DUPLICATE m0m HXOOO 17248 ^•^V*': • r. ■ vi * ■ ■. ' •• * * ^ " , k liifei^^^i^^r IS Il2d CoUese of S^f^v^itimi anb ^urseonsi mhxatv EYESIGHT GOOD AND BAD. EYESIGHT: GOOD & BAD. A TREATISE ON THE EXERCISE AND PRESERVATION OF VISION. BY ROBERT BRUDENELL CARTER, F.R.C.S., Late H^iiiteriaii Professor of PatJwlogy and Surgery to the Royal College of Sit7-gcons of England ; Ofihihalniic Surgeon to St. George's Hospital ; Corrcsj>onding Mejuber of the Royal Medij^a-^tirin'gi^al.^cicty of Edt?iburgh. V OF THV ^ ->€ ITH NUMEROUS ILLUSTRATIONS. MACMILLAN AND CO. 1880. The Right of Translation and Reproduction is Rcscj'zcd. LONDON : R. CLAV, SONS, AND TAYLOR, BREAD STKEET HILL. PREFACE. A LARGE portion of the time of every ophthalmic consultant is occupied, day after day, in repeating to successive patients precepts and injunctions which ought to be universally known and understood. The following pages contain an endeavour to make these precepts and injunctions, and the reasons for them, plainly intelligible to those who are most concerned in their observance. It is proper to mention that some of the diagrams in the second and third chapters have been taken, with slight alterations, from Mackenzie's Physiology of Vision; and also that I have derived valuable suggestions from Die Pflege der Angen of Professor Arlt. 69, WiMPOLE Street, December, 1879. Digitized by the Internet Archive in 2010 with funding from Columbia University Libraries http://www.archive.org/details/eyesightgoodbadtOOcart CONTENTS. CHAPTER I. PAGE THE STRUCTURE OF THE EYES AND OF THEIR APPENDAGES . . I CHAPTER II. ON LIGHT, AND THE FORMATION OF IMAGES BY LENSES . . . l8 CHAPTER HI. THE FORMS AND PROPERTIES OF LENSES 37 CHAPTER IV. THE FORMATION OF IMAGES IN THE EYE ; THE ACUTENESS AND FIELD OF VISION ; THE KLIND-SPOT 49 CHAPTER V. NEAR AND DISTANT VISION ; REFRACTION AND ACCOMMODATION ; PRESBYOPIA, OR AGED SIGHT 6_^ CHAPTER VI. SINGLE VISION WITH TWO EYES ; CONVERGENCE , 8 1 viii CONTENTS. CHAPTER VII. PAGE DEFECTS OF VISION PRODUCED BY FAULTY SHAPE OF THE EYE- BALLS ; MYOPIA, HYPERMETROPIA, AND ASTIGMATISM ... 92 CHAPTER VIII. ASTHENOPIA, OR WEAK SIGHT 1 39 CHAPTER IX. COLOUR, COLOUR-VISION, AND COLOUR-BLINDNESS I $2 CHAPTER X. THE CARE OF THE EYES IN INFANCY AND CHILDHOOD .... 165 CHAPTER XI. THE CARE OF THE EYES IN ADULT AGE ; NATURAL AND ARTIFICIAL ILLUMINATION ; ACCIDENTAL INJURIES ; THE INFLUENCE UPON SIGHT OF THE GENERAL HEALTH AND HABITS OF LIVING , 188 CHAPTER XII. CONTRIVANCES FOR SAVING VISUAL EFFORT . 23O CHAPTER XIII. PRACTICAL HINTS ON SPECTACLES , 244 ' OF TMt ^ ; v.. EYESIGHT: GOOD AND BAD. CHAPTER I. THE STRUCTURE OF THE EYES AND OF THEIR APPENDAGES. I DO not intend, in this chapter, to enter with any minuteness into matters of anatomical detail, but merely to write such a general description of the parts and the construction of the organs of vision as I think every educated person should possess. In giving opinions about patients, and still more frequently in giving evidence in courts of law, medical men are often placed in positions of difficulty by the want of knowledge of those whom they address ; and it is not long since I myself was asked, by a learned judge, why it was that pictures of the interior of the eye, which were before him as matters of evidence, were painted of a red colour. The obvious answer, that they were so painted because the structures represented were themselves red, appeared to be the very last which his lordship was prepared to B 2 EYESIGHT. [chap. receive ; and it was impossible not to feel that his power of appreciating my testimony would have been increased, if he had known something more about the eyes than the fact that they are somehow useful in seeing. In speaking to patients, and more especially in giving opinions or advice which they have to communi- cate to others, similar difficulties are of frequent occur- rence; and, now that the schoolmaster is abroad, and that the elements of physiology will before long be taught in Board Schools, it will not be seemly that those who receive what professes to be a superior education should be left entirely unacquainted with facts which will be made familiar to people of smaller opportunities and of inferior social station. The organs that are sub- servient to the bodily functions which we prize most highly, and which are of inestimable importance to our welfare, cannot be unworthy of some study at the hands of every person of even ordinary intelligence. The function of each eye, taken singly, and as a condition antecedent to the act of vision, is to form upon the retina, or nervous membrane which lines the back of the organ, a sharply defined inverted image of any object which is looked at ; and, if we take the almost transparent eye of a white rabbit, removed from a recently killed animal, we may see this picture shining through its outer coats, and therefore plainly visible. The mode in v/hich the image is formed, optically speaking, will be considered in a subsequent chapter ; and it is sufficient here to say that in principle the eye almost precisely resembles the common camera-obscura I.] STRUCTURE OF THE EYES. 3 of the photographer, and that the image produced upon the retina is precisely analogous to that which is pro- duced upon the sensitive plate, in order that it may be fixed there by the chemical action of light. By means of the optic nerve, the retina, which receives the image, is in direct ccnnection with the brain; and it is the brain which interprets the visual appearances and com- pletes the act of seeing. How this completion is effected we do not at present know; but we do know that it is dependent upon the sharpness and complete- ness of the retinal image ; and that, if this image is blurred or imperfect, accurate seeing is impossible. We shall have no concern in these pages with the element of consciousness or of intelligence, which forms part of the visual act ; and we have only to take note of the share in vision which belongs to the eyes themselves, that is to say, of the retinal images, and of the circum- stances which contribute to or detract from their perfec- tion. Among these, the first to be considered are the formation, shape, and proportions of the eyeball. The human eye, as shown in an enlarged diagrammatic horizontal section in Fig. i, is a nearly spherical body, which, in the adult, measures a little less than an inch in diameter. It consists chiefly of transparent fluids contained in membranes, which are called the tunics of the eyeball. Of these tunics, which lie over one another concen- trically, like the scales of an onion, the external is divided into two parts, called respectively the sclerotic and the cornea. The sclerotic (S, Fig. i) covers the B 2 EYESIGHT. [chap. posterior four-fifths, the cornea (c, Fig. i) covers the front of the eyeball only. The front portion of the sclerotic is visible through the conjunctiva, or delicate ON Fig. I. skin covering it, a portion of which is shown at Ca, and forms what is called the white of the eye. In early childhood it has a bluish tint, and towards the decline of life it generally becomes somewhat yellow. I.] STRUCTURE OF THE EYES. 5 The back of the sclerotic rests upon a cushion of fat which occupies the cavity of the orbit, and which surrounds and protects the optic nerve. In texture, the sclerotic is tough, flexible, and opaque. It is thickest at the hinder part of the eyeball, and becomes gradually thinner towards the front. The cornea is inserted into the front margin of the sclerotic something in the manner of a watch-glass into its setting. It is not only as transparent as the most pellucid water, but as smooth as the most polished mirror ; and to these qualities the eye is indebted for its brightness. The transparency, polish, and proper curvature of the cornea, are all of them of essential importance to vision. These two membranes, taken together, com^plete the outer coverings of the eyeball, and they are maintained, by the fluids within their cavities, and by the blood which circulates in the vessels of the internal mem- branes presently to be described, in a certain state of tension or fulness, the degree of which can be estimated by pressing upon the eye with the points of the fore- fingers through the upper lid. By the maintenance of proper tension, the spherical shape of the eyeball is preserved ; and this shape has led to a custom of describing the several parts of the organ by words borrowed from the language of geography. Thus, the eyeball as a whole is often called the " globe" of the eye ; and its central points in front and behind are" its anterior and posterior " poles." A line uniting these two poles is the axis of th*e globe ; which, it may here 6 EYESIGHT. [chap. be observed, is not quite coincident with the axis of vision hereafter to be mentioned. Upon the axis of the globe, a little behind its centre, and near the letters V H, is a point which remains stationary in all movements of the eye, and is called the centre of rotation. An imaginary plane, midway between the poles, and dividing the globe into an anterior and a posterior half, is called the equator ; and the halves themselves are the anterior and posterior hemispheres. It would be possible, of course, to speak also of the upper and lower hemispheres ; but all planes which are at right angles to the equator, such, for example, as would divide the cornea vertically or horizontally through its centre, are called meridians. For descriptive purposes, too, the cornea is divided into imaginary quadi^ants ; and, by means of these several technicalities, it is pos- sible to describe localities in the eyeball with great precision and exactness. The inner surface of the sclerotic is lined by the next tunic, C/2, which is chiefly composed of a network of blood-vessels, and is called the choroid. The spaces between the blood-vessels of the choroid are filled up by a dark coloured substance called pigment, which consists of fine granules, and which, together with the circulating blood, gives to the interior of the eye, as shown by the ophthalmoscope, a reddish brown or red tint, which varies, in some degree, with the complexion of the individual. Somewhat in front of the equator the choroid beconies thicker, and its inner surface is thrown up into a circle of plaits or folds, Pr, which are I.] STRUCTURE OF THE EYES. 7 known as the ciliary processes, and which constitute, collectively, the ciliary body. These processes, in the human eye, are seventy in number, and they form a ring- or circle about a fifth of an inch in breadth, ex- tending forwards from their origin to the insertion of the cornea. The pigmentation of the choroid renders it a perfectly opaque lining to the sclerotic, so that no light can enter the cavity of the eye excepting through the cornea. The front border of the ciliary body is closely united with both the cornea and the sclerotic at their line of junction, and from this line the choroid is prolonged in the shape of a circular vertical curtain, I, called the iris, which has a central perforation, and which bears to the cornea something of the relation of the dial of a watch to the glass. The space between the iris and the cornea, A C, is the anterior chamber ; and this, together with the smaller space or posterior chamber, P C, behind the iris, is filled by a clear fluid, called the aqueous humour, v/hich consists almost entirely of pure water, and which is in contact with the iris on both sides. The iris is so called from the various tints, such as blue, grey, brown, hazel, and others, which it displays in different people ; and, as seen through the cornea and the intervening aqueous humour, it forms the most con- spicuous part of the eye. The central perforation in the iris is the pupil, a circular opening of variable size, and, in young and healthy eyes, of a bright black. The pupil undergoes dilatation in passing from a bright into a dull illumination, and in looking at distant objects ; 8 EYESIGHT. [chap. and it contracts In passing from a lesser to a greater brilliancy of light, or in looking at near objects, and during sleep. The dilatation and contraction of the pupil are performed by two sets of muscular fibres which enter into the structure of the iris, the circular and the radiating ; and the changes which are thus produced serve to regulate, and approximately to render uniform, the quantity of light which is admitted into the interior of the eye. Like the choroid, the iris is abundantly supplied with pigment, especially on its posterior surface ; and it is thus rendered wholly im- pervious to light, which finds entrance into the eye only through the pupil. The pupil appears black, be- cause the cavity inclosed by the dark choroid and iris contains transparent humours, which suffer light to pass into them freely, but reflect back little in return, and that little only in such a direction that it eludes obser- vation unless looked for in a particular way. If we look from a gas-lighted place at a hole opening into an adjoining chamber, we receive but little light from thence, even although the hole may allow it to enter freely. On the inner surface of the choroid, and closely in contact with it, we find the retina, R, the third and most important of the ocular tunics, to which, indeed, the others are merely protective or containing mem- branes. The retina is the immediate continuation of the optic nerve, ON, which extends from the brain to the eyeball, perforates the sclerotic and choroid, and immediately spreads out into a thin lamina over the I.] STRUCTURE OF THE EYES. 9 surface of the latter. The point of entrance of the optic nerve is nearly on the horizontal meridian of the eyeball, and about the tenth of an inch inwards from the posterior pole, so that it is the right eye which is represented in the figure. The office of the retina is to receive the pictures which are formed within the eye by the light reflected from objects of vision, and, through the medium of the optic nerve, to transmit the resulting visual impressions to the brain. Precisely as the sense of touch is not diffused uniformly over the surface of the body, but is more acute in some parts, as in the finger tips, than in others, as in the back of the hand ; so also the retina is not equally sensitive to luminous impressions over its whole surface, but in the highest degree in the vicinity of the posterior pole, in a part called the yellow spot, f s. A line drawn from any object looked at to the yellow spot is called the " axis of vision ;" and from the spot the sensitiveness of the retina gradually diminishes to the equator. The retina does not extend so far forwards as the choroid, but terminates a little in front of the equator, at the pos- terior border of the ciliary body, by an irregular margin, OS, the indentations of which have received the name of Ora serrata. Immediately behind the iris, and in contact with the margin of the pupil when it is contracted, is the lens, or crystalline lens, L, a solid body which is inclosed in a delicate, transparent, and structureless membrane, the capsule, and is connected, through this, by an equally delicate membrane, the suspensory ligament, with the lo EYESIGHT. [chap. anterior border of the ciliary processes. In shape, the lens resembles an ordinary biconvex glass lens, except that it is less strongly curved in front than behind. In youth, it is a soft or moderately firm and highly elastic body, perfectly transparent and colourless, and as bright as the brightest crystal. With the advance of life it becomes harder, and sometimes of a slightly yellow tint, without losing its transparency ; but in old age it often becomes opaque or nearly so, a change which constitutes the affection known as senile cataract. The opaque lens varies in colour from a yellowish brown to a grey or white tint, and in some few cases has been almost black. Whatever its colour, it prevents vision by intercepting the passage of the rays of light to the retina ; but the sight may in such cases be restored by removing the opaque lens from the eye by a surgical operation. Its place will then be supplied by watery fluid ; and, as this does not possess the refracting power of the organ which it replaces, vision is indistinct unless aided by a convex lens of the kind known as a cataract glass. Such a lens, when properly selected, performs, to some extent, the function of the natural lens which has been taken away. The large cavity of the eyeball behind the lens, which is called the vitreous chamber, is filled by a colourless, transparent, gelatinous substance, V H, som.ewhat re- sembling the white of egg, but less fluid, and known as the vitreous humour. Its office appears to be to main- tain, between the crystalline lens and the retina, an interval, like that which, in a camera, intervenes between 1.] STRUCTURE OF THE EYES. ii the lens which forms the imag^e and the screen upon which it is received. This interval, in the camera, might be filled with fluid as well as with air, except that the substitution would render it necessary to place the screen farther from the lens. The vitreous humour, although to the naked eye it appears homogeneous and transparent, yet contains numerous fine filaments distributed throughout its sub- stance ; and these filaments carry upon them the re- mains of the cells from which they were produced, or the germs of other cells which might in favourable circumstances be called into activity. The cells and filaments are themselves transparent, but it sometimes happens that the degree or index of their refraction difi'ers from that of the fluid in which they float, and they are then liable to cast shadows upon the retina, precisely as a glass rod might cast a shadow when immersed in a tumbler of water. Such shadows are projected outwards by the sense perceptions as visible floating objects. The magnitude of each shadow will depend partly upon the magnitude of the cell or filament producing it, and partly upon the distance of the latter from the retina, or screen which receives the shadow ; and hence such shadows are larcrest and most con- spicuous in short-sighted eyes, which, as a matter of formation, are longer from front to back than others. The shadows appear as floating fibres or particles, which are most plainly seen against a white cloud or white wall, or other bright and uniform surface. They have recci\'ed the name of Miisccs volitantes, and 12 EYESIGHT. [chap. it is one of their characteristics that they never actually- shut out any point which is being looked at, but float a little above, below, or to one side of it ; the reason of this being that the filaments do not appear to exist in the axis of vision, but only in the lateral parts of the humour. Muscse may be rendered more than usually conspicuous by any circumstances which alter the density of the fluid part of the humour, and thus in- crease, either temporarily or permanently, the difference of refraction between this fluid part and the filaments or cells. Musc^ exist, more or less, in all eyes, and may easily be found by looking at a white cloud through a pinhole in a card ; but they are usually harmless, and only require mention on account of the anxiety which they sometimes cause to persons who are unacquainted with their nature. The free movements of the eyes are of such a kind that they involve no change of place, but only rotation upon a central point in each eyeball. The pupils can be directed upwards, downwards, outwards, and inwards, and also in intermediate directions, as upwards and outwards, without the eyeball as a whole changing its position. These movements are accomplished by the agency of six muscles. By muscles, we mean certain flat or round structures, composed of fibres or threads, which, under the influence of the will or of other stimuli, can be made to contract or shorten themselves, and which relax and return to their former length as soon as the will or the stimulus ceases to act. By such shortening or contraction, all our voluntary movements I.] STRUCTURE OF THE EYES. 13 are performed ; and there are also muscles of a some- what different kind, which act independently of the will. The flesh of animals, which we use as food, consists almost entirely of muscles, and these may easily be unravelled into the fibres of which they are composed. Four of the six muscles of each eye are called the 7'ecti^ or straight muscles ; and these have their fixed points of insertion behind the eyeball, upon the bones of the skull at the apex of the orbital cavity. Passing for- wards, they embrace the eye closely; and are inserted into the sclerotic, one on the inner, or nasal side, one on the outer, or temporal side, one above, and one below, at a distance of about a quarter of an inch from the margin of the cornea. When the inner straight muscle contracts, it rotates the anterior pole of the eye inwards, or towards the nose ; and the posterior pole outwards, or towards the temple. When the contraction ceases, the eye is brought back, by the external straight muscle, to its position of rest in the centre of the lid-opening. The other two muscles, which from their respective directions are called the superior and inferior oblique, have their fixed points in front of the equator of the eye, on the inner side of the orbit, the one above, the other below, the inner angle of the lids ; and they arc inserted into the sclerotic, on the outer side, between the equator and the posterior pole. By the single or combined contraction of one or more of these pairs of muscles, the eyeballs can be moved in every direction with greater rapidity than any other portion of the body ; and there is no other group of muscles which, in 14 EYESIGHT. [chap. the waking condition, is called upon to perform so many and such fine gradations of movement as those of the eyes. The stimulus by which these movements are called for is mainly that of the images produced upon the retina, and communicated as visual sensations to the consciousness ; and the response to such sensations is often almost inconceivably rapid. For example, a case is recorded in which a man Avho was seated on a chair was stabbed with a penknife by another who was standing over him, and who struck downwards upon his eye. The blade pierced the upper lid, but pierced the eyeball below the cornea ; so rapid had been the action of the superior straight muscle in rolling the globe upwards in order that it might be less exposed to injury. Nearly all other voluntary muscles require to be rested, even during waking hours, by occasional repose or by change of effort ; but from this requirement those which move the eye appear to be practically exempt. Next after the muscles, the parts most important from our present point of view are the eyelids. Each of these is formed or moulded upon a thin plate of sub- stance resembling cartilage, or gristle, which preserves the shape and firmness of the lid. They are covered externally by very fine and more or less wrinkled skin, internally by a delicate membrane called the conjunc- tiva. Between the skin and the cartilage the lids are crossed by thin bundles of muscular fibres, directed in curves from the inner to the outer angle, and extending from near the margin of the lids to that of the orbital I.] STRUCTURE OF THE EYES. 15 opening both above and below. These fibres collec- tively form the muscle which closes the lids, and which is named the orbicularis by anatomists. The upper lid is raised or held open by another muscle, the antagonist of the orbicularis, which passes along the roof of the orbit from the bones of the skull, and is inserted into the whole length of the upper border of the cartilage ; but the lower lid sinks to the open position by its own weight, as soon as the action of the closing muscle ceases. This muscle is not only called into action during sleep, or to shut the eyes designedly, but also for the performance of that very frequent involuntary closure and re-opening of the lids by which the surface of the eyeball is moistened, and freed from particles of atmospheric dust or other impurity ; a provision which is of the highest importance for the preservation of the polish and transparency of the cornea. From the outer borders of the lids spring the eyelashes, which afford protection against the entrance of coarse dust, insects, and other foreign bodies or intruders ; and, immediately within the eyelashes, on the very margins of the lids, are the openings of numerous fat glands which are situated in the substance of the cartilage. These glands, which are called Meibomean, after the name of their discoverer, furnish a greasy secretion which lubricates the edges of the lids, and which acts as a barrier to the escape of moisture from the eyes over the cheek. The free mobility of the lids over the eyes is per- mitted by the conjunctiva, which, after lining the lids as i6 EYESIGHT. [chap. described, is formed into loose folds at their attached margins, and from these is reflected, or carried over, to the surface of the eyeball itself, where it covers the whole of the front portion of the sclerotic, and, as .shown at Ca, furnishes an extremely delicate coating even to the cornea. The conjunctiva secretes a portion of the fluid by which the surface of the eyeball is moistened ; but the greater part of this fluid is furnished by the lacrymal or tear gland, which is situated above and to the outer side of the globe in the cavity of the orbit, and discharges its secretion by several fine tubes beneath the upper and outer portion of the upper lid. The tear secretion, when formed only in natural quantity, is partly evaporated by the air, and partly absorbed by the conjunctival surface ; but the residue is conveyed away from the eye by a series of tubes which conduct it into the nose, so that the tears do not flow over the cheeks except during weeping or other periods of excessive secretion. The tear passages commence near the inner angle of the lids by two fine round white openings, which are plainly visible when either the upper or lower lid is drawn somewhat outwards, and which lead into two delicate tubes or channels. These channels terminate in the tear bag, a small pouch or cavity beneath the skin of the inner side of the nose, immediately below the inner angle of the eyeHds. From this pouch a single duct leads downwards into the nose. The tear passages are liable to become closed or obstructed by local inflammation ; and, when this I.] STRUCTURE OF THE EYES. 17 happens, a troublesome overflow of moisture is pro- duced. If this condition is not rectified, it may be followed by the establishment of a new and often per- manent opening upon the cheek, which becomes a source of much disfigurement and inconvenience. CHAPTER II. ON LIGHT, AND THE FORMATION OF IMAGES BY LENSES. It has been stated In the foregoing chapter that vision is dependent upon the formation of images upon the retina, by h'ght reflected from the objects which are seen ; and, before considering how these images are produced within the eye, it is necessary to explain the formation of analogous images by artificial means. We mean by light a mode of force which is resolvable into molecular wave movements, and which is well known to us in many of its manifestations. Disregarding the wave movements, an account of which is not necessary for the purpose now before us, we may speak of light as a force which emanates from luminous bodies, and which travels in straight lines in all directions. If we hght a candle, and place it in the middle of an otherwise dark room, all parts of the room equidistant from the flame will be equally illuminated ; and the amount of light falling upon equal surfaces will be inversely as the squares of their distances from the source of illumination. CH. II.] THE FORMATION OF IMAGES. 19 If we take two equal sheets of paper, say each six inches square ; and place one of them six inches from the candle and the other twelve inches from it, the latter will only receive one fourth as much light as the former; and, if we remove the latter to a distance of eighteen inches, it will then only receive one ninth part as much light as the former. For equal distances, however, the illumination afforded by a central source of light is equal in all directions. In considering the course of the rays of light which proceed from any luminous object of appreciable magni- tude, such as a candle flame, we must not regard this as a single source of radiation, but as an assemblage of luminous points of infinite minuteness, from every one of which rays are given out equally in all directions. The rays from these several points must necessarily cross one another as they diverge. As long as the rays of light pass only through the atmosphere, they continue to travel in straight lines ; but, when they fall upon any other substance, they may be variously diverted from their original course. If the substance is opaque, the rays falling upon it are arrested, and are partly absorbed into its material, partly re- flected or turned back ; and the relative proportion in which one or the other of these results is produced is greatly dependent upon the quality of the surface, whether it be dull or polished. If the substance is transparent, much of the light passes through it, either with or without a change in its direction, but part is always reflected. It is by means of the light reflected C 2 20 EYESIGHT, [chap. from surfaces, whether they are opaque or transparent, that images are formed upon the retina, so that the surfaces themselves are rendered visible. A substance capable of absorbing or transmitting all the light which it received, if any such substance existed, would be invisible. A ray of light which strikes upon any surface, and is turned back, obeys precisely the same physical law as a billiard ball under the same conditions ; that is to say, the angle of reflection is equal to the angle of incidence. In Fig. 2 the line A B represents a reflecting surface, C D is a line perpendicular to this surface, and E is a luminous point. A ray from the point E, which falls upon the surface A B in the point D, making with the surface the ancrle E D B, and with the perpendicular the angle E D C, II.] THE FORMATION OF IMAGES. 21 will be reflected in the direction D F, making the angle A D F equal to B D E, and the angle CDF equal to C D E^ When the reflection takes place from a polished surface, and is tolerably complete, the reflected ray appears to an observer who is situated in its course, as at the point F, to proceed from a point in a backward prolongation of the line D F, and as much behind the surface A B as the real source of light is in front of it. Thus, if A B were a mirror, and E a candle, an observer at F who was unconscious of the presence of the mirror, and from whom the actual candle was screened, would think that it was standing at the point G, instead of in its actual situation. Illustrations of the operation of this law- must be familiar to every one ; and it forms the basis of a great number of common optical illusions, employed by conjurors and others. A ray of light which falls upon a transparent sub- stance, and which passes through it, or is said to be transmitted, is seldom transmitted in an unchanged direction. This can only happen, indeed, under one of two conditions : first, if the new medium is of the same refracting power as the atmosphere ; secondly, if the ray falls perpendicularly upon the surface. Under all other conditions, the transmitted ray is more or less bent out of its course ; and it is then said to be refracted. The law is that light, passing in an oblique direction, either out of a rare into a dense medium, or out of a dense into a rare medium, is refracted in different degrees according to the relative refractive powers of the two media ; towards the perpendicular, if the new 22 EYESIGHT. [chap. medium is the more dense, and from the perpendicular, if the new medium is the more rare. Let A B, Fig. 3, represent a ray of hght, passing Fig. through air, and incident obHquely on the surface of water at B. Instead of pursuing its original course to c, Fig. 4. it will be refracted into the direction B D, and drawn towards / r, a line perpendicular to the surface of the water at the point of incidence B. Let AB, Fig. 4, il] the formation of images. 23 represent a ray of light falling in like manner obliquely at B on the surface of a denser medium than water, say glass. It will be drawn still more towards the perpen- dicular/ r, being refracted into the direction BD, instead of pursuing its original course to C. If the dense medium is bounded by plane surfaces, parallel to each other, as is represented in Figs. 3 and 4, on quitting the dense medium, to enter one which is less dense, say air, the ray will undergo a second and opposite refraction. The ray B D, quitting obliquely the second surface of the dense medium, is refracted from the perpendicular p' r\ and takes the direction D E, which, in both cases, is parallel to C F, the original course of the ray. By comparing the angle of incidence B D/' with the angle of refraction E D /, in the two cases, it will be evident that the refraction of the ray B D, on quitting the second surface, is greater when the refracting medium is glass than when it is water. It being understood that different transparent bodies, whether solids or liquids, possess very different degrees of refractive power, not measurable by mere density, and that the amount of the deviation of the refracted ray from its original course is always proportionate to the refractive power of the medium, it is next necessary to explain that by varying the obliquity of the surface of the refracting medium, in respect to the incident ray or rays, we are able to produce any particular deviation we wish to obtain, whether in respect of degree or direction. If we wish to produce a great degree of deviation, we give an increased obliquity to the refracting surface ; if 24 EYE^GHT. [chap. we wish a small degree of deviation, we employ a re- fracting surface of which the obliquity is slight. This may be illustrated by Fig. 5, in which A, B, C, D, are supposed to be rays of light passing from a rare into a dense medium. The ray A meets the surface of the medium perpendicularly, that is, without any obliquity ; therefore, there is no deviation. The ray B. meets the D C Fig. 5. Fig. 6. refracting surface with a slight obliquity ; therefore, there is a small degree of deviation. The deviation of C from its original direction is greater than that of B, and that of D greater than that of C, in proportion to the increas- ing obliquity of the refracting surface. In the figure, all the rays are represented as coming to a focus F, but this is not essential to the principle. 11.] THE FORMATION OF IMAGES. 25 If the rays are passing from a dense into a rare medium, as in Fig. 6, the same principle is appHcable. To produce a great degree of deviation, we must give an increased obliquity to the surface relatively to the ray. In A the deviation is null, because there is no obliquity. In B, C, and D, the deviation increases with the obliquity of the refracting surface. A slight consideration of the facts above stated will A\, be sufficient to suggest that, if the surfaces of a dense medium be inclined to one another, the refraction which the ray will undergo at the second surface, instead of re- storing it to its original course, as was the case in Figs. 3 and 4, where the surfaces were parallel, will augment its deviation. Fig. 7 represents a dense medium, with two surfaces inclined to each other. The ray A B is at the first surface refracted into the direction B D, towards 26 EYESIGHT. [chap. the perpendicular / ; but at the second surface, it is carried into the direction D E, away from the perpen- dicular /'. The second refraction, then, carries the ray still farther from its original course. By a simple appli- cation of this principle, it is manifest that, when a pencil of rays passes through a refracting surface or surfaces, the rays may be bent into determinate directions by determinate outlines of the surfaces, and that variations in the outline will produce corresponding variations in the refraction. It is also manifest that the surface of any medium intended to bring a pencil of rays to a focus must be a curve ; and, further, that the surface of a dense medium employed for this purpose must be convex, both in the case in which we wish to produce convergence by transmitting the rays from a rare into a dense medium, and also when the transmission is from a dense into a rare medium. We may now apply the principles above explained to the determination of the figure of a dense medium, which shall be fitted for collecting rays to a focus. Let the luminous object be very remote, so that the rays proceeding from it may be considered as parallel ; for, although all the unrefracted rays existing in nature are more or less divergent, yet, when they proceed from great distances, their actual deviation from parallelism is insensible. Let A, B, c, D, E, Fig. 8, represent these rays. Only one of them, c, by continuing its straight course, can arrive at the point F. The surface of the dense medium should be presented at right angles to this ray, at i, so that the ray may pass through the medium without II.] THE FORMATION OF IMAGES. 27 deviation. Those rays, B and D, which are situated near to the direct or central ray C, will require but a small degree of refraction to enable them to reach the focus F, and this small refraction will be produced by a small degree of obliquity in the dense medium at the points h and k. In proportion as the rays A and E are more distant from the central ray, a greater amount of refrac- tion, and consequently a greater obliquity of surfaces at g and /will be required to bring them to the same focus. On the presumption that the rays passed through a Fig. b. ' medium ot uniform density, they would converge to a focus, then, at F. Let Fig. 9 represent the same rays A, B, C, D, E, entering the same medium as before, but instead of the medium being continued, let it be supposed to terminate at the curved surface 7;/, ;/, o,p, q, so that it now forms a double convex lens. The central ray C proceeds at right angles through both surfaces, and reaches f' or F, without deviation. The rays B, D, A, E are refracted towards the perpendiculars on passing into the dense medium at the 28 EYESIGHT. [chap. points //, k,g, I, but on quitting it they are refracted from the perpendiculars to the surface of the rare medium, at the points n, /, m, q. This new refraction increases the convergence of the rays, and brings them to a focus F', nearer to the dense medium than the former focus F. The result of the continual change of direction in the refracting medium is a regular curvilineal surface, ap- proaching to the spherical. By giving such surfaces to refracting substances they become fitted to produce, with Fig. 9. more or less exactness, the convergence of parallel rays to a focus ; and, by making the dense medium convex on both sides, both conspire to produce the desired effect. The distance of the focus behind the medium depends on the refracting power of the substance employed, and on the degree of convexity of its surfaces. The greater the convexity of the two surfaces, and the greater the refractive power, the nearer will be the focus. We have next to apply the foregoing principles to the il.] THE FORMATION OF IMAGES. 29 formation of images by lenses ; and for this purpose it is necessary first to consider the results of the ordi- nary transmission of light in straight lines from its source. If we take a card, as A, Fig. 10, perforate it by a central hole about one-tenth of an inch in diameter, and then place it between a lighted candle and a white screen, in an otherwise darkened room, we shall see an inverted image of the candle flame upon the screen. The light, radiating from the flame in every direction, strikes partly upon the imperforated part of the card, Fig. 10. and is thereby either reflected or absorbed ; but that which passes through the hole continues its original direction unchanged. Hence, if we suppose the hole to be opposite the centre of the flame, the ray of light from the top of the flame, which descends to reach the hole, continues descending after having passed through it, and thus arrives at the lower part of the screen B ; while that from the lower part of the flame, ascending to reach the hole, arrives at the upper part of the screen. The rays from intermediate points of the flame cross in the same manner, but of course, strike the screen nearer 30 EYESIGHT. [chap. to its centre ; and the rays from lateral points cross right and left in the same way. The result is that the surface of the screen is illuminated in a patch which copies the shape of the flame from which the rays proceed, but in an inverted position. The inversion may be still farther illustrated by taking another screen, and gradually in- terposing it between the flame and the perforation. If we bring this second screen slowly downwards, as it comes in front of the top of the flame, the bottom of the image will disappear. If we bring it upwards, the top of the image will disappear ; and, if we bring it across from right or left, the opposite side of the image will in each case be obliterated. Hence we see that in every direction, laterally as well as vertically, the inver- sion of the image is complete. If we place two lighted candles before the card, two images will be formed upon the screen ; and, if we move the candle which stands nearer to us, the position of the image which is farther from us will be altered, showing that the images are completely reversed. If we use a row of candles, rays of light flowing through the hole from all of them will form on the screen as many images as there are flames, each image being as clear and distinct as if there were only one. By this experiment we obtain evidence of the great tenuity of light, which allows the rays from several distinct sources to pass together through the hole, travelling on slightly different lines, and without any commingling or confusion. The size of the image formed in the manner above described is dependent upon two factors ; the distance II.] THE FORMATION OF IMAGES. 31 of the screen from the hole, and the distance of the flame from the hole. A glance at the figure will show that, as the lines of light are continually divergent after passing through the hole, they become more widely separated the farther they go, and the size of the image which they form must be increased in a corresponding degree. On the other hand, the nearer the flame itself is to the hole, the greater will be the inclination of the lines which proceed from its boundaries, and the more rapid their divergence afterwards. The law is, that the linear magnitude of the image will bear the same pro- portion to that of the object, as the distance between the aperture for transmission and the screen on which the image is received bears to the distance between the same aperture and the object. The absolute magnitude of the image, or the surface covered by it, increases directly as the squares of the distances of the screen from the aperture of transmission ; and decreases in- versely as the squares of the distances of the object from the same aperture. Thus, at one inch from the aperture, the image covers a certain extent of surface ; at the distance of two inches it covers four times that surface, at the distance of three inches, nine times that surface, and so on. If the object is at the distance of one inch from the aperture, the image will have a certain absolute magnitude ; if the object be removed to a dis- tance of two inches, the image will be diminished to one-fourth ; if the object be removed to the distance of three inches, the image will be diminished to one- ninth, and so on. 32 EYESIGHT. [chap. The images formed in this manner, or, as it is called, by radiation, are never of any great distinctness. It will be obvious that all their brightness is dependent upon the amount of light which passes through the small aperture described ; and it is easy to see that by dimin- ishing this aperture we may diminish light to the extent of obtaining no image at all ; while, if we seek more light by enlarging the aperture, we render the outlines Fig. II. so indistinct that they soon cease to be recognisable. In Fig. II we have the same conditions as in Fig. IQ, but with a larger aperture in the screen A. This aper- ture, CD, would then receive from every point of the flame not a ray of light only, but a cone of rays, and this cone would continue to widen its base as it pro- ceeded towards the screen B, on which it would ulti- mately form a round patch of light, as at gh, ik, and II.] THE FORMATION OF IMAGES. 33 each of these patches would mingle with the similar patches coming from other points of the flame, in such a manner as to produce merely a general illumination of the surface, without any definiteness of outline. In order to obtain an image which is at once sufficiently luminous and sufficiently defined in shape, it Is necessary to have recourse to some contrivance by which the rays of light proceeding from each point will be united in a Fig, 12. corresponding point upon the screen, and this require- ment is fulfilled by means of that- bending or refraction of the rays which we have seen to be a property of lenses. Let us repeat, in Fig. 12, the conditions shown in Fig. II, with the exception that we fill the aperture C D by a double convex lens, and that we place the screen B at the focal distance of this lens. In these circumstances, the cone of light issuing from the summit D 34 EYESIGHT. [chap. of the flame, being refracted by the lens, will be re- united at the point I, the direction of the axial ray of the cone remaing unchanged, but all the others being brought to unite with it. The cone of light from the base of the flame will in like manner be brought to a focus at k^ and the cones from every intermediate point will form corresponding focal points, so that an inverted image of the flame will appear upon the screen B at \ky the whole of the light from the flame which is received by the surface of the lens will be concentrated upon the space \k, and the image will of course be much more vivid than when formed by the mere ex- clusion of the lateral parts of the cones, as in Fig. lo. On the principles already laid down, the shorter the focal length of the lens, and therefore the nearer the image, the smaller it will be. A powerful lens, or one of short focal length, gives a small and very bright inverted image ; a lens of low power gives an image which is larger, but which is less bright, because, assuming the lenses to be of the same superficial magnitude, each image is formed by the same quantity of light. A perfect illustration of the mode In which an image is formed by a lens is furnished by the photographer's camera, now so universally familiar. This instrument con- sists essentially of the lens and screen shown in Fig. 12, with the difference that these form portions of a box or chamber, that the screen is of ground glass, so that, being transparent, the image can be seen through it, and that the lens is furnished with an adjusting screw, II.] THE FORMATION OF IMAGES. 35 by which it may be focussed, as it is said, for objects at different distances. By means of such an apparatus, any one who so desires may become well acquainted, after a few simple experiments, with all the conditions under which inverted images are ordinarily produced. Among the first things which may be observed by the aid of a camera is, that the nearer the object is to the lens, the greater must be the distance between the lens and the screen, in order that a clear and well-defined image may fall upon the latter ; and the reason of this is not far to seek. There is, for every lens, a constant distance at which it will bring to a focus rays which fall upon it in a state of parallelism. Let us suppose, in the case of a given lens, that this distance is ten inches. It is obvious that, if the rays which fall upon it are not parallel, but divergent or spreading out from their point of issue, a certain portion of the power of the lens will be consumed, so to speak, in rendering them parallel, before it can begin to render them divergent ; and hence their union in a focal point will be delayed, or will only occur farther away from the lens, than if they were parallel originally. In like manner, if the rays are already convergent when they reach the lens, part of its work will be already done ; and the focal union will occur sooner, or nearer to the lens, than if the rays were parallel. In estimating the power of a lens, we always take its focal length for parallel rays as the basis of com- putation ; and this is called its principal focal length, or, more commonly, its focal length only. It is, of course, invariable ; while the distances of its foci for convergent D 2 36 EYESIGHT. [cu. ii. or divergent rays will depend, in every case, upon the degree of the deviation of these rays from parallelism. Strictly speaking, all light exists in nature in the form of divergent rays, but those which proceed from a far distant point to fall upon so limited an area as that of a small lens may, as already said, be considered and treated as parallel. As soon as the luminous point or other object approaches the lens, however, the diver- gence of the rays becomes very appreciable ; and so the camera, when arranged to give a clear image of the horizon, would give only a blurred and confused image of objects on the other side of a room. In order to render the latter image as clear as the former, either the distance between the lens and the screen must be in- creased, or else the power of the lens itself must be increased, as by the addition of a second one. Unless one or other of these changes were made, the screen would intercept the rays of light before they were brought to union, as if it were in the position shown by the dotted line B' in Fig. 12, and an imperfect or indistinct picture would be produced. We shall find, hereafter, that a similar provision for adjustment is necessary in the eye itself, in order that it may receive, with equal clearness, images of the many and variously distant objects to which it is from time to time directed. CHAPTER III. THE FORMS AND PROPERTIES OF LENSES. We have seen already, in Fig. 7, that a dense trans- parent medium, the two sides of which are indined towards one another, refracts a ray of Hght away from its thinner and towards its thicker portion ; and to this elementary fact all the varieties of lefraction by lenses may be reduced. The word lens is used to signify any piece of glass or other transparent substance, which is used for the purpose of producing refraction ; and it is manifest that lenses may be of various figures. The lens with inclined sides, which forms the basis of them all, is called a prism. A, Fig. 13, and the other best known forms are the following : — A Double-convex Lens, B, is bounded by two convex spherical surfaces, whose centres are on opposite sides of the lens. It is equally convex when the radii of both surfaces are equal, and unequally convex when the radii are unequal. A Plano-convex Lens, C, is bounded by a plane sur- face on one side, and a convex on the other. 38 EYESIGHT. [chap. A Menisms (that is, a little moon, or crescent), D, is bounded by a concave and a convex surface, and these two surfaces meet, if continued, so that the element of convexity preponderates. A Double-concave Lens, E, is bounded by two concave spherical surfaces, whose centres are on opposite sides of the lens. A Plano-concave Lens, F, is bounded by a plane surface on one side, and a concave on the other. A Concavo-convex Lens, G, is bounded by a concave and a convex surface, the radius of the concave surface being shorter than that of the convex, so that the sur- faces do not meet if continued^ and the element of concavity preponderates. It has already been explained that, when a ray of light passes through a prism, denser than the surround- ing medium, the total deviation of the ray is in all cases from the vertex. The general effect of any lens may be understood by resolving it into two prisms. If the bases of the prisms, of which the lens is supposed to be III.] PROPERTIES OF LENSES. 39 formed, be turned towards each other, the lens must be convex, and the total deviation of the rays which Fig. 14. pass through it will be towards its central axis ; but if the bases are turned from each other, the lens must be Fig. 15. concave, and the rays of light will be bent from its axis. The rays of light, A B, Fig. 14, are refracted by the 40 EYESIGHT. [chap. convex lens, C, precisely as they would have been by the two inscribed prisms, D, E ; and in the same way the concave lens, F, Fig. 15, resembles in its operation the effect of the prisms G and H. On this principle, the six lenses, B, C, D, and E, F, G, Fig. 13, form two classes ; the first three hting co7tvergent and the last three divergent. The first three either cause rays of light to converge, or lessen their divergence; and the last three either cause them to diverge, or lessen their convergence. The lenses which are thinner at the edge than in the middle are convergent, and those which are thicker at the edge than in the middle are divergent. The first class are often called magni- fying glasses y and are used by those whose eyes are too flat in shape, as well as by persons advanced in life ; while the second class are often called diminishing glasses, and are used to aid the distant vision of people who are short-sighted. In all the foregoing varieties, the lenses are portions of spherical surfaces, and curve equally in all directions from their centres or axes to their edges, so that all the light which falls upon them, above, below, or on either side of the centre, is equally refracted. They are solid bodies, such as would be formed by the revolution of the sections, shown in Fig. 13, around the horizontal axis on which they are drawn. Besides these spherical lenses, much use has been made of late years of lenses of another form, which are portions of cylindrical sur- faces, and which refract only those rays of light which fall upon them in certain definite directions. The III.] PROPERTIES OF LENSES. 41 cylindrical lenses in common use are the plano-convex and the plano-concave varieties ; and these would be formed, not by the revolution of sections C and F, Fig. 13, but by building up these sections so as to give them an appreciable height from the surface on which they rest. In Fig. 16, A A represents a plano-convex cylindrical lens, and the dotted lines show the cylinder of which it forms a part. A plano-concave cylindrical lens is of n — V . V Ji V r' -^^j k ^^^ ^ ^^^^ Y i. ^-^ Fig. 16. Fig. 17. analogous form, only its curved or refracting surface is concave instead of being convex. The action of a plano-convex cylindrical lens upon light is shown in Fig. 17. In this figure, the parallel rays of light, h h h, which fall upon the convex surface of the lens in a vertical plane, or in a plane coincident with the axis of the cylinder of which it forms part, are not refracted at all. They only encounter and pass through a piece of glass with parallel sides, and this does not appreciably change their direction. They enter the lens 42 EYESIGHT. [chap. parallel, and they emerge parallel on the other side. But the rays vv, vVy vv, which fall upon the glass in successive horizontal planes, that is, in planes perpen- dicular to the axis of the cylinder, encounter a strongly curved refracting surface, and are brought to foci accord- ingly. Hence, while a spherical convex lens, such as a common magnifying glass, brings the rays which fall upon it, say the rays of the sun when the lens is used as a burning glass, to a ioQ^X point, a cylindrical convex glass brings such rays only to a focal line ; since those only are united which pass through it in a direction transverse to its axis, and those which pass through it in the same direction as its axis are left without any deviation from their former course. We have already seen that the power of a convergent or convex lens is expressed by the distance of its prin- cipal focus, that is, of the point at which it would cause parallel rays to unite after they had passed through it, as at F, Fig. 14. The power of a divergent lens is expressed, in a similar manner, by the distance of the point f'. Fig. 15, from which parallel rays appear to diverge after they have passed through it. In both cases this distance is called the focal length ; and a convex and a concave lens of similar focal length, if combined, completely neutralise one another, and produce the effect of a piece of plane glass. In order to describe the power of a lens, we say that it is of such or such a focal length, and, until very recently, this length was commonly expressed in inches. For microscopes, telescopes, and optical instruments IIL] PROPERTIES OF LENSES. 43 generally, the inch scale is still in use ; but for spectacles it has been superseded by a new and more convenient nomenclature. Spectacle lenses are chiefly made in three countries, England, France, and Germany, and the inches of these three countries are not coincident. The test cases used by surgeons were until recently fitted with lenses graduated in French inches ; and a prescription based upon these was often taken to an optician who supplied lenses graduated in English or in German inches, and who therefore failed to fulfil the intentions of the prescriber. Moreover, as, in the inch scale, a lens of one inch focal length was the unit of the series, all weaker lenses, being parts of this unit, were necessarily expressed in fractions — a system which gave much unnecessary trouble. The faults of the inch scale had long been sources of inconvenience ; but the manu- facturers of lenses were able for a time to place difficulties in the way of the adoption of a better system, which they feared would render their inch-scale grinding tools obsolete, and would thus entail upon them the expense of manufacturing new ones. By the energy of Professor Bonders, of Utrecht, this difficulty was at last overcome ; and we now use, for spectacle purposes, a scale which is based upon the French metre, and which has a lens of one metre focal length for its unit. This unit is called a dioptric ; and, by having a weak lens as the unit instead of a strong one, nearly all others come to be multiples of this unit instead of being parts of it, so that they can be expressed in whole numbers, instead of in fractions. The lens of one dioptric being No. i of the series. 44 EYESIGHT. [chap. No. 2 Is a lens equal to two of the former — that is, it is of double the power of the dioptric, or of half the focal length. It is two dioptrics, and its focal length is half a metre. No. 3 is equal to three dioptrics — that is, its focal length is one-third of a metre. Throughout the series, every whole number expresses the number of dioptrics to which the lens so numbered is equal ; and hence, from whole number to whole number, all the intervals are the same. There is a difference of one dioptric between No. 6 and No. 7 ; and there is equally a difference of one dioptric between No. i and No. 2. This difference, however, is sometimes too great for prac- tical purposes ; and hence a few quarter and half dioptrics have been added to the lower power's of the scale. These introduce tile simple decimal fractions 0*25, 0*50, and 075 ; but these fractions are $0 easily manipulated that they cause no inconvenience. A quarter-dioptric is a lens having a focal length of four metres ; a half-dioptric has a focal length of two metres ; and three-quarters of a dioptric has a focal length of orie metre and one-third. For ordinary purposes the quartet-dioptrics of the series go up to No. 3 ; and the half-dioptrics to No. 6. For higher powers, such fine divisions are seldom needed but the uniformity of the scale affords a ready means of supplying them. If we want a lens of six and a half dioptrics, or of eight and a half dioptrics, we have only to add 0-50 as an addition to 6 or 8, and the requirement is at once fulfilled. In former times, before the principles which should govern the employment of spectacles had been investi- III.] PROPERTIES OF LENSES. 45 gated with scientific accuracy, and when people expected opticians to prescribe spectacles as well as to make and to sell them, the lenses in customary use were commonly distinguished by arbitrary names or numbers. A manu- facturer made, say, twelve different powers, of no definite relation to one another, and he numbered them from one to twelve as a matter of convenience. The numbers really meant nothing, so that the number two of one maker might be the number three of another, or vice versa. Sometimes the glasses, especially in the lower powers of convex lenses, were called by arbitrary names expressive of their supposed properties, as " preservers," *' clearers," and such like rubbish. The period of a definite inch nomenclature commenced about i860 ; and the public are now beginning to think of their spectacles in terms of inches. When a patient says " I have been using No. 12," he generally means that he has had glasses of twelve inches' focal length, not some indefinite number twelve of the pre-scientific period. For those who have thus become accustomed to think in inches, the change to the new nomenclature, like all other changes, may at first be a little confusing ; and there are many who, on hearing of a lens of five dioptrics, will desire to refer it to the old scale before they can perfectly realise its optical value. It will much facilitate the required translation of the thoughts if we consider that a metre is equal to 39*337 English inches ; that is, for all practical purposes, it is equal to forty inches. This means, using English instead of French inches, that the No. i is the old No. 40, or, more correctly, the old i-40th. In like 46 EYESIGHT. [chap. manner, No. 2 of the dioptric scale is the old i-2othi and No. 4 is the old i-roth. There are many of these coincidences, the chief of which are shown in Fig. 18, where the horizontal line represents a metre. Besides the three instances already mentioned, it will be seen that three dioptrics coincide, nearly, with 13 inches ; 5, with eight inches ; 8, with five inches ; 10, with four inches ; and so on throughout. If we wish to translate dioptrics into French inches, we must consider the metre equal to 36 inches instead of to 40. The coincidences obtained by either method are important, and show that 10 a * s S -4 8 10 13 20 Fig. i! Dioptrics English Inches the difficulties raised by manufacturers in the way of the adoption of a metrical scale were mostly imaginary ; since every glass ground upon the old inch tools has its proper value in the dioptric scale, and all that is neces- sary is to define and register this value. For the purpose ■of doing so, Dr. Snellen has invented a very ingenious instrument, called a " Phakometer," which it is not necessary here more precisely to describe, but by means of which it is possible to ascertain the focal length of any lens in dioptrics in a few seconds ; and also to as- certain, what is often of great importance, whether the centre of the lens corresponds with its optical axis. We III.] PROPERTIES OF LENSES. 47 shall see hereafter that inaccuracies in this respect are frequent and fruitful sources of inconvenience. In writing about lenses, in order to avoid the constant repetition of the words convex and concave, it is usual to distinguish lenses of the former kind by the prefix of the phis sign, and those of the latter kind by the prefix of the mimis sign. Thus, + 3'0 signifies a convex of three dioptrics; and -30 signifies a concave of three dioptrics. The materials of which lenses are composed are either glass, or the form of natural rock-crystal which is commonly called pebble. In former times, when the manufacture of glass was less perfect than at present, this material did not long retain complete transparency ; when exposed to atmospheric influences it became more or less turbid from some spontaneous decomposition or re-arrangement of its elements ; and hence pebble, which always retained its transparency, was much more highly prized. For many years, however, a perfectly stable, transparent, and homogeneous glass has been available for the purposes of the optician ; and the only advan- tages of pebble over glass are such as depend upon the greater hardness of the former, which is therefore not liable to become scratched in use. The refractive power of pebble is also greater than that of glass, so that for equal focal lengths the convexity or concavity of a pebble lens is somewhat less than that of a glass one, and on this account, as well as from the greater strength of the material, the pebble lenses may be made com- paratively thin and light. A pebble lens may readily be 48 EYESIGHT. [cH. ill. distinguished from a glass one by its greater coldness to the tongue, pebble being a better conductor of heat than glass ; and also by placing it between two plates of tourmaline and holding it up to a window. With .the glass, no effect is produced, but with pebble the light is polarised, and rings of colour become visible. The ordinary advantages of pebble lenses may be more than neutralised if they have not been cut from the original block in the right direction. The material has the curious property of being bi-refringent in one particular direction ; that is, the ray of light passing through it in this direction is split up into two ; and two images of the object from which it proceeds are produced. In order to make a perfect pebble lens, its axis must be at right angles to the axis of double refraction ; for otherwise, although the thickness will not be sufficient for two images to be produced, the single image may nevertheless be more or less blurred or bordered. The only security against this for the ordinary purchaser is to buy of an optician of repute, who will be more desirous to supply lenses of the best quality than to make the largest possible number out of a given piece of pebble ; but the matter may easily be tested by the plates of tourmaline already mentioned. If the lens be properly cut, the rings of coloured light will be circular : if not, they will be elliptical or more or less irregular in shape. A little instrument containing the tourmahne plates is kept by all opticians, and the purchaser may always ask to try his pebble lenses in this way for himself. CHAPTER IV. THE FORMATION OF IMAGES IN THE EYE ; THE ACUTENESS AND THE FIELD OF VISION ; THE BLIND-SPOT. The crystalline lens of the eye is an instrument precisely resembling, in its optical effects upon light, a double convex lens made by art ; and, together with the cornea and the aqueous and the vitreous humours, which somewhat assist its operation, it forms what are called collectively the refractive media. For the sake of sim- plicity, we may regard the collective media as forming a single lens ; and, in an ideal or natural human eye, the focal length of this lens is precisely equal to the length of the axis of the eyeball, so that its focus is precisely upon the retina. When this is so, the retina receives a sharply defined image of all objects before the eye which are within the lateral range of its vision, and which are sufficiently remote to transmit approximately paral- lel rays. Like that of a camera, this image is, of course, inverted ; and it may be seen in the eye of any recently killed animal by the simple expedient of removing a ^ E so EYESIGHT. [chap. piece of the opaque sclerotic and choroid without inter- fering with the retina itself If we hold up the eye so prepared, we shall see from behind, upon its retina, an inverted image of the object to which its cornea is directed. If we use the eye of a white rabbit, in which the choroid is destitute of pigment and the sclerotic itself is transparent or translucent, we may see the inverted image without any preparation. Recalling what has been already stated, that every visible object is rendered visible by the light proceeding from it, whether this be self-luminosity or light derived Fig. 19. from other sources and reflected ; and also that every such body must be regarded as consisting of an infinite number of luminous 'points, from each of which light radiates equally in all directions, so as to form a cone the base of which rests upon any surface larger than the point itself, we may easily trace out the forma- tion of the retinal image. In Fig. 19, let us suppose that the disc A B is a visible object, radiating light from its surface in the manner described. Of the innumer- able luminous points in this surface we will take only two, the extremities of the diameter A B. From the IV.] FORMATION OF IMAGES IN THE EYE. 51 point A, a cone of light falls upon the surface of the cornea, but the outer portions of this cone are inter- cepted by the opaque iris, and, being reflected from its surface, render this surface visible. The central portion of the cone, between in and n, falls upon the area of the pupil, which in the diagram is some- what dilated, and passes through it to enter the chamber of the eye. The axial ray of the cone proceeds with but little deviation to the retina, which it strikes at the point a, a little above the centre ; and the outer rays of the cone are made to converge, by the action of the refracting media, to this same point a as their focus. Precisely the same thing happens with the cone of rays proceeding from the point B, which is in like manner brought to a focus on the retina at b, a little below the centre. Assuming the disc to be vertical before the eye, with D to the right and C to the left of the spectator, the rays from D would be focussed upon the left side of the retina, and those from c upon the right. The rays from all intermediate points would be brought to foci upon intermediate points of the retina, and the resulting image would be absolutely inverted. That which applies to a disc applies equally, other conditions being the same, to a visible body of any other form ; and we only obtain clear retinal images when the light from every point of the object can be brought to a focus upon the corresponding point of the retina in an inverted position. It was formerly supposed that the blackness of the E 2 52 EYESIGHT. [chap. pupil, and the invisibility and apparent darkness of the Interior of the eye, were due to the absorption of light by the pigment of the choroid. This is not the case ; it being the fact that the eye returns a portion of the light which It receives. The returning rays, however, retrace their paths of entrance, and thus go back to the luminous point from which they issued. Hence an observer, standing in front of an eye and endeavouring to look into Its Interior, must himself intercept the light by which that interior was pre- viously Illuminated ; or, if he fails to Intercept It, he will no longer be in the path of the rays, and will see nothing. By the simple contrivance of a perforated mirror, fitted with lenses adapted to different conditions, the late Mr. Charles Babbage discovered a means of rendering every detail of the Interior of the eye visible ; but his Instrument fell Into the hands of a practitioner who failed to appreciate its value, and It thus remained unnoticed until, four years later. Professor Helmholtz applied himself to the same problem, and contrived what he called an ophthalmoscope. The instrument of Helmholtz gave only a feeble illumination ; and that of Babbage was shortly afterwards re-Invented by Professor Ruete, and has been the occasion of highly Important discoveries with regard to the diseases of the eye and their treatment. The ophthalmoscope is now indispensable to the medical practitioner, but its uses are foreign to the subject matter of these pages. Assuming the eye to be directed straight forward, as along the line EF Fig. 19, it is manifest that it will not IV.] ACUTENESS AND FIELD CF VISION. 53 only receive rays of light from the point F, but also from many points above and below, to the right and to the left of F, and that these, for a considerable lateral range, will be brought to foci within the eye. Of the resulting picture, however, onl}- a small part is clearly visible ; for the reason that the sense of sight is far more acute in the central than in the lateral parts of the retina. If we suppose the disc, ABCD, in Fig. 19, to be of large dimensions, and the eye to be directed to its central point F, this would be seen clearly. The lateral parts of the disc would also be seen, in the sense that they would be recognised as existing, and as forming parts of the object of vision, but they could not be seen with any exactness or minuteness. If there were anything attractive in any of these lateral parts, the eye of the observer, in order to scrutinise this and to see it accurately, would have to turn away from the central point to the new object of vision ; and the portion of the picture which can be seen accurately at one time is much smaller than most people, prior to special observation, would believe. As far as our sense perception is concerned, the retinal picture might be exquisitely finished in the centre and only roughly sketched at the borders, but this is. not so, and it is the acuteness of sight that fails, not the clearness of the lateral parts of the image. In this book, at the ordinary reading distance of fifteen inches, it is not possible to decipher more than about ten letters on the same line without a change in the direction of the eye, although this change is so sliHit that some 54 ■ EYESIGHT. [chap. care Is needed in order to abstain from making it. In technical language, the whole lateral extent of vision is called the Field of vision ; and we are said to see directly with the central part of the retina, and in- directly with the lateral parts. Indirect vision is of great value for many purposes, • and especially for giving us information as to the directions in which it is desirable for direct vision to be exerted. On this account, the indirect is sometimes called the de- fensive part of the field ; since it gives warning of the approach of large objects, and saves people from being exposed to many dangers. There are certain diseases of the eye in which the outer part of the field of vision is lost, so that the sight is circumscribed as if by look- ing through a tube ; and, in these cases, although central vision may be good, and the patient able to read small print, there is yet great difficulty in guiding the footsteps and in avoiding obstacles, especially moving obstacles as in the street. There are many persons with contracted visual field who in one sense can see tolerably, and yet who would not be safe in a crowded thoroughfare. The loss of lateral or indirect vision renders them unable to ascertain correctly the relative positions of objects, and entirely conceals from them many which they would require to see in order to guide their steps with safety. An exceedingly curious example of the effect of con- traction of the field of vision was lately related to me by an old gentleman, who had suffered from a malady which produces this effect, but whose remaining central IV.] ACUTENESS AND FIELD OF VISION. 55 vision I had been able to preserve by an operation. With the aid of spectacles he could read such type as that of this book perfectly, but he was somewhat short- sighted, and without spectacles even his central vision was a little doubtful. Standing one day at the entrance to the garden in front of his house, he was much puzzled by the odd movements of two things on the ground ; things which he thought were two black birds of unknown species, hopping about and behaving very strangely. They turned out to be the feet of a market woman who had brought something for sale, and whose body was invisible to him so long as her feet were in view. Much ingenuity has been expended, at various times, in endeavours to explain how it is that inverted images upon the retina produce erect vision of the objects which the images represent. The most probable of these explanations is that the nerve elements of the perceptive layer of the retina, which consist of rods or cones, having their long axes directed towards the centre of the eyeball, naturally refer the impressions which are made upon each of them to the directions from which the light proceeds. Thus, the cones of the upper portion of the retina, being directed downwards, and receiving the rays of light from below, naturally refer or trace them to objects at a lower level than the eye ; while those of the lower portion, being directed upwards and receiving rays of light from above, as naturally refer or trace them to objects above the level of the eye. In this way, it can readily be under- stood that the inversion of the retinal image may be 56 EYESIGHT. [chap. self-correcting, when it is regarded as a source of the sensory impressions which are communicated to the brain. The central part of the retina, at the posterior pole of the eyeball, is called, on account of its colour, the yellow spot, and it is over the yellow spot, that is to say, for the parts of the general image which fall there, that vision is most acute. Within the yellow spot itself there is a central depression in which the acuteness reaches its maximum, and the limit of acuteness appears to de- pend upon the size of the smallest nerve elements of which the retina is here built up. These elements are cone-shaped, and are of extreme minuteness ; and it seems to be essential to the visibility of an object that its retinal image should be large enough to cover at least one cone. The size of the image depends upon two factors, the actual size of the object and its dis- tance ; and these determine what is called the visual angle, or the angle formed between two lines drawn from the extremities of the object to meet at the nodal point or optical centre of the eye, a point near the posterior surface of the crystalline lens. In Fig. 20, C being the nodal point of the eye, and A B an object, A C B is the visual angle of that object, and ab is, the magnitude of its image upon the retina. But the smaller object, a'b', which is nearer to the eye, is seen under the same visual angle and forms an image of the same magnitude ; while the object D E, which is equal in size to AB, but nearer, is seen under the larger visual angle D C E, and forms the larger retinal image de. In IV.] ACUTENESS AND FIELD OF VISION. 57 order that the retinal image may be of the necessary- size to excite perception, the object producing it must, of course, be seen under a certain visual angle ; and it has been experimentally determined that square letters, which have limbs and subdivisions equal in breadth to one-fifth the height of the letters, and which are at such a distance that their height is seen under a visual angle of five minutes, are distinctly legible to the normal eye. This principle has been applied, by Dr. Snellen of r~-~~.. A' D B B Fig. 20. Utrecht, to the construction of test-types which afibrd a means of testino; the acuteness of vision with exact- ness. His letters are drawn of the proportions men- tioned, and of various magnitudes, each distinguished by a number which indicates the distance, in metres or parts of a metre, at which the height of the letter will be seen under a visual angle of five minutes, the breadth of its limbs under an angle of one minute, and at which the letter as a whole should be legible. The acuteness of vision is then expressed by the distance of the 58 EYESIGHT. [chap. test-types from the eye, divided by the number of the smallest letter which can be recognised with certainty at that distance; the resulting fraction being reduced to its lowest terms. If the distance be six metres, the person who can read letters of the corresponding num- ber at that distance has vision equal to 6-6ths, or equal to I ; and this is taken as the normal standard, although it is not a severe one, and, especially by young people, Fig. 21. is often exceeded. A person who can only read number 9 at the six metres has vision equal to 6-9ths, or 2-3rds of the normal. One who can only read No. 6o has vision equal to 6-6oths, or i-ioth of the normal, and so on. Fig. 21 exhibits two examples of Snellen's letters, which should be legible at twenty-four and at nine metres respectively. The possession of an accurate test of this kind is of IV.] ACUTENESS AND FIELD OF VISION. 59 great use to surgeons, who formerly had no means of estimating vision exactly, or of saying in what degree it had improved or had undergone deterioration. The im- pressions of individuals are often utterly untrustworthy, and nothing is more common than for people who never had half vision in their lives to represent that they have always been remarkable for surpassing and unusual powers in this respect. It is obvious that a man who cannot tell a cow from a horse across a street may be perfectly satisfied with himself as long as he has no pre- cise standard by which to measure himself with others ; and, through the operation of a pardonable human frailty, while we find numbers of people who admit that their sight is not as good as it once was, it is rare indeed to find any one who will admit that his or her sight was never good at all. In the case of children, as I shall hereafter have occasion to say at greater length, nothing is more common than for defective sight to be punished as obstinacy or stupidity ; and the existence of a visual defect seems to be about the last thing which ever sug- gests itself to the blundering self-sufiiciency of an ordinary schoolmaster or teacher. For my own part, I have long learned to look upon obstinate and stupid children as mainly artificial productions ; and shall not readily forget the pleasure with which I heard from the master of the great elementary school at Edinburgh, where 1,200 children attend daily, that his fundamental principle of management was that there were no naughty boys and no boobies. I think it very desirable that all parents should test the vision of their children from time 6o EYESIGHT. [chap. to time ; and I have appended to this volume a page of print for the purpose, which may be detached and hung up against a wall. A child who cannot read every letter at seven feet distance, has not normal vision, and advice should be sought for him. In order to ascertain what his vision is, he should approach the letters slowly until he can read them : and then his distance from them in feet, divided by seven, will give the fraction that is required. In addition to the measurement of the acuteness of central vision, the measurement of the extent of the field is often required in the investigation of disease, and is accomplished by means of instruments' for the purpose, called Perimeters, which are made m various forms con- trived by myself and others. The measurement of the field, however, has no- domestic value ; and it is not necessary to dwell upon it further. Before leaving the subject of the field of vision, I may briefly mention the curious gap, lacuna, or " blind spot," which it contains, and which corresponds with the termi- nation of the optic nerve within the eye. This termination, which is nearly circular in its outline, is absolutely blind, having even no perception of light ; and perhaps the most curious thing about it is that, although men had used their eyes for an unknown number of thousands of years, the existence of such a blind spot in each of them was left to be discovered by Mariotte, in the reign oi Charles the Second. When the existence of the blind- spot is known, its place is easily determined. If we take Fig. 22, and, closing the left eye, look steadily with the right at the cross to the left of the figure, at the same IV.] BLIND-SPOT. 6i time moving the page gently to and fro, we shall soon find a distance, about eleven or twelve inches, at which the black circle to the right of the figure entirely dis- appears from view, the light reflected from it all falling upon the blind spot. Another method is to close the left eye, hold out the arms to the front, with the fingers doubled in and the thumbs in contact with each other Fig. 22. and upright. We then look steadily at the left thum.b with the right eye, and slowly move the right arm out- wards. \\'hen the thumbs are about six inches apart, the right thumb will disappear from view, to become visible again as soon as it is either brought back or moved farther away. Many of the phenomena con- nected with the blind spot and its vicinity are of great interest to physiologists, but they have little place in so elementary a treatise as the present. It is not only in acuteness of ordinary perception that the peripheral parts of the retina are inferior to the central parts, but also, and even especially, in the per- ception of colour. There are few colours which can be identified with certainty if the coloured object is held at 62 EYESIGHT. [ch. iv. one side while the gaze is directed forward, although the object itself may be distinctly seen ; and the differ- ence between black and red in such circumstances soon becomes wholly undistinguishable. A bunch of mixed ribbons affords a good test object for determining this fact experimentally. CHAPTER V. NEAR AND DISTANT VISION ; REFRACTION AND ACCOMMODATION; PRESBYOPIA, OR AGED SIGHT. I HAVE spoken, in the preceding chapter, of the formation of images upon the retina by the parallel rays which proceed from a far-distant object, and under the supposition that the focal length of the eye as an optical instrument is identical with the measurement of its axis, so that the focus of parallel rays falls precisely upon the retina ; but it is obvious that these conditions cannot be fulfilled in all cases. There is no necessary relation between the focal length and the axial length of the eye ; and our vision is constantly required for near objects, from which we receive divergent rays. When the axial and the focal length of the eye are precisely the same, so that, as shown in Fig. 23, the focus of parallel rays falls precisely upon the retina, the eye is said to be emmetropic, from Greek words signifying that it is " in measure ; " and when this correspond- ence between the two magnitudes is not exact, the eye is said to be ametropic, or " out of measure/' It is 64 EYESIGHT. [chap. obvious that the disparity of measure may be in two opposite directions ; one in which the length of the axis is less than the focal length, and one in which the length of Fig. 23. the axis is greater than the focal length. The first of these conditions is shown in Fig. 24, the second in Fig. 25. The former depends upon flatness of the eyeball from Fig. 24. front to back ; and is called flat-eye, or hypermetropia ; the latter depends upon elongation of the eyeball from front to back, and is the occasion of short sight, or Fig. 25, myopia. Both of these conditions; or departures from the right proportion, may exist to a very great degree ; and, in Fig. 26, the dark line shows the outline of an v.] REFRACTION OF THE EYE. 65 emmetropic eye of the natural size ; while the dotted lines show the corresponding dimensions in the most extreme case of hypermetropia, and in the most ex- treme case of myopia, in which actual measurements have been made and recorded. If the eye were a rigid and passive organ, it is plain that its influence in refracting the rays of Hght which entered by its pupil would be always the same. In the \ Fig. 26. emmetropic formation the focus of parallel rays would be always upon the retina ; in the myopic formation, the focus of parallel rays would be always within the eyeball but in front of the retina ; and in the hyper- metropic formation the focus of parallel rays would be always behind the retina, behind and outside of the eye- ball if they could pass through its tunics. This action of the passive eye upon light is called its Refraction ; and by the refraction of the eye we m^ean to express whether, when at absolute rest, it is emmetropic, hypermetropic, or myopic. It will be manifest, from Figs. 24 and 25, that in neither of the latter cases could there be defined images of distant objects upon the retina. In the hypermetropic eye, the rays falling upon the retina F 65 ■ EYESIGHT. [chap. would not yet have been united m a focus ; and in the myopic eye the rays so falling would have united and would afterwards have overcrossed. In both cases the retina would receive a patch of light, technically called a diffusion circle, instead of the defined image which is necessary to vision. We have seen already that it is a property of con- vex lenses to render parallel rays of light convergent, and that it is a property of concave lenses to render them divergent. From this it follows, if we place a convex lens before a hypermetropic eye, that the convergence caused by the lens will enable the eye to bring the rays of light to an earlier focus than by its unaided refraction ; and, if we place a concave lens before a myopic eye, the divergence caused by the lens will contribute to bring the rays of light to a more distant focus. For every case of hypermetropia, therefore, there is a con- vex lens of such a strength that it will exactly suffice, when added to the eye, to place the focus of parallel rays upon the retina ; and for every case of myopia there is a concave lens which, when added to the eye, will produce the same result. As shown in Figs. 27 and 28, such lenses at once correct and measure the ametropia : so that we commonly describe the degree of the ametropia in terms of the lens which will correct it. A hypermetropia of two dioptrics is one which requires a convex lens of two dioptrics for its complete correction ; and a myopia of two dioptrics is one which requires a concave of two dioptrics for its complete correction. It is by no means uncommon for the two eyes of the v.] REFRACTION OF THE EYE. 67 same person to be of unlike refraction, especially in those instances in which there is some difference of formation between the two sides of the face. The difference may be of every possible kind, one eye being emmetropic and the other ametropic ; or the two being ametropic in Fig. 27. different degrees, or even in the contrasted forms ; neither is it uncommon for the same eye to be ametropic in different degrees in different meridians. It is almost a natural formation that the curvature of the vertical meri- dian of the cornea should be a little sharper than that of Fig. 28. the horizontal. When this difference is very slight it is productive of no inconvenience ; but when it amounts to a dioptric, or even less, it becomes disturbing to vision, and constitutes the state known as astigmatism. It is F 2 68 EYESIGHT. [chap. obvious that an eye so formed may be myopic or hyper- metropic in one direction only ; and hence that it may not see a vertical line as plainly as a horizontal one, or vice versa. The test of astigmatism is that all the lines in such a diagram as Fig. 29, or other similar test object, are not seen at once with equal distinctness ; and, in high degrees of the defect, one particular line may appear so confused and blurred as to be scarcely dis- tinguishable, although the one at right angles to it is Fig. 29. clearly visible. It is noteworthy that the meridians of least and of greatest curvature are always at right angles to each other ; and the cornea of an astigmatic eye may be roughly likened to the bowl of a spoon, in which the curve in the direction of the handle is less sharp than that transverse to the handle. Astigmatism always dimijiishes the acuteness of vision in a marked degree, and sometimes in a very curious manner. As an example v.] REFRACTION OF THE EYE. 69 of this, and to show how easy it may be to mistake a surmise for a fact, I may mention that a gentleman once consulted me for what he described as a " periodical obscuration of vision." I found that he sat in an office which commanded a view of a large clock-dial on the other side of a quadrangle. When the hands of the clock were approximately vertical, he could see them plainly ; but, when they were approximately horizontal, he could scarcely see them at all. This, which was the fact, he confounded with his own surmise that he saw differently at different hours of the day; and hence he had been induced to read all he could find written upon the subject of " vital periodicity," and to regard himself as a curious physiological phenomenon. A pair of cylin- drical spectacles at once removed his defect, and restored his vision to a natural state. Proceeding now to consider the vision of near objects, and taking the emmetropic eye as a starting point, we shall see on brief reflection that such an eye, although capable of uniting parallel rays upon its retina, could not unite divergent rays if its condition remained un- changed. The divergent rays, such as proceed from all near objects, require more bending than the parallel rays in order to unite them in the same distance ; and the nearer the object, and consequently the more divergent the rays, the greater amount of bending will be required. An emmetropic eye, therefore, must possess some power of increasing its action upon light in order to obtain clear vision of near objects ; or, in other words, in order to fulfil the requirements of a function which it is 70 EYESIGHT. ' [char constantly and almost unconsciously performing. That the eye does possess such a power of variation may be shown not only by reasoning, but in many ways by direct experiment. An emmetropic person, not too old, after looking at the horizon and seeing all its details with perfect clearness, will immediately, and with equal clear- ness, read a page of small type held at a convenient distance. If he brings the type nearer and nearer to the eye, he will become conscious of a sense of effort ; and presently he will reach a point so near that he can no longer overcome the divergence of the rays, and at this point the characters become indistinct. If he now takes a card perforated by a pin-hole, and looks through the pin-hole at the type, thus cutting off the outer or more divergent portion of the cone of rays, and receiving only the axial portion, which is less divergent, he will be able to read at a much smaller distance than before, notwithstanding the greatly diminished illumination which he receives from the page. Again, to borrow an illustration from Professor Bonders, if we take a piece of net, and hold it between the eyes and a printed page, we may at pleasure see distinctly the fibres of the net, or the printed letters on the page through the inter- stices of the net ; but we cannot clearly see both at once. When we are looking at the letters, we are only conscious of the net as a sort of intervening film of an uncertain character ; and when we are looking at the net, we are only conscious of the page as a greyish background. In order to see first one, and then the other, we are quite aware of a change which occurs in the adjustment v.] ACCOMMODATION. 71 of the eyes ; and if the net is very near, and we look at it for any length of time, the maintenance of the effort of adjustment becomes fatiguing. By such observations as the foregoing, and others of a similar kind, the existence of a power of adjustment had been established, and the adjustment itself had received the name of Accommodation^ long before the mechanism by which the act was effected was understood. The actual change produced might be of two kinds, either an increase in the refractive power of the media, or an increase of distance between the cornea and the retina. It is unnecessary to dwell upon the various hypotheses which were successively suggested and abandoned ; and at last, by the labours chiefly of Cramer and Donders, the act of accommodation was shown to depend upon an increase in the convexity, and hence also in the power, of the crystalline lens ; and this in- crease was shown to be brought about by the action of a muscle which forms part of the ciliary body, and is named the ciliary muscle, or the muscle of accommo- dation. Fig. 30 represents, merely in a diagrammatic form, the exact nature of the change, in which the anterior surface of the crystalline lens becomes more convex, and the pupillary opening smaller. The left hand half of the figure shows the parts at rest, the right hand half shows them as they are when accommodation is being exerted, and the letters cm indicate the position of the ciliary muscle. Putting aside the mechanism by which accommodation is performed, it may be broadly said that the effect of the 72 EYESIGHT. [chap. change is precisely that of placing an additional convex lens within the eye, and the amount of refracting power which can thus be added is very definite, and admits of easy measurement. For every eye there is a point within which clear vision is no longer possible without optical assistance ; and this, which is called the near- point, marks the limit of the power of accommodation. Assuming the eye to be emmetropic, so that it can unite Fig. 30. , parallel rays upon its retina, or, in other words, can see clearly objects which are infinitely distant, let us further suppose that it can also see small objects clearly at twenty centimetres, or one-fifth of a metre, but not at any shorter distance. The effort of accommodation which is exercised in seeing at this near-point produces precisely the same optical results which would be at- tained by placing within the eye an additional convex lens of the same focal length as the distance from the v.] ACCOMMODATION. 73 eye to the near-point. In the case supposed, therefore, the accommodation is equal to a convex lens of five dioptrics. If the near-point were at twenty inches, or half a metre, then the accommodation would be equal to a lens of two dioptrics. As life advances, the crystalline lens gradually loses its elasticity and becomes more and more rigid ; and, as a necessary consequence of the change, the power of accommodation constantly diminishes, and the near- point recedes farther and farther from the eye. Taking the mean of many observations, we find that at ten years of age the accommodation is equal to a lens of 13 dioptrics, and the near-point is at 3 English inches. At twenty-one, the accommodation has fallen to 9 dioptrics ; and the near-point has receded to 4-5 inches. At forty, the accommodation has fallen to 4*5 dioptrics, and the near-point has receded to 9 inches. At fifty, a great change has taken place. The accommo- dation is then only 3 dioptrics, and the near-point has receded to 13 inches; at sixty, the accommodation is only 1*5 dioptrics, and the near-point is at 26 inches; while at seventy-five the accommodation is wholly lost, the eye is passive, and the near-point is therefore at infinite distance. The diagram in Fig. 31 shows the efi"ect of declining accommodation in a simple and intelligible form. The strong vertical line, marked with the figure CC, indicates infinite distance, and the vertical lines to left and right of it indicate dioptrics of refraction. These are numbered consecutively above ; and the figures on the same vertical 74 EYESIGHT. [chap. u^ v.] PRESBYOPIA. 75 lines below indicate the distance of the near-point from the eye, in English inches, for each successive amount of accommodation. The stronger horizontal lines show the effort of accommodation in each of several instances ,' and the figures to the left of the diagram serve to number the instances for reference, and to show the ages of the persons to whom they relate. The first six instances refer to emmetropic eyes, and exemplify the diminution of accommodation by age which is described in the pre- ceding paragraph. In No. i, a person ten years old brings the near-point to 3 inches by the exercise of 13 dioptrics of accommodation. In No. 2, a person of twenty-one has 9 dioptrics of accommodation, and brings the near-point to 4*5 inches. Nos. 3, 4, and S, are the other examples ; and in No. 6 we have total loss of accommodation at seventy-five, the far-point and near-point being the same. The remaining groups illustrate the accommodation in hypermetropia and in myopia, and to these we shall return hereafter. The recession of the near-point, which depends upon hardening of the crystalline lens, and consequent cur- tailment of the accommodation, is a phenomenon which occurs in all eyes, of whatever refraction, and its effects are known as constituting the condition called Presby- opia, or aged sight. Presbyopia is not felt as an incon- venience until the near-point has receded to such a distance that small objects no longer afford light enough for comfortable vision ; and this generally occurs between the ages of forty-five and fifty. The change being a gradual one, it is not possible to say, except arbitrarily, 76 EYESIGHT. [chap. when presbyopia should commence ; but it is held to do so when the near-point has passed beyond eight inches ; that is to say, when there are only five dioptrics of accommodation left. It is found, as a general rule, that not more than half the power of accommodation can be used continuously, and that any attempt to use more, except for a few minutes at a time, is attended by fatigue, which soon compels the abandonment of the effort. A person, therefore, who has five dioptrics of accommodation for possible or brief use, will only have two and a half dioptrics for continued use ; or, in other words, his practical working near-point will not be at eight inches, but at sixteen. There are many pursuits which require the object of vision to be nearer to the eye than sixteen inches ; and, unless for large objects or with abundant illumination, this distance would almost always be too great to be convenient. We have seen, however, that the effect of accommodation is precisely that of adding a convex lens to the passive eye ; and so, when accommodation fails, we can supply its place by adding the required lens by art. To do this is the ordinary function of the spectacles which are required by all people, if their eyes were originally natural, as time rolls on ; the principle on which such spectacles should be selected is that they should be strong enough to be effectual ; and they should be used as soon as they are required. Opticians often supply glasses which are too weak to accomplish what is needed, and which leave the eyes still struggling with an infirmity from which they ought to be entirely relieved ; while the public v.] PRESBYOPIA. >j'j frequently endeavour to postpone what they look upon as an evil day, and do not obtain the help of glasses until they have striven hard and fruitlessly to do without them. These are important practical errors. It cannot be too generally understood that spectacles, instead of being a nuisance, or an encumbrance, or an evidence of bad sight, are to the presbyopic a luxury beyond de- scription, clearing outlines which were beginning to be shadowy, brightening colours which were beginning to fade, intensifying the light reflected from objects by permitting them to be brought closer to the eyes, and instantly restoring near vision to a point from which, for ten or a dozen years previously, it had been slowly and imperceptibly, but steadily declining. This return to juvenility of sight is one of the most agreeable experiences of middle age ; and the proper principle, therefore, is to recognise presbyopia early, and to give optical help Hberally, usually commencing with lenses of +i"25, or +1*50, so as to render the muscles of accommodation not only able to perform their tasks, but able to perform them easily. When, as will happen after a while, in consequence of the steady decline of accommodation, yet more power is required, the glasses may be strengthened by from half a dioptric to a diop- tric at a time, and the stronger glasses should at first be taken into use only by artificial light ; the original pair, as long as they are found sufficient for this purpose, being still worn in the daytime. A popular, but entirely unfounded prejudice, which exists amongst the public with regard to the hurtful 73 EYESIGHT. [chap. effects of wearing convex glasses which are too strong, appears to be traceable to an error founded upon a curious coincidence. There is a disease of the eye termed glaucoma, which formerly ended in complete and irremediable blindness, but which, for twenty years past, has been cured by operation when recognised suf- ficiently early. One of the first or even of the pre- monitory symptoms of glaucoma is a rapid failure of the accommodation, and hence a frequent demand for stronger and stronger glasses. At a time when this disease was very imperfectly understood, opticians saw many examples of people who came to them for stronger glasses every two or three months, who were helped by them for a time, but who soon became totally blind ; and it was not unnatural for them to associate the blind- ness with the use of the strong glasses. The opinion thus formed upon a misinterpretation of facts was con- firmed by an elaborate article upon spectacles which appeared, many years ago, in the Quarterly Review, and which did little more than give new life to a variety 'of erroneous beliefs. Regarded by the light of modern knowledge, the facts lend no support to a belief that strong spectacles can be hurtful ; except in so far that they may produce some fatigue of the eyes and be un- comfortable, when, in a way presently to be explained, they disturb the natural harmony between the two facul- ties of accommodation and of convergence. Remarkable evidence of the harmlessness of continuous working by the aid of a single convex glass is furnished by watch- makers, among whom such work is an unavoidable v.] PRESBYOPIA. 79 condition of their calling, and who appear to me to enjoy an enviable immunity from eye diseases. It is exceedingly uncommon to see a working watchmaker among the patients of the ophthalmic department of a hospital ; and I entertain little doubt that the habitual exercise of the eye upon fine work tends to the develop- ment and to the preservation of its powers. It must not be forgotten, however, that a premature demand for spectacles may arise from the existence of some diseased condition ; and hence the circumstance that they were required at an unusually early age, or that stronger and stronger powers were asked for, at short intervals, by a person not originally hypermetropic, showing that the power of accommodation had declined at more than its average rate of diminution, would be a reason for dread- ing the possible approach of glaucoma, and for seeking skilled advice in time. The persons who suffer most from popular prejudice and ignorance on the subject of spectacles are men of the superior artisan class, who are engaged on work which requires good eyesight, and who, at the age of fifty or sooner, find their power of accomplishing such work is diminishing. It is a rule in many workshops that spectacles are altogether prohibited, the masters ignorantly supposing them to be evidences of bad sight ; whereas the truth is that they are not evidences of bad sight at all, but only of the occurrence of a natural and inevitable change, the effects of which they entirely obviate, leaving the sight as good for all purposes as it ever was. In many shops, in which they are not 8o EYESIGHT. [CH. v. prohibited, they are still made an excuse for a diminu- tion of wages ; and the result of these practices is that hundreds of good workmen struggle on perhaps for years, doing their work imperfectly, when a pair of spec- tacles would instantly enable them to do it as well as at any former period. In the present state of knowledge there is no excuse for rejecting a man's services, or for diminishing his payment, because he requires spectacles ; unless it can be shown that, even when he is furnished with them, his sight is below the natural standard of acuteness. A person who is emmetropic and presbyopic does not, of course, require spectacles for distance, but only for near objects ; and the glasses are placed out of the way of distant vision by putting them somewhat low on the nose, so that for distance the eyes look over them. The same result may be obtained by making the lenses flat on the top instead of regularly oval. CHAPTER VI. SINGLE VISION WITH TWO EYES; CONVERGENCE. I HAVE hitherto considered the act of vision only as it is performed by a single eye ; but, before proceeding to the effects and the relief of the several forms of ametropia, it is necessary to become acquainted with the mechanism by which the two eyes are so closely combined in the performance of their functions that they act almost like a single organ. The power of the brain to unite into a single perception the two images of an object of vision which are formed upon the two retinae is called the power of fusion ; and it only exists when the two images are received upon corresponding parts of the two retinae. This condition, in its turn, is only fulfilled when the eyes are both directed to the same point in space ; and the moment they cease to be so directed, double vision is a result. In Fig. 32, the two eyes, A and B, are directed to the same point C, and the image of C being formed at s, upon the yellow spot of each eye, the two images are fused, and C is recognised as a single point. In Fig. 33, G 82 EYESIGHT. . [chap. on the contrary, the eye A is still directed to C, but the eye B is directed along the line B b', to the left of C. In this position, the image of C falls upon the yellow spot of A as before ; but, as shown by the dotted line, it falls upon a part of B which is internal to the yellow spot, and which therefore, in the natural position of the two eyes, would receive an image from an object situated Fig. q2. at c'. Hence the eye B refers its image of C to the position c', or to the right of its real position ; and the pair of eyes see two points C instead of one only ; one of them in its natural position, the other outwards or to the right in the position C'. The false image, it will be observed, appears to He in an opposite direction VI.] SINGLE VISION, 23 to that of the misdirection of the eye which receives it : so that if the eyes squint convergently, or in an inward direction, the double images belong each to the eye on the side on which it appears, and if the eyes squint out- wards, or divergently, the double images are crossed, and each belongs to the eye on the opposite side. The extent of separation of the images is greater, of course, Fig. as the distance of the object increases ; and, for any given object, it depends upon the degree of the dis- placement. When this is very slight, so that the false image falls near to the yellow spot, double vision is very distressing, because the two images are so nearly alike in intensity and clearness that the sufferer cannot readily say which is which, and they render it difficult G 2 84 EYESIGHT. [chap. to guide the steps correctly, or even, sometimes, to touch objects with the hand. When the displacement of one eye is considerable, there is no difficulty in see- ing which of them is displaced ; but, when it is slight, this may sometimes be for a time a matter of doubt. The point can always be decided by simple means ; and the question to which eye this or that image belongs is solved by taking a candle flame as the object, and by looking at it with a^ piece of red glass held before one eye and with the other eye bare. The red image of the flame, whether it be on the right or the left, will necessarily appertain to the eye before which the red glass has been placed. Besides being displaced inwards or outwards, either eye may be displaced upwards or downwards ; and the maintenance of the correct relative position of the two, in whatever direction they may turn their gaze, is de- pendent upon the preservation of perfect balance and harmony of action between all the six muscles, the four straight and the two oblique, which were mentioned in the first chapter. It may be said, however, to be more essentially dependent upon the internal straight muscles, shown diagrammatically at I, I, in Fig. 34, than upon any others ; since these are chiefly concerned in the function of convergence, by which the eyes are rolled inwards, and are thus moved, at will, from the position nearly of parallelism which they assume when looking at an infinitely distant object, to that which they assume when directed to some nearer point. If we bring an object of vision slowly near to the eyes of another VI.] CONVERGENXE. 85 person, who looks at it as it approaches, we may watch the movement of convergence ; and we shall see that, when the effort to look at the near object ceases, the eyes are rolled back to the position of repose, nearly in the middle of each eyelid opening. This rolling back is done by the external straight muscles, E, E, Fig. 34, as soon as the action of the internal muscles is suspended ; and any continuance of forced or extreme convergence soon becomes excessively fatiguing. For a short time, however, the eyes may be rolled inwards Fig. 34. in a very considerable degree ; so that, especially in young people, they may be fixed upon an object at a distance of only two or three inches from them. In the emmetropic eyes of a young and healthy person, the two functions of accommodation and con- vergence go hand in hand, so that they can scarcely be separately performed. Their objects are totally different, but their harmonious co-operation is none the less essential. The function of accommodation has for its purpose the formation of a clear or defined image on the retina of each eye singly; the function 86 EYESIGHT. [chap. of convergence has for its purpose the fusion of the two retinal images into a single sensory perception. But an object at a given distance will always require, from the same pair of eyes, the same amount of ac- commodation, and it will always require from them the same amount of convergence. Say the object is at ten inches, where it will require four dioptrics of accommodation. It is evident that it must also require convergence to such a degree that the lines of direction of the eyes would intersect at a point ten Inches distant from them ; and further that, if the object were brought nearer, the accommodation effort and the convergence effort would increase in an equal degree. Hence it follows that accommodation for a point, and conver- gence to that point, become strictly correlated efforts ; and that one can hardly be accomplished without the other. The physiology of associated muscular move- ments Is not yet thoroughly understood, and it is by no means certain whether the connection and inter- dependence of accommodation and convergence are natural or acquired ; whether, that is, they are the results of laws written upon the nervous centres by Vi^hich the muscles are called into activity, or only upon the educational influence of constant consenta- neous action. However this may be, it is at least certain that the two functions. In young and healthy emmetropic eyes, are always performed in unison, and that they cannot be dissociated without fatigue and distress. When presbyopia comes on, the tie which unites them is gradually relaxed ; and, as we shall see VI.] CONVERGENCE. 87 A' hereafter, in many instances of ametropia it is either weak or wanting from the beginning". The power of convergence, with its antagonist, the power of divergence, or in other words, the strength of the internal and external muscles, may be conveniently tested by means of prisms. A prism, as we have already seen, deflects a ray of light away from its vertex and towards its base, as shown in Fig. 7. In Fig. 35, a ray of light from the point A, on its way to the left eye C, in which it would fall naturally upon the yellow spot S, is deflected by passing through the prism B. The ray, being bent towards the base of the prism, reaches the eye as if it came from the point a' ; and it is plain that in order to receive the image of A upon its yellow spot, the front of the eye must be rolled out- wards, in the direction a'. Hence, if we place before one eye a prism with its base inwards, or towards the nose, we shall produce double vision unless the eye can be rolled outwards sufficiently to counteract the refraction of the prism, and to keep the image still upon the yellow spot ; while in like manner, if the base of the prism is turned outwards, towards the temple, the eye will be called upon to execute a convergence movement in order that single vision may be preserved. In testing the powers of 88 EYESIGHT. [chap. convergence and of divergence, it is better to use a pair of prisms than one only; and the strongest pair which can be overcome, so as to preserve singleness of vision, when their bases are turned inwards, afford the measure of the strength of the divergence function ; while the strongest pair which can be overcome when their bases are turned outwards afford the measure of the strength of the con- vergence function. The strength of the prisms themselves is expressed by the angle which their sides form with one another. Although, as we have seen, the original purpose of convergence is simply the accomplishment of fusion, or the maintenance of a single image from vision with two eyes, yet the convergence becomes so closely associated with the accommodation that a mere accommodation effort without reference to fusion will excite and main- tain a convergence effort of nearly or quite the same degree. Moreover, in order to exert the accommodation to the fullest extent, it is necessary to exert the conver- gence also ; and this circumstance, as we shall see here- after, furnishes the explanation of the common form of squint in the great majority of the cases in which it occurs. The intimate connection between accommodation and convergence may be illustrated by a simple experiment. If emmetropic eyes are furnished with concave glasses, these, by rendering the rays of light divergent, as if they came from a near point, require accommodation to over- come this divergence, even when the gaze is directed to a distant object. The eyes are then accommodating VI.] CONVERGENCE. 89 without convergence; and the effort soon becomes weari- some and distressing. If we add to the concave lenses a pair of prisms with their bases outwards, so that the eyes are called upon for convergence as well as for accommodation, the added effort of convergence instantly relieves the fatigue and strain which were due to the maintenance of accommodation alone. In like manner, if we place convex glasses before the eyes of a young emmetropic person, and direct him to look at a near object, upon which he must converge for the sake of single vision, but which calls for no accommodation on account of the presence of the convex glasses, then we find that convergence without accommodation soon be- comes as fatis^uincr as accommodation without conver- gence. By taking away the convex glasses, so that accommodation is required together with convergence, the fatigue is relieved ; and it may also be relieved by leaving the glasses, but adding to them prisms of proper strength with their bases inwards, so that the demand for convergence is taken away, as well as the demand for accommodation. We learn, from these experiments, that any departure from harmony between the two functions may become a source of fatigue and strain in the use of the eyes ; and the cases are very numer- ous in which this knowledge is of important practical application in the relief of visual defects. Assuming the immediate purpose of convergence to be the accomplishment of fusion, the fusion itself greatly increases the usefulness of vision as a source of informa- tion concerning the external world. It is by comparison 90 EYESIGHT. [chap. of the different aspects of any object which are received by the two eyes, on account of their sHghtly different positions with regard to it, that we obtain the idea of solidity; and, as of late years has been rendered familiar by the stereoscope, the eyes may even be deceived by the apparent solidity which is produced by placing before them tv/o slightly dissimilar pictures. Moreover, it is by estimating, through the medium of the faculty called " muscular sense," the amount of effort which the muscles of convergence and of accommodation are exerting, that we learn to judge of the distances of the various objects which surround us. When an object is very remote, the images of it which are formed upon the two retinae can be fused almost without effort ; and, as the object comes nearer, the necessary effort steadily increases. If we watch the actions of young children, especially the extreme uncertainty of their endeavours to touch an attractive object of vision, we shall speedily be convinced that the power of estimating distance is in the human species an acquired and not an instinctive faculty; although, in many of the lower animals, it seems to be chiefly of the latter character. If we disturb the ordinary convergence effort by means of prisms, we can produce many curious illusions with regard to the size and the distance of objects, these being, of course, strictly correlated quantities. Thus, if we look at a near object through a pair of prisms with their bases inwards, which render the eyes approximately parallel, as if they were directed to the horizon, the object will appear to be larger than it is, because, although its retinal images are VI.] CONVERGENCE. 91 of considerable magnitude, they are fused without con- vergence effort, as if the object itself were upon the horizon. In Hke manner, the use of prisms with their bases outwards will make an object appear smaller than it is, because its distance from the eyes, judged by the convergence effort necessary to obtain fusion of the images, will be under-estimated. Several curious experi- ments of this kind may be made by the simple original or reflecting stereoscope of the late Professor Wheat- stone, in which a slight movement of the arms, causing a difference in the angles at which the rays of light from the two figures are reflected into the eyes, will produce great differences in the apparent magnitude or apparent distance of the resulting picture. CHAPTER VIL DEFECTS OF VISION PRODUCED BY FAULTY SHAPE OF THE EYEBALLS; MYOPIA, HYPERMETROPIA, AND ASTIGMATISM. In the fifth chapter, ametropia has already been de- fined as a condition in which the length of the axis of the eyeball, and the focal length of its refracting media, are not precisely the same ; and it has been explained that, when the axial length is greater than the focal length, the effect of the disparity is to produce myopia ; while the opposite disparity, or flat eye, is known as hypermetropia. The word myopia is a trivial designa- tion for short or near-sight, derived from the Greek ixvio, to shut ; and is based upon the practice, common to the short-sighted, of nipping their eyelids together for the attainment of better distant vision. In a scientific sense the word has not a single recommendation ; but it is understood alike by scientific and by unscientific people, and is probably too deeply rooted in usage to be abandoned for any other. The ideal or diagrammatic formation of the myopic eye CH. VII.] DEFECTS OF VISION. 93 has been shown in Figs. 25, 26, and 28 ; and the effect of this formation is to place the far-point of distinct vision at some measurable distance from the eye. The myopic person cannot see to the horizon or to the fixed stars ; but he can see quite clearly within a certain fixed limit. The degree of short-sight is determined solely by the farthest boundary of vision, and has nothing to do with the near-point ; which, in short-sighted eyes, as in all others, gradually recedes as life goes on, on account of presbyopic changes which diminish the power of accommodation. Let it be supposed that a person has not normal vision at any point more remote than forty inches, but that within forty inches he can read easily at any point up to four inches. The condition of his eyes will then be precisely what is shown in line ten, in the third group of Fig. 31. His vision does not attain to infinite dis- tance, but is limited to a far-point at forty inches ; and within this distance he has nine dioptrics of accom- modation, which bring his near-point to four inches. If we supply him with concave lenses of one dioptric, his distant vision is instantly restored ; because these render parallel rays of light as much divergent as if they radiated from a point forty inches distant, and therefore allow such rays to be brought to a focus upon his retina. We say, in these circumstances, that there is myopia equal to a dioptric. The concave lenses not only restore dis- tant vision, but they also, when the eyes are directed to any object nearer than the horizon, consume a dioptric of accommodation in order to neutralise them ; 94 EYESIGHT. [chap. and hence, v;hen they are being worn, the near-point is farther from the eye by that dioptric, or is at 4*5 inches instead of at 4. Myopia of only one dioptric is comparatively un- common, and much higher degrees are constantly met with in practice. The eleventh line of Fig. 31 shows a much more common degree, in which the far-point or distant limit of vision is at ten inches instead of at forty. In this case, a lens of four dioptrics would be necessary in order to restore distant vision ; and, sup- posing the person to be twenty-one and to have nine dioptrics of accommodation, the near-point would be at three inches instead of at four, because the whole of the nine dioptrics would be exercised within the ten-inch limit. Whenever a person who can see perfectly well within a certain limit has less than normal vision beyond this limit, myopia is to be suspected ; and the suspicion be- comes certainty if a concave lens restores distant vision nearly or quite to the natural standard. The power in dioptrics of the lens which does this, or which affords the best result, is the measure of the degree of the myopia, w^hich may then be described as equal to so many dioptrics. Around short-sight several erroneous beliefs have gathered themselves together, and these still retain a vigorous hold upon the credulity of the public. They are, chiefly, that short-sighted eyes are good or strong eyes, that short-sight improves v;ith advancing life, and that short-sighted people need not use spectacles for vii.J MYOPIA. 95 reading- or other near work, if they can see to accomphsh it without them. The first of these behefs, that short-sighted eyes are good or strong, admits of easy explanation. It rests upon the fact that short-sighted people can see smaller objects than others, and that they also see in a fainter light than others. Both of these conditions depend upon their power to approximate the object, and upon that power alone. The emmetropic person, shown in line two of Fig. 31, has nine dioptrics of accommodation, which suffice to bring his near-point from infinite distance to 4-5 inches. The myopic person, shown in line eleven of the same figure, has also nine dioptrics of accommodation ; but they bring his near-point from ten inches to three inches. The former of the two cannot see anything clearly if it is nearer than four and a half inches, the latter can see clearly at three inches. In the latter position, from the larger size of the visual angle and from the approximation of the object, the myope obtains a retinal image about one-third larger than, and with more than twice the illu- mination of, that of the emmetrope. To make the cases equal, the emmetrope should be furnished with a convex lens of four dioptrics, and, when he was thus placed under similar optical conditions, it would generally be found that he could see the near object better than the myope. The better sight of the latter is apparent and not real. If the myopic eye were in other respects equal to the emmetropic eye in endurance, the power to approximate an object without the use of a lens might 96 EYESIGHT. [chap. occasionally be convenient ; but, in actual fact, this advantage, if such it may be called, is more than counterbalanced by disadvantages of no uncertain char- acter. The power to see by a comparatively dim light is also dependent upon the power to approximate the object. The belief that short-sight improves with advancing years has even less foundation. The only respect in which it does so is that the pupil is usually smaller after middle life than before ; and its diminished size dimin- ishes the diffusion circles upon the retina which are formed by the rays coming to it from distant objects. The retinal images, therefore, may appear somewhat more clear than in youth ; but the degree of the myopia, as measured by the lens which corrects it, never dimin- ishes. There is one sense in which it appears to do so, and this is explained by the twelfth line of Fig. 31. Myopic eyes, like all others, become presbyopic, and thus their near-point recedes, and they lose the nearest portion of their range of vision. The person shown in line twelve with a myopia of four dioptrics, and with, at the age of twenty-one, an accommodation of nine dioptrics, would be able, by using half that accommoda- tion, to read comfortably at a distance of about five inches. At the age of forty, as shown in line twelve, when he is deprived of half his accommodation by pres- byopia, he will not be able, by the use of half of the remainder, to read comfortably at a nearer point than about six inches. He will have lost a bit of the front of his visual range ; and, in this condition, many people VII.] MYOPIA. 97 say, " I am not so short-sighted as I was." Those who do so are not aware that the measurement of short- sight is from the far-point, and that this remains at ten inches, where it ahvays was. The one advantage which may be conceded to myopia, when it amounts to as much as three dioptrics, is im- munity from the ordinary requirement of spectacles to read with, in consequence of faihng accommodation as hfe advances. With a myopia of three dioptrics, which gives a far-point at thirteen inches, as even a total loss of accommodation could not place the far-point farther away than this, so no convex glass can ever be required. Thirteen inches is a very convenient reading distance, and no wish to bring a book nearer than this would be likely to arise. When, however, the myopia is of small degree, as a dioptric and a half, the loss of accommodation adds the discomfort of presbyopia to that of the original defect. A person so situated will need concave glasses for distance, and convex glasses for reading. I am acquainted with more than one clergyman who is in this condition, and who has the two pairs of lenses in the same frame, the concaves above, and the convex below. He thus looks up through the former at his congregation, and down through the latter at his offices or sermon-book. When we hear of old people who have "wonderful sight," so that they can read without spectacles at seventy or eighty years of age, we may be perfectly sure that there is nothing wonderful in the matter, but that such persons are short-sighted to at least three H 98 EYESIGHT. [chap. dioptrics, and are enjoying the usual immunity of their state. If we turn now to the other side of the picture, we find that it is never an entirely favourable, and is some- times a very gloomy one. The elongation of the eyeball on which myopia depends occurs almost exclusively at the posterior hemisphere, and chiefly in the vicinity of the posterior pole. If we look at the position of the internal straight muscles, as shown at 1 1, in Fig. 34, we shall see that their strong contraction, so as to produce the degree of convergence necessary in order to give single vision to the short-sighted, must produce also a strain upon the tunics of the eyeball at a point remote from the actual insertions of the muscles, and that this strain will fall upon the posterior pole of the eyeball, and chiefly upon the region between the yellow spot and the entrance of the optic nerve. When approximation of the object of vision, and consequent convergence effort, are habitual and long continued, the tunics of the eye begin to yield to the strain, and the elongation of the globe tends steadily to increase. By this increase, the myopia is rendered ''progressive," and progressive miyopia is an affection extremely threatening to the sight. The continued strain upon the posterior pole of the eyeball, and the stretching and elongation of the delicate tunics, produce wasting of the tissue of the choroid, sometimes attended or followed by inflam- matory changes, and even by detachment of the retin^ and loss of sight. It is exceedingly desirable that all young persons who are myopic should be carefully VII.] MYOPIA. 99 watched to see whether their defect is progressively increasing, in order that, if it should present this character, the use of the eyes may be carefully regu- lated with a view to the progress being arrested. Every myopic eye should be looked upon as a weak organ, capable, indeed, of being preserved in a state of use- fulness, but liable to many dangers and mischances from which natural, and even hypermetropic, eyes are comparatively free. Concerning the causes of myopia, we know little. The affection often descends from parent to child ; and, as it depends essentially upon the shape of the eyeball, it is reasonable to suppose that it may be inherited, and that infants may be more or less myopic at birth. Evi- dence upon this point would be attainable with difficulty, and only by a careful series of examinations of the eyes of new-born infants with the ophthalmoscope, such as has never yet been attempted upon a sufficient scale. It is possible that the birth condition, or inheritance, may not be one of developed myopia, but only of weakness of the tunics of the eye at the posterior pole, so that they are disposed to yield and undergo elongation as soon as the strain of the internal straight muscles is brought to bear upon them in the act of convergence. When myopia is once established, and unless its in- fluence is speedily counteracted, it provides for its own increase by the effect of convergence effort ; and it may attain a considerable degree before its very existence is discovered by parents of only ordinary want of observa- tion. The first attempts to trace myopia to a cause H 2 loo EYESIGHT. [chap. were made in Germany, where the condition is more common than in most other countries, by Dr. Cohn, of Breslau, who set himself, to examine the state of the eyes of the children in the local schools. He examined the eyes of 10,060 children, and in this number he found 1,004 who were myopic. He found also, what was far more important, that the myopia increased steadily, both in the relative number of cases and in the degree of the defect, as he ascended the schools from the elementary to the higher classes. He further found that there were relatively more cases of myopia, and higher degrees of the affection, in schools which were badly lighted ; and he finally referred the facts to the operation mainly of two causes, defective lighting and badly constructed desks and forms, both agencies being alike in causing the children to stoop over their work, and to bring their eyes as close to it as possible ; both being alike, that is to say, in compelling the main- tenance of a great amount of convergence effort. The publication of Dr. Cohn's work led to similar researches elsewhere ; and corresponding facts have been brought to light in Russia by Dr. Erismann, and in America by Drs. Agnew and Loring. There is no longer any room for doubt that badly-lighted and badly-fitted schools form a great machinery for the development of myopia ; and it is probable that this machinery, where, as in Germany, it has for a long time been in unchecked operation, may have an important influence upon the form of eyeball which will be inherited by large numbers of the population. The English school-boards have not vii.] MYOPIA. loi been left unwarned upon the subject. As soon as Dr. Cohn's work was published, I placed his facts and figures before the medical profession through more than one channel ; and attention has since been called to these facts and figures by others. Some two or three years ago Lord Monteagle brought the subject before the House of Lords, by a question to the Duke of Richmond, the Lord President of the Privy-Council ; but the Education Department had very little to say in reply. The attention of the great majority of school- boards, in the intervening time, has been chiefly given to questions which have no doubt seemed to them more important than the health and eyesight of the children ; but it is not unreasonable to hope that these may, in their turn, eventually receive some small degree of consideration. For the prevention of myopia in schools, there can be no doubt that good and well-placed windows are essential, and that fittings of judicious design will be useful ; but neither of these will be effectual, or will prevent children from drooping over their work, unless the matter receives the constant and vigilant attention of teachers, and unless the sanitary state of the buildings, and the time relatively given to work and to play, are such as to meet the require- ments of physical health. It is a curious illustiation of the essentially mechanical character of certain minds that the progress of the myopia should in Germany have been referred to the enforced convergence alone, and that better l:ght and better fittings should have been put forward as sufficient to bring about I02 EYESIGHT. [cHAP. a better state of things. Dr. Agnew, of New York, with more practical knowledge and with deeper wisdom, pointed out that a feeble and easily extensible character of the ocular tunics would be a condition largely de- pending upon general debility ; and that the treatment of this debility by food, tonics, and exercise, as well as by an ample supply of pure and often renewed air in the schoolrooms, a judicious abbreviation of tasks requiring the close application of the eyes, and the use of books prmted in bold characters, would be of great assistance in b-ringing about a much needed reform. The robust faith of the average schoolmaster in the efficacy of what he calls teaching is probably not destined to survive the time when a somewhat better acquaintance with the nature of mental operations will become diffused abroad ; and, in the meanwhile, and with reference to the frequent sacrifice of the physical side of the development of the young, it is not uninteresting to recall the results of an experiment made, some ten or twelve years ago, in the village school at Ruddington in Nottinghamshire, under the direction of the late Mr. C. Paget, sometime M.P. for Nottingham. In this school Mr. Paget introduced a half-time system as an experiment, to which only a portion of the children were subjected, and which amounted to a substitution of garden work for about one half of the ordinary school hours. The children who were so treated were found, after a short period, altogether to outstrip in their school-work those who devoted, or who were supposed to devote, twice as much time to it. The prevention of the increase of short-sight VII.] MYOPIA. 103 in schools is less, in my judgment, an affair of desks and fittings than of careful and judicious sanitation : for I have no doubt that the optical conditions which would produce myopia in weakly children would fail to do so in the robust. None the less, however, should these optical conditions, together with the lighting and the distance of the work, receive a due share of attention ; although such mechanical matters must not be expected to supersede the necessity for the constant supervision of a directing intelligence. When myopia is fairly established, the principle upon which it should be treated is very simple. The great cause of the increase of the affection, and of all the dangers to sight which such increase may entail, is the strain thrown upon the tunics of the eyes by undue and prolonged convergence ; and therefore this convergence must be prevented. The prevention is to be accomplished only by means of spectacles ; and the object of the spectacles is not to make the patient see any better, but to compel him to keep his work farther away. The myope-will often allege, with perfect truth, that he can see near things better without the spectacles than with them, and will often complain of some discomfort from their use. It is obvious that his eyes will have been gradually trained, by necessity, to exert a degree of convergence in excess of their accommodation; and that this, however certain to be ultimately injurious to the eyes themselves, may have become easy as an accus- tomed muscular effort. The spectacles call upon the muscles of the eyes to work under new conditions, and I04 EYESIGHT. [chap. these, although more wholesome than those which pre- ceded them, may be irksome as long as they are new ; for this reason amongst others, that the external straight muscles, long accustomed to be almost passive, will be brought into use for the maintenance of fixation at the more distant point of vision. It is obvious that the use of spectacles for reading, writing, and such like occu- pations, will be most important in early life, before growth is completed, and while the tunics of the eyes, like all other bodily structures, are comparatively lax and yielding. At this time, too, in many instances, the demands of education require a closer application to books than at any subsequent period ; so that, when maturity is attained, the glasses may often be laid aside. In some instances, the internal straight muscles of a myope are unable to meet the convergence demand required of them for the maintenance of binocular vision, and early give up the endeavour. In these cir- cumstances the eyes are used singly, and objects can be brought close to either of them without producing con- vergence. Vision with one eye only is always imperfect ; and hence, in the cases referred to, an endeavour to produce fusion of the images by means of glasses which require a smaller degree of convergence should generally be made. When this does not succeed, glasses for near work may be abandoned ; because near vision without convergence has no apparent tendency to produce in- crease of the myopia. The object of vision is then generally held to the right or left of the median line, in front of the eye which is being employed. vii.] MYOPIA. 105 The use of glasses for distant objects will generally at once commend itself to the short-sighted, by the pleasure which they receive from the consequent widening of their visual horizon. Apart from this, glasses for constant wear are greatly to be recommended, especially to the young, as a means of unconscious education. If we consider for a moment what must be the state of a person who has grown up to manhood or womanhood with an uncorrected myopia say of only two dioptrics, that is, with a far-point twenty inches away, we shall perceive the edu- cational importance of the early correction of the defect. An emmetropic person may produce the condition arti- ficially by placing convex spectacles of two dioptrics before the eyes. The artificial myopia thus produced would be deprived of half its inconvenience by the previously acquired knowledge of the exact forms and characters of numerous objects which would be only dimly seen ; but the subject of it would find, for example, that instead of being able to tell the hour by an ordinary drawing- room clock from any part of the room, he would have to approach within three or four feet of the dial in order to perceive dim indications of the hands. He would lose all the play of expression on the faces of persons with whom he was engaged in conversation. I once prescribed glasses to correct the myopia of a lady who had for many years been engaged in teaching, and who had never pre- viously worn them. Her first exclamation of pleasurable surprise, as she put on her spectacles and looked around her, was a curious commentary on the state in which her life had until then been passed. She said, " Why ! I io6 EYESIGHT. [chap. shall be able to see the faces of the children I '* If we think what this exclamation meant, and if we apply the lesson which it teaches to other pursuits, we shall not fail to perceive that the practical effect of myopia is to shut out the subject of it from a very large amount of the unconscious education which the process of seeing the world involves, and thus to occasion losses which can hardly be made up in any other way. Taken in detail, these losses, the mere not seeing of this or that seeming trifle, may appear insignificant ; it is their aggregate which becomes important. A young lady was lately brought to me by her parents, on account of the way in which the effects of her myopia had forced themselves upon their notice during a Continental tour. Two schoolboys were of the party, and they subjected their sister to an unceasing chorus of " Don't you see this .? " and " Don't you see that ? " and " How stupid you are ! " until it became manifest to the elders that a state of things which at home had always been accepted as a matter of course was really a very serious evil. A distinguished man of science, who is myopic in a high degree, and who did not receive glasses until he was nineteen or twenty years old, has often told me how much he had to do in order to place himself upon the same level, with regard to experience of quite common things, with many of his normal-sighted contemporaries; and it will be manifest on reflection that the matters which are lost by the short-sighted, as by the partially deaf, make up a very large proportion of the pleasures of existence. I am accustomed, on this ground, strongly VII.] MYOPIA. 107 to urge upon parents the necessity of correcting myopia in their children ; and I am sure that a visual horizon limited to ten or even twenty inches, with no distinct perception of objects at a greater distance, has a marked tendency to produce habits of introspection and reverie, and of inattention to outward things, which may lay the foundation of grave defects of character. Landscape painters are the only persons to whom a small degree of myopia can be useful. I once accompanied a landscape painter on a sketching expedition, and after a time asked him whether he intended to omit a certain house from his drawing. He looked up with surprise and said, ** What house ? There is no house there." I at once understood a curious haziness of aspect with which it was his custom to clothe distant scenery in his pictures, and which was greatly admired by many persons who mis- took it for a skilful rendering of an uncommon atmo- spheric effect. In fact, it was only what the short-sighted man saw always before him ; and I am sure he must himself have been greatly puzzled by much of the praise which he received. Soon after I first published, in the Practitioner iox 1874, a reference to this effect of myopia upon painting, an endeavour was made to account for some of the peculiarities of Turner's style by the pecu- liarities of his vision ; but, as I shall have to explain more fully when speaking of astigmatism, the views advanced in this endeavour appear to me to be entirely erroneous. In order to display the world it is necessary, generally speaking, to give glasses which fully correct the myopia ; io8 EYESIGHT. [chap. that is, glasses of the same number of dioptrics. An eye which is myopic to four dioptrics, and which is fur- nished with a concave lens of the same power, would, if its accommodation were of natural range, be placed in the position of an emmetropic eye. When at rest, it would obtain perfect images from parallel rays ; and it would require to exert accommodation for divergent rays. We often find, however, that the accommodation of myopic eyes is very defective ; for the simple reason that the muscle by which the adjustment is effected has fallen into a state of weakness, or even of imperfect or arrested development, from disuse. A person who is myopic to four dioptrics can read at ten inches without any accommodation effort at all, and would seldom desire to read at a nearer point ; so that his faculty of accom- modation might be suffered to fall into almost complete abeyance. When this has happened, the lens of four dioptrics, which corrects the myopia for distance, cannot be overcome for near vision, and the person cannot read with it at all, or only for a short time and at the cost of much fatigue. Recurring to line eleven. Fig. 31, we have there the diagram of a myopia of four dioptrics in a person twenty-one years old, and with the full accom- modation of nine dioptrics. Supposing his myopia to be corrected by a lens of four dioptrics, his state will then be identical with that of the eye in Hne two of the same figure ; and an exercise of half his accommodation will bring his near-point to nine inches, or nearer than it need be. He will be able to read at the convenient dis- tance of fifteen or sixteen inches by using only two and VII.] ■ MYOPIA. 109 a half dioptrics, or less than one third, of his accommo- dation ; and hence glasses which afforded him complete correction would be useful and available for all purposes for which they were required. They w^ould show him the horizon with perfect clearness, and would enable him to read easily without undue convergence. But if, in- stead of the natural accommodation of nine dioptrics, he had only four dioptrics, the use of half of this would leave his near-point at twenty inches, a distance some- what too great ; and, in order to bring his book to fifteen inches he would have to exert miore or less strain, so as to bring more than half his accommodation into play. The eyes would then soon become fatigued, and reading with the glasses would be difficult or impossible. Hence, when the accommodation of myopic eyes is weak, it is necessary to give weaker glasses for reading than for distance. It is not possible to lay down any rule which will be of universal application, and every case must be considered and treated upon its own merits ; but, in a general wa}^, it may be said that when a myopia does not much exceed four dioptrics, spectacles which com- pletely correct it may be used for all purposes, and may be worn constantly, as if they were parts of the eyes. When the degree exceeds four dioptrics, it will often be necessary to use weaker glasses for reading ; and such as leave two dioptrics of the myopia uncorrected will be found generally available. Thus, if the myopia be of six dioptrics, the subject will generally read comfortably in glasses of four dioptrics, and the same thing will apply to the higher grades. no EYESIGHT. [CHAP. Notwithstanding the presence of spectacles, we fre- quently observe that myopic children have a tendency, the result of habit, to let their books creep up to the eyes, or their eyes go down to the work, even although this entails no small strain of the accommodation. In fact, the importance of doing away with a habitually stooping posture is second only to the importance of doing away with over-convergence ; since the stooping not only tends to fill the eyes with blood, and to subject their blood-vessels to a hurtful degree of distension, but it also contracts the chest, and interferes with the growth of the body and with the complete aeration of the blood by breathing. Parents and teachers should therefore be careful to see, not only that the glasses are worn, but also that they fulfil their purpose of keeping books and work at a distance, which should seldom or never be less than fourteen inches from the eyes. In order that this injunction may be fulfilled, it is necessary to pro- hibit the use of books printed in very small type, and also to prohibit all attempts to read by defective light, whether this be natural or artificial. Myopic people, because they can see at a nearer point than others, can also see by a smaller degree of illumination ; and hence, as children, they more frequently than others contract a habit of reading by twilight, or moonlight, or firelight. Such a habit cannot be too carefully repressed ; nor can too much stress be laid upon the principle that the use of glasses for reading, '' not that the patient may see better, but that he may see farther off," while it is an absolute necessity in all cases of progressive myopia. VII.] MYOPIA. Ill and should always be enjoined during schocl life, or during periods of close study, as the only means of preventing increase of the defect, in consequence of habitual over-convergence, is not less valuable as an antidote to contraction of the chest and stooping habits. It is the more necessary to render this clearly under- stood, because patients are naturally most disposed to prize and to use glasses for what cannot be accomplished without them, that is, for seeing distant objects. They are often unwilling to use them for near work, alleging, and for a time with perfect truth, that they can see better and more comfortably without them. It is not uncommon, indeed, for short-sighted people, when asked if they have used glasses for reading, to assume a tone almost of self-righteousness in their denial of the impu- tation. They say, " Oh, no, I have never done that ! " and are often greatly exercised in their minds w4ien the urgent necessity for a total change of their habits in this respect is explained to them. In some cases of myopia, we find the defect of shape of the eyeball complicated and increased by spasm of accommodation, the effect of which is to make the myopia appear to be of a higher degree than it actually is. This happens chiefly in cases in which the natural union or correlation between accommodation and con- ve'rgence has survived the production of the myopic elongation of the eye. We find, in such instances, that the convergence which is necessary for fusion of the images of the two retinae is attended by a corresponding effort of accommodation ; which, by approximating the 112 EYESIGHT. [chap. near-point, renders the apparent greater than the actual myopia. The artificial approximation of the near-point calls for a still greater effort of convergence ; and thus a sort of vicious circle is established which tends to the increase of the degree of myopia with excessive and alarming rapidity. In such cases, it is a necessary part of the treatment to restrain the accommodation effort by means of atropia ; but this can only be done under careful medical supervision. I will pass on now to the opposite form of ametropia, in which the axis of the eyeball is shorter than the focal length of the media, and which has already been described as hypermetropia. A less formidable name, and one which has the advantage of describing the physical condition accurately, is '* flat-eyed ; " for hypermetropia appears to depend always upon flatness of the eyeball, and not upon deficient refracting power in the media. The conditions are shown diagrammatically in the second group of lines in Fig. 31, where we have a hypermetropia of four dioptrics at three periods of life : at twenty-one, at forty, and at fifty. At the first period, it will be seen that four dioptrics of accommodation are required in order to bring parallel rays to a focus upon the retina ; the far-point being a mathematical negation on the other side of infinity. Four dioptrics, or nearly half of the total range, being thus used up for parallel rays, it is obvious that the eye, to use a happy expression taken from Professor Bonders, begins life with a deficit for all the purposes of near vision. Assuming that it can only use half its power of accommodation continuously, and VII.] HYPERMETROPIA. 113 its total being nine dioptrics, when tlie requirements of distant vision are fulfilled it has only half a dioptric left, and this would bring its near-point no closer than to eighty inches, a distance at which small type would be illegible from the smallness of the visual angle which it would subtend. The subject, in the case supposed, would labour under two disadvantages as compared with a person whose eyes were of emmetropic formation. The latter would have his accommodation muscle at rest when the eyes were not directed to some near object, while the former would never have his accommodation muscle rested excepting during sleep ; or never, at least, when the eyes were being used for any seeing purpose. Secondly, while the emmetropic eyes would see near objects continuously and without painful effort, the flat eyes would only see them by an effort of accommodation which, even if it could be maintained, would be always irksome and would soon become painful. At the age of forty, when half the accommodation had been lost by the operation of time, the remainder would be barely sufficient for objects at the horizon, and would leave absolutely no residue available for nearer work ; while, at the age of fifty, the accommodation would be insufficient even for the horizon, and the subject would no longer have clear vision at any distance. It follows from this condition of things that the pre- sence of flat eyes is generally made known, even in early life, by the fatigue, pain, and speedy dimness of vision which attend upon their employment over any matters requiring close application. The flat-eyed person will I 114 EYESIGHT. [chap. read fairly well for a time, but, after a period varying with the degree of the defect and with the strength of the muscular system, there comes to be a consciousness of effort in the act of seeing. The eyes feel strained, and the letters become somewhat blurred, and are only restored to clearness by a distinct and often strenuous effort. There is an instinctive desirfe to rest the eyes, to close them firmly for a moment or two, and, often, to compress the closed eyes with the hand. After doing this, a fresh start is made, only to terminate in another compulsory stoppage after a period shorter than the first. In some instances, a habit is acquired of unduly approxi- mating the book or other object of vision, insomuch that the condition has actually been mistaken for the precisely opposite one of myopia, and concave glasses have been prescribed for its relief. The explanation of this is that the nearness of the object increases the magnitude of the retinal images, and in this way assists the dim sight in a greater degree than the diminished definition impairs it, If the subject is compelled to work continuously, as by the demands of some occupation which cannot be laid aside, the eyes are apt to become red, blood-shot, and irritable, and to suffer from obstinate forms of super- ficial inflammation or irritation. The symptoms increase progressively with the natural diminution of the power of accommodation, or they not unfrequently undergo sudden increase as an effect of the general debility produced by some form of enfeebling illness. In persons who follow occupations such as needlework, which throw stress upon the sight, there is often a perceptible amelioration VII.] HYPERMETROPIA. 115 on Monday, after the Sunday's rest, and a gradual increase of the symptoms throughout the remainder of the week. The condition thus described was formerly called asthenopia, or weak sight ; and it was well known for many years, and was regarded as incurable, until Bonders discovered the nature of hypermetropia, and saw at once how asthenopia might be a mere result of over-strain of the accommodation. Save for the fact that it sometimes depends upon muscular strain of a different kind, it is speedily and permanently curable by the use of convex spectacles of adequate power, which supply the place of the irksome accommodation effort, and render the muscular exertion superfluous. It is a curious fact that many patients, prior to the discovery by Bonders, had experienced relief by putting on the spectacles of aged people; but, under the influence of the erroneous notions which formerly prevailed, they were strictly pro- hibited, under the threat of I know not what dreadful consequences, from availing themselves of the remedy which chance or instinct had disclosed to them. The unfortunates were commonly subjected to a great variety of futile and often of severe treatm.ent; such as bleeding, blistering, and the administration of various reputed remedies ; and, in the long run, they were generally advised to become agricultural emigrants, or in some way to seek an occupation in which they w^ould never be required to look critically at near objects. It is essential to the complete relief of flatness of the eyeball that spectacles should be worn always ; but it will often happen that the patient cannot bear full I 2 ii6 EYESIGHT. [chap. correction of his defect at first. The instinctive desire for clear images is so strong, that the muscle of ac- commodation is forced into constant and almost spas- modic contraction to whatever extent may be necessary for seeing the horizon ; that is to say, in the case shown in Fig. 31, to the extent of at least four dioptrics. If we now place a lens of four dioptrics before the eye, the muscle Is not at first able to relax itself; and the person has lens and accommodation too. This is equal, of course, to an accommodation of eight dioptrics, which would bring the visual distance to ten inches, and would render all beyond indistinct. In very many cases, the flatness of the eye may in young people be wholly concealed by the accommodation ; in which state it is said to be latent, and is only revealed or rendered manifest by paralysing the accommodation with atropla. Hence, in examining a flat-eyed person for spectacles, it is often necessary to apply atropia in order to determine the precise degree of the affection, or even in order to discover with certainty the fact of its existence ; and, when the effect of the atropia has subsided, it may still be some time before the eye learns that the presence of a convex lens renders the accus- tomed accommodation effort unnecessary, and acquires the power of laying the effort wholly aside, except when the attention is directed to near objects. It is therefore often necessary to use only a partial correction of the flatness in the first instance, as by means of a lens only one-third or one-half as strong as that which will eventually be required ; and, under this VII.] HYPERMETROPIA. 117 treatment, the eye learns its lesson gradually, and the accommodation is relaxed by degrees. The glass chosen should always be sufficient to relieve, in great part or entirely, the distress which was experienced ; and it will usually be found that some return of the old symptoms will point out the time when the glasses first used may be strengthened, either to the extent of full correction or by another step towards it ; the full power sometimes not being used excepting by three gradations. It is manifest, however, that the eyes will be little likely to learn to relax their accommodation unless the glasses are always before them ; since, if they are liable to be every now and then called upon for all the former effort, the causes of the old spasm will be retained in constant operation. In early life, while the accommodation is still powerful, it is seldom or never necessary to give stronger glasses than those which are required to correct the hypermetropia for distance ; but, as years pass on, and presbyopia becomes developed, the failing power of accommodation must be supplied. Thus the subject whose case is shown in Fig. 31, on reaching the age of forty years, as in line eight, even if he had glasses of four dioptrics, to enable him to obtain clear images from parallel rays, would not be able, without further help, to read continuously at any nearer point than about eighteen inches ; and at fifty, (see line nine,) at any nearer point than thirty inches. As soon, therefore, as the presbyopic period of life is reached, we supplement the original correction of the flatness by whatever additional power the presbyopia may require. ii8 / EYESIGHT. [chap. A curious effect of hypermetropia is the production of the common internal or convergent squint, an affec- tion which depends upon flatness of the eyeballs in fully 90 per cent, of all the cases in which it occurs. The rationale of squint is that when a flat-eyed child first begins to employ his vision carefully, say at three or four years of age, it is necessary for him to exert a large amount of accommodation. This is more readily done if it is associated with convergence effort ; and hence the latter becomes as habitual as the former. In ac- cordance with a well-knowm law of the animal organism, muscles which are habitually exercised, without being over-strained, increase in strength and in volume ; and so it befalls that the convergence muscles undergo this physiological change, and, by reason of their increased power and their frequent or constant exertion, they, like the muscles of accommodation, fall into a sort of spasm, from which it follows that the resting or quies- cent position of the eyes comes to be one of conver- gence, say to a point eight or ten inches distant. If this convergence cannot be relaxed, it is obvious that the two eyes will not receive images from any more distant point upon corresponding retinal regions, and hence, for the reasons already stated, double vision of all such objects will be produced. Let it be supposed that the object of vision is at A, Fig. 36, and that the eyes are habitually convergent to the nearer point B, in which position they would have double images of A, which, neither of them being formed on the yellow spot, would neither of them be of the highest degree vri.] HYPERMETROPIC SQUINT. 119 of distinctness. If, now, the right eye, R, were to make a slight rotation outwards, as in the effort of looking to the right with both, it would receive its image of A upon its yellow spot, and would see clearly. There is, how- ever, no power to make one eye move alone ; and we can only look to the right by moving both, an act in which Fig. t,6. the external straight muscle of the right eye, and the internal straight muscle of the left, work together, and by a common impulse. Hence, when the right eye rolls to the right, or outwards, the left eye rolls to the right or inwards ; and the movements are effected under such conditions, both eyes starting from a position of con- vero-ence, that the same amount of effort which rolls I20 EYESIGHT. [chap. the right eye a little outwards so as to bring it into the middle of the eyelid opening, rolls the left eye inwards to a much greater degree, so as to make it squint. In consequence of this movement, the image received by the left eye falls upon a comparatively insensitive part of the retina, and is easily neglected by the conscious- ness ; so that the inconvenience of double vision is re- moved by the act of squinting. In Fig. 37, at A, we Fig. 37. have the natural position of squinting eyes, and at B and C we have the positions which they may assume when in use. At B they are looking to the right, making the right the working eye and the left the squinting eye ; and at C these positions are reversed ; but it is evident that in all three figures the relations of the eyes to one another are alike, and the Figures E and F show the effects of the same movements to right and left VII.] HYPERMETROPIC SQUINT. 121 when the eyes are natural, as at D, and therefore start from parallehsm instead of from convergence. The right eye at B has travelled as far as the right eye at E, and the left eye at C as far as the left eye at F ; and nothing but the difference in the positions from which they start produces the difference in the results. When squinting is first developed it generally affects the eyes alternately ; that is, the child will sometimes look to the right, making the right the working eye and the left the squinting eye, and sometimes to the left, making the left the working and the right the squinting eye, in order to see, singly and clearly, an object which is in front of him. It seems for a time to be a matter almost of accident which eye may squint ; but, after a while, either because one eye has better vision than the other, or because, from some inequality betw-een the muscles, one is more easily rotated outwards than the other, we see, as a rule, that one is made always the working and the other always the squinting eye. As soon as this occurs, the vision of the squinting eye usually begins to undergo progressive impairment ; and the squint should then be cured without delay by an ap- propriate operation, in order that the impairment may be arrested and the vision of the squinting eye preserved. As long as the vision of both eyes is fairly good, a perfect result may always be obtained by an operation ; that is to say, the natural position and the proper har- mony of movements of the eyes may always be restored. If, on the contrary, the operation is too long delayed, until the vision of the squinting eye, and the tone of its 122 EYESIGHT. [chap. muscles, have alike suffered, nothing better than a coarse result, the removal of a gross and obvious deformity, can in the majority of instances be certainly secured. Although about 90 per cent, of all the cases of convergent squint are directly produced by hyperme- tropia, yet the reverse of this proportion does not hold good ; and the hypermetropic persons who squint are probably not more than two-thirds of the whole number, reckoning those only in whom the flatness of the eyeball is distinctly recognisable. The reason why some flat- eyed persons squint, and others do not, appears to depend mainly upon the closeness or laxity of the tie between the functions of accommodation and conver- gence. When this tie is very close, so that the child cannot accommodate without also converging, squint must almost of necessity be produced ; but, where it is more lax, it is quite conceivable that the necessary accommodation effort may be made, although the eyes are nevertheless kept properly directed by the external muscles in the interests of fusion, that is, for the avoidance of double images. I have entered at some length into this subject of squinting, because it is only lately that the cause of the affection has been made known, and because the wildest errors are still uttered and believed about it even by intelligent and educated people. Flat eyes are a matter of formation, like the shape or proportions of other features ; whence they are often common to several members of a family. Upon this slender basis squint was formerly ascribed to imitation ; just as it has been VII.] ASTIGMATISM. 123 ascribed to teething, to fright, and to a countless multi- tude of other conditions which could have nothing to do with it. Children have repeatedly been punished for squinting ; when it would have been as reasonable and as effectual to punish them for having blue eyes instead of brown ones. It cannot be too generally known that children with a certain degree of hypermetropia, and with a certain closeness of association between accom- modation and convergence, must squint by a necessity of their organisation, as the only means by which they can overcome the consequences of their ocular defect ; while those who are differently formed have no cause or temptation to squint, and would probably be unable to do so even by any amount of carefully directed effort. F'or the sake both of vision and of appearance, every squint should be subjected to operation ; but the defect does not afford the smallest ground of objection to the squinting person as a companion for children, who will not copy it unless they are compelled to squint by their own formation. One of the most remarkable instances of the prevalence of squint in a family with which I am acquainted came under my notice at St. George's Hospital, where, in the course of a few months, I operated upon six sisters and upon their mother. I have already described astigmatism as a state in which the curvature of the cornea is different in two different meridians, which are often vertical and hori- zontal or nearly so, and alwa}s at right angles to each other. It is necessary to amend this description by saying, further, that a very small amount of astigmatism 124 EYESIGHT. [chap. is probably the natural condition of the human eye, and that the defect must exist in an unusual degree in order to be disturbing to vision. As contrasted with a portion of a sphere, the most familiar example of an astigmatic surface is furnished by the bowl of a spoon, which is turned upon a shorter radius, in a direction transverse to the line of the handle, than in a direction continuous with it. The former is the meridian of greatest cur- vature ; the latter is the meridian of least curvature. In the human eye, the meridian of greatest curvature is most frequently vertical ; the meridian of least curvature is most frequently horizontal. This rule applies not only to the slight or customary degrees of astigmatism, but also to the excessive degrees ; although many in- stances are upon record in which the ordinary, directions of the meridians have been reversed, and the greatest curvature has been in the horizontal direction. As a rule, again, the astigmatic formation is symmetrical or nearly so in the two eyes, but to this rule we find numerous exceptions. When parallel rays of light are refracted by passing through a medium which presents a convex spherical surface, all the rays of which the light is composed become united in a single focal point, except for some trifling irregularities due to what is called aberration. Disregarding these, and assuming the refracting medium to be of circular outline, the light forms a cone between the medium and the focus, and any section of this cone in a plane perpendicular to its axis is necessarily a circle. Hence the diffusion patches which are formed on the VII.] ASTIGMATISM. 125 retina of a hypermetropic or of a myopic patient, from rays of light which have not united or which have united and over-crossed, are ordinarily circular ; but those formed in astigmatism are only circular if the retina happens to coincide with one particular point of the refracted bundle. In Fig. 38 A a' represents a lens which is more strongly curved in the vertical than in the horizontal direction. The result is, that parallel rays vv, which fall upon the surface of the lens in a vertical plane, are brought to an earlier focus, at v'., than the rays Jih, which fall upon the lens in a horizontal Fig. 38. plane, and are brought to a focus at //. If we suppose A A to be the cornea, v' will be the focus of the meridian of greatest curvature ; Ji the focus of the meridian of least curvature, and the space between the two, v Ji y is called the focal interval. If we were to intercept the course of the rays by a screen, placed between the cornea A A and the focus v\ the diffusion patch would not be a circle, but an ellipse with its major axis horizontal. The rays ^z' would have approached each other more nearly than the rays JiJi. At the point v\ where the rays v v are united, h h not being yet united, 126 EYESIGHT. [chap. the diffusion patch would be a horizontal line. A little farther, v v having over-crossed, and h h approaching each other more nearly, the diffusion patch is a smaller ellipse in the same position as before ; and this passes into a circle as soon as the over-crossing of vv^ and the approach of hh, form equal magnitudes. A con- tinuance of the same process causes the rays to pass into a small upright ellipse, beyond the circle, into a vertical line at the point //, and into a larger ellipse when both the vertical and the horizontal rays have over-crossed jj' Fig. 39. each other. These successive forms of the diffusion patches are represented in the series of diagrams which make up P'ig. 39, and which are taken from Professor Bonders. When a cornea is so curved as to bring the rays which fall upon it in one meridian to an earlier focus than those which fall upon it in a meridian at right angles to the former, the resulting astigmatism may assume five different forms, which are governed by the position of the retina with regard to the two foci. In the follow- ing figures, ^' always represents the focus of the meridian of greatest curvature, and k the focus of the meridian of least curvature. It will be convenient to assume that the form.er is vertical, and that the latter is horizontal. vir.] ASTIGMATISM. 127 In the first form, Fig. 40, the focus of the vertical meridian is in front of the retina, and that of the horizontal meridian is upon the retina. In other words, the eye is myopic for parallel rays refracted in a vertical plane, and is emmetropic for parallel rays refracted ■/ /i Fig. 40. Fig. 41. in a horizontal plane. This is called simple myopi( astigmatism. In the second form, Fig. 41, the focus of the vertical meridian is upon the retina, and that of the horizontal meridian is behind it. In other words, the eye is emmetropic for parallel rays refracted in a vertical plane, and hypermetropic for parallel rays refracted in Fig. 42. Fig. 43. Fig. 44. a horizontal plane. This is called simple hypermetropic astigmatism. In the third form, Fig. 42, the foci of the two meridians are both in front of the retina, but the focus of the vertical meridian is anterior to that of the horizontal. In other words, the eye is myopic for 128 EYESIGHT. [chap. parallel rays refracted in both meridians, but in a greater degree for rays refracted in a vertical plane than for rays refracted in a horizontal plane. This is called compound myopic astigmatism. In the fourth form, Fig. 43, the foci of the two meridians are both behind the retina, the focus of the vertical meridian being anterior to that of the hori- zontal. In other words, the eye is hypermetropic for parallel rays refracted in both meridians, but in a greater degree for rays refracted in a horizontal plane than for rays refracted in a vertical plane. This is called compound hypermetropic astigmatism. In the fifth form, Fig. 44, the retina is situated in the focal interval, so that the focus of the vertical meridian is in front of the retina, and the focus of the horizontal meridian is behind it. In other words, the eye is myopic for parallel rays which are refracted in a verti- cal plane, and is hypermetropic for parallel rays which are refracted in a horizontal plane. This is called mixed astigmatism. Besides these, there are no other forms of regular astigmatism, excepting only that the meridians of least and of greatest curvature may be in any other directions, so long as they are always at right angles to each other. There are certain de- partures from true curvature which are situated in the crystalline lens, or in structures other than the cornea, and which are called irregular astigmatism, but these hardly admit of being reduced to any general description. The degree of astigmatism is the measure of the VII.] ASTIGMATISM. 129 distance between the foci of the two chief meridians ; or, that is, the measure of the difference between the refraction of these meridians, expressed in dioptrics in the usual way. Thus, in a case of simple myopic astigmatism, where the eye is emmetropic for rays refracted in a horizontal plane, w^ith a myopia of two dioptrics for rays refracted in a vertical plane, we say that the astigmatism is equal to two dioptrics. In a case of compound myopic astigmatism, with myopia of two dioptrics for rays in the horizontal plane, and of three dioptrics for rays in the vertical plane, we say that the astigmatism is the difference between the re- fraction in the two planes, or one dioptric. The degrees of hypermetropic astigmatism are expressed in a pre- cisely similar way ; and those of mixed astigmatism by the sum of the two forms of ametropia. Thus, if the eye shown in Fig. 44 had one dioptric of myopia for rays refracted in a vertical plane, and one dioptric of hypermetropia for rays refracted in a horizontal plane, the resulting astigmatism would be equal to two dioptrics. The correction of astigmatism, that is to say, the equalisation of the refraction in the two chief meridians, is effected by means of piano-cylindrical lenses. We have already seen {page 41) that a plano-convex cylin- drical lens, with its axis vertical, exerts no influence upon the rays of light which fall upon it in a vertical plane, while it refracts the rays which fall upon it in a horizontal plane. In Fig. 41, which illustrates simple hypermetropic astigmatism, we have an eye which exerts K I30 EYESIGHT. [chap. a contrary action, bringing rays which fall upon it in a vertical plane to an earlier focus than rays which fall upon it in a horizontal plane. It is manifest that there must be a plano-convex cylindrical lens of such a power that it will just correct the error of refraction of such an eye ; and this lens, if placed in front of the eye with its axis vertical, will increase the total refraction of the rays in the horizontal meridian until their focus coincides with that of the rays which were previously more strongly refracted in the vertical meridian. In other words, there must be a lens which will bring the focus h, Fig. 41, back to the retina, and render it coincident with focus v, the position of which will be left unchanged. In like manner, there must be a plano-concave cylindrical lens which will postpone the refraction in the vertical plane of the eye shown in Fig. 40, and will therefore put back the focus v to the retina, leaving the position of focus h unchanged. A plano-convex cyHndrical lens of the proper strength both corrects and measures simple hypermetropic astig- matism, and a plano-concave cylindrical lens of the proper strength both corrects and measures simple myopic astigmatism. If we look again at Figs. 40 to 44, we shall see plainly that the correction of either of the two forms of simple astigmatism leaves the eye emmetropic as far as its two chief meridians are concerned; and al- though such emmetropia is not absolute, on account of the impossibility of exactly correcting the refraction to the intermediate meridians, it is yet sufficient for all VII.] ASTIGMATISM. 131 the practical requirements of life. In the compound forms, however, the correction of the astigmatism still leaves the eye ametropic; and the correction of the mixed form may be accomplished in such a manner as to produce the same effect. Thus, in Fig. 42, the astigmatism may be corrected in two ways, either by a plano-convex cylinder which w^ould bring forward the focus h to v, and leave the eye highly myopic, or by a plano-concave cylinder which would put back the focus V to /^, and would still leave the eye myopic, although in a less degree. Hence, however the astig- matism is corrected, there must still be a myopic eye ; and the myopia will require a concave lens of the ordinary spherical kind. In like manner, the astigma- tism of Fig. 43 may be corrected by a plano-convex cylinder, to bring the focus h up to v ; or by a plano-concave cylinder, to put back the focus v to h. Either course would leave the eye hypermetropic, the former in a much less degree than the latter ; and in either case the hypermetropia would require a convex spherical lens for its correction. Both the above de- scribed conditions are of frequent occurrence, and they are met by the so-called spherico-cylindrical lenses, which are ground to be portions of a spherical surface on one side and to be portions of a cyhndrical surface on the other. Such lenses are made, of course, of any desired curvature on either side. Mixed astigmatism, as shown in Fig. 44, may be corrected either by a plano-concave c}'linder, to put back focus v to h, combined with a spherical convex K 2 132' EYESIGHT. [chap. to correct the resulting hypermetropla, or by a plano- convex cylinder, to bring up focus h to v, combined with a spherical concave to correct the resulting myopia. Usually, however, a better way is to employ a bi- cylindrical lens, that is, one which has a convex cylindrical surface on one side, and a concave cylindri- cal surface on the other, the axes of the two being precisely at right angles. In this arrangement the concave side sho-uld be of the power required to put back focus v to the retina, and the convex side should be of the power to bring up focus h to the retina. The two foci will then coincide, and the eye will be emmetropic, or nearly so. There is, however, a difficulty in this mode of correction, arising from the fact that it will not be accurate unless the axes of the two cylindrical surfaces are precisely at right angles to one another. This requirement may be defeated by the smallest rotation of the lens during the process of grinding; and its fulfilment can only be secured by the employment of an optician of adequate skill and carefulness. The influence of astigmatism upon the sight is very considerable, and is exerted in various ways. Its first and most obvious effect is to produce differences in the apparent distinctness of equal lines, which are drawn in different directions ; and in this way it produces indistinctness of some of the linear boundaries of figures, leaving others clearly defined. Thus, an astigmatic person, in reading a printed page, may be able to see clearly the vertical lines which enter into the formation VII.] ASTIGMATISM. 133 of many letters, and so, for example, to distinguish an m from an n. But he would have to place the page at a different distance, or to alter the accommodation of his eyes, in order to distinguish the horizontal lines with equal clearness, and to tell readily an n from an u. The indistinctness of many boundary lines produces a corresponding diminution in the acuteness of vision ; and the necessity constantly to alter the adjustment in looking at the same object produces great fatigue of the ciliary muscles. Hence, defective sight, coupled with weariness and aching of the eyes, are the symptoms of which astigmatic people most commonly complain ; and these symptoms, which may be kept in comparative abeyance while the accommodation is strong and active, tend constantly to increase and to become every year more irksome as the accommodation is curtailed by advancing life. The defective vision of astigmatism is primarily for lines which are at right angles to the meridian of the preponderating ametropia. In the case shown in Fig. 40, for example, there is myopic curvature in the vertical meridian ; and in Fig. 41 there is hypermetropic curvature in the horizontal meridian. The effect of myopia, that is, of myopic curvature, in the vertical meridian is to render the eye short-sighted for horizontal lines ; and the effect of hypermetropic curvature in the horizontal meridian is to render the eye dim-sighted for vertical lines. The reason is, that a line is formed, optically speaking, of a succession of points, from each of which light is reflected. In the case of a vertical 134 EYESIGHT. [chap. line,- any diffusion circles which may be formed from the rays which diverge from successive points of the line in a vertical plane will overlap and conceal one another in that plane, and will not impair the distinct- ness of the retinal image ; but any diffusion circles from the rays which diverge in a horizontal plane will serve to widen out the retinal image in a horizontal direction, and will cause the line to appear diffused or spread. Hence, the eye which has hypermetropic formation in the horizontal meridian has hypermetropic vision for vertical lines; and the eye which has myopic formation in the vertical meridian has myopic vision for horizontal lines. In writing upon the subject, it is necessary to adopt some uniform system of nomenclature in order to avoid confusion ; and my own practice is to neglect the formation and to describe the vision, as the element which is of the greatest practical concern. By vertical myopia, therefore, 1 mean myopia for vertical lines, and I disregard the fact that the vertical myopia is a result of faulty curvature in the horizontal direction. This method of description has an additional advantage in the promotion of simplicity ; since the axis of the correcting cylinder must always be placed in the same direction as that of the visual defect. The fact of the existence of astigmatism, although it may often be inferred from the fatigue and distress which are experienced in using the eyes, and from the imperfect relief afforded by common spectacles, can only be determined by the greater improvement produced by cylindrical glasses, or by giving the patient an object vii-j ASTIGMATISM. ^35 of vision which contains equal lines running in different HoRtZONTAL. /^* ^'^ 90 105 120 "'."•'II i as eo' 73 T\ T C /35 /SO fS5 Fig. 45- directions. Of test-objects of this description I have found none more convenient than the series of striped 130 EYESIGHT. [chap. letters shown in Fig. 45, which were designed for the purpose by an American physician, Dr. Orestes Pray. If, on looking at these letters from a good distance, say of six or eight feet, according to the degree of illumination, the stripes in some of them are decidedly more distinct than those in others, the presence of astigmatism is declared ; and the directions of the meridians of least and of greatest curvature will be shown by the directions of the stripes which are most and least distinct. For a line in each of these two directions the refraction must be tested separately, and the difference will declare the degree of the astigmatism, and will show the power of the cylindrical lens required for its correction. The efficacy of this lens, however accurately it may be prescribed, will depend upon the position of its axis ; since a position at right-angles to the proper one will double the defect. For this reason it is usual to make cylindrical lenses circular, so that any slight deviation from the best position may be rectified by turning them to the required extent in" the rings of the spectacle frame. When the spectacles are otherwise ready, the patient should himself give the lenses their final adjustment for position before they are fixed ; and in this way the most precise accuracy may be attained. Of course it is quite possible to have the cylindrical lenses cut into oval form after the best position of the axis is determined ; but the London workmen are apt, from want of adequate knowledge of what they are about, to fail in this endeavour, to displace the position of the axis, and to VII.] ASTIGMATISM. 137 spoil lens after lens before they succeed. In the great cities of America the case is different. Astigmatism is not uncommon in the United States, and Americans, as a rule, will not wear spectacles with circular glasses. The effect of their determination is that the opticians of New York and Philadelphia, not to mention other places, make cylindrical lenses of oval shape with absolute correctness. As a cylindrical lens corrects astigmatism, so, of course, it renders a natural eye astigmatic ; and, by wearing cylindrical lenses as an experiment, it is easy to feel and realise the conditions of vision which astig- matism produces. It has been said, and is often sup- posed, that astigmatism produces distortion of the shapes of objects, but, in reality, it has no such effect. If we place a cylindrical lens in the tube of a magic lantern, we shall obtain distortion of the images cast upon the screen ; but this is due to the fact that the rays of light, after their refraction, travel a sufficient distance to allow the distortion to occur. The distance between the cornea and the retina is not sufficient for this purpose ; and the only effect of astigmatism is to obscure certain boundary lines, rendering them hazy and ill-defined. If we place cylindrical glasses, with their axes vertical, close to the eyes, and look through them at any familiar object, such as a postage-stamp, we shall see no alteration in its shape; but its lateral boundaries, and the lateral boundaries of the Queen's head, will look fuzzy and con- fused. If we remove the lenses a little way from the eyes, so as to give the distance necessary for distortion. 138 EYESIGHT. [cH. vii. distortion will be produced ; the object appearing to be extended in a vertical direction if the lenses are con- cave, and in a horizontal direction if they are convex. It is unimportant whether the object looked at is near or distant. As long as the cylindrical lenses are close to the eyes, an obscuration of the boundary lines of the object, in a direction transverse to the axes of the cylinders, is the only effect observed ; but, as soon as the lenses are held at a distance, alteration of shape becomes apparent. As the seat of ordinary astigmatism is in the cornea, what is true of the lens held close must be true, in a still greater degree, of the astigmatic eye itself; and hence the suggestion that correctness of shape in drawing can ever have been disturbed by the astigmatism of an artist is one which appears to me to have no sort of foundation in the facts of the case. CHAPTER VIII. ASTHENOPIA ; OR WEAK SIGHT. We have seen that the vision of the natural eye ranges from infinite distance, or the fixed stars, to a near-point the nearness of which depends upon the amount of the accommodation, which should be about four-and-a-half inches distant at the age of twenty-one, and which becomes progressively more remote as life advances. Within the limits of the range thus specified, or at least within the narrower limits determined by the impossibility of using the whole of the accommodation, the natural eyes can be exerted continuously for inde- finite periods ; and it is characteristic of most forms of ametropia that the use of the eyes is not thus free and unrestrained, but is conditioned by limits of time, or by limits of distance, to which natural eyes are strangers. Limitations of distance may be obvious con- sequences of ametropia ; as when the short-sighted person has his far-point brought within a range determined by the degree of his myopia, and is shut up within a visual horizon of ten or twenty-inches, as the case may be. I40 EYESIGHT. [chap. Limitations of time are imposed, as a rule, by some disturbance of the natural harmony between the muscles by which the eyes are accommodated and directed ; or, in other words, by want of proper co-ordination between accommodation and convergence. Hence, in a large proportion of cases of ametropia, and in some in which no ametropia is discoverable, we find, after a period of use, longer or shorter according to circumstances, either that the vision becomes indistinct, or that pain is ex- perienced in the eyes themselves, in the regions im- mediately around them, or even generally over the head. In many instances the symptoms of distress commence with indistinctness of vision which, if the effort to see be continued, leads on to pain. In many there will be found sore lids, blood-shot conjunctiva, or some other sign of superficial irritative or inflammatory disturbance. In a few the headache is apt to be followed by sickness, giddiness, palpitation, and other symptoms ; and these may even be so severe as to occasion a belief that the patient is suffering from some obscure disorder of the heart or brain. The foregoing conditions are conveniently included under the general term of asthenopia, or weak sight, depending, as they all do, upon some cause which renders sustained vision impossible. The word is only a convenient way of saying that the person cannot use his eyes for long together ; and it requires to be ex- plained, in every case, by some other which is descriptive of the precise nature and the apparent cause of the inability. In some instances asthenopia is associated with VIII.] ASTHENOPIA; OR WEAK SIGHT. 141 manifest defect of vision ; in others the vision is Httle, if at all, below the natural standard. It has been stated in a former chapter that asthenopia was for many years regarded as incurable ; and that Donders, when he discovered the nature and the fre- quent existence of hypermetropia, was at first disposed to trace nearly all examples of asthenopia to the strain which was thrown upon the muscle of accommodation by its endeavours to correct the flatness of the eyeball. In an immense number of instances this explanation was verified by experience, and was found to furnish a clue to successful treatment by the use of convex glasses ; but the great attention which was soon given to the matter speedily led to the conclusion that some asthen- opic persons were not hypermetropic, but myopic, or even emmetropic ; and it was suggested by Von Graefe that in these cases the fatigue might be fatigue of the internal straight muscles by the act of maintaining con- vergence for some definite distance, instead of being fatigue of the accommodation. Von Graefe proposed a division of asthenopia into the accommodative and the muscular, accordingly as the effort which occasioned distress was that of accommodation or of convergence. He supposed that, in cases of the latter class, the internal straight muscles were the subjects of a peculiar weakness which he called " insufficiency ; " and he suggested tests by which he thought this insufficiency might be demon- strated, and its degree measured. Later observations have neither confirmed his hypothesis nor established the value of his tests ; but have tended to show that the 142 EYESIGHT. [chap. cause of asthenopia, probably in almost every instance, is a want of harmony between the accommodation effort and the convergence effort ; and that the muscular fatigue is due, not to the absolute exertion of either function, but to endeavours to combine the two in proportions which disturb the natural relations between them. When measured by the natural or emmetropic standard, it is evident that in every case of hypermetropia the eyes will be called upon to exert accommodation in excess of their convergence ; and that in every case of myopia they will be called upon to exert convergence in excess of their accommodation. Referring back for examples to Fig. 31, we shall see that the person whose state is shown on line seven would require to exert five dioptrics of accommodation (an amount which would bring the vision of an emmetrope to a point only eight inches away), when his eyes were convergent to a point forty inches away. In like manner, the myope shown on line eleven would require to render his eyes convergent to a point only ten inches distant when his accommodation was absolutely at rest ; and an accommodation effort of one dioptric, which would place the emmetropic visual dis- tance at forty inches, would require from the myope convergence to eight inches. It has been already stated that the correlation between accommodation and con- vergence is much closer and more complete in some eyes than in others ; as if in some the natural relations had survived the change in the shape of the outward organ, while in others they had yielded to the pressure exercised upon them by the requirements of vision. It seems VIII.] ASTHENOPIA; OR WEAK SIGHT. 143 that, in cases of the former kind, in which the natural relation between accommodation and convergence has been preserved, the combination of the two functions in unnatural degrees cannot be maintained ; and that this, rather than the absolute exercise required from either, is the cause of the asthenopic symptoms. No emme- tropic person could maintain convergence to eight or ten inches, in repose of the accommodation, even for five minutes ; although nearly every emmetrope of the age of twenty-one could maintain accommodation and con- vergence for a distance of ten inches even for hours. It is not surprising that what would be impossible for all emmetropes should also be impossible for some myopes, and there can be no doubt that such is the fact. Some few years ago I saw a very remarkable illustration of this, which I have already published in detail, but the chief features of which it is worth while here to repeat. A young gentleman of good position, who was reading for honours at his university, suddenly broke down, with symptoms which were attributed to some form of brain disease, and was advised to give up his studies and to go home. After a period of rest, being no better, he sought advice in London, where the opinion previously given was confirmed ; and, as a means of affording the most complete possible rest to his brain, he was advised to make a voyage to Australia and back. He did so, and returned in the same condition. He was then con- sidered to be incurable, was told that he must abandon a career which had been open to him, and a matrimonial engagement which he had formed. In a word, his whole 144 EYESIGHT. [chap. life was blighted. Ultimately, he was brought to me, not from any idea that his eyes were at fault, but merely that I might examine their internal circulation with the ophthalmoscope, in order to see whether this examination would throw any light on the state of the circulation in his brain. I found his eyes healthy, but myopic to five dioptrics ; and, on making inquiry into his symptoms, ascertained that they resolved themselves into simple inabihty to read. As soon as he took up a book, he became giddy ; and the giddiness brought on intense headache, palpitation of the heart, and some- times sickness. The case was of the simplest kind. The patient had never used spectacles, and, up to a certain point, he had been able to read well and easily ; that is, he had been able to converge to eight inches in repose of his accommodation. When he began to work for honours, and to read eight or ten hours a day, the disproportionate exertion of the two functions could no longer be continued. The convergence muscles gave w^ay ; and then, as the two eyes were no longer directed to the same point, there was double vision. This, in its turn, produced giddiness; and the giddiness produced headache and sickness by disturbing the circulation. The strained muscles which had once given way became prompt to give way again when they were unduly called upon ; and the grave view which was taken of the symptoms by medical men filled the patient with alarm. As soon as he tried to read, the old troubles were brought back by fear and expectant attention. I assumed him that he had no brain disease, tried to make him viii.] ASTHENOPIA; OR WEAK SIGHT. 145 understand his condition, prescribed spectacles to correct his myopia and to diminish his convergence, and told him to wear them constantly and to read in them three times a day for half an hour at a time. He was to report progress in three weeks ; and at the end of that time he returned cured. He could read as much as he liked, he was going to be married in the following week, and on returning from his wedding trip was to take up the career which he had fancied closed to him for ever. All these pleasant anticipations were in due time fulfilled, and the cure was permanent and complete. If, in every case of ametropia, the faculties of ac- commodation and convergence became disjoined and capable of independent exercise, asthenopia would not occur ; and, if these faculties preserved in every case their natural relations, asthenopia would occur in all ametropic persons, but would always be curable by complete correction of the ametropia. Unfortunately, an intermediate condition commonly exists ; in which the natural relation is disturbed, but not completely broken through ; and it is this which occasions the greatest amount of perplexity. At one time, it was thought that much good might be done in cases of troublesome asthenopia by having convex and concave lenses ground upon prisms ; a practice which I intro- duced to the profession in this country in 1S69, by a translation of a work upon the subject by Dr. Schcfflcr of Brunswick, who, although he was not the originator of the idea, was the first to suggest its systematic appli- cation in practice. Starting from the proposition that L 146 EYESIGHT. [chap. we can diminish convergence by prisms with their bases inwards, and can increase it by prisms with their bases outwards, Dr. Scheffler proposed that prisms should be used in almost all cases, as means of bringing accommo- dation and convergence into harmony. The method was beset by many inconveniences, was of little practical utility, and was soon abandoned ; so that the use of prismatic spectacles is now scarcely known except as an instrument of quackery, and to diminish convergence effort in a few cases of presbyopia. The true principle on which asthenopia should be treated is, in the first place, to prescribe appropriate spectacles for the ame- tropia alone, and to have them used steadily for two or three weeks. If, after the lapse of such a time, as- thenopia is still troublesome, it is nearly certain that the patient is exerting either too much accommodation for his convergence, or too much convergence for his accom- modation. In order to redress the balance, it is best to make the convergence a fixed quantity, say to a point fifteen inches distant, and to modify the accommodation to harmonise with it. It is not so much the absolute accommodation effort that occasions distress, as its dis- parity with the convergence effort ; and hence we may always, if necessary, call upon the eyes for a little more accommodation in order that the disparity may be removed. To test the nature of the condition, it is neces- sary to have frames from which the glasses may be shifted at pleasure, and which will carry two lenses before each eye. We place in such a frame the lenses which were first prescribed, and cause the patient to read with them, VIII.] ASTHEx\OPIA; OR WEAK SIGHT. 147 at a distance of fifteen inches, until he is tired. As soon as symptoms of distress appear, we add to the lenses prisms of about four degrees, with their bases inwards ; thus call- ing upon the eyes for less convergence. If the previous accommodation was in excess of the convergence, it will be left still more in excess, and the distress will be increased. If the previous convergence was in excess of the accommodation, the excess will be diminished and the symptoms relieved. The result, one way or the other, will be declared in a few minutes ; and it may be checked by reversing the experiment, and placing the prisms with their bases outwards, so as to increase the convergence. In this way, we may find that the pre- viously exerted accommodation was either too much or too little for convergence to fifteen inches ; and we may then rectify the disparity by altering the convex or con- cave lenses. It is manifest that stronger convex lenses or weaker concaves diminish the demand upon the ac- commodation ; and that weaker convex lenses or stronger concaves increase it. By this means it is generally pos- sible to bring the accommodation and the convergence into harmony by common lenses alone, which may be obtained everywhere, and to abandon the use of prisms except for testing purposes. The prisms are not only heavy and unsightly; but, unless they are cut and fitted into the spectacle-frames with perfect accuracy, they are liable to produce disturbing double images in an upward and downward direction. In asthenopia of great severity or of long standing, even when we have corrected every discoverable ocular L 2 148 EYESIGHT. [chap. defect, and have brought the accommodation effort and the convergence effort into harmony, we shall sometimes find that the use of the eyes is as painful or difficult as before. For a time this will be so in many cases ; and it is always well for sufferers to understand that their glasses will call upon the eyes to work under new con- ditions, which, although better than those which they supersede, may yet be irksome as long as they are new. We must not, therefore, reckon too confidently upon immediate relief; and it is best to insist upon the dili- gent use of the prescribed glasses for at least a fortnight, before Vv^e form a definite opinion about their probable efficacy. After the lapse of that time, if the patient is still complaining, and if we are sure that we have placed the ocular mechanism under the most favourable attain- able conditions, we have next to develop the powers of that mechanism by carefully regulated exercise. The patient has often been encouraged, by previous advisers, to '' rest the eyes ; " advice which is readily followed on account of the present relief which it affords, but which, several years ago, I described as being always wrong in principle, and always injurious in practice. Rest is necessary in many states of inflammation or disease ; but to rest a muscle which is only weak is the surest way of increasing its weakness, since its nutrition and vigour are alv/ays greatly dependent upon proper exer- cise. We have often to combat a totally groundless fear of blindness, and to deal with organs v/hich have been already " rested " until all their nervo-muscular apparatus has been brought by disuse into a state at once of debility VIII.] ASTHENOPIA; OR WEAK SIGHT. 149 and of excitability. Such conditions can only be relieved by careful strengthening of the weakened muscles ; for which purpose the employment of the eyes must be so regulated as not to impede nutrition by occasioning undue fatigue. The best method of fulfilling the required conditions was first clearly described by Dr. Dyer, of Pittsburgh, Pa., whose rules for the purpose have proved so useful, and their value has been so thoroughly esta- blished by experience that, in the United States, the process is commonly called *' Dyerising." I will give a portion of Dr. Dyer's instructions in his own words, which, on this side of the Atlantic, are less generally known than they deserve. " The exercise of the muscles is best accomplished by reading. The patient is directed to select a book of good type, but not too absorbing, and to read regularly with the prescribed glasses three times a day. He must determine by trial the number of minutes he can read without discomfort. He may find this to be thirty seconds, five minutes, ten minutes, or even more. He must, however, find this initial point. Starting at this point, he must read regularly, and always with the p-lasses. The first reading; must not be until one half- hour after breakfast, the second at noon, the third finished before sundown. The periods of reading must be regu- larly increased from day to day. No other use of the eyes should be allowed. In cases where discomfort occurs in less than five minutes, the increase should not be more than one half-minute per day until ten minutes are reached. In other cases the patient may increase ISO EYESIGHT. [chap. one minute each day until he can read thirty minutes three times a day without pain. If this can only be done with pain, the patient must be encouraged to persist, notwithstanding the pain ; the surgeon, however, exercising his judgment in not pushing the treatment too rapidly. Should the pain continue from one period to the next, it is evidence that he has gone beyond the maximum of his ability, and that he should fall back to a period at which he can read without discomfort, should regard that as a new point of departure, and proceed as before. As said above, reading is the best exercise ; but it frequently happens that the patient is very desirous to write or to sew. This may be attempted when thirty minutes has been reached in the middle period. After the exercise has begun by reading ten minutes, sewing or writing may be tried for ten minutes, and the period finished by reading. From this point I permit an increase of two minutes a day, and a relative increase in the time of writing. This may be gradually introduced into the morning and evening period. I do not consider the treatment completed until an hour and a half has been reached. " I have found it of great assistance to explain the rationale of the treatment to the patient. These cases rarely occurring except in the educated classes, they readily understand their nature and are anxious to assist the surgeon. I tell them that, in reading, pure muscular action is required as much as In lifting a weight ; that, through want of use, debility, or some derangement of the system, they have lost the power to exert the VIII.] ASTHENOPIA; OR WEAK SIGHT. 151 reading muscle without fatigue ; that they can strengthen this muscle and increase its power of endurance by- regular, constant, and systematic exercise, as well as with any other muscle in the body. The course of treatment serves to distract the mind of the patient and restores his confidence in his ability to use his eyes. He has become discouraged ; he has had the horror of blindness carefully instilled by friends, and sometimes by well-meaning physicians, who, not feeling quite sure of their ground, err on the safe side and prescribe entire rest. In these cases, the safe side is the wrong side. When the glasses are procured, and the patient is assured that there is no absolute disease of the eye as revealed by the ophthalmoscope, he commences his course of treat- ment with hope and zeal. The mere fact that he is told he must use his eyes gives him, to a certain extent, the power to do so." Dr. Dyer elsewhere lays great stress upon the import- ance of restraining impatience when improvement is beginning to be declared. The patient who finds that he can read for ten minutes without distress is very likely to go on for twenty minutes, or until pain warns him to stop ; but to do this is to invite relapse. With an increase of only one minute a day, the duration of the treatment would be about three months ; and it is better to submit quietly to this period of modified use, and of self-restraint, than to lose time at the beginning by fruitless endeavours to hasten a process which depends essentially upon the gradual improvement of muscular nutrition. CHAPTER IX. COLOUR, COLOUR-VISION, AND COLOUR-BLINDNESS. The phenomena which have been described in the preceding pages have had reference to vision as it deals with the perception of form alone ; but it is hardly necessary to say that the information which we receive through our eyes with regard to visible objects is materially enlarged by the different way in which these objects appeal to the sense of colour. The light by which the things around us are illuminated and rendered visible is, in all ordinary circumstances, composed of rays of different degrees of refrangibility and of different wave-lengths ; and these differences are perceived, by the great majority of persons, as the causes of many different sensations which we include under the general v/ord colour, and to which a great number of distinctive names have been attached. It was discovered by Newton that, when a ray of sunlight is made to pass through a transparent prism, the different parts or constituents of the ray undergo unequal degrees of refraction. In order to observe the en. IX.] COLOUR. 153 phenomena in the most perfect manner, the sunHght should be admitted into an otherwise darkened room, through a small circular aperture; and, after refraction, should be received upon a v>hite surface. Instead of the ray as a whole being merely displaced towards the base of the prism, in the way already described, so as to produce a circular spot of white light in a position different from that in which this spot would appear if the prism were absent, the light is spread out into a horizontal bar, called the prismatic spectrum, which is . again divided vertically into bands of colour, succes- sively red, orange, yellow, green, blue, indigo, and violet, and which contains also invisible rays, called " ultra-" red, and " ultra-" violet, beyond its apparent boundaries. Of the several bands, the red portion is the least, and the violet portion the most, diverted from the original track of the ray ; so that the former is said to be com- posed of the least refrangible rays, and the latter of the most refrangible rays. Investigations of much later date have appeared to show, first, that differences in refrangibility are due to, or at least are associated with, differences in the lengths of the undulations or waves by which the phenomena of light are believed to be pro- duced, the longer waves being less refrangible, or less liable to be diverted from their original course, than the shorter ones ; and, secondly, that four of the seven ap- parent colours of the spectrum are compound, the results of the admixture, or of the overlapping of the margins, of the three simple colours. With regard to these simple colours, there has not been entire agreement 154 EYESIGHT. [chap. among physicists ; but the latest researches point to red, green, and violet, as those from which all others are produced. It is necessary to remem^ber, in dealing with this subject, that it has reference to light alone ; and that no inferences can be drawn from observations made upon pigments, which are always compound colours, even when they produce to the eye the effect of simple ones. The colours of all the objects in nature are due to the different degrees in which they absorb or reflect the several elements which enter into the form- ation of white or solar light ; and it will be manifest that an infinite variety of compounds may be produced by the breaking up and disturbance of light waves by the crossing or admixture of others of different length, or of the same length but not vibrating in unison. In the normal eye, the retina is capable of being excited to sensation by light waves of a great variety of lengths and vibrations, and is therefore susceptible of a great number of different shades and intensities of colour, for the due differentiation of which a somewhat formidable vocabulary has been invented and is constantly being enlarged. But there are some persons whose eyes are blind to colour ; that is to say, whose retinae are unable to distinguish between different wave-lengths, but receive the same sensory impression from them all. These persons see no colour ; and only perceive through their eyes the forms of objects, and differences in the absolute amount of light reflected from their surfaces, without reference to its quality ; differences, that is, of light and shade, but nothing more. Such truly colour-blind IX.] COLOUR. 155 people are very rare ; but there are others, far more numerous, who are bhnd to one colour only, sometimes entirely, sometimes only partially. Of these, some are blind to red, some to green, and a few to violet. For the sake of illustration I will assume what is not strictly the case ; namely, that the three simple colours enter into the formation of white light in equal proportions. It is manifest, on this assumption, first, that the person who is blind to one colour will perceive all white objects under one-third less illumination than they present to normal eyes ; secondly, that objects will appear to him less illuminated in proportion as they reflect a larger amount of the colour to which he is blind and less of the others ; and thirdly, since there is no object in nature which absorbs the whole of any of the primary colours, and reflects only a dual combination, that every colour will assume an unnatural aspect in the eyes of the persons who are blind to one only, and who therefore lose one of the elements which enter into and modify all compound colours in the eyes of the normal sighted. The question of colour-blindness has been brought into great prominence by the demands of railway and steam- boat signalling ; since it is necessary to use red and green lights or signals for these purposes, and the dif- ference between red and green is that with regard to which the partially colour-blind most frequently fall into error. Blindness to red and blindness to green are the most common forms of the defect ; and either of these would involve a liability to mistakes by which many lives might be sacrificed. 156 EYESIGHT. [chap. To determine the fact of colour-blindness is less easy than might be supposed ; and many inquirers have adopted methods of so faulty a kind that the conclusions based upon them are entirely valueless. For example, ignorant people, or children, have been asked to name different colours at sight, and have been set down as colour-blind when they named them incorrectly ; al- though the errors, in many cases, were afterwards shown to have been due only to an imperfect acquaintance with the chromatic vocabulary. A test which at first seems more trustworthy, although it is not so in reality, is to place before an intending signal-man, or outlook- man, red and green discs, or lights, alternately, as he would see them when on duty, and to ask him, again and again, which is which. The source of error here is that the coloured objects are presented to the sight under the same conditions of illumination, so that, to the colour-blind, one will look brighter than the other. The colour-blind person will see a difference between them, although not the difference ; and thus, if he happens to be right the first time, he v/ill keep right all through as long as the same conditions of illumination are preserved. Suppose the man thus tested to be blind for red. He will not see any difference of colour between the red disc and the green one, but the former will look darker to him than the latter, because he will be blind to the greater portion of the light which is reflected from it. In the same way, a red .lamp will .shine with a much fainter light than a green one, sup- posing the flames behind the two glasses to be of equal IX.] COLOUR-VISION. 157 intensity. The perception of this difference in the degree of illumination, as a matter of comparison between two signals, would not in the least assist him to discover whether a single signal, seen under different conditions of illumination, was of one colour or the other; and the test is therefore worse than valueless, because it is misleading. The only way in which the fact and the nature of colour-blindness can be certainly determined is one which has been devised by Professor Holmgren of Upsala ; and which is as much superior to other methods in simplicity as in accuracy. The examiner is provided with a large number, about 150, skeins of Berlin wool; the collection including all the chief colours and several shades of each. The skeins are placed in a heap on a table covered with a white cloth, and in good daylight. The examiner selects two or three skeins of decided colours from the heap, and places them aside ; and the person examined is then told to select and place beside them other examples of the same colours. There is no ques- tion of nomenclature, but only of selecting matches, not absolute matches, but other skeins which conv^ey to the eye under examination a similar impression to those selected as tests. If the person examined matches cor- rectly, his colour-vision is assured ; if he makes mis- takes, those mistakes demonstrate not only the fact of his colour-blindness but also its nature ; whether, that is, he is blind to colour absolutely, to red, to green, or to violet. The limits of this treatise will not allow me to pursue 158 EYESIGHT. [chap. this very Interesting subject to its proper limits ; but readers who desire further information with regard to it may obtain it from a volume on the subject by Dr. Joy Jeffries, of Boston, v/hich is pubHshed in London by Messrs. Triibner. Dr. Jeffries has for many years been engaged in original investigations upon the subject, and has also largely availed himself of the works of Professor Holmgren and other authorities. He has brought together a number of facts showing the frequent heredit- ability of colour-blindness, and also the curious prepon- derance of the male sex among the subjects. Taking the mean of trustworthy observations in all countries, it would appear that rather more than four per cent, of males, and not more than a fifth per cent, of females, are the subjects of some form or degree of the defect. Colour-blindness properly so-called is a matter of formation, and cannot in any way be modified by train- ing or practice. It would be no more possible to enable a colour-blind person to recognise colour, by any amount or kind of teaching, than it would be possible to enable a totally deaf person to recognise sound. The organ of the faculty is wanting or inactive, and the faculty itself is absent. Many well-meaning but in this matter igno- rant people have talked and written about educating the sense of colour, which can no doubt be done when it is present, but not when it is absent ; and their efforts in several instances have been manifestly directed to the not very difficult task of teaching the names of colours to children who were always perfectly well able to dis- tinguish them, but who blundered when asked what IX.] COLOUR-BLINDNESS. 159 they ought to be called. A remarkable intellectual phenomenon has lately been exhibited in a letter ad- dressed to the Times paper by a gentleman whose name escapes my recollection, but who described himself as an inspector of schools, and who set forth that he never had been able to establish the fact of the existence of colour-blindness in children under seven years of age. He apparently wished this inability on his part to be ac- cepted as evidence that all children are born with correct colour perception, and that colour-blindness is a trick, or habit, or incapacity, acquired after the mysterious age of seven years has been passed. It requires unusual intelligence in the subject, and great patience and tact on the part of the examiner, even to determine the degree of acuteness of ordinary vision by letter tests in a child under seven ; and to ask little children questions about colours, or to attempt to test their colour vision except by Holmgren's method, is merely to add one more to the many possible methods of wasting time which the ingenuity of former generations has dis- covered. Even Holmgren's test, which requires practice and aptitude in its application to uneducated adults, would require these in far greater proportion, if any attempt were made to introduce it successfully within the walls of an infant school. Besides true or congenital colour-blindness, there are forms of incapacity to distinguish colour which may be produced by disease. In jaundice the transparent media of the eye may become tinged with yellow, and all objects of vision will then appear as if seen through i6o EYESIGHT. [chap. yellow glass. The drug santonine produces yellov/ vision in certain circumstances. In some forms of disease of the brain or spinal cord, the loss of colour- vision is an early and a very important symptom, which is often perceived because it is often looked for ; and it is highly probable that the same defect occurs, more frequently than is supposed, in connection with transient forms of brain disturbance, and passes away again as these are relieved or disappear. In certain diseases of the retina colour-blindness is observed as a symptom ; and it is manifest that the natural colour perception is capable of being disturbed, either permanently or temporarily, by a great variety of conditions. The letter from the inspector of schools, to which I have just referred, was followed or preceded by another from a hospital physician, relating the case of a costermonger who had failed, to his own great amusement, to distinguish colours when first tested, but who, by perseverance, and by the diligent care of the sister of the ward, was taught to recognise them before he was discharged. The inferences sua"g;ested were that colour-blindness, at least in this case, was merely another word for ignorance depending upon the imperfect cultivation of a faculty, that the man could not distinguish colours because he had not been taught to distinguish them, and that those who can distinguish them are indebted, more or less, to education for their capability. The narrator apparently failed to perceive the bearing of what was incidentally mentioned as part of the history of the case ; namely, that the patient was admitted into hospital on account IX.] COLOUR-BLINDNESS. i6i of a " nervous affection. " There can be little doubt that he was rendered temporarily colour-blind by some disturbance of the circulation in his brain, that he recovered his colour-perception as he recovered his health, and that his recovery was promoted by judicious cultivation and exercise of the impaired faculty. The very fact of the amusement which was afforded to him by his own mistakes is sufficient to show that he had been able to perceive the differences between colours at some former period ; for, if not, he would not have been sufficiently aware of the marked nature of these differences to have been amused at his mistakes when he was told of them. If a man has once seen red, it is intelligible that he should be amused when he is told that he has mistaken it for some other colour, or some other colour for it ; but, if he has never seen red, and has no idea what it is like, such a mistake will appear to him to be perfectly natural, and will afford no matter for amusement or even for surprise. The difficulty would be to avoid the mistake, not to fall into it ; and this part of the case in question would alone be sufficient to afford conclusive evidence of its real character. If we reflect upon the nature of visual impressions, and even upon the little certainty afforded by verbal descriptions that different people see things precisely in the same manner, we shall readily recognise that colour- blindness, which is a mere negation, the mere non- recognition of a particular quality in a particular manner, may easily escape detection unless it is carefully looked for. What the world in general calls red or M i62 EYESIGHT. [chap. green or violet presents, to the sense perceptions of the colour-blind, an appearance of some sort, and this appearance he comes to associate with the sound. Many of the colour-blind have remained for long periods unconscious of their defect, until its existence has been established by accident ; and some have shown great ingenuity in the recognition of colour by other qualities of the coloured surfaces. It is very important that a defect, which disqualifies for certain positions in life, should be recognised early ; and it is therefore desirable that parents should test the colour-vision of their children from time to time, not necessarily in any systematic or obvious way, as a set examination to be gone through, but by observing whether they are capable of matching colours correctly. The principle of Holmgren's method, which is the only sound one, maybe applied sufficiently for all domestic requirements without having recourse to any other coloured objects than those which every ordinary household will supply ; and any mistakes which seem to indicate defective colour-vision should not at once be made subjects of comment, but should rather suggest experiments to determine whether similar mistakes will be unconsciously repeated. By a little care and patience parents would soon be able to arrive very nearly at certainty ; and the complete application of Holmgren's test would only be called for in cases where the existence of some degree of defect had been rendered obvious in the more simple way. In ever}^ such Instance, of course. It would be desirable to apply the test completely at as early an . IX.] COLOUR-BLINDNESS. 163 age as the intelligence would allow ; so as to ascertain the precise degree and character of the defect, as well as the mere fact of its existence. The different elements which compose white light are not only different in the sensations which they excite, but also in the degree in which they stimulate the retina. It is a matter of ordinary experience that some colours are more " trying " to the eyes than others ; and those in which red, yellow, or light green predominate are the most generally condemned. On this point Dr. Boehm of Berlin obtained some interesting evidence from the women who were employed, at a large establishment in that city, in embroidering ecclesiastical vestments ; and he was told that, while they could work upon a ground of blue satin until their fingers and heads were weary, they were soon compelled to lay aside work upon light green, on account of the distress which it occasioned to their eyes. On the whole, it may probably be said that the light of long wave-lengths, or of wave-lengths broken and disturbed by the interference of others, is more dis- tressing than that of shorter or more regular vibrations • and it is well known that red, which has the lono-est wave-lengths, both impresses the retina more powerfully, and exhausts it more completely, than any other colour. Many toys have been founded upon the fact that a sort of temporary colour-blindness may be thus produced ; and the resulting phenomena have lately been pressed into the service of trade advertising. If we look steadily for a time at a red spot, seen under bright illumination, and then turn the eyes to a white surface, we shall first M 2 i64 EYESIGHT. [ch. ix. see a similar red spot reproduced there from the per- sistence of the original impression, and then, as this fades away, it will be replaced by another spot of the complementary colour, due to the circumstance that the portion of the retina on which the red image fell has become so far exhausted that it is for the time unable to perceive the red element in the white light which falls upon it, but sees only the bluish green produced by the admixture of the still visible green and violet. The properties of the different colours, and the different ways and degrees in which the eyes are affected by them, are matters not without some practical bearing upon the furnishing and decoration of rooms, as well as upon the choice of the means of obtaining artificial light ; and in the eleventh chapter it will be necessary to return in some measure to the subject. CHAPTER X. THE CARE OF THE EYES IN INFANCY AND CHILDHOOD. The number of blind persons in every civilised com- munity is exceedingly large ; and, of the total number of the blind, the loss of sight dates, in a very consider- able proportion of cases, from the first few days or weeks of life, or, as it is often erroneously said, from birth. I am not aware of the existence of any trust- worthy statistics which show the precise proportion which those who become blind in early infancy bear to the rest of their fellow-sufferers, but common experi- ence, or a visit to any one of the institutions for the education of blind children, will show that it must be large. The most recent statistics of blindness with which I am acquainted are derived from the census of the State of Massachusetts in 1875 ; and there, in a total population of 1,651,912, there were 2,806 blind persons, or i in 588. If we assume that the same pro- portion will apply to this country, there must be over 52,000 blind persons in the United Kingdom. Of the persons who are commonly said to have been blind from birth, the enormous majority, probably at i66 ' EYESIGHT. [chap. least ninety-nine out of every hundred, would bring with them into the world eyes as good and useful as those of their neighbours. The causes of infantile blindness are more frequently to be found in careless- ness and ignorance than in all possible injuries and diseases put together; and the carelessness and the ignorance are displayed, most frequently, in the neg- lect of proper precautions about light, cleanliness, and temperature. It has been known from the earliest times, and has been abundantly confirmed by recent experience, that exposure to intense or dazzling light may not only pro- duce temporary or permanent diminution of the sensi- tiveness of the retina, but that it may also partially or completely destroy the power of vision. The eyes of infants are at least as sensitive to light as those of adults, possibly even more sensitive ; and they are far less protected against it. From the imperfect deve- lopment of the bones of the infant skull the eyes are placed, so to speak, on the surface instead of being in hollows ; the eyebrows and eyelashes are short, thin, and pale, the eyelids are almost transparent, and the irides are imperfectly furnished with opaque pigment. In the first weeks of life, moreover, infants are unable to shelter themselves from dazzling light by changing the position of the head. The importance of these several conditions should be more considered by parents and nurses, with reference to the regulation of the light w^iich is admitted to the cradle, than seems usually to be the case. We find only too often that an infant is X.] CARE OF THE EYES IN INFANCY. 167 placed close to a window in the full light of day, and even with the sun shining directly upon its face. This should never be jDcrmitted, although there is no reason for falling into the opposite errors of covering the face so as to impede the access of fresh air, or of keeping the room so dark as to render the eyes preternaturally sensitive. Next after the precautions which are essential to the maintenance of life, the cleansing of the eyes of the new-born should receive early and careful attention. This cleansing should be finished before any attempt is made to wash the head or body ; and it should be performed with water of a gentle warmth, and with pieces of soft linen set apart for the purpose, or with a small fine soft sponge first carefully scalded and purified. The water employed should be contained in a convenient basin, into which no part of the infant should be dipped ; and the not uncommon practice of placing the infant in a bath, and then of washing the eyes, first, indeed, in point of time, but with the water of the bath in which the body is already immersed, cannot be too strongly con- demned. The eyes should be completely cleansed and dried before any other cleansing is commenced, and they should not again be touched until the next jDeriod of washing, which should be performed in the same manner as at first, so as to prevent the possibility of injury from soap or any other irritant. The prompt washing of the eyes has the important advantage of speedily removing any hurtful matters which may have come in contact with them; so that i68 EYESIGHT. [chap. the smarting or discomfort thus occasioned, as well as the possible danger, if not entirely relieved or obviated, will at least be greatly diminished. The best liquid for the purpose is simple warm water; and by preference river or rain water, since the spring water of some localities contains mineral substances, the presence of which mlgfht not be a matter of Indifference to such delicate organs. The ingredients sometimes added to water by nurses, such as white of egg, infusion of marsh - mallows, milk, and so forth, are at the best only harm- less ; and pure warm water should always be preferred. Impure air may affect the eyes of Infants injuriously. The foulness due to overcrowding, or to the presence of dirty clothes and such matters, is even v/orse than smoke or dust ; and the vapour produced by the washing of clothing may often be loaded with particles of noxious organic matter, which may thus be conveyed into the eyes as readily as into the lungs. Exposure to cold, either by draught upon the face or by the subjection of the whole body to a sudden change of temperature, is among the injurious influences which may injure the eyes ; and a chill of the whole body may be occasioned by putting on damp or cold napkins or garments. But the greatest danger to which the eyes of Infants are exposed Is from the Inflammatory disease called purulent ophthalmia, which may occur notwithstanding the greatest care. The inflammation begins, generally speaking, between the second and the fifth day, or it may be somewhat longer delayed. It shows Itself by X.] CARE OF THE EYES IN INFANCY. 169 redness and swelling of the eyelids, and by the formation of a thick yellowish discharge, which at first resembles mucus, but soon assumes the characters of matter. At first this discharge is scanty, and glues the eyelids to- gether as it dries, but it quickly becomes more abundant, and may then escape freely. As soon as swelling and discharge are observed, no time should be lost in ob- taining proper medical assistance. The inflammation may subside, and the eyes escape, even without treat- ment ; but in many cases irreparable mischief is done even in a few hours, and no unskilled person can dis- tinguish the most trivial from the most dangerous case. It must never be forgotten that this disease is the chief cause of blindness in infancy ; and that a short period of neglect or of unskilful management may lead to partial or complete destruction of the cornea, escape of some of the contents of the eyeball, and subsequent wasting or other deformity. Every ophthalmic surgeon can speak from only too much experience of the eyes which are lost in consequence of this affection, especially among the poor, simply from neglect or delay to obtain proper advice. Scarcely a week passes in which I do not see at the hospital some poor little baby in whom purulent ophthalmia has been left for three or four days or longer to the devices of grandmothers or nurses, and whose sight, in one or both eyes, is in consequence hopelessly destroyed. I will not go so far as to affirm that sight would never be lost under skilful medical treatment, but the cases in which it would even suffer would be extremely (cw in number ; and I say without I/O EYESIGHT. [chap. hesitation that I have never myself seen a single instance in which an eye has been permanently damaged, if it were taken under proper care before irremediable mischief was already done. Until medical assistance can be obtained, the mother should turn a deaf ear to all suggestions of domestic remedies, and should content herself by keeping the in- fant in a comfortably warmed (65° to 70° Fahrenheit) and moderately darkened room, and by the preservation of the most perfect cleanliness. The window-blinds should not be red or yellow, and should be double if there is direct exposure to the sun. The eyes should be cleansed and bathed with lukewarm water as often as any con- siderable quantity of fresh secretion is formed, and this, in severe cases, will be perhaps every half-hour. The secretion is itself actively irritating, so that it can hardly be removed too carefully. For this purpose, the lower lid should be gently drawn down towards the cheek with the forefinger of one hand, while from the other a slender stream of water is allowed to trickle Upon the inside of the lid, and is caught either by a sponge or a small cup held against the cheek by another person. If the lids are much sv/oUen, or if the child is very sensitive to light, it may be necessary that the upper lid should also be raised by one finger of the second person, who should tenderly draw it upwards towards the eyebrow, and must be most careful to exert no pressure upon the eyeball. In order to avoid any sudden movements of the head, the spare hand and fingers of the assistant may steady it on either side. When all discharge has X.] CARE OF THE EYES IN INFANCY. 171 been washed away, the lids must be dried without friction, by gentle pressure with an old handkerchief or with some other soft and absorbent material. Those who are in charge of the child must remember :hat the matter from the lids is extremely contagious, and that the smallest portion of it, introduced into a healthy eye by the medium of a finger or handkerchief, would be prone to excite violent and dangerous inflam- mation. They must therefore be exceedingly careful not to touch their own eyes whilst engaged in the cleansing process described above, to wash their hands immediately after it, and to prevent any careless use by others of sponges, linen, or towels, which may have been soiled by the discharge. The more severe the inflammation the greater will be the danger of infection ; and there- fore, both in the very rare cases in which only one eye is attacked, and in the more common ones in which one eye is far worse than its fellow, an endeavour should be made to prevent accidental inoculation of the sound or of the better one by the flowing of the discharge over the bridge of the nose. Where the difference between the two is marked, each one should have its separate basin, sponges and linen. The only local application which should be ventured upon, before medical assistance is obtained, is a soft compress of folded linen, moistened in cold water, laid over the closed lids, and changed as often as it becomes warm. There can be no doubt that this is always com- forting, and often beneficial. The medical treatment consists chiefly in the use of astringents ; but any details 1/2 EYESIGHT. [CHAP. with regard to it would be wholly foreign to the scope of the present treatise. When the period of infancy is passed, and as soon as children begin to employ their eyes intelligently about surrounding objects, the time has arrived when the char- acter of the visual function should be made the subject of observation. It is well known that the differences which exist among adults, in respect to the distance, the acuteness, and the duration of vision are exceedingly great. One person, who reads the finest print near to the eyes, will scarcely recognise friends when they are two or three yards away ; while another, who can see the hands of a turret-clock half a mile off, may require spectacles in order to read at all. One person can read, write, or otherwise apply the eyes to near objects, for fifteen hours or more daily without inconvenience ; while another can- not work in a similar manner for as much as a single hour. There are even many, in whom the eyes present no trace of disease, but who have not acute vision at any distance. In some of the preceding chapters the causes of such differences have been fully explained ; and they have been traced to differences in the original formation of the organs of vision, by which these are more or less calculated for continued effort. Apart from these dif- ferences, it must be admitted that the powers of the eyes, like those of the other senses, are capable of being improved by judicious use and cultivation, and of being impaired by the operation of various adverse influences. A delicately organised system may break down under a kind or an amount of labour which would serve to X.] CARE OF THE EYES IN CHILDHOOD. 173 call forth and develop the strength of the strong. Parents are too much accustomed to think of and to treat children as if they were all born with eyes of similar formation and endurance ; and this error is productive of many evils, which begin to show themselves about the time when systematic instruction is commenced. Such evils are especially produced in cases of ordinary myopia, hypermetropia, or astigmatism ; for these departures from the natural shape and proportions of the eyeball • do not declare themiselves, to the perceptions of un- skilled people, with the same readiness as the varieties of imperfect vision which depend upon cloudiness of any of the refracting media, upon absence of pigment, or upon other conspicuous defect. When any of the latter are present, the behaviour of the children with reference to small objects is generally sufficiently declared to excite, even in the minds of the unobservant, some suspicion of the truth. In children with eyes of the hypermetropic formation, squinting is very frequently produced, and will be of the convergent or in-turned variety. It is especially prone to occur when there is any other cause of imperfection of sight, as from slight cloudiness of the cornea left behind by inflammation ; and the tendency to deviation inwards may be much promoted by very small play- things, which are necessarily brought close to the face. The tendency is still more promoted, if such things arc played with in dim light ; or if the child is taken but little into the open air, where he has a wider field of vision, or if, when there, he is not induced to look 174 EYESIGHT. [chap. about him at distant objects. The cutting out of small figures from pictures, and the putting together of fine dissected puzzles, are in the same way disad- vantageous ; and it is better to give hypermetropic children amusements which do not call upon their eyes for any great amount of accommodation. Outdoor pursuits and exercises, skipping, building with wooden bricks, games with balls, and the direction of the attention to natural objects, are all especially to be recommended. The majority of the short-sighted are to be found, without doubt, among the dwellers in towns and among the more educated classes. This depends upon the fact that the occupations, especially in childhood, exert a great influence upon the development of the higher grades of the affection. The lesser grades constantly pass undetected by common observation, and the eyes in which they might be discovered are supposed to be natural. In circumstances which early promote the application of the eyes to near objects we find a com- paratively large number of children who, even at the age when they begin to learn to read and write, are already more or less short-sighted. It would appear that their eyes are not only naturally somewhat larger than normal ones, but that they are also inclosed in weaker and more distensible tunics, so that, under the same conditions which promote the occurrence of squint in the hypermetropic, they undergo a gradual stretching at the posterior pole, and a consequent elongation in the direction of the axis. There is X.] CARE OF THE EYES IN CHILDHOOD. 175 every reason to believe that the deviation of such eyes from the normal form would not occur, or would not occur in so great a degree, if their gaze were not habitually directed to near objects ; and also, as the firm- ness of the tunics increases with the increasing strength of the body, that mere delay in the application of the eyes to near work would be decidedly advantageous. We must therefore strongly condemn the practice of teaching children to read and write at too early an age, as at five or even four years. The premature acquisition of these merely mechanical powers is of no real advantage ; especially when they are only to be obtained at the cost of some imperfection of de- velopment, either in the body as a whole or in some single organ. There is yet another matter to which reference may be briefly made, although, in strictness, it does not belong to this part of the subject. The proper use of the eyes is, as has already been set forth, a matter of education ; and, although this education is generally an unconscious process, it is nevertheless one which ma}^ be much promoted by judicious interference. When children are noticing, or playing with, their toys, or pictures, or any common objects around them, it is very desirable to take some trouble in order to guide them towards the acquirement of habits of careful visual observation. If, for example, a child has a picture of a dog, it is well to direct his attention successively, by questions or remarks, to all parts of the drawing ; to the head, the tail, the feet, the eyes, the ears, and so 176 EYESIGHT. [chap. forth, in order to accustom him, when looking at a picture or an object, to take note of all its details. Somewhat later, attention may be directed to the dif- ferences in size, colour, shape, and other particulars, say between a cat and a dog ; and again, when the child begins to think, he may be asked about the length or the height of an object, or the distance of one object from another, so as to promote the develop- ment of the muscular sense by which we judge of distance and magnitude. In showing pictures, children should not be suffered to pass hurriedly from one to another, but should be induced to fix their attention for a while upon the salient points of each, so as to form a habit of minute and careful, rather than of superficial observation. The power of mentally re- calling the peculiarities of objects depends, at least in a great degree, upon the way in which visual obser- vation is habitually exercised ; and many persons in other respects of good abilities would be far more efiicient in their respective callings, if they had been taught to observe carefully, minutely, and correctly, at the time of life when they were learning to see. If two people witness the same incidents, or examine the same thing, how great will often be the difference between their attempts to recall and describe the pecu- liarities which have thus been brought under their notice. Robert Houdin, in a well-known passage in his autobiography, gives a highly interesting account of the manner in which his powers of visual observation were cultivated ; and, in a book which was the delight X.] • CARE OF THE EYES IN CHILDHOOD. 177 of children fifty years ago, the Evenings at Home of Mrs. Barbauld and Professor Aiken, there was a story called " Eyes and no Eyes, or the Art of Seeing," which deserves to live in literature, and which admirably en- forced the lesson that it has been the object of this paragraph to teach. The time at which children are permitted to begin close and sustained attention to ocular impressions exerts a very decide'd influence upon their power of maintaining visual effort at a later period. It is not at all uncommon for the eyes, during the years of school life, to become less acute, weaker, and more or less short-sighted. It is very important, whether at home or at school, to see that children in reading, and more especially in writing, maintain a posture in which the head is not suffered to fall too far fonvards. With print or writing of the usual size, the paper need never be nearer than twelve or at least ten inches from a normal eye. If it is noticed that any child brings his work habitually nearer than this, or that he cannot decipher print at this distance surgical advice should be sought for him without delay. The approxi- mation of the work may depend upon, several causes ; upon opacities or turbidities left behind by previous inflammation, upon simple myopia or hypermetropia, or upon absolute weakness of sight. The existence of any of these conditions requires either medical treatment, or especial care in the conduct of the education. In myopia or hypermetropia, the use of properly selected spectacles may be desirable or even necessary. In learn- ing to write, it may be necessary to employ none but N 178 EYESIGHT. [chap, large-hand copies, and to see that these are reproduced in the same magnitude. F.or this purpose it may be desirable to give double lines ; and the use of a somewhat thicker pen-handle than usual may also be advisable. Children should never be permitted to use books with stinted margins, printed in small and closely compressed type, hke too many of the hand-books which are supposed to be prepared expressly for their benefit. There are few schools in which the seats and desks are adapted to the sizes of the children, and in many they are all alike for children of widely different ages. Those for young children should be lower than others ; and all should be so proportioned as neither to bring the heads of the occupants too near the desk, nor to require irksome stooping of the body in order to place the eyes at the proper distance of twelve inches or so from their work. Too much bending of the neck will impede the return of blood from the head and from the eyes, as may be seen by observing the flushing of the face which occurs when the head is kept for some time in such an attitude. Next, the children should never be suffered to read, to write, or even to draw, by an insufficient light. Nothing distresses the eyes more readily than failure in this re- quirement, and there is no other in which failure is more common. There are unfortunately many schools, from the elementary to the highest, which are so badly con- structed as regards their windows that twilight com- mences in them quite early in a winter's afternoon, even when it does not exist all day. Whenever it is X.] CARE OF THE EYES IN CHILDHOOD. 179 not possible to effect alterations which will admit more light, the master should strive to render the comparative darkness as little as possible injurious to his scholars, by such a re-arrangement of their work as may bring the tasks which require the least eyesight to the darkest hours. Where the light is always defective, the reading, writing, and drawing should not be pursued for more than an hour without interruption ; and short periods of other work, or even intervals of complete repose, should be made to intervene. Most important of all, however, is the general prin- ciple that children should not be overburdened with any tasks which call upon the eyes for close application. " How many of my schoolfellows," writes Professor Arlt, "have had occasion to deplore the doctrine of our teacher, ' that by writing we learn. ' " A teacher who considers the eyes of his pupils w^ill discover some better punishment than that of making them write out, ten or twenty times, an imperfectly learned lesson. The greater number of eyes will bear much exertion without sustaining injury, when once adult age is at- tained ; but a large proportion of them, during child- hood, are too delicate for the amount and kind of work which is now commonly exacted from them. There has lately appeared, in the Times, a correspondence upon the possible effect of written impositions in spoiling the handwriting ; but those who took part in the controversy seemed to have no notion of the more important issues which it might raise. The knowledge of the injurious effects of certain kinds of schooling upon vision is not N 2 i8o EYESIGHT. [chap. a new acquisition ; for Beer wrote, more than sixty years ago, "He who has taken the fruitless pains, as often as I have done, to try and impress upon parents and friends, in the most friendly manner and upon the most con- vincing grounds, the mischievous effect upon the eyes of growing children of the forcing-house system of the present day, will still be disheartened to find his well- intended counsel, based upon long experience, and often repeated, either entirely neglected or listened to only by a few. . . . .Because people hold the imperfectly-understood principle that children should be constantly occupied, there is at all hours of the day a master at hand. There is reading, writing, language-learning, drawing, arith- metic, embroidery, singing, piano- and guitar-playing without end, until the persecuted victims are rendered pale, weak, and sickly, and to such an extent short- sighted or weak-sighted, that finally medical counsel must be obtained Of what avail is it to many charming girls, many estimable women, that as children they were regarded as prodigies, when the soundness of their eyes and the acuteness of their vision have been sacrificed I have seen pictures, worked upon a tobacco-pouch in the so-called pearl-stitch, which were scarcely inferior to miniature-painting, and which I examined with much pleasure until I remembered the eyes of the em- broideress. I hope that the publicity of my opinion may secure to some poor children the daily enjoyment of even a single hour of fresh air and of the free move- ment of their limbs In the present daily teaching of children, the work most injurious to their sight is the X.] CARE OF THE EYES IN CHILDHOOD. i8i constant piano-practice from engraved notes ; since the uniformity and the small size of these notes are calcu- lated soon to fatigue and weaken the strongest eyes, as any one may ascertain by experiment." When framing the last sentence, Beer was probably recollecting some cases of astigmatism which he had seen in the course of his practice. At the time when he wrote the passages quoted above, the nature of astigmatism was not under- stood ; and it is manifest that this condition renders the effort of reading music, which consists chiefly of the horizontal stave and the vertical lines of the notes, especially fatiguing and injurious. In commenting upon the above passages, Professor Arlt writes : " If the illustrious Beer were now with us, he would not fail to call attention to the injurious print of many books, as the stereotyped editions of Latin, Greek, and German classics, the pocket dictionaries, and the small maps, which require a magnifying lens to render the names of places discernible. Parents and teachers should be very careful that such books and maps are not used by the children under their charge. The number of those who, in consequence of these books, have suffered in the extent, duration, and clearness of their vision, is not inconsiderable ; and I remember that I myself, when I had completed my school education, was no longer able to see a mountain an hour's journey distant, and which, in my thirteenth year, I had seen from the same place with perfect distinctness." In the choice of a profession for children, the capa- bilities of their eyes should never be left out of account. 1 82 ^ EYESIGHT. [cHAP. The state of a young man, whose eyes refuse to per- form his accustomed work, may be even more painful than if he were bhnd ; and we should find fewer persons in this condition if more care were taken to consider the powers of the eyes before deciding upon an occupa- tion. Eyes which within a few years would fail an en- graver, a goldsmith, or a watchmaker, would last their possessor his lifetime if he were an agriculturist, a gar- dener, or employed in many other callings. He who has sound and normal eyes may choose his occupation without reference to them ; but he who is short-sighted, or weak-sighted, or whose eyes are inclined to be in- flamed, must endeavour fully to realise the claims which an otherwise desirable calling will make upon his sight ; and to understand the different ways in which this or that kind of work may be injurious to him. It may perhaps be laid down as a general principle that a child who is simply short-sighted, and who can employ his eyes continuously, and with clear vision, upon small objects, such as very fine print, so long as it is near enough, may undertake work which requires accu- rate and continued seeing. Experience teaches that merely short-sighted eyes, when the short sight has not reached a very high degree, say not more than four dioptrics, in early life, will bear without injury very fine and continuous work In the higher degrees of short- sight, however, it is undesirable to engage in any occu- pation in which the vision must be directed by turns to near and to distant objects, since the latter require X.] CARE OF THE EYES IN CHILDHOOD. 183 the use of concave lenses, which will increase the strain thrown upon the accommodation by the former. Children who are the subjects of weak sight or hyper- metropia, and who either cannot see near and small objects clearly, or cannot see them for long together, or only by the aid of convex glasses, should be dissuaded from engaging in occupations which will demand from them the application of the eyes to uniform work upon fine or small objects. The hypermetropic can, indeed, be greatly assisted by glasses, but these are not available in all pursuits. Children who have often suffered from any of the various forms of inflammation of the eyes, which are incidental to early life, especially if they show any ten- dency towards relapse, or if they are still prone to irrita- tion of the margins of the lids, should never be allowed to undertake any kind of work in which they will be exposed to dust, particularly woollen dust, to smoke, or to excessive perspiration from fire or heat. Even when the eyes are of natural formation and acuteness, it would be improper to forget how much the power of sustained visual effort is dependent upon the general vigour of the muscular system. Girls of feeble frames and late development should avoid, on this account, the more sedentary forms of industry ; and should rather find employment in work that is compara- tively coarse than in sewing, embroidery, or the like. The caution herein contained applies, also, in a still greater degree, when the eyes have been weak or inflamed during childhood. 1 84 EYESIGHT. [chap. In addition to the foregoing general principles, the whole education of children with delicate eyes should be regulated with some reference to their deHcacy. For those who attend a day school, the distance and manner of the journey, and the protection to be afforded upon the way, require careful consideration, since various forms of inflammation of the eyes are caused, or at least promoted, by exposure to wet or to vicissitudes of weather. In all day schools there should be arrange- ments to allow of the removal of wet or damp clothing, and especially of wet or damp boots or shoes, before the children are suffered to settle down to their tasks. The atmosphere of schoolrooms, and the due supply of fresh air to them, are matters which probably will not be regarded until school-boards and school managers have no political or polemical questions left to dispute about. With regard to the actual conduct of the teaching, it must be remembered that there is no reasonable doubt of the injurious influence of premature exertion of the brain in retarding the development of the body, the eyes, of course, included ; and I myself entertain none that such premature exertion is at least equally injurious to the mental faculties themselves. Many years ago, I wrote an essay upon " The Artificial Production of Stupidity in Schools," which had for its purpose to show the manner in which the proceedings of teachers may defeat their supposed objects ; and this essay has now been so often reprinted, in this and other countries, that I would fain hope it may have induced some few teachers to reconsider their ways. For the present X.] CARE OF THE EYES IN CHILDHOOD. 185 purpose, it is sufficient to observe that any excess of school work impHes, almost of necessity, an undue application of vision to near objects ; and that hence, when the eyes are either weakly or diseased, such excess should be strictly prohibited. It is very worthy of note that, in the experience of ophthalmic surgeons, it is exceptional to meet with a child suffering from defective vision who has not, before the defect was discovered, been repeatedly and system- atically punished by teachers or schoolmasters for sup- posed obstinacy or stupidity. The very reverse of this practice is that which ought to obtain ; and apparent obstinacy or stupidity should lead, from the first, to the question, '' Can he see perfectly } " Children have an indefeasible claim upon their elders for friendly and considerate treatment. If they are harshly or unjustly dealt with, punished for errors which they cannot avoid, or forced to undertake tasks, either mental or bodily, which are beyond their powers, they will suffer either in mind or body or in both. Unfortunately, the work of teaching seems to exert a destructive influence upon the imagination, using that word in its true scientific sense ; and the average schoolmaster has often done an amount of wrong which can hardly be repaired, before the sur- geon has any opportunity of interposing to put the saddle upon the right horse, and to assign the palm of stupidity to the pedagogue instead of to the pupil. It would be improper to leave the subject of the care of the eyes in childhood without some reference to the frequently^ contagious character of the forms of i86 EYESIGHT. " [chap. superficial ophthalmia from which children are especially liable to suffer, and which are often widely diffused through the agency of schools. There is a contagious inflammation affecting the whole surface of the con- junctiva, and secondarily the cornea, which has again and again prevailed as an epidemic in workhouse schools, more especially in the very large ones which are attached to city unions, and which has destroyed the sight of hundreds of children. There is also a contagious form of inflammation of the lid margins, and of the roots of the eyelashes, which does an immense amount of mischief, and which is common among poor children who live in large towns. The sufferers from this affection come in numbers to the ophthalmic departments of the London hospitals ; but the great difficulty in the way of curing them is the perpetual worry in which their parents are kept by the officers of the School Board. That august body has so much respect for the medical pro- fession as to attribute to its members the gift of prophecy ; for it has actually put forth a blank form of certificate of illness, in which the doctor who signs it is asked to state, not only that the child referred to is then unfit to attend school, but also how long it will be before this unfitness will terminate. With so high an opinion of medical knowledge as this demand indicates, the School Board might surely allow its agents to receive a hospital letter, dated in such a manner as to show actual and regular attendance, as a sufficient excuse for temporary absence from the ministrations of the teacher ; for it is hardly fair to call upon hospital medical officers, whose X.] CARE OF THE EYES IN CHILDHOOD. 187 time is generally very fully occupied about more im- portant matters, to write or sign certificates by the dozen in proof of the existence of conditions which are sufficiently evident to the meanest capacity. To call upon a child with sore eyes to go to school is in most cases an act of almost inconceivable folly as regards the child itself, and of very manifest cruelty to its unfortunate classmates. CHAPTER XL THE CARE OF THE EYES IN ADULT AGE ; NATURAL AND ARTIFICIAL ILLUMINATION ; ACCIDENTAL INJURIES ; THE INFLUENCE UPON SIGHT OF THE GENERAL HEALTH AND HABITS OF LIVING. The eyes may suffer injury in adult age from a great variety of causes ; from defective or excessive illumina- tion ; from excessive application ; from unclean or impure air ; from exposure to cold ; from mechanical or chemical injury ; from mental conditions, and from many-so-called pleasures ; from unnatural conditions of the general system, which either occasion determination of blood to the eyes and to the head, or which depend upon abnormal states of the blood, by which the strength of the whole body, and with it that of the eyes, is reduced ; and finally from the want or the misuse of spectacles. The general principles which should be borne in mind, in order that the evils hence arising may be avoided, are of importance to every one who wishes to preserve his eyes in a state of health and of efficiency. CH. XI.] CARE OF THE EYES IN ADULT AGE. 189 With reference to ligJit and illiLinination, we must carefully distinguish the natural or white light which comes to us from the sun, from the artificial and coloured light which we obtain by the burning of various sub- stances. The natural light is as congenial and necessary to the eye as food to the digestive organs ; but, just as these organs may be so enfeebled by long abstinence that they can only tolerate food in very small quantities, so the eyes, by the prolonged exclusion of daylight, may be brought into a state in which even a very moderate amount of light serves to irritate them, and a strong light is absolutely unendurable. In this way it is possible for the prolonged exclusion of daylight to prove highly injurious, even such exclusion as is produced when the eyes are bound up for a long period, or from the continued wearing of dark-coloured glasses. Some cases have been recorded, notably that of Dr. Harley, which show that the amount of light necessary to afford vision depends rather upon the amount to which the eyes have become habituated than upon the actual quantity. We all know how soon vision is recovered after coming into a comparatively dark room ; and Dr. Harley and others, after having been kept for a time in what would be described in common parlance as complete darkness, have been able to distinguish the objects around them. Persons imprisoned in dungeons, to which no light found access but by some single fissure, have after a while been able to see in this darkness, and to distinguish the mice which were attracted thither in search of crumbs. In such cases, the return to daylight 190 EYESIGHT. [chap. has at first been very painful ; but, if cautiously and gradually managed, has seldom been productive of any permanent injury. On the other hand, vision has often been enfeebled or destroyed by exposure to a dazzling light, either suddenly on leaving comparative darkness, or even with- out the aid of the contrast thus afforded. Every one knows the discomfort which is experienced from a sudden increase of light ; as when we come out of a dark place into one which is brilliantly illuminated, or when a flash from some reflecting surface is suddenly allowed to fall upon the eye. Such discomfort should be regarded as a warning of the dangerous character of the circumstances which call it forth. Many people have expiated, by impairment or loss of sight, the indiscretion of looking at the sun, especially when an eclipse has been observed through a piece of insuf- ficently obscured glass. After looking at the sun there remains, in less disastrous cases, an appearance as of a dark disc or cloud, which may assume a fiery or a violet tint when the lids are closed. This appearance may continue for a variable time, hours, days, or weeks ; and may even leave a permanent dark spot in the middle of the field, in such a position that it will interfere more or less with the vision of minute objects. In other instances, the disturbance of sight is not limited to a single spot, but extends, either gradually or imme- diately, over the whole of the retina ; and the most prompt treatment has sometimes barely sufficed to pre- vent the occurrence of complete blindness. Professor XL] NATURAL AND ARTIFICIAL ILLUMINATION. 191 Arlt records that he saw three cases of this description after the solar ecHpse of 185 1. Temporary or permanent loss of vision has been occasionally produced by the reflection of the solar rays into an eye from a mirror or other like surface. Mischievous children have sometimes produced blindness in this manner by playing with a mirror. A long continued gaze upon the full moon is said to have produced marked weakness or distress to very sensitive eyes, and it is easy to conceive that exposure to strong artificial light, as that of fireworks, Bengal lights, the electric light, white hot metal, and so forth, may prove injurious. Beer has recorded instances in which exposure to bright sunlight on awaking in the morning has produced troublesome inflammation ; and everybody knows how much tem- porary distress may be occasioned to the eyes in this manner, either by sleeping in a bed facing the morning sun, with the windows of the room insufficiently covered by curtains, or when these curtains are suddenly thrown open by a servant. Persons who use a night light should be careful so to place and to screen it that its rays do not fall directly upon the eyes ; but there can be no doubt that such an appliance is much better omitted than used ; and that perfect darkness is eminently conducive to the attainment of sound and refreshing sleep. The solar light when reflected from white surfaces has often been injurious. The inhabitants of polar countries have found by experience the necessity of 192 EYESIGHT. [ctiAP. protecting themselves against the glare from snow ; and surfaces of sand, of chalk, or even of water, have often done harm to voyagers or travellers by the bright- ness of the reflection which they have afforded. Sailors, who are inhabitants of temperate climates, and who have been much exposed to glare in the tropics, have not seldom been affected by night-blindness ; a con- dition which depends upon over-stimulation of the retina, by which it has been rendered insensitive to anything less than the illumination of full daylight. All travellers going to the tropics, or into countries where there is likely to be snow upon the ground together with sunshine, should provide themselves with dark-coloured protecting glasses, or with some similar protective contrivances. It follows from the foregoing considerations that the manner of admitting light into the rooms of a dwelling house is by no means a matter of indifference to the inhabitants. Of artificial light I shall have to speak hereafter ; but we may often find rooms in which there Is far too much daylight. It should always be remem- bered that the position of the eyebrows, and the general structure and arrangement of the eyes and their appen- dages, are calculated chiefly to protect them from light coming down from above, and that they are compara- tively defenceless against that which comes up from below. On this account, very low windows are rather to be avoided ; or, if used, they should be fitted with blinds made to draw up rather than down ; and the floors should not be covered with very bright coloured XI.] NATURAL AND ARTIFICIAL ILLUMINATION. 193 materials, or with any which possess reflecting surfaces. The blinds, too, by which the admitted light is tempered, should be of a suitable colour ; neither white nor white striped with red ; but of a blue or grey tint, and of sufficient thickness to be really effectual for the purpose for which they are designed. Those whose occupations or manner of life call upon them for regular use of the eyes upon small matters, as in reading, writing, keeping accounts, embroidering, or any similar pursuits, have not only, like all other persons, to consider the question of light in general, but also the kind of illumination, whether natural or artificial, by which their work is done ; and to consider this the more carefully, the finer the work itself may be, the less it is diversified by other pursuits, and the shorter the periods of intermission. Under defective illumination all eyes are prone to suffer in acuteness of vision and in the power of sustained effort ; and the eyes of young persons are likely to suffer also in visual distance. The conditions of illumination which are most pre- judicial to the eyes compelled to work in them are chiefly these : when the light is too feeble, or insufficient ; when it is too strong, so as to be dazzling ; unsteady, some- times strong and sometimes weak ; irregularly dis- tributed, as when broken by shadows ; of bad quality, as compared with white daylight ; and, lastly, when it falls upon the eyes or the work in a wrong direction. Since all the foregoing forms of defect are most liable to be incidental to the use of artificial light, such as is o 194 EYESIGHT. ' ' [chap. afforded by candles, lamps, or gas, It follows that those who have no alternative but to work by such artificial light should be especially careful v/ith regard to its quality, direction, and general management. It has been mentioned in an earlier chapter that the solar light is a compound of different colours, which can be separated by passage through a prism ; and which are ultimately resolvable into the three primaries, red, green, and violet. These three, like the seven of the spectrum, can be reunited so as to produce white light ; and in the solar light the three primary colours are combined in the proportion of five parts of red, three of green, and eight of violet. When this proportion is changed, as it generally is in artificial light, the light is no longer pure, or white, but is coloured ; and the colour of that kind of prim.ary light which is present in excess is of course that which preponderates. In common candle or lamplight, the red rays are in excess ; so that the light itself is more or less reddish or orange. This peculiarity of artificial light is productive of many important consequences. The different colours produced by dyes or pigments assume quite a different appearance, when seen by lamplight, to that which they have by daylight ; green appears yellowish ; blue, greenish, more or less according to the greater or less quantity of violet in the artificial light employed ; dark-blue, purple-red, and orange appear much brighter. In proportion to the excess of red rays, and so on. In bright moonlight, if we look at a surface, say of the paved causeway of a street, which is partly shaded from the moon, but lighted XL] NATURAL AND ARTIFICIAL ILLUMINATION. 195 everywhere by gas, we shall see that the part lighted only by gas will appear reddish, by contrast with the adjoin- ing part which receives the moonbeams also, and which appears of its natural greyish tint. It is a pretty experiment to admit into a room with white walls the light from a white cloud, and to hold up a staff in such a manner as to cast a shadow upon the wall. This shadow will appear simply dark ; but, if we bring in a lighted candle, and throw a second shadow from its flame by the side of the first one, then the first shadow will appear yellow, and the second will appear blue. The reason of this is that the first shadow, formed by the interception of daylight, will be lighted up by the light of the candle, in which yellow preponderates ; while the shadow formed by the interception of the candle light will be illuminated by the daylight, and the two being near together, produce an effect of contrast which causes the second to appear blue by comparison. If we surround the candle by a blue chimney glass, then both shadows will be alike, simply dark, from whence we may infer that both the sources of light contain the same colours, but in different proportions ; and that the blue glass, by absorbing red and green rays from the flame, restores something like the natural or solar proportion between its colours. It follows from these conditions that working with colours by artificial light is either impossible or highly fatiguing, especially with dark colours. The dark colours absorb too much of the light which they receive, and return too little, for them to be sufficiently illuminated from artificial sources. O 2 195 EYESIGHT. [chap. Whenever possible, dark surfaces should be worked upon only by daylight, and lighter surfaces should be reserved for the hours of artificial light ; but embroidery with and upon colours, which is sufficiently trying even by daylight, should never be attempted by artificial light at all. Precisely as the powers of all other organs are ex- hausted by exertion, and require to be renewed by rest, so the eyes become dulled by the prolonged incidence of light, and require to be renewed in like manner, more especially if the light is not of the natural colour and quality. The necessary rest is afforded, to a great extent, by the mode in which the two eyes work to- gether, it seeming that they see actively and passively by turns, each coming to the support of its fellow in time to save it from exhaustion. An observer who is placed in an astronomical chair, which supports the head and neck without fatigue ^in an appropriate position, may fix the eyes upon a single star, and may keep it in view for a very long period. If, however, the star is looked at with one eye only, it will before long disappear from view ; the reason being that the portion of the retina upon which the image has fallen has been rendered insensitive to it by exhaustion, and requires a period of rest. If the lids are closed, and the gaze turned away, the star may again be seen with the same eye after a few minutes ; but it will be lost on the second occasion more quickly than on the first. When the light to which the eyes are exposed is of an unnatural colour, the power of the second eye to support the first is less declared ; XT.] NATURAL AND ARTIFICIAL ILLUMINATION. 197 and those who work much by artificial Hght, in which there is the usual relative preponderance of red and green rays, are sometimes found to become comparatively insensitive to these colours, so that, when they are in white light, or looking upon white surfaces, they see an appearance of points, lines, or clouds, which are dark blue, or dark reddish-blue, or almost black. The prin- ciple of these appearances has been explained in a previous chapter, and there is a familiar child's book in which this principle is applied to the production of so- called apparitions. The book contains pictures in bright colours, and after looking at these pictures steadily for a time, if the eyes are turned towards a white surface, the same pictures are seen in the air in colours complemen- tary to those in the book. Assuming the picture to be red, the eyes, by looking at it for a time, are rendered insensitive to the red rays. They are then turned towards the white light, in which they are capable of being impressed by the violet and the green rays only. Hence they see the appearance of a bluish green or green figure, precisely resembling the red one at which they have been looking. Such experiments are harmless when neither too often repeated nor too long continued ; but they show how easily the sight might be permanently injured by the constant employment of the eyes under artificial conditions. Furthermore, it follows from the composition of artificial light that its illuminating power does not bear any definite proportion to its quantity. Where, for example, there is an excess of the red rays, which afford but a feeble illumination as measured iqS eyesight. [chap. by their power of displaying objects, it is manifest that there must be more of the hght as a whole, to allow of any occupation being carried on, than would be necessary if it were white ; and hence that the eye may be irritated and dulled, not only by the primary excess of red, but also by the secondary excess which depends upon the greater quantity of the light that is rendered necessary by reason of its inferior quality. Lastly, the heating qualities of light must by no means be left out of account. It is well known that all combustion which affords light affords heat also ; although the relative proportions of light and heat differ in flames of different descriptions. A gas flame, for example, gives much more heat than a candle flame of the same magnitude. It has been made known by accu- rate experiments that the violet rays are the least heat- ing, the green next, and the red the most, in the relative proportions of 56, 58, and 72 ; and it follows that artifi- cial light, in relation to its illuminating properties, must be warmer, the greater the proportion of red and green rays, and especially of red rays, which it contains. Besides this, every ray of light is accompanied by a cer- tain amount of free heat, which is found chiefly at the red end of the prismatic spectrum, outside the light rays themselves. Near objects, at which we have to look for a considerable time, seldom derive their light directly from the solar ray, which before reaching them has been reflected from innumerable objects, such as atmo- spheric particles, clouds, the earth, plants and animals, the furniture of rooms, and so forth, by each of which XL] NATURAL AND ARTIFICIAL ILLUMINATION. 199 the attendant heat rays have been more or less absorbed, so that the ultimate illumination is accompanied by comparatively little heat. When, however, artificial light is used, its rays generally fall directly from their source upon the object, and their source is commonly near to the worker ; so that both the light and its accom- panying heat are usually reflected from the object of vision directly upon and into the eyes. This heat acts especially upon the surfaces of the eyes and upon the lids, drying and irritating them, and powerfully predis- posing to various inflammatory affections. In certain callings, such as that of an engraver on wood, in which the source of artificial light is required to be near the worker, it is a common practice to interpose between the lamp and the object a globular glass vessel full of water, from which two advantages are obtained. In the first place, much of the heat is absorbed by the water ; and, in the second place, the globe serves to focalise or condense the light upon the point which is being looked at. On this account, a smaller and less heating flame than would otherwise be required may be employed, and the disturbance of vision by scattered illumination is wholly prevented. Another common plan is to place a large sponge, soaked in water, on the table near the worker. By this, no economy of light is obtained ; but the evaporation from the sponge serves to keep the sur- rounding air cool and moist, notwithstanding any heat which may proceed from the lamp. In order to improve the colour of the artificial light from a lamp, it has been a common practice to surround 200 EYESIGHT. [chap. the flame with a blue chimney, or to wear blue glasses. The glass for this purpose is coloured by oxide of cobalt, and, while it allows nearly all the violet rays to pass through, it arrests a considerable proportion of the red and of the green. Although such glasses have the advantage of rendering the artificial light nearer the colour of the natural, they have also the disadvantage of diminishing the total quantity of light which falls upon the object, and hence of entailing all the ordinary consequences of insufficient illumination. The blue glass, it must be remembered, supplies nothing, but only intercepts. Its peculiarity is to be very transparent to violet rays, and more or less opaque to others. Hence, when it transmits light in which red and green prepon- derate, it will to some extent restore the balance by arresting the preponderating elements. Generally speak- ing, it arrests too much of them to transmit white light; and it must be obvious that, the blue being originally deficient, and the red and green partially arrested in transmission, the total amount of light is considerably reduced. Unless the original light be very intense, the use of blue media will finally resolve itself into the use of defective illumination. The other chief disadvantages of artificial light are its unsteadiness, its variableness, and the faulty directions from which it often proceeds. All the older forms of artificial light are more or less unsteady, from the absence of any means of preserving an uniform propor- tion between the air-supply and the combustible. This unsteadiness is always objectionable, and, in its worst XI.] NATURAL AND ARTIFICIAL ILLUMINATION. 201 form, as in working by the light of a flickering candle, it may be highly distressing. Attempts were formerly made to diminish the effect of this unsteadiness by the use of two or more candles, so that the inequalities of illumination depending upon irregularities of the wicks might be less noticeable, while the total quantity of light was increased. For this purpose, the candles should be placed as near together as possible, and should not be d'stributed, or divided to the right and left, since the latter arrangements not only diminish the total quantity of light which falls upon the work, but also interfere with its equal distribution. Candles, however, are now so little used, except as auxiliaries to other means of lighting, that it is hardl)' necessary to dwell upon their disadvantages. A faulty position of the source of artificial light either sends the rays too directly into the eyes, or else affords only an insufficient illumination of the object looked at. An ordinary candle or lamp should have its flame a few inches higher than the eyes, in order to utilise the shelter of the brows ; and should be somewhat to the left front of the worker. If placed higher than this, the illumination is commonly insufficient for close ap- phcation. The central gas or oil chandelier, by which rooms may be conveniently lighted for meals or con- versation, seldom affords enough light for any purpose which requires accurate vision ; and I have constantly to warn parents against allowing their children to pre- pare lessons for the next day in a sitting room which is only lighted in this manner. A flam.e on the same level 202 EYESIGHT. [chap. with the eyes, or a little below them, sends too much light directly into the pupils, and too much heat upon the surfaces of the eyeballs, and thus tends to produce dryness, dazzling, and irritation. Even upon these grounds alone, the otherwise objectionable practice of reading in bed should be prohibited ; since both the light and the book are almost of necessity placed too low and too much on one side, and the eyes are brought into a strained and unnatural position. Where the habit cannot be relinquished, it is at least desirable to have a lamp which is capable of being set at any desired height. When the light is too much on one side, it affords to one eye a comparatively feeble, and to the other a com- paratively strong illumination.' The tendency of this in- equality is to produce unequal contraction of the pupils, and to render the visual act exceedingly fatiguing. Of late years, great attention has been given to the means of obtaining artificial light, and many improve- ments have been effected. The electric light furnishes the nearest approach to the actual solar ray which we possess, since it contains almost the same proportion of violet ; but it is rendered dazzling by the directness with which it comes to us from its source, and by the conse- quent absence of those multitudinous reflections to which, in the case of sunHght, reference has previously been made, and by which both its colour and its brightness are much modified. Moreover, for domestic purposes, the electric light is not yet practically accessible ; and, even if it were, we should still have to learn by experience the effects of continued exposure to an illumination which is as XL] NATURAL AND ARTIFICIAL ILLUMINATION. 203 rich in the actinic or chemical rays as in those which are re- cognised by our senses in the form of hght. It is by virtue of these actinic rays that light holds its place as one of the most powerful of the stimulants to vital action ; and nature is careful to withdraw them from us for long periods. They might prove to be as harmless as the illuminating rays themselves ; but that they are so has yet to be ascertained by direct observation. In the mean- while, the best artificial light is that which most nearly approaches diffused daylight in colour ; and this cha- racter belongs, I believe, to that which is furnished by the various forms of burners invented and patented by Mr. Silber. It may be said, generally, that the quality of light will be proportionate to the completeness of the combustion by which it is afforded ; and complete com- bustion is only to be secured by the precise adaptation of the quantity of the combustible to the quantity of atmospheric air which finds access to the flame. When the combustible is in excess, part of it must remain unconsumed, or only partially consumed, and the flame will be dark, smoky, and ill-coloured. When the air is in excess, its superabundance will lower the tem- perature of the flame, and will thus diminish chemical activity, with the same ultimate result as in the former case. In the original Argand burners, although they were a great improvement on all which had preceded them, the air supply was in excess, so that the flame was unduly cooled ; and no precise regulation was attained until the construction of the Silber Argand burner, which, in the form adapted for gas, is shown in elevation 204 EYESIGHT. [chap. and in section in Figs. 46 and 47. It will be seen that the air only obtains access to the flame through a square opening of carefully calculated dimensions ; that the entering current is divided, and that part of it is conveyed through the inner tube to the interior of the flame near its summit. By this contrivance, any volatilised particles of carbon which are being driven off unconsumed are brought into contact with a fresh supply of oxygen, and are completely burned. Another point of difference from the old Argand is that the supply of Fig. 46. Fig. 47. air can only reach the flame by passing through metal channels of considerable length, in which the air becomes heated to such a degree that, even if superabundant, it would exercise no refrigerating effect. Coincidently with the researches of Mr. Silber, much good work in the same direction has been done by another inventor, Mr. Sugg, who also makes Argand gas- burners of great excellence. As between the Silber and the Sugg burners it would be difficult for the unaided eye to assign the palm of superior merit ; but, according to a photometric investigation made by Dr. Wallace, and XL] NATURAL AND ARTIFICIAL ILLUMINATION. 205 the results of which were embodied in a paper read by him before the Society of Arts, and pubHshed in the Journal of the Society for the 7th February, 1879, the Silber burners are decidedly, although not very greatly, the better of the two. For the same consumption of the same kind of gas, the light given by the best form of Silber Argand burner exceeds that given by the best form of Sugg Argand in the proportion of about twenty- five to twenty-two. It is only in gas-burners that the two inventors are competitors ; and in burners for oil, to which the Silber principle is also very completely applied, I am not acquainted with any others which at all approach them in either the quantity or the quality of the resulting illumination. The weak point common to all forms of artificial light, other than the electric, is their original deficiency in violet rays ; and some time ago, with the kind assistance of Professor Barff, I commenced a series of experiments, which I was unfortunately not able to complete, for the purpose of ascertaining the relative merits of different kinds of burners and of different kinds of illuminating agents, with especial reference to this single question. The tests which may be used are two in number ; the first being the examination and comparison of known colours by the artificial light, the second being its actual analysis by a prism, ■ I < - z I q: 111 > LJ LU z h h < UJ UJ '^ I- LJ CO Q O h o Q. LJ I H < LjJ < O O X Q. Q. 1-3 3 LJ CC o CO G o I O I CC UJ < o LJ o < -J CL LU CC D h O D CC co UJ CC o u. o o CC < H CO UJ LJ LJ X H CQ O UJ Q O Q. X -J D c !n -^ ^ rt O O CI, ^ s >• Oh O J 2 o T3 O ^ ;;; O C/5 .2 'tn o Uh O aj o in •T3 g3 Bedford Street, Covent Garden, London, March, 1879. MACMILLAN S CO:S MEDICAL CATALOGUE, Works in Physiology, Anatomy, Zoology, Botany, Chemistry, Physics, Midwifery, Materia Medica, and other Professional Subjects. ALLBUTT (T. C.)— ON THE USE OF THE OPHTHALMOSCOPE in Diseases of the Nervous S3'stem and of tlie Kidneys ; also in certain other General Disorders. By Thomas Clifford Allbutt, M.A., M.D., Cantab., Pliysician to the Leeds General Infirmary, Lecturer on Practical Medicine, &c., &c. Svo. 15s. ANDERSON.— Works by Dr. McCall Andeesox, Professor of Clinical Medicine in the University of Glasgow, and Physician to the Western Infirmary and to the Wards for Skin Diseases. ON THE TREATMENT OF DISEASES OF THE SKIN : with an Analysis of Eleven Thousand Consecutive Cases. Crown Svo. 5s. LECTURES ON CLINICAL MEDICINE. With Illustrations. Svo. 10s. 6d. ON THE CURABILITY OF ATTACKS OF TUBERCULAR PERITONITIS AND ACUTE PHTHISIS (Galloping Consumption). Crown Svo. 2s. 6d. ANSTIE.— ON" THE USE OF WINES IN HEALTH AND DISEASE. By F. E. Anstie, M.D., F.R.S., late Phj'sician to Westminster Hospital, and Editor of Tlic Practitioner. Crown Svo. 2s. BALFOUR.— ELASMOBRANCH FISHES ; a Monograph on the Development of. By F. M. Balfour, M.A., Fellow and Lectiu-er of Trinity College, Cambridge. , With Plates. Svo. 21s. BARWELL.— ON CURVATURES OF THE SPINE : their Causes and treatment. By Richard Barwell, F.R.C.S., Surgeon and late Lecturer on Anatomy at the Charing Cross Hospital. Third Edition, with additional Illustrations. CroAvn Svo. 5s. BASTI AN.— Works by H. Charlton Bastian, M.D., F.R.S., Professor of Patholo£;ical Anatomy in University College, London, &c. : — THE BEGINNINGS OF LIFE : Being some Account of the Nature, Modes of Origin, and Transformations of Lower Organisms. In Two Volumes. With upwards of 100 Illustrations. Crown Svo. 2Ss. EVOLUTION AND THE ORIGIN OF LIFE. Crown Svo. 6s. 6d. ON PARALYSIS FROM BRAIN DISEASE IN ITS COMMON FORMS. Illustrated. Crown Svo. 10s. 6d. " It would be a good thing if all such lectures were as clear, as systematic, and as interesting It is of interest not only to students but to all who make nervous diseases a study." — Journal of Mental Science. BUCKNILL.— HABITUAL DRUNKENNESS AND INSANE DRUNKARDS. By J. C. Bccknill, M.D. Lond., F.R.S., F.R.C.P., late Lord Chancellor's Visitor of Lunatics. Crown Svo. 2s. 6d. CARTER.— Works by R. Brudenell Carter, F.R.C.S., Ophthal- mic Surgeon to St George's Hospital, &c. A PRACTICAL TREATISE ON DISEASES OF THE EYE. With Illustrations. Svo. It3s. " No one will read Mr. Carter's book without having both, his special and general knowledge increased. " — Lancet. ON DEFECTS OF VISION WHICH ARE REMEDIABLE BY OPTICAL APPLIANCES. Lectures at the Royal College of Sui-geous. With munerous Illustrations. Svo. Os. 3,000.3.79. 2 MACMILLAN A]N"D CO.'S CHRISTIE— CHOLERA EPIDEMICS m EAST AFRICA. An Aecoinit of the several Diffusions of the Disease in that country from 1821 till 1872, -with an Outline of the Geography. Ethnologj-, and Trade Connec- tions of the Regions through which the Epidemics jjassed. By J. Christie, M.D., late Physician to H.H. the Sultan of Zanzibar. With Maps. Svo. 15s. COOKE (JOSIAH P., Jim.).— FIRST PRINCIPLES OF CHEMICAL PHILOSOPHY. By Josiah P. Cooke, Jun., Ervine Professor of Chemistry and Mineralogy in Harvard College. Third Edition, revised and corrected. Crown Svo. 12s. CREIGHTON — CON"TRIBUTIONS TO THE PHYSIOLOGY AND PATHOLOGY OF THE BREAST AND ITS LYMPHATIC GLANDS. By Charles Creighton-, M.D., Demonstrator of Anatomy in the University of Cambridge. With Illustrations. Svo. 9s. "It is impossible not to see at once that the work is deserving of all praise, both from tlie originality and from the care which has been bestow^ed upon it." — Practitioner. FLOWER (W. H.).— AN INTRODUCTION TO THE OSTEOLOGY OF THE MAMMALIA. Being the substance of the Course of Lectures delivered at the Royal College of Surgeons of England in 1870. By W. H. Flower, F.R.S., F.R.C.S., Hunterian Professor of Comparative Antatomy and Physiology. With numerous Illustrations. Second Edition, revised and enlarged. Crown Svo. 10s. 6rf. FOSTER.— Works by Michael Fosteji, M.D., F.R.S. :— A TEXT BOOK OF PHYSIOLOGY, for the use of Medical Students and others. Second Edition, revised and enlarged, with additional Plates and Illustrations. Svo. 21s. " Dr. Foster has combined in this work the conflicting desiderata in all text- books — comprehensiveness, breAity, and clearness. After a careful perusal of the whole work we can confidently recommend it, both to the student and the practi- tioner as being one of the best text-books on physiology extant." — Lancet. A PRIMER OF PHYSIOLOGY. Illustrated. 18mo. Is, FOSTER and L ANGLE Y.— AN ELEMENTARY COURSE OF PRACTICAL PHYSIOLOGY. By Michael Foster, M D., F.R.S., assisted by J. N. Langley, B.A. Thii-d Edition, enlarged. Crown Svo. 6s. "Equipped with a text-book such as this .... the beginner cannot fail to acquire a real, though of course elementary, knowledge of the leading facts and principles of Physiology." — Academy. FOSTER and BALFOUR.— ELEMENTS OF EMBRYOLOGY. By Michael Foster, M.D., F.R.S., and F. M. Balfour, M.A., Fellow of Trinity College, Cambridge. With numerous Illustrations, Part I. Crown Svo. 7s. Qd. " Both text and illustrations are alike remarkable for their clearness andfreedom from error, indicating the immense amount of labour and care expended in the .production of this most valuable addition to scientific literature." — Medical Press and Circular. FOTHERGILL, — Works by J. Milner Fothergill, M.D., M.R C.P., Assistant Physician to the Victoria Park Chest Hospital, and to the West London Hospital : — THE PRACTITIONER'S HANDBOOK OF TREATMENT : or, THE PRIN- CIPLES OF RATIONAL THERAPEUTICS. Svo. 14s. "We have every reason to thank the author for a practical and suggestive work." — Lancet. THE ANTAGONISM OF THERAPEUTIC AGENTS, AND WHAT IT TEACHES. The Essay to which was awarded the Fothergillian Gold Medal of the Medical Society of London for 1878. Crown Svo. 6s. MEDICAL CATALOGUE, 3 FOX.— Works by Wilson Fox, M.D., Lond., F.R.C.P., F.R.S., Holme Professor of Clinical Medicine, University College, London, Physician Extraordinary to her Majesty the Queen, &c. :— DISEASES OF THE STOMACH: being a new and revised Edition of "The Diagnosis and Treatment ok tue Varieties of Dyspepsia." Svo. Ss. 6d. ON THE ARTIFICIAL PRODUCTION OF TUBERCLE IN THE LOWER ANIMALS. With Coloured Plates. 4to. 5s. 6d. ON THE TREATMENT OF HYPERPYREXIA, as Illustrated in Acute Articular Rheumatisni by means of the External Ap])lication of Cold. Svo. 2s. 6d. GALTON (B.).— AK ADDRESS 0^ THE GENERAL PRIN- CIPLES WHICH SHOULD BE OBSERVED IN THE CONSTRUCTION OF HOSPITALS. By Douglas Galton, C.B., F.R.S. Crown Svo. Ss. Qd. GEGENBAUR— ELEMENTS OF COMPARATIVE ANATOMY. By Carl Gcgenbaur, Professor of Anatomy and Director of the Anatomical Institute, Heidelberg. A translationaby F. Jeffhey Bell, B.A., revit-ed, with Preface by E. Ray Lankester, M.A., F.R.S., Professor of Zoology and Comparative Anatomy in University College, London. With numerous Illus- trations. Medium Svo. 21s. GRIFFITHS. -LESSONS ON PRESCRIPTIONS AND THE ART OF PRESCRIBING. By W. Hansel Griffiths, Ph.D., L.R.C.P.E- New Edition. 18mo. 3s. 6d. *' We recommend it to all students and junior members of the profession who desire to understand the art of prescribing. "—Medical Press. HANBURY.— SCIENCE PAPERS, chiefly Pharmacological and Botanical By Daniel Hanburv, F.R.S. Edited with Memoir by Joseph Inge, F.L.S.. F.C.S. Svo. 14s. HOOD (^Vharton.).— ON BONE-SETTING (so-called), and its Relation to the Treatment of Joints Crippled by Injury, Rheumatism, Intlamination, fcc, (be. By Wharton P. Hood, M.D., M.R.C.S. Crown Svo. Illustrated. 4s. Qd. "Dr. Hood's book is full of instruction, and should be read by all surgeons.**— Medical Times. HOOKER (Dr.).— THE STUDENT'S, FLORA OF THE BRITISH ISLANDS. Bv Sir J. D. Hooker, K.C.S.L, C.B., M.D., D.C.L., President of the Royal Society. Second Edition, revised and corrected. Globe Svo. 10s. 6d. HUMPHRY.— ^Vorks by G. M. Humpiiey, ]\LD., F.R.S., Professor of Anatomy in the University of Cambridge, and Honorary Fellow of Downing College : — THE HUMAN SKELETON (including the Joints). With 260 Illustrations drawn from Nature. ;Medium Svo. ^Ss. OBSERVATIONS IN MYOLOGY. Illustrated. Svo. 6s. THE HUMAN FOOT AND HAND. Illustrated. Fcap. Svo. 4s. Gd. HUXLEY and MARTIN. — A COURSE OF PRACTICAL INSTRUCTION IN ELEMENTARY BIOLOGY. By T. H. Hvxi.ev, LL.D. Sec. R.S., assisted by H. N. Martin, M.B., D.Sc. New Edition, revised. Crown Svo. Us. " To intending medical students this book will prove of great value." — Lancet. HUXLEY (Professor).— LESSONS IN ELEMENTARY PHY- SIOLOGY. By T. H. Huxley. LL.D., F.RS. With numerous Illustrations. New Edition. Fcap. Svo. 4s. 6d. 4 MACMILLAN AND CO.'S KEETLEY.-THE STUDENT'S GUIDE TO THE MEDICAL PROFESSION. By C. B. Kbktley, F.R.G S., Assistant Surgeon to the "West London Hospital. With a Cliapter for Women Students. By Mrs. Gaekett Anderson. Crown Svo. 2s. 6d. KUHNE— ON THE PHOTOCHEMISTRY OF THE RETINA AND ON VISUAL PURPLE. Translated from the German of Dr. KIjhne, and Edited, with Notes, by Michael Fostee.,;^M.D., P.R.S, Svo. 3s. 6cl. LANKESTER.— COMPARATIVE LONGEVITY IN MAN AND THE LOWER ANIMALS. By E. Ray Lankestek B.A, Crown. Svo. is. 6d. LEISHMAN.— A SYSTEM OF MIDWIFERY, including the Diseases of Pregnancy and the Puerperal State. By William Leishman. M.D., Regius Professor of Midwifery in the University of Glasgow : Physician to the University Lying-in Hospital: Fellow and late Vice-President of the Obstetrical Society of London, &e., &c. Svo. Illustrated. Second and Cheaper Edition. 21s. MACLAGAN. — THE GERM THEORY APPLIED TO THE EXPLANATION OF THE PHENOMENA OF DISEASE. By T. Maclagan, M.D. Svo. 10s. 6d. "We think it well that such a book as this should be written. It places before the reader in clear and unmistakable language what is meant by the germ theory of disease." — Lancet. MAC N AM AR A. —Works by C. Macnamaea, F.C.U., Surgeon to Westminster Hospital : — A HISTORY OF ASIATIC CHOLERA. Crown Svo. 10s. 6d "A very A'aluable contribution to medical literature, and well worthy of the place which it is sure to assume as the standard work on the subject." — Medical Examiner. DISEASES OF BONE.— Clinical Lectures. Crown Svo. 5s. MACPHERSON.— Works by John Macphekson, M.D. :— THE BATHS AND WELLS OF EUROPE : their Action and Uses. With Notices of Climatic Resorts and Diet Cures. With a Map. New Edition, revised and enlarged. Extra fcap. Svo. Qs. Qd. OUR BATHS AND WELLS : The Mineral Waters of the British Islands. With a List of Sea-Bathing Places. Extra fcap. Svo. 3s. Qd. MANSFIELD (C B.).— A THEORY OF SALTS. A Treatise on the Constitution of Bipolar (two-membered) Chemical Compounds. By the late Charles Blachford Mansfield. Crown Svo. Ids. MAUDSLEY.— Works by Henry Matjdsley, M.D., Professor of Medical Jurisprudence in University College, London : — BODY AND MIND : An Inquiry into their Connection and Mutual Influence, specially in reference. to Mental Disorders: being the Gulstonian Lectures for 1S70. Delivered before the Royal College of Physicians. New Edition, witb Psychological Essays added. Crown Svo. Qs. Qd. THE PHYSIOLOGY OF MIND. Being the First Part of a Third Edition, revised, enlarged, and in great part re- written, of " The Physiology and Patho- logy of Mind." Crown Svo. lOs. 6i. THE PATHOLOGY OF MIND. [In the Press. MI ALL.— STUDIES IN COMPARATIVE ANATOMY. No -.1— The Skull ;of the Crocodile. By L. C. Miall, Professor of Biology in the Yorkshire College of Science. Svo. 2s. 6d. No. II.— The Anatomy of the Indian Elephant. By L. C. Miall and F. Green- wood, Curator of the Leeds Scliool of Medicine. lUustrattd. Svo, 5s.. MEDICAL CATALOGUE. 5 MIVART (St. George).— Works by St. George Mivart, F.R.S., &c., Lecturer in Comparative Anatomy at St. Mary's Hospital : — ON THE GENESIS OF SPECIES. Second Edition, to which notps have been added in reference and reply to Darwin's " Descent of Man." With numerous Illustrations. Crown Svo. 9*. LESSONS IN ELEJIENTARY ANATOMY. With upwards of 400 lUustrations. New Edition. Fcap. Svo. 6.?. (jd. "It may be questioned whether any other work on anatomy contains in like compass so proportionately great a mass of information." — Lancet. M'KENDRICK.— OUTLINES OF PHYSIOLOGY IN ITS RELA- TIONS TO MAN. By John Gray M'Kexdrick, M.D., F. R.S.E., Professor of the Institute of Medicine and Physiology in the University of Glasgow. Illustrated. Crown Svo. 125. Gd. MUIR— PRACTICAL CHEMISTRY FOR MEDICAL STUDENTS. Specially arranged for the first M. B. Course. By M. M. Pattison Muib, F.R.S.E., Prffilcetor in Chemistry, Caius College, Cambridge. Fcap. Svo. Is.Gd. " This little book will aid the student not only to jjass his professional examina- tion in practical Chemistry more easily, but will give him such an insight into the subject as will enable him readily to extend his knowledge of it should time and inclination permit." — Practitioner. OLIVER.— LESSONS IN ELEMENTARY BOTANY. By Daniel Oliver, F.R.S., F.L.S., Professor of Botany in University College, London, and Keeper of the Herbarium and Library of the Royal Gardens, Kew. With nearly 200 Illustrations. New Edition. Fcap. Svo. 4«. 6d. PARKER and BETTANY.— THE MORPHOLOGY OF THE SKULL. By W. K. Parker, F.R.S., Hunterian Professor, Royal College of Surgeons, and G. T. Bettanv, M.A., B.Sc, Lecturer on Botany in Guy's Hospital Medical School. Crown Svo. 10s. 6d. PETTIGREW.— THE PHYSIOLOGY OF THE CIRCULATION IN PLANTS, IN THE LOWER ANIMALS, AND IN MAN. By J. Bell PETriGREW, M.D.. F.R.S., etc. Illustrated by 150 Woodcuts. Svo. 125. "A more original, interesting, exhaustive, or comprehensive treatise on the circulation and the circulatory apparatus in plants, animals, and man, has never, we are certain, been offered for the acceptance of the anatomist physiologist or student of medicine." — Veterinary Journal. PIFFARD.— AN ELEMENTARY TREATISE ON DISEASES OF THE SKIN, for the Use of Students and Practitioners. By H. G. Piffard, M.D., Professor of Dermatology in the University of the City of New York, &c. With Illustrations. Svo. 16s. RAD CLIFFE.— Works by Charles Bland Radcliffe, M.D., F.R.C.P., Physician to the Westminster Hospital, and to the National Hospital for the Paralysed and Epileptic :— VITAL MOTION AS A MODE OF PHYSICAL MOTION. Crown Svo. Ss. 6d. PROTEUS : or UNITY IN NATURE. Second Edition. Svo. 7s. 6rf. RANSOME.— ON STETHOMETRY. Chest Examination by a more Exact Method with its Results. With an Appendix on the Chemical and Microscopical Examination of Respired Air. By Arthur Ransome, M.D. With Illustratious. Svo. 10s. 6d. " We can recommend his book not only to those who are interested in the graphic method, but to all who are specially concerned in the treatment of diseases of the chest."— BritUih Medical Journal. REYNOLDS (J. R.).— A SYSTEM OF MEDICINE. Edited by J. RfssFLL REYNOT.ns, M.D., F.P.S. London. In 5 Vols. Vols. I. to IlL, 25s. each ; Vol. IV., 21.=.. ; Vol. V., 25s. 6 MACMILLAN AND CO.'S EEYNOLDS (J. 'R.).—contimted. Vol. I. — Part I. General Diseases, or Affections of the Whole System. Part !!> Local Diseases, or Affections of Particular Systems. § I. — Diseases of the Skin. Vol. II. — Part II. Local Diseases (continued). § I. — Diseases of the Nervous System. § II. — Diseases of the Digestive System. Vol. III. — Part II. Local Diseases (continued). § II. — Diseases of the Digestive System (continued). § III.— Diseases of the Respiratory System.^ Vol. IV. — Diseases of the Heart. Part II. Local Diseases (continued). § IV. — Diseases of the Organs of Circulation. Vol. V. — Diseases of the Organs of Circulation. — Diseases of the Vessels. — Diseases of the Blood-Glandular System. — Diseases of the Urinary Organs. — Diseases of the Female Reproductive Organs. — Diseases of the Cutaneous System. Also, now publishing in MONTHLY PARTS, Price 5s. each, to he completed in 24 Parts. (Part 1, April 1st, 1879.) KICHARDSON.—Works by B. W. Eichaedson, M.D., F.E.S. :— DISEASES OF MODERN LIFE. Fifth and Cheaper Edition. Crown 8vo. 6*. ON ALCOHOL. New Edition. Crown 8vo. Is. HYGEIA, A CITY OF HEALTH. Crown 8vo. Is. THE FUTURE OF SANITARY SCIENCE. Crown Svo. Is. TOTAL ABSTINENCE. A course of addresses. Crown Svo. 3s. 6d. ROSCOE.— Works by Henry Koscoe, F.E.S., Professor of Chemistry in Owens College, Manchester : — LESSONS IN ELEMENTARY CHEMISTRY, INORGANIC AND ORGANIC. With numerous Illustrations, and Chromolithographs of the Solar Spectrum and of the Alkalies and Alkaline Earths. New Edition. Fcap. Svo. 4s. 6d. CHEMICAL PROBLEMS, adapted to the above. By Professor T. E. Thorpe, M.D., F.R.S.E., with Preface by Professor Roscoe. Fifth Edition, with Key. ISmo. 2s. PRIMER OF CHEMISTRY. Illustrated. 18zno. Is. ROSCOE and SCHORLEMMER.-A TEEATISE ON CHE- MISTRY. By Professors Roscoe and Schorlemmer. Vol. I. The Non- Metallic Elements. With Numerous Illustrations and Portrait of Dalton. Svo. 21s. Vol. II. Metals. Part I. With numerotzs Illustrations. Svo. 21s. SCHORLEMMER.— A MANUAL OF THE CHEMISTEY OF THE CARBON COMPOUNDS, OR ORGANIC CHEMISTRY. By C. ScHORLEMMER, F.R.S., Lccturer in Organic Chemistry in Owens College, Manchester. Svo. Ms. SEATON.— A HANDBOOK OF VACCINATION. By Edward C. Seaton, M.D., Medical Inspector to the Privy Council. Extra fcap. Svo, Ss. t5d SEILER.— MICEO-PHOTOGEAPHS IN HISTOLOGY, Normal and Pathological. By Cari- Seiler, M.D., in conjunction with J. Gibbons Hunt, M.D., and J. G. Richardson, M.D. 4to. 31s. 6d. SPENDER.— THERAPEUTIC MEANS FOE THE EELIEF OF PAIN. Being the Prize Essay for which the Medical Society of London awarded the Fothergillian Gold Medal in 1874. By John Kent Spender, M.D. Lend., Surgeon to the Mineral Water Hospital, Bath. Svo. Ss. 6d. MEDICAL CATALOGUE. 7 STEWART (B.).— LESSOXS I:N' ELEMENTAEY physics. By Balfour Stewart, F.R.S., Professor of Natural Philosophy in Owens College, Manchester. With Numerous Illustrations and Chromolithograph of the Spectra of the Sun, Stars, and Nebulae. New Edition. Fcap. 8vo. 4*. 6d. PRIMER OF PHYSICS. By the same Author. Illustrated, 18mo. Is. TUKE.— INSAiS^ITY IN ANCIENT AND MODERN LIFE, with Cliapters ou its Prevention. By D. Hack Tuke, M.D., F.R.C.P. Crown 8vo. 6s. " This work exhibits deep research in various directions, and teems with allusions and quotations which prove the author to be not only an accomplished psycho- logical physician, but a scholar of no mean order." — Medical Times. WEST.— HOSPITAL OPGANISATION. With special reference to the organisation of Hospitals for Children. By Charles "West, M.D. Founder of, and for twenty-three years Physician to, the Hospital for Sick Children. Crown Svo. 2s. 6d. WURTZ.— A HISTORY OF CHEMICAL THEORY from the Age of Lavoisier down to the present time. By Ad. Wurtz. Translated by Henry "Watts, F.R.S. Crown Svo. Qs. MANUALS FOR STUDENTS. THE MORPHOLOGY OF THE SKULL. By W. K. Parker, F. R.S., Hunrerian Professor, Royal College of Surgeons, and G. T. Bettany, B. Sc, Lecturer on Botany in Guy's Hospital Medical School. Illustrated. Crown Svo. 10s. Gd. THE OSTEOLOGY OF THE MAMMALIA: A Series of Lectures by Prof W. H. Flower, F.R.S., F.R.C.S. With numerous Illus- trations. New Edition, enlarged. Crown Si'O. 10s. 6rf. THE ELEMENTS OF EMBRYOLOGY. By Michael Foster, M.D., F.R.S., and F. M. BALFOUR, M.A. Part I. 7s. 6d. PRACTICAL PHYSIOLOGY : an Elementary Course of. By Dr. M. Foster, assisted by J. Langley. New Edition. Crown Svo. 6s. ELEMENTARY BIOLOGY : a Course of Practical Instmction in. By Prof. Huxley and H. N. Martin. New Edition. Crown Svo. (3s. PHYSIOGRAPHY : an Introduction to the Study of Nature. By Prof. lIixLEY, F.R.S. With Coloured Plates and Woodcuts. New Edition. Crown Svo. 7s. 6d. 8 MACMILLAE" AlTD CO.'S MEDICAL CATALOGUE. PRICE EIGHTEENPENCE, MONTHLY, THE PRACTITIONER: g. |0trntal oi ®^-era;pextfcs aitir public pi^alll^* EDITED BY T. LAUDER BEUXTON, M.D., F.R.S., Fellow of the Royal College of Physicians : Assistant Physician to St. Bartholomeio's Hospital; and Lecturer on Materia Medica and Therapeutics in St. Bartholomew's Hospital School. CONTENTS. Original Commnnications — Reviews of Books— Clinic of the Month— Extracts from British and Foreign Journals— Notes and Queries — Bibliography — and the Public Health Department. In Quarterly Farts, price 3s. 6d. BRAIN: A JOURNAL OF NEUROLOGY. EDITED BY J. C. BUCKNILL, M.D., M.R.C.P., F.RS. J. CRICHTON-BROWNE, M.D., F.R.S.E. D. FERRIER, M.D., F.R.C.P., F.R.S. J. HUGHLINGS-JACKSON, M.D., F.R.C.P. Contents — Original Articles, consisting mainly of Clinical and Pathological Records and Anatomical and Physiological Researches, Human and Comparative, on the Nervous System. Signed Critical Digests and Reviews of Clinical, Experi- mental and other Researches in this department of Science, both at home and abroad. Foreign Correspondence. It will be the object of "Brain" to keep its readers well abreast of modern progress in Neurology, and to advance the knowledge of a class of disease respecting which it is universally admitted that much has yet to be learnt. THE JOURNAL OF PHYSIOLOGY. EDITED (With the co-operation in England of Prof A. GAMGEE, F.R.S., of Manchester; Prof W. RUTHERFORD, F.R.S.. of Edinburgh; Prof. J. B. SANDERSON, P.R.S., of London; and in America of Prof. H. P. BOWDITCH, of Boston ; Prof. H. N. MARTIN, of Baltimore ; and Prof, H. C. WOOD, of Philadelphia) by DR. MICHAEL FOSTER, F.R.S., Of Trinity College, Carribriclge. It is proposed to publish it in parts, not at rigidly fixed times, but according to the supply of material. Every eifort, however, will be made to prevent any unnecessary irregularity in the appearance of the successive parts. About four or six parts, the exact number depending on the size of the several parts, will form a volume of about 500 pages. The volume will not necessarily coincide with the year ; its issue, like that of the constituent parts, wiU depend on the abundance of contributions. The subscription-price for the volume, post free, will be, when paid in advance — For Great Britain or America £l Is., or §5,25 (gold). Each part, as well as each volume, may also be obtained in the usual way through the trade, at the rate of £1 lis. 6d. per volume, the exact price of each part, being dependent on its size, &c. COLUMBIA UNIVERSITY This book is due on the date indicated below, or at the expiration of a definite period after the date of borrowing, as provided by the rules of the Library or by special ar- rangement with the Librarian in charge. DATE BORROWED DATE DUE DATE BORROWEO DATE DUE "J i^AE 1 «,-. C28(63a)MS0 C2^ COLUMBIA UNIVERSITY LIBRARIES 0047951362 m 1'^*.*