o iowed 
 
 Ml 
 
 300 
 
 C791r 
 
 1306 
 
 11 
 
 ill!
 
 THE LIBRARY 
 
 OF 
 
 THE UNIVERSITY 
 OF CALIFORNIA 
 
 LOS ANGELES
 
 .'V
 
 if! V, 
 
 REFRACTION 
 
 INCLUDING 
 
 MUSCLE IMBALANCE AND THE ADJUSTMENT 
 OF GLASSES 
 
 ROYAL S. COPELAND, A. M., M. D., 
 
 PROFESSOR IN THE UNIVERSITY OF MICHIGAN 
 
 ADOLPH E. IBERSHOFF, M. D., 
 
 INSTRUCTOR IN THE UNIVERSITY OF MICHIGAN 
 
 PHILADELPHIA : 
 
 BOERICKE & TAFEL 
 1906
 
 COPYRIGHTED 
 BY 
 
 BOERICKE & TAFKL 
 1906
 
 
 PREFACE. 
 
 With no word of explanation, it would be presumptuous 
 to add to the large and growing list of excellent and ex- 
 haustive treatises on the subject of refraction. No student 
 of medicine, we think, will deny that the study of this 
 branch of practical therapeutics is beset with many difficul- 
 ties, and, when viewed through the meshes of a multiplicity 
 of diagrammatic cobwebs, it appears especially unattract- 
 ive. We feel safe in adding that any real help in simplifying 
 the teaching of refraction will be welcomed by student and 
 practitioner. An attempt to accomplish this end is our only 
 hope of justification, and if, in a small measure, we succeed 
 in elucidating some of the difficult principles and problems 
 of the subject, we shall feel that our efforts have not been 
 vainly spent. 
 
 Refraction is an eminently practical science and we aim 
 to treat it as such, divesting it, so far as possible, of such 
 theoretical demonstrations, logarithmic computations, and 
 minor technicalities as are not deemed absolutely essential 
 to a thorough comprehension of the subject. To strip it of 
 its embellishments is to invite criticism, but the relative im- 
 portance of what has been omitted is, of course, a matter of 
 judgment. To some of our readers, occasional statements 
 may seem too broad, but if we have failed to limit them suf- 
 ficiently, -or to state all known exceptions, we have acted in 
 the hope of avoiding confusion without sacrifice of essentials. 
 To lead the student along practical lines to an accurate un-
 
 IV PREFACE. 
 
 derstanding of the applied principles of refraction has been 
 the object of our labor. To this end, it is strongly recom- 
 mended that the student early acquaint himself with the 
 test-case and, so far as possible, supplement his studies by 
 practical verification of the facts and theories set forth in 
 the text. 
 
 University of Michigan, June, 1905.
 
 CONTENTS. 
 
 CHAPTER I. 
 
 PAGE. 
 
 OPTICAL PRINCIPLES 9 
 
 Wave Theory of Light. 9 ; The Light Ray, 9 ; Refraction, 10 ; Index of 
 Refraction, 11; First Principles, 11. 
 
 CHAPTER II. 
 
 LENSES, 13 
 
 A Lens, 13; The Prism, 13; Spherical Lenses, 14; Cylindric Lenses, 20; 
 Properties of Lenses, 23 ; Varieties of Lenses, 23 ; Systems of Lens 
 Measurements, 24; The English System, 25; The Metric System, 
 25 ; Prescription for Lenses, 26 ; Neutralization of Lenses, 26 ; 
 Other Methods, 28; Measuring the Strength of a Prism, 29; Me- 
 chanical Devices for Measuring Lenses, 31 ; The Relationship of 
 Lenses, 32; The Relationship Between Spheric and Cylindric 
 Lenses, 32; The Relationship of Cylinders, 33; The Relationship of 
 Spheres to Prisms, 34; The Relationship of Cylinders to Prisms, 34. 
 
 CHAPTER III. 
 
 THE NORMAL EYE, 35 
 
 Introduction, 35; Requisites of Vision, 35 ; Dioptric System of the Eye, 
 35 ; Static Refraction, 36 ; Dynamic Refraction, 36 ; Physiology of 
 Accommodation, 37 ; Convergence, 37; Relationship of Accommo- 
 dation, Convergence and Pupillary Contraction in the Emmetropic 
 Eye, 40; Optical Defects of the Normal Eye, 41 ; Changes Occur- 
 ring During the Life History of the Eye, 42 ; Visual Acuity, 44. 
 
 CHAPTER IV. 
 
 ASTHENOPIA, " 47 
 
 Symptoms, 47 ; Asthenopia as a Cause of General Disease, 48. 
 
 CHAPTER V. 
 
 THE TRIAL CASE AND ITS USES, ... 49 
 
 Introduction, 49 ; Spherical Lenses, 49; Cylindric Lenses, 49; Prismatic 
 Lenses, 50; The Trial Frame, 52.
 
 VI CONTENTS. 
 
 
 
 CHAPTER VI. 
 
 HYPEROPIA OR FAR SIGHT, 55 
 
 Introduction, 55; Causes, 57; Symptoms and Sequela;, 57; Spasm of 
 Accommodation, 57; Varieties of Hyperopia, 58; Determination 
 of Hyperopia, 58 ; Correction of Hyperopia, 58; Cycloplegics and 
 Mydriatics, 60 ; Contraindication for the Use of Mydriatics, 60 ; 
 Correction of Results Obtained Under a Cycloplegic, 61; Myotics, 62. 
 
 CHAPTER VII. 
 
 MYOPIA OR NEAR SIGHT, 63 
 
 Introduction, 63 ; Causes of Myopia, 65 ; Symptoms of Myopia, 65 ; 
 Determination of Myopia, 66 ; Correction of Myopia, 66 ; Pro- 
 phylaxis, 67. 
 
 CHAPTER VIII. 
 
 ASTIGMIA, 68 
 
 Introduction, 68; Forms of Regular Astigmia, 69; Causes of Regular 
 Astigmia, 71; Symptoms, 71; Determination of Astigmia, 71; 
 Correction of Astigmia, 74 ; Simple Hyperopic Astigmia, 75 ; 
 Simple Myopic Astigmia, 76 ; Compound Hyperopic Astigmia, 77; 
 Compound Myopic Astigmia, 78 ; Mixed Astigmia, 79. 
 
 CHAPTER IX. 
 
 ANISOMETROPIA, APHAKIA, AMBLYOPIA AND MALINGERING, 82 
 
 Anisometropia, 82; Aphakia, 82; Amblyopia and Amaurosis, 82 ; Ala- 
 lingering, 83. 
 
 CHAPTER X. 
 
 PRESBYOPIA, 84 
 
 Causes and Symptoms, 84; The Relationship Between Presbyopia and 
 the Age of the Patient, 86 ; Correction of Presbyopia, 86. 
 
 CHAPTER XI. 
 
 CONFIRMATORY TESTS (OBJECTIVE), . . 88 
 
 Introduction, 88; Mirror Test (Direct Method), 89; Mirror Test (In- 
 direct Method), 90; Retinoscopy or Skiascopy (Keratoscopy), 91.
 
 CONTENTS. Vll 
 
 CHAPTER XII. 
 
 MI/SCULAR IMBALANCE 94 
 
 Ocular Movements, 94; Muscular Balance or Orthophoria, 95 ; Hetero- 
 tropia or Strabismus, 95 ; Heterophoria, 96 ; Forms of Hetero- 
 phoria, 96; Determination of Heterophoria, 96 ; Prism Test, 98; 
 Maddox Test, 98 ; Cobalt Test, 99 ; Cover Test, 99 ; Determina- 
 tion of Cyclophoria, 100 ; Correction of Heterophoria, 100 ; Cor- 
 rection of the Error of Refraction, 100 ; Prism Exercise, 101; Wear- 
 ing of Prisms, 101 ; Operation, 102. 
 
 CHAPTER XIII. 
 
 SPECTACLES AND EYE GLASSES, 103 
 
 Introduction, 103 ; Spectacles, 103 ; Requisites of Properly Fitting 
 Glasses, 105 ; Measurements and Data for Ordering Spectacles, 
 105 ; Inter-pupillary Distance, 106 ; Size of Lenses, 107 ; Angle 
 of Lenses, 107 ; Bridge or Nose Piece, 107 ; Temples, 108 ; Eye 
 Glasses, 108 ; Measurements and Data for Ordering Eye Glasses. 
 109; Springs. 109; Studs, 110; Guards, 110; Other Forms of 
 Glasses, 111; Half Oval Eye Frames, 111 ; Reversible Spectacles, 
 111; Hook or Grab Fronts, 111; Clerical or Adjustable Eye 
 Glasses, 112. 
 
 CHAPTER XIV. 
 
 SPECIAL FORMS OF LENSES, 113 
 
 Bifocal Lenses, 113; Periscopic Lenses, 114; Meniscus Lens, 115; Toric 
 Lens, 115. 
 
 APPENDIX. 
 
 MECHANICAL AIDS TO REFRACTION, 118 
 
 Ophthalmoscope, 119; Luminous Ophthalmoscope, 119; Luminous 
 Retinoscope, 120; Ophthalmometroscope, 122; Ametropometer, 
 125; Refractometer, 125; Placido's Disc, 126; Ophthalmometer, 
 127; Risley's Rotating Prism, 129; Phorometer, 129; Perimeter, 
 130; Stigma tome ter, 132.
 
 INDEX OF ILLUSTRATIONS. 
 
 FIGURE. PAGE. 
 
 1. The Refraction of Light, n 
 
 2. A Prismatic or Wedge-shaped Lens, .' . . . 13 
 
 3. The Deflection of a Light Ray Toward the Base of a Prism, .... 14 
 
 4. Sections of a Sphere to Represent Convex Spherical Lenses, ... 14 
 
 5. A Concave Spherical Lens, 15 
 
 6. The Convergence of a Beam of Light in Passing Through a Spherical 
 
 Convex Lens, . 15 
 
 7. The Divergence of a Beam of Light in Passing Through a Concave 
 
 Lens, . . 16 
 
 8. The Virtual Focus of a Concave Lens, .... 16 
 
 9. Sections of Spherical Lenses as Made Up of Two Prisms, ... 17 
 
 10. Spherical Convex Surfaces as Made Up of Prism Sides, 18 
 
 n. Spherical Concave Surfaces as Made Up of Prism Sides, 18 
 
 12. Method of Determining the Optical Centre of a Lens, 19 
 
 13. Convex Cylindrical Lens, . . 20 
 
 14. How a Beam of Light Becomes Wedge-shaped After Passing 
 
 Through a Cylinder, . . 21 
 
 15. A Concave Cylindrical Lens, 22 
 
 16. How the Axis of a Cylindrical Lens is Indicated, 23 
 
 17. Varieties of Lenses 24 
 
 18. Break in the Continuity of a Line Caused by Tilting a Cylindrical 
 
 Lens, 2y 
 
 19. Break in the Continuity of a Line Caused by a Prismatic Lens, . . 30 
 
 20. Authors' Axis Finder, . 31 
 
 21. Lens Measure, 31 
 
 22. Prismatic Effect of Decentering a Lens, 34 
 
 23. Mechanism of Accommodation, 38 
 
 24. Method of Constructing Snellen's Test Letters, . . . 44 
 
 25. Principle Involved in Snellen's Test Letters, -44 
 
 26. Test Card, 45 
 
 27. Changeable Test Card, 46 
 
 28. Trial Case, . . 49 
 
 29. Cylindrical Test Lens, 50 
 
 30. Pin Hole Disc, .' . . . 51 
 
 31. Stenopaeic Disc, ..... 51 
 
 32. Maddox Multiple Rod, ... 51
 
 X INDEX OF ILLUSTRATIONS. 
 
 FIGURE. PAGE. 
 
 33. Double Prism, 5 1 
 
 34. Trial Frame, 52 
 
 35. Trial Frame Properly Adjusted for Conducting a Test, . 53 
 
 36. Authors' Trial Frame Tilted for Reading, .... 54 
 
 37. The Focus of Rays in a Hyperopic Eye, . . 
 
 38. A Hyperopic Eye Corrected for Distant Vision, . . 56 
 
 39. A Hyperopic Eye Corrected for Near Vision, . . 56 
 
 40. The Focus of Parallel Rays in a Myopic Eye, .... .63 
 
 41. Why a Myopic Eye Sees Clearly at Close Range, . . 64 
 
 42. A Myopic Eye Corrected for Distant Vision, .... . . 66 
 
 43. The Effect of a Double Cylinderas Illustrating the Refractive Effect 
 
 of an Astigmic Surface, 68 
 
 44. Varieties of Astigmia, . . 70 
 
 45. Fan of Rays Used in Testing for Astigmia, 7 2 
 
 46. Astigmic Dial, 73 
 
 47. Dial as Seen by an Astigmic Eye, 74 
 
 48. Position of Focal Lines in Simple Hyperopic Astigmia and Position 
 
 of Correcting Cylindric Lens, 75 
 
 49. Position of Focal Lines in Simple Myopic Astigmia and Position of 
 
 Correcting Cylindric Lens, 76 
 
 50. Position of Focal Lines in Compound Hyperopic Astigmia and Posi- 
 
 tion of Correcting Cylindric Lenses, 77 
 
 51. Position of Focal Lines in Compound Myopic Astigmia and Position 
 
 of Correcting Cylindric Lenses, 78 
 
 52. Position of Focal Lines in Mixed Astigmia and Position of Correct- 
 
 ing Cylindric Lenses, 80 
 
 53. Gradual Recession of the Near Point Owing to Hardening and 
 
 Flattening of the Lens Presbyopia, ... 85 
 
 54. Argand Burner with Thorington Retinoscopy Chimney, ...... 88 
 
 55. Retinoscope, 89 
 
 56. Apparent Displacement of an Object Toward the Apex of a Prism, . 97 
 
 57. Full Frame Spectacle Mounting, ... 104 
 
 58. Rimless or Skeleton Spectacle Mounting, 104 
 
 59. Method of Measuring Interpupillary Distance, 106 
 
 60. Standard Lens Sizes, 107 
 
 61. Rimless or Skeleton Eye Glass Mounting, 108 
 
 62. Full Frame Eye Glass Mounting, 109 
 
 63. Half Oval Eye Glass Frame, . . . in 
 
 64. Grooved Bifocal Lens, 113 
 
 65. Different Forms of Bifocal Lenses, . . 114 
 
 66. A Tor with Sections to Represent Piano -Con vex and Piano-Concave 
 
 Toric Lenses, . . . . 116 
 
 67. A Toric Lens, 116 
 
 68. Rear View of Ophthalmoscope, 118 
 
 69. Luminous Ophthalmoscope, 120
 
 INDEX OF ILLUSTRATIONS. XI 
 
 FIGURE. PACK. 
 
 70. I/utninous Retinoscope, ... 121 
 
 71. Oplithalmonietroscope,. . . 122 
 
 72. Ametroporueter, '. . . 124 
 
 73. Refractometer, . . 125 
 
 74. Placido's Disc, ... 127 
 
 75. Ophthalmometer, 128 
 
 76. Risley's Rotating Prism, 129 
 
 77. Improved Phorometer, 130 
 
 78. Perimeter, . . . 131 
 
 79. Stigmatometer, .... 132
 
 INDEX OF TABLES. 
 
 TABLE. PAGE 
 
 I. Showing Strength of L,enses and Their -Focal Length in English 
 
 Inches and in Centimeters, 26 
 
 II. Showing the Relationship of Accommodation, Convergence and 
 
 Pupillary Contraction, 41 
 
 III. Showing Strength of Cycloplegics and Mydriatics, Time Required 
 
 to Act, and Duration of Drug Effect, 61 
 
 IV. Showing the Relationship Between Presbyopia and the Age of the 
 
 Patient, 86 
 
 V. Showing Ocular Muscles Concerned in the Several Excursions of 
 
 the Eyeball, 94 
 
 VI. Showing Relative Position of Streak to Flame in the Diagnosis of 
 
 Heterophoria, 99
 
 REFRACTION. 
 
 CHAPTER I. 
 OPTICAL PRINCIPLES. 
 
 WAVE THEORY OF LIGHT. 
 
 The long- time accepted view regarding light production is 
 based on the theory that all space is pervaded by a medium 
 called the luminiferous ether. In the terms of this hypothe- 
 sis, the emanations from a luminous source involve the ether 
 in \vave-like motion, resulting in the phenomenon of light. 
 
 This theory answers very well most of the observed phe- 
 nomena, but recently it has been considerably revised. The 
 newer view is that the oscillations are of an electrical 
 nature, that electro-magnetic radiations and light radiations 
 are practically identical. The nature of the so-called "elec- 
 trical displacement" is not thoroughly understood, but in 
 its terms all of the phenomena of light can be explained. 
 
 It is believed that the particles constituting the ether are 
 in constant motion. These ethereal waves, according to 
 their length, frequency, form and energy, manifest themselves 
 by stimulation of the organs of special sense. In one in- 
 stance, their impingement upon the terminal nerves of the 
 body is proved by a sensation of feeling. Again, they mani- 
 fest themselves as heat producers, or may serve to induce 
 chemical changes. Under other conditions they serve to 
 stimulate the retina of the eye, effecting the sensation of 
 light. 
 
 THE LIGHT RAY. 
 
 A ray of light may be considered as the path described by 
 the emanation from a single point in the luminous source. 
 2
 
 10 REFRACTION. 
 
 Such a path may be described as a succession of waves pro- 
 ceeding along a straight course. This has been compared to 
 the oscillations of the various points of a rope along which 
 a wave is passing, the vibration of the ether being at right 
 angles to the direction of the wave itself. For the sake of 
 convenience in description, these oscillations may be con- 
 sidered as consisting of "wave fronts," or "wave crests." 
 So long as the optical density of the transmitting medium is 
 constant, the velocity of the oscillations is uniform. As 
 would be expected, their speed is lessened in passing from a 
 rare into a dense medium, as, for instance, from the air into 
 glass or water. Their direction remains unchanged, pro- 
 vided they enter the denser medium perpendicularly. This 
 is explained by the fact that the entire transverse vibration, 
 " wave front," or " wave crest," whatever it may be called, 
 enters the denser medium at the same time and, hence, en- 
 counters the retarding influence equally at every point. 
 
 REFRACTION. 
 
 Should a ray of light encounter in its course a transpar- 
 ent medium of greater density and enter it at any angle less 
 than 90 (i. e., not perpendicularly), one end of the "wave 
 front," or "wave crest," will be retarded, while the other 
 end proceeds at the original velocity until it, too, enters the 
 denser medium. (Fig. 1.) In this way the course of the ray 
 is deflected from its straight path and is spoken of as a Re- 
 fracted Ray. The reverse of this takes place when the ray, 
 at any angle less than 90, emerges from the denser medium 
 to enter one more rare. The end of the " wave front" first 
 liberated immediately increases its velocity and consequently 
 a deflection occurs in its course. When the entire "wave 
 front" has passed out into the new medium, it again pro- 
 ceeds in a straight line. 
 
 The ray before its deflection is known as the Incident 
 Ray, and the substance causing its change of course is 
 called the Refracting Medium. A ray of light entering such 
 a refracting medium perpendicularly, as has been said, will 
 not be deflected, although its velocity will be diminished ; 
 neither will the refracted ray alter its course while travers-
 
 OPTICAL PRINCIPLES. 
 
 11 
 
 ing the refracting medium. It is only in passing from one 
 medium into another of different density that any change of 
 the ray's course is possible, and this deflection takes place at 
 the surface of the refracting medium. 
 
 Refraction, then, is the deflection of luminous rays in 
 passing obliquely from one medium into another of different 
 optical density. 
 
 The Index of Refraction. Any transparent substance is 
 a refracting medium. Some media deflect rays of light more 
 
 FIG. i. Showing the Refraction of a Light Wave. 
 
 than does air, the adopted standard. This relative refractive 
 power of a substance, as compared with air, is expressed in 
 figures and is known as the index of refraction. It varies 
 directly with the optical density of the medium. Air having 
 an index of 1 and crown glass having half again as much 
 refractive power, because of its greater density, is said to 
 have a refractive index of 1.50. 
 
 First Principles. From what has been said we ma}' de- 
 duct the following principles:
 
 12 REFRACTION. 
 
 1. From any luminous source, light rays pass out in 
 every direction. 
 
 2. Light rays always proceed in a straight course, if the 
 medium through which they pass be homogeneous; if de- 
 flected from their original direction by passing through an 
 optically denser medium, they resume a straight course 
 upon release. 
 
 3. The velocity with which the rays proceed varies in- 
 versely with the optical density of the medium through 
 which they pass. 
 
 4. Light rays passing perpendicularly from a rare into 
 an optically dense medium, or vice versa, do not alter their 
 direction. 
 
 5. In passing obliquely from one medium into another of 
 different optical density, rays of light are deflected from 
 their course. 
 
 6. The degree of such deflection is determined by : 
 
 (a) The angle of entrance ; 
 
 (b) The difference in optical density (refractive in- 
 
 dices) of the two media.
 
 CHAPTER II. 
 
 LENSES. 
 
 A Lens. A lens, named from its resemblance to a lentil, 
 may be any transparent substance, the surfaces of which 
 cause parallel light rays, passing through it, to be diverted 
 from their course. The effect of a lens to change the direc- 
 tion of a light ray, as has been said in a previous chapter 
 (see p. 11), is known as its power of refraction. To under- 
 stand this it is well to consider the simplest form of a lens, 
 the optical unit, viz., the wedge-shaped lens commonly 
 known as a prism. 
 
 FIG. 2. Showing a Prismatic or Wedge-shaped Lens. 
 
 The Prism. The prism is a transparent substance whose 
 plain surfaces are not parallel to each other. (Fig. 2.) The 
 thick edge of such a lens is spoken of as the base ; the thin 
 edge towards which the surfaces converge being the apex. 
 Rays of light passing through a prism will be deflected 
 towards its base. (Fig. 3.) When a light ray strikes the 
 inclined surface of the prism one end of the wave crests is 
 held back while the other proceeds uninterruptedly, until 
 it, too, encounters the more resistent lens surface. The ray 
 proceeds along its new course through the lens until the
 
 14 
 
 REFRACTION. 
 
 other surface is reached. One end of the wave crests emerg- 
 ng into the less resistent air advances faster than the other, 
 'until the whole wave-front has emerged, when th'e ray pro- 
 ceeds along its new course. 
 
 FlG. 3. Showing the Deflection of a Light Ray Toward the 
 Base of a Prism. 
 
 Spherical Lenses. When the surface of a lens represents 
 a section of a sphere, it is called a Spherical Lens. (Figs. 4 
 and 5.) Such a lens deflects light rays equally in all meridians. 
 
 FIG. 4. Showing Sections of a Sphere to Represent 
 Convex Spherical Lenses.
 
 LENSES. 15 
 
 Spherical lenses may be Convex or Concave. When thick- 
 est at the centre, the lens is convex, commonly called " plus," 
 
 FIG. 5. Showing a Spherical Concave Surface. 
 
 and designated by the sign + . The effect upon a beam of 
 light in passing through a convex spherical lens is to cause 
 it to become a converging cone. (Fig. 6.) The apex of the 
 
 FIG. 6. Showing the Convergence of a Beam of I<ight in Passing 
 Through a Spherical Bi-Convex L,ens. 
 
 cone, the point to which the rays centre, is known as the 
 Focus of the lens.
 
 16 
 
 REFRACTION. 
 
 When thickest at its periphery, the lens is concave, com- 
 monly called a "minus" lens, designated by the sign . A 
 beam of light in passing through a concave spherical lens 
 
 FIG. 7. Showing the Divergence of a Beam of Light in 
 Passing Through a Spherical Bi-Concave Lens. 
 
 becomes shaped like a diverging cone truncated at the sur- 
 face of the lens. (Fig. 7.) Projected backward through the 
 lens the boundary lines of this cone reach a point on the side 
 
 FIG. 8. Showing the Virtual Focus of a Concave Lens. 
 
 from which the light enters. This point is known as the 
 
 "negative" or " virtual focus" of the concave lens. (Fig. 8.) 
 
 It has long been customary to consider a convex lens as
 
 LENSES. 
 
 17 
 
 made up of prisms, placed base to base, and a concave 
 lens as made up of prisms placed apex to apex, (Fig. 9.) 
 Their effect upon rays of light is thus readily explained. But 
 to be more exact, the two surfaces of a spherical lens should 
 be considered as made up of the sides of an infinite number 
 of prism sections. When the bases of these prism sections 
 are directed toward the centre of the lens, it is convex; when 
 they are directed toward the edge of the lens, it is concave. 
 (Figs. 10 and 11.) In both concave and convex lenses, the 
 
 FIG. 9. Showing How Spherical Lenses may be Considered 
 as made up of Prisms. 
 
 angle formed by the two surfaces of the prism sections in- 
 creases from the centre outward. At the centre the two sur- 
 faces are parallel and, therefore, have no refractive effect at 
 that part of the lens, if the light enters perpendicularly. At 
 the periphery the prismatic angle is largest, hence the refrac- 
 tive effect is greatest. 
 
 A straight line, perpendicular to both surfaces of the lens 
 and passing through its centre of curvature, is called the 
 Principal Axis. The Focus, whether it be the real focus of 
 the convex lens, or the virtual focus of the concave lens, is
 
 18 
 
 REFRACTION. 
 
 FIG. 10. Showing Convex Spherical Surfaces as made up of 
 Prism Sides. 
 
 FIG. ii. Showing Concave Spherical Surfaces as made up of 
 Prism Sides.
 
 LENSES. 
 
 19 
 
 found in the Principal Axis. The distance of the focus from 
 the lens is called the Focal Distance. 
 
 The Optical Centre of a lens is that point on the principal 
 
 FIG. 12. Showing Method of Determining the Optical 
 Centre of a L/ens. 
 
 axis and within the substance of the lens, through which rays 
 coming from all directions pass without altering their 
 course. It may be found by viewing the corner of a test
 
 20 REFRACTION'. 
 
 card or other object presenting a right angle. When the two 
 lines seen through the lens are continuous with those outside 
 of it, the point of the angle will coincide with the optical 
 centre of the lens. (Fig. 12.) 
 
 Cylindric Lenses. When a lens diverts light rays in one 
 plane only, it is called a Cylindric or Astigmic Lens. This 
 is made by cutting lengthwise through a solid cylinder or 
 
 FIG. 13. Showing Convex Cylindric Lens. 
 
 rod of glass, splitting off a shaving, as it were. (Fig. 13.) 
 The resulting segment, of course, will have one plane and 
 one cylindric surface, constitutingaplano-cylindric lens. The 
 axis of such a lens coincides with the axis of the rod from 
 which it is cut; in other words, it corresponds to the lead of 
 the lead-pencil, were that the cylinder from which the shav- 
 ing came.
 
 LENSES. 
 
 21 
 
 A cylindric lens causes transmitted rays to meet, not at a 
 focal point, as happens with a spherical lens, but in a focal 
 line. A beam of light, after passing through such a lens, be- 
 comes wedge-shaped. (Fig. 14.) The sharp edge of the 
 wedge is the focal line and this lies parallel with the axis of 
 the lens. The plane in which light rays are deflected by a 
 
 FIG. 14. Showing Effect on a Beam of Light Passing 
 Through a Cylindric Lens. 
 
 cylindric lens is, therefore, at right angles to the axis; the 
 rays lying in a plane with the axis remain unchanged. 
 
 Cylindric lenses, or Cylinders, as they are called, may be 
 convex or concaves according as they cause transmitted 
 rays to converge or diverge.
 
 22 REFRACTION. 
 
 In describing a cylindric lens, it is necessary to indicate 
 its axis (the non-refracting meridian). With the lens in its 
 
 FIG. 15. Showing a Concave Cylindric Lens. 
 
 proper position before the eye, the axis is indicated in de- 
 grees of a semi-circle numbered from right to left. If the 
 
 FIG. 16. Showing How the Axis of a Cylindric Lens is Indicated. 
 
 axis lies horizontally, it is denoted as 180, if vertically, as 
 90, etc. (Fig. 16.)
 
 LENSES. 23 
 
 PROPERTIES OF LENSES. 
 
 Convex Spherical Lenses. From what has been said it 
 will be seen that : 
 
 1. Parallel rays passing through a spherical convex lens 
 are made to converge to a point, or focus. The distance of 
 this point from the centre of the lens, measured along the 
 principal axis, is called the focal distance. 
 
 Conversely, rays proceeding from the principal focus of a 
 convex lens are rendered parallel by the action of the lens. 
 
 2. Divergent rays, after traversing such a lens, unite at a 
 point farther away from the lens than the principal focus. 
 
 Conversely, rays proceeding from a point farther from 
 the lens than its principal focus emerge from it as con- 
 vergent rays. 
 
 3. Convergent rays, refracted by a convex lens, meet at a 
 point nearer to the lens than its principal focus. 
 
 Conversely, rays emanating from this point are, upon re- 
 fraction, made less divergent. 
 
 Concave Spherical Lenses. Parallel rays after passing 
 through a concave lens diverge as though emitted from a 
 point on the side from which the light emanates. This point 
 is the "virtual" focus of the lens and its distance from the 
 lens is the " negative " focal length. 
 
 Rays converging to meet at this point become parallel 
 after passing through the lens. 
 
 Cylindrical Lenses. The same properties characterizing 
 spherical lenses apply to cylinders, but in one meridian only, 
 viz., at right angles to the axis. 
 
 VARIETIES OF LENSES. 
 
 Every lens has two refracting surfaces, the shape and 
 relative position of these surfaces to each other determining 
 the power and refracting properties of the lens. (Fig. 17.) 
 Both surfaces may be plane, spherical, or cylindrical, or one 
 surface ma\ T differ from the other; therefore, a large variety 
 of lenses may be ground. Those in common use are the fol- 
 lowing :
 
 24 
 
 REFRACTION. 
 
 1. Piano; 2. Prismatic; 3. Plano-convex Spherical; 4. 
 Plano-concave Spherical; 5. Plano-convex Cylindrical; 6. 
 Plano-concave Cylindrical ; 7. Biconvex Spherical ; 8. Bi- 
 concave Spherical ; 9. Concavo-convex (two varieties): (a) 
 The Convex surface having a greater radius of curvature 
 than the concave, forming a lens called the Converging 
 Meniscus, (b) The Convex surface having a lesser radius of 
 curvature, forming the Diverging Meniscus; 10. Convex 
 Sphero-cylindrical; 11. Concave Sphero-cylindrical ; 12. Bi- 
 convex Cylindrical ; 13. Biconcave Cylindrical ; 14. Con- 
 cavo-convex Cylindrical (with axes crossed). A prism may 
 
 FIG. 17. Showing Varieties of Lenses, viz.: i, Piano; 2, Bi-Convex 
 
 Spherical; 3, Piano-Con vex Spherical; 4, Bi-Concave Spherical; 
 
 5, Piano-Concave Spherical; 6, Converging Meniscus; 7, 
 
 Diverging Meniscus; 8, Coquille; 9, M'Coquille; 
 
 10, Prism; n, Convex Cylindrical; 
 
 12, Concave Cylindrical. 
 
 be introduced as an element in each of the above named 
 lenses, thus doubling the list of possible forms of lenses. 
 And, lastly, we might add the so-called "toric" lenses, men- 
 tion of which will be made in a subsequent chapter. (See p. 
 115.) 
 
 SYSTEMS OF LENS MEASUREMENTS. 
 
 Up to this point we have been dealing with the subject in 
 the abstract. To make the science of Refraction practical, 
 it is necessary to determine exact values. To accomplish 
 this end we must adopt a standard of measurements.
 
 LENSES. 25 
 
 The English System. Inasmuch as a lens of great refrac- 
 tive power causes the rays of light to converge at a shorter 
 focal distance than does one of less refractivity, the focal dis- 
 tance expresses the strength of a lens. Thus, a spherical 
 lens with a focal distance of ten inches is twice as strong as 
 one with a focal distance of twenty inches. The strength, 
 likewise, of a cylindrical lens signifies the distance of the 
 focal line from the optical centre of the lens. The strength 
 of a lens, then, may be expressed in terms of its focal dis- 
 tance in inches. Under the old English system this was the 
 accepted method. Since the strength of a lens is inversery 
 proportional to its focal length, this method necessitates 
 the use of fractions in making deductions. Consequently, it 
 is confusing and laborious. Furthermore, the inch varies in 
 different countries, and measurements under this standard 
 are uncertain and scientifically unreliable. 
 
 THE METRIC SYSTEM. 
 
 That in common use to-day is a better system, and has 
 almost completely supplanted the inexact and cumbersome 
 methods of the past. It was introduced by Nagel in 1867, 
 and is an adaptation of the metric system of measurement. 
 Instead of computing focal lengths, its values designate the 
 refractive power of a given lens as compared with one of 
 known strength adopted as a standard. This standard is 
 a lens whose focal distance is one meter (39.36 inches), the 
 "Meterlens" of Xagel, and is called a Diopter (Monoyer). 
 
 A lens, then, having a focal distance of one meter is said 
 to have the refractive power of one diopter. The strength 
 of any given lens is expressed in terms of decimals or multi- 
 ples of this standard. For instance, a lens of 2.50 D. 
 (diopters) has two and one-half times the refractive power 
 of the standard lens, or a focal distance two and one-half 
 times shorter; a lens of .50 D. strength has one-half the re- 
 fractive power of the standard, and, of course, a focal length 
 of two meters. 
 
 The following table shows the strength of lenses in 
 diopters and their corresponding focal lengths in English 
 inches and in centimeters : 
 3
 
 26 REFRACTION. 
 
 
 TABLE I. 
 
 
 Diopters. 
 
 Focal length 
 in English 
 inches. 
 
 Focal length 
 in c e n t i- 
 meters. 
 
 Diopters. 
 
 Focal length 
 in English 
 inches. 
 
 Focal length 
 in c e n t i - 
 meters. 
 
 0.25 
 
 160 
 
 400 5.0 8 
 
 20 
 
 0.50 
 
 80 
 
 200 5.50 7.50 
 
 18 
 
 0.75 60 133 6.0 
 
 7 
 
 16 
 
 i.o 40 ico 
 
 7.0 
 
 6 
 
 14 
 
 1-25 
 
 35 
 
 80 
 
 8.0 
 
 5 
 
 12.5 
 
 1.50 30 
 
 66 
 
 9.0 
 
 4-50 
 
 II 
 
 1-75 25 
 
 57 
 
 10.0 
 
 4 
 
 10 
 
 2.O 
 
 20 
 
 5o 
 
 II. 
 
 3-75 
 
 9 
 
 2.25 
 
 18 
 
 44 
 
 12.0 
 
 3-50 
 
 8-3 
 
 2.50 16 
 
 40 
 
 
 
 
 2-75 H 
 
 36 
 
 14.0 
 
 2.25 
 
 7-i 
 
 3.0 12 
 
 33 
 
 15-0 
 
 3.00 
 
 6.6 
 
 3-50 II 
 
 28 
 
 16.0 
 
 2.50 
 
 6.2 
 
 4.0 10 
 
 25 
 
 18.0 
 
 2.25 
 
 5-5 
 
 4-50 
 
 9 
 
 22 
 
 20. o 
 
 .2 
 
 5 
 
 PRESCRIPTION FOR LENSES. 
 
 For the purpose of writing prescriptions the following 
 signs and abbreviations are used : 
 
 ABBREVIATIONS USED IN REFRACTION. 
 
 V Vision (seeing power of the eye). 
 
 O. D Oculus dexter (right eye). 
 
 O. S Oculus sinister (left eye). 
 
 O. U. . ... . . Oculi utrique (both eyes). 
 
 S. or Sph Spherical lens. 
 
 C. or Cyl Cylindrical lens. 
 
 D Diopter. 
 
 -J- Plus or Convex. 
 
 - Minus or Concave. 
 
 = Equal to. 
 
 O Combined with. 
 
 The prescription " + 1 D. S. C + .25 D. C. Ax. 90," de- 
 ciphered by the aid of this table, would read: " Plus one 
 Diopter Spherical combined with plus one-quarter Diopter 
 C3 r lindrical at axis 90 degrees " (vertical). 
 
 NEUTRALIZATION OF LENSES. 
 
 To gain an intelligent conception of a lens, three points 
 must be taken into account, viz., its character, its form, and
 
 LENSES. 27 
 
 its strength. According to the first, a lens is either convex 
 or concave ; its form determines whether or not it is spheri- 
 cal, cylindrical or both; while its strength expresses in 
 diopters its refractive power. 
 
 If a convex 4ens be moved from side to side in front of the 
 eye, an object viewed through it will appear to be in motion, 
 its displacement being in the direction opposite the move- 
 ment of the lens. With a concave lens the reverse is true, 
 the object apparently moving with the lens. This truth, 
 known in physics as Parallactic Displacement, affords a 
 ready means of determining the character, kind and strength 
 of a lens. 
 
 When the object viewed through the lens moves equally 
 in all meridians, the lens is spherical. If a meridian is found 
 at which there is no apparent displacement, the lens is cy- 
 lindrical, and the meridian producing no displacement indi- 
 cates the axis of the cylinder. If the object moves in all direc- 
 tions, but more in one direction than in another, the lens 
 examined is a sphere-cylinder, known also as a compound or 
 combination lens. The same simple method, therefore, which 
 served to determine the character of the lens examined will 
 also indicate its kind. 
 
 W r hen a convex and a concave lens of equal strength are 
 placed together, they neutralize each other. Moving them 
 from side to side in front of the eye will produce no ap- 
 parent displacement of an object viewed through them. The 
 strength of a lens, therefore, may be determined by placing 
 over it successively lenses of opposite character and of differ- 
 ent degrees of refractive power until a lens is found which 
 nullifies the apparent motion of the viewed object. Know- 
 ing the focal distance of each neutralizing lens, the strength 
 of the lens in question is readily determined by this method. 
 
 The following are a few hypothetical cases, intended to 
 assist in mastering this method of identifying the kind, 
 character and strength of lenses : 
 
 (a) On moving the lens through different meridians be- 
 fore the e3 T e, objects are apparently displaced in an opposite 
 direction and equally in all meridians. It is, therefore, a con- 
 vex spherical lens.
 
 28 REFRACTION. 
 
 Beginning with the weakest lens in the trial case, and 
 trving lenses of increasing powers, we find the lens in ques- 
 tion is neutralized by placing over it a concave spherical lens 
 of 1.25 diopters strength. The prescription for the lens 
 tested is, therefore, + 1.25 D. S. 
 
 (b) Viewed through this lens, we find the object appar- 
 ently moving with the lens in all meridians except the hori- 
 zontal. We have, therefore, a concave cylinder. 
 
 Neutralizing with convex cylinders of increasing powers 
 and placed horizontally, we find that all apparent displace- 
 ment ceases with a lens of + 1.62 diopters. The examined 
 lens, therefore, is a -1.62 B.C. Ax. 180. 
 
 (c) The next lens causes an apparent movement against 
 the lens, more pronounced in one meridian than in another. 
 It is, therefore, a combination of a convex spherical and a 
 cylindrical lens. 
 
 We now have two values to determine, and for the sake 
 of convenience, we begin with the spherical. It is found that 
 after a concave spherical lens of .50 D. strength is placed 
 over the lens in question, there remains a slight movement, 
 in the opposite direction, of the object viewed ; a concave 
 lens of .75 D. causes a slight displacement in the same direc- 
 tion. The former is, therefore, too weak, the latter too 
 strong, while a .62 D. is found to neutralize the lens exactly, 
 777 one meridian. In the meridian at right angles to this the 
 object still continues to move in the opposite direction. In- 
 stead of one lens, the .62 D., we now add concave cylinders 
 of different strength, with the axis of each coinciding with 
 the non-refracting meridian of the unknown lens, until one 
 is found which does away with any motion of the viewed 
 object. This result is met with a 1 D. cylinder at axis 75. 
 The strength of the lens in question, then, is expressed by 
 the prescription : + .62 D. S. C + 1 D- C. Ax. 75. 
 
 Other Methods. Another method of recognizing a cylin- 
 drical lens is by viewing through it a straight line, e.g., a 
 vertical window sash, steam pipe or the edge of a door. If 
 the lens be rotated in its plane, that portion of the line seen 
 through the lens will be tilted to a greater or less degree, de- 
 pending upon the strength of the lens and the extent of its
 
 LENSES. 
 
 29 
 
 rotation. (Fig. 18.) In one position only will the line appear 
 continuous, viz., in the non-refracting meridian of the lens or 
 its axis. This fact is utilized, not only in determining the 
 kind of lens, but also in estimating its strength, for having 
 found the axis, by superimposing known cylinders of oppo- 
 site character with axes coincident, the lens can be readily 
 neutralized. When so neutralized, it may be revolved with- 
 
 FIG. 18. Showing Break in Continuity of a Line Caused by 
 Tilting a Cylindrical Lens. 
 
 out causing any break in the continuity of the straight edge 
 mentioned. 
 
 Measuring- the Strength 01 a Prism. It is often neces- 
 sary to determine the strength and position of a simple 
 prism, or of a prism incorporated in a given lens. A prism 
 has no focussing power, forms no images and shows no par- 
 allactic displacement. That the given lens is a prism or con-
 
 30 
 
 RK PR ACTION. 
 
 tains a prism may be determined by first fully neutralizing 
 the refractive features of the lens and then viewing through 
 the lens the edge of an open door, or other surface having a 
 straight border. The portion' of the straight edge seen 
 
 FIG. 19. Showing the Effect of a Prismatic Lens. 
 
 through the lens will be displaced in the direction of the 
 apex of the prism. (Fig. 19.) By neutralizing with known 
 prisms, their bases overlying the apex of the unknown lens,
 
 LENSES. 31 
 
 when the straight edge appears continuous both in and out 
 of the lens, the strength of the prism has been determined. 
 Mechanical Devices for Measuring- Lenses. A number of 
 
 FIG. 20. Showing Authors' Axis Finder. 
 
 devices have been marketed for the purpose of facilitating 
 the neutralization of lenses and determining with accuracy 
 the axis of a cylinder. Such a device is illustrated in Fig. 20, 
 
 FIG. 21. Showing a Lens Measure. 
 
 and is known as an Axis Finder. The rear aspect shows a 
 number of chambers adapted to lenses of standard sizes, 
 while the front view shows two springs for the reception of
 
 32 REFRACTION. 
 
 neutralizing lenses. The radiating lines indicate the meridi- 
 ans in degrees of a circle. When the unknown lens has been 
 properly neutralized the non-refracting meridian of the neu- 
 tralizing cylinder will be continuous with one of these radi- 
 ating lines, the degree number indicating the axis of the 
 cylinder tested. 
 
 The strength of a lens may also be determined by means 
 of a lens measure. (Fig. 21. ) The -three pointed rods of the 
 instrument are brought against the surface of the lens to be 
 tested, the middle rod lying against the center of the lens. 
 II either the instrument or the lens be slowly revolved so 
 that the three pointed rods shall successively lie in each and 
 ever} T meridian, the curvature, i. e., the refractivity, of each 
 will be indicated on the dial. The surface .of a spherical lens 
 is curved equally in all meridians and gives but one reading. 
 That of a cylindric lens is not curved in the meridian of its 
 axis, while the meridian at right angles to the axis gives the 
 greatest reading and is the measure of the strength of the 
 C3 r linder. 
 
 The average measure, although a convenient contrivance, 
 is not entirely accurate in its results, especially when testing 
 lenses of high refractivity. 
 
 THE RELATIONSHIP OF LENSES. 
 
 There exists between the different kinds of lenses a well 
 defined relationship which we may here discuss with profit. 
 There is a similarity of refractive effect by which one lens, or 
 a combination of lenses, may sometimes be made to do the 
 work of another entirely different lens or combination. In 
 studying the neutralization of lenses, we found that lens 
 values may be added and subtracted the same as any other 
 values. Not only is this true with regard to lenses of the 
 same character, as, for instance, the addition of a 4 1 D. S. 
 to a - .50 D. S. making a + .50 D. S., or a - 1.50 D. S. to a 
 f 2.50 D. S. making a -f 4 D. S., but also lenses of different 
 character may be added or subtracted in a similar manner. 
 A - .50 D. Cyl. may be added to a + .50 D. S., leaving a 
 f .50 D. Cly. at the opposite axis. We will consider, first : 
 
 The Relationship Between Spheric and Cylindric Lenses.
 
 LENSES. 33 
 
 A cylindric lens of any given strength has just one-half the 
 total refraction of a spherical lens of the same strength 
 or number. This will at once become clear if we consider 
 even- spherical lens as composed of two cylindrical lenses of 
 the same character and number with their axes at right 
 angles to each other. Thus a + 1 D. spherical may be con- 
 sidered as made up of a -h 1 D. cylinder at one axis with 
 a r 1 D. cylinder at right angles to the former. Likewise, 
 if we place over a -r 1 D. spherical lens a 1 D. cylindrical, 
 at the horizontal axis, what have we left? A -f 1 D. cylin- 
 der at the vertical axis ; in other words, we have neutralized 
 one of the two cylinders of which the sphere was composed 
 and we have left the other component cylinder. It follows 
 as a self evident fact, that in adding two cylinders of like 
 characters, but of opposite axes and different powers, to 
 form a sphero-cylindrical lens, that the weaker cylinder 
 determines the strength of the sphere, the remainder of the 
 stronger cylinder retaining its identity. Thus: -f 1 D. C. 
 Ax. 90 3 + .75 D. C. Ax. 180 == + .75 D. S. C - -25 D. C. 
 Ax. 90. 
 
 To become proficient in lens computations it will be well 
 for the student to work out a number of different examples 
 similar to the following: 
 
 Example: + 1.25 D. S. - .50 D. C. Ax. 180 = = + (1.25D. 
 C. Ax. 90) + (1.25 D. C. Ax. 180) - .50 D. C. Ax. 180 = 
 -r 1.25 D. C. Ax. 90 + .75 D. C. Ax. 180. R + .75 D. S. C 
 f .50 D. C. Ax. 90. 
 
 But the relationship between spheric and cylindric lenses 
 is even closer, for the effect of a spherical lens placed ob- 
 liquely in the path of luminous rays is actually identical 
 with that of a cylinder. In fact, Young, who was the dis- 
 coverer of ocular astigmia, believed the cylindric effect 
 (Astigmia) of the refracting media of the eye to be due 
 wholly to the oblique position of the crystalline lens. This 
 cylindric effect increases with the degree of inclination of the 
 lens. 
 
 The Relationship of Cylinders. If a + 2 D. cylinder and a 
 2 D. cylinder are so placed that their axes correspond they 
 will neutralize each other. When, however, they are turned 
 about their centres so that their axes are at right angles to
 
 34 
 
 REFRACTION. 
 
 each other, they vary constantly in their respective refrac- 
 tive powers, as the two cylinders approach right angles. 
 When they lie in opposite meridians they exercise a combined 
 cylindric effect equal to the sum of the power of the two 
 lenses, viz., 4 D. 
 
 The Relationship of Spheres to Prisms. A spherical lens 
 is theoretically composed of a large number of prisms, base 
 in or out, according as the lens is convex or concave. This 
 was mentioned and illustrated when considering spherical 
 lenses (q. v.). 
 
 There exists, however, a practical, as well as a theoretical 
 relationship between spherical and prismatic lenses. 
 
 Between the optical centre of a spherical lens and the 
 periphery, the refractive effect is that of a sphere with the 
 addition of a prism. This phenomenon is most apparent in 
 a lens in which the thickest or thinnest point does not lie 
 equi-distant from the periphery. Such a lens is said to be 
 decentered. A glance at the accompanying Fig. 22 shows 
 
 FIG. 22. Showing Prismatic Effect of Decentering a Lens. 
 
 a section of a decentered lens which is seen to be precisely 
 the same as though a normally centered lens had been split 
 flatwise and a prism had been introduced between the 
 halves. The strength of the added prismatic effect depends, 
 of course, upon the strength of the lens and the extent to 
 which it has been decentered. This relationship has been 
 accurately determined and tabulated and may be found in 
 detail in some of the more pretentious works on Refraction. 
 The Relationship of Cylinders to Prisms. A cylindrical 
 lens, or the cylindrical element of a sphero-cylindrical lens, 
 when decentered in a direction vertical to its axis, shows 
 the prismatic effect of a spherical lens of the same strength.
 
 CHAPTER III. 
 THE NORMAL EYE. 
 
 INTRODUCTION. 
 
 The percentage of eyes approaching more or less closely 
 to a condition which would be counted as normal is rela- 
 tively small (not more than 2 to 10 per cent.), yet it will be 
 necessary to consider the optical properties of such an eye 
 before entering upon a discussion of abnormal conditions. 
 As health and disease are relative terms there must be a 
 standard upon which to base our comparisons and compu- 
 tations. Emmetropia is the ideal state of refraction and the 
 Emmetropic eye is one of perfect morphology and function. 
 In such an eye parallel rays of light fall exactly upon the 
 retina without effort of accommodation. The Emmetropic 
 eye, then, is the end and aim of the refractionist. 
 
 REQUISITES OF VISION. 
 
 In the emmetropic eye the three requisite conditions to 
 perfect vision are met, viz., 1. The pupillary aperture is free 
 from all obstructions and the media are clear and trans- 
 parent; 2. The refracting surfaces are such as to focus 
 parallel rays exactly upon the retina ; 3. The retina is -suffi- 
 ciently sensitive to perceive the image created. 
 
 DIOPTRIC SYSTEM OF THE EYE. 
 
 We shall here consider briefly the individual factors enter- 
 ing into the Dioptric System, or optical mechanism of such 
 an eye. Primarily the dioptric system is composed of the 
 Cornea, Aqueous Humor, Crystalline Lens and Vitreous. 
 Since the two surfaces of the cornea are practical!}' parallel 
 and its index of refraction is the same as that of the aqueous, 
 these two media may be considered as one. The anterior 
 surface of the cornea is the most active of all the refracting 
 surfaces of the dioptric system, because the difference be-
 
 36 REFRACTION. 
 
 tween the refracting indices of air and the aqueous humor 
 is greater than that of any two contiguous media in the in- 
 terior of the eye (see Optical Principles). 
 
 The Crystalline Lens is not homogeneous, but consists of 
 a spherical nucleus having a very high refractive index and 
 of a great number of superimposed layers whose curvature 
 and refractive index decrease from within outward. This 
 structure has the effect of: 1st, Diminishing the spherical 
 aberration and, 2d, Giving the lens a greater refractive 
 power than w r oulcl be possible with a homogeneous body of 
 the same shape (Landolt). The vitreous is a homogeneous 
 medium, the refractive index of which is the same as that of 
 the cornea and aqueous. 
 
 STATIC REFRACTION. 
 
 Acting together the dioptric surfaces effect the Static Re- 
 fraction of the eye, producing distinct retinal images, with 
 the optical mechanism in a state of repose. The pupil maybe 
 slightly contracted, but otherwise muscular activity is sus- 
 pended and no effort is needed for distinct vision. The visual 
 axes are practically parallel and the eye is focussed for its far 
 point or Punctum Remotum (which in the emmetropic eye 
 lies at infinity). 
 
 DYNAMIC REFRACTION. 
 
 While the eye is focussed for distant vision a near object 
 will appear indistinct. To see this near object clearly, the 
 distance between the observer and the object viewed re- 
 maining constant, it is necessary to change the focus. A 
 photographer has to focus his camera in order to obtain 
 a clearly defined image on his plate. To do this he has the 
 choice of four methods of procedure, viz.: 1. To move the 
 object viewed, the lens and plate remaining stationary; 2. 
 To move the lens, the object and plate remaining station- 
 ary; 3. To move the plate, the object and lens remaining 
 stationary ; 4. To increase or decrease the refractive power 
 of the lens. 
 
 In the human eve, inasmuch as the lens and retina are 
 normally immovable, only two of the four means of focuss-
 
 THE NORMAL EYE. 37 
 
 ing are possible. By approaching toward, or receding from 
 the object viewed, and more especially by increasing to a 
 greater or less degree the refractivity of the lens, the diverg- 
 ing rays of objects less than twenty feet distant are brought 
 to a focus on the retina. This power of the eye to adjust 
 itself for near vision is known as the Dynamic Refraction or 
 Accommodation. When exercised to its highest degree the 
 accommodation focusses the eye for its near point, or 
 Punctum Proximum. The difference in the refractivity of the 
 eye during maximum and minimum accommodation is 
 called the range or Amplitude (Bonders) of Accommodation 
 and is, of course, expressed in terms of diopters. 
 
 PHYSIOLOGY OF ACCOMMODATION. 
 
 Two factors enter into the accommodation of the eye, 
 viz.: 1. The normal resiliency of the Lens, and 2, the action 
 of the Ciliary Body. 
 
 We are indebted to Helmholz for our knowledge of the 
 physiology of accommodation and so complete were his re- 
 searches as to leave little for modern investigators to add. 
 The Ciliary Body consists essentially of two sets of muscles, 
 one of which constitutes a sphincter (annular muscle of 
 Miiller), the other having a radial, longitudinal arrange- 
 ment and tying parallel with the sclerotic. These fibres ex- 
 tend back into the choroid and have as their fixed point of 
 attachment the corneo-scleral junction. The suspensory 
 ligament of the lens, or Zonule of Zinn, is attached to the 
 Ciliar\ r Body under tension so that when this ligament is 
 cut the lens, left to its own elasticity, becomes more convex. 
 
 The same phenomenon occurs by the action of the ciliary 
 muscles as follows : When the lens is released from the ten- 
 sion of the Zonule of Zinn by the contraction of the sphincter 
 fibres principally, the lens bulges. Both surfaces increase 
 their convexity, the anterior more than the posterior. 
 (Fig. 23.) 
 
 CONVERGENCE. 
 
 To secure binocular vision for near objects, it is not only 
 necessary that the focus be regulated, but that the eyes
 
 38 
 
 FIG. 23. Showing Change in the Shape of the Lens During Accommoda- 
 tion. (After Landolt. )
 
 THE NORMAL EYE. 39 
 
 should rotate inward, the degree of rotation depending 
 upon the nearness of the object. This phenomenon is known 
 as Convergence. In the exercise of this function of the eye, 
 the fovea centralis, which is the most sensitive portion of 
 the retina, is so directed toward an object that the image is 
 formed upon it. The eye is then said to "fix" the object; 
 the images formed in the two eyes fall upon identical points 
 and fuse so that the brain perceives but one. Should the 
 images fall upon corresponding points above, below, to 
 the right or left of the fovea they should likewise produce 
 but a single impression. If, however, the image should fall 
 upon points bearing a different relation to the fovea, as 
 would be the case if the function of convergence were sus- 
 pended or improperly performed, both images would be per- 
 ceived. In other words, we would have a condition known 
 as Diplopia or double vision. 
 
 For binocular distant vision the axes of the eyes must be 
 parallel. For binocular vision at distances less than twenty 
 feet the Rectus Internus muscle draws each eye inward to a 
 degree inversely proportional to the distances of the object 
 looked at. To measure the convergence intelligently we em- 
 ploy as a unit the angle through which the visual axis of the 
 eye moves to fix on a point one meter distant the meter- 
 angle of convergence (Nagel). If the object viewed be only 
 one-half meter distant the amount of convergence necessary 
 will be two meter-angles, and conversely at two meters dis- 
 tance the convergence would be one-half meter-angle. The 
 same number which expresses the diopters of accommoda- 
 tion necessary for distinct near vision also expresses the 
 amount of convergence in meter-angles. For when the ob- 
 ject viewed is one-half meter distant and the convergence is 
 two meter-angles the amount of accommodation brought 
 into play is two diopters, and again, conversely, at two 
 meters distance the amount of convergence is one-half meter- 
 angle and the accommodation required one-half D. 
 
 The amplitude of convergence is the number of meter- 
 angles of convergence of which the eyes are capable in pass- 
 ing from a condition of complete relaxation far point of 
 convergence to one of maximum convergence or the near 
 point.
 
 40 REFRACTION. 
 
 RELATIONSHIP OF ACCOMMODATION, CONVERGENCE AND PU- 
 PILLARY CONTRACTION IN THE EMMETROPIC EYE. 
 
 From the above facts it will be seen that accommodation 
 and convergence are very closely associated, inasmuch as 
 both increase and decrease together and to the same degree. 
 This relationship, however, is not absolute, for it is possible 
 for the eye to fix upon a given point and with convergence 
 stationary to increase or decrease its accommodation and, 
 conversely, to focus for a given distance and without alter- 
 ing this focus, to increase or decrease the convergence angle. 
 The former is known as the relative amplitude of accommo- 
 dation, the latter as the relative amplitude of convergence. 
 
 Throughout the physiological range of accommodation 
 and convergence in the emmetropic eye their relationship 
 may be said to be absolute, beyond this limit, relative. For, 
 when the eye is adjusted for its Punctum Remotum, the ac- 
 commodation may be relaxed still more, and when the ac- 
 commodation is at its maximum, the amount of convergence 
 associated is not only relatively greater, but is capable of a 
 considerable increase. Advancing age, as will be shown 
 hereafter, diminishes the power of accommodation to a point 
 of total suspension, yet has no effect upon the function 
 of convergence. 
 
 The Iris plays an unimportant role in connection with the 
 accommodation, in fact, this membrane may be entirely 
 wanting without greatly disturbing the power of accommo- 
 dation. Normally, however, the pupil contracts during ac- 
 commodation, and dilates when the latter is relaxed. The 
 nerves supplying the ciliary muscle belong to the Motor 
 Oculi, which contain also the fibres distributed to the 
 sphincter of the pupil. Their origin seems to be in the floor 
 of the fourth ventricle. Irritation of the anterior .portion of 
 the floor produce s accommodation ; irritation farther back 
 causes contraction of the pupil and, when that part, where 
 the fourth ventricle passes into the Aquasductus Sylvii, is ir- 
 ritated, contraction of the Rectus Internus results. Gencrallv 
 speaking, pupillary contraction is more closely associated
 
 THE NORMAL EYE. 
 
 41 
 
 with convergence than with accommodation, for it takes 
 place with convergence in the absence of accommodation. 
 The following table will serve as a summary of these facts : 
 
 TABLE II. 
 
 SHOWING THE RELATIONSHIP OF ACCOMMODATION, CONVERGENCE AND 
 PUPILLARY CONTRACTION. 
 
 The Eyes. 
 
 Accommodation. 
 
 Convergence. 
 
 Pupillary Action. 
 
 When adapted Relaxed, but has Completely relax- 
 to the Punc- some reserve ca- ed. Visual axes 
 turn Remotum pacity. parallel. 
 
 When adapted Maximum power E x e re i se d to a 
 to Punctum exerted. greater degre e 
 
 Proximum. than is accommo- 
 
 dation and can 
 be increased. 
 
 Pupil dilated. 
 
 Pupil contracted. 
 
 Under Mydria- Relaxed, 
 tics. 
 
 Under Myotics. Increased. 
 
 Activity decreased. Pupil dilated. 
 
 Possible action in- 
 creased. 
 
 Advancing Age. Decreased to a Not affected, 
 point of total sus- | 
 pension. 
 
 Pupil contracted. 
 
 Pupil contracted. 
 
 OPTICAL DEFECTS OF THE NORMAL EYE. 
 
 We have spoken of the Emmetropic eye as one of perfect 
 morphology and function, yet this is a theoretical concep- 
 tion. Such an eye does not exist and, from a practical stand- 
 point, instead of associating the idea of " perfection " with 
 the term Emmetropia, we must be content to accept the Em- 
 metropic eye as the "normal" eye, the standard by which 
 we are to judge anomalies. This normal eye, although the 
 most wonderful of all the organs of the body, the most intri- 
 cate and complex in its construction, marvelous in its detail 
 and delicate in its adjustment, nevertheless shows many 
 glaring defects. "Many of these," says Bid well, "are the 
 more striking because they are so obvious. The external sur- 
 face of the lens formed by the aqueous humour and the 
 4
 
 42 REFRACTION. 
 
 cornea is not a surface of revolution, such as would be fash- 
 ioned by a turning lathe or a lens grinding machine; its 
 curvature is greater in a vertical than in a horizontal direc- 
 tion, and the distinctness of the focussed image is conse- 
 quently impaired. Again, the crystalline lens is constructed 
 of a number of separate portions which are imperfectly 
 joined together. Striae occur along the junctions, and the 
 light which traverses them instead of being uniformly re- 
 fracted is scattered irregularly. Moreover, the system of 
 lenses is not centered upon a common axis; neither is it 
 achromatic, while the means employed for correcting spheri- 
 cal aberration are inadequate." 
 
 These defects, being inherent in the design or structure of 
 the eye itself, produce anomalies which we may classify as 
 physical in comparison with those of psychic origin. The 
 latter result from the erroneous interpretations placed by 
 the mind upon the phenomena presented to it through the 
 medium of the optic nerve and the brain. Under this head 
 may be classified the many strange manifestations known 
 as "Optical Illusions." 
 
 In spite of the many defects set forth, however, our eyes 
 do us excellent service because with incessant practice we 
 have acquired a very high degree of skill in their use. Bid- 
 well sa3 7 s, tersely: "If anything is more remarkable than 
 the ease and certainty with which we have learned to in- 
 terpret ocular indications when they are in some sort of 
 conformity with external objects, it is the pertinacity with 
 which we refuse to be misled when our eyes are doing their 
 best to deceive us." 
 
 CHANGES OCCURRING DURING THE LIFE HISTORY OF THE EYE. 
 
 In addition to the ocular defects enumerated there must 
 be mentioned certain changes occurring in the eye at differ- 
 ent periods of its life history, changes which are prejudicial 
 to good vision. By reason of the regularity and uniformity 
 with which these changes take place, they are looked upon 
 as physiological, and are necessarily incident to the two 
 periods of life, viz., that of growth and development and 
 that of decline (senility). When we consider that the eye
 
 THE NORMAL EYE. 43 
 
 continues to develop for many years and that throughout 
 this growth a very accurate and definite relationship must 
 be maintained between its component parts, we can readily 
 appreciate the wide range of possible refractive errors to 
 which the dioptric system is subjected. And when we take 
 cognizance of the many and varied uses and abuses to which 
 the eyes are subjected during the life of the individual, the 
 frequency of refractive errors ceases to be an object of 
 wonder. 
 
 Nearly all eyes are far-sighted at birth. This is owing to 
 the fact that the eye-ball has not reached its full size and is, 
 therefore, too short to bring parallel rays to a focus on the 
 retina. In this condition, known as Axial Hyperopia, or 
 far-sightedness, parallel rays focus too far behind the retina. 
 Not only is this condition found to exist in the new-born, 
 but it continues for a longer or shorter period and is still 
 the rule at ten years, although the power of accommoda- 
 tion, which has a wider range of activity in youth than at 
 any other time of life, usually serves to overcome the anom- 
 aly in whole or in part. 
 
 The pupil, which is very much contracted in new-born in- 
 fants, soon becomes more dilated, smaller again in adult life 
 and still more contracted in old age. Its activity is greatly 
 lessened in the decline of life, owing to the unyielding char- 
 acter of the tissues of the iris, especially that of the Sphincter 
 Pupillae. 
 
 The accommodation of the eye shows a steady decline 
 throughout life. The lens apparently partakes of the general 
 tendency of the tissues of the body to become more rigid 
 and unyielding as the age of the individual increases and 
 hence the degree of convexity caused by the action of the 
 ciliary muscle is proportionately decreased. While this is 
 undoubtedly the prime factor in causing the functional de- 
 cline of the accommodative mechanism, the laxity of fibre 
 and muscular atrophy incident to old age must also be con- 
 sidered as pla} r ing an important role. 
 
 This gradual failure of the focussing power of the eye, 
 known as Presbyopia, bears a very definite relationship to 
 the age of the individual. Table 4, page 86.
 
 44 
 
 REFRACTION. 
 
 VISUAL ACUITY. 
 
 In order that an object may be distinctly seen by the 
 normal eye, it must be of such size as to subtend an 
 angle of at least one minute, the eye being at the point 
 of the angle. This fact is utilized in the construction of 
 the Snellen test types, the limbs of which subtend an 
 angle of one minute and the whole letter an angle of five 
 minutes. (Fig. 24.) The size of the letter or object included 
 
 FIG. 24. Showing Construction of Snellen's Test Letters. 
 
 in an angle of five minutes is, of course, constant for a given 
 distance, increasing as it recedes from and decreasing as it 
 approaches nearer the eye. (Fig. 25.) A Snellen test card 
 
 FIG. 25. Showing Principle Involved in Snellen's Letters. 
 
 contains a number of letters of varying sizes arranged in 
 rows, each row bearing a number expressing the distance in 
 feet or meters at which its letters subtend an angle of five
 
 THE NORMAL EYE. 
 
 45 
 
 minutes, hence the distance at which they should be read by 
 the normal eye. (Fig. 26.) 
 
 Sea ting a patient at a distance of twenty feet from the test 
 card, (Fig. 27) (this distance being chosen because at this 
 range rays of light are practically parallel), and testing one 
 eye at a time, the lowest line the patient is able to read cor- 
 rectly is the measure of his visual acuity. This is recorded 
 in terms of a fraction, the numerator of which expresses the 
 distance of the patient from the test card and the denomi- 
 nator the number above the lowest line read. Hence V !&, 
 
 ^^ 
 
 C 
 
 R B 
 T r P 
 
 5 C C O 
 
 4 K B E R 
 3 v y r P T 
 
 FIG. 26. Showing Test Card. 
 
 would signify that at 20 feet the patient reads the line which 
 he should be able to read at 40 feet. Visual acuity of f$ should 
 be looked upon as the minimum of the normal adult eye, most 
 eyes of young people having an acuity of f f and even f$. 
 When the acuity falls below f, except in aged individuals, 
 the eye is no longer to be considered normal. 
 
 In testing the acuity of vision the best possible illumina- 
 tion is necessary. When the test is made on cloudy days due 
 allowance must be made for the apparent decrease in vision.
 
 46 
 
 REFRACTION'. 
 
 Tests may be conducted in a subdued light by means of 
 porcelain test cards bearing transparent letters which are 
 illuminated from the rear. 
 
 FIG. 27. Showing Changeable Test Card.
 
 CHAPTER IV. 
 ASTHENOPIA. 
 
 Under the term, Asthenopia, may be grouped the many 
 and varied subjective symptoms complained of by patients 
 suffering from some error of refraction. The word signifies 
 "painful vision;" the underlying cause of the condition 
 being exhaustion of: 
 
 (a) The Ciliary Muscle, producing what may be called 
 Accommodative Asthenopia, or 
 
 (b) The Internal Recti Muscles, producing Muscular or 
 Convergence Asthenopia, or 
 
 (c) A combination of both (a) and (b). 
 
 In the first form, the Ciliary Muscle is overtaxed by its 
 efforts to overcome some refractive error. In the second, the 
 Internal Recti Muscles are unequal to the task imposed on 
 them. This failure of perfect function may be due to con- 
 genital weakness of the muscles ; or, too wide separation of 
 the eyes necessitates an excessive amount of convergence in 
 order to bring each eye into perfect alignment. In the third 
 form there is a combination of both these conditions and 
 consequently a doubled cause for painful vision. 
 
 SYMPTOMS. 
 
 The more common symptoms complained of are head- 
 ache, pain at the back of the eyes and around the eyes, any 
 of the symptoms usually termed ''neuralgia" of the head or 
 face, conjunctival irritation, photophobia, lachrymation, 
 spasm of the lids, blepharitis, apparent movement of objects 
 steadily gazed at, blurring of vision, running together of the 
 type, feeling of dryness of the lids, sensation of sand in the 
 eyes requiring the patient to rub them for temporary relief, 
 dizziness, and even pronounced functional nervous disturb- 
 ances. While they vary greatly with the patient's general 
 health, all of the symptoms are usually worse after a day's
 
 48 REFRACTION. 
 
 work, or on first waking in the morning. Sometimes a 
 marked error of refraction becomes manifest only as a sequel 
 to some exhausting illness. 
 
 The symptoms induced by hyperopia usually follow ex- 
 cessive use of the eyes for close -work, such as reading, writ- 
 ing, drawing, sewing, etc., while those of myopia are more 
 commonly induced when the patient uses his eyes excessively 
 for distant vision. The strain caused by astigmia is always 
 present, although it usually becomes more manifest after 
 the excessive convergence and accommodation incident to 
 near work. 
 
 ASTHENOPIA AS A CAUSE OF GENERAL DISEASE. 
 
 Much has been written on the subject of asthenopia as a 
 cause of general disease. The argument is based on the fact 
 that in eye strain the constant expenditure of nerve energy, 
 due to the patient's efforts to see distinctly, results in nerv- 
 ous exhaustion. It certainly seems reasonable that a pro- 
 nounced error of refraction when uncorrected, especially in 
 patients who make more than ordinary demands upon their 
 eyes, like any other source of continued irritation, may so 
 far deplete the economy as to render it liable to the invasion 
 of disease which, under normal conditions, would be success- 
 fully combated by the protective forces of the body. Every 
 physician -will testify to pronounced cases of general dis- 
 turbance which resisted all forms of treatment until the 
 error of refraction or the muscular imbalance of the eyes 
 had been corrected.
 
 CHAPTER V. 
 THE TRIAL CASE AND ITS USES. 
 
 INTRODUCTION. 
 
 The trial or test-case consists of an assortment of convex 
 and concave spherical and cylindrical lenses. (Fig. 28.) 
 
 Spherical Lenses. The spherical lenses are arranged in 
 two double rows, convex on the right side of the case, con- 
 cave on the left. Between these rows is a double column of 
 
 FIG. 28. Showing Trial Case. 
 
 figures, one denoting the strength of the lenses opposite in 
 terms of Diopters, the other specifying the focal distance in 
 inches. In a full case these lenses range from .12 D. to 2O D., 
 the weakest lenses occupying the front of the case. Each 
 lens has stamped upon its handle, or cut into the glass, the 
 figure expressing its strength in diopters. 
 
 Cylindrical Lenses. The cylindrical lenses are similarly 
 arranged in double rows in the center of the case, convex on
 
 50 REFRACTION. 
 
 the right, concave on the left, and are numbered in like man- 
 ner. They are usually fewer in number than the spherical 
 lenses. In the edge of the lens a little cut into the glass in- 
 dicates the axis of the cylinder. Each one is so marked, or 
 the lens has a segment at each side rendered opaque by 
 grinding the glass, the edge of the opacity corresponding to 
 the axis of the cylinder. (Fig. 29.) 
 
 Prismatic Lenses. Prismatic lenses usually occupy a sep- 
 arate compartment at the back of the case. They are num- 
 bered from 1 to 20 to express in degrees the angle formed by 
 their two sloping surfaces. 
 
 FIG. 29. Showing Cylindrical Test L,ens. 
 
 There are in addition a number of accessories, as follows : 
 
 (a) An obturator, an opaque disc made of metal or hard 
 rubber, used to cover one eye when examining the other. 
 
 (b) Ground glass disc used for the same purpose. 
 
 (c) Pin-hole disc, a metal or hard rubber disc having in 
 its center a minute aperture. (Fig. 30.) It has the effect, 
 when placed before the eye, of cutting off the peripheral 
 rays, thereby reducing the size but sharpening the outline of 
 the image produced on the retina. When vision can be im- 
 proved in this manner it can also be improved -by lenses.
 
 THE TRIAL CASE AND ITS USES. 
 
 51 
 
 (d) Stenopseic disc. This is an opaque disc in the center 
 of which is cut a narrow slit. (Fig. 31.) It is used in testing 
 for astigmia (q. v.). 
 
 FIG. 31. Showing Stenop- 
 
 FiG. 30. Showing Pin Hole 
 Disc. 
 
 (e) Maddox rod. This is an opaque disc with one or 
 more parallel stenopseic slits, in front of each of which and 
 
 FlG. 32. Showing Maddox 
 Multiple Rod. 
 
 FlG. 33. Showing Double 
 Prism. 
 
 parallel to it is a glass rod or cylinder. (Fig. 32.) A flame, 
 when looked at through the maddox rod, appears as a
 
 52 
 
 REFRACTION. 
 
 luminous streak passing through the center of the cylinders 
 and at right angles to them. Its use will be explained in dis- 
 cussing the subject of muscular imbalance. 
 
 (f) Double prism. This is a disc, at the centre of which 
 are two prisms, base to base. (Fig. 33.) Objects looked at 
 through these prisms will appear double. Its use will be ex- 
 plained in the study of muscular imbalance. 
 
 The Trial Frame. The trial frame is a contrivance, re- 
 sembling n pair of crude spectacles, Fig. 34, used in conduct- 
 ing a test of vision. It has a lens chamber for each eye, each 
 chamber being capable of receiving two or three lenses. 
 These mav be revolved so that the cvlindrical lenses mav be 
 
 FIG. 34. Showing Trial Frame. 
 
 placed at any axis from to 180 degrees. The angle of the 
 cylindrical axis is indicated in degrees of the circle marked on 
 a celluloid arc above the lens chamber. The trial frame is so 
 arranged that it can be adjusted to suit the patient's nose, 
 pupillary distance, length of head and width of face. 
 (Fig. 35.) In addition to the features common to all, the 
 authors' trial frame, Fig. 36, is fitted with a temple adjust- 
 ment by which the frame and the included lenses may -be 
 tilted to any desired angle. In making the test of vision 
 and in prescribing spectacle frames, especially for near work, 
 this is frequently of great advantage.
 
 THE TRIAL CASE AND ITS USES. 53 
 
 FIG. 35. Showing Trial Frame Properly Adjusted for Conducting a Test.
 
 54 
 
 REFRACTION. 
 
 FIG. 36. Showing Authors' Trial Frame Adjusted for Reading.
 
 CHAPTER VI. 
 HYPEROPIA OR FAR SIGHT. 
 
 INTRODUCTION. 
 
 The Emmetropic eye has been mentioned as one which, 
 when at rest, /. e., with accommodation relaxed, focusses 
 parallel rays in a sharp image upon the retina. This is true, 
 however, only of central rays which unite in, or very near 
 to the fovea centralis or macula lutea, that point on the 
 retina affording most distinct vision. Peripheral parallel 
 
 FIG. 37. Showing the Focus of Parallel Rays Behind the Retina of a 
 Hyperopic Eye. 
 
 rays do not converge to a sharp image upon the retina, even 
 in an emmetropic eye. But when, in the absence of accom- 
 modation, all parallel rays fail to focus on the retina the 
 condition is known as Ametropia. 
 
 The principal forms of Ametropia are : Hyperopia, My- 
 opia and Astigmia. 
 
 Hyperopia is also known as Hypermetropia or Farsight- 
 edness, expressed by the letter H. In this form of Ametropia
 
 56 
 
 REFRACTION. 
 
 the refractive power of the eye is insufficient. Parallel rays 
 focus behind the retina and only by the aid of the accommo- 
 
 FIG. 38. Showing a Hyperopic Eye Corrected for Distant Vision. 
 
 dation can they be made to focus on the retina. (Fig. 37.) 
 The divergent rays of close objects focus at a still greater 
 distance back of the retina. In this form of Ametropia a 
 
 FIG. 39.Showing a Hyperopic Eye Corrected for Near Vision. 
 
 convex lens is required to bring the focus forward to the 
 retina. (Fig. 38 and Fig. 39.)
 
 HYPEROPIA OR FAR SIGHT. 57 
 
 Hyperopia may be due to : 
 
 (a) Deficient curvature of the refracting media, called 
 Curvature Hyperopia., or 
 
 (b) Abnormal shortness of the eyeball, called Axial Hy- 
 peropia, or 
 
 (c) A combination of both (a) and (b). 
 
 CAUSES. 
 
 Hyperopia is usually congenital. By the growth and de- 
 velopment of the eyeball, it may be changed to a condition 
 of emmetropia, or more rarely, it may even pass into my- 
 opia. While it may remain as a latent error, it becomes 
 manifest in later life. 
 
 The extraction of the lens for cataract, leaving the eye in 
 the condition known as Aphakia, produces one form of 
 hyperopia. 
 
 SYMPTOMS AND SEQUELAE. 
 
 The patient finds it difficult to maintain a distinct image 
 of small objects, and consequently complains of blurring 
 vision. He is forced to stop reading and rub his eyes ; after 
 this his sight seems better for a moment, but very soon the 
 haziness of vision recurs. The accommodation finally be- 
 comes exhausted and work must be discontinued. Children 
 frequently hold their books near to the face and partially 
 close the eyes to cut off the divergent rays. Too often this 
 leads to an erroneous diagnosis and the prescription of con- 
 cave glasses. Accommodation and convergence are such 
 closely related acts, nervously, that the excessive accommo- 
 dation, used in high degrees of hyperopia, and its associated 
 convergence may result in permanent displacement of the 
 eye, a condition known as Strabismus. 
 
 SPASM OF ACCOMMODATION. 
 
 This annoying complication is due to the persistent con- 
 traction of the ciliary muscle by which the patient actually 
 makes himself myopic. Distant vision is rendered indistinct 
 and the patient suffers much pain, discomfort, and the loss 
 of muscular and nervous energy. Spasm is apt to occur in 
 individuals of a neurasthenic type. It is attended by con- 
 5
 
 58 REFRACTION. 
 
 junctivitis, blepharitis and congestion of the retina and 
 choroid. Other symptoms are headache, aggravated by 
 using the eyes, functional nervous disturbances and, some- 
 times, convergent squint. While concave glasses will im- 
 prove the vision, the error of such a prescription is readily 
 seen. 
 
 VARIETIES OF HYPEROPIA. 
 
 According to Bonders, Hyperopia is Facultative when it 
 can be overcome by accommodation without squinting; 
 Relative when it can be overcome by accommodation only 
 when the subject squints; and Absolute when it cannot be 
 overcome by any efforts of the accommodation. 
 
 Manifest hyperopia, indicated by the letters H. M., is that 
 amount of hyperopia, which becomes apparent without 
 paralyzing the accommodation by means of a mydriatic. It 
 is measured by the strongest convex lens through which the 
 eye retains distinct vision. 
 
 Latent hyperopia, indicated by the symbols H. L.,is that 
 form which is discovered only after the accommodation has 
 been arrested by a cycloplegic. This, when added to the 
 manifest, is the total hyperopia, Hyperopia Totalis, indicated 
 by the letters H. T. 
 
 DETERMINATION OF HYPEROPIA. 
 
 Hyperopia always exists : 
 
 1. When distant vision is not reduced by a convex lens. 
 
 2. When the patient can read fine print through a convex 
 glass at a greater distance than its focal length. 
 
 3. And usually when the near point lies at a greater dis- 
 tance from the eye than is proper for the age of the patient. 
 
 CORRECTION OF HYPEROPIA. 
 
 With the patient seated six meters (twenty feet) from the 
 Snellen test card and wearing a properly adjusted test frame, 
 place before the uncovered eye convex glasses, passing 
 gradually from weaker to stronger ones until the best vision 
 has been obtained. The strongest convex lens giving this 
 result is the measure of the manifest hyperopia (H. M.).
 
 HYPEROPIA OR FAR SIGHT. 59 
 
 It will be found frequently that a hyperopic person, be- 
 cause his accommodation overcomes the error, has perfect 
 vision with the unaided eye. In such a case, of course, it is 
 impossible to improve the vision by means of convex lenses, 
 indeed, concave lenses may appear to afford some improve- 
 ment. Such an eye does its work satisfactorily as to vision, 
 but at too great a cost in energy ; glasses will enable it to 
 perform its function with a much smaller expenditure of mus- 
 cular effort. In such a case the manifest hyperopia is meas- 
 ured by the strongest convex lens with which the patient can 
 see as well as he does with the unaided eye. The very fact 
 that he sees as well at a distance with a convex glass as 
 without it, proves that the eye is hyperopic ; the emmetropic 
 eye and, to a greater degree, the myopic eye sees less dis- 
 tinctly through even a weak convex lens. 
 
 In testing the eye for distant vision by means of the Snel- 
 len types at a range of six meters, it is essential that the ac- 
 commodation be relaxed. It is desirable throughout the 
 test to disturb the natural conditions as little as possible. 
 While it is the practice in most public clinics to employ in 
 every case a mydriatic, as a matter of routine and to 
 economize time, such procedure in private practice is neces- 
 sary only in exceptional cases. The refractionist who is 
 skilled in the use of the test case and the confirmatory tests, 
 and who has tact and patience, may learn to coax out the 
 most obstinate accommodation and accurately measure the 
 degree of eye strain without the routine use of a mydriatic. 
 The more skillful the refractionist, the less will he depend 
 upon the drug. 
 
 In order to relax the ciliary muscle, there are some meas- 
 ures which will be found of value. The following may be 
 employed : 
 
 1. Place before one eye, its fellow being covered, convex 
 lenses of increasing strength until the image is blurred, then 
 gradually weaken to a point of distinct vision. In other 
 words, overcorrect the error and then return to a lens which 
 gives clear and distinct vision. 
 
 2. Place before each eye a strong convex lens, for instance, 
 a plus 4 D. S., allowing the patient to wear them ten or
 
 60 REFRACTION. 
 
 fifteen minutes, then gradually neutralize by superimposing 
 concave lenses of increasing strength to the point of best 
 vision. . 
 
 3. Sometimes a two-degree prism, base in, will serve to 
 relax the Internal Rectus muscle and the accommodation. 
 
 In every case, the lenses should be increased in strength 
 very gradually, the accommodation relaxing by this method. 
 Great deliberation is of positive value. It is absolutely im- 
 portant to quiet the nervousness of an excitable patient; 
 in every case, to win the confidence of the patient makes the 
 task much easier. 
 
 In myopes and in the majority of hyperopic and astigmic 
 patients, the above measures will be sufficient. If, however, 
 the statements of the patient are hesitating, varying and 
 inconsistent, no definite conclusions can be reached by the 
 examiner; in such a case a cycloplegic must be employed. 
 
 CYCLOPLEGICS AND MYDRIATICS. 
 
 In view of the fact that paralysis of accommodation is 
 usually accompanied by a simultaneous dilatation of the 
 pupil, the terms cycloplegic and mydriatic are ordinarily used 
 synonymously. The majority ofcycloplegics are also mydriat- 
 ics. True mydriatics are those which dilate the pupil with- 
 out affecting the accommodation, e.g., Euphthalmin, Ephed- 
 rin, Mydrin and Cocain. The most commonly used cyclo- 
 plegics are Atropin Sulphate, Homatropin Hydrobromate, 
 Scopolamin and Duboisin. 
 
 In choosing between cycloplegics, the chief factor to be 
 considered is the age of the patient. Atropin (in 1 per cent, 
 solution usually), being the strongest, is best suited to the 
 more active accommodation of children and young adults; 
 Homatropin (in 1 per cent, solution), alone, or in combina- 
 tion with Cocain (2 per cent, solution), being employed to 
 combat the less active accommodation of patients from 
 twenty-five to thirty years of age. After the age of thirty- 
 five, spasm of accommodation is very rare and hence the 
 use of a cycloplegic is rarely required. 
 
 Contraindications for the Use of Mydriatics. In any case 
 of increased tension of the eyeball at any age, the use of
 
 HYPEROPIA OR FAR SIGHT. 
 
 61 
 
 a mydriatic is interdicted. This includes suspected cases of 
 glaucoma as well as those in which the disease is actually 
 present. 
 
 TABLE III. 
 
 SHOWING STRENGTH OF CYCI/)PI,EGICS AND MYDRIATICS, TIME REQUIRED 
 TO ACT AND DURATION OF DRUG EFFECT. 
 
 Drug. 
 
 Strength Employed. 
 
 Time Required t o 
 Reach Maximum Ef- 
 fect. 
 
 Duration of Drug 
 Action. 
 
 Atropin Sul- 
 phate. 
 
 I % solution. 
 
 48 hours. 
 
 7 to 10 days. 
 
 H omat ropi n 
 Hydrobromate 
 (several instal- 
 lations at in- 
 
 I % solution. 
 
 2 hours. 
 
 I to 2 days. 
 
 tervals of ten 
 
 
 
 
 minutes). 
 
 
 
 
 Scopolamin Hy- 
 drobromate. 
 
 i-io % solution. 
 
 i hour. 
 
 4 days. 
 
 Duboisin Sul- 
 phate. 
 
 % % solution. 
 
 Yz hour. 
 
 4 days. 
 
 Euphthalmin 
 Hydro chlor- 
 
 5-10 % solution. 
 
 20 minutes. 
 
 5 hours. 
 
 ate. 
 
 
 
 
 Ephedrin H y - 
 
 10 % solution. 
 
 40 to 60 minutes. 
 
 5 to 20 hours. 
 
 drochlorate. 
 
 Cocain Hydro- 
 chlorate. 
 
 2 % solution. 
 
 I hour. 
 
 3 to 4 hours. 
 
 Correction of Results Obtained Under a Cycloplegic. 
 That lens or combination of lenses which gives the patient 
 the best distant vision while under the influence of a cyclo- 
 plegic expresses the total error of refraction, i. e., the mani- 
 fest plus the latent error. The latter was brought out by 
 the cycloplegic and represents thetonicity of the ciliary mus- 
 cle. To prescribe the full correction, i. e., the strength of 
 lens giving most perfect vision under mydriasis, would mani- 
 festly be a grave error, because :
 
 62 REFRACTION. 
 
 1. The ciliary muscle, which in hyperopic eyes is more or 
 less hypertrophied, without drug effect is unable to fully re- 
 lax ; therefore, the accommodation is never fully relaxed in 
 the hyperopic eye. 
 
 2. If the ciliary muscle could be fully relaxed, to keep it so 
 by prescribing glasses to perform the functions normally its 
 province, would not only cause the patient great discomfort, 
 but would result sooner or later in muscular atrophy from 
 disuse. 
 
 For this reason, it is necessary to deduct from the meas- 
 urement under atropin or other cycloplegic, from 1 D. to 1.50 
 D. in the case of children and young adults, and from .75 D. 
 to 1 D. in older individuals. For example, if the total error 
 of refraction, as revealed by the cycloplegic, is + 3 D. S., the 
 prescription for glasses would be from + 1.50 D. S. to + 2 
 D. S. in a child, and from + 2 D. S. to + 2.25 D. S. for an 
 adult. 
 
 MYOTICS. 
 
 Myotics are drugs which produce a contraction of the 
 pupil and may induce spasm of accommodation. Those 
 generally emplo3*ed in medical practice are Eserin (in 1 per 
 cent, solution) and Pilocarpin (in Vi and y% per cent, solu- 
 tion). They are used to counteract the effect of mydriatics 
 and to draw the iris away from the iris angle in glaucoma 
 and in peripheral lesions of the cornea. 
 
 After using a cycloplegic like homatropin, its annoying 
 effect may be temporarily overcome by the use of a myotic. 
 Frequent installations of eserin, for instance, will counteract 
 the action of the cycloplegic.
 
 CHAPTER VII. 
 MYOPIA OR NEAR SIGHT. 
 
 INTRODUCTION. 
 
 As the correction of hyperopia necessitates an increase of 
 the refractive power of the eye by means of convex glasses, 
 so that of Myopia demands a decrease of the refractivity, ac- 
 complished by the use of concave lenses. In Myopia the re- 
 fractivity is too great for the length of the eyeball, so that 
 parallel rays focus in front of the retina. (Fig. 4-0.) In this 
 
 FIG. 40. Showing the Focus of Parallel Rays in Front of the Retina of a 
 
 Myopic Eye. 
 
 condition the only rays falling on the retina emanate from 
 objects nearer the eye than infinity. The more the rays di- 
 verge, i. e., the nearer the object viewed is to the eye, the 
 more distinct will be the vision. Hence, in myopia of mild 
 degree, the patient requires correcting lenses for his distant
 
 64 
 
 REFRACTION. 
 
 vision only, no glasses being needed for close work. (Fig. 41.) 
 In nvyopia of high degree, the glasses prescribed for distant 
 vision are too strong for reading. A second pair of glasses 
 of much weaker power are, therefore, required for close 
 work. 
 
 Myopia may be due to : 
 
 (a) Increased length of the eyeball, known as Axial 
 Myopia, or 
 
 (b) Increased curvature of the refracting media, known 
 as Curvature Myopia, or 
 
 FIG. 41. Showing Divergent Rays of a Near Object Focussed 
 on the Retina of a Myopic Eye. 
 
 (c) A combination of both these conditions. 
 
 The great majority of cases of Myopia are probably 
 axial, those cases due to increased curvature of the refract- 
 ing media being much less frequent. Curvature Myopia may 
 be of two kinds: 
 
 (a) That affecting the cornea, and 
 
 (b) That affecting the lens. 
 
 As an example of the former may be mentioned the condi- 
 tion known as Keratoconus or Bulging Cornea, while the 
 latter is well illustrated by the swelling of the lens which
 
 MYOPIA OR NEAR SIGHT. 65 
 
 sometimes takes place in incipient cataract. As a result of 
 the increased refractivity caused b}^ this swelling, the patient 
 is able to read without the aid of glasses, a condition 
 known to the laity as " second sight." 
 
 CAUSES OF MYOPIA. 
 
 % 
 
 Myopia is very largely the result of the unreasonable de- 
 mands which civilization and higher education make upon 
 the eyes. The excessive and improper use of the eyes 
 for near work during the period of development seems 
 to be the chief predisposing cause. Heredity is also an 
 important causative factor. The exciting cause is usually 
 found in improper hygienic conditions, e.g., poor ventilation 
 and insufficient illumination of school rooms, faulty position 
 of the head and body during study and sedentary habits. 
 
 The lengthening of the eyeball found in axial myopia may 
 be brought about in two ways: 
 
 (1) Individuals, whose eyes are far apart, are compelled 
 to exercise an excessive amount of convergence when fixing 
 upon near objects. By this excessive convergence the extra- 
 ocular muscles exert undue pressure on the eyeballs, causing 
 a bulging of the less resistent posterior pole, thereby pro- 
 ducing the condition known as Posterior Staphyloma. 
 
 (2) As a result of chronic inflammatory processes and 
 the destructive changes caused thereby, the coats of the eye- 
 ball become softened and less resistent and finally yield to 
 intra-ocular and extra-ocular (muscular) pressure. This 
 form of myopia is usually progressive, often attaining a 
 high degree and sometimes resulting in total blindness. 
 
 SYMPTOMS OF MYOPIA. 
 
 The patient's distant vision is poor. The eyes are sensitive 
 to light and tire easily, becoming painful after near work. 
 Patients frequently complain of black spots before the eyes 
 and occasionally of flashes of light. 
 
 In myopia of high degree the eyeballs are often prominent 
 and staring, giving the face a rather stupid expression. The 
 patient, when reading, moves his whole head instead of his 
 eyes alone, thus following the lines back and forth across
 
 66 
 
 REFRACTION. 
 
 the page. The ophthalmoscope shows the optic disc to ap- 
 pear enlarged. There may be divergent squint, the internal 
 recti being unequal to the task of maintaining the ex- 
 cessive amount of convergence necessary. 
 
 DETERMINATION OF MYOPIA. 
 
 
 
 Myopia always exists when, in the absence of spasm of 
 the ciliary muscle : 
 
 1st. Distant vision is improved by concave lenses and 
 blurred by cqnvex lenses ; 
 
 2d. The far point (punctum remotum) lies nearer the eye 
 than infinity. 
 
 FIG. 42. Showing a Myopic Eye Corrected for Distant Vision. 
 CORRECTION OF MYOPIA. 
 
 With the patient seated twentv- feet from the test card, 
 place before the eye, testing each eye separately, concave 
 lenses of different strength until the most distinct vision is 
 obtained. The weakest lens giving this result is the measure 
 of the mvopia and should be prescribed for distant use. 
 (Fig. 42.)" 
 
 Should the error be one of high degree, e. g., from 5 D. 
 to 8 D., or the accommodation feeble, a weaker lens af-
 
 MYOPIA OR NEAR SIGHT. 67 
 
 fording good vision at 14 to 16 inches should be prescribed 
 for reading. This is a point deserving special emphasis, be- 
 cause it is too frequent-lv- disregarded. 
 
 PROPHYLAXIS. 
 
 A myope should be instructed in a proper hygienic 
 regime, e. g., the use only of books having large distinct 
 type, the best possible light, proper ventilation of the study 
 or school room, plenty of out-door exercise and the limita- 
 tion of close work to the minimum. He should be instructed 
 to wear his glasses only for distant vision, or if two correc- 
 tions are prescribed, the weaker one for reading should be 
 the invariable practice.
 
 CHAPTER VIII. 
 ASTIGMIA. 
 
 INTRODUCTION. 
 
 A beam of light on passing through a convex cylindrical 
 lens becomes wedge-shaped, the sharp edge of the wedge 
 being the focal line (See p. 21). The base of the wedge lies 
 against the lens. (See Fig. 14.) By placing together two con- 
 vex cylindrical lenses of different refractive strength, with 
 their axes at right angles to each other, there will be formed 
 
 FIG. 43. Showing the Effect of Crossed Cylinders as Illustrating the 
 Refractive Effect of an Astigmic Surface. 
 
 two wedges, a short one by the stronger lens and a longer 
 one by the weaker lens. The edges or focal lines will lie some 
 distance apart and at right angles to each other. (Fig. 43.) 
 The same result is obtained by combining a spherical lens 
 with a cylinder (See Relationship of Lenses). 
 
 This represents the refractive condition of some eyes. 
 Either the cornea, or the crystalline lens, or both, so change
 
 ASTIGMIA. 69 
 
 the transmitted light rays as to have the effect of a sphero- 
 cylindrical lens. The result is that, instead of focussing the 
 rays of light at one point upon the retina, two focal lines 
 are formed some distance apart and at right angles to each 
 other. One of these lines may be upon the retina or both 
 may be behind or in front of that structure. An eye with 
 this peculiar refractive power is said to be astigmic, and 
 the condition is known as Astigmia or Astigmatism. The 
 cause of astigmia usually lies in the cornea, which, instead 
 of being like a section of a sphere, resembles more the bowl 
 of a spoon, /. e., one meridian is more convex than that 
 which is at right angles to it, having, therefore, a shorter 
 focal distance. 
 
 In view of the fact that the majority of astigmic eyes 
 show a more pronounced curvature vertically than hori- 
 zontally, astigmia at this angle is said to be "according to 
 the rule." The opposite condition, viz., that in which the 
 horizontal meridian shows the greater curvature, is spoken 
 of as astigmia " against the rule." 
 
 In a general way Astigmia may be divided into two 
 classes, as follows : 
 
 Regular Astigmia is that form in which all the meridians 
 of the refracting surfaces of the eye are regular in curvature, 
 i. e., they are the arcs of circles. This is the condition which 
 especially concerns the refractionist because it can be cor- 
 rected by the use of glasses. 
 
 Irregular Astigmia, of interest to the ophthalmologist, 
 but hopeless, optically speaking, is caused by a deformity of 
 the cornea resulting from a burn, injury, ulcer or other dis- 
 eased condition, followed by the formation of scar tissue. 
 The latter, by its contraction, produces an irregular, refract- 
 ing surface, the effect of -which is to so distort the image 
 formed on the retina that no lens, or combination of lenses, 
 will serve to bring the different parts of the image into one 
 plane. It is rarely improved by the use of glasses. 
 
 FORMS OF REGULAR ASTIGMIA. 
 
 There are five principal forms of regular astigmia, viz.: 
 (1) Simple Hyperopic, in which one meridian focusses on 
 the retina, the opposite meridian behind it.
 
 70 
 
 REFRACTION. 
 
 (2) Simple Myopic, in which one meridian focusses on the 
 retina, the opposite one in front of it. 
 
 FIG. 44. Showing Varieties of Astigmia I. Emmetropia; 2. Simple 
 
 Hyperopic Astigmia; 3. Simple Myopic Astigmia; 4. Compound 
 
 Hyperopic Astigmia; 5. Compound Myopic Astigmia; 
 
 6. Mixed Astigmia. 
 
 (3) Compound Hyperopic, in which both meridians focus 
 behind the retina, but at different distances from it.
 
 ASTIGMIA. 7 1 
 
 (4) Compound Myopic, in which both meridians focus in 
 front of the retina, but at different distances from it. 
 
 (5) Mixed, in which one meridian focusses behind and the 
 other in front of the retina. 
 
 The different forms are illustrated by Fig. 44. (Also by 
 Fig. 43, the retina being interposed at the points indicated.) 
 
 CAUSES OF REGULAR ASTIGMIA. 
 
 Astigmia is usually congenital and frequently associated 
 with other defects of vision. Ordinarily it is found to affect 
 both eyes, usually to the same degree and in the same me- 
 ridian. When acquired, it is usually caused by pressure upon 
 the eyeball of the lids or external eye muscles, or by opera- 
 tion, e.g., cataract extraction or iridectomy. Astigmia may 
 also be caused by an oblique position of the lens of the eye, 
 due to accidental or natural subluxation. (See Relation- 
 ship of Lenses.) 
 
 SYMPTOMS. 
 
 These are the usual symptoms of Asthenopia. When the 
 error is comparatively slight, i. e., below 1 D., no indistinct- 
 ness of vision is noticed. When more than 1 D., however, 
 circular bodies will appear elliptical and straight lines fall- 
 ing in the faulty meridians will appear indistinct. The pa- 
 tient makes frequent mistakes in reading the test types at 
 twenty feet. Astigmia should always be suspected if the pa- 
 tient in attempting to read the letters on the test card, 
 calls some correctly and makes guesses as to others. There is 
 more or less blurring of the letters when reading at close 
 range. It is not unusual to find assymetry of the face asso- 
 ciated with astigmia, the eye apparently sharing the facial 
 irregularity. 
 
 DETERMINATION OF ASTIGMIA. 
 
 The examination proceeds exactly as in the determination 
 of Hyperopia or Myopia. The patient is given the best 
 vision possible by the use of spherical lenses. When it is im- 
 possible to render vision normal by the use of spherical 
 lenses, it is probably owing to the presence of astigmia.
 
 72 
 
 REFRACTION. 
 
 A simple way of detecting astigmia is by rotating before 
 the eye a weak cylinder, say a plus .50 D. If the vision is 
 made worse in all meridians no appreciable amount of as- 
 tigmia is present. If the vision is better at any one meridian, 
 
 90 
 
 50 Feet 
 
 P5T2E6R 
 
 30 Feet 
 
 8F4O9G3T5 
 
 20 Feet 
 
 D2R6A5Z8P9H4E 
 
 3N9V4T8PZ 1 B5K3R6D2 
 
 E5C8L2R4Y3T6 et A2N4Z5P3V8R 
 
 FIG. 45. Showing Fan of Rays. 
 
 astigmia is present and the axis of the cylinder giving most 
 distinct vision is at right angles to the faulty meridian. 
 
 A very useful aid in determining astigmia is the fan of 
 rays (Fig. 45) or the astigmic dial (Fig. 46.) This is used at
 
 ASTIGMIA. 
 
 73 
 
 a distance of twenty feet and all the tests are made with one 
 eye at a time. When astigmia is manifest, i. e., when it is 
 not concealed by the patient's accommodation, one or two 
 rays of the fan or radiating lines of the dial appear more dis- 
 tinct than the remainder and indicate that the normal 
 meridian lies at right angles to these. (Fig. 47.) When the 
 
 FIG. 46. Showing Astigmic Dial. 
 
 astigmia is latent, i. e., concealed by the patient's accommo- 
 dation, it can usually be made manifest by revolving cylin- 
 ders of varying degrees of strength before the eye. The cylinder 
 at some axis will cause one or two of the rays or lines to ap- 
 pear blacker. This axis coincides with the faulty meridian. 
 6
 
 74 REFRACTION. 
 
 When this method fails a cycloplegic must be employed to 
 put the patient's accommodation entirely at rest. 
 
 CORRECTION OF ASTIGMIA. 
 
 General Remarks. In the correction of astigmia, just as 
 in the neutralization of a cylindric lens (see Neutralization of 
 
 FIG. 47. Showing Dial a* Seen by an Astigmic Eye. 
 
 Lenses), three things must be ascertained regarding the cor- 
 recting lens, viz., the kind, the strength and its position be- 
 fore the eye, i. e., the axis. The latter is indicated in degrees 
 of an arc numbering from right to left. (See Fig. 16.) By 
 placing at the proper axis before the eye convex or concave
 
 ASTIGMIA. 
 
 75 
 
 cylinders of varying degrees of strength, the strongest con- 
 vex, or the \veakest concave, cylinder through -which vision 
 is clearest and all of the rays or lines are seen with equal dis- 
 tinctness, is the measure of the astigmia. 
 
 Simple Hyperopic Astigmia (H. As.). By placing before 
 the eye a convex cylinder, of the right strength and at the 
 proper axis, the rays which formed a focal line behind the 
 retina are converged to meet on the retina. (Fig. 48.) The 
 strongest convex cylinder which makes the patient's vision 
 
 FIG. 48. Showing Position of Focal Lines in Simple Hyperopic Astigmia 
 and Position of Correcting Cylindric Lens. 
 
 clear and all the lines on the dial appear equally distinct is 
 the correction of the condition and should be prescribed for 
 constant wear. When the lens prescribed is weak and 
 the patient's symptoms slight, an exception may be made 
 and the patient be permitted to wear his glasses for near 
 work only. 
 
 In some cases in which the error is one of very high degree, 
 the full correction cannot be prescribed, owing to the great 
 discomfort experienced by the patient. It then becomes
 
 76 
 
 REFRACTION. 
 
 necessary to reduce the strength of the lens sufficiently to in- 
 sure comfort, and, at the same time, render vision as good 
 as possible. The final prescription in such a case can be as- 
 certained only by repeated trials. 
 
 Simple Myopic Astigmia (M. As.). By placing before the 
 eye a concave cylinder, of the right strength and at the 
 proper axis, the rays which formed a focal line in front of the 
 retina, Fig. 49, are made less convergent so that they meet 
 
 FIG. 49. Showing Position of Focal Lines in Simple Myopic Astigmia and 
 Position of Correcting Cylindric Lens. 
 
 on the retina. The weakest concave cylinder which makes 
 the patient's vision clear and all the lines on the dial appear 
 equally distinct is the correction of the condition. 
 
 In cases of low degree, this correcting cylinder may be 
 prescribed for constant wear. When the strength of the 
 lens prescribed exceeds .50 to .75 D. a second pair must be 
 prescribed for near work. Usually it is sufficient to prescribe
 
 ASTIGMIA. 
 
 77 
 
 convex cylinders of the same strength at the opposite axis 
 for near work. In this way the emmetropic meridian is 
 brought forward to the same plane with the faulty meridian, 
 thus changing the condition to one of myopia, which the pa- 
 tient will disregard for reading, if the error be of low degree. 
 
 Example. For distant vision : .75 D. C. Ax. 180 ; for 
 reading: + .75 D. C. Ax. 90. 
 
 When the error is one of high degree, e. g., 4 D. to 6 D., it 
 is frequently necessary to prescribe, for reading, a crossed 
 cylindrical lens, i. e., a concave cylinder to carry the faulty 
 meridian toward the retina and a convex cylinder at right 
 
 FIG. 50. Showing Position of Focal Lines in Compound Hyperopic Astig- 
 mia and Position of Correcting Cylindric Lenses. 
 
 angles to the above to carry the emmetropic meridian for- 
 ward to the same plane, thus again producing a condition of 
 myopia. 
 
 Example. For distant vision: --6 D. C. Ax. 75; for 
 reading- 3 D. C. Ax. 75 C + 3 D. C. Ax. 165. This pre- 
 scription may also be written: 6 D. C. Ax. 75 + 3 D. S. 
 (See Relationship, Page 33.) 
 
 Compound Hyperopic Astigmia (C. H. As.). In this con- 
 dition both focal lines are behind the retina, but at different 
 distances. (Fig. 50.) Ry placing a convex spherical lens of
 
 78 
 
 REFRACTION. 
 
 the proper strength before the eye, both focal lines are moved 
 forward, one falling on the retina, the other still behind the 
 retina, but nearer than before. Placing a convex cylin- 
 dric lens of the right strength over the spherical lens at the 
 proper axis, the rays which formed the focal line behind the 
 retina are converged to meet in a point on the retina. The 
 strongest convex spherical and cylindrical lenses, which com- 
 bined make the patient's vision clear and all the lines on the 
 dial to appear equally distinct, are the correction for the 
 condition. 
 
 FIG. 51. Showing Position of Focal Lines in Compound Myopic Astigmia 
 and Position of Correcting Cylindric Lenses. 
 
 When the error is of low degree, the glasses prescribed 
 may be worn for close work only. In the majority of cases, 
 however, they should be prescribed for constant wear. 
 
 Compound Myopic Astigmia (C. M. As.). In this con- 
 dition both focal lines are in front of the retina, but at differ- 
 ent distances from it. (Fig. 51.) By placing a concave spheri-
 
 ASTIGMIA. 79 
 
 cal lens of the proper strength before the eye, both focal lines 
 are moved back, one falling on the retina, the other still 
 in front of it, but nearer than before. Placing over the 
 spherical lens a concave cylindric lens, of the right strength 
 and at the proper axis, the rays which formed the focal line 
 in front of the retina are made less convergent, so that they 
 meet in a point on the retina. The weakest concave spheri- 
 cal and cylindrical lenses, \vhich combined make the patient's 
 vision clear and all the lines in the dial to appear equally dis- 
 tinct, are the correction for the condition. 
 
 When the error is of low degree, one prescription may be 
 sufficient, the patient being instructed to wear the glasses 
 constantly. When moderately strong lenses are prescribed 
 for distance, the spherical part may be dropped and convex 
 cylinders be ordered for reading. 
 
 Example. For distant vision: - 1 D'. S. C ~" 1.25 D. 
 C. Ax. 45; for reading: - 1.25 D. C. Ax. 135. 
 
 When the error is one of high degree : 
 
 (a) The same cylinder and axis may be prescribed for 
 reading, or, 
 
 (b) The same cylinder and axis, combined with a weaker 
 spherical lens. 
 
 Examples, (a) For distant vision: 3 D. S. O ~ 4 D. 
 C. Ax. 15; for reading: 4 D. C. Ax. 15. 
 
 (b) For distant vision: - 6 D. S. C - 3 D. C. Ax. 15; 
 for reading: - 2 D. S. C - 3 D. C. Ax. 15. 
 
 Mixed Astigmia (M. A.). When it is found impossible by 
 means of spherical or cylindrical lenses, or combinations of 
 the same, to secure distinct vision, mixed astigmia is prob- 
 ably present. Cases which at first appear to be high degrees 
 of myopic or Iryperopic astigmia, on careful examination, 
 may be found to be mixed. 
 
 In this condition, one focal line lies behind the retina, the 
 other in front of the retina. (Fig. 52.) By placing before the 
 eye a convex cylinder, of the right strength and at the proper 
 axis, the rays which formed the focal line behind the retina 
 are made more convergent so that they meet in a line on the 
 retina. Placing before the eye a concave cylinder, of the 
 right strength and at the proper axis (at right angles to the
 
 80 REFRACTION. 
 
 convex cylinder), the rays which formed a focal line in front 
 of the retina are made less convergent so that they meet on 
 the retina. 
 
 Glasses prescribed for Mixed Astigmia should be worn 
 constantly. When the myopic cylinder is of high degree, a 
 separate correction should be made for reading, in which 
 case the concave cylinder is decreased and the convex cylin- 
 der correspondingly increased. 
 
 FIG. 52. Showing Position of the Focal Lines in Mixed Astigmia and 
 Position of Correcting Cylindric Lenses. 
 
 Example. - 3.50 D. C. Ax. 180 C + 1-75 D. C. Ax. 90. 
 For reading: - 1.50 D. C. Ax. 180 C + 3.75 D. C. Ax. 90. 
 Stenopseic Disc. Another method of determining the 
 presence of astigmia is by revolving before the eye the steno- 
 paeic disc. (See p. 51.) When in the position affording most 
 distinct vision, place before the slit convex and concave 
 spherical lenses; the strongest convex or the weakest con- 
 cave lens, giving the greatest improvement, represents the 
 measure of the refraction in this meridian. The slit is then
 
 ASTIGMIA. 81 
 
 revolved to a position at right angles to its former axis and 
 the refraction of this meridian ascertained in the same man- 
 ner. The sum of the two results obtained is the total error 
 of refraction. 
 
 Example. Vertical meridian : f .25 D. S. 
 
 Horizontal meridian : + .75 D. S. 
 
 Total error: -r 25 D. S. C + .50 Ax. 90.
 
 CHAPTER IX. 
 
 ANISOMETROPIA, APHAKIA, AMBLYOPIA AND 
 MALINGERING. 
 
 ANISOMETROPIA. 
 
 By the term, Anisometropia, is meant a difference in the 
 retractive power of the two eyes. One eye may be normal, 
 or both hyperopic, myopic or astigmic, one more so than 
 the other. When the difference is slight, both eyes may be 
 corrected. When the difference is pronounced, the glass re- 
 quired to correct the poorer eye is often too strong to be 
 worn with comfort, causing diplopia or producing other un- 
 comfortable symptoms. Therefore, it is frequently necessary 
 to prescribe a partial correction or even none at all for the 
 poorer eye, and complete correction for the better eye. 
 
 APHAKIA. 
 
 After the extraction of acataract, the patient is compelled 
 to wear two pairs of glasses, one pair for distance, the other 
 for near vision. The crystalline lens of the eye having been re- 
 moved, all accommodation is suspended and the refractive 
 power of the eye is practically limited to that of the cornea 
 alone. Rays of light entering such an eye are not deflected 
 sufficiently to meet on the retina. A strong convex lens 
 (from 5 D. to 12 D.) is, therefore, necessary for distant 
 vision, and a stronger one (from 12 D. to 20 D.) for read- 
 ing. There is usually present, also, a considerable degree of 
 astigmia, caused by the incision through the cornea and the 
 resulting alteration in its curvature. This must also be cor- 
 rected in the ordinary way and the proper cylindrical lens 
 added to the spherical lens prescribed. 
 
 AMBLYOPIA AND AMAUROSIS. 
 
 When, in the absence of any visible changes in the eye, 
 there is a partial loss of vision which cannot be improved 
 by the use of glasses, the condition is known as Amblyopia.
 
 ANISOMETROPIA. 83 
 
 When there is a complete loss of vision without evident 
 changes in the structure of the eye, the condition is known 
 as Amaurosis. 
 
 Amblyopia and Amaurosis may be the result of disuse, as 
 in the case of the squi'nting eye, or it may be caused by in- 
 jury, loss of blood or cerebral disease. It is also occasionally 
 met in hysterical individuals ; in such cases it is temporary 
 and accompanied by other hysterical symptoms. 
 
 MALINGERING. 
 
 For the purpose of collecting damages or insurance after 
 a trivial injury, or to gain an increase of pension, a patient 
 may pretend to be blind. When complete blindness of both 
 eyes is claimed, it is often difficult to disprove, except by ob- 
 serving the patient without his knowledge. Perhaps the 
 most effective method in such a case is by pretending a blow 
 over the eye. If the patient sees, the eye will be suddenly 
 closed to prevent injury. 
 
 When blindness in one eye is claimed, several tests may be 
 made to disclose the true condition. A ten degree prism, 
 base out, may be placed before the eye under suspicion. If 
 the eye sees, it will rotate inw^ard, in an unconscious effort 
 to fuse the two images into one. 
 
 If, with a strong convex or concave lens before the good 
 eye and a plane lens before the pretended poor e} r e, the pa- 
 tient is able to read the Snellen type at twenty feet, blind- 
 ness is disproved. 
 
 It is needless to add, perhaps, that the examination of a 
 patient suspected of simulating blindness must be conducted 
 without arousing suspicion as to the true purpose of the ex- 
 aminer. The examiner should act as if convinced that blind- 
 ness exists and that he is seeking for its cause.
 
 CHAPTER X. 
 PRESBYOPIA. 
 
 CAUSES AND SYMPTOMS, 
 
 The hardening of the lens, previously mentioned in our 
 consideration of the subject of accommodation (p. 43), is a 
 physiological change, which, beginning in early life, shows a 
 steady and almost constant increase as the age of the indi- 
 vidual advances. Near vision becomes more difficult, the 
 patient ultimately finding that he is no longer able to see 
 distinctly at close range. Gradually he is compelled to hold 
 his book or newspaper at a greater distance than formerly 
 (Fig. 53) or he finds it necessary when reading fine print to 
 hold it in a bright light, the resulting contraction of the 
 pupil rendering vision more distinct. By these means the 
 wearing of glasses for near work may be deferred, distant 
 vision remaining unimpaired. But the time comes when, even 
 by these means, the threading of a needle or the reading of a 
 newspaper is no longer possible. This is due to the fact that 
 the lens of the eye has so far lost its elasticity that by no 
 effort of the ciliary muscle can the lens be made sufficiently 
 convex to focus on the retina the divergent rays of a nearby 
 object. This failure in the functional activity of the crystal- 
 line lens usually becomes manifest between the ages of forty- 
 three and forty-five, although in some cases in which the 
 eyes are used but little for close work and the general health 
 is good, it may not require correction until later. 
 
 The phenomenon by which patients of advanced age are 
 able to read fine print without glasses is frequently a source 
 of wonder among the laity and by them considered a sign of 
 unusual constitutional powers. It signifies merely that the 
 individual in his earlier years has been nearsighted, but his 
 myopia was gradually overcome by the increasing presby- 
 opia until the patient finally had normal vision for the first 
 time. The eyeball being too long, the crystalline lens had
 
 PRESBYOPIA. 
 
 85 
 
 55 Ye 
 
 65 Yec 
 
 FIG. 53. Showing Gradual Recession of the Near Point Owing to Harden- 
 ing and Flattening of the Lens Presbyopia.
 
 86 REFRACTION. 
 
 been able to focus clearly on the retina only very divergent 
 rays of close objects. Having lost its elasticity, the lens be- 
 came more flattened, its refractive power decreased so that 
 rays which formerly focussed in front of the retina now 
 form a clear image upon it. 
 
 A very definite relationship exists between the amount of 
 presbyopia, or failure of accommodation, and the age of the 
 patient, as the following table will show : 
 
 TABLE IV. 
 
 SHOWING THE RELATIONSHIP BETWEEN PRESBYOPIA AND THE AGE 
 OF THE PATIENT. 
 
 Age. Failure of Accommodation. 
 
 45 * D. 
 
 50 . . 1.75 to 2.00 D. 
 
 55 2-50 to 3.00 D. 
 
 60 3.00 to 4.00 D. 
 
 65 3.50 to 4.50 D. 
 
 CORRECTION OF PRESBYOPIA. 
 
 From what has been said, it is evident that patients, who 
 until the age of forty -five years have worn no glasses, after 
 that time should be provided with the proper correction for 
 reading. Provided no astigmia or other more serious error 
 of refraction be present, such patients can readily select a 
 "reading glass "'from among an assorted stock without the 
 assistance of a refractionist. From the very nature of pres- 
 byopia, it will be seen that the patient's glasses must be 
 changed every three to five years, their strength being 
 steadily increased to make up for the gradual decrease inci- 
 dent to the flattening of the crystalline lens. In selecting 
 glasses for presbyopia, the following rules will be helpful : 
 
 (a) When no other error of refraction exists the strength 
 of the reading glasses will be determined by the age of the 
 patient and will conform approximately to the table. 
 
 (b) When the patient has previously worn glasses for 
 distant vision on account of astigmia or myopia of low de- 
 gree, the reading correction prescribed will be the distance 
 prescription increased by the correction for presbyopia at 
 the patient's age.
 
 PRESBYOPIA. 87 
 
 (c) When the patient has previously worn glasses for 
 both distant and near vision on account of myopia of high 
 degree, or of myopic or mixed astigmia, of high degree, the 
 reading correction prescribed will be the previous reading 
 lens added to the presbyopic correction for the given age. 
 
 Examples, (b) A patient who had been wearing 4 D. 
 S. C - 2.75 D. C. Ax. 15 for distance and - 1.50 D. S. C 
 - 2.75 D. C. Ax. 15 for reading would, at age forty-five 
 years, require for reading the addition of + 1 D. Sph., mak- 
 ing his prescription read .50 D. S. C~ 2.75 D. C. Ax. 15. 
 
 (c) A patient who had been wearing 4 2.75 D. C. Ax. 
 45 C 1.50 D. C. Ax. 135 for distance and 4- 4.25 D. C. 
 Ax. 45 for reading would, at age fifty years, require for 
 reading + 2 D. S. C - 4.25 D. C. Ax. 45. 
 
 In deciding upon the strength of glasses for reading, it is 
 not always practicable merely to add to the patient's distance 
 glass the strength of lens necessary at the age of the patient. 
 Too strong a lens for near work would be uncomfortable. 
 Here, as elsewhere in the practice of refraction, no hard and 
 fast rules can be followed out, the patient's comfort being 
 the first consideration. A good rule to follow is to take 
 the refraction of each eye separately for near work, selecting 
 such lens or combination as will give the most acute vision. 
 Then try the two eyes together, reducing the total correction 
 to one which gives good vision, without causing the patient 
 any uncomfortable symptoms.
 
 CHAPTER XL 
 CONFIRMATORY TESTS (OBJECTIVE). 
 
 INTRODUCTION. 
 
 The tests heretofore described have been entirely subject- 
 ive, L e., dependent in every case upon the replies of the pa- 
 tient to the questions of the examiner. In children and 
 illiterates, these tests are not always practicable, and object- 
 ive tests must be substituted. In fact, as a matter of 
 
 FIG. 54. Showing Argand Burner with Thorington Retinoscopy Chimney. 
 
 routine, it is advisable to employ one or more confirmatory 
 tests in all cases of refraction. This reduces the possibility 
 of error to the minimum. 
 
 Objective tests are those which depend solely upon the ex- 
 aminer. They consist essentially of a series of maneuvers 
 carried out in a dark room by means of a suitable lamp (an 
 Argand burner is well suited to this purpose, Fig. 54,)
 
 CONFIRMATORY TESTS. 
 
 89 
 
 retinoscopic mirror (Fig. 55) and objective lens. In place of 
 a retinoscope, the mirror of an ophthalmoscope may be em- 
 ployed, while an ordinary case lens of + 16 D. or + 18 D. 
 will serve the purpose of an objective lens. 
 
 MIRROR TEST (CONCAVE MIRROR). 
 
 Direct Method. The examiner, if not emmetropic, must 
 first correct his own error of refraction by means of glasses 
 or the proper lens placed behind the sight-hole of the oph- 
 
 FIG. 55. Showing Retinoscope. 
 
 thalmoscope. He should then seat himself opposite his pa- 
 tient, in a darkened room and at a distance of fifteen inches, 
 directing the patient to look toward the right side of 
 the observer's head when the right eye is being examined, 
 and to the left side if the other eye. The fundus is now 
 illuminated, and the reflex observed; if no details of the 
 fundus are visible, the patient is emmetropic. If any part of 
 the disc or retinal vessels can be seen, the patient is ame- 
 7
 
 90 REFRACTION. 
 
 tropic. The observer now moves his head in various direc- 
 tions, and notes the resulting movement of the retinal ves- 
 sels; according as these move in the same direction with the 
 head of the observer, or in a direction opposite to the latter, 
 the error of refraction is determined. 
 
 Direction of Movement of Retinal Vessels. 
 
 Retractive Error. 
 
 (a) Movement with in all directions. 
 
 (b) Movement with in one meridian. 
 
 (c) Movement against in all merid- 
 
 Hyperopia. 
 
 Simple Hyperopic Astigmia. 
 
 Myopia. 
 
 (d) Movement against in one merid- Simple Myopic Astigmia. 
 
 ian only. 
 
 (e) Movement with in one meridian Mixed Astigmia. 
 
 and against in the other. 
 
 If the image is blurred, the strongest convex or the weak- 
 est concave lens, which will render it distinct, is the measure 
 of the error. 
 
 Indirect Method. This test should be conducted under a 
 mydriatic. The conditions are the same as in the previous 
 test, with the addition of an object lens which is held as near 
 the patient's eye as possible. Reflecting the light into the 
 eye to be examined and keeping the optic disc in view the 
 lens is gradually withdrawn to a distance of several inches, 
 and the resulting change in the size of the disc noted. The 
 following table will serve to show the diagnostic significance 
 of the change in the size and shape of the optic disc on with- 
 drawing the object lens : 
 
 Conduct of the Optic Disc. 
 
 (a) If the disc .remain the same size 
 
 throughout. 
 
 (b) If the disc diminish in size. 
 
 (c) If the disc diminish in size in one 
 
 meridian only. 
 
 Refractive Error. 
 
 Emmetropia. 
 
 Hyperopia. 
 Hyperopic Astigmia.
 
 CONFIRMATORY TESTS. 91 
 
 Conduct of the Optic Disc. 
 
 Refractive Error. 
 
 (d) If the disc diminish in all merid- 
 
 ians, but more so in one than 
 in the others. 
 
 (e ) If the disc increase in size. 
 
 (f) If the disc increase in one merid- 
 
 ian only. 
 
 (g) If the disc increase in all merid- 
 
 ians, but more so in one than in 
 the others. 
 
 Compound Hyperopic Astigmia. 
 
 Myopia. 
 Myopic Astigmia. 
 
 Compound Myopic Astigmia. 
 
 (h) If the disc increase in one merid- Mixed Astigmia. 
 ian and decrease in the opposite. 
 
 RETINOSCOPY OR SKIASCOPY (KERATOSCOPY). 
 
 This test is known also as the shadow test. To obtain 
 accurate results by its use, a mydriatic should be employed. 
 The observer is seated opposite the patient, at a distance 
 of one meter in a dark room. The lamp should be placed 
 above and a little to the rear of the patient, leaving his 
 face in the shadow. The test frame should be adjusted 
 to the patient's face and the eye not under examination 
 should be covered by means of the obturator. The pa- 
 tient directs his gaze at the observer's forehead, the light is 
 reflected into the eye as in the previous tests and is made to 
 pass across the retina by slightly rolling the handle of the 
 mirror between the thumb and forefinger, thereby causing 
 the mirror to revolve in the axis of the handle or vertical 
 meridian. Repeat the test, holding the handle horizontally 
 and diagonally, noting in each case: 
 
 (a) The shape and size of the shadow surrounding the 
 light reflex. 
 
 (b) The brightness of the image. 
 
 (c) The relative direction of the movement of the shadow 
 as compared \vith the direction of rotation of the mirror. 
 
 (d) The rate of motion of the reflex. 
 
 Generally speaking, if the reflex is bright, the surround- 
 ing shadow narrow or crescentic, and the rate of motion 
 slow, the refractive error is one of low degree and vice versa.
 
 92 REFRACTION. 
 
 The mirror used may be either plane or concave. When a 
 plane mirror is used the results are the reverse of these, but 
 the following are the results obtained with a concave mir- 
 ror: 
 
 Emmetropia. Theshadowmay^ show no apparent move- 
 ment, or a slight movement against the mirror in all merid- 
 ians. The luminosity is pronounced, and the edge of the 
 shadow is more or less crescentic. 
 
 Hvperopia. The shadow moves against the mirror in all 
 meridians. Place before the eye convex lenses of different 
 strength until the movement is reversed. From the weak- 
 est convex lens which reverses the movement deduct -f 1 D. 
 and the result represents the measure of the Hvperopia. 
 
 Myopia (Low degree). The shadow moves against the 
 mirror when the error is less than 1 D. Place before the eye 
 convex lenses of different strength until the movement is re- 
 versed. From the weakest convex lens which reverses the 
 movement deduct -f 1 D. and the result represents the meas- 
 ure of the Myopia. 
 
 Myopia (High degree). The shadow moves with the mir- 
 ror in all meridians. Place before the eye concave lenses 
 of different strength until the strongest lens is found with 
 which the shadow continues to move with the mirror. 
 From this likewise deduct ~r 1 D. (add 1 D.) and the result 
 is the measure of the myopia. 
 
 Astigmia (Hyperopic). The shadow moves against the 
 mirror in all meridians. The weakest convex lens which re- 
 verses the movement in one meridian will not be strong 
 enough to reverse the movement in the opposite meridian. 
 Note the strength of the former, then try stronger lenses 
 until one is found which will reverse the movement in the 
 opposite meridian. By deducting + 1 D. from the two lenses 
 which reversed the movement in the two principal meridians, 
 the refraction is obtained. 
 
 Astigmia (Myopic). When of low degree, the image 
 moves against the mirror in all meridians. The test is the 
 same as the preceding. 
 
 When the error is of high degree and the case is one of 
 simple myopic astigmia, the image moves rapidly with the 
 mirror in one meridian and slightly against in the other.
 
 CONFIRMATORY TESTS. 93 
 
 When the error is of high degree and the case is one of 
 compound myopic astigmia, the image moves with the 
 mirror in both meridians, more rapidly in one than in the 
 other. The strongest concave lens with which the shadow 
 continues to move with the mirror in all meridians is the 
 measure of one of the principal meridians (after deducting 
 4- 1 D., i. e., adding ID.). Now increasing the strength of 
 the neutralizing lens, the shadow will be reversed in all but 
 one meridian, viz., the other principal meridian. The strong- 
 est concave lens with which the shadow continues to move 
 with the mirror is the measure of the refraction in this me- 
 ridian (after deducting + 1 D., /. e., adding 1 D.). 
 
 Astigmia (Mixed). When the error in each of the princi- 
 pal meridians is less than 1 D. the image moves against the 
 mirror in all meridians. W T hen the error is greater than 1 D. 
 the movement in one meridian will be with the mirror, in 
 the opposite meridian against it. The test is made like the 
 preceding ones. 
 
 When astigmia is of high degree the retinal reflex has the 
 appearance of a band of light, so that the edge of the 
 shadow is straight and sharply defined. This band of light 
 coincides with one of the principal meridians and with the 
 axis of the correcting cylinder.
 
 CHAPTER XII. 
 MUSCULAR IMBALANCE. 
 
 OCULAR MOVEMENTS. 
 
 When the eyes are directed toward an object they are 
 said to "fix" this object. When in this position the image 
 falls upon the retina of each eye at the Macula Lutea, or 
 point of most acute vision. This is necessary for two pur- 
 poses, viz.: First, in order that the brain perceive but one 
 image of the object looked at; and, second, in order that the 
 image be as clear as possible. 
 
 The act of fixing is accomplished by the extrinsic muscles 
 of the eye, consisting of four straight and two oblique mus- 
 cles, viz., the Superior Rectus, Inferior Rectus, Internal 
 Rectus, External Rectus, Superior Oblique and Inferior Ob- 
 lique. The Recti move the eyes upward, downward, in- 
 ward or outward, while the oblique muscles cause the eye to 
 rotate in the plane of the equator of the eyeball. More than 
 one muscle takes part in each of these movements, as is 
 shown by the following table : 
 
 TABLE V. 
 
 SHOWING OCULAR MUSCLES CONCERNED IN THE SEVERAL EXCURSIONS OF 
 
 THE EYEBALL. 
 
 Movement. 
 
 Direction. 
 
 Muscles. 
 
 Elevation. 
 Depression. 
 
 Upward. 
 Downward. 
 
 Superior Rectus. 
 Inferior Oblique. 
 
 Inferior Rectus. 
 Superior Oblique. 
 
 Adduction. 
 
 Inward. 
 
 Internal Rectus. 
 Superior Rectus. 
 Inferior Rectus. 
 
 Abduction. 
 
 Outward. 
 
 External Rectus. 
 Superior Oblique. 
 Inferior Oblique. 
 
 Sursumduction. 
 Sursumduction. 
 
 Rotation of the upper | Superior Oblique, 
 part of the eye inward. I Superior Rectus. 
 
 Rotation of the upper 
 part of the eye outward. 
 
 Inferior Oblique. 
 Inferior Rectus.
 
 MUSCULAR IMBALANCE. 95 
 
 MUSCULAR BALANCE OR ORTHOPHORIA. 
 
 Under normal conditions both eyes move simultaneously 
 and to the same extent. Whether focussed for near or dis- 
 tant objects, the eyes may be moved up, down, in or out, 
 and in so doing, both eyes move at the same time, without 
 altering their positions as regards each other. This is 
 known as "associated" movement of the eyes. When the 
 muscles of the two eyes are in perfect equilibrium or balance, 
 the condition is known as Orthophoria, 
 
 The amount of movement of which the normal eye is 
 capable is about 45 upward, inward and outward, and 
 about 55 downward. This latitude of motion constitutes 
 what is known as the Field of Fixation. 
 
 HETEROTROPIA OR STRABISMUS. 
 
 When the two eyes are directed toward the same object 
 the image falls upon corresponding points of the two retinae, 
 so that the brain perceives but one, a condition known as 
 Binocular Single Vision. This is, of course, the normal state. 
 When the two eyes are not directed toward the same object 
 the condition is known as Heterotropia, Strabismus, Squint 
 or Cross-eye. The image in such a condition does not fall 
 upon corresponding points of the two retinae, so that the 
 brain perceives the image from each retina, and the patient 
 has double vision or Diplopia. 
 
 That image produced on the retina of the eye which fixes, 
 falls upon the macula and is, therefore, called the true image, 
 while that produced on the retina of the deviating eye does 
 not fall on the macula and is, therefore, known as the false 
 image. After the condition of Heterotropia has* existed for 
 some time, the patient learns to disregard or suppress the 
 false image and depends entirely upon the vision of the nor- 
 mal eye. The functional disuse to which the retina of the 
 squinting eye is subjected, results ultimately in a loss of sen- 
 sitiveness to light stimulation, a condition known as 
 Amblyopia and one which cannot be improved by the use 
 of glasses.
 
 96 REFRACTION. 
 
 HETEROPHORIA. 
 
 In the absence of an actual deviation of either e3 r e, there 
 may yet be present a muscle imbalance or Heterophoria, ow- 
 ing to the fact that one muscle or set of muscles is weaker 
 than another. An almost constant effort is, therefore, neces- 
 sary to compel the weaker muscle to do as much work as its 
 stronger opponent. This continued strain leads often to 
 pronounced symptoms of asthenopia. 
 
 Heterophoria bears much the same relation to hetero- 
 tropia or squint as a latent error of refraction does to one 
 which is manifest, i. e., it is concealed and overcome by the 
 exertion of sufficient muscular and nervous energy, w r hereas 
 a squint is manifest and cannot be overcome by any mus- 
 cular effort. Heterophoria is a tendency toward deviation, 
 while heterotropia is an actual deviation. 
 
 FORMS OF HETEROPHORIA. 
 
 The varieties of Heterophoria are : 
 
 Esophoria, a tendency of the eye t'o turn inward. 
 
 Exophoria, a tendency of the eye to turn outw r ard. 
 
 Hyperphoria, a tendency of the eye to turn upward. 
 
 Hypophoria, a tendency of the eye to turn downward. 
 
 Hyperesophoria, a tendency of the eye to turn upward 
 and inward. 
 
 Hyperexophoria, a tendency of the eye to turn upward 
 and outward. 
 
 Cyclophoria, a tendency of the eye to turn in the plane of 
 the equator of the eye. 
 
 DETERMINATION OF HETEROPHORIA. 
 
 The pres'ence of latent errors of refraction was determined 
 by making such errors manifest. This was done by over- 
 coming the activity of the muscles of accommodation, which 
 concealed the underlying conditions. In a similar way the 
 presence of heterophoria is made manifest. Numerous 
 methods have been devised and many instruments have been 
 constructed for the determination of heterophoria, but all 
 are based on the following fundamental principles:
 
 MUSCULAR IMBALANCE. 97 
 
 (a) T$y producing artificial diplopia and determining 
 whether the two images fall upon corresponding points of 
 the two retinae. 
 
 (b) By creating two different images, one falling upon 
 the retina of one eye, the second upon the retina of the other, 
 and determining whether the two images fall upon corre- 
 sponding points of the two retinas. 
 
 For the purpose of creating an artificial diplopia strong 
 prisms are employed. An object looked at through a prism 
 appears displaced toward the apex, the extent of the displace- 
 ment depending upon the strength of the prism. (See Fig. 
 19,p.3O.) This apparent displacement is due to the fact that 
 light rays in passing through a prism are deflected toward 
 
 FIG. 56. Showing Apparent Displacement of an Object Toward the Apex 
 
 of a Prism. 
 
 the base. The eye, perceiving the rays as they emerge along 
 their altered course from the prism, sees the image in the 
 direction of this altered course (Fig. 56.) Hence, if a strong 
 prism be placed before one eye while the other eye remains 
 uncovered, two images will be seen ; one image will be 
 deflected toward the apex of the prism, while the position 
 of the other remains unchanged. 
 
 A double prism, two prisms placed base to base, may be 
 employed. When placed before one eye, two deflected images 
 are produced, one toward the apex of each prism, while the 
 image of the uncovered eye will lie between these two de- 
 flected images. 
 
 The tests for Heterophoria most frequenth- used are: 
 (1) Prism test; (2) Maddox rod test; (3) Cobalt test.
 
 98 REFRACTION. 
 
 PRISM TEST. 
 
 The right eye being uncovered, by placing before the left eye 
 an eight degree prism with its base up or down, two images 
 will be seen, one above the other. If both images lie in the 
 same vertical plane, one exactl}' above the other, those 
 muscles which move the eyes to the right and left (hori- 
 zontally) are properly balanced. If they do not lie one ex- 
 actly above the other, the horizontal muscles are not bal- 
 anced, and that prism, base in or out, which causes them to 
 lie in the same plane, one exactly above the other, is the 
 measure of the heterophoria. When the base of the prism 
 which corrects the error is directed inward, the condition is 
 one of Exophoria.* If the base is directed outward the con- 
 dition is Esophoria. 
 
 Example: If a four degree prism, base out, is found to 
 place the two images one exactly above the other, the con- 
 dition is one of Esophoria and its measure is four degrees. 
 
 To test the equilibrium of the vertical muscles place be- 
 fore the left eyeaten degree prism, base in. If the two images 
 are on the same level, perfect equilibrium exists. If one is 
 higher than the other, heterophoria is present, and that 
 prism, base up or down, which causes the two images to lie 
 on the same level, is the measure of the error. 
 
 MADDOX TEST. 
 
 This test is best executed in a darkened room. Place the 
 simple or multiple Maddox rod, Fig. 32, before the left eye, 
 the right eye remaining uncovered. Direct the patient to look 
 at a flame twenty feet distant. The uncovered eye will per- 
 ceive the image of the flame, while the other will see a streak 
 of light. When the Maddox rod is horizontal this streak 
 will be vertical and vice versa. 
 
 If the streak does not pass directly through the flame, 
 heterophoria is present and the relative position of the 
 streak to the flame will serve to diagnose its kind. That 
 
 *The apex of the correcting prism, like the sharp edge of an axe, is al- 
 ways directed toward the muscle which would be cut were the case one of 
 actual deviation or strabismus.
 
 MUSCULAR IMBALANCE. 
 
 99 
 
 prism which causes the streak to run directly through the 
 flame is the measure of the error. 
 
 The following table will serve to indicate the diagnostic 
 value of the several positions assumed by streak and flame : 
 
 TABLE VI. 
 
 SHOWING RELATIVE POSITION OF STREAK TO FLAME IN THE DIAGNOSIS 
 OF HETEROPHORIA. 
 
 (Maddox Rod over Left Eye. ) 
 
 Position of Mad- Direction of Light : Position of Streak with 
 dox Rod. Streak. Regard to Flame. 
 
 Heterophoria. 
 
 Horizontal. 
 
 Vertical. 
 
 Vertical. 
 
 To the left of flame. 
 
 To the right of flame. 
 Horizontal. Above the flame. 
 
 Below the flame. 
 
 Esophoria. 
 Exophoria. 
 
 Hypophoria 
 (left eye turns 
 down ). 
 
 Hyperplioria 
 (left eye turns 
 up). 
 
 COBALT TEST. 
 
 This is similar to the Maddox Rod test, a blue image of 
 the flame being substituted for the light streak. By placing 
 the Cobalt blue glass found in the test case before the left 
 eye, the right eye being uncovered, and directing the patient 
 to look at a flame twenty feet away, two images will be per- 
 ceived, one yellow and one blue. The relative positions of the 
 two images will serve to diagnose the condition. If the 
 colored image is at the left of the true image, the condition 
 is that of Esophoria. If the colored image is at the right, 
 the condition is that of Exophoria. If above, the left eye 
 turns down ; if below, the left eye turns up Hyperphoria. 
 This is a simple procedure which will serve to demonstrate 
 the presence of heterophoria. 
 
 COVER TEST. 
 
 While one eye is covered, the patient should be directed to 
 look at some object about twelve inches distant. If, on. re-
 
 100 REFRACTION. 
 
 moval of the obturator, the eye is seen to move, muscle im- 
 balance exists. The direction of this movement indicates the 
 weak muscle, for it was the weak muscle which was relaxed 
 while the eye was at rest. When the eye was uncovered this 
 weak muscle, in response to excessive stimulation, drew the 
 eye to its proper position to re-establish muscular balance. 
 Hence, if the eye moves inward on uncovering, outward 
 deviation had taken place while the eye was at rest and the 
 condition is, therefore, one of Exophoria. 
 
 DETERMINATION OF CYCLOPHORIA. 
 
 Insufficiency of the oblique muscles of the eye is seldom 
 found to exist alone, but usually in conjunction with an in- 
 sufficienc}' of one of the recti muscles. It is best determined 
 by means of the double prism found in the test case. This 
 is placed before one eye, the other eye being uncovered. A 
 card on which a horizontal line is drawn is held at a distance 
 of eighteen inches. By the action of the double prism two 
 lines are seen parallel to each other. Between them is a third 
 line, the image on the retina of the uncovered eye. If the 
 line is parallel with the other two, perfect balance is present. 
 But if this third line is oblique to the other two, cyclophoria 
 is indicated. That prism, by the aid of which the three lines 
 are made parallel, is the measure of the error. The position 
 of the correcting prism before the eye will be oblique, with 
 the base in or out. 
 
 CORRECTION OF HETEROPHORIA. 
 
 According to the degree of the insufficiency, four methods 
 are available for the relief of muscular imbalance, viz. : 
 
 (a) Correction of the error of refraction. 
 
 (b) Prism exercise. 
 
 (c) Wearing of correcting prisms. 
 
 (d) Operation. 
 
 Correction of the Error of Refraction. In mild cases cor- 
 rection of the error of refraction will be sufficient for the re- 
 lief of the condition. The muscles which turn the eyes in- 
 ward are supplied by the third nerve, which also supplies the 
 ciliar\' muscle. This will explain the very close relationship
 
 MUSCULAR IMBALANCE. 101 
 
 existing between accommodation and convergence. Any in- 
 crease or decrease in the stimulation of the muscles of ac- 
 commodation likewise affects the muscles of convergence. 
 
 Esophoria is usually the result of uncorrected Hyperopia. 
 The constant contraction of the ciliary muscle in its effort to 
 overcome the hyperopia is accompanied by increased nerve 
 stimulation of the internal recti, resulting in too great con- 
 vergence. 
 
 Exophoria is usually associated with myopia. The re- 
 laxation of the ciliar\ r muscle being necessar}^ to clear vision, 
 the muscles of convergence do not receive the requisite 
 amount of stimulation and, in consequence, relax. 
 
 Hyperphoria is frequently found associated with marked 
 assymetry of the face and with some forms of anisometropia. 
 
 Prism Exercise. By means of persistent, regular and sys- 
 tematic exercise of the weaker muscle, the same can fre- 
 quently be strengthened sufficiently to do its work properly, 
 especially after any existing refractive error has been cor- 
 rected. In conducting prism exercises, the prism should 
 always be inserted in the position which actually increases 
 the error, i. e., base over the stronger muscle. Prism exer- 
 cise is most promising of success in the treatment of exo- 
 phoria, less so in correcting esophoria or hyperphoria. 
 
 These exercises are conducted as follows : Weak prisms 
 are placed before the eyes, base over the stronger muscle, and 
 the patient is directed to look at a flame twenty feet awav. 
 As soon as the double images come together, the result of 
 considerable effort on the part of the patient, the prisms are 
 removed. After a moment's rest, they are again replaced. 
 These exercises should be conducted five or ten minutes 
 daily, the strength of the prism being gradually increased. 
 
 Wearing" of Prisms. When the kind and amount of 
 heterophoria has been determined, it is frequently necessary 
 to assist the weaker muscle by prescribing a prism for con- 
 stant \vear. No hard and fast rule can be followed in deter- 
 mining the strength of the prism to be prescribed. The fol- 
 lowing suggestions will be useful : 
 
 Exophoria. From one-half to three-quarters of the 
 strength of the correcting prism may be prescribed.
 
 102 REFRACTION. 
 
 Esophoria. When of low degree no prism need be worn. 
 When of high degree, about one-quarter of the strength of 
 the correcting prism may be prescribed. 
 
 Hyperphoria. The full correcting prism should be pre- 
 scribed. 
 
 For the sake of comfort and cosmetic effect, it is custom- 
 ary in prescribing prisms to divide the prism correction be- 
 tween the two lenses. Instead of placing a four-degree 
 prism before one eye with an ordinary lens before the other, 
 two two-degree prisms are ordered. If the base of the pre- 
 scribed prism is directed inward, both prisms must be di- 
 rected inward; if outward, both prisms are prescribed base 
 out. The effect of a two-degree prism base out before each 
 eye is the same as that of a four-degree prism base out before 
 one eye, because in each case the images falling on the two 
 retinas are deflected inward an equal amount. 
 
 Only weak prisms can be worn ; strong prisms have the 
 effect of breaking up the light which passes through them, 
 producing what is known as Chromatic Aberration. The 
 weight of such prisms is a further objection to them. 
 
 When a patient presents a refractive error in addition to 
 his heterophoria, instead of prescribing prisms the lens may be 
 ordered decentered. (Fig.22.) (See "Relationship of Lenses," 
 p. 34.) Decentering a convex lens inward or a concave 
 lens outward produces the effect of a prism, base in, and, of 
 course, a convex lens decentered outward, or a concave lens 
 inward, produces the effect of a prism base out. The 
 strength of the prismatic effect obtained depends upon (a) 
 the curvature of the surface, /. e., upon the strength of the 
 lens, and (b) upon the extent of the decentration. 
 
 Operation. Only in extreme cases, in which the suggested 
 methods have been tried without relief to the patient, should 
 recourse be had to operative procedure. A partial tenotomy 
 of one or both of the stronger muscles may be made. This 
 phase of the subject, however, falls beyond the scope of this 
 treatise.
 
 CHAPTER XIII. 
 SPECTACLES AND EYE GLASSES. 
 
 INTRODUCTION. 
 
 The most satisfactory method of wearing glasses is in 
 spectacle frames. The advantages of spectacles over eye 
 glasses are : (a) They can be worn \vhen eye glasses cannot 
 be kept in place; (b) They maintain cylindrical lenses at 
 their proper axes; (c) They cannot fall off, no matter what 
 may be the position of the patient's head. 
 
 On the other hand, the fact that eye glasses, especially the 
 rimless variety, are much less conspicuous and more becom- 
 ing to most people, gives them the advantage whenever the 
 cosmetic effect is an important consideration. Glasses will 
 be more cheerfully endured if they are becoming than if they 
 are not so. The lighter weight of eye glasses is an added ad- 
 vantage. Eye glasses are of doubtful value and ordinarily 
 should not be prescribed when the patient's correction calls 
 for strong cylinders, because a slight drooping of the lenses, 
 by changing the axes of the cylinders, alters their refractive 
 effect. 
 
 SPECTACLES. 
 
 There are two principal varieties of spectacle mountings, 
 viz., those having metal rims encircling the lenses, called 
 "full frames," and those without such rims, known as "rim- 
 less" or "skeleton" mountings. 
 
 Full frame spectacles are more durable than the rimless, 
 owing to the protection afforded the lenses by the metal 
 rims. (Fig. 57.) The chief objection to this form of spec- 
 tacle is the fact that the rims interfere with vision, thus 
 proving a source of annoyance to the patient. A further ob- 
 jection is the fact that the rims render the frames more con- 
 spicuous and also add to their weight. 
 
 The rimless or skeleton mountings are neater in appear- 
 ance, less conspicuous, of lighter weight, and do not inter- 
 fere with vision. (Fig. 58.)
 
 104 
 
 REFRACTION. 
 
 Spectacle mountings consist of a bridge or nose piece, 
 two lenses, with or without rims, and two temples or side 
 pieces which pass backward over the ears and serve to hold 
 
 FIG. 57. Showing Full Frame Spectacle Mounting. 
 
 the frame in place. The frames may be constructed of gold, 
 silver, nickel or steel; various alloys are also used, the chief 
 constituent of which is tin. Gold, by virtue of its superior 
 wearing qualities, has proved most satisfactory. Silver 
 
 FIG. 58. Showing Rimless or Skeleton Spectacle Mounting. 
 
 frames are soft, easily bent out of their proper shape, and 
 too readily tarnish, while steel and nickel plated frames are 
 liable to rust.
 
 SPECTACLES AND EYE GLASSES. 105 
 
 REQUISITES OF PROPERLY FITTING GLASSES. 
 
 For glasses to occupy their proper position, and to be 
 worn with comfort, the following conditions must be met : 
 
 1. The optical center of each lens must be directly in 
 front of the pupil of each eye. 
 
 2. The lenses must be so far removed from the eyes that 
 the patient's lashes do not come in contact with them. 
 
 3. Both lenses must lie in the same plane and this plane 
 must be at right angles to the direction of vision. 
 
 4. The long diameter of the lenses must be horizontal. 
 
 5. The lenses must be large enough to afford an ample 
 field of vision. 
 
 6. The bridge or nose-piece of spectacles must be of the 
 proper height and width to lie flush with the nose, and the 
 temples must be long enough to pass loosely over the ears, 
 and far enough apart to avoid any pressure on the sides of 
 the face. 
 
 7. The guards of eye glasses must lie flush with the sides 
 of the nose and must neither irritate nor exert undue press- 
 ure upon the tissues. 
 
 MEASUREMENTS AND DATA FOR ORDERING SPECTACLES. 
 
 In ordering spectacle frames it is necessary that the 
 manufacturer be furnished with certain data and measure- 
 ments. These are : 
 
 (a) The prescription for each lens. 
 
 (b) The kind of frame, whether full or rimless. 
 
 (c) The material of construction, whether gold, silver, 
 nickel, steel or alloy. 
 
 (d) The interpupillary distance. 
 
 (e) The size of the lenses desired and their angle, i. e., 
 whether vertical or with their upper edges tilted forward. 
 
 (f) The dimensions, the height and breadth, of the bridge 
 or nose piece. 
 
 (g) The form of the temples, whether straight or riding 
 bows. When the latter are ordered the kind must be speci- 
 fied, whether plain or cable. 
 
 (h) The length of the temples. 
 8
 
 106 REFRACTION. 
 
 Interpupillary Distance. This is the distance between 
 the two pupils. It is best measured by holding a small 
 ivory rule before the patient's eyes, Fig. 59, and noting the 
 
 FIG. 59. Showing Method of Measuring Interpupillary Distance. 
 
 distance between the outer edge of one pupil and the inner 
 edge of the other, or from the center of one to the center of 
 the other. When the iris is very dark, or the illumination
 
 SPECTACLES AND EVE GLASSES. 
 
 107 
 
 poor, it is sometimes easier to measure the distance from the 
 outer corneo-scleral junction of one eye to the inner corneo- 
 scleral junction of the other. In the manufacture of the 
 frames the distance between the optical centers of the lenses 
 must correspond with this measurement. 
 
 Size of Lenses. The majority of manufacturers now con- 
 form to one standard S3 r stem of designating the size of 
 lenses. The latter are numbered, beginning with the small- 
 est: 4, 3, 2, 1, 0, 00, 000, 0000 and Jumbo. (See Fig. 60.) 
 The sizes most commonly used are: For male adults, 00 
 and 000 ; for female adults, 1, and 00 and for children 2, 1 
 and 0. 
 
 000 EYE 
 41x33 
 
 FIG. 60. Showing Standard L,ens Sizes. 
 
 Angle of Lenses. When the glasses are to be worn for 
 distant vision only, the lenses should lie in a vertical plane, 
 i. e., at right angles to the direction of vision. 
 
 When the glasses are prescribed for reading, the frames 
 should be tilted to lie in a plane more nearly parallel with 
 the patient's book, see Fig. 36. The lenses should also be 
 from a sixteenth to an eighth of an inch lower than for dis- 
 tance, because the eyes are directed downward. This lower- 
 ing of the lenses is effected by increasing the height of the 
 bridge. 
 
 Bridge or Nose Piece. This is best measured by trying 
 on a sample frame and making the needed corrections. The 
 width of the bridge is the distance between its lower ex- 
 tremities, while the height is the vertical distance between
 
 108 REFRACTION. 
 
 the crest of the bridge and the pupillary line (an imaginary 
 line passing horizontally through the center of each pupil). 
 
 The distance of the lenses from the eyes is regulated by 
 the crest of the bridge. To place the lenses farther from the 
 ej'es, the crest of the bridge is ordered "inset,'' /. e., it is 
 made to lie behind the plane of the lenses. To place the 
 lenses nearer the eyes the crest of the bridge is ordered " out- 
 set," i. e., it is made to lie in front of the plane of the lenses. 
 Prominent eyes and small, flat noses require an inset, while 
 deep-set eyes and prominent noses require an outset. 
 
 Temples. These are the side pieces which hold the frame 
 in place. For reading, straight temples may be ordered. In 
 the majority of cases, however, riding bows will be prefer- 
 able. These pass backward over and around the ear. Plain 
 riding temples are made of stiff, tempered wire, while cable 
 
 FIG. 61. Showing Rimless or Skeleton Bye Glass Mounting. 
 
 temples are spirally wound with a narrow, thin, metal strip, 
 making them more flexible and less liable to exert pressure 
 on the ear. 
 
 Temples are usually made in three lengths, viz., short, 
 medium and long, these being respectively five and one-half, 
 six, six and one-half inches in length. Short temples are 
 required for children and medium temples for the average 
 adult. Only when the head is unusually large need long 
 temples be supplied. 
 
 EYE GLASSES. 
 
 There are two principal varieties of eye glasses, viz., the 
 full and the rimless frames, corresponding to the two varie- 
 ties of spectacle frames already described. The rimless
 
 SPECTACLES AND EYE GLASSES. 109 
 
 mounting, on account of its lightness, simplicity and beaut}-, 
 is usually preferred. (Fig. 61.) 
 
 Eye glasses consist of two lenses, a spring, two studs and 
 two nose guards. The frames are constructed of the same 
 material as that used in the manufacture of spectacles. 
 
 MEASUREMENTS AND DATA FOR ORDERING EYE GLASSES. 
 
 In ordering eye glasses the following data and measure- 
 ments must be furnished the manufacturer : 
 
 (a) The prescription for each lens. 
 
 (b) The kind of frame, full or rimless. 
 
 (c) The material of construction. 
 
 (d) The interpupillary distance. 
 
 (e) The size of the lens desired. 
 
 FIG. 62. Showing Full Frame Eye Glass Mounting. 
 
 (f ) The kind of guards, and their angle with the plane 
 of the lenses. 
 
 (g) The distance between the upper ends of the guards, 
 (h) The distance between the lower ends of the guards. 
 
 ( i ) The shape and length of the spring. 
 
 ( j ) Whether or not a handle or hole for cord or chain is 
 desired. 
 
 Springs. The spring connects the two halves of the 
 frame. (Fig. 62.) It may be either vertical or horizontal. 
 Vertical springs are made in two shapes, the one circular, 
 called a "hoop," the other elliptical, called the "oblong" 
 spring, the choice between them lying with the patient. 
 
 Vertical or regular springs are either short, medium or 
 long. Medium springs are suited to the majority of cases. 
 Only in exceptional cases are short or long springs required.
 
 110 REFRACTION. 
 
 Studs. These are the small metal pieces which serve to 
 hold the spring and guards together and to join them to the 
 lenses. They are made of different lengths for the purpose of 
 regulating the distance between the lenses to make this coin- 
 cide with the interpupillary distance. Offset studs are de- 
 signed to place the lenses in a plane in front of the spring. 
 This is necessary in patients having prominent eyes and long 
 lashes in which the guards do not remove the lenses far 
 enough from the eyes. 
 
 Guards. These are the two pieces which rest against the 
 sides of the nose. They are measured by placing a sample 
 frame on the patient's nose and ascertaining the distance be- 
 tween the two upper ends and that between the two lower 
 ends. 
 
 There are three principal varieties of guards, viz. : 
 
 (a) Shell-faced guards, in which the bearing surface, or 
 that which lies against the nose, is covered with tortoise 
 shell or celluloid. 
 
 (b) Cork-faced guards, in which this surface is covered 
 with cork. 
 
 (c) All-metal guards. The latter are most modern, and 
 in the majority of cases the most satisfactory, because: 1. 
 They do not become foul with the accumulation of perspira- 
 tion and dirt, as do the others; 2. The}' are made with a 
 wide bearing surface to lessen the irritation on the nose, and 
 3. They are much neater in design. 
 
 The position of the lenses is largely regulated by the 
 guards. To set the lenses out sufficiently to prevent the 
 lashes from coming in contact with them, "off-set" guards 
 are used, the arms of which place the nose-pieces or shanks 
 back of the plane of the lenses. The amount of off-setting 
 depends upon the length of the arms. 
 
 To hold the lenses in a perpendicular position for distant 
 vision, or to tilt them for reading, the arms are attached to 
 the shanks at varying angles. Sample glasses fitted with 
 guards at different angles should be tried until one is found 
 which meets the requirements of the case. 
 
 When it is desired to lower the lenses, guards of the 
 proper angle and off-set, with arms attached to the lower
 
 SPECTACLES AND EYK GLASSES. Ill 
 
 end ot the shank, should be ordered. The lenses of eye 
 glasses may be still further lowered by "dropping" them, 
 that is, by attaching them to the frame above the pupillary 
 line; or in the case of rimless eye glasses, by boring the holes 
 for studs above the horizontal axis. 
 
 OTHER FORMS OF GLASSES. 
 
 In addition to the above described regular forms of spec- 
 tacles and eye glasses, there are a number of modifications 
 designed to meet the special requirements of certain patients. 
 The most important among these are the Half Oval Eye 
 Frames, Reversible Spectacles, Hook or Grab Fronts and 
 Clerical Glasses. 
 
 Half Oval Eve Frames. This form, Fig. 63, is convenient 
 
 FIG. 63. Showing Half-Oval Eye Glass Frame. 
 
 when no distant correction is used and when the patient's 
 occupation requires correction for near work alternately 
 with distant vision. These frames are used by teachers, 
 preachers and those engaged in certain kinds of office work. 
 
 Reversible Spectacles. When a patient has sight in only 
 one eye and this eye requires correction for both distant and 
 near vision these frames are useful. It is fitted with a double 
 bridge. The lens for distant vision is placed' on one side, 
 that for reading on the other. To change from the dis- 
 tant lens to that for near work the patient has merely to re- 
 verse the frame, i. e., to put them on upside down. This 
 form of spectacle is especially useful to old people having one 
 aphakic eye, the other being cataractous. 
 
 Hook or Grab Fronts. These frames, as the name
 
 112 REFRACTION. 
 
 indicates, may be hooked over a spectacle frame for read- 
 ing. They are fitted with lenses of such strength that, 
 when added to the distance lenses, the combined effect is the 
 patient's reading correction. 
 
 Clerical or Adjustable Eye Glasses. These are useful for 
 lecturers, ministers and others who are compelled to remove 
 their glasses frequently. The guards lie in the plane of the 
 lenses and readily adjust themselves to the nose.
 
 CHAPTER XIV. 
 SPECIAL FORMS OF LENSES. 
 
 BIFOCAL LENSES. 
 
 When the patient requires correction of both distant and 
 near vision, the necessity of two pairs of glasses may be ob- 
 viated by prescribing bifocal lenses. The original bifocal, in- 
 vented by Benjamin Franklin, consisted of two half-oval 
 lenses, the upper half being the patient's correction for dis- 
 tant vision, the lower half the correction for reading. This 
 form is now known as the "split" bifocal. (No. 3 of Fig. 65.) 
 
 A great many other forms have been developed, all based 
 on the original idea of affording, in one frame, both distant 
 
 FIG. 64. Showing Grooved Bifocal Lens. 
 
 and reading correction. In some of these the reading correc- 
 tion is ground on the lower part of the distance lens, in 
 others a small reading lens fits into a notch cut out of the 
 distance lens (Fig. 64), while in a third form a small "wafer" 
 lens is cemented on the back or front of the distance lens 
 (No. 1 and 2 of Fig. 65). This wafer is of such a strength 
 that, when added to the distance glass, the total refractivity 
 is the correction for reading. This form, known as the 
 cement bifocal, is the most satisfactory and is now almost
 
 114 
 
 REFRACTION. 
 
 universally prescribed. The size and shape of the wafer 
 must be suited to the taste and requirements of the patient. 
 
 Although bifocal lenses are a great convenience, they pre- 
 sent several objectionable features. The most serious of 
 these lies in the fact that the patient, when looking at the 
 floor, when ascending and descending stairs, and getting in 
 and out of vehicles, is confused by the blurring effect of the 
 reading wafer. At such times he is, therefore, compelled to 
 look under his glasses, or over the upper edge of the wafer. 
 
 The edge of the wafer, in a properly adjusted spectacle 
 frame, is below the pupil, in order that the patient's distant 
 vision be unimpaired. The wafer may be decentered some- 
 what to increase the field of distant vision. But even with the 
 most perfect adjustment, the bifocal lens is frequently a great 
 
 FIG. 65. Showing Different Forms of Bifocal L,enses. 
 
 
 
 annoyance to the patient. The annoyance of the wafer is 
 partially overcome by constructing the distance lens of two 
 thin lenses, each having a groove in its lower part and then 
 cementing these two lenses together, their plane surfaces in 
 apposition. The reading wafer is then inserted into the 
 groove between the two lenses. This form is known as the 
 "invisible" bifocal. In the heavy glasses required in cases of 
 aphakia this is the only practicable arrangement. Its chief 
 objection lies in the expense of manufacture. 
 
 PERISCOPIC LENS. 
 
 A periscopic lens is one which is convex on one side and 
 concave on the other. The concave surface has a spherical 
 curvature of 1.25 D. This curvature, conforming closely to 
 the arc of rotation of the eye, brings the border of the lens
 
 SPECIAL FORMS OF LENSES. 115 
 
 nearer the eye and does away with the large gap between 
 the edge of the lens and the face. In this way the field of 
 vision is enlarged, and the refracting surfaces of the lens are 
 maintained at right angles to the line of vision, irrespective 
 of the movements of the eye. For these reasons, a periscopic 
 lens is to be preferred to the ordinary flat lens, only the cen- 
 tral part of which is optically correct. Objects seen through 
 the outer portions of the common lens appear more or less 
 distorted, a phenomenon known as Spherical Aberration ; 
 this is more apparent in the stronger convex lenses. There 
 is in a flat lens also a prismatic effect which increases the 
 more the eye is turned from the centre of the lens. This is 
 due to the fact that the line of vision is not at right angles 
 to the refracting surface. These objections are removed by 
 the use of the periscopic lens. 
 
 Meniscus Lens. When a periscopic lens has a deeper con- 
 cave surface than 1.25 D. and a spherical refractive effect, it 
 is called a Meniscus. A converging meniscus is one in which 
 the convex surface has a greater curvature than the concave, 
 (No. 6 of Fig. 17) having, therefore, the refractive effect 
 of a convex lens. A diverging meniscus is one in which the 
 concave surface has a greater curvature than the convex, 
 (No. 7 of Fig. 17) having therefore, the refractive effect of a 
 concave lens. 
 
 TORIC LENS. 
 
 A toric surface is one which is shaped like the bowl of a 
 spoon, i. e., with one of the principal meridians more convex 
 than the other. In the case of a spoon, the meridian lying 
 crosswise of the bowl is more convex than the meridian ly 
 ing lengthwise of the bowl. The convex side of the bowl 
 represents, crudely, a convex toric surface ; the concave side, 
 a concave toric surface. 
 
 The term "Toric" is derived from the latin "tor," signi- 
 fying the base of an architectural column. Such a base has 
 two curvatures, the horizontal one being the arc of a larger 
 circle and hence less convex than the vertical one. Making 
 a vertical section of such a tor, /. e., cutting off a segment, 
 we have, roughly speaking, a plano-convex toric lens. (Fig.
 
 116 
 
 REFRACTION. 
 
 66.) By grinding into a flat block a surface conforming to 
 the tor, we have a plano-concave toric lens. (Fig. 66.) 
 
 A toric lens (Fig. 67) is usually ground with the toric sur- 
 
 FIG. 66. Showing a Tor with Sections to Represent Piano- 
 Convex and Piano-Concave Toric Lenses. 
 
 face on the anterior side, the posterior surface being a con- 
 cave sphere of either 3 D., 4.50 D., 6 D. or 9 D. strength. It 
 is used exclusively for the correction of simple, compound, or 
 mixed astigmia. 
 
 FIG. 67. Showing a Toric Lens. 
 
 As has been stated, one of the principal meridians of 
 a toric surface has a stronger refractive power than has the 
 other. The weaker is called the " base curve " and is always 
 in the same meridian as the axis of the cylinder prescribed.
 
 SPECIAL FORMS OF LENSES. 117 
 
 When the base curve has the same refractive power as 
 that of the concave spherical surface, there will be no refrac- 
 tion in this meridian. The lens will, therefore, have the ef- 
 fect of an ordinar}' cylinder, the axis of which coincides with 
 the meridian of the base curve. 
 
 Example. + 1 D. C. Ax. 90 in toric form may be ground 
 as follows : 
 
 Anterior surface Base curve : plus 6 D. in the meridian 
 of 90 ; 4 7 D. in the meridian of 180. 
 
 Posterior surface 6 D. Sph. 
 
 When the base curve has a different refractive power than 
 that of the concave spherical surface, the lens is a sphero- 
 cylinder, and the difference in refractivit3 r between the base 
 curve and the concave surface determines the strength of the 
 spherical part of the lens. 
 
 Example. + 1 D. S. C + -50 D. C. Ax. 90 in toric form 
 may be ground as follows : 
 
 Anterior surface Base curve: -r 7 D. in the meridian of 
 90 ; + 7.50 D. in the meridian of 180. 
 
 Posterior surface 6 D. Sph. 
 
 One great advantage of toric as well as periscopic lenses, 
 over the ordinary flat lens, is their lessened weight. This 
 feature is especially valuable in the prescribing of cataract 
 lenses or those having a strong spherical element, with or 
 without a cylinder. This is owing to the fact that the curves 
 are equally divided between the two sides of the lens. The 
 spherical aberration of such a lens is less than in one having 
 the entire curve on one surface. 
 
 In the prescribing of toric lenses, it is well to use an inset 
 stud, thus taking advantage of the hollow of the lens to 
 bring the latter as near the eye as possible.
 
 APPENDIX. 
 MECHANICAL AIDS TO REFRACTION. 
 
 A number of instruments mentioned in the text, and 
 others considered of value in confirming the results of the 
 test case, are here described. 
 
 FIG. 68. Showing Rear View of Ophthalmoscope.
 
 APPENDIX. 119 
 
 OPHTHALMOSCOPE. 
 
 Since the invention of the ophthalmoscope by von Helm- 
 holz some fifty years ago, the instrument has been greatly 
 improved and is made in many different forms. The Loring 
 pattern (Fig. 68) typifies the modern instrument. It con- 
 tains two independent rotatable discs, the lower disc carry- 
 ing fifteen lenses, and the superposed disc four supplementary 
 lenses. By combining the lens powers in both discs, all of 
 the positive and negative equivalents from .50 D. to 24 D. 
 are quickly obtained and automatically recorded, the posi- 
 tive and negative quantities being expressed in white and 
 red figures respectively. The lenses in the ophthalmoscope 
 are used principally for the neutralization of errors in the re- 
 fraction of the observed eye, and hence the acquirement of a 
 clear view of the intra-ocular tissues. The use of the oph- 
 thalmoscope for determining errors of refraction has been 
 fully outlined. (See Objective Tests.) 
 
 LUMINOUS OPHTHALMOSCOPE. 
 
 In this instrument (Fig. 69) the light, instead of being re- 
 flected into the eye from a wall lamp, as with the ordinary 
 ophthalmoscope, is supplied by a small electric lamp encased 
 within the handle. It passes through a strong condensing 
 lens and is then reflected into the observed eye. The ex- 
 aminer looks above the edge of the mirror, thus avoiding 
 the usual corneal reflex which frequently embarrasses a clear 
 view of the fundus. 
 
 This form of instrument has several advantages over the 
 ordinary form, viz.: 
 
 1. It can be used in either daylight or darkness. 
 
 2. It affords a larger field of view. 
 
 3. It can be brought very close to the eye. 
 
 4. It can be used at the bedside.
 
 120 
 
 REFRACTION. 
 
 The chief objection to this form of instrument as com- 
 pared with the ordinary type is the time and trouble re- 
 quired to keep it in perfect working order. 
 
 FIG. 69. Showing Luminous Ophthalmoscope. 
 LUMINOUS RETINOSCOPE. 
 
 In the Luminous Retinoscope, as in the case of the Lumin- 
 ous Ophthalmoscope, the source of light is combined with 
 the instrument (Fig. 70). An electric lamp is encased in a 
 tube facing a diagonally placed mirror by means of which
 
 APPENDIX. 121 
 
 the light is reflected into the eye. The use of the retinoscope 
 for diagnosing refractive errors is described under Retin- 
 oscopy. (P. 91.) 
 
 FIG. 70. Showing Luminous Retinoscope. 
 
 The advantages claimed for this instrument over the 
 ordinary form are : 
 
 1. It can be used in daylight. 
 
 2. It has stronger illumination. 
 
 3. It is portable. 
 9
 
 122 
 
 REFRACTION. 
 
 THE OPHTHALMOMETROSCOPE. 
 
 This instrument (Fig. 71) consists of a Luminous Oph- 
 thalmoscope with the addition of an illuminated test object 
 of multiple radiating lines, so arranged as to be projected 
 
 FIG. 71. Showing Ophthalmometroscope. 
 
 and focussed upon the retina. The rays from a small electric 
 lamp are gathered by a strong, adjustable condensing lens 
 directly over it, and projected upon the translucent test ob- 
 ject. The light from the illuminated test object is directed
 
 APPENDIX. 123 
 
 by the obliquely fixed reflector into the eye, where, by looking 
 through the peep aperture above the mirror, a perfect repro- 
 duction of the test object may be seen upon the retina. 
 
 In applying the instrument for the measurement of errors 
 of refraction the operator moves the test object up or down 
 in the tube, as required to focus it sharply on the retina, em- 
 ploying at the same time such lens power at the peep aper- 
 ture as may be found necessary to give a clear view of the 
 fundus. 
 
 In Emmetropic eyes all the radiating lines of the test ob- 
 ject will appear uniform and most distinct with the index at 
 zero. 
 
 In Simple ftyperopia all the radiating lines will appear 
 uniform and most distinct with the index upon the scale of 
 white figures, which is below zero. 
 
 In Simple Myopia all of the radiating lines will appear 
 uniform and most distinct when the index is upon the scale 
 of red figures, which is above zero. 
 
 In Simple Astigmia the radiating lines in one meridian 
 will be in focus at zero, while those in the opposite meridian 
 will appear blurred, and if, to focus the blurred lines, the test 
 object is moved down, the error is Simple Hj-peropic As- 
 tigmia, while if moved up, Simple Myopic Astigmia. 
 
 In Compound ttyperopic Astigmia the Hyperopia is 
 equal to the smallest number among the white figures, indi- 
 cating the focus of the lines in one meridian, and the astig- 
 mia to the difference between this number and that greater 
 one on the same scale, at which the lines in the opposite me- 
 ridian are focussed. 
 
 In Compound Myopic Astigmia the same conditions pre- 
 vail, excepting that the foci of both meridians are recorded 
 upon the scale of red figures. 
 
 In Mixed Astigmia the focus of one meridian is recorded 
 by the white figures, and that of the opposite meridian 
 upon the red ones. 
 
 To obtain the best results with this instrument the pupil 
 should be dilated.
 
 124 
 
 REFRACTION. 
 
 FIG. 72. Showing Ametropometer.
 
 APPENDIX. 
 
 125 
 
 AMETROPOMETER. 
 
 This instrument (Fig. 72) consists essentially of a metal 
 tube and a disc fitted with a double prism which can be re- 
 volved to any desired axis. A metal target, bearing a white 
 ring on a black background, is hung at a prescribed dis- 
 tance. The patient is instructed to place his eye close to the 
 eye-piece, his head being steadied by means of a chin rest. 
 Directing his gaze toward the target he will see two white 
 rings, their relative positions in the different meridians being 
 determined by revolving the metal disc. The position of the 
 two rings in the several meridians serves to diagnose the re- 
 fractive condition of the eye examined. 
 
 REFKACTOMETER (OEZENG's). 
 
 This instrument (Fig. 73) is designed for the purpose of 
 measuring refractive errors and is adapted especially to 
 
 FIG. 73. Showing Refractonieter. 
 
 cases in which a cycloplegic is contraindicated. It consists 
 of two metal tubes, one sliding within the other, and regu-
 
 126 REFRACTION. 
 
 lated by a rack and pinion adjustment. The outer tube con- 
 tains a concave lens of 20 D. strength, the inner one a con- 
 vex, achromatic lens. These lenses, at different distances 
 from each other, have a combined refractive effect varying 
 from + .12 D. to + 18 D. and from .12 D. to 9 D. The 
 convex spherical effects are recorded on a revolving dial at 
 the side, the concave effects on the top of the inner tube. At 
 the front of the tube is a revolving head, composed of two 
 revolving discs, containing a stenopseic slit and a number of 
 concave cylinders. 
 
 The best method employed in conducting a test is that 
 known as "fogging," /. e., overcorrecting a hyperopic or un- 
 dercorrecting a myopic eye. This causes the type to blur 
 and the accommodation to relax, so that the latent errors 
 are made manifest. By adjusting the instrument until the 
 vision is clear the amount of refractive error is readily 
 noted. By fogging and again clearing the vision while the 
 patient's gaze is directed toward the astigmic dial, the pres- 
 ence of latent as well as manifest astigmia can be deter- 
 mined. When astigmia is present it can be measured and 
 the axis verified by the use of the cylindric lenses contained 
 in the revolving discs. 
 
 PLACIDO'S DISC OR KERATOMETER. 
 
 Placido's disc consists of a target attached to a handle 
 and pierced through the centre by a small sight-hole (Fig. 74). 
 On the reverse side is an attachment for receiving lenses. To 
 use the disc the patient is placed with his back to a strong 
 light. The examiner holds the disc with the sight-hole di- 
 rectly in front of his eye, and, with the target brightly 
 illuminated, approaches the eye to be examined until the 
 outer edge of the image on the cornea coincides with the 
 corneo-scleral junction of the examined eye. By placing a 
 convex spherical lens of 3 D. or 4 D. behind the sight-hole, 
 the image will be magnified, when, if astigmia be present, 
 the black and white circles will appear oval. This test is of 
 use in determining corneal astigmia only. It has, of late 
 years, been largely supplanted by more accurate procedures.
 
 APPENDIX. 
 
 127 
 
 FIG. 74. Showing Placido's Disc. 
 OPHTHALMOMETER. 
 
 The Ophthalmometer (Fig. 75) is a device used for the de- 
 termination of corneal astigmia. It consists of a horizontal 
 tube or telescope mounted upon a movable tripod. Within 
 this tube is a combination of lenses and prisms and two 
 reflectors, technically called "mires," known as the "steps" 
 and "parallelogram." The mires are illuminated by means 
 of electric lamps. 
 
 The patient is placed so that the eye to be examined is in 
 proper position at one end of the tube, the observer focussing
 
 128 
 
 REFRACTION. 
 
 from the other end. When the instrument is properly focussed, 
 the observer will see, reflected from the cornea of the pa- 
 tient's eye, a double image of the two mires. The central 
 image, in which the mires are close together, is the one to 
 be observed. 
 
 In the "primary" or first position, adjustment is made 
 so that the edges of the mires touch and the black lines in 
 each are continuous. When these conditions are met, the 
 tube is rotated ninety degrees to the right or left. In this 
 way any difference in curvature in the meridians of the cor- 
 nea will be indicated, either by the overlapping or separa- 
 tion of the mires. 
 
 FIG. 75. Showing Type of Improved Ophthalmometer. 
 
 The mire known as the " steps " is made up of eight equal 
 divisions. The amount of astigmia is measured by the num- 
 ber of the steps overlapped or separated, one step corre- 
 sponding to 1 D. of astigmia. The amount and kind of 
 astigmia, as well as the axis of the correcting cylinder, are 
 indicated on a graduated arc attached to the tube. If the 
 image remains the same in all meridians no corneal astigmia 
 is present. 
 
 In view of the fact that lenticular astigmia is not meas- 
 ured by the Ophthalmometer the results obtained do not, as
 
 APPENDIX. 129 
 
 a rule, indicate the total error of refraction. The instrument 
 is essentially a " keratometer " and its use constitutes merely 
 a confirmatory test. 
 
 RISLEY'S ROTATING PRISM. 
 
 This instrument (Fig. 76) used in testing insufficiencies of 
 the external ocular muscles, is intended to do away with the 
 necessity of a large number of prisms. It consists of two 15 
 prisms which are revolved in opposite directions by a milled 
 head screw, thus furnishing a prism the strength of which 
 can be increased from to 30. It is made of the diameter 
 of trial lenses and can be placed before the eye in the trial 
 frame. 
 
 FIG. 76. Showing Risley's Rotating Prism. 
 PHOROMETER (STEVENS'). 
 
 The Phorometer is an instrument designed for the testing 
 of muscular insufficiencies. It contains two cells in each of 
 which rotates a 5 prism. Each cell has a border of teeth, a 
 small wheel gearing the two cells together and causing them 
 to rotate in opposite directions, thus furnishing a prism the 
 strength of which can be increased from to 10. A spirit 
 level is placed beneath the prisms to maintain them in a 
 horizontal position. 
 
 The improved instrument (Fig. 77) has added to the 
 above, Risley's Rotary Prism and Maddox Multiple Rod in 
 such manner that the Phorometer can be used independently
 
 130 
 
 KEFRACTION. 
 
 of the other two or in conjunction with them. It is equipped 
 with a wall bracket swinging freely by means of a hinge 
 connection and can be readily raised or lowered by means of 
 a geared adjustment. 
 
 FIG. 77. Showing Improved Phorometer. 
 PERIMETER. 
 
 Of no valiie in obtaining the refraction, this instrument 
 (Fig. 78) is used to measure and outline the visual field of the 
 eye. It consists essentially of a wide metal arc fastened to a 
 disc, the two being supported by an upright. A small black
 
 APPENDIX. 
 
 131 
 
 metal plate bearing a white spot is clutched to the arc in 
 such a manner that it can be moved freely along the arc 
 from end to end. The arc can be readily rotated to lie in any 
 meridian. The disc, being fastened to the arc, moves with 
 it, its purpose being to hold a small chart on which the pa- 
 tient's visual field is traced. A chin rest is supplied for the 
 purpose of steadying the patient's head. 
 
 The patient, having been placed in position, is instructed 
 to direct his gaze toward the centre of the arc which is first 
 placed in the vertical meridian. The small metal plate bear- 
 
 FiG. 78. Showing Perimeter. 
 
 ing the \vhite spot is then moved up and down along the arc 
 until it disappears from the patient's sight. The two points 
 on the arc where this occurs will correspond with two of 
 the degree numbers on the back of the arc and are indicated 
 on the chart by means of dots or pin pricks. The arc is then 
 rotated to another meridian and the same procedure re- 
 peated. When all meridians have been tested the dots on 
 the chart are joined together by means of aline, when the 
 field will have been outlined. For the purpose of comparison, 
 the chart bears the outline of a normal visual field. The im-
 
 132 REFRACTION. 
 
 proved instrument is fitted with an attachment by which 
 the results are automatically recorded on the chart. The 
 color of the slide may be changed for the purpose of testing 
 the color field. 
 
 STIGM ATOMETER . 
 
 The Stigmatometer (Fig. 79) is an instrument for testing 
 the refraction of the eye by the objective method. It casts 
 an image on the patient's retina which is plainly visible to 
 
 FIG. 79. Showing Stimatometer. 
 
 the observer. This image is focussed by adjusting the instru- 
 ment to conform to the refraction of the eye under observa- 
 tion, the adjustment being read off on the scale and the re- 
 fractive error indicated. This instrument is also a complete 
 ophthalmoscope for the direct examination. It consists of 
 a mirror, a lens, a screen, light and an operator's lens plate.
 
 APPENDIX. 133 
 
 Use as an Objective Test. The patient is placed with his 
 chin on the rest and the mirror adjusted before the eye 
 to be examined, the light being reflected into the pupil so 
 that the patient sees the image of the object. With the 
 holder containing the astigmic dial at a given point, the 
 object is rapidly moved from right to left until the lines (or 
 part of them) are visible to the operator. This image is 
 focussed by moving the screen, the figures on the bar indicat- 
 ing in diopters the amount of refractive error. If in this 
 position but one line is clearly seen, astigmia is present and 
 the object is now moved back until the line at right angles 
 is distinct, when the difference in the two readings indicates 
 the cylinder to be prescribed in addition to the sphere indi- 
 cated. 
 
 As a Subjective Test. In using the stigmatometer as a 
 subjective test, the same principles apply as in its use by the 
 objective method, the difference being that the patient is 
 questioned as to his vision. 
 
 By withdrawing the screen, which is readily removable, 
 the instrument becomes an ophthalmoscope.
 
 GENERAL INDEX. 
 
 Abbreviations, 26 
 
 Abduction, 94 
 
 Aberration, chromatic, 102 
 
 spherical, . 115 
 
 Absolute hyperopia, 58 
 
 Accommodation, 37 
 
 amplitude of, 37 
 
 diminution of, . ... 43, 84 
 
 in hyperopia, 57 
 
 in myopia, 66 
 
 in presbyopia, 84, 86 
 
 iris in, 40 
 
 mechanism of, 37 
 
 muscle of, .... 37 
 
 near point of, 37 
 
 paralysis of, 60, 61 
 
 physiology of, ... - 37 
 
 relationship of, to age, .... 86 
 
 to convergence, . 40, 41, 57, 101 
 
 to pupillary contraction, . 40, 41 
 
 relative amplitude of, ... 40, 41 
 
 spasm of, 57 
 
 Actual deviation, 95, 96 
 
 Acuity of vision, 26, 44 
 
 Acuteness of vision, .... 26, 44 
 
 definition of, 45 
 
 in astigmia, 71 
 
 in emmetropia 45 
 
 in hyperopia, 58 
 
 in myopia, 65 
 
 method of recording, 45 
 
 Adduction, 94 
 
 Adjustable eye glasses, 112 
 
 Advantages, of eye glasses, . . . 103 
 
 full frames, 103 
 
 luminous ophthalmoscope, . 119 
 
 retinoscope, 121 
 
 periscopic lens, . 114, 115, 117 
 
 rimless or skeleton frame, .. 103 
 
 spectacles 103 
 
 toric lens, ... . . 117 
 
 Age, effect of, on accommodation, 
 
 40, 41, 43, 86 
 
 convergence, . . 40, 41 
 
 pupillary contraction, .... 41 
 
 size of pupil, 43 
 
 Aids to refraction, 118 
 
 Amaurosis, 82, 83 
 
 causes of , 83 
 
 Amblyopia, 82, 83, 95 
 
 causes of, 83 
 
 Ametropia, 55, 89 
 
 forms of, 55 
 
 Ametropometer, 123 
 
 Amplitude of accommodation, 37, 40 
 
 of convergence, 39, 40 
 
 Angle of convergence, .... 39 
 
 of lenses, 107 
 
 meter, 39 
 
 prismatic, 17 
 
 visual, 44 
 
 Anisometropia, 82 
 
 correction of, 82 
 
 Annular muscle, 37 
 
 Anomalies, physical, 42 
 
 Apex of prism, ... . . 13, 97, 98 
 
 Aphakia, . 57, 82 
 
 cause of, . . .... 82 
 
 glasses for, 82 
 
 Appendix, 118 
 
 Aqueductus Sylvii, 40 
 
 Aqueous humor, 35 
 
 Argand burner, .88 
 
 Associated movement, .... 95 
 Assymetry of face in astigmia, . 71 
 
 in hyperphoria, 101 
 
 Asthenopia, 47, 96
 
 136 
 
 GENERAL INDEX. 
 
 accommodative, 47 
 
 as a cause of general disease, . 48 
 convergence or muscular, . . 47 
 symptoms of, 47 
 
 Astigmic dial, . . . . 73 
 as seen by an astigmic eye, . 74 
 lens, 20 
 
 Astigmia, 68 
 
 according to rule, . . 69 
 
 against the rule, 69 
 
 acquired, 71 
 
 assymetry of face in, 71 
 
 causes of, 71 
 
 compound hyperopic, . . . 70, 77 
 
 myopic, 71, 78 
 
 congenital, 71 
 
 corneal, 69, 126, 127 
 
 correction of, ......... 74 
 
 description of, 68, 69 
 
 determination of, 71, 72 
 
 fan of rays in, 72 
 
 forms of 69, 70 
 
 high degree of, ... 92, 93 
 
 irregnlar, 69 
 
 latent, 73 
 
 lenticular, 33, 71, 128 
 
 low degree of, 92 
 
 manifest, 73 
 
 measurement of, 75 
 
 mixed, 71, 79 
 
 movement of shadow in, . . . 93 
 principal meridians of, . 69, 70, 71 
 
 regular, 69 
 
 forms of, 69 
 
 simple hyperopic, 69, 75 
 
 myopic 70, 76 
 
 symptoms of, 71 
 
 tests for, . 72, 73, 90, 91, 92, 93 
 varieties of, 69, 70 
 
 Atropin, 60, 61 
 
 Authors' axis finder, 31 
 
 trial frame, 52, 54 
 
 Axial hyperopia, 43 
 
 myopia, 64 
 
 Axis finder 31, 32 
 
 of astigmia, 74 
 
 cylinder, 20, 22 
 
 eye, 39 
 
 principal, 17 
 
 B 
 
 Balance, muscular, 95 
 
 Band of light, 93 
 
 Base, curve, 116, 117 
 
 of prism, 13, 102 
 
 Beam of light, 15 
 
 Bi-concave lens, 24 
 
 Bi-convex lens, 24 
 
 Bidwell, 41, 42 
 
 Bifocal lens, 113 
 
 forms of, 113 
 
 Franklin, 113 
 
 invisible, 114 
 
 objections to, 114 
 
 Binocular vision, 37, 95 
 
 Blepharitis, 47 
 
 Bridge, 104, 107 
 
 crest of, 108 
 
 height of, 105, 107, 108 
 
 width of, 105, 107 
 
 Bulging cornea, 64 
 
 Burner, Argand, ... 
 
 c 
 
 Cataract lenses, 82, 117 
 
 Cement bifocal, 113 
 
 Centre, optical, . 19 
 
 Chimney, re tinoscope, . . . 88 
 
 Choice of mydriatics, 60 
 
 Chromatic aberration, 102 
 
 Ciliary, body, 37 
 
 muscles, 37 
 
 in the hyperopic eye, ... 62 
 nerve supply of, .... 40, 100 
 
 Circular spring, 109 
 
 Clerical eye glasses,' 112 
 
 Cobalt test, 99 
 
 Cocain, 60, 61 
 
 Compound lens, 27 
 
 Concave lens 15, 16, 17 
 
 mirror, 89
 
 GENERAL INDEX. 
 
 137 
 
 Cone, converging, 15 
 
 diverging, 16 
 
 Confirmatory tests, 88, 89, 90, 91,92,93 
 
 Conical cornea, 64 
 
 Conjunctival irritation, .... 47 
 
 Contraction of pupil, 62 
 
 Contraindications for use of myd- 
 
 riatics, 60 
 
 Convergence, . .... 37, 39 
 
 amplitude of, ... ... 39 
 
 angle of, 39 
 
 asthenopia, due to, 47 
 
 in hyperopia, 57 
 
 measure of, 39 
 
 near point of, 39 
 
 relationship of, to accommoda- 
 tion, ... 40, 41, 57, 101 
 to pupillary contraction, . 40, 41 
 relative amplitude of, ... 40 
 
 Converging cone, 15 
 
 meniscus, 24, 115 
 
 Convex lenses, 15) 17 
 
 Coquille, 24 
 
 Cork guards, no 
 
 Cornea, 35 
 
 astigmia of, 69, 126, 127 
 
 bulging or conical, 64 
 
 Correction of results under myd- 
 
 riatics, 61, 62 
 
 Cover test, 99 
 
 Crest of bridge, 108 
 
 Cross-eye, 95 
 
 Crossed cylinders, 79 
 
 Crown glass, n 
 
 Crystalline lens, 35, 36, 82 
 
 hardening of, 84 
 
 Curve, base, 116, 117 
 
 Curvature, hyperopia, 57 
 
 myopia, 64 
 
 Cycloplegics, 60, 61 
 
 choice of, 60 
 
 contraindications for, .... 60 
 correction of results under, 61, 62 
 
 duration of effect of, 61 
 
 strength of, 61 
 
 time required to act .61 
 
 10 
 
 Cyclophoria, 96 
 
 determination of, 100 
 
 Cylinders, crossed, ..... 79 
 
 Cylindric lens, 20 
 
 axis of, 20, 22 
 
 displacement by, 27 
 
 effect of, 21 
 
 neutralization of, . . . . 28, 29, 33 
 
 D 
 
 Data for ordering, eye glasses, . 109 
 
 spectacles, 105 
 
 Decentering a lens, ... . 34 
 
 prismatic effect of, 115 
 
 Defects, optical, of normal eye, 41 
 Deflection of light rays, .10, u, 12 
 
 Density, optical, 10 
 
 Deviation, actual, 95, 96 
 
 tendency toward, 96 
 
 Devices for measuring lenses, 31, 32 
 
 Dexter, 26 
 
 Dial, 73, 74 
 
 Dilatation of pupil, 60 
 
 Displacement by lenses, .... 27 
 
 Diopter, 25 
 
 Dioptric system, 35 
 
 Diplopia, 39, 95 
 
 artificial, 97 
 
 Direct method, 89 
 
 Disc, optic, 90, 91 
 
 pinhole 50, 5 1 
 
 Placido's, 126, 127 
 
 Displacement, by cylinders, . . 27 
 
 by prisms, 30, 97 
 
 by spheres, ........ 27 
 
 by sphero-cylinders, 27 
 
 electrical, 9 
 
 parallactic, 27 
 
 Distance of lenses from the eye, 108 
 
 Diverging, cone, 16 
 
 meniscus, 24, 115 
 
 squint, 66 
 
 Double prism, 52, 97 
 
 Dropping lenses, .... . . in 
 
 Duboisin, 60, 62 
 
 Duration of effects of mydriatics, 61 
 Dynamic refraction, 36
 
 138 
 
 GENERAL INDEX. 
 
 E 
 
 Elasticity of lens, 37, 43 
 
 Electrical displacement, .... 9 
 Electro-magnetic radiations, . . 9 
 Elongation of the eyeball, . . 63, 65 
 
 Emtuetropia, 35, 4 1 
 
 Emmetropic eye, .... 35 
 
 changes in, 42, 43 
 
 defects of, . . 41 
 
 life history of, 42, 43 
 
 English system, .. 25 
 
 Ephedrin, 60, 61 
 
 Eserin, 62 
 
 Esophoria, 96 
 
 cause of, 101 
 
 Ether, luminiferous, 9 
 
 vibrations of, 10 
 
 Euphthalmin, 60, 61 
 
 Exophoria, 96 
 
 associated with, myopia, . . 101 
 
 External rectus 94 
 
 Extrinsic muscles, 95 
 
 Eye, astigniic, ... . . 69 
 
 changes during life history of, 42 
 
 defects of, . . 41 
 
 dioptric system of, 35 
 
 emmetropic, 35 
 
 hyperopic, 43, 56 
 
 myopic, . . 63, 64 
 
 normal 35 
 
 Eye-strain, 47 
 
 Eye glasses, 109 
 
 adjustable, 112 
 
 advantages of, 103 
 
 clerical, 112 
 
 data for ordering, ..... 109 
 
 half oval, in 
 
 full frame, 108, 109 
 
 measurements for, 109 
 
 parts of , . 109 
 
 requisites for properly fitting, 105 
 rimless or skeleton, ....'. 108 
 
 F 
 
 Face, assy metry of, . . 71,101 
 
 Facultative hyperopia, . . 58 
 
 False image, ........ 95 
 
 Fan of rays, ..... ..... 72 
 
 Far point, of accommodation, 36 
 of convergence, ...... 39 
 
 in myopia, ......... 66 
 
 Farsight, .... 43, 55 
 
 Faulty meridian, ... 73 
 
 Field of fixation, ........ 95 
 
 of vision, .......... 115 
 
 First principles, ... 12 
 
 Fixation, ........ 39, 94 
 
 field of, . . . ........ 95 
 
 Focal, distance, ........ 23 
 
 length, .......... 23 
 
 line, ............ 21 
 
 Focal lines, position of, in com- 
 
 pound hyperopic astigmia, 77 
 myopic astigmia, .... 78 
 
 mixed astigmia, ...... 79 
 
 simple hyperopic astigmia, . . 75 
 myopic astigmia, ...... 76 
 
 Focus,. ........ . . 15 
 
 negative, .......... 16 
 
 principal, ....... 15, 17, 22 
 
 virtual, ........... 16 
 
 Fourth ventrical, ....... 40 
 
 Fovea centralis, ... 39 
 
 Frames, eye glass, ....... 104 
 
 spectacle, ........ 109 
 
 Franklin bifocal, ........ 113 
 
 Functional, disuse, ...... 95 
 
 failure, ......... 84 
 
 Fundus reflex, ...... 89, 91 
 
 Glass, crown, ...... n 
 
 Glasses, ..... . ...... 103 
 
 requisites for properly fitting, 105 
 Glaucoma, ......... 60 
 
 Guards, ............ no 
 
 II 
 
 Half oval frames, 
 Helniholz, . . 
 Heterophoria, . . 
 correction of, . 
 
 . . Ill 
 
 37, JI 9 
 
 .... 96 
 
 . IOO, IOI, IO2
 
 GENERAL INDEX. 
 
 139 
 
 determination of, 96, 97, 
 
 forms of, 96 
 
 Heterotropia, . . 95 
 
 Hematropin, 60, 61 
 
 Hook fronts, . . . . . in 
 
 Hoop spring, 109 
 
 Humor, aqueous, . . -35 
 
 vitreous, 35, 36 
 
 Hygiene of myopia, . . 65, 67 
 
 Hyperesophoria, 96 
 
 Hyperexoplioria, 96 
 
 Hyperopia, . 43, 55 
 
 absolute, 58 
 
 accommodation in, 57 
 
 axial, 43 
 
 causes of, . . . 43, 57 
 
 ciliary muscle in, ... . . 62 
 
 convergence in, . . 57 
 
 correction of, ...... 58 
 
 curvature, . ... 57 
 
 description of , . . . . 56 
 
 determination of, 58 
 
 facultative, 58 
 
 latent, .... 58 
 
 manifest, 58 
 
 near point in, 58 
 
 relative, ....... 58 
 
 sequelae of, . . 57 
 
 symptoms of, 57 
 
 total, 57 
 
 varieties of, .... ... 58 
 
 Hyperopic astigmia, . 69, 70, 75, 77 
 
 Hyperphoria, 96 
 
 associated with assymetry, . . 101 
 Hypophoria, 96 
 
 Illiterates, 88 
 
 Illusions, optical, 42 
 
 Image, movement of, .... 90, 92 
 
 Imbalance, muscular, . . 94 
 
 Inch system, 25 
 
 Index of refraction, ... . . n 
 
 Indirect method, 90 
 
 Inferior oblique, 94 
 
 rectus, . 94 
 
 Inset bridge, 108 
 
 Instruments used for refraction, 118 
 
 Internal rectus, ... . 39, 40, 94 
 
 nerve supply of, .... 94 
 
 Invisable bifocal, .114 
 
 Iiiterpupillary distance, .... 106 
 Iris in accommodation, ..... 40 
 Irregular astigmia, 69 
 
 K 
 
 Keratoconus, 
 Keratometer, 
 Keratoscopy, 
 
 64 
 129 
 
 Latent astigmia, 73 
 
 hyperopia, 58 
 
 Latitude of motion, . . 95 
 
 L/ens, crystalline, . . 35, 36, 82 
 
 hardening of, 43, 84 
 
 resiliency of, 37 
 
 striae in, . 42 
 
 suspensory ligament of, . . 37 
 Lenses, abbreviations for, . . 26 
 
 action of, 13, 23 
 
 angle of, 107 
 
 astigmic, 20 
 
 bi-concave, 24 
 
 bi-convex, .... . . . 24 
 
 bifocal, 113, 114 
 
 cataract, 82, 117 
 
 character of, 27 
 
 concave, 15, 16, 17 
 
 convex, 15, ^7 
 
 compound, - . .... 27 
 
 coquille, 24 
 
 cylindric, 20, 24 
 
 effect of, 21, 23, 28, 29 
 
 measuring strength of , . . 28 
 properties of f ... . . 23 
 
 relationship of, . . . . 32, 33, 34 
 
 decentered, 34 
 
 prismatic effect of, 115 
 
 displacement by, 27 
 
 distance from the eye of, 105, 108 
 
 dropping, . in 
 
 effect of, 13, 23
 
 140 
 
 GENERAL INDEX. 
 
 , forms of, 24, 27 
 
 measuring, . . . . 28, 29, 30 
 
 mechanical devices for, 31,32 
 
 meniscus, converging, . . 24, 115 
 
 diverging, 24, 115 
 
 minus, - . 16 
 
 neutralization of, 26, 27 
 
 numeration of, 26, 49, 50 
 
 periscopic, 114 
 
 plane of, 105 
 
 piano, 24 
 
 plano-concave, .... . 24 
 
 -convex, 24 
 
 -cylindric, 20 
 
 plus, 15 
 
 prescription for, ...... 26 
 
 prismatic, 13, 24 
 
 effect of, 29, 34 
 
 measuring strength of, ... 29 
 
 properties of, 29 
 
 relationship of, . . . 32, 33, 34 
 
 sizes of, 107 
 
 special forms of, 113 
 
 spheric, 14, 15 
 
 effect of, 14, 23 
 
 measuring strength of, ... 28 
 
 properties of, 23 
 
 relationship of, . . .32, 33, 34 
 
 sphero-cylindric, 24 
 
 strength of, ... ... 27 
 
 surfaces of, 17, 23 
 
 systems of measurement of, 24, 25 
 
 toric, . . 24, 115, 116 
 
 varieties of, 23, 24 
 
 Lenticular astigmia, . . . 33, 71, 128 
 
 Ligament, suspensory, 37 
 
 Light, band of, 93 
 
 deflection of, 10, n, 12 
 
 ray 9 
 
 sensation of, . . 9 
 
 wave theory of, . 9 
 
 Line, focal, . 21 
 
 Loring ophthalmoscope, . . . .118 
 
 Luminiferous ether, 9 
 
 Luminous ophthalmoscope, . .119 
 retinoscope 119 
 
 Macula lutea, 94, 95 
 
 Maddox rod, 51 
 
 test, 97, 98 
 
 Malingering, 83 
 
 Manifest astigmia 73 
 
 hyperopia, 58 
 
 Measurements for ordering, eye 
 
 glasses, . . . . 109 
 
 spectacles, 105 
 
 Measuring lenses, .... 28, 29, 30 
 
 Mechanical, aids to refraction, . 118 
 
 devices for measuring lenses, 3 1 , 32 
 
 Mechanism of accommodation, . 37 
 
 Medium, refracting, 10 
 
 transmitting, 10 
 
 Meniscus, 115 
 
 converging, 24, 115 
 
 diverging, .... . . 24, 115 
 
 Meridian, faulty, ....... 73 
 
 normal, 73 
 
 principal, 115, 116 
 
 Metal guards, no 
 
 Meter, 25 
 
 angle, .... 39 
 
 lens, 25 
 
 Metric system, ... . . 25 
 
 Minus lens, .... . . 16 
 
 Mires, 127, 128 
 
 Mirror, concave, 92 
 
 piano, 92 
 
 retinoscopic, 89, 92 
 
 test 89 
 
 Mixed astigmia, 71, 79 
 
 Monoyer, 25 
 
 Motor oculi, 40 
 
 Movements, associated, .... 95 
 
 ocular, 94 
 
 Miiller, annular muscle of, . . 37 
 
 Muscles, annular, 37 
 
 balance of, 95 
 
 ciliary, 37 
 
 extrinsic, 94 
 
 imbalance of, 94 
 
 ocular 94 
 
 moving the eyeball, .... 94
 
 GENERAL INDEX. 
 
 141 
 
 of accommodation, 37 
 
 tests of, .... 96, 97, 98, 99, 100 
 
 Mydriatics, ... . . . 60, 61 
 
 choice of, . . . . . 60 
 
 contraindications for use of, . 60 
 correction of results under, 61, 62 
 
 duration of effect of, 61 
 
 strength of, 61 
 
 time required to act, 61 
 
 Mydrin 60, 61 
 
 Myopia, 63 
 
 accommodation in, 66 
 
 axial, . 64 
 
 causes of, .... 65 
 
 correction of, 66 
 
 curvature, ... 64 
 
 description of, : 63, 64 
 
 determination of, . . . . 66 
 
 forms of, 64 
 
 hygiene of, .... . . 65, 67 
 
 measurement of, . .... 66 
 
 ophthalmoscopic appearance in, 66 
 progressive, . . .... 65 
 
 prophylaxis, ... . 67 
 
 symptoms of .......... 65 
 
 treatment of, ... . . 67 
 
 varieties of, 64 
 
 Myotics, 62 
 
 Nagel, 25 
 
 Near point, of accommodation, . 37 
 
 of convergence, 39 
 
 in hyperopia, 58 
 
 recession of, ...... 84, 85 
 
 sight, 63 
 
 Negative focus, 16 
 
 Nerve, motor oculi, 40 
 
 Neutralizing lenses, . . 26 
 
 Normal eye, 35 
 
 changes in, ... 42, 43 
 
 defects of, 41 
 
 life history of, .... 42, 43 
 
 Nose piece, 104, 107, 108 
 
 Numeration of lenses, . . 26, 49, 50 
 
 O 
 
 Objective, lens 89 
 
 tests, . . 88, 89, 90, 91, 92, 93 
 Oblique, inferior, . . 94 
 
 superior, 94 
 
 Oblong spring, 109 
 
 Obturator, . . .... 50 
 
 Ocular, movements, 94, 95 
 
 muscles, . . . 94 
 
 Oculi utrique, 26 
 
 Oculus dexter, . . ... 26 
 
 sinister, 26 
 
 Operation for strabismus, .... 102 
 
 Ophthalmometer, 127, 128 
 
 Ohthalmometroscope, . . 122, 123 
 
 Ophthalmoscope, . . 118, 119 
 
 Loring's, .... . . . . 118 
 
 luminous, 119 
 
 Ophthalinoscopy , 89, 90 
 
 Optic disc, 89, 90 
 
 Optical, centre, . . . 19, 105 
 
 defects, 41 
 
 density, . . . . . . 10 
 
 illusions, 42 
 
 principles, 9, 12 
 
 Orthophoria, 95 
 
 Oscillations of ether, ... 10 
 
 direction of, 10 
 
 rate of, 10 
 
 uniformity of, 10 
 
 Outset bridge, ... 108 
 
 Offset stud, ... no 
 
 Parallactic displacement, ... 27 
 
 Parallelogram, 127, 128 
 
 Paralysis of accommodation, 60, 61 
 
 Perimeter, 130, 131 
 
 Periscopic lens, ... . 114 
 
 Phorometer, . 129 
 
 Physical anomalies, . . .42 
 Physiology of accommodation, 37 
 
 Pilocarpin, 62 
 
 Pin hole disc, 50, 51 
 
 Placido's disc, 126, 127 
 
 Plane of lenses, 105
 
 142 
 
 GENERAL INDEX. 
 
 Piano, 24 
 
 concave, 24 
 
 convex, ... 24 
 
 cylindric, 20 
 
 Plus lens, 15 
 
 Position, primary, . . . . 128 
 
 Presbyopia, 43, 84 
 
 age of onset, 84 
 
 causes of, 43, 84 
 
 correction of, .86 
 
 description of , 43,84, 85 
 
 glasses for, 86, 87 
 
 relationship of, to age, . . 86 
 recession of near point in, 84, 85 
 
 symptoms of, 84 
 
 Prescription writing, ... 26 
 
 Primary position, 128 
 
 Principal, axis, 17 
 
 foCUS, . . 15, 17, 22 
 
 Principles, optical, 12 
 
 Prism, apex of, ... 13, 97, 98 
 
 base of, 13, 102 
 
 correcting, 98, 102 
 
 deflection by, .... 14, 29, 97 
 displacement by, ... 30, 97 
 
 double, 52, 97 
 
 effect of, 29, 30 
 
 exercise, 101 
 
 neutralization of, . . . 30 
 
 numeration of, 50 
 
 position of correcting, . 98, 102 
 
 Risley's, 129 
 
 sides forming spherical sur- 
 faces, 17, 18. 
 
 uses of, 97, 98 
 
 wearing of, 101 
 
 Prismatic, angle, .... 17 
 
 effect of decentering, . 34, 115 
 
 Properties of lenses, 23 
 
 Prophylaxis of myopia, . . 67 
 
 Punctum proximum, ... . . 37 
 
 of accommodation, 37 
 
 of convergence, 39 
 
 in hyperopia, 58 
 
 recession of, 84, 85 
 
 Punctum remotum, 36 
 
 of accommodation, 36 
 
 of convergence, .... 39 
 
 in myopia, 66 
 
 Pupil, activity of, 43 
 
 con traction of, ... 40, 61, 62 
 
 dilatation of, 60 
 
 relationship of size to age, 43 
 
 to accommodation, 40 
 
 to convergence, . . . . 40 
 sphincterof, 40 
 
 Pupillary distance, 106 
 
 line, .... . . . . 108 
 
 R 
 
 Radiations, electro-magnetic, . 9 
 
 light, ... 9 
 
 Range, of accommodation, . . 37 
 of convergence, . -39 
 
 Rays, 9 
 
 convergent, 15, 23 
 
 course of, 12 
 
 deflection of, . 12 
 
 divergent, 23 
 
 fan of, 72 
 
 incident, . 10 
 
 parallel, 23 
 
 refracted, 10 
 
 velocity of, . . . . 12 
 
 Recession of near point, . . 84, 85 
 
 Rectus, externus, 94 
 
 inferior, 94 
 
 internus, 39, 40, 94 
 
 nerve supply of, roo 
 
 superior, . . ... 94 
 
 Reflex, fundus, .... . . 89 
 
 brightness of, 91 
 
 movement of, .... . . 91 
 
 Refraction, ...... TO 
 
 aids to, . . 118 
 
 by cylinders, 21 
 
 by prisms, 14, 29, 97 
 
 by spheres, 15, 16 
 
 defined n 
 
 dynamic, ... 36 
 
 index of, ... . . . . n 
 
 laws of, 12 
 
 static, . . ....... 36
 
 GENERAL INDEX. 
 
 143 
 
 Regular astigniia, ....... 69 
 
 Relationship of lenses, . . 32, 33, 34 
 Remotum, punctum, .... 36 
 
 Requisites of, properly fitting 
 
 glasses, . 105 
 
 vision, 35 
 
 Retinal image, in astigmia, . 90 
 
 in hyperopia, 90 
 
 in myopia, 90 
 
 Retinoscope, 89 
 
 luminous, 120, 121 
 
 Retinoscopy, . . 91 
 
 chimney, . . 88 
 
 Reversible spectacles, in 
 
 Risley's prism, 129 
 
 S 
 
 Scopolamin, 60, 61 
 
 Second sight, 65 
 
 Sequelae of hyperopia, 57 
 
 Shadow, movement of, . . 91, 92 
 
 shape of, 91, 92 
 
 size of, 91, 92 
 
 test, 91 
 
 Shell guards, no 
 
 Simple, hyperopic astigmia, 69, 75 
 myopic astigmia, . ... 70, 76 
 
 Sinister, .26 
 
 Sizes of lenses, 107 
 
 Skiascopy, '91 
 
 Snellen test letters, 44 
 
 principle involved in, .... 44 
 Spasm of accommodation, ... 57 
 
 symptoms of, 58 
 
 treatment of, 59 
 
 Spectacles, 103 
 
 advantages of, 103 
 
 full frame, 103 
 
 measurements for, 105 
 
 requisites for properly fitting, 105 
 
 rimless or skeleton, 103 
 
 Spherical, aberration, . ... 115 
 
 lens, 14, 17, 27 
 
 Sphincter, of ciliary body, . . -37 
 
 of pupil, 40 
 
 Split bi-focal, 113 
 
 Springs, 109 
 
 Squint, 95 
 
 divergent, 66 
 
 Standard lens, 25 
 
 sizes, 107 
 
 Staphyloma, 65 
 
 Static refraction, 36 
 
 Stenopseic disc, 50, 80 
 
 Steps, 127, 128 
 
 Stevens' phorometer, 129 
 
 Strabismus, . 57, 95 
 
 divergent, 66 
 
 Streak of light 98, 99 
 
 Striae of lens, 42 
 
 Studs, , . no 
 
 Superior, oblique, 94 
 
 rectus, 94 
 
 Surface, refraction at, n 
 
 Sursumduction, . . 94 
 
 Suspensory ligament, 37 
 
 Sylvii, aqueductus, 40 
 
 Symptoms of, asthenopia, ... 47 
 
 astigmia, 71 
 
 hyperopia, 57 
 
 myopia, 65 
 
 presbyopia, 84 
 
 System, dioptric, 35 
 
 English, 25 
 
 metric, ... 25 
 
 T 
 
 Temples, 104, 108 
 
 forms of, . .' 105, 108 
 
 length of, 108 
 
 Test, card, 45, 46 
 
 frame, 52 
 
 letters, 44 
 
 mirror, . 89 
 
 shadow, 91 
 
 Tests, confirmatory, 
 
 88, 89, 90, 91, 92, 93 
 
 cover, . . . 99 
 
 for astigmia, 71-80 
 
 cyclophoria, 100 
 
 field of vision, 130, 131 
 
 hyperopia, 58
 
 144 
 
 GENERAL INDEX. 
 
 malingering, 83 
 
 muscles, 96, 97, 98, 99 
 
 myopia, 66 
 
 Theory of light, 9 
 
 Third nerve, 100 
 
 Thorington chimney, 88 
 
 Total hyperopia, 57 
 
 Tor, 115 
 
 Toric lens, 115, 116 
 
 advantages of , 117 
 
 Trial, case, 49 
 
 frame, 52 
 
 True image, 95 
 
 U 
 Utrique, 26 
 
 Varieties, of astigmia, .... 69, 70 
 
 bi-focals, 113, 114 
 
 heterophoria, 96 
 
 hyperopia, 58 
 
 lenses, 23, 24 
 
 myopia, 64 
 
 Ventricle, fourth, 40 
 
 Vertical springs 109 
 
 Vibrations of ether, 10 
 
 Virtual focus, 16 
 
 Vision, acuteness of, 26 
 
 binocular, 37, 95 
 
 field of .... 115 
 
 requisites of, 35 
 
 Visual, acuity, 26, 44 
 
 measure of, 45 
 
 angle, 44 
 
 field, 115 
 
 Vitreous humor, 35 , 36 
 
 W 
 
 Wafer lens, . 113, 114 
 
 Wave theory of light, 9 
 
 Wedge-shaped, beam, . . . . 21, 68 
 lens 13 
 
 Young, . 33
 
 
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