must have been apparent. And, in the second place, I have excluded the action of the cornea by plunging my eye into water in a little bowl, bounded by a convex glass replacing the cornea, and the phenomena have then continued under the usual form. If the cause be thus situated in the lens, the question suggests itself, how these phenomena are to be explained by it. In the first place we observe, that the form of the rays, under similar circum- stances perfectly constant for each eye, immediately reminds us of the pecuhar structure of the lens, namely of the radiating figure from which its fibres proceed. Those of the anterior surface we can observe in any one in the living eye, by lateral focal illumination (Helmholtz), especially by employing a lens, and better still with the aid of the phacoidoscope (compare p. 16). The lines of the posterior surface differ in form and in direction. Meanwhile the crystalline lens is by those lines divided into irregular sectors. Now the explanation of the polyopia is this, that each sector forms a sepa- rate image. The proof of this I have given twelve years ago, by moving a rather small opening (about i mm., or 0'01968 of an En- gUsh inch, in diameter) before the pupil, while the multiple image of a point of Hght falls upon the retina. We thus see a simple image when the opening corresponds to a given sector, and when by shifting the opening we come to the boundary between two sectors, two faint images appear, of which, on further displacement, that first seen dis- appears, while the one which has supervened remains alone andbrighter. On more rapidly moving the opening it appears as if the little image of light jumps, which really happens in the transition from one sector to another. — ^In proportion as we accommodate with more precision, the multiple images approach one another, and finally coalesce into one image. However, even with the most perfect accommodation, they do not exactly cover each other. In the first place, regular astig- matism, and in so far the cornea also, here plays a part. This regular astigmatism manifests itself precisely by the fact, that the images CAUSES OF THE ANOMALY. 547 placed opposite to each other more speedily reach each other in one direction than in the opposite. The result of this is, that a point of light always appears somewhat angular, and even a black spot undergoes a pecuhar change of form on a slight play of accommo- dation, without at the same time ceasing to be black. But even when we completely correct the regular astigmatism by means of a suitable cylindrical glass, all the multiple images do not meet pre- cisely in one place : in one direction or another a single one projects beyond the rest into the centre, and accurate consideration of a point of light shows that all have not even their focus exactly in the same axis. In this, therefore, lies, in the first place, an element of irregular astigmatism. A second element we find in the image of each sector in itself. It is very diificult by experiments to get a correct idea of the image of each sector. The impression of light on each point is, in fact, not proportional to the strength of the light, and consequently we ob- tain a different result with respect to the distribution of light, in pro- portion to the brightness of the light with which we experiment. On repeating the experiments above stated we had abundant opportunity to satisfy ourselves of this. While, for example, a bright fixed star (Sirius I have often taken as the object), with slightly hypermetropic arrangement of my right eye, gives seven or eight extremely fine bright rays, partly ramifying towards the periphery, and terminating at a short distance from the centre, a less bright luminous point appears rather as a circle of spots, with comparatively very strong illumination in the periphery, about agreeing with the circle of spots under which, with a similar arrangement, we see a black spot. The rule, however, remains with each illumination, and also in experi- menting with a spot of a certain extent, that each image is elongated in the direction of the rays of the whole circle of difl'usion, so that we can in it distinguish an outer and an inner margin, which last is turned towards the centre of the circle of diffusion. We can now further satisfy ourselves : — 1°. That the image of each sector is astigmatic. Through an opening of about 0'5 mm., held before a given sector, a fixed star, which under the greatest magnifying power remains a point, forms, with the most perfect accommodation, an image on the retina, which, were it accessible to our investigation, would certainly be very per- ceptible. The light of a lantern, seen at a great distance through a single sector, is nearly as great as if we had approached it by half 2 548 IRREGULAR ASTIGMATISM. the distance. By using monochromatic light, the astigmatism of each sector remains unmistakeable. 2°. That the image has an aberration, agreeing with the spherical aberration. In the circle of diffusion, formed by a spherical lens from a monochromatic point of light, the light is not uniformly dis- tributed. Before the focus of the rays (as both construction and direct experiment readily show) the illumination is strongest at the outside, behind the intersection, in the centre of the circle of diffusion. Now the same is true of the light, refracted by one of the sectors. Therefore a point of light, and even a little luminous spot, seen through the whole lens, is more strongly illuminated in the centre or at the circumference, accordingly as the eye is accommodated for a shorter or longer distance than that at which the spot is. We have already remarked that, consequently, the magnitude of the circles of diffusion being equal, the acuteness of vision suffers more when the retina lies behind than when it lies in front of the focus. We have thus indicated two causes of normal irregular astig- matism, namely, the imperfect coincidence, even after accommodation, of the images of the different sectors, and the astigmatism proper to the image of each sector in itself. In the image of each sector we can, moreover, easily recognise the chromatic aberration. In front of the intersection each image is red at the outer, and blue at the inner margin; behind the intersection the outer margin appears blue, and the central light is reddish. Now, if we examine the multiple images of a thin line, the lateral images have also coloured edges, and only the central image is uncoloured. It is very instructive, as Helmholtz has done, to combine a Hne with a point, placed'near the extremity of the line. We now see, with imperfect accommodation, especially with adjustment for a greater distance, different lines close to one another, and we can satisfy our- selves that these, in each direction of the line, correspond to the multiple images of the point of light to which they are directed. It is now evident, that the edges of the central line will be colourless, because the radiating elongation of the sector-image, whence it arises, lies in the direction of the line, and the colours thus fall over one another. We obtain the sharpest, brightest, and most achromatic line, by giving the line such a direction, that two opposite radiating sector-images cover one another on the line. On the contrary, the lateral, images of the line have coloured edges, and are at the same time fainter and broader : their section is equal to the longi- tudinal section of the elongated image, to which they corre- OBSERVATIONS OF DE LA HIRE, YOUNG, AND LISTING. 549 spond. — The phenomena here described may also be observable at the boundaries of brightly illuminated surfaces, with imperfect ac- commodation, as the transition from the bright to the dark takes place through two or three degrees. Even with perfect accom- modation, some can satisfy themselves by the fact, that they see the bright moon as images covering one another. I was parti- cularly struck with the distinctness and well-defined boundary, over the whole surface of the round images covering one another, of an opening, through which the nearly homogeneous light of the flame of alcohol containing salt was seen. But more especially when the accommodation is not perfect, we see in that experiment a number of circles, for the most part covering one another, and by covering a portion of the pupil we can never make one of these circles partly disappear — cut a segment off it : the circle only grows faint, to dis- appear suddenly and completely, when the whole sector of the lens belonging to that circle is entirely covered. It needs scarcely to be remarked that, on covering the pupil, the figure, as a whole, disap- pears on the same side, when the eye is accommodated for a nearer object, but on the opposite side, when, on the contrary, it is accom- modated for a more remote point. In the foregoing the cause of the peculiar distribution of light of the circles of diffusion was found in the crystalline lens, and was in general terms brought into connexion with the radiating figure, from which the fibres of the lens proceed. The perfect explanation is, however, not thus attained. The multiple images of a point were seen so early a? by de la Hire.* Thomas Young t examined the circles of diflusion of a point of light at different- distances, even delineates them, and said respecting their cause : " The radiating lines are probably occasioned by some slight inequalities in the sur- face of the lens, which is very superficially farrowed in the direction of its fiibres." Listing J discovered that, in many persons, in the entoptic spectrum of rays of Hght, which, proceeding from the ante- rior focus, run parallel in the vitreous humour, some bright lines, mostly in the form of an irregular star, occur with some off-shoots, which he looks upon as the image of an umbihcal body with suture- like and elevated ramifications, derived from the separation, in the foetal state, of this part of the capsule from the inner surface of the cornea. * Mimoires de I'acadSmie de Paris, 1694, p. 400. f- Philosophical Transactions for 1801, i. p. 43. t Beitrag zur physiologischen Optik. Qottingen, 1845. 550 IRREGULAR ASTIGMATISM In examining the entoptic phenomena I found* that multiple images, lines radiating from points of light and the entoptic star of Listing pass imperceptibly into one another, and therefore have one and the same origin ; but respecting the proper cause in the structure of the lens I could form no satisfactory idea, and even now I have not been sufBciently successful in my attempts, to make known their result. Further investigation on this point appears to me to be necessary. The astigmatism, of which we have thus far spoken, may be con- sidered to be normal. The acuteness of the power of vision suffers very little under it, and least of all when we look with both eyes together, and when these have about the same refraction. We never find the astigmatism of both eyes exactly equal. The images of the same point, formed on the two retinas, therefore, deviate a little from one another. Both, however, coalesce in idea, and the correctness of the judgment respecting the form of a point or of a very small object, sometimes gains considerably thereby. Thus the acuteness of vision, apart from the stereoscopic effect, is greater with two eyes than with one ; if vision takes place with only one eye, the form of the retinal image is, at least in the, vicinity of the* yellow spot, pro- jected with greater accuracy ; in vision with two eyes, on the con- trary, the object is more correctly estimated. If the acuteness of the power of vision, in monochromatic light, suffers Httle by ordinary astigmatism, the achromatism of the eye, with imperfect accommodation, may even gain thereby, as different coloured images fall over one another, and are thus partially neutralised. This also is especially the case in using both eyes. II. Abnormal irregular astigmatism. — This has its sent either in the cornea or in the lens. As to the cornea, the herato-conus or cornea conica first comes under observation. High degrees strike the eye at once. Slight degrees, on the contrary, are often enough overlooked. The disturbance of the power of vision frequently suggests the idea of amblyopia, combined with myopia. Three cases have already occurred to me which were long treated as amblyopia. That in this instance an anomaly of refraction, and indeed astigmatism, is the cause of the diminished sharpness of sight, is evident. A complete theoretical de- velopment would, even did we know precisely the surface of curvature, which is to be ascertained only by examination with the ophthalmo- meter, be very difficult. In high degrees, the mere inspection of the * Nederlandsch Lancet, 1846-1847, D. ii. p. 432. DETECTED BY THE OPHTHALMOSCOPE. 551 curvature and profile at once satisfy the observer, that the radius of curvature in the centre of the cornea is much shorter, so that the rays falling thereon from each cone of light must much sooner unite. Especially in reference to these rays the eye is myopic. There must, however, be not only a difference in focal distance, but the foci are also imperfect even for small portions of the refracting surface, and moreover, certainly do not all lie in the same axis. The high degree of astigmatism connected with this state, therefore, needs not to be further proved. — It would be very troublesome if, in order to recog- nise slight degrees, we should be obliged to have recourse to the oph- thalmometer, in order to determine the radius of curvature in different parts of the cornea. Fortunately, we have a more practical auxiliary. The already long-existing disturbance of vision leads us to resort to the ophthalmoscope, chiefly with the idea of finding the cause in the fundus oculi, and unexpectedly we discover the anomaly of the refracting surface. This has happened to me more than once. Sometimes the degree was still so slight, that even after the discovery of the true cause, the observer, on taking a profile view, could not satisfy himself as to the state of things, so that full certainty as to the existence of the anomaly was attainable only with the ophthal- mometer. How the ophthalmoscope exhibited it is very simple. In the inverted image, where there is a tolerably wide pupil, we over- look, at the same time, a rather large portion of the fundus oculi ; the image, therefore, of one part or other, for example of the optic disc, remains in the field of vision both on moving the head of the observer, and on shifting the lens held before the observed eye. At the same time, however, the rays, which, proceeding from the optic disc, strike the eye of the observer, pass each time through other parts of the cornea : now if its curvature is irregular, the result is, that the form of the disc each time alters, that it shortens in this direction, extends in that direction, and, moreover, is never seen acutely in its integrity. In somewhat higher degrees, too, the side of the conical projection opposite to the incidence of the light is darker, — as if shaded. That it is important to recognise these slight degrees of conical cornea, and not to treat them as amblyopia, is evident. Where the form is favourable and the position advantageous, stenopseic spectacles may produce considerable improvement. If this assistance be not sufficient (the field of vision too small, the spectacles an annoyance), an improvement by operation may be 552 IRREGULAR ASTIGMATISM. attempted. The chief object of the operation is easily stated : we desire to place the pupU before that part of the cornea, whose curvature is most uniform and approaches most nearly to the sphericalj in order that a sharper image may be formed in the visual line, and especially that direct vision may be improved. A priori it will be evident, that this object will be more easily attained by a small pupil, not only because the circles of diffusion are thus rendered smaller, but particularly because we may expect the less difference in the radius of curvature, the smaller the portion of the cornea is, which participates in the formation of the image. Bow- man* has made the pupil slit-Kke by double iriddesis.f Von Graefef confined himself to iridectomy. Both obtained favourable results with respect to the acuteness of vision. Von Graefe proposed be- sides to procure diminution of the pressure in the eye, which result of iridectomy had in his hands obtained such brilliant and useful application in glaucoma: he hoped thus to oppose the further development of the conicity, if not to lessen the existing degree of it. Bowman, on his part, has actually seen diminution of the conicity take place after iriddesis. The results of the latter operation upon the power of vision are still more favourable, so that at present iriddesis, by which we also obtain a small pupil, seems to deserve the prefer- ence. Theoretically, however, the slit-like pupil, obtained by double ireddesis, appears to me, narrow as it may be, not the most favour- able : when the direction of the slit is horizontal, the diffusion for ver- tical lines will still be considerable, and though for horizontal lines the eye will have little diffasion, it will be highly myopic. It certainly seems better, by simple iriddesis, to exclude the apex of the cone. By means of stenopseic spectacles (with artificial mydriasis) the most suitable place and form of the pupil can be discovered, and perhaps also the seat of most favourable curvature may be sought with the ophthalmometer; and when by this or any other mode we have ascertained where and in what form the pupil must act most advantageously, the further task of operative surgery is, to apply the means so as to realise what is found to be desirable.g * Ophthalmic Hospital Reports, etc., TSo. ix. 1859, p. 154. t Compare, upon this operation, also Critchett, Ophthalmic Hospital Reports, etc., No. ix. 1859, p. 145. I Archivf. Ophthalmohgie, B. iv. H. ii. p. 271. § In this respect, very ingenious proposals have been made by Bowman, the object of which is to bring the pupil by means of a second iriddesis on the same side, proceeding from the margin of the pupil obtained by the first SPOTS ON THE CORNEA A CAUSE OF IT. 553 To be classed with conical cornea, though usually producing less disturbance, are pa/rtial hulgings or flattenings of this membrane, which, in consequence of suppuration or of softening, not unfre- quently occur. These are often accompanied with so much opacity as to render the displacement of the pupil by iriddesis or iridectomy desirable. But the astigmatism is not thereby removed, since the clear part of the cornea has lost its regular curvature. Von Graefe has remarked, that after iridectomy the form of the cornea* gradually improved — and I have repeatedly found this confirmed. Moreover, it appears, that in these cases improvement is often to be obtained by cylindrical glasses, the asymmetry being partly reducible to regular astigmatism. To the very ordinary causes of altered, and consequently irregular arching of the cornea, belongs the extraction of cataract. Especially when prolapsus iridis or threatening prolapsus has existed, whereby the pupil has lost its central position, or where, with forward projec- tion of the flap, the wound- surfaces do not perfectly correspond, we seldom obtain a completely normal arching of the cornea. If the deviation is slight, the power of vision may still be quite sufiicient ; but on accurate investigation it now appears that the acuteness is defective, and the astigmatism is in this instance also partly capable of correction by giving an oblique position to the convex glass, or by combination with a cylindrical one. A common cause of irregular astigmatism we find further in spots on the cornea. That slight spots cause much more disturbance by scattering diffused light in the eye, than by reflecting and cutting off a part of the light, has been shown many years ago, and hereupon the indication for stenopeeic spectacles was subsequently founded. Even in my first communication I had referred to the irregular refraction of light, connected with spots. How much effect this has, the ophthal- moscope afterwards taught me. Through a rather transparent spot we distinguish the fundus oculi with tolerable accuracy ; but, while, in the mode described above, the rays are brought consecutively through different parts of the spot to the eye of the observer, he is surprised iriddesis, • still nearer to the edge of the cornea. He has also proposed to try, by means of a glass with corresponding curvature, still further to improve the astigmatism for this place, at least for the stationary eye. * In the acute process of softening or suppuration it is a matter of re- cognised importance, to keep the form of the cornea as perfect as possible. To attain this object, repeated experience shows that timely support by means of a bandage causing pressure, cannot be suflB.ciently recommended. 554 IRREGULAR ASTIGMATISM. at the extremely irregular displacement, shrinking and distortion of the forms, connected with a peculiar glancing, — very characteristic for any one who has once seen it. Spots, whose existence was not per- ceived on superficial inspection, sometimes, on examination with the ophthalmoscope, produced in a remarkable degree the phenomena just mentioned. Thus by ophthalmoscopic investigation we are led to examine the cornea with focal illumination, and then we find, in a scarcely perceptible opacity, the cause of the astigmatism, and at the same time of the diminished acuteness of vision, which had at first suggested the presence of other causes. — Under these circumstances the surface of the cornea is often not perfectly smooth, as readily appears from the irregular images of a flame reflected on the opaque part. This occurs chiefly when superficial ulceration of the cornea has existed. But this inequality is not necessary to produce astig- matism, and therefore local change of the coefficient of refraction, with , condensation of the corneal tissue, seems equally capable of taking part in its production. — If we have not examined the astig- matism with the ophthalmoscope, we shall often think we find a disproportion between the degree of opacity and the disturbance of the acuteness of vision, which must be explained partly by irregular astigmatism. Pinally, in some acute affections of the cornea, particularly with transparent ulcers, a high degree of irregular astigmatism coexists. So much with respect to the cornea. As to the crystalline lens, irregular astigmatism may by it in two ways attain a high de- gree, namely, by a change in the lens itself, and by displacement of the lens. With reference to the change of the lens itself, my friend Bowman recently wrote to me as follows : " I have often thought that some defects of vision may depend on physical altera- tions of the lens, sometimes independent of cataract, sometimes attending the earlier stages of cataract ; making the changes of shape of the lens under the same action of the ciliary muscle more or less incomplete." To these remarks I give my full adhesion. Even in- dependently of the changeability by accommodation irregularities are developed in the lens, which when the eye is still in a state of rest mani- fest themselves as irregular astigmatism. Usually, as Giraud-Teulon observed, this astigmatism increases at a more advanced time of life, especially when opacity of the lens is superadded. Even Mackenzie says : "■ Uniocular diplopia is sometimes a precursor of cataract," and Ruete very correctly explains, how radiating opacity of the lens may give rise DISPLACEMENT OF THE LENS A CAUSE OF IT. 655 to diplopia. We hear the latter complained of mostly, when one eye no longer takes much part in vision, which is very easily explicable on the principles already laid down (p. 550). Besides, it very seldom occurs, that multiple images producing any disturbance remain, when by the assistance of suitable spectacles the accommodation has been made as perfect as possible. In youth the phenomena of irregular astigmatism may, both without and with ten- sion of accommodatiGn, be almost wholly absent. — The irregular astig- matism which depends on displacement of the lens, produces much more disturbance, especially when the lens has only partially remained in the plane of the pupil, and the rays, therefore, in part, refracted solely by the cornea, penetrate to the retina. This may take place in incomplete luxation, whether spontaneous, or produced by ex- ternal injury; but it appears to occur more frequently as the result of congenital ectopia of the lens. Of this I have seen remarkable cases, three of which belonged to the same family. In such instances the power of vision is very imperfect. Just like highly hypermetropic individuals, the patients see near objects comparatively better, though still very defectively. Even when the lens occupies half of the plane of the pupil, they comport themselves as hypermetropes. This led me to infer that convex glasses would be beneficial to them, and it appeared that in this I was not mistaken. The glasses required were similar to those indicated in aphakia. On accurate examination with the ophthalmoscope and with focal illumination this result cannot appear strange. If the lens even in the normal eye is less homo- geneous near the equator than near the axis, this is especially true of the abnormally situated lens. The reflection is, moreover, very strong, particularly when the lens lies obliquely. At the side of the lens the fundus ocdi is seen perfectly clear j through the crystalline lens it usually appears less clear and acute. This we observe especially in examining the inverted image, as, we can then see the optic disc in two closely adjoining pictures, one larger and brighter by the side of the crystalline lens, the other smaller and less brilliant, formed by the co-operation of the crystalline lens. Thus, also, the image, formed by the medium of a glass lens on the retina, is clearer, more per- fect, and at the same time larger than that produced through the edge of the natural lens ; and, on the other hand, the disturbance from the rays, which form no sharp image on the retina, must in the latter case be greater than in the first. A stenopeeic apparatus, by which either the rays entering the eye through the crystalline lens. 556 IRREGULAR ASTIGMATISM. or those passing in beside it, are cut off, very mucli improves the acuteness of vision. In a still youthful lad, labouring under conge- nital ectopia of the lens, cataract was developed ; in proportion as the lens became more opaque, the sight improved. I had absolutely no inducement to operate on this cataract, even after it had become ripe, although the diffuse light stiU. continued somewhat inconve- nient. The literature and history of our knowledge of irregular astigmatism have been sufficiently dwelt upon in the foregoing. I shall here add only that the literature of normal irregular astigmatism, with a brief statement of its contents, is given by Helmholtz {I. c, p. 146), and that the cases of normal irregular astigmatism which have occurred, are communicated in Stellwag von Carion's monograph {Denhschriften der k. h. Akad. v. 2, p. 172. Veher doppelte Brechung und davon ahhdngige Polarisation des Lichtes im men- schlichen Auge (on Double Refraction and Polarisation of Light dependent thereon in the Human Eye), pp. 19 et seq., Wien, 1853) as diplopia (polyo- pia) monophthalmica. The theory of Stellwag von Carion, which is evident from the title of his work, is refuted by Gut ( Ueber Diplopia monoph- thalmica, Zurich, 1854). CHAPTER IX. DIFFERENCE OF EEFEACTION IN THE TWO EYES. § 41. OcCUEEENCEj PHENOMENA^ RESULTS. Like the organs of aniinal life in generalj the eyes present a great symmetry between the right and the left side. The statement so often made, that the right and the left eye usually differ considerably is an error, or, more strictly speaking, an exaggeration. In all respects, similarity is rather to be met with. This extends, not only to the magnitude of the eyeball, the diameter of the cornea, the colour of the iris, the size of the pupil, and other external properties,- even some congenital morbid deviations, such as microphthalmos, cataracta congenita, iridemia, and acquired changes of form, such as cornea conica, often occur in both eyes in about the same way. The same is found to obtain with respect to the refractive condition of both eyes. Even the degrees of progressive myopia in most cases differ little in the two eyes. We have already seen that the majority of eyes are nearly emmetropic, and this generally holds good, in fact, for both. This emmetropia is the result of the convexity of the cornea, of the position and of the focal distance of the crystalline lens, and of the length of the visual axis, each of which may in itself differ considerably in the normal emmetropic eye, and then mutually compensate each other. But the similarity in the same individual usually goes so far that, as numerous measurements have shown me, the radius of curvature of the cornea coincides almost perfectly for both eyes ; whence we may infer that for two eyes of the same individual the crystalline lens and the length of the visual axis approach more nearly to each other than they do for emmetropic eyes in general. A certain harmony also unmistakeably exists in general in the course of the subcon- junctival blood-vessels, in many peculiarities of the optic disc and its blood-vessels, in the entoptic figure of the lens, and in the position of the yellow spot (angle a). Even in the asymmetry of the cornea there exists symmetry between the right and the left side. All this is the rule. Exceptionally it occurs that both eyes differ much originally from each other, particularly with respect to their 558 DIFFERENCE OF REFRACTION IN THE TWO EYES. refractive condition. Thus, as we have already remarked (p. 252), this asymmetry of the eyes is usually combined with asymmetry of other parts, especially of the 'orbit, and of the bones composing it, so that the difference of the eyes is reflected both in the form of the forehead and of the face. Since my former observa- tions on this subject, I have taken much pains in endeavouring to discover fixed rules on this point. In this, liowever, I have not succeeded. I can only in general maintain, that at the side where the strongest refraction, or rather the longest visual axis occurs, the orbit (and with it the eye) is situated closer to the median line, while its surrounding edges are placed more forward. If the left and right half differ in this respect from one another, there exists also in general a difference in the refractive condition, and vice versa. Hence there is evidently a connexion between the two. But that the connexion is not absolute is not strange, for just as with differing form and position of the orbits the two eyes may be emmetropic, it must be possible that equality of the eyes should coexist with difference of the orbits of the same individual. Though homo dexter and Jiomo sinister may be dissimilar, they may both be emmetropic or equally ametropic. The differences occurring in refraction may be divided into con- genital and acgui/red. We shall speak first of the congenital, which are the more important. Among these must be reckoned the differ- ence in myopia, though in youth this is often but slight. The pre- disposition was original and the further development was included in it : where both eyes become highly myopic, that high degree of myopia was not present also in youth. All imaginable combinations of refraction occur in fact. "With emmetropia of the one eye, the other may be either myopic or hyper- metropic; hypermetropia or myopia may occur in very different degrees in the two eyes ; lastly, the one eye may be hypermetropic and the other myopic. It is remarkable that, when astigmatism occurs only on one side, there is in general in other respects harmony of refraction on both sides ; that is, with H of the one eye, we find hypermetropic astigmatism of the other j with M of the one, myopic astigmatism of the other; with emmetropia, the astigmatism is mixed. — When with difference in refraction of the two eyes, the cornese have nearly equal radii, this is to be considered as acci- dental : in general, under these circumstances, the difference is as great as is usually the case in eyes of different individuals. The VARIOUS MODES OF USING UNEQUAL EYES. 559 same may be admitted also respecting the crystalline lens, since the length of the visual axis for each eye is connected with the nature and with the degree of the ametropia. As to the use of the eyes, with difference of refraction, this is possible in three ways : 1°. binocular vision, 2°. vision with each of the two eyes alternately, 3°. constant exclusion of the one eye. 1. Simultaneous vision with two eyes, even when the eyes were similar, was formerly doubted. It was asserted, that, although both eyes are properly directed, only one eye sees at the same time, and that in this the eyes relieve one another. This assertion has long since been refuted. But it certainly is true, that we usually abstract from the one eye more easily than from the other. If any one causes the sight to be directed to a remote object, it appears, on subsequent closing of the left eye, almost always, that the right eye has been used. If a distant object be covered with the extended finger, this covering will by most people be effected for the right eye. Now where there is difference in refraction that eye is used, with which, at the required distance, vision is most acute and easiest. But if the ordinary observation of an object be in question, there may be binocular vision even with unequal eyes, within the limits of easy convergence. This occurs in many cases, even where there is con- siderable difference in refraction. Experience, in fact, shows that in spite of the unequal magnitude and unequal acuteness, the images of the two retinas assist one another in observation : not only are the solidity and the distance more correctly estimated, but even acuteness of vision and the facility of reading, writing, &c., may gain thereby. This, indeed, cannot surprise us. In the first place, even for normal and equal eyes, there are no absolutely identical or corresponding points, and it is certain that such are much less still to be expected, when, from original inequality of the two eyes, the condition for connecting these points by practice more and more perfectly in a symmetrical position, has been wanting (compare p. 165). In the second place, as will more particularly be seen, the feeble tints of diffuse images forthwith disappear, when the acute image of the second eye is combined with them. In speaking of irregular astig- matism, I have already alluded to a remarkable co-operation of two unequal images (compare p. 550). How unequal in magnitude and acuteness the two retinal pictures of a near object, viewed laterally by both eyes, often are in equal eyes ! In truth, the second eye is rarely disturbing to vision, unless, in consequence of an opacity, it 560 DIFFERENCE OF REFRACTION IN THE TWO EYES. admits much diffused light to the retina; and that even then this disturbance is by no means the rule, is proved both by the rarity of the deviation of an eye affected by cataract, and by the possibility of the development of cataract in one eye being totally unobserved. To satisfy ourselves, whether both eyes take part in vision, we cover them alternately, having fixed an object, by putting the hand before them. Whichever eye we cover, that which has remained uncovered must continue to fix without movement, and if the covered eye had deviated behind the hand, it must, on removing the latter, immediately again occupy its former place. If the result of this examination leaves any doubt, we place a weak prismatic glass with the. angle inwards before the one eye, whereupon, if vision is binocular, double images arise, which are overcome by a distinct movement inwards. When there is a difference in refraction, we can determine the farthest and nearest point of each eye separately. If the acuteness of vision is sufficient in both, we usually find the ranges of accom- modation also equal. And if these are greater than the difference in refraction, they faU partly on one another : the nearest point of the least refracting eye lies at a shorter distance than the farthest point of the most refracting eye. But still we should be very much deceived, if we supposed, that in binocular vision the distance for which accommodation takes place could be equal. Buffon held this opinion, but we have called it an error. Even a slight difference in refraction we are not able to adjust by accommodation, if this has an equal range in both eyes, so inseparably is the tension of accommo- dation in one eye connected with that in the other. Of this we can easily satisfy ourselves. Let any one who has equal eyes, hold only a weak negative or positive glass before the one eye, look at any ob- ject, and then close alternately the one and the other eye. Experi- ments of this nature are reaUy important. We perceive in the first place, that we continue to accommodate acutely with the one eye, and indeed by preference with the eye, which, with less tension of the relative accommodation, forms the sharpest and largest images. I myself, for example, read the finest diamond type for hours together, without fatigue, in the evening, without spectacles, but if I bring a glass of ^ before one eye, I nevertheless, by preference use this eye for looking at near objects. Now it further appears, that with the unaided eye we have in this case a diffusion-image. By no CO-OPERATION OF. THE SECOND EYE. 561 tension whatever can we succeed in getting a sharply-defined image for both eyes together. Mnally, we observe, if we now again open the^ assisted eye, that the feeble parts of the diffuse-image almost entirely disappear, while the darker parts coincide with those of the acute image. With glasses of ^I experience no disturbance what- ever ; with those of ^ there is something misty, which disappears on covering the eye which is not properly adjusted ; but in spite of that mistiness, both the solidity and the distance of objects are more cor- rectly estimated, and with the stereoscope a stereoscopic image is obtained. Undoubtedly the advantages of binocular vision extend much farther, if the difference in refraction existed originally, the reasons of which have already been given. Besides, just as in the experiments with artificial difference by means of glasses, the one eye in this case also accommodates sharply, at the expense of the other, rather than by average tension of accommodation to obtain half acute images in both eyes. This, however, does not prevent the acuteness of vision, when it is imperfect in both eyes, being rendered greater by the assistance of the less correctly accommodated eye ; this I have ob- served especially in disturbance, the result of astigmatism. But even when in consequence of too great difference in refraction, the second eye no longer assists, it at least produces no disturbance. I recently made the acquaintance of an optician of great merit, who told me, that he was emmetropic in one eye, while in the other, he had M = 1 : 5"5. The eyes were properly directed for any distance. In ordinary vision he experienced no disturbance and used his emmetropic eye. With the emmetropic eye a small light at a distance appeared to him to be, in fact, very small ; with the myopic eye it presented a large diffused image. But if he now opened the emmetropic eye, the diffused image diminished to one-half. He asked me for the ex- planation of this fact. I found it partly in the diminution of the pupil of the myopic eye, on opening the other, partly in the circumstance that the most external and fainter portion of the diffusion-image actually became invisible j the outermost part was fainter, because the eye was accommodated for a nearer point. Not unfrequently it has occurred to me, that the patient has thought that with one eye he could scarcely distinguish anything, notwithstanding that the acuteness of vision was still tolerably good. I have found this in high degrees both of myopia and of hyperme- 36 562 DIFFERENCE OF REFRACTION IN THE TWO EYES. tropia. That the power of vision of a strongly hypermetropic eye, which requires glasses of „, or even of ■% in order to have defined images on the retina, has been overlooked, cannot surprise us ; but it is singular that clever and well-informed men have so often continued ignorant that they still see satisfactorily with their strongly myopic eye, when they only bring the object near enough. In these cases the unused eye has often deviated somewhat, and indeed almost without exception in the outward direction. This direction I have even met with, when the deviated eye was strongly hyperme- tropic, provided that myopia, or at least emmetropia, existed in the other used eye. Under these circumstances the direction of the eye may for certain distances have stiU. continued correct. In general, I must observe that deviation is never produced by difference in refrac- tion. At most the latter may be the cause, why the deviation was not prevented, and it is, in fact, no longer prevented, so soon as the difference in refraction is so great that the one eye loses all im- portance for binocular vision : this eye is then equivalent to a blind eye, and just Kke the latter, therefore deviates outwards". But if the eye has still any co-operation in binocular vision, vision is im- proved by it, and certainly the deviation never arises in order to prevent binocular vision, as has been asserted. 2. TAe eyes are alterndtely used,. — In difference in refraction it not unfrequently occurs, that the one eye is employed for near, and the other for distant, vision. I allude to those cases in which a certain deviation exists, and where, consequently, binocular vision, properly speaking, does not occur : evidently this alternation exists as a rule, so long as binocular vision is maintained, in wliich, as we have seen, the one eye is always acutely accommodated, and there- fore, in the whole region of accommodation, each eye in part takes the of&ce on itself. Now, in these cases, it may appear as if the range of accommodation is extraordinarily great. Thus one of my friends boasted, that at a distance he saw with perfect acuteness, and that in vision for near objects he was inferior to none. On exami- nation this was at once explained. His right eye was emmetropic, and his left presented myopia = ^^- He was himself not aware of this. After his twenty-eighth year this last eye began to deviate outwards, and was thus excluded from binocular vision ; but he con- tinues to use it, when he wishes to distinguish very small objects. Thus a deviated myopic eye is most certainly preserved from ambly- EXCLUSION OF ONE EYE. 5G3 opia, while, on the contrary, that which has deviated inwards becomes amblyopic throughout the greater part of its field of vision. That it is of importance, if possible, to preserve it from amblyopia, need scarcely be observed. 3. One eye may, in observation, continue wholly excluded. — Under this head two kinds of cases are to be distinguished : those in which a morbid condition of the eye (e. g., detachment of the retina) has set in, and has given rise to the exclusion with deviation, and those where the deviation was primary, influenced by the tension of the muscles, and the disturbance of vision is the result of want of use. With respect to the first we may be silent. As to the last, we must distinguish between the deviation inwards and that outwards. In the deviation outwards the field of vision is enlarged, and extends over objects, which are not seen by the other eye. In the deviation inwards the field of vision is diminished, and that of the deviated eye falls more over the other. This may produce confusion, and therefore we men- tally neglect the impressions received on the corresponding part of the deviated eye, which consequently, so far as the common binocu- lar field of vision extends, becomes amblyopic. In the deviation outwards only a small portion of the field of vision is common ; and, moreover, abstraction is less required, because the usually strongly myopic eye receives very diffuse images. Therefore, in these cases the power of vision is generally tolerably satisfactorily maintained, even though the eye is not used. A word still as to the acquired differences in refraction. They are limited chiefly to aphakia and to loss of accommodation in one eye. The mode in which vision takes place in aphakia of one eye has been investigated by von Graefe, particularly with reference to the ques- tion, whether it is desirable to operate for cataract in one eye, while the other is sound ? His answer is, that, taking everything together, " the operation for cataract in one eye, with important advantages, is attended with no essential injury, and is therefore always indicated, if we can, with tolerable certainty, reckon upon a favourable result.'" In this opinion I can cordially concur. Particularly in active young persons, in whom at the same time the danger of the operation is slight, the advantages of a wider field of vision, diminishing the risk of wounds upon the second eye, the removal of the deformity and the greater self-confidence inspired by the possession of two eyes, throw a considerable weight into the scale. Moreover, I can con- firm the observation of von Graefe, that in young persons the existence of combined vision can often be established, whereby the 2 564 DIFFERENCE OF REFRACTION IN THE TWO EYES. estimation of distance and of solidity is improved, and that where combined vision is wanting, the lensless eye at least extremely rarely causes any disturbance. On a single occasion double vision pro- duced disturbance where there was deviation outwards. Besides, that strabismus should occur as a result of the operation, I cannot admit. At most, it is comprehensible that it should increase where it had already existed, and produced, immediately after the operation, double images, near to one another, which it was difficult to uuite again by muscular action. Rnally, where the power of accommodation is lost or diminished in the one eye, the distance of distinct vision does not proceed equally on both sides. On looking to a distance both eyes may, for exam- ple, be properly accommodated; this proper accommodation disap- pears in proportion as the object approaches. The inequality which has usually suddenly set in, probably still more its changeable degree, gives rise to complaints of dazzling. The large pupU is doubly injurious, as it makes the circles of diffusion larger and in- creases the strength of the Hght in the eye which does not see acutely. In speaking of the anomalies of accommodation we shall revert to this point. § 42. Treatment and Optical Eemedies in Difference of Refraction in the Two Eyes. In the establishment of the indication, the principal thing is to determine, whether binocular vision exists, or not. Where binocular vision is present, at any distance, the point is to maintain this, and if possible, to render it capable of extension over a greater region. In the choice of glasses we start from the more acutely-seeing eye, to which the other must remain subordinate. Where there is little difference in acateness of vision, it may also be considered which eye needs the weaker glass for correction, that is indeed also in general the eye which has the more acute vision. Por this eye now, according to its refraction and accommodation, quite independently of the other, all the rules hold good, that guide us in the choice of glasses (compare §§ 18, 23, and 32). The question then remains, what glass the other eye requires ? At first view we might suppose, that for this latter we should have simply to choose the glass that brings the farthest point to the same distance, at which it lies for the first eye. This is in fact the opinion of laymen: "My eyes differ, consequently I need different CHOICE OF GLASSES. 565 glasses," — such is the ordinary reasoning. It is so evident, so pal- pable, and apparently so logical, that we cannot be surprised at it, the less so, as the so-called "opticians" support it, and are quite prepared to put two different glasses in the same frame. It is, how- ever, far from being the case, that we should keep to this rule. Even from habit a great difficulty arises. One person has, in spite of hypermetropia in one eye, in his youth always seen and read with- out spectacles, and has never experienced any difficulty in binocular vision. Another reads admirably and suffers no fatigue, although his eyes are in different degrees myopic. If we now give such people convex or concave glasses similar for, both eyes, they will be satisfied, for the relation between the two eyes, to which they are ac- customed,, remains 4°iost unchanged, If, on the other hand, we give them different glasses, whereby the range of accommodation in both eyes becomes more equal, we shall often enough find the proverb confirmed " le noieux est Tenuemi du bien." The cause of this is principally that when the distance of distinctness is made equal, the images of the two eyes are not equal, but di^erent, particularly in magnitude. Within certain Hmits a difference in magnitude, such as, with equal eyes, can be produced by a combination of a positive and negative glass; for: the one eye, is, as is shown by the experiment, attended with no essfcntial difficulty, especially not, in looking at objects situated ia the same ^ plane, as in reading, writing, &c. If we consecutively close each eye, . it is found that the letters appear with one eye larger, with the other smaller, while both eyes combine them in a medium magnitude. It is well known that this is equally the case when we look at two uniform figures, with a slight difference in magnitude, through the stereoscope. It is the result of the varying and imperfect correspondence of the symmetrical points which must result from the ordinary use of. the eyes,,iu which letters and other forms are so often met with at somewhat unequal distances from the two eyes. But if the difference in magnitude exceeds a certain degree, double vision becomes evident, with larger letters corresponding to one, smaller to the other eye, which cannot be brought to cover one another, and which the person is, therefore, incHned, by deviation of one of the visual lines, to separate still further. The same thing occurs when the same distance of dis- tinct vision has been produced in cases of great differences in refrac- tion by difference of glasses.* This has led me to adopt the rule * We might turn higUy perisoopie glasses (whose nodal points lie out- side the body of the glass) for the one e^e with the convex, for the other 566 DIFFERENCE OF REFRACTION IN THE TWO EYES. to give similar glasses for both eyes, when the binocular vision of eyes of different refraction is acute and easy at any distance without glasses, and shifting of this distance is necessary, and I find this to answer very well. But may we never deviate from this rule ? Undoubtedly. In the first place we may do so when the difference in refraction is slight, amounting to not more than n or ^. I have met, particularly, with myopes, who have for distant vision given the preference to such corresponding difference in glasses. More- over, where there is a greater difference in refraction, we may, by a moderate difference in glasses, partly correct this ; for example, with M = ITS in one eye, and M = J in the other eye, we may give a glass of — ra for the first (if there is no centra-indication to neutralisation), and at the same time one of — is for the other eye ; but the difference in glasses can rarely exceed w or w- Lastly, there may often be an advantage in producing by different glasses, in imperfect acute- ness of vision, nearly accurate images on the two retinas, by whose co-operation then the power of distinguishing is sometimes really in- creased. This refers especially to hypermetropes : these, even, are also the cases in which the patients, under the existing difference in refraction, were not satisfied with their power of vision at any distance whatever. But under all circumstances the combination of the dif- ferent glasses ought to be tried, to ascertain if it is really suitable. A priori we can give no certain opinion, as the existing difference of the eyes has become habitual, and may have had an influence on the corresponding points. When an eye takes part in combined vision, its function is main- tained, even when it constantly receives very imperfect diffused images. Particularly the field of vision continues in its integrity, and if the acuteness of sight may somewhat diminish, it returns, when more is required from this eye, for example, when disturbance occurs in the other and on systematic practice. This last I consider in every case desirable, especially in high degrees of ordinary hyper- metropia, hkewise in aphakia ; it is accomplished, the eye ordinarily used being closed, simply with a convex glass. Thus remaining un- enfeebled, this eye is then always ready to afford assistance so soon as the other may come to fail, and at the same time it can stiU better with the concave surface towards the eye, or by a peculiar combination of a concave and a convex glass, different for each eye, we might endeavour to bring the resulting nodal points on both sides to an equal distance from the retina, and thus to make the images equally large ; but the method is verv delicate, and I can scarcely imagine that it will ever be adopted in practice. ABSENCE OF COMBINED VISION. 567 aid in binocular vision. A highly myopic eye can practise without a glass. This, howeverj has usually deviated outwards, and it then belongs to another category, namely, to those cases in which Binocular vision is absent. — It is chiefly under this head that the remarkable examples fall, in which a defect in refraction is looked upon by the patient as total uselessness. I will communicate a couple of them. I. Mr. E., aged 58, an architect, has from his youth been much occupied ■with architectural drawing. In this he has always made use of Ms left eye. Since an attack in the eyes, from v?hioh he suffered, he has seen less acutely with that eye. '' It is my only eye," he says, " and I am much per- plexed." I establish the existence of M = 1 : n-5, S ^ 0-6, diffuse light being warded off, and of synechia, with opacity of the anterior surface of the crystalline lens, the result of iritis. On glancing with the ophthalmo- scope also into the right, somewhat outwardly deviated eye, I find it free from synechia and see with ^ (compare my method at p. 337) the fundus oculi scarcely diffused in the non-inverted image: the eye was therefore hypermetropic. On asking him how he saw with that eye, I received for an answer that at a distance everything was confused, and that he could distin- guish nothing near with it. His amazement, when he looked at a distance through a glass of -^, is still vividly before me. " I actually see better with this eye," he exclaimed, " than I ever did with the right, even with the aid of my concave glass." Objects appeared to him at the same time much larger, and if he looked at those with which he was acquainted, he thought the distance shorter than it really was. With \ he read without difficulty ; with \ he distinguished the smallest type. His hypermetropia amounted to not more than J ; S being at the same time == 0-7. Two points here deserve our attention. In the first place, that in apparent disuse the eye had continued so good : I am, however, convinced that in looking at distant objects the patient not unfrequently made use also of this eye. In the second place, that with the comparatively slight degree of H, he did not know the value of the eye. This is certainly to be ascribed to the fact, that the other eye was myopic, and that in youth, its degree of myopia being somewhat less, it saw near objects almost without exertion, and at the same time distinguished at a distance better than the hyperme- tropic eye did. In this way there was no occasion to put the accommoda- tion much upon the stretch. If both eyes had been in the same degree hypermetropic, Mr. R. would undoubtedly have read with them. II. Mrs. L., aged 40, has a number of general ailments, and to them she ascribes it, that during the last six months her sight has fallen off' very much. "With the right eye she has never seen." It is rather strongly deviated outwards. The left eye has M = 1 : 5, S ^ 10 : 70 ; the diminished aouteness of vision is dependent on chorioiditis disseminata and motes in the vitreous humour. A glance into the right eye with the ophthalmoscope shows me with ^ at a distance the inverted image of the fundus oeuli, bearing 568 DIFFERENCE OF REFRACTION IN THE TWO EYES. a rather considerable circular atrophy. It therefore appears that the myopia has here a much higher degree ; and while I now approximate No. 1 to 2' , she reads it without any difficulty, to her own amazement : " She had never tried this." In this eye M was = 1 : 2-5, S = 10 : 40, and with a glass of — J she could still read satisfactorily with this eye. While the left eye was under treatment, I advised her to practise the right eye cautiously, partly without a glass, partly with — J. Thus S within some months increased to 9 : 20, and the eye was, and continued, much more useful than the left. In general, the optical treatment in difference in refraction, is mucli easier where there is deviation of one eye than when there is binocular vision. In this case, we keep the better eye for ordina,ry use, and keep up the other by regular practice, with exclusion of the better. In rare cases, with deviation outwards, the one eye is used exclusively for distant, the other for near, objects. This task they must continue to fulfil, and we keep up each, so far as is necessary, according to the general rules. This is, therefore, stiU. simpler. Lastly, almost always with deviation inwards, and not unfrequently also with deviation outwards, the one eye is wholly out of use : perhaps it was even originally less sharpsighted, and now it is quite amblyopic. If it no longer fixes on closing the other eye, there is nothing to hope for. Practice is then in vain. — The question, when it is expedient, in deviation, to perform tenotomy, cannot be here investigated at length. On this subject, I may refer to the weU- known essay of von Graefe. A couple of remarks may here be made. As the result of my investigation, I have assumed that difference of refraction never produces strabismus, but only does not prevent its occurrence. Consequently, from the difference in refrac- tion, a decided contra-indication to tenotomy is never to be deduced. We must say only, that binocular vision can acquire no particular value. But is not tenotomy performed for appearance' sake, even when one eye has lost its sight ? — A second remark is, that in the highest degrees of myopia an eye deviated outwards acquires, through simple tenotomy, a better position in looking at distant objects, but seldom learns to converge. In aphakia of the one eye, with normal acuteness of vision of the other, especially with deviation of that eye, some practice with a convex glass is to be recommended, in order to prevent retrogression of the acuteness of vision. Some minutes daily are sufficient. To the indication, in loss of accommodation of one eye, I shall revert, in speaking of the anomalies of accommodation, at the con- sideration of which we have now arrived. n-ANOMALIES OF ACCOMMODATION. INTRODUCTION. We commenced the second chapter of this work by demonstrating that the anomalies of refraction and those of accommodation form two different categories. We laid stress upon the fact that, as the first are anomaHes of the form, the latter disturbances in the func- tion of muscles, they must be rigidly distinguished from one another. But with this difference in nature, the connexion between the two is not to be overlooked. Thus we had, in speaking of the emmetropic eye, to treat of presbyopia, which, although the normal condition at a more advanced time of life, is, with respect to its essential nature, related to the anomalies of accommodation. Thus, in aphakia, the refraction is abnormal, but, ut the same time, the accommo- dation is removed. Thus we saw further how hypermetropia, and sometimes also myopia, leads to spasm of accommodation, and how, in the two forms, the connexion between accommodation and con- vergence is peculiarly modified. If we add that the symptoms differ, accordingly as the anomaly of accommodation occurs in emmetropic or ametropic eyes, and that the differential diagnosis between the two categories is not always free from difficulty, it will certainly be admitted that it is practically useful to append, in the same work, to the detailed treatise on the anomalies of refraction, a short descrip- tion of the disturbances of accommodation. Accommodation depends upon muscular action. We are here there- fore to expect the anomaHes which are proper to muscles in general : paralysis and spasm. In connexion with the former, the action of mydriatics ; in connexion with the latter, that of myotics, is tp be studied. Each investigation must be based upon a knowledge of the nerves, which are implicated in either condition, and we have to treat thereof in connexion with the movements of the iris, which are associated with those of accommodation. We therefore treat, in three chapters, of : — I. The influence of the nerves upon accommodation, and upon the movements of the iris. II. Paralysis and debihty. III. Spasm. CHAPTEE X. INFLUENCE OF THE NERVES UPON ACCOMMODATION AND UPON THE MOVEMENTS OF THE IRIS. § 43. The Movements of the Ieis. The mechanism of accommodation has already come under our consideration (§ 4). Here we may therefore confine ourselves to the movements of the iris. The movements of the iris are of two kind,s : reflex and voluntary. Reflex action is exhibited as constriction, of 'the pupil, in donse- quence of the stimulus of incident light upon the retina. Foiitana* has shown that the light falling upon the iris pi^oduces Do remark- able contraction. We have confirmed this resjolt by cailsing the image of a sm^U distant light to fall, by means of a' convex lens, upon the iris, where'by during slight perception df light, a doubtful contraction occurred, which gave way to a strong contraction so soon as the light, entering through the jpupil, excited a vivid perception.f That this contraction takes place by reflex action of the optic nerve upon the ocuio-motor nerve in the brain was proved by Mayo,f by striking experiments upon pigeons^ Nevertheless, the experiments of Harless§ and of Budge || have shown, that even after death, so long as irritability remains, the pupil still contracts upon the con- tinued action of light. Of the correctness of this observation we have satisfied ourselves. In a dog, killed by loss of blood, the one eye was closed, the other was opened and turned to the light : after the lapse of an hour the pupil of the opened eye was perceptibly smaller than that of the closed eye. The latter now remained also exposed to the light, and on the following day the diameter of both * Dei Moti dell' Iride. Luoca, 1765. Compare also Pro^romma, etc., cui inest Diss. E. H. Weberi, Summam doctrince de motu iridis continens, 1821. t Compare de Ruiter, De actione Atropai Belladonnce in iridem. Trajecti ad Rhenum, 1853, and in Nederl. Lancet, III. p. 433. From these investiga- tions, made under my direction and with my assistance, upon the iris and belladonna, I have borrowed, when I use the plural " we," and quote the dissertation. t Mayo, Anatomical and Physiological Commentaries, No. II. 4to. London, 1823. ^ Die Muskelirritahilitdt, Miinohen, 1850. II Cumptes rendus, T. xxxv. p. 661. DIRECT AND CONSENSUAL CONTRACTION. 573 eyes was equal. The upper jaw, alone with the eyes, was taken out of some frogs, one eye was exposed to the light, whUe the other was covered with a closely-folded piece of black paper : after the lapse of haK-an-hour, the pupil turned to the light was narrow, the other was wide. But the latter also contracted almost immediately after the removal of the paper.* When light falls upon one side, the pupil contracts on both sides : the contraction on the same side we call direct, that on the opposite side, we call consensual.f We may accurately study these two, as well as the accommodative (Listing), by ourselves, after the entoptic method (compare p. 197). A small opening in an opaque plate, held at about 6" from the eye, and turned towards the light, gives, in the vitreous humour a bundle of nearly parallel rays, of the size of the pupil, and is therefore seen as a round, illuminated disc, whose diameter increases and diminishes with that of the pupil. If both eyes had been closed, and if one be now opened, the pupil is seen almost immediately to contract, and then slowly and vibratingly to dilate again : the little light entering the eye through the opening is sufficient to excite strong contraction. The consensual contraction, on the contrary, according to Listing,f does not begin until § second after the opening of the other eye, lasts about i second, after which the pupil again dilates slowly and vibratingly for some seconds. The consensual dilatation, he observed to commence about J second after the closing of the other eye, and with diminishing rapidity to con- tinue for one or two seconds. This last continues in my eyes con- siderably longer. § The whole course of consensual contraction and dilatation (opening of the left eye, contraction of the right, closing of the left, dilatation of the right), which, with Listing, lasts |-f-i-(-J-(-l to 2 = 2'1 to 3'1 seconds, with me occurs ten times in the minute, and therefore lasts six seconds. The dif- * Further investigations on this subject have been instituted by Kuyper {I. c), H. Miiller ( Wurzh. Ahhandlungen, x. p. L.), and especially by Brown- Sequard {Journal de physiologie de I'homme et des animaux, 1859, T. ii. pp. 281 et451). f When, in the absence of the direct, consensual contraction is present in one eye, we are justified in inferring the existence of blindness in that eye. If both are present, or both are wanting, no certain conclusion is to be drawn as to the power of vision. Compare, with respect to this last, Mac- kenzie, The Physiology of Vision, London, 1841, p. 198. t Beitrag zur physiologischen Optik,Q:'6itmgen, 1845. § Conf. Nederlandsch Lancet, 2" Serie, 1846, D. ii. p. 442. 574 INFLUENCE OF THE NERVES UPON ACCOMMODATION. ference lias reference especially to the duration of the consen- sual dilatation, whose maximum it is difficult exactly to determine. — In these experiments the closing must, for more than one reason, be effected only by holding a screen before the eye. The accommodative movement is, as well as the accommodation itself, to be considered voluntary. It is true, we contract our pupil, without being conscious of the contraction of muscular fibres, but this holds good for every voluntary movement. When a person raises the tone of his voice, he is not conscious that by muscular contraction he makes his chordae vocales more tense; he attains his object without being aware of the means by which he does so. The same is applicable to accommodation for near objects, and to the contraction of the pupil accompanying it. The fact that this last is only an associated movement, does not deprive it of its voluntary character, for there is perhaps no single muscle which can contract entirely by itself. E. H. "Weber* has discussed the question, whether the contraction of the pupil is associated with the convergence of the visual lines, or with the accommodation. Prom his experiments of seeing acutely the same object, alternately through concave and through convex glasses, he came to the conclusion that the pupil neither contracts nor dilates without change of convergence. Cramer f repeated these experiments, but without taking sufficient care that vision took place through the axes of the glasses, so that, on removing the latter, some change of convergence was readily produced. Nevertheless, in my experiments with Dr. de Euiter, I came to the same conclu- sion as Cramer, namely, that tension of accommodation, even without increase of convergence, is attended with contraction of the pupil. Now, on repeating the experiments, also without the use of glasses, and being able, the fixation of the same point remaining unchanged, to put my accommodation alternately more and less upon the stretch, I find that, especially in looking at a remote object^ each stronger tension is combined with contraction of the pupil. These experiments, in which the contraction of the pupil presents so completely the voluntary character, are still more un- assailable than those with glasses, in which the change of the intensity of light is not altogether avoidable. — That increased con- vergence ,of the visual lines without change of accommodation also makes the pupil to contract, is easily proved by simple experiments with prismatic glasses. • I. c. p. 12. t Met accommodatie-vermogen der oogen. Haarlem, 1853, p. 115. FUNCTION OF THE CILIARY SYSTEM. 575 Listing observed that the accommodative contraction of the pupil takes place almost contemporaneously with the will, just as is the case in movements of ordinary muscles. It is, however, easy to show that, even if contraction and extension commence almost simultane- ously with the will, they by no means take place with the rapidity which is peculiar to voluntary muscles. By alternating the accommo- dation for a remote and a near object, I cannot voluntarily strongly contract and dilate the pupil more than thirty times in the minute. §44. The Ciliary System and its Function. We have already shown that accommodation is dependent on muscular action in the eye. In this organ there are no other muscles than those of the iris and the ciliary muscle, which have been described at p. 23. Now, accommodation is produced by the ciliary muscle, probably even exclusively by this muscle (compare p. 26). But the movements of the iris are associated with the accom- modation; they are governed by the same nerves as the latter; even a direct relation cannot be considered impossible, so long as the mechanism of accommodation has not been clearly elucidated ; if we add that the disturbances of accommodation are most distinctly revealed in deviation of the movements of the iris, it will certainly be allowable to treat in common of the nerves of the iris and of the ciliary muscle. The parts enumerated derive their nerves from the ciHary, also called the ophthalmic, ganglion. This ganghon gives off from 10 to 16 minute branches, the ciliary nerves, which perforate the sclerotic, not far from the optic nerve, and proceeding straight forwards between the sclerotic and the chorioidea, reach the ciliary muscle and the iris, and give some filaments to the cornea. One or two ciliary branches come directly from the naso-ciliary nerve, perforate, as well as the others, the sclerotic, and, according to Bernard's statement,* finally pass into the conjunctiva and the iris, but not into the cornea : their origin indicates that they act chiefly as nerves of sensation. — Into the ciliary ganglion three so-called roots enter : the short root from the oculo-motor nerve, the long root (often existing doublet) from the naso- ciliary nerve; and, lastly, a branch, derived from the sympathetic nerve * Bernard, Lemons sur la physiologie et la pathologie du sysUme nerveux, Paris, 1858, T. ii. p. 86. t Conf. Hyrtl. Serichtigungen uber das CUiarsystem des menschhchen Auges,i.D. Med. Jahrb. Oesterr. B. xxviii. s. 1. 576 INFLUENCE OF THE NERVES UPON ACCOMMODATION. in the neck. In the ciliary ganglion numerous gangKonic cells are met with. What the connexion is between the three kinds of nerve- fibres enumerated and these ganglionic cells, has not been ascertained^ . nor whether fresh nerve-fibres here have their origin, joining the ciliary nerves. All the ciliary nerves, the commencement of whose course we have already described, divide in the first in- stance near at hand and farther up the outer surface of the ciliary muscle into two, and afterwards into more numerous branches, which form a rich plexus {orbicuhis ciliaris of W. Krause), whence many little fasciculi penetrate into the ciliary muscle. I examined them in 1853, with Dr. de Huiter, in white rabbits, and as the result of that investigation, the following was noted :* " Many branches pro- ceed, after having formed a plexus on the ciliary muscle, to the peri- phery of the iris, and there again form, near the margin of the latter, a plexus of tolerably large branches, whence smaller branches take their origin and establish a third plexus of the iris, in that part where the muscular fibres are observed to run a circular course. The nerve-tubes belong for the most part to the slighter kind, be- come by ramification still slighter, have a very long isolated course through the iris, which is particularly true of the thicker nerve-tubes, and in their progress form many loops, from whose extremities new fibres again arise by division, which again form loops, not, however, to be considered as terminal loops, as we can often observe further ramifications of the tubes, which, after having lost the medullary sheaths, in many places terminate in an invisible manner.'"' This re- sult has been confirmed by subsequent investigators. I have now, in reference to the nerve-fibres, nothing essential to add. In the mean- time, in the peripheral distribution of different nerves, especially in those of the involuntary muscles, ganglionic cells are found, and this is true also of the ciliary nerves. Not only have they been demon- strated by H. Miiller and Schweigger (compare p. 381), and recently also by Saemisch,t in the chorioidea, where, too, the muscular fibres are not wholly wanting, but also particularly by H. Miiller,! in tie orbiculus ciliaris, where C. Krause § already mentions them. Miiller saw, in the ramifications of the first and second order of the ciliary nerves in the ciliary muscle, some beaufiful and distinct cells, some- * Nederlandsch Lancet, D. iii. p. 436. t Beitrdge zur normalen und pathologischen Anatomie des Auges, Leipzig, 1862. PI. II. Figs. 2 and 3. t Wurzb. Abhandl. x. p. 108. § C. Krause, Handhuch der Anatomie, 2' Aufl. B. i. p. 526. ACTION OF THE OCULO-MOTOR NERVE. 577 times with two or three efferent brauches^ whose transition into nerve-fibres with medullary sheaths was not, however, with certainty re- cognised. He thinks, nevertheless, that they may be regarded as gang- lionic cells. This discovery was confirmed by W. Krause* in all its parts. Moreover, Miiller (l. c.) found even in the finest nerve-bundles of the ciliary muscle, where the primitive fibres also ramify, little thick- enings of the fibres, in which a small round or oval body was to be seen, presenting the appearance of a bipolar ganglionic cell. While Miiller remaius in doubt as to the nature of these corpuscles, W. Krause {I. c), who found them constantly present in twelve subjects, thinks they must be regarded as genuine ganglionic cells, although he agrees that they are not connected with the axes of the fibres. It is not improbable that, in connexion with the ciliary nerves, stiU more of these groups of ganglionic cells will be found in the eye. After this brief anatomical exposition, we may now proceed to the consideration of the function of the ciliary system. Except the above-mentioned one or two ciliary nerves of separate origin, all the branches destined for the iris and for the ciHary muscle proceed from the ciliary ganglion. In this ganglion consequently the function is comprised. The question is therefore, properly, what influence each of the three nerves exercises on the ganglion, and what conditions of the internal muscles of the eye correspond to the conditions of the ganglion. On this subject, however, nothing can with certainty be said, and we can at present investigate only the influence of the roots of the ganglion, as this manifests itseK indirectly upon the muscles. TAe action of the oculo-motor nerve upon the sphincter of the pupil is established beyond doubt. Not only is the pupil dilated and immovable in paralysis of this nerve, but on irritating the nerve in the base of the brain in animals, we see it strongly contract. If Volkmann and E. Weber had, in opposition to previous investigations, seen dilatation produced by irritation, Budget showed that this was to be ascribed to simultaneous irritation of the sympathetic branch running in the neighbourhood, and retaining its irritability longer, and Nuhn,f who in a decapitated criminal had likewise seen dilatation, recognised, after experiments on different animals, the same source of error as Budge had indicated. In the beheaded man also, contraction * Anaiomische Untersuchungen, Hannover, 1861, p. 91. f Arehiv f. physiol. Heilhunde, 18.53, B. xi. p. 780. X Zeitschrift f. rationelle Medicin, N. F. 1853, B. iii. 37 578 INFLUENCE OF THE NERVES UPON ACCOMMODATION. of the pupil on irritation of the nerve in question was subsequently seen.* Most decisive, however, are the cases of complete para- lysis, because they show that this nerve is the condition sine qua non, both for reflex and accommodative movement of the pupil, and for the accommodation itself : no trace thereof remains, when its paralysis is complete. If other nerves also have influence upon the sphincter of the pupil and the ciliary muscle, they have it only by the intervention of the oculo-motor nerve. Nor are these positive facts shaken by the negative results of the investigations of Bernard,t who saw no change take place in the diameter of the pupil, either upon dividing the oculo- motor nerve, or upon irritating its peripheral portion, in rabbits. — No other disturbances than those above-mentioned take place on irri- tating the oculo-motor nerve. The only fact which exhibits the in- fluence of the intervening ganglion, is the comparative slowness of the contraction of the pupil. — Whether the oculo-motor nerve sends also some sensory filaments to the internal eye, cannot be ascertained. The influence of the sympathetic nerve upon the pupil '^ as discovered even before 1727, by Petit :% after dividing the nervus vagus he found the pupil smaller. That Petit had correctly ascribed this phe- nomenon to the division of the sympathetic nerve, which, in many animals, is in the neck united with the nervus vagus, was proved by Dupuy,§ who observed the same phenomenon after extirpation of the first ganglion. The accuracy of the fact was still further de- monstrated by the careful experiments of Eeid. |{ Budge and Waller^ have the merit of having shown, that the filaments of the sympathetic nerve acting on the pupil arise from the spinal cord, and pass into the anterior roots of the two inferior cervical, and the six superior dorsal, nerves. In frogs and rabbits the contraction consequent on division of the nerve is slight (and in the last it is certainly not greater * By Budge and Waller (Archivf. phy&iol. Seilk. B. xi. p. 775), and by Duval, Eochart and Petit (Gazette medicale de Paris, 1852, p. 457). t Lefons sur la physiologie et la pathohgie du systeme nerveux, Paris, 1858, t. ii. pp. 207 and 209. J Memoire dans lequel il est demontrg, que lea nerfs interoostanx four- nissent des rameaux qui se portent des esprits dans les yeux, in Histoire de I'Academie royale des sciences, Annee 1727. § Journal de midecine, de chirurgie, etc. 1816, T. xxxvii. p. 340. II Edinburgh Medical and Surgical Journal, August, 1839 ; and Physio- logical and Pathological Researches, Edinburgh, 1841, p. 291. T Comptes rendus, 1852, tomes xxxiv., xxxv., in different places. ACTION OF THE SYMPATHETIC ROOT. 579 when the first ganglion also is extirpated), in dogs it is very consider- able ; but the dilatation on irritation of the sympathetic nerve of the neckj which, though it does not directly follow, rapidly attains its maximum, is still the most striking phenomenon. — The difference of - the two pupils after division of the nerve is greatest while the eyes are exposed only to faint light : where there is a strong action of the sphincter muscle, the resistance even of the undiminished action of the radiating fibres, at least in rabbits, is almost annihilated. The difference in magnitude is, however, permanent; at least we have seen it continue in dogs and rabbits longer than six months j Budge has observed it even for a year. — Budge and Waller showed, that after division of the nervus vagus and of the sympathetic in the neck, in connexion with the difference in origin, of the former the inferior part, of the latter the superior, pass into degeneration (fatty metamorphosis). We have confirmed this in many cases, but in the first ganglion itself, and in the efferent fasciculi, whose number seems far to exceed that of the afferent, no change was met with. Irritation of the ganglion then also still produces dilatation of the pupil after the lapse of many weeks. If the first ganglion itself is extirpated, the efferent bundles too pass into a state of degeneration, but then also the ciliary nerves continue unchanged even into the iris, where we ex- ' amined them many weeks after extirpation of the ganglion, which is most probably due to the influence of the ciliary ganglion. Thus, too, it is intelligible, that the extirpation of the first gangUon is not followed by more contraction, than the division of the nerve below the ganglion. The foregoing shows that the action of the sympathetic root con- sists in a persistent exaltation of the tone of the radiating fibres. Thus the dilatator pupillse is with constant force the antagonist of the sphincter muscle. The action of the sphincter changes, as we have seen, both with the incidence of Ught and with accommodation ; but if the sphincter is paralysed, the pupil is immovable. Meanwhile we may assume that, just as for the vaso-motor nerves, the tonic action in the dilatator muscle may, under certain still unknown circum- stances (irritation of the fifth pair ?) somewhat fall and rise. — It has, indeed, hypothetically been assumed that the sympathetic nerve acts also upon the accommodation. Though it might appear some- what rash absolutely to deny an influence, for which the analogy of the iris may be appealed to, yet we find that it is supported by no fact whatever. We are acquainted with no muscle capable of 2 580 INFLUENCE OF THE NERVES UPON ACCOMMODATION. acting antagonistically to the ciliary muscle ; nor have we any reason to admit the existence of aciive accommodation for distance (com- pare p. 20). On the other hand, the action of the sympathetic branch on the tone of the blood-vessels is fully established. It is known that division of the sympathetic nerve in the neck is followed by considerable dilatation of the vessels of the head, most distinctly observable ia the ears of rabbits, while irritation of that nerve is attended vrith contraction of the same vessels (Bernard). With Dr. van der Beke Callenfels,* I showed that the vessels of the pia mater are governed by the same nerve ; and I subsequently satisfied myself, with Dr. Knyper,t that the vessels of the iris also contract on irrita- tion of the sympathetic nerve, even when they are distended under the influence of the instillation of digitaline or in consequence of discharge of the aqueous humour, or, as I recently found in conjunction with Mr. Hamer, when after the action of the Calabar bean, the same irrita- tion scarcely makes the pupil dilate. This last confirms my opinion, that this contraction of the vessels cannot be the mechanical result of dilatation of the pupil, but that it occurs independently. Formerly, indeed, I was inclined to attribute the dilatation of the pupil attendant on irritation of the sympathetic nerve to the con- traction of the vessels, as I supposed that the diminution of blood in the iris would at the same time lessen the contraction of the sphincter muscle. The phenomenon is, however, equally evident, when the circulation of the blood has already ceased, and the dilata- tion is moreover, too considerable, to be explained by the contraction of the vessels. We must, therefore, take refuge in contraction of the radiating fibres of the iris, which, as they are in some animals highly developed, can with certainty be demonstrated. The influence of the nervus trigeminus upon the iris and upon the accommodation is still doubtful. By exclusion we may assume that this nerve gives sensation to the iris ; for neither the oculo-motor nor the sympathetic possesses sensitive filaments by which the great sensibility of the iris might be explained. Moreover, sensation ceases when the nervus trigeminus is divided. The difficulty lies in the determination of the influence of the nervus trigeminus upon the motion of the iris. It has been found experimentally that irritation of the trunk of the fifth pair, as well as of its ophthalmic division • Nederlandseh Lancet, 1855, D. iv. p. 689. t Onderzoelcingen over de kunstmatige verwijding van den oogappel, Diss. naug. 1859. ACTION OF THE TRIGEMINUS. 581 (the nervus ophthalmicus Willisii) causes the pupil to contract. Now we are acquainted with no other contractions of the pupil than those produced by the reflex action of light, and by accommo- dation, and these both wholly disappear on paralysis of the oculo- motor nerve. The existence of a direct influence of the fifth pair upon the sphincter of the pupil (through motor-filaments) is therefore improbable. We are consequently led to assume that stimulation of the nervus trigeminus acts, both in its trunk and in its branches, upon the ciliary ganglion, so as there either to increase the action of the fibres of the oculo-motor, or to diminish that of the sympathetic nerve. The influence still takes place, if the sympathetic and oculo-motor nerves have previously been divided. This is, however, by no means strange, since the ciliary ganglion and the internal nervous system of the eye continue permanently normal after the sections aUuded to, as is proved, with' respect to the latter, by the unaltered action of atropia and of the Calabar bean dropped into the eye. The mechanism, whereby the nervus trigeminus acts upon thei ciliary ganglion is, however, rather obscure. Since this influence, as we have seen, continues after division of the oculo-motor and sympathetic nerves, it must be capable of taking place without reflexion in the central organs. Indeed, we can, from the fact that on stimulation of a nerve the change of the electrical condition is continued in both directions, very well comprehend the direct influence of a stimulus, without assuming in the nervus trigeminus the existence of fibres, whose ordinary function should be centrifugal conduction towards the ciliary ganglion. But if such exist (in the nervus lachrymaHs centrifugally conducting fibres are undoubtedly present), the contraction of the pupil which occurs in an irritated state of the peripheral sensitive filaments of the eye, might be explained by reflex action, upon those centrifugally conducting fibres, in the Gasserian ganglion. At any rate, in an irritated condition of the cornea, to which the ciliary nerves are distributed, reflex action even in the ciliary ganglion may be assumed from analogy, as, with reference to the secretion of saliva, reflex action through the submaxillary ganglion has been demonstrated by Bernard. The principal experiments on which the ahove view of the action of the nervus trigeminus upon the iris is based, are the following : — a. After division of the n. trigeminus at the base of the skull, the pupil contracts in rabbits (in dogs ?) ; but this contraction often does not occur before the lapse of some minutes, and in a few days or hours for the most 582 INFLUENCE OF THE NERVES UPON ACCOMMODATION. part disappears again (Longet, Budge). The same is true of frogs (Budge) : also on division of the half of the medulla oblongata (Joh. Mueller). h. On merely compressing the nervus ophthalmicus Willisii, Budge and Waller observed contraction of the pupiL Bernard observed the same after division of this nerve. The mobility of the pupil on the incidence of light is not impaired in this experiment, and if no inflammation of the eye en- sues, the diameters of the two pupils are soon again nearly equaL c. If the oculo-motor nerve be previously divided (Budge), or torn out (Bernard), contraction of the pupil nevertheless ensues, on division of the n. trigeminus. d. Bernard {I. c. T. II. p. 90) tore the oeulo-motor nerve at one side, whereupon the pupil became dilated, acquired, however, the same dilatation on both sides upon the instillation of extract of belladonna, and subsequently, upon dividing the n. trigeminus on the same side on which the oculo-motor nerve was torn, he saw the pupil contract. e. In a young rabbit he divided the optic nerve, and all the motor nerves of the eye ; even thea stimulation of the fifth pair produced con^sraction of the pupil. f. In another animal the first ganglion of the sympathetic nerve was removed, whereupon the pupil became small, with long vertical diameter; the nervus trigeminus having been then divided, the pupil became round, while the contraction increased. In all that I had seen and read respecting the influence of division of the n. trigeminus, the doubt had occurred to me, whether this section did not act upon the pupil principally in consequence of the filaments of the sympa- thetic nerve being at the same time divided, which filaments, according to Budge, reach the ciliary, through the Gasserian ganglion. I therefore de- termined, in concert with Dr. P. 0. Brondgeest, my assistant in the physio- logical laboratory, to make some experiments upon the subject. In rabbits the sympathetic nerve was exposed on one side of the neck, and was very gently stimulated for a moment (with the aid of the galvanic apparatus of du Bois-B,eymond) in order to ascertain that the exposed nerve acted on the pupil ; thereupon the skin was again closed with serresjines, and the trigeminus was divided on the same side, in the manner indicated by Ber- nard. If anaesthesia of the eye was obtained without further general dis- turbance, the sympathelio nerve was, after a shorter or longer interval, again stimulated, in order to see whether it had maintained its influence upon the pupil. The experiment succeeded in eleven rabbits. The following results were obtained, upon the bearing of which I need not further dwell. 1°. Division either of the Gasserian ganglion, or of the n. ophthalmicus WiUisii produces constant contraction of the pupil, with longer vertical dia- meter. Even with imperfect anesthesia of the surrounding parts of the eye, the contraction is still present, provided the cornea be insensible. 2°. The contraction diminishes within a few hours, but does not disappear, if irritation of the eye occurs, and if the blood-vessels of the iris are highly distended. The pupil then sometimes becomes angular. 3°. The contraction gives way to some dilatation when, even with com- plete anaesthesia, the eye being properly protected, the state of irritation is EXPERIMENTAL PROOFS. 583 absent. Dr. Snellen (De invloed der zenuwen op de ontsteking, TJtreoht, 1857) showed that the inflammation which is known to occur after division of the nervus trigeminus is the result of external injuries, which are not avoided owing to the existing anaesthesia, and that it can he prevented by systematic protection from all external injury. 4°. From comparative experiments on both sides, it appeared that the contraction is much greater on division of the n. trigeminus than on that of the sympathetic nerve in the neck, or on extirpation of the first ganglion. 5°, The tension of the eyeball remains at first the same, sometimes be- comes even rather greater, while the pupil is very narrow, and the iris lies near the cornea. The tension, however, in a short time, regularly dimi- nishes, and indeed in the highest degree, when the eye by proper protection remains free from irritation. This diminished tension is in accordance with my theory of glaucoma, as being originally neurosis of the nerves of secretion. 6°. After division and acquired ansBstheaia, stimulation of the sympathetic nerve produced, in seven out of eleven cases, dilatation of the pupil, though in a much slighter degree than when the n. trigeminus is not divided. In four instances the dilatation of the pupil on stimulation of the sympathetic nerve was entirely absent. In all these cases the cornea was completely insensible. In three of the seven cases, in which dilatation followed stimu- lation of the sympathetic nerve, there was sensibility of the lower, and in one there was, in addition, sensibility of the upper, eyelid ; in the three other cases there was no sensibility. 7°. In all cases, too, where the pupil continued immovable, the vessels of the ear contracted on stimulation of the sympathetic nerve, showing that the nerve was sensitive. Before the division of the trigeminus it was seen, that on stimulation simultaneous dilatation of the pupil and contraction of the vessels of the ear occurred. 8". After division Calabar produces contraction, atropia, dilatation of the pupil, — the latter not in a high degree. 9°. Dissection showed that in general, the nervus ophthalmicus Willisii liad been divided in front of the ganglion; sometimes also the second, and partly the third branch. In other cases the Gasserian ganglion itself was touched. It did not account satisfactorily for the cases in which little or no dilatation of the pupil occurred on stimulation of the sympathetic nerve. CHAPTER XI. PARALYSIS AND DEBILITY OF ACCOMMODATION. § 45. Mydeiatics and their Action. A COMPAKATIVE examination of a number of substances and of preparations has shown, that the most useful mydriatics are to be found among the Solanese, and that of these the Atropa Belladonna is, for various reasons, to be preferred to all others, even to the Datura Stramonium and the Hyoscyamus niger.* Above all, where strong action is not required, atropia (soluble in 450 parts of water), and where a stronger effect is indicated, the very soluble sulphate of atropia, which were first introduced into practice in England, are to be recommended. Eor the full effect, a drop of a solution of one part of sulphate of atropia in 120 parts of water (we express this strength by 1 : 120), is quite sufficient.t The internal exhibition of the remedy also, with which it is necessary to be cautious, produces my- driasis. The principal phenomena consequent on the instillation of sulphate of atropia, are : 1°, increasing dilatation, followed by insensibility of the pupil ; 2°, diminution, and soon total loss of accommodation. The dilatation of the pupil, considerable in man (especially in youth), the dog, and the cat, is less so in the rabbit, slight ia birds, in whom it was formerly overlooked, very perceptible in frogs, and not at all, or scarcely so, in fishes. After the instillation of 1 : 120 the dilatation begins in man within fifteen minutes, and in the course of * Compare Kayper, Onderzoehingen, over de kunstmatige verwijding van den oogappel. Diss, inaug. Utrecht, 1849. t If the instillation is followed in the course of an hour and a-half by pain and injection of the vessels, the preparation is not suitable, and there then arises, on its repeated employment, a peculiar inflammation, described by me as atropinism. Even when suitable, it produces in some persons, after having been used for many months, a similar inflammation, and it must then be altogether laid aside. In such cases other mydriatics are then seldom borne. Chemical reagents were not decisive in distinguishing the inapplicable sulphate of atropia. Compare Kuyper, I. c. EFFECTS OF ATROPIA. 58 from twenty to tweiity-five minutes attains its maximum, with ab- solute immobility. The younger the individual, and the thinner the cornea is, the more rapidly does the action occur. In frogs and in birds the dilatation disappears within one or two hours, and gives place to a brief contraction. The course of the dilatation in man, after the instillation of 1 : 120, is represented by the line dd oi Fig. 166, with the return to the normal diameter in 167. On the absciss a a (Fig. 166) the minutes are indicated after the instillation, which Fig. 166. i s 12. 16 io * I'f j^ Si 40 tfi 4s SI St eo ■~iirpirT"" jn ■■■■■■■■■■■■■■■BH^HinBBH ■SRSBBIiBpa ■s * ■§ 6 took place at 0, the lengths of the ordinates downwards perpendicular to a a', are the transverse diameters of the pupil, to be read off in mm. before the line. The pupil was always accurately measured at short intervals, with the ophthalmometer, with perfectly equal illu- mination of the eye, the other being closed. Fig. 167, whose absciss marks in days the total duration of the change of the pupil, in like manner gives the diameters in the curve under aa. The diminution of the accommodation commences somewhat later than the dilatation of the pupil. The accommodation gradually returns after some days, together with the mobility of the pupil. Fig. 166 indicates by the curve pp, the course of the absolute nearest point j by the curve r r, that of the farthest point. It will be seen that the latter undergoes scarcely any change; the nearest point, on the contrary, removes from the eye. This removal commences in from twelve to eighteen 586 MYDRIATICS AND THEIR ACTION. minutes after the instillation, is in twenty-six minutes, when the dila- tation is already nearly complete, still little remarkable ; then rapidly. Fig, 167. and subsequently slowly, proceeds, and attains its maximum in one hundred and three minutes after the instillation, when p and r coin- cide and the accommodation is therefore whoUy removed. When, after forty-two hours, the pupil is somewhat smaller, a slight degree of mobility has also returned, and, at the same time, some accommo- dation is to be observed, which now rather rapidly increases until the fourth day, but is not perfect until after the lapse of eleven days. The observation was made on the eye of my assistant, Mr. Hamer, who has practised himself in very accurately determining Ms absolute nearest point at the maximum of convergence, while one eye is closed (compare p. 117). — ^Besides the results to be deduced from the figures, we have still to remark : — 1°. After the return of the accommodation with the third and following days, the relative range of accommodation has a positioii similar to what it occupies in myopes : with moderate convergence, only a very small fraction of the existing accommodation can be ob- tained. Thus, Mr. Hamer, on the sixth day found, that, while with convergence to 9" of his eye which had not been subjected to instil- lation, about the half of the total accommodation came into play, the eye which had undergone the operation attained only a fifth of what EFFECTS OF WEAK SOLUTIONS OF ATROPIA. 587 it was now again capable of at the maximum of convergence. 2°. The farthest point has here, with a slight degree of M, continued nearly unaltered. Usually, it removes somewhat further from the eye. If there exists permanent tension of accommodation, such as is proper to H, and not unfrequently occurs in some amblyopes, as- tigmatics and in young myopes, this gives way under the influence of atropia, and r then removes to a much greater extent. 3°. With tension of accommodation, objects appear to become smaller (micro- pia) : in this case we imagine the object nearer, and as the visual angle has not become greater, we suppose the object to be smaller.* 4°. To the eye which has undergone instillation, objects appear much more strongly illuminated, especially when both eyes are at the same time open, under which circumstances, in consequence of consensual reflexion, the pupil of the eye not subjected to instillation is narrower than usual. The comparison is effected by looking at a bright object upon a darker ground, a prismatic glass, with the angle outwards, being brought before one of the eyes. The loss of accommodation, after the action of atropia, is the more troublesome, because the pupil is so extremely wide, which, even in slight variation of accommodation, produces great circles of diffusion ; therefore for each distance different glasses are neces- sary for acute vision, affording scarcely any range. The dlslmrlance, moreover, differs according to the refraction of the eye. Emme- tropes see well at a distance, but can distinguish nothing near with- out convex glasses. Myopes complain less, because, although their distant vision is much more diffuse, they can still often read, their farthest point remaining unaltered. In hypermetropes, even the slightest action of the mydriatic produces such disturbance, that without convex glasses nothing is distinguished. — If the mydriatic have been dropped into only one eye, the disturbance is the greater, because the defined image of the eye not operated on is so feebly illuminated, compared with the diffuse image of the other: the mydriatic eye is then by preference closed. In atropia-mydriasis of both eyes there is no diificulty in using convex glasses. With respect to the action of weaker solutions of sulphate of atropia. Dr. Kuyper has made some investigations. His researches have yielded the following results : — a. The solution of 1 : 1800 produces good dilatation within 30 * I observed this phenomenon first in myself, and gave the above expla- nation of it in Ned. Lancet, 1851, D. vi. p. 607. 588 MYDRIATICS AND THEIR A.CTION. minutes ; this attains its maximum, with complete immobility of the pupil, and generally with almost total annihilation of the power of accommodation, at the end of from 45 to 60 minutes. So early as on the following day, some mobility has returned. On the third day, no inconvenient disturbance remains, although two, three, or more days later, a difference in the diameters of the pupils is still perceptible. h. The solution of 1 : 2400 produces dilatation after from 25 to 33 minutes; this attains almost the maximum (diameter = 8j- mm.), after the lapse of from 45 to 50 minutes, without com- plete loss of mobility and accommodation ; after 55 minutes, sometimes no mobility is perceptible : after 70 minutes, the power of accommodation is very much diminished, but by no means removed. Even in a few hours later, the dilatation again diminishes. The following day it is still distinctly perceptible ; on the third day it is slight; it is not until the fourth day that it has wholly disap- peared. c. The solution of 1 : 9600 produces dilatation after 60 minutes, which slowly increases; after the lapse of 90 minutes, the pupU is perfectly dilated, but is not immovable ; the power of accommodation is but little diminished. The following day the pupil is very movable, and is much smaller. On the third day there is no trace of mydriasis. d. The solution of 1 : 14,400 causes commencing dilatation, after from 35 to 40 minutes; in from two to four hours the diameter of the pupil is 2 mm. greater ; after the lapse of seven hours, contrac- tion has again taken place, and on the following day no trace of mydriasis is perceptible. The mod^ of action of mydriatics I have investigated with Dr. de Euiter. Independently of qut researches, von Graefe, as he stated to me by letter, had arrived at the same results. Our experiments have established beyond doubt the passage of atropia into the aqueous humour. a. The action exhibits itself the more rapidly, the thinner the cornea, and the younger the animal is. Removal of the outermost layers of the cornea hastens the action (von Graefe). When some dilatation ensues upon inunction above the eye, a trace of the matter rubbed in may always have reached the organ. On strong action in the wounded skin (in dogs) some dilatation (the result of general action) rapidly sets in also on the other side. h. The application, confined to the cornea, in the eyes of frogs, after excision of the NERVES UPON WHICH THEY ACT. 589 heart, after decapitatiorij after removal of the brain and spinal cord, and even after complete isolation of the eyes, produces distinct dila- tation within a few minutes. The same took place in a chopped-off calf's head, and in recently killed rabbits, c. A. trace of an extremely dilute solution, introduced by means of a capiUary tube into the anterior chamber of the eye, produced dilatation in rabbits, d. After repeated instillation in a rabbit, the whole eye was washed out with a broad jet of water: the aqueous humour then discharged, introduced ' into the eye of a dog, and long kept in contact with it (von Graefe also injected it into the chamber), produced considerable dilatation of the pupil. This is incontestably the experimentum crucis. The quantity which penetrated is, however, extremely small, for the solution of 1 : 120,000 kept equally long in contact with the cornea, acted still more strongly. On internal administration and consequent mydriasis, the evacuated aqueous humour was in- efficacious. Lastly, the question is, through the intervention of what nerves the absorbed atropia acts. In the first place, we cannot admit that the matter acts directly upon the muscular fibre-cells : the similar nature of these contractile elements in the sphincter and the dilator should then lead us to expect a similar influence upon both, and strong dilatation of the pupil could not take place. We therefore infer that the atropia acts on the nerve fibres, or on the ganglionic cells, a. The sphincter muscle becomes paralytic: reflex and ac- commodative movements are abolished ; and, moreover, paralysis of accommodation (of the m. ciliaris) ensues, which, however, remains much longer incomplete than that of the sphincter of the pupil. Hence it follows, that the elements of the oculo-motor nerve are paralysed, — the more deeply-seated (of the ciliary muscle) being the last to be affected (an additional argument for the direct action of the atropia on the nerve-elements), h. The dilator muscle becomes strongly contracted. The proof consists in the fact that, as Euete* was the first to show, in complete paralysis of the oculo-motor nerve, the size of the pupil is still considerably increased by atropia; additional dilatation also occurs under atropia after removal of the nerve in question in animals. To explain this we assume a stimulating action on the sympathetic nerve, which we can scarcely imagine to be persistent unless it takes place by the intervention of ganglionic * Klinische Beitrdge z. Pathohgie und Physiologie der Augen und Ohren. Braunschweig, 1843, p. 250. 590 MYDRIATICS AND THEIR ACTION. cells. Of these it is known that they are specific in their action, and of a condition of persistent stimulation by a given substance we have an example in the action of strychnia brought into direct contact with the grey substance of the spinal cord.* After a powerful effect of atropia, I saw still further dilatation of the pupil arise on stimulation of the sympathetic nerve in the neck in rabbits, a proof that this nerve at least is not paralysed. If division of the sympathetic nerve have previously taken place, the pupil on the same side is not so fully dilated by atropia as that upon the other. Biffit and Cramer J saw also in this phenomenon a proof of the stimulating action upon the sympathetic nerve. We cannot admit it to be such : the difference between normal and increased action of the sympathetic nerve, as opposed to paralysis of the oculo-motor nerve, may be sufficient to explain the difference observed. The influence of atropia on the n. trigeminus is probably nar- cotising. However, both in cases of paralysis in man, and after the division of this nerve in the rabbit, the action of atropia is as usual : if the pupil was, as in Dr. Snellen's case, originally wider than in the other eye, it remained wider also after instillation in both eyes; if it was narrower, as in experiments on rabbits, it remained narrower also upon instillation. Eespecting the influence upon the vaso-motor nerves of the iris, nothing can with certainty be stated. The ancients (Conf, Plinius, Sist. Naturalis, liber xxv. cap. 13) were acquainted with the mydriatic action of some plants, and employed it in the depression of cataract. Such an action was ascribed especially to Anagallis, which, however, has not been confirmed. Of the influence of belladonna on the pupil we find mention first made by van Swieten ( Comment, in Boerhavii Aphorismos, t. iii.) ; moreover, by Reimarus (conf. Daries. Diss, de Atropa Belladonna. Lipsise, 1776), Mellin, Ray and others; and Loder (conf. Schi- ferli, Ueher den grauen Staar, p. 85) employed the infusion in the extrac- tion of cataract. Karl Himly ( Gott. Gelehrte Anzeige, 1800), who discovered the mydriatic action of hyoscyamus, has, however, the merit of being the first to make mydriatics in an extended sense available in ophthalmic surgery. Almost at the same time Darwin (Zoonomia, iii. 132, London, 1801) sug- * That Harley did not obtain this eflfect, is probably to be attributed to the employment of too strong a solution (conf. KoUiker, Verhandl. der Geselhch. Wiirzburg, B. ix. p. xvii.). t Intorno all'influenza che hanno sulVocchio i due nervi grandi sympathico e vago. Favia, 1846, p. 12. \ Set accommodatie-vermogen, 1853, p. 127. PARALYSIS OF ACCOMMODATION. 591 geated the advantage to be derived therefrom in some forms of ophthalmia. — The influence of mydriatics on the accommodation was not investigated until a later period, and then, indeed, with wonderful accuracy, by Dr. Wells {Philosophical Transactions, 1811, p. 378). In his experiments upon Dr. Cutting, it appeared that the accommodation was wholly lost, and even that the farthest point somewhat receded. In a couple of myopes, too, diminution of accommodation with unaltered farthest point was established. In connexion with the observation which we here find, that, on the passing off of the effect, the diminution of the pupil and the increase of the range of vision did not keep regular pace with each other (the occurrence of dilatation before the loss of accommodation was unobserved), Brewster came to the conclusion that, after the application of belladonna, another organ than the iris must also be paralysed. — Numerous other investigations were com- municated from many quarters, but contributed nothing essential beyond what is comprised in the foregoing section. Only it still deserves to be mentioned that, according to von Graefe, diminished tension of the fluids of the eye might also be admitted as a result of the use of atropia, of the cor- rectness of which view Dr. Sohneller {Archiv f. Ophthalmologie, B. II., Abth. 2., p. 95.) thought he found a proof in the dilatation of the vessels of the retina observed by him. I suggested a doubt whether, supposing that the observation is correct, it indicates diminished tension (conf.Kuyper, I. c). The paralysis of the accommodation certainly could not explain this, for, even if with accommodation temporary increased tension may arise, the relation between absorption and secretion, just as we observe in consequence of pressure with the finger on the eye, would immediately remove it by the absorption of a little fiuid. The tension of the eye depends upon the action of the secretory nerves, and these lie in the path of the n. trigeminus, the division of which, as numerous experiments have shown me, considerably lessens the tension of the eye. We should thus, a priori, think it possible that atropia, by paralysing these nerves, should diminish the tension, but I have never been able with certainty to satisfy myself of it. §46. MoEBiD Paralysis of Accommodation. Paralysis of accommodatioii as disease is by no means an unusual occurrence. Emmetropic and ametropic eyes are alike liable to it. It occurs too at every age, but in old persons, who have already lost their accommodation by senile change-s, it is of little importance. As we tnow, that the accommodation is effected exclusively by the internal muscles of the eye, we can seek paralysis also only in the fibres of the short root of the ciliary ganglion. Now, in fact, it often occurs that only these fibres are paralysed, and in this case we have paralysis of accommodation alone : except that paralysis of the sphincter pupillse, which derives its motor fibres S92 PARALYSIS OF ACCOMMODATION. from the same root, is usually combined therewith. But in about an equal number of cases there exists at the same time paralysis of other fibres of the oculo-motor nerve, and not unfrequently the paralysis extends even to all the branches of this nerve. It is re- markable, that while paralysis of accommodation very often occurs separately, paralysis in the domain of the oculo-motor nerve is com- paratively rarely met with, without paralysis of accommodation. I may add, that so far as my experience goes, uncomplicated paralysis of accommodation occurs much more frequently in women, often too in children; paralysis of the oculo-motor nerve on the contrary, including paralysis of the accommodation, is much more frequently found in men, and ordinarily not until after the twenty-fifth year. In either case, the paralysis is rarely complete : generally speaking, it is only paresis, inasmuch as a certain, though usually only a slight degree of accommodation, has remained. Uncomplicated paralysis of accommodation has only one objective symptom : dilatation and immobility of the pupil. The dilatation is not considerable ; for even with complete paralysis, a wider pupil than the normal in the dark is not to be expected. Nevertheless, in complete paralysis not a trace of either accommodative or of reflex movement is to be seen. But I may add, that these cases are ex- tremely rare. Purther, the connexion between paralysis of the pupil and of accommodation cannot be called absolute : once I found satisfactory accommodation still coexistent with absolute immobility of the pupil. In one instance too, paralysis of accommodation disap- peared without a return of the mobility of the pupil, — and, on the other hand, with perfect or almost perfect loss of accommodation, the motioQ of the pupil may be but little disturbed. From all this it is evident, that the subjective phenomena are the most important. Now, upon these the refraction of the eye has a considerable influence. Myopes, whose farthest point is not more than 14" from the eye, find no difficulty in reading, for this point remains unchanged, and although their nearest point then coincides with it, they see, with unalterable refraction, perfectly acutely at the distance of 14" or less. The disturbance is confined to this, that on the one hand, objects at a greater distance appear, on account of the greater circles of diffusion of the larger pupil, more diffuse than usual, — on the other, that within the distance of their combined nearest and farthest point, they cease to see acutely. Both disadvantages are in IN HYPERMETROPES. 593 great part removed when the paralysis of accommodation is incom- plete, and we then hear few complaints from myopes. It is only when they wear neutralising spectacles, and use them at their work, that they are on a footing with JEmmetrqpes. — These, on the occurrence of paralysis of accommo- dation, immediately resort to the oculist. They can no longer read nor write, and they are aware that an important disturbance exists : even when, as is usual, only one eye is affected, a certain dimness is, on account of the acute origin of the paralysis, forthwith observed, causing each eye to be separately tried, and thus the lesion is discovered. If we find that vision at a distance is acute, and with either concave or convex glasses becomes diffuse, while for near objects convex glasses are necessary, the diagnosis is made, which finds only a still further confirmation in the torpidity of the dilated pupil. The paralysis of accommodation is productive of yet greater dis- turbance in Sypermel/ropes : not only for near, but also for distant objects, with respect to which an involuntary accommodation for- merly easily overcame their hypermetropia, is their vision diffuse. It is evident, that such a condition suggests the idea of amblyopia, and I have already (p. 386) communicated a case in which the patient''s father, himself a medical man, feared the worst. By attending to the direction, in any disturbance of vision,* systematically to define with glasses the refraction, and the acuteness of sight jn distant vision (compare § 9), we shall be sure to avoid error : aj^yopia is thereby forthwith excluded, and while the glasses requiredrdr distant vision are insufficient for seeing near objects, the paralysis of accom- modation is recognised. The phenomena are less characteristic when no complete paralysis, but only paresis is present. The myope then often experiences no actual disturbance ; the emmetrope complains of fatigue only on tension for near objects, resembling the asthenopia of the hypermetrope (com- pare Chap. Y., p. 287) ; but the hypermetrope very rapidly experiences considerable asthenopia for near objects, and even difficulty in seeing acutely at a distance. In general with paresis of accommodation, asthenopia very quickly occurs ; in the first place, because the wider pupil requires more accurate accommodation to distinguish satisfac- torily ; in the second place, because, just as in atropia-paresis, the relative range of accommodation is very unfavourably situated : while with the maximum convergence, the closest point is found 38 594 PARALYSIS OF ACCOMMODATION. comparatively little farther from the eye, with medium convergence, only a slight tension of accommodation appears to be possible. — Sometimes in paresis of accommodation micropia is also complained of;* the explanation of which is given above (p. 586). The foregoing refers more particularly to the uncomplicated paralysis of accommodation. Often, however, this is, as we have seen, only a part of a more general morbid condition, and most frequently dtparal/yBis of the oeulo-motor nerve. If this is complete the upper eyelid hangs, and the outer angle of the slit even stands considerably lower than that of the other side (compare Kg. 168, taken from a photograph) — a proof that the (now paralysed) elevator of the upper eyelid is also the elevator of the conjoined lower lid. A very slight upward movement of the eyelid remains, in consequence of the fact that, on endeavouring to raise it, the musculus orbicularis palpebrarum, which is governed by the facial nerve, can relax itself still more : the latter muscle remains in any case capable of strong contraction. If we raise the paralysed eyelid, we find the cornea deviated outwards (Fig. 169), and on looking to the opposite side it Fig. 168. scarcely reaches the middle of the slit (paralysis musculi recti interni). On endeavouring to look upwards, the paralysed eye remains unal- tered in its place (paralysis mm. recti superioris et obliquiinferioris). On endeavouring to look downwards, the m. obhquus superior, * Demours in particular {Traite des maladies des yeux, T. i. p. 444) has remarked this. Compare also Duval, Ann. W Oculist , T. xxiii. p. 154. IMPLICATION OF THE OCULO-MOTOR NERVE. 595 whicli is governed by the n. trocUearis, alone acts, and tliis, especially when the eye is turned outwards, produces a slight inward rotation around the visual axis rather than a downward movement of this axis.* With these phenomena a misapprehension of the position, and an apparent movement of the objects, on every attempt at move- ment of the eye, are combined — ^the first dependent on an incorrect estimation of the position of the eye, the second on the loss of adequate correspondence between the attempt at movement voluntarily made and the real movement of the field of vision on the retina. The complaint of vertigo is thus explained. — In other cases only a part of the muscles, influenced by the third pair, is imphcated in the paralysis; sometimes the m. levator palpebrse superioris, which does not readily remain wholly free, and also very easily shows the disturbance, alone is affected, sometimes the m. rectus internus; very seldom the m. obliquus inferior is imphcated, notwithstand- ing that it is precisely the branch destined for this muscle, which gives off the short root of the cihary ganghon, of which wholly uncomplicated paralysis very often occurs. — Not unfre- quently, too, in both eyes, different muscles governed by the third pair are at the same time or consecutively, more or less para- lysed, whether with the accommodation or not. — Purther, we ob- serve that the fourth and sixth pairs are also affected (I have even seen a case in which only the musculus rectus externus and the * The rotation mentioned here, was observed and analysed/by me in a case described in the Nederlandsch Lancet, 1850, D. vi. p. 4l and credulous people have admitted hysteria and hypo- chondriasis Into the list of causes. On the subject of treatment we may be brief. The so-called rheu- matic paralysis often gives way spontaneously, most frequently after two or three months. It is pretty generally the custom, under such circumstances, to apply an ointment with Veratria around the eye, and to give secale cornutum internally, and in this respect I myself follow the example of others; but it is very hard to satisfy one's self by comparative observation that this plan is attended with any benefit. TREATMENT. 597 The myosis, which is the result of the employment of Calabar, even in paralysis of the oculo-motor nerve, may in every case pro- duce symptomatic improvemen.t. How far this remedy is other- wise indicated, further experience must decide (compare the close of § 48 on Myosis and Myotics). On the supposition of the existence of constitutional syphilis, an antisyphilitic treatment is often tried, and the preference is generally given to a short course of inunction, which, however, as every other treatment, is frequently unattended with any marked result. — Where the nervous system is more generally implicated, fegimen and treatment are directed to that condition, without special attention to the paralysis of accommodation. Eespecting the use of spectacles in paralysis of accommodation, it is almost sufficient to observe, that there is scarcely ever any objection to bringing the point of distinct vision to the distance which the existing acuteness of vision and the nature of the work to be performed render desirable. Sometimes, however, especially when the paralysis is in- complete, we give weaker glasses, so that the tension required ensures practice of accommodation. But if asthenopia then occurs, we do not withhold stronger glasses. Whether in paralysis of accommodation of one eye the assistance of a convex glass is to be afforded, must be judged from what has been said as to the use of glasses in difference of refraction of the two eyes (compare § 42). We should bear in mind, that in the stationary refraction of the one eye, the same glass can in this case be useful only for a given distance. The principal objective phenomenon of paralysis of accommodation — the wide, immovable pupil — was the first to attract attention, and was designated bythe ancients under the term mydriasis. But a dilated pupil being a symptom of amaurosis, the disturbance of vision in simple paralysis of accommodation was also looked upon as a slight degree of amblyopia, or was ascribed to the excess of incident light. Even in our own day this error is not unusual. It was in fact incredibly long, before ophthalmologists in general had a sufBciently correct idea of accommodation properly to comprehend its devia- tions. And this in spite of the excellent example of Dr. Wells (^Philo- sophical Transactions, vol. ci., p. 378, London, 1811), who, in 1811, cor- rectly recognised and understood a case of paralysis of accommodation, and terminated his description with the following pregnant words : — " Prom these circumstances it was plain that this gentleman, at the same time that his pupils had become dilated, and his upper eyelids paralytic, had acquired the sight of an old man, by losing suddenly the command of the muscles, by which the eye is enabled to see near objects distinctly ; it being known to those who are conversant with the facts relating to human vision, that the eye, in its relaxed state, is fitted for distant objects, and that the seeing 598 PARALYSIS OF ACCOMMODATION. of near objects accurately is dependent upon muscular exertion." Now, as Ruete states, we find mention made by some writers, as E. Home, Sichel, and Canstatt, in oases of paralysis of the ooulo-motor nerve, of diminished accommodation for near objects ; but still we miss the accurate determina^ tion, and, in general, any correct idea of the state of the case. James Hunter {Edinburgh Medicaland Surgical Journal, Jan. 1840, p. 124) even thinks that a case of mydriasis occurring suddenly in a chUd, in which near objects could be distinguished only with the aid of convex spectacles (J), must be ascribed to spasm. With reference to this case, Himly (I. c, B. 2, p. 481) proposes the question, whether paralysis should not rather produce presbyopia, and, in fact, he describes paresis of accommodation under the name of suddenly-occurring presbyopia. In like manner, what was described by von Walther (Journal von Graefe und Walther, B. iii. p. 22), as amaurosis ciliaris; by Sichel (Ann. d'oeuUst., 1853) as amblyopie presbytique, might also be nothing else than paresis of accommodation. Meanwhile Euete (I. c. 1843, p. 246) treated in detail of the influence of paralysis of the third pair upon the accommodation ; but, as in the three cases which he happened to meet with, scarcely any disturbance existed, he thought that no very great influence upon the accommodation is to be attributed to this nerve. Thus ignorance or doubt remained. I know not who was the first to suggest clearer and correct ideas. But it seems as if these were included in the discovery of the principle of accommodation, and on that discovery, as it were, spontaneously came to light in different quarters, although Cramer, who discovered that principle himself, was still deceived as to the nervous influence in accommodation. A more accurate observation of the cases which presented themselves certainly proceeded from the introduction of the determination of the range of accommodation. Mydriasis, as a symptom of blindness, almost always depending on a cerebral cause, does not belong to my subject. With respect to mydri- asis, independent of paresis of accommodation, I might probably also be silent. I believe, in fact, that in the great majority of cases, either the paresis of accommodation was overlooked, or the existence of mydriasis was lightly assumed. Certainly it has not been sufficiently kept in view, that young children usually have large pupils, and thus all sorts of causes have been called upon, but especially the presence of worms, to explain a phenomenon which, in childhood, needed no explanation. Accurate determi- nations of the size of the pupil, the light being the same, may in each case be required : in my own investigations, in which the measurement was effected with the ophthalmometer, I was struck with the great similarity of the pupil in the same individuals at different times, the illumination and the accommodation only being equal. Moreover, cases are communi- cated in which, with a general tendency to spasm, with anaesthesia, also with irritation of the fifth pair, there was a particularly wide pupil, but respecting the accommodation nothing is here noted. A remarkable case, with irregular, almost whimsical, sometimes periodically recurrent, mydriasis, for a time alternating in the two eyes, with accompanying paresis of accommo- PARESIS AFTER DIPHTHERIA. 599 dation, is recorded by von Graefe (ArcMv f. OpUhalmoloyie, B. iii., H. 2, p. 359). In this instance the power of vision was permanently disturbed, and probably a cerebral affection was at the bottom of it. Von Graefe says he has also observed mydriasis to be a precursor of mania. — That an irritated condition of the sympathetic nerve in the neck (irritation of the abdominal portion in animals does not produce mydriasis) might lead to uncomplicated mydriasis, without any influence on the accommodation, is, a priori, very probable ; but the cases in which the occurrence thereof should be proved are almost wholly wanting in literature, and in nature I have so far not met with them. §47. Pabesis of Accommodation after Diphtheuia faucium and weakening of accommodation. For some years past a malignant disease, known by the name of diphtheria faucium, or angina diphtheritica (better diphtherina) , has prevailed, in certain countries of Europe. In the commencement of 1860 it began to manifest itself in several places in the Netherlands, where it is still prevalent, and even in the course of the past year carried many to the grave.* In France and elsewhere, different forms of paralysis had been observed, as sequelae of this diphtheria. Among these mention had been made of disturbance of vision, but without just appreciation of its nature.t Soon after the occurrence of the disease, cases presented themselves to me, to the connexion of which with the angina in question I was led by a particular circum- stance. It immediately appeared to me, that what was considered as disturbance in the function of the retina, was a simple paresis of accommodation; and I subsequently had opportunities of satisfying myself of the correctness of this view in a great number of patients. * See the Reports in the Nederlandsch Tydschrift voor Geneeskunde, 1863. f Mention had already been incidentally made of paralysis after diph- theria, by Bretonneau, Trousseau and Blache, and a detailed description of a case observed by him, with a brief report of six others, was given by Faure {L' Union Mid., Nos.. 15 and 16, 1857). Although general paralytic symptoms are the most prominent, mention is made also of paralysis of the palate and of " weakness of sight," once too of strabismus. Cases are given also by Richard and by Mayer (conf. Eisenmann, Canstatt's JahresbericM, 1858), in which mention is made of disturbance of vision. — Since my own investigations, confirmatory observations have reached me from various quarters. 600 PARESIS AFTER DIPHTHERIA. The foUowing is a history of the course of my observations on this subject : — * I. Miss D., aged 26, applied to me on the 22nd of May, 1860, complain- ing of disturbance of vision. On examination, it was found that this dis- turbance depended on diminution of the power of accommodation. Remote objects were seen quite acutely, and were rendered diffuse both by convex and concave glasses : emmetropia, therefore, existed. The distance P of the nearest point (at 26 years of age, with normal range of accommodation of the emmetropic eye = from 4J to 5 Parisian inches) was reduced for the right eye to about 24", for the left to 12". With glasses of j'j, R was for the right eye ^ 12"; for the left R was = 8"- The pupils were wider than usual, particularly the right one ; the reflex movement was tolerably good, the accommodative movement, particularly in the right eye, was very limited. Rather more than five weeks previously the patient, then lodging at Bennekom, had suffered from inflammation of the throat. Having returned to Utrecht, she for the first time remarked, about fourteen days before, that she could no longer see near objects acutely. She could read only a few lines, and that not unless at a comparatively great distance ; there- upon everything ran together ; the letters were unrecognisable ; the lines appeared to be strokes j the eyes were as if fatigued. These symptoms resemble those of asthenopia. But there was no manifest hypermetropia ; and with atropia-paralysis scarcely Hm ^ appeared, which is usual with emmetropic eyes. We had, therefore, not to do with ordinary asthenopia. Indeed, both the history of the case and the present symptoms were opposed to the affection being of that nature. As to the first, — the disturbance had occurred suddenly, at least within a few days, without any particular fatigue having been remarked before, even on continued work. In asthe- nopia from hypermetropia, on the contrary, the troublesome symptoms come on either very slowly, at first apparently periodically, or after particularly weakening causes. And as to the symptoms : the nearest point was too far from the eye ; reading, &c., was, even on the first attempt, too difficult; rest of the eye was attended with too little temporary improvement. Moreover, the peculiar feeling of pressure in the forehead, which brings the hand involuntarily to that part, was wanting j while, on the other hand, the wide and, in accom- modation, too slightly movable pupils, pointed directly to paresis. The cause of this paresis was meanwhile obscure. In children, • The particulars have been given in detail by me in the Archivfur die Holldndische Beitrage mr Natur. und Seilkunde, published by F. C. Bonders and W. Berlin. B. 10, 1861. PECULIAR LESION OF SPEECH. 601 according to my experience^ loss or diminution of the accommodation, without paresis of the muscles of the eye, in both eyes alike, does not occur so rarely, and usually gets well within two or three months : the cause then remains quite unknown, and recovery ensues without anything particular having been done. In adults this is quite different : paralysis of accommodation in hoth eyes together is in such persons a very unusual occurrence, certainly still rarer without further paralysis of the muscles of the eye and eyelids. I was therefore unable to give any certain prognosis from experience. The prognosis, however, could not be favourable, inasmuch as the simultaneous occurrence of the affection on both sides led me to suspect the existence of a central cause, to which dulness in the head, slight attacks of vertigo, and sometimes even violent headache, gave stiU further support. Derivation by the intestinal canal, pediluvia, stimulating frictions in the frontal region, were prescribed, and rest was recommended. Subsequently glasses of =-^ were permitted for near objects, by which all difficulty for close vision was removed. In examining this patient, a peculiar lesion of speech did not escape me. I suspected that it must have depended on a congenital defect. Through motives of delicacy I was unwilling to. ask the patient directly about it ; nor did she allude to it. II. About a fortnight after, a youth named E.., aged 15, of fan* complexion, pale, and rather slight, was brought to me. His complaints were in all respects the same as those of Miss D. The power of vision was, however, still more limited : at a distance he saw acutely ; as for near objects, on the contrary, he could absolutely not read ordinary type. The closest point of distinct vision could not be directly determined ; with glasses of \ it lay at 7". The pupils were large, reflex motion was slight, accommoda- tive movement was scarcely perceptible. It struck me that in this boy there was a lesion of speech similar to that in the case of Miss D. He, too, had suffered from sore throat. More- over, he oame from Ede, a village in the immediate 'neighbourhood of Bennekom. Still more : I heard that in the same Bennekom several other people, who had likewise suffered from inflammation of the throat, presented both disturbance of vision and difBculty of speech. This fact appeared to me to be truly important. In R., I now, in the first place, investigated everything which had refer- ence to the modification of the voice and speech. The disturbance had in this boy remained directly after the inflammation of the throat ; in Miss D. it had not, as I subsequently ascertained, been developed until some time after the sore throat had given way. The mucous membrane of the mouth and throat was normal, rather pale 602 PARESIS AFTER DIPHTHERIA. than red ; the tonsils were scarcely swollen. The uvula, however, was extremely long and was absolutely immovable. — If we look at the palate in the normal state, the tongue heing a little depressed and the nose closed, so that respiration must take place through the widely-opened mouth, the palate hecomes retracted, and the uvula is at the same time usually short- ened and elongated alternately. On an effort at swallowing, which mechanism succeeds best in this position when the lower jaw is steadily fixed, the soft palate ascends stiU more, the arches contract, and the first time at least the uvula is simultaneously retracted. Ahout the same is observed on every effort to speak : it is best to make the patient utter the sound a (as in art), which is very possible with wide-opened mouth and depressed tongue. StUl greater are the contraction of the arches and the elevation of the palate, with retraction of the uvula in the commencement of the vomiting movement, which occurs on tickling the pharynx with a fine feather. Now, in all these experiments the uvula of our patient remained equally long and immovable, the ascent of the palate was very limited, and the pharyngo-staphyline arches approached each other but slightly. Evidently, therefore, the azygos uvulae was paralysed, and the other muscles of the palate were more or less affected by paralysis. With respect to speech, a double abnormity presented itself, namely, speaking through the nose, and the accompaniment of many sounds with a rattling or snoring^ accessory sound. The rattling sound was evidently dependent on a quivering of the uvula which had come into contact with the root of the tongue, and it was strongly heard with particular consonants, as well as with the vowel a (as in able). The speaking through the nose was in itself a proof that, in consequence of the paresis of the soft palate, the nasal cavity was not closed. The nasal sound was most strongly heard in the pronunciation of o ; but with all vowels it existed more or less, and it was also audible with all the soft con- sonants. (I use here the terms employed by Dr. R. Lepsius. Standard Alphabet, etc. London, 1855.) Moreover, the communication with the nose in the pronunciation of each vowel was shown by the movement imparted to the down of fine feathers, when this was kept under the nostrils upon a sheet of paper held against the upper lip. Also, in the compression of the alae nasi, the slight descent of the tone in each sound, as well as the increase of the nasal sound was distinctly established. Lastly, the impeded or retarded movement of the palate appeared from the impossibility of giving a soft pro- nunciation to the explosive consonants. These consonants are uttered by closing the opened, or opening the closed, mouth. On closing between the lips,^ is uttered ; on closing between the anterior part of the tongue and the anterior part of the hard palate, t is pronounced ; on closure between the more posterior parts of the tongue and palate, k is expressed. If at the same time we make the voice ring, p, t, and k give way to the soft con- sonants b, d, and g (as in game). Now, to produce these sounds, the nose must be shut off from the throat. If this separation be wanting, we produce, instead of b, d, and g, the resonants m, n, and ng. This appeared to be actually the case in our boy : a soft explosive consonant at the end PECULIAR LESION OF SPEECH. 603 of the word gave way to the corresponding resonant. English words, whose explosive sounds are expressed also at the ends of the words with a ring- ing voice, hest illustrate this,— for example, ruh, head, and egg. Now, if E. tried to pronounce these words well, he invariably said rum, hen, and eng ; or, rump, hent, and enk. Even after having heard how much these sounds differed from the required, rup, het, and ek were sometimes elicited. But it was only necessary to impress upon him that the final consonant should he expressed with a ringing voice, and then it again became each time, rum, hen, and eng. If the soft explosive consonants were not at the end of the words, their sound was characteristically heard, but yet it began with the resonant ; 1/and was pronounced mband, door as Mdoor, give as n^give. On the contrary, scarcely any deviation was perceptible in the pronunciation of the hard non-ringing explosive consonants, p, t, and k.* * I formerly paid much attention to the subject of articulation, and was therefore in a position to observe accurately the phenomena connected with that function. They often give the first indication to the oculist. I may therefore be allowed here briefly to explain them. When after a vowel, pronounced through the nose, the soft explosive consonant is to follow, the nose must be completely separated from the throat, at the same moment as the cavity of the mouth between the lips (with h), or between the tongue and palate (with d and g) is closed. If the separation does not take place at the same time, we hear the resonants m, n and ng, in place of the explosive consonants h, d, and g. For the mechanism is precisely the same for both, except that in the resonants there is a continuity between the cavity of the nose and that of the throat, which is wanting in the explosive consonants. Now if there be paresis of the palate, in the articulation of the vowels the nose is not com- pletely closed, and it is evident, that on the effort immediately after to pro- duce a soft ringing explosive consonant, the complete closing follows either not at all, or at least too late. Instead of rub, head, and egg, we therefore hear in our patient, rum, hen, and eng. On increased exertion they become rump, hent, enk (properly engk — for the n before k is always ng) : the hard explosive consonant is added. The mechanism of this last, in fact, presents no difficulty. After the sound of the voice has ceased, the closing of the mouth needs only to be interrupted by some impulse of air, to make the hard explosive consonants heard. The nose indeed remains open, but the sound of an explosive consonant is with a non-ringing voice much stronger than that of a resonant, and therefore we hear the first distinctly, the latter not so. Hence it will be understood that our patient had no difficulty in pro- ducing rup, het, and eh. For this the voice needed only to be brought to silence immediately after the vowel, then the resonant was not heard, and the interruption of the closing of the mouth caused the hard explosive con- sonant to be heard. If it be asked, lastly, why the soft explosive consonants could be produced better at the beginning of a word than at the end, the answer is simply that in that case the closing of the cavity of the mouth and the removal of the continuity of the cavities of the throat and nose needed not to take place precisely at the same moment. In pronouncing hand, door, &o., the cavity of the mouth was first closed and, before the voice 604 PARESIS AFTER DIPHTHERIA. The semiparalytio condition of the palate led me to suppose, that swal- lowing also would not take place regularly. When questioned on the subject, the patient stated, too, that he could swallow solid food only with, great effort, and that in drinking he was obliged to proceed slowly and cautiously. Fluids passed very readily into Oie nose, and that usually gave rise to regurgitation into the larynx, and, consequently, to cough. So much, as yet, with respect to the boy E. Soon after I had an opportunity of again seeing Miss D. The agreement of the phenomena relating to the palate, speech, and deglu- tition, with everything noted above respecting E., was striking. The rattling accessory sound was, however, less, and she complained more of the secretion of viscid mucus in the throat, which there was nmch difficulty in removing. ^ The two cases here described kept up my interest. There could, it seemed to me, be no doubt as to the connexion between the para- l3''tic symptoms and the preceding inflammation of the throat. Further investigation appeared, however, desirable, and I therefore repaired to Bennekom, where the physicians, Dr. Thomas and Mr. Ketting, with the greatest readiness, gave me all the information I requested ; they also afforded me the opportunity of examining some other patients, in whom secondary paralytic symptoms had manifested themselves. The first four cases had proved fatal in the acute stage of the disease. III. I saw in the first place (on the 10th of June) a girl aged 17, who had been attacked on the 7th of April, and after the separation of con- siderable gangrenous spots, appeared within a fortnight to have recovered. I heard that immediately after the infl.ammation of the throat, a slight lesion of speech seemed to exist, but that it nevertheless had not be- come considerable until a fortnight later, that then for the first time the rattling sound was heard, and the nasal tone of the voice had become really distinct. Moreover, that about a week after the termination of the disease, difficulty in reading was for the first time observed. This sym- ptom also soon increased ; the patient had always, however, been able to read some lines. During the month of May the symptoms connected with speech and sight remained almost unchanged. In the commencement of was heard, a powerful effort was made to close the palate by the way to the nose. If this in great part succeeded, a sound was heard, which held the medium between the explosive consonant and the resonant, or, rather, because the closing still went on during the sound of the voice, it was as if the explosive sound was preceded by a resonant : mband, ndoor, &c. It needs no proof that, if the continuity had remained equally free, at the beginning of the words also, in place of the explosive consonant, only the resonant would have been heard. PECULIAR LESION OF SPEECH. 605 June evident improvement took place. At the time of my visit, on the 10th of June, the movements of the palate were normal ; a rattling accessory sound of the voice was no longer audible. Yet the nasal tone was usually still per- ceptible in the pronunciation of the vowels ; rub, head, and egg were still heard as rump, hent, and engk ; sometimes rub was well pronounced ; head, never ; he sounded almost always as pe ; if the nose was held close exter- nally, she could say be, de, and ge better. — The power of accommodation was still far from having attained its normal range. Myopia existed about = ^. The nearest point still lay, however, at 6". The range of accom- modation, therefore, amounted to ^ — ^=. rather more than l, ; it ought, at the patient's age, to have amounted to more than \, and was consequently reduced by about one-half. There still also existed difficulty in continuing reading and fine work. On the whole, the symptoms were similar to those of ordinary asthenopia, — they were only so far different that reading was much easier when the book was held somewhat further off. Reflex and accommodative movements of the pupil were little disturbed. — After my visit the improvement steadily progressed. On the 7th of September I received from Mr. Ketting the following report : — G. v. N. again sees per- fectly well ; the pronunciation of rub, head, and egg, cannot yet, however, be called quite perfect. Strengthening regimen and treatment. Respecting MissD., (the patient of i.),I learned here that she came under treatment on the 15th of April, that the symptoms were in every respect moderate : there was little pain in the throat, there was but slight fcetor, nor was there much swelling ; — that, nevertheless, diphtheritic spots had appeared in the throat, that in this case also mineral acids were employed, whereupon separation had ensued, and the patient rapidly recovered. In the beginning of the month of May she returned to Utrecht. It has above (p, 599) been stated at length how, some time subsequently, loss of the power of accom- modation and lesion of speech and deglutition had appeared. In this, however, improvement gradually occurred. I saw her on the first of Sep- tember. The rattling sound had quite disappeared ; the uvula was easily moved ; deglutition was normally performed. The vowels had still a weakly nasal sound ; the tone changed a little on closing the nose externally. Rub and head were still often heard as rumb and hend ; egg was better pro- nounced. — The accommodative power had returned : on accurate optometric investigation the nearest point was found for both eyes at 5*, for the right eye at 54", for the left, at 5-3'. Nor was close work attended with any inconvenience whatever ; the pupils were normal. The headache, dulness, &c., had quite disappeared. IV. Moreover I saw a boy, aged 9, who had been attacked on the 16th of April with violent symptoms and with great swelling of the external glands of the neck. At the end of three weeks he had recovered, but was still feeble. About a fortnight later the lesion of speech was observed ; little complaint was made as to vision. Meanwhile he was and continued weak, and about a month after his illness it was observed that he began to run badly. On the 9th of June I find him a pale, emaciated boy, with sunken eyes, somewhat hanging under jaw, unhealthy complexion, and a painful 606 I'ARESIS AFTER DIPHTHERIA. expression of countenance. His gait is tottering ; in running lie often falls on his knees, and then jSnds difficulty in rising up again. For the last three days, too, he could not turn in bed ; he had to be raised in order to be placed upon his side. At the same time he complains of pain in the forehead, sometimes also in the neck. All this does not prevent his being very cheerful and gay, running and playing about, nor when among his companions does he think of illness. Nevertheless he finds chewing, and particularly swallowing, especially of solid food, very difficult ; there- fore he always wishes to drink with his meals, the morsel invariably goes wrong, the water comes out through his nose, and cough, and sometimes nausea and vomiting, ensue. In fine, he gets but little food into his sto- mach. His voice is strongly nasal, the rattling accessory sound is almost constantly heard, — in a word the lesion is the same as that above described at length, and the limited movement of the palate testifies of a semiparalytic condition. — His power of accommodation is less interfered with than was the case with the other patients. He sees well at a distance, and his near- est point lies at 6". It ought to lie at 4" or 3'5". Stress was laid upon the necessity for nourishing diet and tonic treat- ment. Qeneral improvement gradually took place. On the 7th of Septem- ber I received the following favourable report : — "P. v. L. is again at school ; he pronounces rvh, head, and egg like an Englishman, no longer totters or falls in running ; all his movements are easy and free ; his appetite is better than it ever was before; he sees acutely up to 6"; there is no nausea whatever." Of the case, communicated under ll., the result was unfortunate. The course of the disease had been violent, and was combined with swelling in both the throat and the salivary glands. The convalescence, nevertheless, appeared to be progressing favourably. Soon, however, the lesion of speech became developed, and shortly afterwards, that of the power of accommo- dation supervened. In other respects the patient now seemed to be perfectly well. But after I had seen him at Utrecht (compare p. 601) weakness of the limbs set in ; the arms became so powerless that he could neither strip nor dress himself. The emaciation increased, and difficulty of breathing not unfrequently occurred. A first attack of violent dyspnoea subsided favour- ably. Some weeks later a second followed. In spite of all the stimulating remedies employed, the breathing became rapidly rattling, and the patient died with symptoms of so-called paralysis of the lungs. A post-mortem examination could not be obtained. Besides those here communicated, I saw a few cases, in which also disturbance of vision had existed, but where the power of ac- commodation appeared to have already completely returned. It is remarkable that in the epidemic in the village visited by me, all those who had recovered, as it seemed without exception, exhibited para- lytic symptoms. Subsequently in other places the mortality was less, and paralysis much more rarely ensued. Nevertheless the number PECULIAR I-ESION OF SPEECH. 607 of those who applied to me was upwards of thirty, and in the majority of these the paresis was confined to the palate and to the accommo- dation. One case I had seen even before those related above. It was that of Miss Y., of Weesp, aged twenty, who called upon me for the second time on the 17th of June. Eeferring to my note- book I read literally : — " Diminution of range of accommodation ; symptoms resembling those of hebetude ; hypermetropia = ^ j she has also had aphonia, has been very hoarse, the cavities of the nose and throat communicate permanently — ^these symptoms were deve- loped after inflammation of the throat, with great external swelling, without pain, but with fever and distress." In this case, too, I had made experiments as to the pronunciation of different sounds, and as the patient now stated, had found that she had difficulty in pro- nouncing d, h, and g (in game) at the end of words. But at that time I had not recognised the connexion either between the paralysis of the internal muscles of the eye and of the palate, or between these two and the preceding angina, — and therefore I had retained no distinct recollection of the case. Now she had completely recovered, with respect both to sight and speech. The hypermetropia, too, had given way. Such patients apply, it appears, to the oculist. The knowledge of their cases is, therefore, important to him. About the same time Dr. !Ples informed me that he had been called at Utrecht in con- sultation respecting a boy who, after angina (as it appeared on sub- sequent investigation, diphtherina) was attacked with paralysis of the palate and of the muscles of accommodation, together with incom- plete ptosis and strabismus divergens, ah indicating paralysis of the oculo-motor nerve ; at the same time, want of power of the limbs, and of the cervical and masticating muscles, had occurred. Only the debility of the muscles of the limbs remained ; the other symptoms of paresis had again disappeared. Among the cases which afterwards occurred to me there were many in which the angina ran its course without important sym- ptoms, several in which the angina was not recognised as diph- theria, and yet the subsequent paresis of accommodation left, I am convinced, no doubt upon the subject. In this we have a hint to the physician, to make an accurate inspection in every case of angina, even when the symptoms appear to be of slight importance. And let the oculist, on his part, never neglect, in paresis of accommodation, to inquire whether angina has preceded the attack or not. It is im- portant for him to ascertain this, both in a prognostic and in a thera- 608 PARESIS AFTER DIPHTHERIA. peutic point of view. As to tlie former, it is favourable ; for in. all the cases which occurred to me, recovery ensued, in one instance, not until ten months after the attack of angina, general debility, too, remaining aU that time. We must not, however, conceal the fact, that besides the case given above, a second, as we were informed, ended fatally two or three months after the attack. The proximate cause of this paresis after diphtheria has not been satisfactorily explained. Besides the contagious character, which in this first epidemic might appear most distinctly, and was also quite clear to me, the subsequent paralytic symptoms also show especially, that the so-called angina is a general disease, and we may sup- pose that the altered blood-crasis has excited a secondary morbid process in the central organ, which becomes the cause of the para- lytic symptoms : pathological anatomy must decide the question. Besides, the supposition of Bretonneau deserves consideration, who looks upon the paralysis as a secondary symptom of the morbid poisoning, to be compared with the secondary phenomena of syphilis. It is certainly remarkable, that since strong cauterisation of the white spots in the throat has been generally adopted, the ratio of mortality has become more favourable, and the consecutive paresis has been of rarer occurrence — as if the absorption of the matters formed in the local process was in truth injurious. I closed my original essay with the following words : — " As to the treatment of the paralysis after angina diphtheritica, little can as yet be said. In every typical deviation, which we encounter for the first time, reason is our guide. It appeared here to recommend tonics — of which, in general, our age has need. Has experience also as yet expressed any opinion ? I would not venture to assert it. It has, however, been found, that with nourishing diet (to ensure which, under the existing difiiculty of chewing and swallowing, double care must be taken), combined with tonic medicines, complete recovery has, in the great majority of cases, been the result." Now I may add that these inferences have been fully confirmed by later cases. Under the use of sulphate of quina combined (the digestive organs being in a normal state) with small doses of sulphuric acid, some- times also of preparations of iron, and with due attention to nutri- tious diet, the lesion almost invariably gave way within a couple of months. The phenomena of paresis after diphtheria have been ascribed to general debility. This explanation appears to me to be unsatisfactory : ITS PROXIMATE CAUSE. 609 the very circumstance, that, besides the soft palate, frequently only the accommodation and the sphincter iridis have suffered, must lead us to suspect the existence of a special process. It is true that after considerable loss of blood, and after exhausting diseases, the accom- modation is often seriously affected, that then in the slightest degrees of hypermetropia, and even in emmetropia, symptoms of asthe- nopia are not unfrequent ; but in such cases the whole muscular system also exhibits a proportionate condition of debility. After diphtheria, on the contrary, we often observe the want of pro- portion in this respect. If individuals are sometimes weak, there are others who feel perfectly well, have resumed their work and run for hours, while the paresis of accommodation continues. They complain of nothing else than lesion of vision, and for it apply to the oculist. Further information we can expect only from anatomical investigation, the opportunity for which, fortunately, rarely presents itself. 39 CHAPTEE XII. SPASM OF ACCOMMODATION. § 48. Myotics and Myosis. The year just elapsed has put us in possession of a myotic, wliich immediately proved itself an able opponent of the best mydriatics. It is the ordeal iean of old Calabar, obtained from the Physostigma venenoswm (Balfour), belonging to the Leguminosse. This remedy has at once superseded all myotics, before tried or recommended as such. The preparations so far known, and sent to me are — a dark brown alcoholic extract, two sorts of paper, brown and violet (pre- pared according to Streatfeild's method) , and two solutions of the extract in glycerine, a weaker c and a stronger d. On the last we read — " 1 Minim equal to 4 grains of Bean." Among the principal phenomena observed after the application of these agents to the conjunctiva, contraction of the pupil and spasm of accommodaiiou are the most prominent. The first effect, immediately after the application, is a brief irrita- tion ; upon this, after the lapse of four minutes, slight spasms super- vene in the lower eyeHd. The contraction of the pupU, and almost simultaneously, the spasm of accommodation now follow. The contraction, after a sufficient dose (J drop of c', diluted or not with water), commences after from five to ten minutes, attains its maximum after from thirty to forty minutes, at which it remains only a short time, diminishes slowly after three hours, and disappears entirely in from two to four days, occasionally being replaced even by some dilatation. The whole process is, therefore, more rapid than that of the effect of atropia, probably in consequence of a greater power of imbibition. The subjoined Figures 170 and 171 (compare those of the action of atropia at pp. 585 and 586) indicate the course of the contraction in Mr. Hamer. Pig. 170 extends over two hours; Fig. 171 over three days. The action was moderately strong, and was almost painless. The pupil I measured with the ophthalmometer, always with equal illumination, and I directed the whole investigation. ACTION OF THE CALABAR BEAN. Fig. 170. 611 With respect to the contraction and the phenomen'a connected with it the following are to be noted : — ' f a. The diameter of the pupil becomes still less (from 1 1 to 2 mm.) than, in the normal condition, with the strongest light which can be borne and with the most powerful accommodation (von Graefe). b. The influence of the light does not, however, cease: one- can easily observe in himself particularly the consensual contraction, employing the entoptic method, by closing and opening the other eye (compare p. 197), as von Graefe also did. The movements are slow ; the consensual contraction lasts three, the consensual dilata- tion lasts four, seconds (compare p. 573). Moreover, the pupil at the same time often appears to be somewhat angular. It is in the middle like a crape, and has a tolerably sharply circumscribed, more strongly illuminated diffuse border (Fig. 172 A), which on consen- sual contraction is broader (B), and has a dark-green tint, while the 2 612 SPASM OF ACCOMMODATION. middle of the surface appears yellow. (In the entoptic observation Fig. 171. 1:3 3-2, 3-4 3-7 4 4-4 4.8 5-3 6 6-9 8 9-6 12 16 24 48 ■^ ■ B B^^^BB m ■ BBBBHBB Si ■ 1 1 ^Hii^H^B BH 1 ■ HBBBBI^H ^H 1 ^^^^Bfl^B Hi ■■ IB^^hIhBhh^^^hi ^9 ■■ 1 ^H Bss^B nl l_ ■■■■^H IH ■ UliBH IB i BBBhh HI HI |M ^B ^Hm |M HJBBi u h| H wl IM Efll |M |n |9 ■ |M HH SI |3 H jBI Ss jB ■! |H ■ si 00 0-6 1 1-6 2 2-5 3 3-5 4 4-6 5 5-5 6 tlie figure is seen inverted, but it is here (Pig. 172) inverted, and therefore gives the true form of the pupil for the left eye). c. Especially in the commencement of the contraction involuntary spasmodic vibrations occur in the diameter of the pupil. d. The illumination of objects is feeble, with an unusual brownish tint (Bowman). The effect is similar to that of solar eclipsesy in which, notwithstanding sunshine with its usual strongly contrastiiig shadows, the light is unusually feeble. If the instillation has been performed on only one eye, the great difference in illumination is best seen on doubling the image by a prism (compare p. 133). e. The circles of diffusion of a point of light, situated beyond the distance of distinct vision, become less the smaller the pupil is, and vision therefore becomes much less diffuse beyond the limits of accommodation. ACTION OF THE CALABAR BEAN. 613 f. After the disappearance of the myosis the pupil sometimes be- comes somewhat larger than before. The, spasm of accommodation appears from the altered position both of the farthest (Figs. 170 and 171 rr) and of the nearest {pp) point of distinct vision, drawn above the absciss a a. The said curves exhibit the whole course of both. We have, moreover, to note the following : — CO. In the determination of the farthest point clonic spasms of accommodation alternately arise, so that objects appear with the same glass (at a distance) alternately distinct and diffused. Now the points of the curve rr correspond to the moments of relaxation. In an hour after the employment of the myotic, the accommodation is again completely under control. The curve r r "shows further, that with a moderate dose, pain being virtually absent, the farthest point has still approached the eye from 56" to 7"'3, that is to nearly two- thirds of the absolute range of accommodation, originally present. ■ b. The determination of the nearest point was effected with the ^id of the most perfect optometrical instruments. The course of the points, as shown by these, is very satisfactory. In an earlier experi- ment of Mr. Hamer the action, after the application of a piece of Calabar paper too strongly impregnated, was much more violent, the painful spasms lasted more than six hours, and the pain increased so much on endeavouring to accommodate, that the idea of determining the nearest point was given up. c. The figures show that, with dimmution of the action, the range of accommodation is absolutely increased, most considerably after about 100 minutes (Kg. 170), and that this increase diminishes only 614 SPASM OF ACCOMMODATION. slowly (Fig. 171). In a previous iavestigation an increased range of accommodation was found also at the commencement of the action, the influence on the nearest point being in the first period greater than on the farthest. In the observation represented by Fig. 170, rather the opposite took place. d. The great effect upon the accommodation with slight impulse of the will is very important. This is still strongly felt when the farthest point has again returned nearly to its original position : 105 minutes after the application, the point of distinct vision lay, with con- vergence to 10", for the right eye naturally at 10'; for the left, on the contrary, at 4"*5,thus nearly attaining the absolute nearest point ; after the lapse of three hours and a-half, the said point lay, with similar convergence, at 5 "'6; after nearly seven hours, at 6'* 2; after rather more than eleven hours at 8""3 ; — and so long as the range of accom- modation remained greater in the left eye, some difference in accom- modation continued perceptible also with equal convergence of the two eyes. (It was determined, what negative glass the eye which had undergone instillation must have before it, in order, in looking at 10' and with rapid alternation pushing the hand before each of the eyes, to obtain equal sharpness of letters and of optometer-wires.) The relative accommodation has, therefore, approached to that of hyper- metropes : much accommodation with slight convergence, — the re- verse of what is met with under the influence of atropia. e. In the condition described under d, the determination of the nearest point with horizontal and vertical lines presents more difference than usual (Bowman) . This apparently exalted astigma- tism is certainly in part dependent on the greater difference in refraction with difference of convergence (compare p. 451). /. So long as with a given tension accommodation takes place for a shorter distance than usual, objects appear larger (macropia), just as in the opposite case (compare p. 587) they appear amaUer (micropia). g. The increased refraction on relaxation usually lasts in any percep- tible degree only an hour. With a very large dose, which produces persistent and violent pain, it may last many hours. With a small dose considerable contraction of the pupil, without any very percep- tible influence on the accommodation, might be obtained. Finally, it remains to be stated : that, according to von Graefe, 1° the acuteness of vision sometimes diminishes, particularly in the period of development of the spasm, probably in consequence of in- sufficient stabihty of accommodation, in any case, independently of NERVES ON WHICH CALABAR ACTS. 615 the want of light from constriction of the pupil; 2° that, also in defect of the iris, the influence on refraction and accommodation remains the same. The action of Calabar is not equally great in all animals. In this respect it presents much analogy to Belladonna, inasmuch as in man, and moreover in the dog and cat, a high degree of myosis is obtained, even with small doses, while in the rabbit, as in birds, and especially in amphibious animals and fishes, a slighter effect is observed. With a stronger dose too, than we can employ in man, still stronger myosis is obtained in these (von Graefe). Immobility of the pupil we could not obtain. With respect to the mode of action, similar experiments lead to a like conclusion as in the case of mydriatics. As to transition into the aqueous humour, however, von Graefe could not directly satisfy him- self from the myotic action on instillation into the eye of another animal. — After repeated strong application, however, it succeeds, if the discharged fluid is long kept in contact with the eye into which it is dropped. We have also been specially engaged in the investigation of the question, h^ the intervention ofwJiat nerves the Calabar acts. It is quite evident that spasm of the m. sphincter pupillse ensues. Indeed the high degree of contraction renders paralysis of the dilator muscle alone insufficient to explain it; and the increased refraction, in which a more powerful auction of the muscles of accommodation is included, puts altogether beyond doubt a spasmodic contraction of the sphincter pupillse, as being governed by the same nerve as the ciliary muscle, and associated with it in its action: moreover, accord- ing to our experiments upon rabbits, after division, both of the sym- pathetic nerve and of the trigeminus, the contraction of the pupil under the influence of Calabar still increases considerably. The nerve alluded to is the oculo-motor nerve, and, more especially, the short root, which this nerve sends to the ciliary ganglion. It is opposed to our idea of the similar nature of all nerve-fibres, that a par- ticular substance, as Calabar, should have on some fibres a paralysing, . on others a stimulating, and still more a tonic stimulating, influence. Consequently, in this case also, we prefer to assume (compare p. 590) an action upon specific nerve-cells present in the eye itself. With a stimulated condition of this internal ciliary system, however, as we have seen, increased action, both voluntary and reflex, is not excluded, — quite analogous to the effect of strychnia upon voluntary muscles. 616 SPASM OF ACCOMMODATION. It is not so easj to show, how far the Calabar exercises upon the dilator muscle also the opposite influence of atropia. In the first place, it certainly does not give rise to complete paralysis of this muscle; for after long and repeated application of Calabar, stimu- lation of the sympathetic nerve in rabbits and in dogs always still produced some dilatation of the pupil. We performed, among others, the following experiment : — the nervus trigeminus was divided on the left side, — insensibihty of the eye and con- traction of the pupil ensued; the cervical sympathetic .nerve of the same side was three times stimulated, — each time there was dilatation of the pupil ; it was then divided, — the pupil appeared still more contracted than before the stimulation ; in both eyes Calabar (J drop of c') was long kept in contact, — after eight minutes there was incipient, at the end of fifteen minutes "there was the strongest con- traction, greater on the left side ; after the lapse of eighteen minutes there were spasms of the extremities, difficulty of breathing, impend- ing suffocation; artificial respiration was kept up, — -four minutes later death ensued with sHght spasms of the limbs ; the sympathetic was again irritated, — dilatation of the pupil still invariably followed. Thus, after slow death, in consequence of poisoning by the instilla- tion of Calabar, the sympathetic nerve in the eye is still excitable. Consonant to this is the fact, that on division of the cervical sym- pathetic nerve, before or after the employment of Calabar on both sides, the pupil appears smaller on the side on which the nerve is divided. — We supposed that the action of Calabar, in cases of paralysis of the oculo-motor nerve, would elucidate the question still farther. In different quarters it was observed that this paralysis did not prevent the myotic action. In a first case, in which, in the ordi- nary investigation, the pupil appeared absolutely immovable, some mobility was stiU seen, under strong incident light, in observation with the ophthalmometer : the paralysis was therefore not complete, and we consequently desisted from an investigation which promised no certain result. In a second case, in a lady aged 32, there was absolute paralysis of the whole right oculo-motor nerve, which had set in gradually six weeks previously, after repeated complaints of headache for four years, often combined with erysipelas of the right half of the face ; in this instance the strongest incident light on one or both eyes gave no appearance of contraction on this side. The Calabar employed (the extract), however, produced contraction, as strong as usual, as the subjoined figure (173) shows, and at the same time somewhat increased refraction. Now, we were disap- ANTAGONISM OF ATROPIA AND CALABAR. 617 poiuted in so far as this considerable contraction teaches nothing respecting the influence of the sympathetic nerve; for evidently Fig. 173. spasm of the sphincter muscle is at the same time in ope'ration. But such local spasm, where there is complete paralysis of the trunk of the oculo-motor nerve, is still an important phenomenon : in the first place, because it confirms the fact that the Calabar acts by direct contact ; and in the second place, because this action cannot well be imagined to occur otherwise than by the intervention of a gangHonic- cell-containing centre in the eye. — Meanwhile it is, on other grounds, more than probable, that Calabar, if it does not paralyse, at least lowers the action of the sympathetic nerve. When, in fact, by a moderate action of Calabar, the accommodation is brought at most by half into tonic tension, the pupil is already narrower than with intense light and strong accommodation ; and this half takes place without the sphincter, which continues sensitive for reflex and accommodative impulses, attaining the maximum of its action : con- sequently, without diminished action of the dilator, such strong contraction is not explicable. The circumstance, too, that the con- traction of the pupil continues for a few days in so much a higher degree than the spasm of accommodation, is in favour of a diminished actioii of the radial fibres, and consequently of a lowering influence of the Calabar upon the sympathetic nerve. There was no evidence of a special action on the trigeminus and on the vaso-motor nerves of the iris ; we observed only that on the side on which the trigeminus was divided, the action of Calabar was not less distinct than on the other side. The struggle between atropia and Calabar, when applied simul- 618 SPASM OF ACCOMMODATION. taneously, or soon after one another, is remarkable. When applied together, some contraction of the pupil and spasm of accommodation first occur, as the efl"ect of Calabar. The spasm of accommodation still continues, when the action of the atropia on the iris gains the upper hand, and the pupil consequently becomes wider. Von Graefe in particular has investigated how, when employed consecutively to the action of atropia, that of Calabar may become intercalated. He showed that in weak action of atropia, and in the period of dimi- nution of its strong action. Calabar is capable of temporarily con- stricting the pupil and increasing the refraction, and that after the giving way of these phenomena, the more tedious atropia-process again follows its regular course. We were particularly desirous to ascertain whether, with absolute atropia-paralysis of the sphincter and of the muscles of accommodation, a powerful employment of Calabar stiU has influence; and this, in fact, we most distinctly found to be the case, observing, moreover, that this influence was still greatef upon the refraction and accommodation, than upon the 1:5-3 6 6-9 8 9-6 12 Fig. 174. i I W 4o 60 io ' '10.0 f^f? ' 1^0 : Ho I ISO ' 2S0 , z^cf j ^'fiJ r- i--l- ■ - 1 ' P 1 -■" ■ ■ ■ ■ ■ [^ 1 i ■■■■■■■■■—■B—l ■■■■■■■ ■■■■■■■■■■■■HiBi 1 ^ M IT M M ■■■■■■■■■■■■■■BH 1 i\ HHlHHn 16 ! 4 24 r 1^' ■-xj """,-.. ::> ■ ^, ' ' ,.: 48 . ^"~S4^ fr/ , 'T^'-i-*-.,^ twmsumBamMnmmmmm 0-5 I pi ■■■mmBBI ^r ^' "' - ^"^=t°' 1 i j_ ! M 1 M- ! I . .. .: 1 i i i 1 It 1 1 .-•■-... 1'6 2 2-5 3 3-5 4 4-5 i ■ 1 — M H j--^ It, i i I ! ! 1 M i 1 i M ! i M 11 M Ti 1 1 i 1 i IB^^IHHBIHIIhHBBBI t i iBBB9BBhhhBBBhihI Ly 1 1 i M 1 1 1 1 i 1 11 1 1 l-.l ! 1 j_ i ■■■■■■■■■BBaaBM -i+- IBBBinnnHBBBBB _ ■!-■ 1 ' o o 6 6-5 1 1-^ Ik d 1 L i ._ ■i ^i \ J. y: " - 1 L^_J_ 1 1 ■■ 1 1 i 1 o !!■ - J - , J diameter of the pupil. Kg. 174, an observation on the eye of Mr. ANTAGONISM OF ATROPIA AND CALABAR. 619 Miiller, surgeon in the navy, fnrnislies an example of this : accord- ing to the cross, on the absciss a a', 137 minutes after the applica- tion of atropia, while p and r coincided and the mydriasis had attained its maximum. Calabar was dropped in, — and ten minutes later the refraction ascends {rr), accommodation is again present {jap), and at the same time the diameter of the pupil is a little diminished. In a second, likewise very practised observer (Mr. van Leent, surgeon in the navy), the effect of Calabar was on the first-day, and particularly on the second day, after the renewed application of atropia, much stronger still. Notwithstanding that the pupil contracted only slightly, he could each time, after some rest, read for a few moments No. 1^ of Snellen's test-types at 12"; and the nearest point was, in fact, determined at about fropi 14" to 16", while, on relaxation, his usual slight hypermetropia had given way to emmetropia. After the cessation of the action of the Calabar, that of the atropia was in these three accurately investigated cases, after fourteen days or longer, still recognisable from the wider pupil, — ^in itself a proof that the mydriatic had been strongly applied. Hence it appears, that the paralysis, produced by a specific agent, may be overcome by the effect of another specific agent, even in such a way that voluntary action again becomes possible. The subjoined Figure 175 indicates the influence of atropia on the Fig. 175. impil, fifty minutes after the employment of Calabar : the complete dilatation of the pupil is produced evidently somewhat more slowly. Kather more than three hours later Calabar is again dropped in in the same person (Dr. Land). The existmg hypermetropia of as en- 620 SPASM OF ACCOMMODATION. tirely gives way, for a short time spasmodic myopia occurs, and with convergence undoubtedly accommodation, which, however, just as in the previous cases, is not perfectly under control, and after a few mo- ments becomes fatigued. The pupil at the same time becomes some- what narrower = 6"75. On the foUowing day H is again ■=m; the pupil is somewhat movable. After seven days the pupil = 6"35 ; it is not until the seventeenth day that both pupils are equal, = 4"37 — thus less than at the commencement. On the discovery of the physiological action of Calabar, it fol- lowed as a matter of course that this agent should be tried in various anomalies. In the first place, it is found to be useful to lessen the inconvenience of atropia-mydriasis ; and according to V. Graefe the atropia process may be shortened by a systematic em- ployment of the Calabar. How far it may lead to cure, or at least afford permanent benefit in paralysis of accommodation and in mydriasis, in which its use had already been suggested by Eobertson, experience only can decide. But it is certainly important, that in ordinary paralysis of accommodation, whether self-existing, or con- nected with further paralysis of the oculo-motor nerve (in cerebral mydriasis the efi'ect of Calabar is found to be entirely absent), the pupil is contracted and the refraction is increased, by Calabar : in a case of unilateral paresis of accommodation which occurred to me, and which very much interfered with binocular vision, the inconve- nience was completely removed by a glycerine solution diluted with eight parts of water, used once daily. Thus, too, the myosis con- nected with the weak action of Calabar, may often be useful in many cases where a stenopeic apparatus improves the sight (compare pp. 128 et. seq.), as in difi'usion of Ught (opacities of the cornea, &c.), in irregular astigmatism (keratoconus, luxation of the lens, &c.) ; (com- pare pp. 553 et. seq.), further in aphakia, especially when the plane of the pupil is not clear. The improvement of the acuteness of vision in ordinary ametropia is indeed remarkable : precisely with very weak action of Calabar myopes distinguish much more accurately at a dis- tance, and hypermetropes, under the double advantage of smaller circles of difiusion and of easier tension of accommodation, lose for a time their asthenopia. — The great question, which practice has to answer is, whether Calabar is permanently as harmless for accom- modation as atropia is, and whether the conjunctiva will perma- nently bear its repeated application. Until these points are decided, the future of Calabar in therapeutics cannot be foretold. THERAPEUTIC EMPLOYMENT OF CALABAR. 621 I have now to add only, that v. Graefe has advantageously applied the contraction of the pupil in glaucoma, in order to faciHtate the performance of iridectomy, and that in his opinion the alternating action of Calabar may probably contribute to tear synechia. The want of an efficient myotic was long felt in ophthalmic surgery. But the longer it was unsupplied, the feebler must the hope of finding such have become. It is true that all myotic action could not be denied to the agents which were formerly placed in this category : Semen Santonicum (Himly), Daphne Mezereum (Hahnemann), Mcotiana Tabacum (Heise), Aconitum Napellus, Secale Cornutum, &c. j but their stimulating action was in itself sufficient to forbid their employment in practice. The same conclusion was arrived at by myself with Dr. Kuyper (Z. c.) in a systematic investiga- tion with respect to nicotin, conia, extract of aconite, and digitaline ; and Ad. Weber {Verhandhinffen der von 3 bis 6 Septemher, 1859, in Heidelberg versammetten Augenarste, Berlin, 1860) had to pay the penalty of an attack of keratitis for an imprudent application of digitaline to his own eye. Fresh tobacco leaves appeared to be somewhat better borne. Finally, a subcutaneous injection of morphia, in whioh von Graefe (^Deutsche Klinih, 20 AprU, 1861) discovered a tolerably powerful agent, not only for contracting the pupil, but also for increasing the refraction, was also certainly inap- plicable to this special object. — ^In the Calabar the wished-for agent appears to have been bestowed upon us. The general effect of this ordeal Bean had already been studied by Dr. Daniell (1846), and more fully by Christison (1855) ; van Hasselt had also (1856) found myosis to be a principal symp- tom in its general action, when Thomas Fraser {Diss, inaug., defended at Edinburgh on the 31 st July, 1862) discovered that its local application contracts the pupil, and Dr. Argyll Robertson {Edinburgh Medico- Chir- urgical Society, 4th February, 1863), pointed out its influence on accom- modation, and introduced the Calabar as a new remedy into ophthalmological practice. Upon this followed investigations by Harley (conf. Med. Times and Gazette, 20th June, 1863), especially with respect to its general action, with some additional remarks by Hulke ; moreover by Bowman and by Soelberg Wells {Medical Times and Gazette, 16th May, 1863) ; lastly, by von Graefe {Deutsche Klinik, 1863, No. 29, and Archiv f. Ophthahnologie, B. is..), Hamer {Geneeskundig Tijdschrift, July, 1863), Rosenthal {Archiv f. Anatomic und Physiologic, Jahrgang, 1863), and Schelske {Kliniache Monatsbldtter /. Augenheilkunde, 1863, p. 380) ; and, in concert with Mr. Hamer, I have continued them. Our object was, rather by the accuracy than by the number of our experiments, in following up the previous valuable investigations, somewhat to increase our knowledge of this re- markable agent. The principal results of our researches are comprised in the foregoing pages. 622 SPASM OF ACCOMMODATION. §49. Spasm op Accommodation. — Myoris. — Painful Accommo- dation. We have to distinguish different forms of spasm of accommoda- tion. That which most frequently occurs is nothing else than an exalted tone of the muscles concerned in the latter function. In healthy emmetropic eyes this tone is, keeping in view the extremely slight diminution of refraction in atropia-paralysis, undoubtedly very trifling. On the other hand, we have in hypermetropia found a per- manent tension, which wholly or partially conceals the abnormal condi- tion : it can exist only in virtue of the accommodation, and must there- fore be destroyed by the paralysing influence of the atropia. The in- creased tension is here the natural result of the persistent effort to over- come the existing anomaly of refraction. In amblyopes and astigmatics where the same sometimes occurs, it is explicable by the constant en- deavour, in accommodating for the nearest point, to see the smaller objects under a greater angle. It is less evident, how myopes also acquire a tonic spasm of their accommodation. That this not un- frequently occurs, especially when the eyes are in a state of irritation, we have already observed. Dr. Pies now informs me, and I readily beheve it, that he met with this condition in many cases in young persons, especially in boys, who had been prepared for an examina- tion at one of the miUtary schools. Partly irritation of the eye, reflected on the accommodating system, partly excessive tension of accommodation during the constant work, particularly with defec- tive light, may be the cause of it. It is invariably the paralysis by atropia which reveals its existence. In young children under these circumstances, I have seen slight degrees of myopia give way even to hypermetropia, so extremely sensitive is their play of accommodation. The tonic spasm of which we have here been speaking, seldom acquires much pathological importance. In myopia we must attend only to the accompanying symptoms of irritation ; in hypermetropia we must recognise it, in order to direct the optical treatment accordingly. We must here once more remind the reader, that von Graefe, where there was an attempt to relax the accommodation, thinks that he has sometimes established the existence of an involuntarily exalted, and therefore spasmodic, action (compare p. 351), and that some very slightly myopic persons, who endeavoured without a negative glass to ILLUSTRATIVE CASES. 623 see the fundus oculi in the non-inverted image, have complained to me of something similar. Of the acute occurrence of myopia, in which the idea of spasm of accommodation naturally suggests itself, we find some examples related by the earher writers, which Ruete * has collected. I am, however, not convinced that, in these instances, anything else than amblyopia had occurred. When in consequence of the latter, smaller objects, as for example letters, could be distinguished only when near, the existence of near-sightedness was often inferred. Generally speaking, among the earlier observations, the determination of the farthest point of distinct vision, whereby nevertheless alone the presence of myopia could with certainty be proved, is omitted. Acute spasm of accommodation, such as, for example, is produced by Calabar, is un- doubtedly very rare. 1 myself have never met with a clear case of it, and this may excuse my scepticism. My task is confined to quoting those few cases which afford satisfactory evidence. Von Graefe t communicates two of them. The first is that of an engineer, in whom the right cornea had been in- jured by the finger-nail of a child. When the irritation consequent thereon had, after some days, wholly subsided, the patient saw indistinctly with this^ eye, and small objects were multiplied. The pupil was of normal diameter, with slow and slight reflex-, and without accommodative move- ment. At the precise distance of distinct vision the polyopia had disap- peared. Accommodation was almost entirely lost, and the eye was, at the same time myopic : with the naked eye the patient was accommodated for 34", with — J for 8", with -^ for 2J'. The left eye had the usual range of accommodation, and was about emmetropic : with ^ vision ranged from 3' to 9J". — The jiatient had often previously satisfied himself that both eyes were equal. Cure ensued, and indeed very rapidly, after Heurteloup- ian abstractions of blood: after the first the eye accommodated from 3 J" 4;o 5i'; after the second, from 4" to 8 J" ; after the third the accommodation was nearly equal to that of the other eye, whose nearest point lay at about Perhaps this affection is to be considered, as von Graefe says, as a reflex neurosis, in like manner as injuries of sensory nerves sometimes excite tonic spasms in ordinary voluntary muscles. The second case is that of a girl aged 18, with painful spasm of the * Ruete, Pathologie und Physiologie der Augen und Ohren, p. 262. f Archivf. OpMhalmologie, B; ii. H. 2, p. 304. 624 SPASM OF ACCOMMODATION. orbicular muscle [of the right side], which on tension of the eyeUds, and also sometimes spontaneously, became more violent. Pressure on the supra- orbital nerve had no effect, a slight pressure on the facial nerve increased the pain and the spasm, strong pressure lessened both. A brief improve- ment, after the application of leeches, -was followed by aggravation of the symptoms, with disturbance of vision. The refraction appeared to be in- creased, and the accommodation to be very limited, namely from 2|" to 3 J'. The right pupil was somewhat narrower than the left, with slight reflex-, and without any accommodative movement. The left eye was normal. The endermic employment of sulphate of atropia behind the ear was at first fol- lowed by no improvement ; but, on the contrary, the same condition in all respects was developed also in the left eye. When, on the third day, bella- donna-symptoms arose, the spasm and pain diminished, and on increase of the intoxication they entirely ceased, and the patient could accommodate with the right eye from 5J" to 14J', vnth the left from 5J' to 17'. How- ever the symptoms returned when the employment of atropia was sus- pended. The result of the case is not communicated. Von Graefe sees in this case a combination of spasm of the muscles of accommodation (oculomotor nerve), with neurosis of the facial nerve, in which it is important that, although we cannot admit the existence of any connexion between the nerves mentioned, the affection increased and diminished pari jpassu in both. A third case was described by Liebreich.* Miss F., aged 21, complained of dazzling before the left eye ; fatigue on exertion and nearsightedness had set in a year before after constant work, often continued at night. On examination it was found that vrith parallel visual lines, sight was acute at a distance with — ^, but that with commencing convergence, such strong accommodation supervened, that she then needed even stronger negative glasses to see acutely in the point of convergence, until finally, at the distance of 6" both eyes could distinguish sharply without glasses. As the existence of spasm of accommodation was hence inferred, atropia was repeatedly dropped in, in consequence of which the myopia gave way to hypermetropia = ^. Meanwhile it ap- peared further, that the convergence was diflBcult, the possible divergence particularly great, while other symptoms indicated insufficiency of the mm. recti interni. The spasm of accommodation might be connected herewith ; but still it was determined first to suppress this with atropia in both eyes, and the employment of the remedy was continued for fourteen days. When the accommodation had subsequently returned, the patient seemed to have recovered : the spasm had altogether given way, and even with glasses of j'j, which were prescribed for her, distant and near objects were acutely seen, and work was continued without fatigue. Archivf. Ophthalmohgie, B. viii. H. 2, p. 2.59, ILLUSTRATIVE CASES. 625 It was evident that in this case the insufficiency of the mm. recti interni had not been the immediate cause of the asthenopia. But might not the spasm of accommodation, with the long-continued excessive tension, have been produced by the effort to overcome that insufficiency ? It is extremely remarkable how accurately the spasm observed in this case agrees with that excited by Calabar : by it too the refraction is increased, and convergence gives a relatively too strong tension of accommodation. The diameter of the pupil is not given. The cases here communicated may suffice to illustrate the different forms of spasm of accommodation. We have stiU to speak briefly of pain with tension of accommo- dation, which is probably combined also with spasm. At p. 289 I have already communicated a case of this nature. I may be allowed here shortly to relate two others. Mrs. O., aged 29, called upon me in October, 1859, complaining of vio- lent pain in the eye on any effort to see near objects, wbicb had already existed in a greater or less degree for more than ten years. She had a flat face and shallow anterior chamber of the eye. The pupils were small, but movable. The nearest point lay at 11", the farthest at oo ; upon artificial mydriasis, H appeared ^ ^. The acuteness of vision was normal. I gave her glasses of ^, believing that after the return of the accommodation she would be able to work with them at a distance. — Some time after she called upon me again. Her state had continued the same, and the spectacles had been of no use to her. Other glasses were tried, but with no better re- sult. A derivative treatment was also adopted quite in vain. I determined then to have atropia dropped in for some time, in order to counteract all tension of accommodation, apd I permitted her to wear light blue glasses of ^, and to use 1, for near objects. Repeatedly, after one month, two months, &c., it was tried whether the mydriatic could be omitted. The results of these trials were unfavorable. But at the end of six months, she observed with joy, that leaving olf the use of atropia, with the return of accommodation, reading without glasses, she was free from pain. She now, moreover, occupied herself at close work with glasses of ^. — When after a year-and-half, a relapse occurred, the employment of atropia for three months was again sufficient to overcome the painful spasm. In this case, the pain is much more prominent than spasmodic contraction. But what suggests the idea of the latter, is the whole latency of the hypermetropia, with comparatively shghfc range of accommodation. At the time of the relapse the patient was at Dresden, and she consulted me by letter. I should have been glad then to have examined her anew, particularly in order to satisfy myself still further respecting the relative range of accommodation. 40 626 SPASM OF ACCOMMODATION. A second case of this nature was tliat of a friend of mine, who had held an important office in the East Indies, in which he had to work hard and to make observations with optical instruments. During his observations, he for the first time felt pain, which made him cautious. His state was considered to be hypersemia of the retina. On reading and in calculating his observations the pain increased. Examination now showed, that after the use of atropia, hypermetropia = ^ existed. At first weak, afterwards strong convex glasses were given, quite in vain. Derivants, leeches, and Heurteloup's artificial leeches were tried, with equal want of success. Reading for a few minutes produced a degree of pain, which compelled him to desist. This state had already lasted for a-^ear and a-half, and his return to his native country was spoken of. But it occurred to those about him, that they might first consult me by letter, for which purpose both the patient and his oculist, formerly a pupU of mine, sent me a very detailed report, dated the 15th February, 1861. My advice was : — for a time to employ sulphate of atropia (1 ; 120) at least twice a-week, and meanwhile during the paralysis of accommodation thus obtained, to make vision at difierent distances possible by means of difierent convex glasses. Some months later I received the report, that " the atropia treatment had instantly afibrded the best results. Hope revives in me," thus wrote the patient, " that we shall overcome the malady. In the morning I can already work tolerably steadily. In the evening, reading or writing inconveniences me less, but I do not yet venture upon it, in order not to retard the progress of my recovery. Your suspicion that my hypermetropia might be more than ^ is not confirmed ; every time we get the same result. I now use ^ for distance, ^ when sitting at table, and ^ when at work. These num- bers are, however, ' nominal ' — (N.B. They are the numbers of Paetz and Flohr, compare p. 142), and are too great : ^ is, properly speaking, ^; This last is still too weak when I have just dropped in atropia ; then I have J,', and ^ as equivalent to J." Thus my friend wrote to me (and, as is evident, he was master of the subject), on the 31st of August, 1861. The hopeful expectation he expressed was fully justified by the result. When, after a first disappointment, the use of atropia was, a few months later, for the second time suspended, the pain on tension of accommodation did not return, and, so far as I am aware, no disturbance whatever has since recurred in making observations, reading, writing, or calculating. This case scarcely needs any comment. It shows that in slight degrees of H a condition may he developed by continued tension, in which the least accommodation for near objects becomes very painful. Spectacles are then of no use, because with the convergence involuntary accommodation is combined, and this again excites the pain. The above-quoted observation of Liebreich has suggested to ILLUSTRATIVE CASES. 627 me tlie possibility, that in these cases, just as in Liebreich's, tension of accommodation too strong in proportion to the convergence sets in. Spasm of accommodation would then really exist. NOTE. From time to time cases occur of extremely contracted pupil, myosis, without established modification of accommodation. Usually it exists in both eyes, and, as Mackenzie says, the small, black, round pupil is at the same time " slow in its motions, scarcely dilating in the dark, and with bella- donna." In the highest degrees not only does the acuteness of vision suffer from want of sufficient light, but the field of vision is also limited by the thickness of the margin of the iris, — ^in any case it is peripherally very dark. Even Plenck {De morhis oculorum, 1777, p. 120) distinguished two forms : myosis spastica and myosis paralytica. Theoretically, in fact, both spasm of the sphincter, and paralysis of the dilator may give rise to myosis. Now it is to be expected, that with spasm of the sphincter, the accommo- dation should often be affected, which is less to be anticipated with paralysis of the dilator. A case probably of paralytic nature has been recorded by Dr. Felix v. Willebrand {Archiv fur Ophthalmologie, B. I. i. p. 319), in which the cause of the myosis was sought in pressure on, and paralysis of the sympathetic nerve by masses of swollen lymphatic glands in the neck, with the diminution of which, under the use of mercurial ointment with iodide of potassium, and of warm alkaline baths, the myosis and the dis- turbance of vision depending on it gave way. To this class, too, a case of myosis appears to belong, communicated by Dr. Gairdner to the Medico- Chirurgical Society of Edinburgh (see ' Monthly Journal of Medicine,' vol. XX. p. 71, 1855), in which an aneurism of the subclavian artery may be supposed to have compressed and paralysed the sympathetic nerve in the neck. The want of an antagonist might in such cases lead to secondary shorteniag of the sphincter. — The narrow pupil met with in inflammatory states of the cornea, and in connexion with intolerance of light, must cer- tainly be considered rather as the result of spasm of the sphincter muscle. The same is probably true of myosis observed ia violent neuralgia of the ophthal- mic nerve (Ruete, Zehrbuch der Ophthalmologie, 1853, B. i. p. 328), particu- larly of that which occurs with a violent attack of prosopalgia, only on the affected side, of which my colleague Prof. Loncq has related two cases to me. It will he important, in the occurrence of such cases, to ascer- tain with precision how far the accommodation is implicated. Those cases are still more obscure which are observed to be attended with permanent headache, especially in weak cachectic subjects, with tabes dorsalis (conf. Romberg, Nervenhranhheiten, B. i. Abth. 3, p. 684), and with different cerebral affections. More than one instance has occurred to me, where I could discover nothing with respect to the cause. — I think I have remarked, that continued close work, at an advanced time of life, gives rise to permanent contraction of the pupil, and slight degrees of myosis may find their explanation in this fact. 2 INDEX. Aberration, chromatic and spherical, 450. Accommodation, in the eye, proofs of the existence of, 7 ; change of the dioptric system of the eye in, 10 ; mechanism of, 20 ; range of, 28, 72, 110; defects of, 80; definition of, 80 ; causes of anomalies of, 84; disturb- ances of the muscles of, 84 ; dia- grammatic representation of the range of, 90; modification by spec- tacles of the range of, 145 j effects of microscopes and telescopes on the, 148 ; modification by spectacles of the region of, 150; inodifieation of the range of, by age, 204 ; question of the existence of, in aphakia, 319; spasm of, a cause of intermittent myopia, 370 ; power of, in myopes, 391; anomalies of, 6S9; influence of the nerves upon, 572 ; effects of mydri- atics upon the, 584; morbid paralysis of, 591 ; paresis of, after diphtheria, 599; spasm of, 610; painful, 622. Acuteness of vision, modification of, by ap;e, 188,225; determination of the, 189, 194. Age, modification of acuteness of vision by, 188; modification of the range of accommodation by, 204. Airy, his method of determining the degree of astigmatism, 483 ; on its correction by cylindrical glasses, 510; on the discovery of astigmatism in his own eye, 539. Alhazen, on the eye, referred to, 444. Ametropia, definition of, 82 ; clinical determination of, 96; by glasses, 97; with the ophthalmoscope, 105. Ammon (yon) on the protuberantia scleralis, 385; on elongation of the visual axis, 447. Andrese, on the motion of muscse voli- tantes, referred to, 202. Angina diphtherlna, paresis of accom- modation the result of, 287, 599. Aphakia, definition of, 84, 309 ; diag- nosis of, 314; selection of glasses in, 316; question of the existence of accommodation in, 319. Aphakia! system, simplicity of the, 310. Apparent strabismus, 244. Aqueous humour. Sir David Brewster on the refracting proportion of the, 38. Arlt, on change of form of the optic disc in myopia, 358; on a peculiar form of the retina in detachment, 399; on elongation of the visual axis in myopia, 447. Artha (von) . See Hasner. Asthenopia, definition of, 102, 259; synonyms of, 269; operative inter- ference in, 273; danger of mistaking apparent for true, 289; mnscularis, caused by insufficiency of the internal recti muscles, 265, 405 ; simulation of, by paresis after diphtheria fau- cium, 600. Astigmatic lens of Stokes, 485. Astigmatism, definition of, 449 ; regular and irregular, 451; derivation of the term, ibid. ; universality of its occur- rence, 455; cause of, ibid.; normal and abnormal, 456; influence of the cornea in, 459; determination of its seat, 462; influence of the crystalline lens in, 467, 496 ; disturbances of vision in, 469 ; phenomena of disper- sion in, 473; diagnosis of abnormal, 479; determination of degree of ab- normal, 481; objective signs of, 488; change of form of the optic disc in, 490; cause and seat of abnormal, ibid.; use of cylindrical lenses in, 501; iriddesis io, 511; nosology of, ibid. ; congenital, 512 ; its effect on vision, 513. Cases illustrative of, 515, et seg.; acquired regular, 533; history of our knowledge of, 539} irregular, 543 ; abnormal irregular, 550. Asymmetrical surfaces, refraction by, 457. Airophy of the chorioidea, in myopia, 354. Atropia, effects of, 584; antagonism of, and Calabar, 617. Aubert andPorster, on the diminution of the acuteness of visit.n, referredto, 193. INDEX. 629 Bean, Calabar — see Calabar bean. Beer, on asthenopia, 270 ; on the here- ditary nature of myopia, 350. Belladonna, history of the influence of, upon the pupil, 590 Berthold's mj opodiorthoticon, 417. Bessel, theoreiical investigations of, referred to, 38. Biconvex lens, refraction by a, 44 ; and spherical surface, refraction through a compound system consist- ing of a, 62. Binocular vision, various writers on, referred to, 161, note. Boehm (Dr. L.) , on the use of blue glasses, referred to, 170 ; on asthe- nopia, 271 ; on the treatment of hypermetropia, 275 ; on the use of convex glasses in strabismus, 306, 327 ; on the hereditary nature of myopia, 350. Boerhaave, on the causes of myopia, 446. Bonnet, on asthenopia, 270. Bowman (Mr.), on the retina, 4 ; on the muscular nature of the ciliary ligament, 23 ; on glaucoma simplex, 214, note; case of extreme hyper- metropia, by, 258, 290 ; on sponta- neous luxation of the lens, referred to, 310; on the Structure of the optic nerve, 375 ; on the detection of regular astigmatism, 490 ; on irid- desis in conical cornea, referred to, 511; on double iriddesis, 552, and note ; on physical alterations of the lens as causing defects of vision, 554 ; on the effects of Calabar bean, 612. Brachymetropia, definition of, 82. Braun on the structure of the retina, referred to, 6. Brewster (Sir David), on the refracting proportion of the aqueous humour, 38 ; on muscse volitantes, 203. Bfuecke, on vision, 5 ; on the muscular nature of the ciliary ligament, 23; on binocular vision, 162 ; dissecting spectacles of, 229. Brussels, asthenopia among the lace- makers of, 272. Budge and Waller on the influence of the sympathetic nerve upon the pupil, 577 et seq. i Buffon, on the cause of squinting, 413. - Buphthalmos, a cause of myopia, 370. Burow, on the position of the centre of motion, 1 80 ; on myopia, 41 7. Calabar bean, useful to myopes by con- stricting the pupil, 388 ; prepara- tions and action of,* 6 10; nerves on which it acts, 615; antagonism of, and atropia, 617; therapeutic em- ployment of, 620. Cardanus, on amorousness of myopes, 389. Cardinal points, 39; of the eye, general indications of the, 62; review of the position of the, in the eye, 67 ; use to be made of the knowledge of the, 68 ; deiermination of the, in astigmatism, 475. Carron du Villards, on asthenopia, 272. Cary, the optician, aware of the correc- tion of astigmatism by giving an oblique direction to suitable glasses, 456. Cataract, varieties of, 226 noie; on iri- dectomy in operating for, 316: ex- traction of, a cause of astigmatism, 553. Centre of motion of the eye, 179. Centre, optical, 49. Chorioidea, atrophy of the, in myopia, 354 ; Existence of muscular fibres and of ganglionic cells in the, 381, note; 576. Chorioiditis disseminata, occurrence of, in myopia, 360. Chromatic aberration, 450. Ciliary ganglion, existence of ganglio- nic cells in the, 576. Ciliary muscle, the, 23. Ciliary system, function of the, 575. Coccius, on the formation of pigment in myopia, referred to, 363; on the condition of the retina in staphyloma scleroticsB, 378. Concave glasses, 135. Conjugate foci, 42. Contraction, direct and consensual, of the iris, 573, Conus, glass, of Steinheil, 424. Convex glasses, 135. Cooper, Mr. White, on asthenopia, 272 ; on hypermetropia, 327; on improve- ment in sight in some cases of myo- pia by giving an oblique direction to suitable glasses, 511. Cornea, conical, a cause of astigmatism, 550. Cornea, fixity of form of the, in accom- modation, 15; measurements of the radius of the, 89 ; condition of the, in myopia, 368; influence of the, in astigmatism, 459 ; spots on the, a caubc of astigmatism, 553. Corresponding or identical retinal points, law of, 164 630 INDEX. Cramer, on accommodation in the eye, referred to, 11. Critehett, Mr., on iriddeeis, referred to, 511. Crystalline lens, change of form in the, in accommodation, 10; the spectrum of the, 200 ; condition of the, in myopia, 369 ; mode of calculating the part played by the, in astigmatism, 496. Cyclitis — see Kyklitis. Cylindrical lenses, use of, in astig- matism, 501. Darwin, on the use of mydriatics, in ophthalmia, 590. Dechales, on muscse volitantes, 202; on myopic vision, referred to, 389. Defects of refraction and accommoda- tion, 80 ; causes of the, 84, 86. De la Hire, on polyopia monocularis, referred to, 549. Depth, estimation of, 158. Diagrammatic eye, the, 175. Dietrichstein, von, his question to Kepler, 444. Dioptric images of the eye, calculation of tie position and magnitude of the, 70. Dioptrics of the eye, 38. Dioptric system of the eye, change of, HSis 6^1- ™ accommodation, 10. . ' Diphtheria faucium, paresis of ac- '\ & |\,i r ^ commodation after, 599. ~ Dispersion, phenomena of, in astig- Aru Pdfc matism, 473. '•-Distance, determination of focal, 140; '^ V'^ estimation of, 153. ' Dupuy, on diminution of the pupil after extirpation of the first ganglion, 578 Elasticity of the lens, 21. Emmetropia, definition of, 81. Emmetropic eye, the, 173. Entoptic observation, 197 ; History of, 202. Epicrisis, note by Professor Hanghton on the term, 517. Eye, muscular elements of the, 23 ; dioptrics of the, 38, refracting sur- faces in the, 38 ; general indication of cardinal points of the, 62; calcu- lation of the position of the principal points in the, 63; the emmetropic, 173; the diagrammatic, 175; centre of motion of the, 179; movements of the, 179 ; hypermetropic, form, position, and movements of the, 244. Eyes, difference of refraction in the two, 557 ; various modes of using unequal, 559. Farsightedness, incorrectness of the term, 209. Fatigue, two forms of, 265 ; theories of, 267. Fechner, on binocular vision, referred to, 161, note. Fick, on the movements of the eye, re- ferred to, 180, note. Focal distance, determination of, 140. Focal interval ot Sturm, 453. Foci, conjugate, 42 ; of a lens, defini- tion of the, 56 ; calculation of the position of the, in the eye, 65. Focus, principal, the, defined, 7 ; a virtual, 135. Form, estimation of, 153. Franklin's glasses, 138. Fronmuller, on " Myopia at a dis- tance," 328, 851. Furnari, on the absence of myopia among the Kabyles, 342. Ganglion, ciliary, presence of gangli- onic cells in the, 576. Ganglionic cells, presence of, in the chorioidea, and in the orbiculus ciliaris, 576. Gauss, on the dioptrics of the eye, referred to, 38. Gendron, on the ellipsoidal figtire of the myopic eye, 447. Giraud-Teulon, on the anomalies of re- fraction, referred to, 331. Glasses, employed in the clinical de- termination of ametropia, 97 ; pris- matic, 132 ; convc"x and concave, 135 ; periscopic, 136 ; Franklin's, 138 ; best material for, 139 ; influence of, on vision, 143 ; green and blue use of, in moderating the light, 170 ; reading, use of, 229 ; convex, not in- jurious, 231. Glaucoma, necessity for iridectomy in, 235 ; development of, with high de- grees of staphyloma posticum, 402. Goode (Dr.), on astigmatism, 540. Graefe (von), on prismatic spectacles in strabismus, 170 ; on the move- ments of the eye, referred to, 180; note ; on asthenopia muscularis, 265 , on strabismus, 307 ; on congenital aphakia, 310 ; on iridectomy prepa- ratory to operation for cataract, 316; on accommodation in aphakia, 320 ; - on hypermetropia, 330 ; optical in- strument invented by, 335, note ; on myopia at a distance, 351 ; on the frequency of atrophy in high degrees of myopia, 365 ; on the limits of the field of vision in amblyopia, referred to, 397 ; on tenotomy of the mnscu- INDEX. 631 lus rectus externus, 428; on the cause of myopia, 447 ; ou the mode of action of mydriatics, 588. Gut, his refutation of Stellwag Ton Cation's theory of irregular astigma- tism, referred to, 556. Haan (Dr. Vroesom de), on the influ- ence of age upon the acuteness ot vision, 189. Haas (Dr. D.), hypermetropia, and ac- commodation of, 241 ; historical re- searches on hypermetropia and its results, referred to, 306. Hamilton, case of abnormal astigma- tism by, referred to, 540. Happe, on the anomalies of refraction, referred to, 331. Hartiug (Prof. P.), on the microscope, referred to, 170. Hasner (yon Artha), imitation of his optometer, 115 ; on the anomalies of refraction, referred to, 331 ; on the effect of social position upon myopia, referred to, 342 ; on the hereditary nature of myopia, 350 ; on atrophy of the retina in childhood, 361 ; on ellipsoidal form of the eye in myopia, 371. Haughton (Eev. Professor), Notes by, 7, 41,45, Sl7. Hajs (J.), cases of astigmatism ap- pended to the American edition of Mr. Lawrence's works by, 541. Helmholtz, on accommodation in the eye, referred to, 13; inventor of the ophthalmometer, 17 ; on the dioptrics of the eye, referred to, 38 ; on flatten- ing of the crystalline lens under the influence of tension of the zonula Zinnii, 369 ; ou the pheuonema of dis- persion in the eye, 473; ou the literature of normal irregular astig- matism, referred to, 556. Hess (J. A.) on " presmyopia," 329. Heurtelonpian abstraction of blood, 436, note. Heymann, on impressions of the ciliary nerves on the atrophic sclerotic, re- ferred to, 382. Himly (Karl), discoverer of the mydri- atic action of hyoscyamus, 590. Homocentric light, 449. . Homogeneous light, 450. Hulke (Mr.), the author's obligations to, 539. Hyoscyamus niger, mydriatic action of, 584. Hypermetropia, definition of, 82, 236 ; acqaisita, 204 ; Subdivisions of, 238; phenomena and diagnosis of, 251; treatment of, 274; prognosis in, 283 ; case of extreme, by Mr. Bowman, 290 ; strabismus convergens, the re- sult of, 291, 306 ; historical remarks, respecting, 325. Hypermetropic eye, form, position, and movements of the, 244 ; tendency to asymmetry of the, 255. Hyperopia, term proposed by Helm- holtz, '331. Hyperpresbyopia, term employed by Stellwag von Carion, 325. Identical or corresponding retinal points, law of, 164. Images, dioptric, of the eye, calcula- tion of the position and magnitudes of the, 70. Insufiiciency of the internal recti muscles, a result of myopia, 402, 426. Interval, focal, of Sturm, 453. Iriddesis in astigmatism, 511; double, Mr. Bowman on, 552, and note. Iridectomy, necessity for, in Glaucoma, 235, in operating for cataract, 315. Iris, muscular fibres of the, 23; move- ments of the, 572; direct and con- sensual contraction of the, 573. Jacob, membrane of, 1. JacobsoD, on iridectomy in operating for cataract, 316. Jaeger (von), his spectacle-frame, 97, note; on hypermetropia as a diag- nostic sign in affections of the central nervous sjstem, 244, note; on the developmentof eyes, 251; on accom- modation in aphakia, 321; ophthal- moscopic determinations of the refrac- tion of the eye, by, 331 ; on the pro- gression of myopia, 349; on the fre- quency of myopia among the lower classes in Austria, 353; representa- tions of the fundus ocnli of the myopic eye, by, referred to, 356, note; on change of form of the optic disc in myopia, 358 ; on hereditary atrophy of the retina, 361; on the frequency of atrophy in high degrees of myopia, 365; on the ophthalmo- scopic examination of the myopic eye, 366; on myopia as a result of partial ax'commodation for near ob- jects, 369; on prolongation of the visual axis in myopia, 371; on the structure of the optic nerve in myo- pia, 377. Janin, early acquainted with hyper- metropia, 326, note. Jones (Wharton), on the cause of astig- matism, 542. 632 INDEX. Jungken, on heietudo visua, 270; on the hereditary nature of myopia, 350. Junge (Professor), his method of de- teraiining the position of the centre of motion of the eye, 185; on spasm of accommodation in high degrees of myopia, 393. Jurin, acquainted with the larger pupil of myopes, 388. Kabyles, absence of myopia among the, 342. Kepler, on the accommodation of the eye, 10; the author of our first know- ledge of myopia, 444. Kerato-conus, see cornea, conical. Kerst, on " myopia at a distance," 328, 351. Knapp (Dr.), on the accommodation of the eye, 16; on the variable form of the optic disc in astigmatism, 489; on a peculiar form of the crystalline lens producing astigmatism, 531. Krause (C), on the presence of gan- glionic cells in the orbicularis ciliaris, referred to, 576. Krause (W.), on the structure of the retina, referred to, 6 ; on the orbiculus ciliaris, 576. Krecke (Dr.), on binocular vision, 133; on prismatic spectacles in strabismus, 170. Kuyper (Dr.), on the mydriatic action of weaker solutions of atropia, 587. Kyklitis, 370; meaning and derivation of the term, ix. Langenbeck (Maximilian), on the re- flected images of the lens, 11. Lawrence (W.), on asthenopia, 270. Lens, biconvex, refraction through a, 44 ; nodal points of a, 45 ; principal points of a, 50; definition of the foci of a, 56; astigmatic, of Stokes, 485. Lenses, varieties of, 135; cylindrical, use of, in astigmatism, 501. Lens, the crystalline, alteration in ac- commodation, 13; elasticity of, 21; influence of, in astigmatism, 467; displacement of the, a cause of irre- gular astigmatism, 555. Liebreich, representations of thefundus oculi of the myopic eye, by, referred to, 356, note ; on change of form of the optic disc in myopia, 358; on the ophthalmoscopical examination of highly myopic eyes, 367; case of recovery by spontaneous replace- ment of detached retiua, by, 400; representation of rupture of the re- tina, referred to, 432, note. Light, homocentric, 449; monochro- matic and homogeneous, 450. Listing, OQ the dioptrics of the eye, re- ferred to, 38, 206; on the co-efficient of refraction, 39; diagrammatic eye of, 177 ; on entoptic phenomena, 203; on the diffusion of light, 549. Mac Gillavry, on the extent of accom- modation, referred to. 111 note, 117, 330; on the relative range of accom- modation in myopia, referred to, 448. Mackenzie, on the muco-lachrymal spectrum, 203; on asthenopia, 270; observations on the hypermetropic condition, 325, 328; on the cause of myopia, 447; on astigmatism, 539. Magnitude, estimation of, 153. Mariotte, blind spot of, 3, 397. Maurolycus, on the action of the crys- talline lens, 444. Meissner, on the movements of the eye, referred to, 180, note. Menisci, advantages of, 136. Meyer, on the reflected images of the lens, referred to, 11. Microscopes, effect of, on the accom- modation of the eye, 148. Middelburg, Dr., on the seat of astig- matism, referred to, 462. Mile, on the position of the centre of motion, 180. Monochromatic light, 450. Montucla, his criticism of Kepler's Astronomies pars optica, 445. Mooren, on the use of convex glasses in strabismus, 305 ; on the coexistence of hypermetropia and strabismus con- vergens, referred to, 309; on iridec- tomy in operating for cataract, 316. Moser, on the dioptrics of the eye, re- ferred to, 38. Motion, centre of, of the eye, 179. Movements of the eye, 179; literature of the subject, 180, note. Muco-lachrymal spectrum, the, 197. Mueller, H., radiating fibres of, 3; on the situation of the perceptive layer of the retina, 5 ; his theory of accom- modation, 27 ; on the formation of elevations on the choroid, 192; on the state of the retina in myopia, 378; on the presence of muscular fibres and of ganglionic cells in the chorioidea, 381 note, 576. Mueller, John, on a connexion between convergence and accommodation, re- ferred to, 110; strabismusjocongmns of, 250; on strabismus, 414. Musca3 volifantes, 197 INDEX. 633 Muscle, the ciliary, 23. Muscles, insufficiency of the internal recti a result of myopia, 402, 426. Muscular elements of the eye, 23. Mydriatics and their action, 584. Myopes, the vision of, 387 ; Calabar bean useful to, 388. Myopia, definition of, 82; diagnosis of, 332, determination of the degree of, 337 ; comparative frequency of its occurrence, 340; influence of social position on, 343; progressive nature of, 344; predisposition to, 349; "at a distance," 328, 351 ; noa-implica- tion of the lens in, 352 ; principal anatomical changes in, 354 ; inter- mittent spasm of accommodation a cause of, 370; question of its cura- bility, 417; choice of spectacles in, 420; therapeutic treatment of, 429; cases illustrative of, 433; historical remarks on, 444. Myopic eye, advantages of the, 213; result of the ophthalmoscopic inves- tigation of the, 353 ; anatomy of the, 367. Myopodiorthoticon, Berthold's, 417. Myotics and myosis, 610. Nagel, on binocular vision, referred to, 161. Nancy, asthenopia among the em- broiderers of, 272. Nerve, sympathetic, influence of the, upon the pupil, 578 Nerves, influence of the, upon accom- modation and upon the movements of the iris, 572; action of mydriatics upon, 588 ; action of myotics on, 615. Nodal points, anterior and posterior, 29; of a lens, 45; calculation of the position of the, in the eye, 65. Nuhn, on the effect of acetate of lead and acetic acid on the zonula Zin- nii, 25 ; on dilatation of the pupil in a decapitated criminal, referred to, 577. Ooulo-motor nerve, action of the, upon the sphincter of the pupil, 577. Ophthalmometer, the, described, 17 et seq. Ophthalmoscope, the, 16; application of the, in the clinical determination of ametropia, 105; use of. in the de- tection of astigmatism, 551. Optic disc, change of form of the, in myopia, 358; variable form of the, in astigmatism, 490. Optical centre, 49 Optometer, the, described, 33; descrip- tion of an, 115. Ordeal bean of Old Calabar — see Cala- bar Bean. Oversightedness, Euete on, 329; Stell- wag von Carion on, 330. Fagenstecher and Saemisch, on corneal spots in strabismus, 296 ; on the co- existence of hypermetropia and stra- bismus convergens, referred to, 309. Fagenstecher, on iridectomy in operat- ing for cataract, 316. Panum, on binocular vision, referred to, 161. Paralysis and debility of accommoda- tion, 584 ; treatment of, 596; Dr. "Wells on, 597. Paresis of accommodation, the result of angina diphtherina, 287, 599; proximate cause of, 608. Percy and Beveillfi-Parise, on the focal distance of the crystalline lens in myopia, 369. Periscopic glasses, 136. Petit, on the influence of the sympathetic nerve upon the pupil, referred to, 578. Fetrequin, on the cause of asthenopia, 270. Phacoidoscope, the, described, 16. Plenck, on asthenopia, 269. Points, nodal, 29; of a lens, 45; car- dinal, 39; principal, of a lens, 50; principal and nodal, conditions for the coincidence and non-coincidence of the, 52, 53; cardinal, of the eye, general indication of the, 62 ; prin- cipal, in the eye, calculation of the position of the, 63; nodal, calcula- tion of the position of the, 65 ; car- dinal, review of the position of the, in the eye, 67 ; use to be made of the knowledge of the, 68; retinal, law of identical or corresponding, 164. Polyopia uniocularis, dependent on the lens, 451, 545. Porta, J. Baptists, inventor of the camera o'bscura, 444. Porterfield, on a connexion between convergence and accommodation, referred to, 110; acquainted with the larger pupil of myopes, 388. Position, stooping, evils of, in Myopia, 419. Presbyopia, definition of 83, 204; ety- mology of the term, 210, necessity for the term, 211; treatment of, 215; causes of early, 223. Presmyopia, term employed by J. A. Hess, 329. 634 INDEX. Prevots, on the motion of muscsB voli- tantes, referred to, 202. Prismatic glass and spectacles, 132. Protuberantia scleralis, in the foetal eye, von Ammon on the, 385. Pupil, action of the oculo-motor nerve upon the sphincter of the, 577. Purkinje's experiment respecting the perceptive layer of the retina, 5 ; on the reflected images of the lens, 11. Radius of the cornea, measurements of the, 89. Kange of accommodation in the eye, 28, 72, 110; absolute, binocular, and relative, 112 ; modification of the, 145. Rankine, Mr., terms borrowed from, 265. Becklinghausen, on the function of the retina and binocular vision, referred to, 161, note, Eefraciing surfaces in the eye, 38. Refraction, coefficient of, as fixed by Listing, 39 ; by a spherical surface, 40; through a biconvex lens, 44; through a compound system, con- sisting of a spherical surface and a biconvex lens, 62; defects of; 80; normal, definition of, 81; causes of the defects of, 86 ; anomalies of, 171 ; cause of the diminution of, 206; by asymmetrical surfaces, 457; differ- ence of, in the two eyes, 557. Region of accommodation, modifica- tion of the, 150. Reid, on the influence of the sympathetic nerve upon the pupil, 578. Retina, function of the, 1 ; structure of the, 2; appearance of the, in myopia, 377 ; detachment of the, in myopia, 398. Retinal points, law of identical, or cor- responding, 164. Reynolds (Sir Joshua}, spectacles used by, 228. Rltterich, on strabismus, referred to, 328; on pear-shaped eye ia myopia, 447. Rnete, on spectacles, referred to, 170; on the movements of the eye; referred to, 180, note; on the cause of asthen- opia, 271 ; on the treatment of hyper- metropia, 275; on "oversightedness," 329; on increase of size of the pupil by atropia in cases of paralysis of the oculo-motor nerve, 589. Euiter (Dr. de), on the mode of action of mydriatics, 588. Saemisch, on the presence of gangli- onic cells in the chorioidea, referred to, 576. Sanson, on the diagnosis of cataract, referred to, 11. Scarpa, on asthenopia, 270 ; on ellipsoi- dal form of the eye in myopia, referred to, 371, 447. Scheiner, experiment of, 33; on the vision of myopes, 446. Schnyder (Pastor), on astigmatism in his own eye, 541. Sehurman (Mr.), on the mobility of the eyes in myopia, 409, note* Scotomata, occurrence of, in myopia, 396. Senif, on the refractive power of the lens, referred to, 39, 206. Sichel, on asthenopia; 270 ; on awbly- opie presbytique conginitale, 326. Smee, visuometer of, 116; pantoscopic spectacles of, 227 ; on the hyperme- tropic condition, 328. Smith (Robert), on accommodation of myopes, referred to, 391, 446. Snellen (Dr.) system of test-types by, 32, 97, 189; on diminution of direct vision in the region of the yellow spot in amblyopia, 397. Sotteau, on the motion of muscse voli- tantes, referred to, 202. Spasm of accommodation, 610. Spectacles, 127 ; stenopseic, 128 ; prismatic, 132; periscopic, 136; choice of materials for, 139 ; influ- ence of, on vision, 143; green and blue, 170; in strabismus, 170; Wijn- gaarden oa, referred