/
 
 OPTICAL DEFECTS OF THE EYE.
 
 (iy(^ ^ c/'/L 
 
 THE OPTICAL 
 
 DEFECTS OF THE EYE 
 
 AND THEIR CONSEQUENCES, 
 
 ASTHENOPIA AND STRABISMUS. 
 
 BY 
 
 JOHN ZACHAEIAH ^LAURENCE, F.E.C.S. 
 
 M.B. (Univ. Lond.) 
 
 SUEOEON TO THE OPHTHALMIC HOSPITAL, SOUTHWAEK ; EDITOR OF THE OPHTnALMIC BEVIKW; 
 
 MEMBER OF THE HEIDELBERG OPHTHALMOLOGICAL SOCIETY; OF THE 80CIBIY OF 
 
 PRACTICAL MEDICINE OF PARIS; OF THE PATHOLOGICAL AND HAEVEIAIT 
 
 SOCIETIES OF LONDON, ETC. ETC. 
 
 LONDON : 
 EOBERT HARDWICKE, 192, PICCADILLY. 
 
 1865.
 
 VI PREFACE. 
 
 writings of those distinguished ophthalmologists. Nor should 
 I omit to specially mention the admirable work of Porterfield, 
 which, after the lapse of upwards of a century, still holds its 
 groun,d as a standard authority on the Optic's of the Eye. 
 
 Finally, I would venture to hope that the following pages 
 may be found a useful Introduction to the comprehensive 
 Treatise of Professor Bonders, in the twenty-second volume of 
 the New Sydenham Society's works. 
 
 J. Z. L. 
 
 30, Devonshire Street, Portland Place, 
 April, 1865.
 
 CONTENTS. 
 
 Chapter . Pago 
 
 I. OPTICAL CONSIDERATIONS 1 
 
 II. — PHYSIOLOGICAL OPTICS. 17 
 
 Til. PATHOLOGICAL OPTICS 30 
 
 IV. MYOPIA 32 
 
 V. HYPERMETROPIA 52 
 
 VI. — ASTIGMATISM 61 
 
 VII. PRESBYOPIA 74 
 
 VIII. — PARALYSIS OF ACCOMMODATION 82 
 
 IX. — ASTHENOPIA 92 
 
 X. — THE CONNEXION BETWEEN CONVERGENT STRABISMUS 
 
 AND HYPERMETROPIA 100 
 
 INDEX Ill
 
 //
 
 OPTICAL DEFECTS OF THE EYE. 
 
 CHAPTER I. 
 
 OPTICAL CONSIDERATIONS. 
 
 TN the present course of lectures I propose giving an 
 ^ account of those defects of vision which depend either 
 upon a misformation or a perversion of function of the optical 
 structures of the eye. Up to a comparatively recent period 
 the pathological deviations of the refractive and accommo- 
 dative powers of the eye were^ from the want of any system 
 in their discrimination^ involved in such hopeless confusion, 
 that the determination of the glasses requisite for their cor- 
 rection became unsatisfactory and empirical in the last 
 degree : the error was, most ophthalmic surgeons rested 
 satisfied with but a superficial knowledge of optics, whilst 
 opticians regarded the eye rather as a production of their 
 workshops requiring some correction, than as a structure of 
 the human body, endowed with all the attributes of vitality. 
 I shall, therefore, not conceive the time ill-spent, if we confine 
 our attention, in the first instance, to optics considered as a 
 pure science,* altogether independent of the appUcations we 
 
 * In the course as actually delivered by me at the Ophthalmic Hosjjital, 
 Southwark, all the leading phenomena of optical science, as the laws of 
 reflection and refraction, &c., were fully discussed ; but as these subjects are 
 but imperfectly intelligible without such experimental illustrations as I then 
 produced, I have omitted them in the lectures as now published, proceeding 
 at once to the consideration of optical lenses, a knowledge of which is 
 essential for the due comprehension of the physiological and pathological 
 optics of the eye. 
 
 B
 
 Z OPTICAL DEFECTS OF THE EYE. 
 
 shall hereafter have to make of our knowledge thus acquired. 
 To attempt to understand the optical construction and defects 
 of the eye without such a previous knowledge, is simply- 
 irrational; and it is to the absence of any such preliminary 
 information that I am led to ascribe the unintelligible cha- 
 racter which most of the communications hitherto published 
 on this subject have possessed for the jjrofcssion at large. 
 
 The eye is, in its most important relations, essentially an 
 optical instrument, through whose medium we take cogni- 
 sance of the most obvious and necessary events of the external 
 world. This, in its turn, becomes the subject of our visual 
 observation by that physical force which we understand by the 
 term "light.'-' 
 
 Light is one of those agents of nature the existence of 
 which is inferred from its effects, just as that of heat, elec- 
 tricity, mechanical or vital force is inferred from their 
 effects. The effect of light which principally concerns us 
 is that which it exercises on the retina ; but photography, 
 magnetism, vegetation, all demonstrate the existence of light 
 as a perfectly independent phj'sical agent. In other words, 
 supposing all the world were blind, still light would exist, 
 even in its purely optical sense : it would still be reflected, 
 refracted, polarised, although we should be unable to re- 
 cognise any such effects. Some of the bodies of nature are, 
 in themselves, sources of light — self-luminous — such as the 
 sun, the stars, bodies in a state of ignition. On the other 
 hand, most bodies are not self-luminous, but derive their 
 luminosity from reflected light, which itself is, in the first 
 instance, derived from self-luminous bodies. Thus, the 
 objects in a perfectly dark chamber only become visible by 
 the introduction of a light ; extinguish this latter, and they 
 are no longer seen. 
 
 For the sake of precision in the investigation of the laws of 
 the propagation of light through space, it is necessary to 
 consider all luminous bodies as composed of an infinite number 
 of infinitely small luminous points, each of these points to 
 give off, in all possible directions — radiate — an infinite number 
 of straight lines of light of extreme tenuity. These lines of 
 light are individually called rays of light, whilst the combined
 
 OPTICAL CONSIDERATIONS. 6 
 
 bundle of rays whicli emanate fronij or converge to, a luminous 
 point is called a pencil of rays.* If the component rays of a 
 pencil of liglit, in their onward progress from their initial 
 source — the luminous point — separate further and further 
 from each other, then we have before us a divergent pencil 
 (Fig, 1) ; if, on the other hand, the rays approach each other 
 closer and closer as they proceed onwards to a point, then we 
 have a ronverr/ent pencil (Fig. 2) ; whilst, lastly, if the rays of 
 a pencil neither diverge nor converge in their onward course. 
 
 Fig. 1. 
 
 Fig. 2. 
 
 Fig. 3. 
 
 
 -> 
 
 > 
 
 Fra. l.-Divorgent Pencil-) i^^i^^us point, or foes. 
 „ 2. — Convergent „ ) r j j 
 
 3.— Parallel 
 
 we have a parallel pencil (Fig. 3). A luminous point is 
 termed, in the language of optics, a focus of rays, or, shortly, 
 a focus. 
 
 As an illustration of divergent and convergent pencils, the 
 case of the flame of a candle before which a convex lens is 
 held may be taken. The rays of light diverge from the con- 
 stituent luminous points of the flame, and are converged to as 
 many points again by the lens, as we shall discuss fully at a 
 later period of these lectures. From what has been before 
 
 * We may obtain an approximate concrete idea of a " ray" of light by allow- 
 ing the light from the sun to pass into a dark chamber through the minutest 
 possible hole drilled in a thin plate of metal ; enlarge this hole, and several 
 rays, constituting a "pencil" of light, will enter the chamber. 
 
 B 2
 
 OPTICAL DEFECTS OP THE EYE. 
 
 said, it may be inferred that all luminous bodies emit their 
 rays of hght in the form of divergent pencils. If a luminous 
 point [0, Fig. 4), be at a certain distance from a recipient 
 
 Fig. 4. 
 
 surface, AB (the pupil of the eye, e.g.), any two divergent 
 rays' form some angle, as AOB, with each other; if the 
 luminous point (o) be fui'ther off, the angle, as AoB, will be 
 less ; if still further off, the angle, as xiojB, still less ; and so 
 on. This fact is obvious to the eye from a mere inspection of 
 Fig. 4; but it is also geometrically demonstrable (Euclid i., 21). 
 Following up the same reasoning, if we suppose the luminous 
 point infinitely far off, any two proximate rays will form no 
 angle at all with one another — will be, in a word, 'parallel, it 
 being in all cases understood that the surface on which the 
 rays of light impinge is of limited extent in comparison with 
 its distance from the luminous point. Thus, if a candle flame 
 is, say, twenty feet from us, the rays of hght from each 
 luminous point which strike the area of the pupil of the eye 
 form practically parallel pencils, and a fortiorc, in ascending 
 progression, those which strike the pupil from a lighthouse, 
 from the sun, from the fixed stars. But if the recipient sur- 
 face on which the rays of Hght from a candle flame fall be, 
 say, a large target, from the lighthouse a large extent of the 
 ocean, from the sun the earth, from the fixed stars the entirety 
 of space, we may, in the first two instances certainly, in the 
 two latter not unreasonably, assume that the pencils of rays 
 are of a divergent character in regard to the magnitude of the 
 surfaces on which they fall. Strictly speaking, then, no such 
 rays as parallel rays exist in nature; but for the recipient 
 surfaces we shall have to deal with, our eyes, lenses, reflectors, 
 &c., we may practically regard all rays of light which proceed 
 from near objects as divergent, those from distant ones as
 
 OPTICAL CONSIDERATIONS. 5 
 
 parallel, whilst convergent rays are not to be found in nature 
 at all, but may be produced bj certain optical means, of which 
 we shall soon come to speak. 
 
 What evidence have we of all the above assumptions T 
 Their truth is inferred from the fact that most optical 
 phenomena are only deducible from such assumptions, and 
 that the laws of optics hence deduced have not only led, 
 by a chain of inferences, to the explanation of more com- 
 plex phenomena, but have served as the landmarks to the 
 investigation of, up to the time, unknown fields of discoveiy in 
 optical science. Two diiferent theories have been advanced 
 of the more intimate nature of light. The Newtonian (corpus- 
 cular) conceives each luminous point to be constantly giving 
 off a succession of luminous corpuscles, which follow each 
 other in uninterrupted succession on an imaginary axis, like 
 a string of beads coursing along a rigid thread. The undu- 
 latory theory, on the other hand, considers space as pervaded 
 by a subtle gaseous fluid or ether ; that luminous bodies have 
 the power of communicating to this ether a wave-motion, 
 which affects the retina, just as the undulations of the atmo- 
 sphere from vibrating bodies affect the auditory nerve, or, to 
 take a ruder simile, as the waves of the ocean impart their 
 movements to the vessel that rides on them. The undulatory 
 theory is of far wider significance, however, than its mere 
 physiological application. Su* John Herschel, speaking of it, 
 says, "It is, a theory which, if not founded in nature, is cer- 
 tainly one of the happiest fictions that the genius of man has 
 yet invented to group together natural phenomena, as well as 
 the most fortunate in the support it has received from all 
 classes of new phenomena, which at their discovery seemed in 
 irreconcileable opposition to it. It is, in fact, in all its appli- 
 cations and details, one succession of felicities, inasmuch that 
 we may almost be induced to say, ' If it be not true, it deserves 
 to be.'^' 
 
 We shall now proceed to the, for our purpose, most essen- 
 tial part of the subject —viz., to the consideration of the 
 properties of optical lenses. 
 
 The double-convex lens is represented in section in Fig. 5. 
 It is formed by the apposition of segments of two spheres.
 
 6 OPTICAL DEFECTS OF THE EYE. 
 
 Witli the centre C and radius C E, describe the circle C RP, 
 and with the centre c and radius c r, describe the circle c rjp ; 
 the part A B, common to both circles, represents the vertical 
 section of a double-convex lens. C c are the centres of curva- 
 
 FiG. 5. 
 
 C, c=centre8 of curTature"^ 
 CS,c r,=radii of curvature 
 
 Cc=principal axis 1 . ., , . „ 
 
 ^B = diameter f°^ ^'^® ^°'^^ ^ ^• 
 
 0= optic centre 
 jSo«=a secondary axis J 
 
 ture ', C R, cr radii of curvature of the lens. A line c, drawn 
 through the two centres, is called the principal axis ; the line 
 A B, perpendicular to the axis, the diameter ; the centre of 
 this Hne the optic-centre of the lens ; whilst lines passing 
 through 0, a,s S s, are secondary axes of the lens. 
 
 Fig. 6. 
 
 In the convex lenses in general use for spectacles, the radii 
 of curvature of the two surfaces are equal. We then have a 
 double equi-convex lens {AB, Fig. 6).
 
 OPTICAL CONSIDERATIONS. / 
 
 Parallel rays {i. e., as I have before mentioned, such as ema- 
 nate from luminous objects at a distance from the lens ; prac- 
 ticalljj I find experimentally that for a lens of about an inch 
 and a-half diameter, this distance is anything beyond four 
 feet) entering one surface of the lens, are found, on then' exit 
 from the second surface, to have converged to a point at a 
 fixed determinate distance from the lens. 
 
 This point F (Fig. 7) is the fonts of parallel rays — the 
 
 Fig. 7. 
 
 -^ 
 
 Fig. 7. — i''^. = principal focal length, in an equi-eouvex glass lens equals the radius 
 of curvature. 
 
 principal focus of the lens, or, briefly, the focus of the lens. 
 The distance of this point from the contiguous surface (strictly 
 speaking, from the optic centre) of the lens, FA, is the j;rin- 
 cipal focal length of the lens, or briefly, ih.Q focal length of the 
 lens. This, in the case of an equi-convex glass lens, equals 
 the radius of curvature. Thus we speak of convex lenses of 
 2, 3, 5, 8, &c., inches focal length, or, as they are commonly 
 termed, 2-inch, 3-inch, 5-inch, 8-inch, &c., convex lenses — 
 meaning thereby that such lenses have the property of con- 
 verging parallel rays to a focus, uniting them to a point, at 
 distances from the lenses of 2, 3, 5, 8 inches respectively.* 
 To make a practical application of our knowledge : — A person 
 comes to you with a pair of convex glasses, but he has forgotten 
 their focal length, and you wish to find this out; all you have 
 to do is to see at what distance the image of any distant 
 object (such as of objects in the street, of the sun, &c.) is 
 formed most distinctly on a sheet of paper. Measure the 
 
 * I strongly recommend all persons who wish thoroughly to understand 
 the subject of these lectures, to actually perfonn themselves all the experi- 
 ments wliich are adduced. A set of trial lenses, and a few stands to hold 
 them, &c., constitute all the apparatus recpiii'ed.
 
 8 
 
 OPTICAL DEFECTS OF THE EYE. 
 
 Fig. 8. 
 
 distance of the lens from tlie image, and you at once have the 
 required information. 
 
 Vice versa, rays of light which diverge from a point F 
 (Fig. 8), situate at the focal length of the lens, issue from 
 
 the other side in a 
 state of parallelism. 
 Many light-house lan- 
 terns are constructed 
 on this principle, the 
 light being placed at 
 the focus of a convex 
 lens.* 
 
 It is necessary to the right understanding of what follows, 
 to here mention a well-known property of convex lenses. 
 If an object be placed behind a convex lens at a less 
 distance than its focal length, an erect magnified image of 
 the object is perceived by an eye placed in front of the lens. 
 Now, the shorter the focal length of the lens, the greater its 
 magnifying power — e. </., a 2 -inch lens magnifies more than 
 a 4-inch lens. In reference to their respective magnifying 
 powers, then, we should be wrong in designating the first lens 
 as 2, the second as 4, which would imply the second were 
 " stronger " than the first j but the more correct expression 
 is to call the first lens a ^, the second a j, the latter fraction 
 being less than the former. This nomenclature is strictly 
 true, if we understand by the word "power" of a lens its 
 deflecting power on the rays of light that strike it ; so that, 
 e. g., a lens of ^l is one that unites parallel rays to a focus 
 at a distance of twelve inches from its sm-face, a lens of -^ one 
 that unites them at twenty-four inches, and so forth. [Vide 
 note to p. 15.] 
 
 Designating, then, in this way convex lenses by fractions. 
 
 * I may here call attention to the fact, that, in the French trial- 
 lenses, the focal lengths are given in Paris inches, whereas English opticians 
 give theirs m English inches. 12 English inches = about 11| Paris inches ; 
 so that, e. g., if M'e find a patient requires a 22|-iuch glass of the French 
 trial-glasses, we should order hun a 24-inch English glass. In the German 
 trial-lenses (Paetz & Flohr's) the Prussian inch is adopted, which differs but 
 slightly from the English inch.
 
 OPTICAL CONSIDEEATIONS. 9 
 
 of whicli the numerators are 1^ and the denominators the 
 focal lengths of the lenses^ let me announce tO you a most 
 important proposition : the optical effect of a combination 
 of lenses in contact, both as regards its power and as 
 regards the distance at which parallel rays impinging on 
 the first lens are brought to a focus by the combination of 
 lenses, is always given by adding the several fractions, which 
 express their respective powers, together. The resultant 
 fraction represents the power of the single lens, which is 
 optically equivalent to the whole set. Thus, suppose we wish 
 to know the focal length of the combination of an 8-inch and 
 of a 5-inch convex lens, placed together, we have, i + i = i- 
 nearly. The equivalent single lens is a 3-inch lens. Parallel 
 rays impinging on such a combination wiU be focussed at a 
 distance of three inches.* 
 
 We have now considered the optical eflFect of a convex lens 
 on parallel rays, such as emanate from objects at any distance 
 greater than four feet from the lens. But suppose the object 
 situate at any less distance, but greater than the principal 
 focal length, from the lens, then the rays of light which pro- 
 ceed from each luminous point are no longer parallel, but 
 divero^ent. It will now be found the lens still retains its 
 power of re-uniting the rays to a point — of focussing them. 
 But the focus is now further oiF the lens than the principal 
 focus. I think I may explain this result by a very simple 
 demonstration. Take the case, e. g., of a 6-inch convex lens. 
 Such a lens unites parallel rays to a focus at six inches. 
 Suppose, however, the rays of light, instead of being parallel 
 to, diverge from, a luminous point, say, twenty inches distant 
 from the lens, then, as I have before said, they would, after 
 passing through a 20-inch lens, issue parallel. Now, a 6-inch 
 lens may (according to the proposition before announced) be 
 considered as equivalent to the combination of a 20-uich lens 
 and a 9-inch lens; for ^=2V + i (nearly). So that we may 
 assume that that part of the 6-inch lens which is equivalent 
 to a 20-inch lens has been employed in converting the rays 
 
 * If any of the lenses are concave ones, their powers must be subtracted. 
 If the resultant fraction is negative, tliis indicates that the entire combination 
 is equivalent to a concave lens.
 
 10 
 
 OPTICAL DEFECTS OF THE EYE. 
 
 diverging twenty inches from its surface into parallel ones ; 
 these tlien strike that remaining part of the 6-inch lens which 
 is equivalent to the 9-inch lens, and are, therefore, brought 
 to a focus at nine inches from this latter. So that the final 
 result is, that rays diverging from a luminous point twenty 
 inches from a 6-inch convex lens are brought to a focus at a 
 
 O 
 
 distance of nine inches from its surface.* 
 
 Fig. 9 illustrates the presumed theory graphically : — 
 
 Fig. 9. 
 
 i^e^rncMenr} Together equivalent to a 6-inch lens. 
 
 F=a. luminous point, twenty incbes from B. 
 JF"=focus of combined lenses, nine inches from A. 
 
 As the luminous point approaches nearer and nearer to the 
 lens, the focus of the emergent rays moves further and further 
 off, till, as I have before said, when the luminous point arrives 
 at the principal focus of the lens, the emergent rays are not 
 united to a focus at all, but issue parallel to one another. 
 
 We are now j)repared to understand how a convex lens 
 possesses the power of forming an image of an object placed 
 in front of it. As I have before explained, every object in 
 nature may be considered as composed of an infinite number 
 of points, either in themselves luminous or rendered so by 
 
 * We may hence deduce the practical rule : To find the focus of rayj 
 diverging from any given distance from a convex lens of kno\™ focal length 
 subtract from the power of the lens the fraction whose numerator is unity 
 and whose denominator is the distance of the object from the lens ; the 
 resulting fraction reduced to a fraction whose numerator is unity gives in its 
 denominator the distance from the lens to which the rays will be converged 
 to a focus by the lens. My friend Dr. Giraud-Teulon, of Paris, has based an 
 ingenious view of the action of lenses generally on the theory in the text. 
 This theory he, I know not how far correctly, has ascribed the origination o:" 
 to myself (Vide Annales (VOculistique, Tome LIL, p. 261).
 
 OPTICAL CONSIDERATIONS. 11 
 
 reflected light, every such luminous point emitting a pencil of 
 rays, either divergent or parallel, according to the distance of 
 the object from the surface oa which the rays fall. If the 
 rays of any one pencil are made to again converge, the 
 luminous point from which they in the first instance emanated 
 is, so to say, again reproduced at their point of union (focus). 
 In this way, each luminous point of the object having its 
 optical representative, an "image" of the object is formed. 
 We understand, then, by the term "image" of an object, its 
 optical reproduction. An object has certain physical pro- 
 perties : its extension in space, its power of affecting our 
 sense of touch. It may possess heating or electrical attributes, 
 and, as a rule, always has an optical existence. Certain 
 bodies, however, such as some gases, having neither any inhe- 
 rent nor acquired luminosity, may be said to have no optical 
 existence ; at any rate, as far as our means of ascertaining any 
 such existence extend, although they may have certain optical 
 properties, such as the power of refracting or polarising the 
 light emitted from other bodies. If you regard yourself in a 
 looking-glass, you see yourself optically reproduced, but not 
 in any other sense; your image conveys to you no direct 
 information of any other physical properties you may possess ; 
 you cannot, hence, directly infer whether your flesh is hard or 
 soft, whether yom* body feels hot or cold, whether you are in 
 any particular electrical state, &c. Your image in the glass 
 is your optical reproduction, and nothing more. 
 
 If in a plate of metal a minute aperture is drilled, and on 
 one side of the plate an object (the flame of a candle) be 
 placed, whilst on the other side is a sheet of paper, an 
 inverted image of the flame will be observed on the paper. 
 To understand this result, we must premise that hght 
 travels only in straight lines. Look at the sky through a 
 flexible tube; bend the tube, and the sky will no longer 
 be perceived. 
 
 Let AB (Fig. 10) be an object from every point of which a 
 pencil of divergent (or parallel, as the case may be) rays pro- 
 ceeds (only those which proceed from the top and bottom of 
 the object are represented in the figure), PP' a plate of metal 
 perforated at its centre C, so as to admit of but a single ray
 
 12 
 
 OPTICAL DEFECTS OF THE EYE. 
 
 of light passing tlirough it, S S' a, sheet of paper. A single 
 ray, as A A and B B, from each point of the object will strike 
 
 Fig. 11. 
 
 the paper, and hence, being reflected to the eye, E, of an 
 observer, a distinct htvertecl image, B A, of the object will be 
 
 perceived. It will be remarked, 
 that the inversion of the image 
 is caused by the fact that rays 
 of light only travel in straight 
 lines.* A simple inspection of 
 the figure shows that if the 
 sheet of paper is as far behind 
 the aperture in the plate as 
 the object is in front of it, 
 the image {B A) will be equal 
 in size to the object; if closer, 
 less (/^ a) ; if further off, greater 
 {ha). As of all the rays that 
 diverge from each luminous point 
 of the object only one of each 
 pencil strikes the paper (the rest 
 being cut off by the opaque plate of metal), it follows that the 
 
 li-ht. 
 
 In such an elementary exposition as the above, I ignore the inflexion of
 
 OPTICAL CONSIDERATIONS. 
 
 13 
 
 image will be greatly inferior in brightness to the object. If 
 we^ as in Fig. 11, increase the aperture in the plate, more rays 
 of the pencils from each point of the object are received 
 on the paper, but each point of the object is no longer 
 represented by a point, but by a circle (the section of each 
 conical pencil of rays) on the paper : these circles (" circles of 
 diffusion ") from contiguous points necessarily overlap ; hence 
 the image is no longer a faithful optical representation of the 
 object, but appears blurred and indistinct.* In other words, 
 we have lost in definition what we have gained in brightness 
 by the enlargement of the aperture in the plate. Suppose we 
 now, in the enlarged aperture C C , place a double convex lens. 
 
 Fig. 12. 
 
 then the rays of each divergent pencil, from every luminous 
 point of the object, will be reunited by the power of the lens 
 into a corresponding luminous point in the image, and we 
 shall obtain (Fig. 12) all the advantages of distinctness of 
 
 * In the case of imperfect accommodation of the eye, the " circles of dif- 
 fusion " on the retina take the form of the pupil. I have found a piece of 
 coarse calico affords a ready method of studying these circles. The inter- 
 stices between the threads appear on imperfect accommodation as multitudes 
 of overlapping cu-cles, which vary in size, illumination, &c., with the size of 
 the pupil, with the addition of convex or concave lenses, &c.
 
 14 OPTICAL DEFECTS OF THE EYE. 
 
 definition combined witli a due degree of luminosity in 
 the image. An eye in front of tlie paper will perceive an 
 inverted image of tlie object by reflected liglit ; if an aperture 
 be made in the paper, an eye placed behind the aperture will 
 see an inverted image of the object, suspended, as it were, in 
 the air (an " aerial image "), formed by the rays of the con- 
 vergent pencils which produce the image, diverging again 
 after their reunion. If the object be at a considerable distance 
 from the convex lens, of course the image will be formed at 
 its principal focal length ; after the object is about four feet 
 from the lens, as it approaches nearer, so the image recedes, 
 and at the same time becomes larger; when it is at the 
 principal focus, no image at all is formed ; a circle of difiused 
 light, in the case of a flame, the size of the lens, is alone 
 visible. A little reflection shows these facts to be an 
 immediate consequence of what has been already said 
 regarding the foci of convex lenses, and the optical effects 
 of a minute aperture. 
 
 I now wish to direct your attention to some phenomena 
 which are the direct consequences of what has preceded, and 
 which are of great importance in the physiology and pathology 
 of the optical apparatus of the eye. 
 
 If we place a candle-flame at a considerable distance from 
 an 8-inch lens, a screen placed eight inches behind this latter 
 will exhibit a distinct inverted image of the flame ; if we now 
 bring the flame closer to the lens, say, to fourteen inches from 
 it, this approximation of the object is accompanied by a 
 corresponding recession of the image;* if the screen still 
 remains at 8", only an indistinct, blurred, quasi image of the 
 flame will be seen; but by placing a 14-inch lens next to the 
 8-inch one, we re-obtain a perfectly distinct (larger) image of 
 the flame on the screen. The rationale of this is very simple ; 
 the 14-inch lens has the effect of rendering parallel the rays 
 which diverge from each luminous point of the object fourteen 
 inches in front of it ; these parallel rays impinging on the 
 8-inch lens are then focussed at eight inches — the original 
 
 * In fact, the distance behind the lens at which a distinct image of the 
 flame is formed is 19", so that the screen must be moved eleven inches further 
 back for the observation of a distinct image.
 
 OPTICAL CONSIDERATIONS. 15 
 
 position of the screen. Of course, for the two (14- and 8-inch) 
 lenses we may substitute a single 5-inch one; forJ^+J^^- 
 (nearly). As we shall see further on, this is precisely what 
 occurs in the human eye, in the act of accommodation for 
 near objects. 
 
 The converse experiment is important in the theoiy of 
 myopia. The flame being at a considerable distance off, and 
 the screen being at eight inches from the 8-inch lens, substi- 
 tute for this latter a 5-iuch one : no distinct image of the flame 
 then appears on the screen. Now place a 14-inch concave 
 lens next to the 8-inch convex one, and a distinct image on 
 the screen is at once produced. After what has preceded, the 
 equation, i — T^=i (nearly), sufficiently explains the above 
 result. 
 
 In all the cases we have been hitherto considering, the rays 
 of light which converge to form the image actually meet, and 
 may be received on a screen ; or, if we remove this latter, the 
 aerial image may, by a little attention, be seen suspended in 
 the air, as it were — to be an " aerial image," as it is termed. 
 But as soon as the object approaches the lens to a distance 
 anything less than the principal focal length of this latter, a 
 remarkable change ensues ; the image all at once transplants 
 itself to the same side as the object, and simultaneously 
 becomes erect and magnified.* (Before, it was on the contrary 
 side of the lens to the object, and inverted.) This image, 
 
 * A convex lens used as a magnifier, in tlie sense referred to in the text, 
 is simply an artificial addition to oiu- accommodation, which enables us to 
 view the object under a greater angle than we otherwise could do. This, 
 in combination with a mental reference of the image to a further plane of 
 projection than the object itself really occupies, explains the " magnifying 
 power" of the lens. The word "power" has been used rather loosely in 
 optical science. In the text we have used it in the sense of magnifying 
 power (M). This M appears to depend on (1) the focal length of the lens, 
 (2) the state of refraction and accommodation and acuteness of vision of 
 the eye, (3) on a formula given by Parkinson at p. 179 of his Treatise on 
 Optics. At p. 89 of this same work, the word " power " of a lens is arbi- 
 trarily defined as the reciprocal of its focal length. Giraud-Teulon has very 
 lucidly explained (in the Annales d'Oculistique for July, 1864) that the word 
 *' power " must, in the latter case, be taken to represent the con- or di-verging 
 power of a lens on rays of light. In this sense the power of a lens is strictly 
 inversely proportional to its focal length.
 
 16 OPTICAL DEFECTS OF THE EYE. 
 
 however, cannot be received on a screen. It is not, as in 
 the case of the inverted imago, a real, but only an imaginarn 
 (virtual) one; or, if I were to designate the real image 
 as a physical, I should call the imaginary a physiological 
 one. 
 
 If this course were one of pure optics, I should now bring 
 before you the subject of concave lenses; but as these find 
 their application in myopia, I shall defer speaking of them 
 till I come to that part of our subject.
 
 }'HYSIOLOGICAL OPTICS. 1 " 
 
 CHAPTER II. 
 
 PHYSIOLOGICAL OPTICS. 
 
 rpHE liumau eyeball is, from before backwards, about i^ of 
 -*- an English inch.* In it exists a system of convex lenses 
 composed of the aqueous chambers, the crystalline lens, and 
 the vitreous humour. These lenses differ, infer alia, from the 
 glass ones we have hitherto been speaking of, in being soft, 
 and thus capable of being modified in their degTces of curva- 
 ture — in their focal lengths t — by certain muscles. When 
 we regard a distant object, these muscles are in a state of 
 relaxation, or, at any rate, of tonic contraction. The eye is 
 then said to be in a " state of rest ; " its lenses preserve one 
 fixed curvature. The three lenses of the eye we may, for all 
 practical purposes, consider as equivalent to a single 1-inch 
 glass convex lens (more accurately to one of i^ inch) . The 
 crystalhne, Monro states to have a focal length of f inch. 
 When the eye is in a state of rest, the image of a distant object 
 is formed exactly on the retina, and may there be seen. J The 
 retina thus closely resembles the screen of our previous optical 
 experiments, or the ground glass of an ordinary camera ob- 
 scuraj§ the curved surface of the retina is however much better 
 
 * Professor Ed. Jager found the average length of the axis of eighty adult 
 eyes to be 24'3 millimetres ; that of seventy eyes of new-born infants to be 
 17'53 millimetres. 
 
 f Both in convex and concave lenses it may be asserted, generally, that the 
 shorter the radius of curvature (or the greater the degree of curvature), the 
 shorter the focal length of the lens (or the greater its power). 
 
 t If we carefully cut away the sclerotic in the sheep's eye, a distinct in- 
 verted image of any distant object may be seen to be formed on the retina. 
 The eye of an albino rabbit shows the same thing without any further 
 preparation. 
 
 § John Baptista Porta, the inventor of the camera obscura, in his " Magia 
 Naturalis" (1560), thought that the pupil played the part of the minute aper- 
 ture in his camera, and that the crystalline lens was the recipient surface of 
 the image. It was reserved for Kepler, forty years later, to prove the true 
 functions of the lenses of the ej^e. 
 
 C 
 
 \
 
 18 OPTICAL DEFECTS OF THE EYE. 
 
 adapted to receive an accurate image than the flat surface of 
 the ground glass of the camera is. 
 
 How it is that the purely physical image thus formed, trans- 
 lated to the mind {sensorium), through the medium of the 
 optic nerve, produces to us an accurate idea of the object 
 itself, as regards position, size, form, colour, &c., is more than 
 we at present know, any more than how the different vibra- 
 tions impressed on the auditory apparatus convey to the mind 
 the various ideas of musical and other sounds.* But, at the 
 same time, it is most important to bear in mind the mental 
 (psychical, nervous) nature of vision — its partly subjective, as 
 contrasted with its partly objective character ; f otheinvise, 
 we might often be misled into ascribing defects of vision 
 either to optical or organic causes, which depended rather on 
 some perversion of the sentient apparatus. Nevertheless, it 
 is remarkable how perfectly the purely optical functions of the 
 eye must be exercised before vision can be accurate. For the 
 distinct perception of objects their images must fall exactly on 
 the retina. If a normal eye be in a state of rest, the images 
 of all distant objects fulfil this condition, and vision is perfect. 
 Suppose, now, the eye, with its lenses still remaining in this 
 state of equilibrium, be directed to a near object, the image 
 would be foinned behind the retina. I say " would be," be- 
 cause, in reality, the retina having no power of locomotion 
 
 * In Porterfield's quaint langiiage, " The Connection betwixt our Ideas and 
 the Motions excited in the Eetina, Oj^tic Nerves, and Sensorium, is unhiown to 
 us, and seems to depend entirely on the Will of God.'" — (IL xviii) 
 
 t Many facts illustrate the subjective nature of vision. Dreams in sighted 
 persons, and still more forcibly in the blind, prove the share of the mind in 
 the act. This, so to say, memory of vision occurs to us awake or asleep. 
 From several inquiries which I have made amongst the blind, I find that 
 their subjective vision is as vivid as in the sighted, but that pei-sons born 
 blind are destitute of any such, so to say, " second " sight. However, as I am 
 only now collecting specific information on this interesting subject, I defer 
 saying anything more on it at present. In this point of view I might fanci- 
 fully say that events are written on the tablets of the mind, as it were, -with 
 a sympathetic ink, which requires but the association of ideas — the fire — to 
 bring out its characters. Ocular spectra {vide Darwin in the PhilosojMcal 
 Transactions, vol. Ixxvi. p. 33, et seq., and a paper by the author in the 
 Glasgoiv Medical Jo^irnai for July 1, 1861) are further IQustrations of 
 subjective vision.
 
 PHYSIOLOGICAL OPTICS. 19 
 
 backwards, only an indistinct quasi-image is formed on it 
 by the circles of diffusion, as I have before fully explained in 
 the case of a convex lens. The retina remaining stationary, 
 an increase of cui'vature — power — of the crystalline would, as 
 you can, from what I have before said (p. 14), now readily under- 
 stand, bring the image back on to the retina ; and this is, in 
 fact, precisely what occurs in the human eye. The images of 
 near objects are brought back on to the retina by the crystal- 
 line lens becoming more convex. This was first partially 
 proved by Young, much more completely so by Max. Langen- 
 beck, in 1849, by observing the changes in size of Purkinje's 
 images, and fully confirmed in the same way by Cramer by his 
 phakoidoscope. With this instrument the images of a taper- 
 flame produced by reflection from the anterior and posterior 
 surfaces of the crystalline lens are seen to increase in size 
 and in their distance apart, or the reverse, according as the 
 observed eye is directed to a distant or to a near object 
 respectively. The corneal image, on the contrary, undergoes 
 no change whatever, either in size or position. This change 
 in the convexity of the crystalline lens is what is known as 
 
 Fig. 13. 
 
 ^ <- 
 
 -==^rM— — \ <- 
 
 ? £rzi~jl^ — "^ 
 
 ^ ^ 
 
 Fig. 13. — <; = crystalline lena. 
 2= retina. 
 
 the " power of accommodation or adjustment " of the eye ; 
 or, as I shall in future term it, simply the " accommodation " 
 of the eye. It is of great importance that you should con- 
 sider this accommodation as a quantity — as a magnitude, in a 
 word — susceptible of increase or diminution. This will be- 
 come clear from an example. If, from having seen a distant 
 object distinctly, I all at once transfer the eye to an object, 
 say six inches oS", and find I see t^ distinctly, it is then evident 
 
 c 2
 
 20 
 
 OPTICAL DEFECTS OF THE EYE. 
 
 . (from our previous purely optical considerations^ p. 14,) that, 
 the retina being possessed of no locomotive power backwards, 
 the lenticular system of the eye must have increased in power. 
 It has, in fact, increased by a quantity equivalent to a 6-inch 
 glass convex lens. If we suppose the crystalline lens un- 
 changeable in its power, Fig. 13 will represent the eye 
 adapted for parallel rays (from the distant object), Fig. 14 the 
 same eye plus a 6-inch glass convex lens placed before it. The 
 effect of this latter is to convert the divergent rays (from 
 the near object) into parallel ones, and these, then, will be 
 focussed, as before, by the natm-al lenticular system of the 
 eye. I have in the diagrams represented this process as if the 
 additional glass lens were in front of the eye ; really, it is as 
 if superimposed on the anterior surface of the crystalline. 
 Now we find in practice that the proximity up to which dif- 
 ferent eyes can accommodate themselves varies greatly in 
 different individuals and at different ages, and even in the 
 two eyes of the same individual. One person may read a 
 given tyiDC as close as three inches, whilst another may not 
 be able to read the same type closer than thirty inches. In 
 
 Fig. 14. 
 
 Fig. 14. — a 4 = 6 inches. 
 
 c = crystalline lens. 
 iJ=retina. 
 i=G.;nch glass convex lens. 
 
 other words, whilst the former person possesses the power 
 of increasing the convexity of his crystalline by a quantity 
 equivalent to a 3-inch glass lens, the second cannot increase 
 its convexity greater than by a quantity equivalent to a 
 oO-inch glass lens. We then say that the accommodative 
 power of the first person is to that of the second as i is to -^, 
 or as 10 is to 1 ; in other language, "the accommodation of
 
 PHYSIOLOGICAL OPTICS. 21 
 
 the first equals i, that of the second equals ■^." The 
 imaginary passive state of the crystalline which we have, 
 for the sake of illustration, depicted in Figs. 13 and 14, is 
 actually now and then met with in practice as the disease 
 ''paralysis of accommodation.^^ This disease will be fully 
 treated of at Chapter VIII. of the present work. To illus- 
 trate, however, what we are now discussing, I may say that 
 in these cases the patients cannot see near objects distinctly, 
 but we may restore their vision for them precisely in the 
 manner delineated in Fig. 1 4. Such a patient to read at four 
 inches requu'es a 4-inch convex glass, to read at six inches a 
 6-inch one, to read at nine inches a 9-inch one ; and so forth 
 I shall hereafter show you that we in atropine possess an 
 agent capable of producing artificially this same paralysis of 
 accommodation . * 
 
 When we, then, speak of a person possessing e. g., \ accom- 
 modation, we simply mean he has the power of increasing the 
 convexity of his crystalline lens by a quantity equivalent to a 
 6-inch convex glass lens ; such a person can see critically 
 distinctly both objects at a distance and those which are six 
 or more inches from the eye, but he cannot see objects criti- 
 cally distinctly nearer than six inches. A young healthy eye 
 is generally endowed with an accommodation of j for the two 
 eyes used together, and say of \ for either used alone. As an 
 illustration of the nature of accdmmodation, I may mention 
 that after a person has brought an object (e. g. a fine print) 
 as close to the eye as he can possibly see it critically distinctly, 
 he may see it equally distinctly still closer by the interposition 
 of a convex lens — adding, in a word, an artificial amount of 
 accommodation to what the eye intrinsically possesses. 
 
 * This property of atropme has been long known {vide Miiller's " Physiolog. 
 rles Gesicht-Sinncs," p. 2( »0). Seven cases of belladonna poisonmg are reported 
 by Dr. Letheby in the Pharmaceutical Journal for 1846, vol. i. p. 25. Dr. 
 Thomas Fraser has recently discovered that the Calabar bean possesses the 
 remarkable power of contracting the pupil. Since his discovery it has been 
 shown by Dr. Argylle, Robertson, and others, to possess a similar power 
 over the ciliary muscle, and of thus producing an artificial spasm of accommo- 
 dation. For a detailed account of the properties of the Calabar bean, vide 
 an analysis of Von Grufe'.s paper (in the ninth volume of the Archiv. /. 
 Ophth.) in the niunber of the Oiihthalmic Review for April, 1864 (p. 36).
 
 22 OPTICAL DEFECTS OP THE EYE. 
 
 How is the amount of accommodation measured ? In the 
 case of a normal or simply presbyopic eye, by merely ascer- 
 taining the nearest point of critically distinct vision, the '^ near 
 point/''* If the near point is e. g., six inches, the accommoda- 
 tion, which we wiU designate by 7, r=i. 
 
 The consideration of accommodation in myopic and hyper- 
 metropic eyes had better be deferred till we come to treat of 
 those diseases respectively. 
 
 In what does the mechanism of accommodation consist? 
 The older observers, as Thos. Young, thought the fibres of the 
 crystalline were muscular, and hence named it the '' musculus 
 crystallinus." But Cramer and Helmholtz have galvanized 
 the crystalline with but negative results. Cramer thought the 
 iris was the agent of accommodation. The image of a flame 
 thrown on the back of the vitreous he found rendered indis- 
 tinct by sending a galvanic current across the cornea ; this 
 change did not occur after division of the iris. It is now, 
 however, the generally-received opinion that it is the ciliary 
 muscle which is the eSective agent of accommodation. 
 Porterfield distinctly states the muscular natiire of the " Uga- 
 mentum ciliare,^' and even goes so far as to compare it to 
 other involuntary muscles. The anatomical researches of 
 H. Miiller, Van Eeeken, and Rouget go to prove that the 
 ciliary muscle, like the iris, is composed of two sets of fibres 
 — in man of the nonstriated, in birds of the striated variety, — 
 a circular and a radiating. The circular fibres would by their 
 contraction increase the convexity of the crystalline ; the 
 radiating would act as their antagonists in restoring the 
 crystalline to its previous degree of curvature. t Has the iris 
 
 * Similarly the furthest point of critically distinct vision is termed the 
 " far point." 
 
 + The precise mode by which this is effected is still an object of discussion. 
 Manuliardt considers " if the iris and corpus ciliare, with its processes, are 
 drawn back, (1) a pressure is exercised on the vitreous body, where it embraces 
 
 the surfaces of the lens ; (2) the zonula zinii is made tense the fluid 
 
 in the canal of Petit subjected to increased pressure, which is transmitted to 
 
 the equator of the lens the tension of the zonula zinii prevents the 
 
 lens advancing, but not its tendency to alteration of form." (Bemerkungen 
 iiber den Accommodations-muskel und die Accommodation, von Dr. J. 
 Mannhardt, in the Archiv. f. Ophth., iv. i. p. 267.) Becker (Wien. Med.
 
 PHYSIOLOGICAL OPTICS. 23 
 
 any share in accommodation? I think not. The erudite 
 author of the article " On the Optical Relations of the Eye " 
 (in the British and Foreign Medico-Ghirurgical Review for 
 January, 1862) has said I have fallen into a mistake by 
 asserting it has. Restricting the word " accommodation ^^ to 
 increase of refractive power of the eye, the reviewer is right. 
 But I, in the paper he refers to,* used the word accommoda- 
 tion in a wider sense — that of the means of attaining distinct 
 
 Jahrb., 1863, ii. 159 ; 1864, i. 3) has arrived at the following conclusions 
 from observations on four albinotic patients, in whom the iris was so trans- 
 parent that the parts behind it could be observed : — 1. At every age the ciliary 
 processes are placed to the outer and anterior side of the edge of the lens. 
 2. Their size varies according to the size of the pupil : they increase Avhen 
 this dUates and diminish when it contracts. This applies to the changes in 
 the size of the pupU, both in accommodation and under the action of atropine 
 or Calabar bean. Wharton Jones has observed calabar to contract the pupil 
 in an extirpated eye, and an increase in width of the space between the 
 ciliary body and the margin of the lens. 3. As the cUiary processes enlarge 
 during dihitation of the pupil, they pass forwards and inwards into the 
 so-Ciilled posterior chamber, between the periphery of the iris and the anterior 
 surface of the lens, but do not touch this structure. It follows from the above, 
 that the ciliary processes have no direct influence in the act of accommodation. 
 (Vide Oiylithalmic Review, Part V., from which this summary from the pen of 
 Mr. Windsor is taken.) It is remarkable that our eminent Dr. Mackenzie 
 thirty years ago arrived at the same conclusion. He says {London Medical 
 Gazette, vol. xiii. p. 631, January, 1834), "In some eyes after death we find 
 the pupil small, and in othei-s we find it large. Now, in those eyes in wliich 
 we find the pupil small or contracted, we find the ciliary circle — that is, the 
 termination of the ciliary processes around the crystalline lens, expanded 
 .... in those eyes in which the pupil after death is large or dilated, we 
 find the cUiary circle contracted." 
 
 Czermak has noticed that if, after havmg m the dark accommodated for the 
 near point, he suddenly relaxed this efi'ort, a fiery ring of considerable size 
 api^eared before him. This appearance was first observed by Purkinje, but 
 can, according to Czermak, only be seen by few persons who possess gi'eat 
 sensibility to such spectra, and have sufficient control over their accommo- 
 dation. He regards it as an " accommodation phosphene," due to " tension of 
 the zonula zinii and traction of the surrounding retinal ring." 
 
 * Glasgow Medical Journal, vol. viii. p. 268, 1860. I have here experi- 
 mentally proved that a pin-hole (1) confers distinctness on objects nearer 
 than the eye, per se, would be otherwise capable of accommodating for (I have 
 since found it also corrects the disturbing influence on vision of both convex 
 and concave lenses for near or distant objects) ; (2) it increases the apparent 
 magnitude of objects.
 
 24- OPTICAL DEFECTS OF THE EYE. 
 
 vision generally. This meaning of the word accommodation, 
 however, I grant, had better be abandoned, Hmiting its sig- 
 nification to increase of convexity of the crystalline. That, 
 however, the iris does play an important part in attaining 
 distinctness of vision I have, in the paper referred to, proved 
 experimentally. You all know the pupil contracts when we 
 regard near objects ; dilates for distant ones. Can you for a 
 moment imagine such a constant occurrence to be devoid of all 
 meaning ? My theory is that the ii'is is an auxiliary organ 
 for distinct vision — that the contraction of the pupil is 
 intended as a corrective supplement to our accommodation, 
 properly so called, by diminishing any slight circles of 
 diffusion in the retinal image that might possibly arise from 
 inaccurate contraction of the ciliary muscle. 
 
 We meet with other such auxihary agents in nature. No 
 one doubts that we should be able to feel with the ends of the 
 fingers without the innumerable papillae with which they are 
 beset; but no one either doubts that these are powerful 
 auxiharies in extending* the fineness and rangfe of our tactile 
 sensibility. I beg, however, to here distinctly state, the chief 
 muscle of accommodation is the ciliary, and that in our future 
 studies we may almost disregard the auxiliary action of the 
 iris. The best evidence of this are those cases of absence of 
 the iris — either congenital or traumatic, in which accommo- 
 dation remained unimpaired. A simple and convincing proof 
 of the purely supplementary share of the iris in accommoda- 
 tion is, that if we place before the eye a thin plate of metal, or 
 a card, perforated with an aperture equal to the minimum size 
 of the natural pupil, we still retain our power of distinct vision 
 for distant or near objects, notwithstanding we have thus 
 altogether eliminated the action of the pupil.* 
 
 * In the Arckiv. f. Ophthalmologie, vol. vii. pt. 2, p. 150, Professor von 
 Grafe relates a case in which a partial separation of the iris having acci- 
 dentally occitrred in attempting to remove a prolapsus, he removed the 
 entire iris. Afterwards the patient read No. 20 at 24 feet, No. 1 at 5| 
 inches, and had an accommodation of 1-5|, which closely corresponded 
 with the normal amount of accommodation of a person of liis age — viz., 
 27 years. A four-grain solution of atropine in one hoiu- completely paralysed 
 all accommodation, and induced a feeble degree of hypermetropia (of l-60tli 
 to l-70th).
 
 PHYSIOLOGICAL OPTICS. 25 
 
 Le Cameus, Roliault, Scliroder van der Kolk, and Arlt were 
 advocates for the power of tlie extrinsic muscles of the eye, 
 the recti, and obliqui, producing accommodation by elongating 
 the axis of the globe. This hypothesis may now be all but 
 disregarded. Yon Griife [Archiv. f. Oplith., ii.) observed 
 the accommodation undisturbed in paralysis of all the ocular 
 muscles. 
 
 The theory that the cornea changes its convexity is one of 
 those traditions that, devoid of all demonstrated truth, has 
 still retained its hold on some conservative minds. Any such 
 change of curvature has been absolutely disproved by Helm- 
 holtz^s exact observations with his ophthalmometer : this 
 instrument would render apparent a change of -—^ of the 
 radius of curvature of the cornea, while an accommodation for 
 five inches demands a change of upwards of j inch.* 
 
 From what has preceded you will natm'ally infer that after 
 operations for cataract all accommodation is lost. Without 
 referring further to the erroneous opinions of Arlt, I may state 
 that Dr. Young, at the commencement of the present century, 
 showed that any quasi accommodation remaining, was, in this 
 instance, really due to the cncles of diffusion being dimin- 
 ished by the smallness of the aperture of vision. In the paper 
 in the Glasgow Journal (October, 1860, p. 272) is said: — '^ In 
 the majority of these cases the patients have been able to 
 accommodate their vision to different distances by bringing 
 the two eyelids into such close apposition as to leave but a 
 
 narrow linear aperture between them If in such cases 
 
 the eyelids be held open, and the patients be told to look 
 through a pin-hole in a card, the same effects will ensue as 
 from their own instinctive approximation of the eyelids.^^ It 
 is not uncommon to find cases of double extraction in which 
 in one eye a portion of opaque capsule with a hole in it remains j 
 in such cases the apparently worse eye often enjoys the best 
 vision on this same principle. Donders has distinctly proved, 
 
 * Notwithstanding the above strongly expressed opinion, I may mention 
 that whilst these sheets are going through the press, those of the fifth number 
 of the Ophthalmic Review are doing so also, and that these latter contain a 
 paper, by Dr. Hemy Lawson, to prove that the cornea is the agent of accom- 
 modation.
 
 26 OPTICAL DEFECTS OF THE EYE. 
 
 what was asserted a century back by Porterfield, that after 
 the operation for cataract not the shghtest true accommoda- 
 tion exists. 
 
 Before quitting this part of our subject I must direct your 
 attention to a point which you may find useful to remember 
 in a subsequent part of the course. I allude to the relation 
 which exists between the degree of accommodation and the 
 directions of the eyes. Speaking in general terms^ as indi- 
 cated long ago by Porterfield* and J. Miiller^t when the axes 
 of the eyeballs converge to an object at an angle (the greater 
 in proportion to the proximity of the object), the crystallines 
 necessarily become more convex by an automatic action of 
 the ciliary muscle consentaneous with that of the internal 
 recti muscles ; and conversely when these muscles are relaxed 
 and the axes of the eyeballs parallel, the two cihary muscles 
 are also at their minimum degree of contraction. 
 
 Binocular Vision. 
 
 The pupils of the eyes are about 2^" apart. If we direct our 
 eyes to a distant object, the axes of the eyeballs are practically 
 parallel, and whether we use each eye alone successively, or the 
 two together, in all three cases we obtain but one and the same 
 view of the object. This may be well illustrated by performing 
 the experiment on any tolerably distant sharply-outlined archi- 
 tectural projection. But if we regard a near object, the two optic 
 axes are no longer parallel, but converge to the object at an 
 angle larger and larger in proportion to the proximity of the 
 object. This angle is what is termed the binocular parallax : 
 at 12 ft. it = 1°; at 18"=8°, &c., and it varies shghtly with 
 the different intervals at which the eyes are set in different 
 individuals. A giant would have a much larger binocular 
 
 * " Thus, when we view any Object at two Feet Distance, we not only accom- 
 modate our Eyes to that Distance, but we also move our Eyes, so as their 
 Axes produced may meet in some Point of the Object ; whence it comes to 
 pass, that these Motions, which at first had no necessary Connection or 
 Dependence on each other, do in time come to cohere so closely . . . . " 
 — (Porterfield's " Treatise on the Eye," ii. p. 8. Edinb., 1759.) 
 
 t " Physiology of the Senses," translated by Baly, 1848, p. 1145.
 
 PHYSIOLOGICAL OPTICS. 27 
 
 parallax ttan an ordinary person. The consequence is tliat 
 witli either eye we see any near object from respectively two 
 different points of view ; the differences being proportionate 
 to the magnitude of the binocular parallax. If we thus regard 
 the frustum of a solid cone with the right eye, we see more of 
 its right side ; if with the left eye, more of its left side. 
 
 Fig. 15. Fig. 16. 
 
 The two figures (Figs. 15, 16) represent the plane projec- 
 tions of the two perspectives, the lesser circles representing the 
 summits, the greater the bases of the truncated cones ; the 
 former eccentrically approximated to each other. 
 
 Fig. 17. Fig. 18. 
 
 Figs. 17, 18 represent the plane projections of the 
 frustum of a hollow cone. In these the positions of the 
 circles representing the summits are eccentrically separated 
 from each other. 
 
 If we present these two perspective drawings to their 
 respective eyes separately, and then combine them by reflec- 
 tors or prisms, as in Wheatstone^s or Brewster's stereoscope, 
 we get mentally reproduced the sohd or hollow cones from 
 which they were delineated. 
 
 These facts were known to Euclid more than two thousand 
 years since ; and quite clearly stated by an eminent member 
 of our profession, Galen, one thousand five hundred years 
 affo.
 
 ( 
 
 28 OPTICAL DEFECTS OE THE EYE. 
 
 It is the mental combination of the two different perspective- 
 projections of the object formed on the retina that conveys to 
 us the idea of solidity. A solid differs from a plane in having 
 a third dimension in space, which the plane has not. A 
 square has height and breadth ; a cube a third dimension, 
 thickness. Now we estimate this thickness, visually, by the 
 distance of the series of luminous points forming its extreme 
 edges. This third dimension, distance, cannot be correctly 
 directly estimated by one eye alone, as every one knows when 
 he, e. (J., attempts to snuff a candle with one eye shut ; but open 
 both eyes and the action becomes one of certainty and pre- 
 cision. This is given by the muscular sense involved in the 
 convergence of the eyeballs by the internal recti. In the same 
 way as we, by the degree of contraction of the flexor muscles 
 of the arm, judge of the weight of an object, do we, by the 
 degree of contraction of the internal recti, judge of its dis- 
 tance.* It would be interesting to see, whether, in cases of 
 insufficiency or paralysis of the internal recti, the power of 
 judging of distance were in any way lost. Do persons who 
 squint judge properly of distance ? 
 
 A very singular application of this negative property of 
 monocular vision is, that we may by closing one eye obtain a 
 certain stereoscopic effect from a well-executed single portrait. 
 With both eyes we at once see the whole portrait is on one 
 and the same plane — is flat. With one eye alone we are unable 
 to so positively recognize this fact : the consequence is that, 
 if the geometrical perspective and the chiaroscuro be good, 
 a considerable degree of relief is imparted by what is really a 
 deficiency in our visual perception. Any one who has ever 
 regarded some of the representations of relief at Versailles, 
 or, to take a humbler illustration, some of the very artistically 
 painted relievo letters over shop-facades, with one eye, must 
 have been forcibly struck with this effect. If you regard 
 
 * An essential difference is, however, tliat whilst in the latter case the 
 contraction of one set of muscles suffices, in the latter those of both sides 
 must be called into action, at any rate, to give a perfectly accurate idea of 
 distance. If we use only one eye, although we may acquire a certain rough 
 idea of solidity — of relief — by the comparison of adjacent objects, still this 
 idea is vastly inferior to that acquired by the use of both eyes.
 
 PHYSIOLOGICAL OPTICS. 29 
 
 a carte cle visile portrait with one eye, and especially with, the 
 aid of a magnifying-glass, you will see a certain amount of 
 rehef imparted.* 
 
 To obtain a perfect idea of solidity both eyes must converge 
 to the object. Brewster, ignoring the duplication of our mus- 
 cular sense in this act, explains it in the following manner : — 
 " We are now prepared to understand generally, how in bino- 
 cular vision we see the difference between a picture and a 
 statue, and between a real landscape and its representation. 
 When we look at a picture of which nearly every part 
 is at the same distance from the eyes, the point of con- 
 vergence of the optical axes is nearly at the same dis- 
 tance from the eyes ; but when we look at its original, 
 whether it be a living man, a statue, or a landscape, the 
 optical axes are converged in rapid succession upon the nose, 
 the eyes, the ears, or upon objects in the foreground, the 
 middle and the remote distances in the landscape, and the 
 relative distances of all these points from the eye are instantly 
 perceived.^^t You will see from this that Sir David Brewster 
 considers the eyes judge of distance, as the muscles of the arm 
 are sometimes used to judge of weight, when the person 
 alternately raising the hand up and down by repeating, so to 
 say, the weight of the body, endeavours to form a more accu- 
 rate idea. 
 
 It appears to me that this is a very gratuitous hypothesis, 
 and that all the phenomena of the stereoscope may be ade- 
 quately and completely explained by the simple mental fusion 
 of two perspectives. The only fact I know of at all supporting 
 Brewster's hypothesis is, that if we, e.fj., attempt to run our 
 eyes down the graduations of a measure, we find we can do so 
 much more rapidly and accurately with two eyes than we can 
 with one eye. Besides which, in the ordinary use of the 
 stereoscope, no convergence of the optic axes takes place 
 at . all ; it is supplanted by an artificial direction given to the 
 rays of hght by two prisms. 
 
 * A series of photographs of celebrated statues is now being exliibited by 
 the aid of the magic-lantern at the Polytechnic Institution, illustrating beau- 
 tifully this effect of monocular relief. 
 
 t Brewster " On the Stereoscope," p. 51.
 
 30 
 
 OPTICAL DEFECTS OF THE EYE. 
 
 CHAPTER III. 
 
 PATHOLOGICAL OPTICS. 
 
 lyrOW that you thoroughly understand the leading optical 
 ■^^ properties of the normal eye^ I may at once proceed 
 to investigate the departures from that standard. To 
 have attempted to do so before would have been simply 
 to have imposed upon you a delusion. I shall, however, 
 even now not think my time ill-spent, if I from time to 
 time again recur to our former optical studies, when any par- 
 ticular defect of vision presents any unusual complication. 
 
 There are two states of the eye which we must carefully 
 distinguish, — that in which it is directed to distant objects, 
 its state of rest, and that in which it is accommodated for a 
 near object. The former condition is a fixed constant one, 
 the latter varies with every degree of convergence of the eyes. 
 We shall therefore, with Professor Donders, propose a system 
 of classification founded upon the refractive condition of the 
 eye when, in a state of rest. The following table exhibits this 
 system : — 
 
 The Eye in a State of Rest {zzcrystalUne at its minimum 
 curvature ; optic axes parallel) . 
 
 1. 
 Eye :- 
 
 2. 
 Parallel rays 
 are focussed : 
 
 3. 
 
 Far-point 
 
 4. I 5. 
 
 Eye in a state Effect of 
 of rest, adapt- glasses for dis- 
 ed for : — I tant objects : 
 
 I. Normal On the retina, 
 
 At an infinite 
 distance. 
 
 Parallel rays. 
 
 Convexes and 
 concaves de- 
 teriorate vi- 
 sion. 
 
 II. JNIyopic 
 
 In front of the At a definite 
 
 retina. distance and 
 
 I positive. 
 
 Divergent rays 
 
 Concaves im- 
 prove vision, 
 
 III. Hj^erme- 
 tropic* 
 
 Behind the re- At a definite 
 
 tiiia. 
 
 distance and 
 negative. 
 
 Convergent 
 rays. 
 
 Convexes im- 
 prove vision 
 
 * The name Hypcrmdropia (adjective = hypermetropic) was introduced
 
 PATHOLOGICAL OPTICS. 31 
 
 The three last lines of tliis table^ read horizontally, give the 
 leading charactei-istics of each class of eye ; read vertically, 
 the leading contrasts. 
 
 I shall now discuss these three different eyes, as they pre- 
 sent themselves to us in a state of rest, leaving for the present 
 entirely out of the question the variable element accommoda- 
 tion. I shall, in other words, entirely separate the two factors, 
 the constant and the variable, the refraction of the eye and its 
 power of adjustment. To adopt a homely simile, if we had 
 three different telescopes submitted to our judgment, our first 
 business would be to test their powers of definition in regard 
 to distant objects, as the heavenly bodies, and we might 
 then proceed to test their individual powers of adjustment 
 for the distinct vision of nearer objects. 
 
 The first step, then, in our investigating the refractive pro- 
 perties of a given eye is to eliminate the element accommoda- 
 tion. This may be effected in two ways. The patient may 
 be able to suspend his accommodation by an effort of the will, 
 or, if not, we may temporarily paralyze the ciliary muscle by 
 a solution of atropine. For this pm-pose it must be of a 
 strength of not less than four grains to the ounce ; and must 
 have had about one hour and a half given to it to act before 
 we can feel certain that the paralysis is complete. 
 
 by Professor Donders : it is synonymous with hyperpresbyopia and hyper- 
 opia, I have retained it, not for its perspicuity, but rather the reverse, for 
 its subtleness of expression, attaching no other meaning to it than what 
 immediately flows from its symptoms.
 
 32 
 
 OPTJCAL DEFECTS OF THE EYE. 
 
 CHAPTEE IV. 
 
 MYOPIA, 
 
 T)EFOE.E we can satisfactorily discuss tliis anomaly of rc- 
 -L' fraction, it ^vill be necessary to liere introdilce the subject 
 of concave lenses. A concave lens has properties precisely 
 the reverse of a convex one. We shall limit our observations 
 to the case of the equi-concave glass lens, of which a sectional 
 figure is subjoined (Fig. 19). 
 
 The two sides of such a section, A B and A' B', are the 
 
 Fig. 19. 
 
 Fig. 19. — J^= apparent focus, the dotted lines indicating the apparent path of the rays 
 when they strike the eye, U. 
 
 concave sides of two circles — the circles of curvature. C, C ; 
 C B, C B', are the centres and radii of curvature respectively. 
 Parallel rays, P, P , B", B'", &c., striking one sm-face of the 
 lens, have a dwerg^nce imparted to them on their exit, as if 
 they proceeded from an imaginary focus, F, which is called 
 the principal focus of the lens. F B is the principal focal 
 length, or briefly the focal length. It is in the case of an 
 equi-concave glass lens, as is evident from the figure, equal to 
 the radius of curvature, C B. It will be remembered that this 
 same relation held good of the eqm-convex-gla,ss lens. In this 
 last, however, the focal length is meaBured fonvanls from the 
 surface of the lens, in the same direction as the rays travel
 
 MYOPIA. 33 
 
 (converge to their focus) ; whilst in the case of the concave 
 lens the focal length is measured haclcwards, from R to F, in 
 the contrary direction to the course the rays diverge from 
 their focus. If, then^ we agree to call the focal length of the 
 convex lens positive, we must call that of the concave one 
 negative, and such is the practice actually adopted.* We thus 
 speak of a convex lens having, say, five inches positive focus, 
 and a concave lens having five inches negative focus. And 
 furthermore, from the entirely opposite properties the two 
 classes of lens possess in every point of view, it is the usage 
 to speak of convex lenses as positive lenses ; of concave lenses 
 as negative lenses. Or, still more briefly, employing the two 
 algebraical symbols + (plus) and — (minus), we should de- 
 signate the convex lens in question simply as a '' ■\-h lens," 
 the concave one as a "—5 lens;" or, as already explained, 
 with reference to their " powers," we might call the convex 
 one a " +\ lens," the concave one a " — i lens." The words 
 positive and negative are used, then, simply to express opposite 
 properties. If I agree to call running forwards positive, 
 running backwards is negative; and I might in this sense 
 
 Fig. 20. 
 
 speak of having run, say, + 5, or — 5 miles. If we agree to 
 call five inches measured forwards from (Fig. 20) 4-5, five 
 inches measured backwards from are — 5. 
 
 I have illustrated the meaning of these two terms perhaps 
 a little more fully than is absolutely necessary — firstly, on 
 account of the misconception that I know so frequently pre- 
 vails on this point ; and secondly, because we shall presently 
 find a very convenient application of these terms when we 
 come to speak of the far-point in the three classes of eyes 
 {vide Table at p. 30, column 3). 
 
 If the rays of light from a luminous point are already 
 divergent, before they strike a concave lens (rays from near 
 objects), they will be found, after their exit from the second 
 
 * In English works on optics the reverse nomenclature prevails. 
 D
 
 31- OPTICAI. DEFECTS OV THE EYE. 
 
 surface of tlie lens, to liave acquired a still greater degree of 
 divergence, to hence appear as if they proceeded from a point 
 nearer the lens than they really do from (Fig. 21). 
 
 It is one of the laws of vision to refer the origin of any rays 
 striking the eye {E) to the position a luminous point {F' in 
 the figure) would occupy which corresponds to the degree 
 
 Fig. 21. 
 
 Fig. 21. — £. = Luminon9 point. 
 
 i*"= Imaginary Focus of the Rays after their Exit from the Lens, the dott»-d 
 lines indicating the apparent course of the rays. 
 
 of divergence the rays [R and R') have when they strike the 
 eye. This luminous point or focus {F') has, then, no real 
 existence ; the rays of light have never met there ; the focus 
 is an " imaginary,^^ '' virtual,'^ or '^ apparent " one, its position 
 in space being fixed by the law of vision just mentioned. 
 The dotted lines in both figures (19 and 21) indicate the 
 apparent dii-ection of the rays to the eye [E) . 
 
 What holds good of a pencil of rays from one luminous 
 point of an object, equally does of all. The consequence is 
 that a concave lens forms an apparent image of any distant 
 object precisely at its focal length ; if the object is a near one, 
 its image appears still nearer. Furthermore, the image is 
 always erect and diminished (Fig. 22). 
 
 There are two ready modes of ascertaining the focal length 
 of a concave lens, — the one is to see the magnifying power of 
 what convex lens it exactly neutralises. The positive focal 
 length of this latter is the negative one of the concave lens 
 in question. This method is not practically applicable to low- 
 powered concaves. The focal length of a concave lens may be 
 ascertained by finding out what convex lens placed in appo-
 
 MYOPIA. 
 
 sition witli it no longer makes distant objects appear to move 
 (or " dance," as opticians call it) as we move the pair of 
 lenses rapidly before tlie eye. To illustrate this method, sup- 
 pose I have a concave lens given to me, and that, when I 
 place a 9 -inch convex lens to it, and look through the pair 
 at distant objects, I find, as I move the combination down- 
 wards, objects appear to rise, and vice versa ; that, on the 
 
 Fig. 22. 
 
 ^ 
 
 Fig. 22.— 0= Object. 
 J= Image. 
 P, P' = Parallel rays from the top and bottom points (A, A') of the object, which 
 after emergence from the lens are changed into rays, D, D', having a 
 divergence as if they proceeded from a, a' of the apparent image. 
 
 contrary, when I place a 13-inch convex to it, the objects 
 appear to move downwards as I move the combination down- 
 wards, and vice versa ; then the concave lens in question has a 
 focal-length between 9 and 13 inches. I now try convex lenses 
 of 10 and 12 inches, but still notice the same apparent move- 
 ments of objects, though to a less degree. I then try an 11- 
 inch convex lens, and find that objects viewed through it and 
 my concave lens no longer execute any apparent movements 
 at all in moving the combination, then the concave lens in 
 question is one of 11 inches focus. In the first case (with the 
 9-inch and 10-inch con vexes) an inverted, in the second case 
 (with the 13-inch and 1 2 -inch convexes) an erect image moves. 
 This method is remarkably easy and quick in practice. The 
 same method is applicable, ■mttiafi's mutandis, to convex lenses. 
 The " numbers " English opticians designate their convex 
 spectacle-lenses by correspond with the actual focal lengths 
 (in English inches), but the numbers they name their concaves 
 
 D 2
 
 86 
 
 OPTICAL DEFECTS OF THE EYE. 
 
 by are arbitrary. The subjoined table (from Mackenzie) gives 
 to what focal lengths of concaves the numbers in question 
 correspond. I give this table for the sake of completeness^ 
 but in my practice I altogether ignore such an illogical 
 nomenclature : — 
 
 Numbers. 
 
 Focal lengths 
 in inches. 
 
 Numbers. 
 
 Focal length 
 in inches. 
 
 1 
 
 48 
 
 7 
 
 9 
 
 2 
 
 36 
 
 8 
 
 7 
 
 3 
 
 24 
 
 9 
 
 5 
 
 4 
 
 18 
 
 10 
 
 4 
 
 5 
 
 14 
 
 11 
 
 3 
 
 6 
 
 12 
 
 12 
 
 . . 2i 
 
 Referring to the previous table of the diagnostic signs of 
 the three different classes of eyes, we find that the myopic 
 one in a state of rest presents characteristics, which we shall 
 now illustrate. 
 
 1. "Parallel rays arefocusscd in front of the retina." This 
 hardly needs explanation : the parallel rays from each luminous 
 point {}}. Fig. 23) of a distant object (o) are re-united (con- 
 
 FiG. 23. 
 
 verged) to a corresponding luminous point (focus, F) in front 
 of the retina (i?) ; from each point the component rays of the 
 pencil, pursuing their paths uninterruptedly onwards, re- 
 separate (diverge) ; thus each point of the object is repre- 
 sented by a circle (0) (of diffusion), the vertical section of a 
 cone of divergent rays, and thus, as before fully explained, 
 vision becomes indistinct. 
 
 2. " The far-i)oint is at a definite distance from the eye and 
 Yiositive." The far-point is the furthest distance at which a 
 luminous point is critically distinctly seen. It is the farthest
 
 MYoriA. 37 
 
 point wliicli can, by the refractive power of the eye, be 
 focussed again to a point on the retina. The normal eye sees 
 critically distinctly a luminous point at a7iy distance (leaving 
 out of consideration for the present the magnitude a point 
 must have to be perceptible to the eye — the minimum 
 visibile) : * the far point is therefore, in this case, indefinite 
 or infinite. Agreeing to call lines measured from the vertex 
 of the cornea forward positive, lines measured fi>om the same 
 point backward negative, the far-point in the normal eye is 
 infinite and positive {vide table at p. 30, column 3). In the 
 myopic eye the far-point is also positive', but it must be at a 
 definite distance. This varies extremely in different cases. I 
 have met with one case where the patient could not see any 
 
 * Drs. Snellen and Giraud-Teulon have devised a series of test-tjqjes in 
 which this element is taken into consideration. In each the minimum angu- 
 lar magnitude of distinct vision is taken as 1 mt. They are further graduated 
 in such a manner that their numbers correspond in Paris feet to the distance 
 at which the breadth of the thick strokes of each letter subtends the above 
 angle on the retina. If we are dealing with a normal eye, No. I. type ought 
 to be distinctly legible as far oft' as one Paris foot, XII. at twelve feet, XV. 
 at fifteen feet, &c. If we designate by S the acuteness of vision of a person, 
 then, in the above examples, 
 
 if, on the other hand, an eye to read I. has to approach half-a-foot to it, to 
 read XII. three feet, to read XV. five feet, &c., then 
 
 ^_1_1. 3 1. _5__1 . 
 
 l.~2 ' ~X1I.~4' XV., ~ 3' ' 
 
 respectively. Or, if an feye at twenty feet only reads X., at fifteen feet only 
 v., &c., then 'S'=^ and g respectively, ^si is considerably influenced by age. 
 The following table exhibits the results of the investigation, by Di*. de Haan, 
 of 281 cases of emmetropic eyes at difl"erent ages : — 
 
 At 10—20 years '^ = ^ 
 
 30 -22 
 
 20 
 
 50 ,. 
 
 " 20 
 
 - " -=ll 
 
 - .=IJ
 
 38 
 
 OPTICAL DEFECTS OP THE EYE. 
 
 object clearly farther off than 1| Enghsh inch ; others, in 
 which the far-point was at 2 inches from the cornea ; and so 
 through all distances up to 80 inches, in which last case the eye 
 does not practically differ from a normal eye. 
 
 Case I. — Progressive Myopia of \i unaccompanied by 
 
 Staphyloma Posticum or Buphthalmos. 
 
 Lewis M., tet. 40, consulted me, in 1862, on account of excessive myopia of 
 his left eye. He had lost the sight of the right eye for about fourteen years. 
 He had been in the habit of using stronger and stronger glasses, tiU he was 
 at last obliged to use a combination of two concave glasses to assist his vision, 
 not having been able to procure any single one sufficiently strong for his 
 purpose. One of these concaves I found was of 2j" focus ; the two together 
 equivalent to one of 1|" focus. His eye was not unduly prominent. There 
 was no staphyloma posticum in the left eye ; the right oni' had a divergent 
 squint, and was filled with iimumerable brOliant floating scales, which pre- 
 sented a very beautiful ophthalmoscopic picture (=sijnchysis scintillans). Ho 
 read with his left eye No. 1 at l| inch from the cornea — not farther, not 
 nearer. He preferred for distance 2-inch concaves to all others, and with 
 these read No. 23 at about 10 feet, No. 22 at 9 feet, and No. 21 at 4i feet. 
 Any tinting of the glasses made him see worse. I regret that I did not 
 mstillate atropine ia order to determine the presence or absence of any spasm 
 of the ciliary muscle. 
 
 High myopics require very strong concaves, and yet, when I 
 teU you that ordinary opticians hardly ever keep glasses of a 
 shorter focus than 6 inches, you will readily understand that 
 there are many persons who may consider themselves practically 
 blind from the want of sujQ&ciently high glasses ; whilst, on the 
 other hand, many low myopics are unaware of their slight 
 optical defect, till they feel surprised at the improvement 
 their vision experiences from a slightly concave (such as 
 a -gL) glass. 
 
 The first thing to ascertain in any suspected optical defect 
 of the eye is to determine to which of the three classes of eyes 
 it belongs. This may be ascertained, speaking generally, by 
 placing the patient at 5 — 7 yards off a graduated series of types, 
 varying from Nos. 23 to 18 of Mannhardt's and Jiiger's 
 scales,* and ascertaining his power of reading such types. If 
 
 * I have had a number of sheets of such types printed off ; they may 
 be liad of the Secretary to the Ophthalmic Hospital, St. George's Cii'cus, 
 Southwark.
 
 MYOPIA. 39 
 
 no concomitant amblyopia be present (whicli^ however, nearly 
 invariably co-exists in all extreme optical defects), he should 
 be able to read No. 18. If he cannot, or even if he can, 
 see whether a low-powered concave (e.^. ^^ improves vision: 
 if so, he is myopic ; or a low-powered convex {e.g. -^) : 
 if so, he is hypermetropic ; * or both deteriorate vision : if 
 so, his refraction is normal. In instituting such an examina- 
 tion some precautions must be taken, to some of which we 
 shall direct attention incidentally hereafter. Many persons 
 fancy any low-powered glass improves their vision ; this may 
 be a mere fancy. If such suspicion exists in the mind of the 
 Practitioner, let him hold a piece of plain glass (of which 
 I have a piece shaped like the real lenses in my trial-case) 
 before the eye. The patient will probably give evidence of 
 his own delusion by declaring it also improves his vision. 
 Again, in all such examinations each eye must be tested 
 separately,t then the two together. I have met with five 
 cases in which the one eye being normal the other was myopic. 
 Cases of different degrees of myopia in the pair of eyes are 
 quite common. Opinions vary as to the advisability of giving 
 patients with unequal refraction of the two eyes two diiferent 
 lenses : I think each case must be judged on its own meritS. 
 
 Having thus ascertained that a patient is myopic, our next 
 object is to determine what particular power glass he requires. 
 This demands the greatest care : to give him too deep a glass 
 would entail an unnatural expenditure of accommodation for 
 
 * You will often find a convex or concave glass improves vision for dis- 
 tance, but still does not enable the patient to read any less/ type than he ^r?*- 
 coidd before. This may depend on astigmatism or amblyopia. 
 
 t " As it very frequently happens that one eye has not exactly the same 
 focal length as the other, and that, when it has, the vision by one eye is less 
 perfect than that by the other ; the picture formed by uniting a perfect with 
 a less perfect picture, or with one of a different size, must be more imperfect 
 than the single picture formed by one eye." (Brewster on the " Stereoscope," 
 p. 46.) — Porterfield (vol. ii. p. 10) dii-ects attention to the different " limits " 
 of vision persons may have in either eye. In testing each eye separately, it 
 is advisable for the surgeon to simjjly hold his hand in front of the eye not 
 under examination ; any compression of the eyeball by the patient's forcible 
 closure of the eyelids may impede the free play of the eyeballs, and thus 
 influence the refractive condition of the eye under examination. — Vide 
 Miiller's "Physiology," Baly's translation, p. 1148.
 
 40 
 
 OPTICAL DEFECTS OF THE EYE. 
 
 distance in order to neutralize the excess of power ; to give 
 him too low a one would deprive him of all the advantages 
 the perfect correction of his myojaia might confer. We obtain 
 a clue to the required glass by a rough approximation to the 
 far-point. For this purpose I in ordinary cases employ as my 
 optometer No. 1 of Jiiger's test-types. In high degrees of 
 myopia larger types must be used^ on account of the nearly 
 constantly co-existing amblyopia. I may here mention a 
 rather unscientific instrument that I have adapted to the 
 purpose. I take an ordinary shoemaker's rule ; the fixed 
 upright I cut down and notch out for the reception of the 
 patient's eye ; to the sliding upright I afiix three small 
 springs, which retain in position an ordinary address-card, on 
 to which the test-type is pasted. The fixed upright then 
 corresponds exactly with the surface of the cornea, and by 
 sliding along the other moveable one with the test-type 
 
 Fig. 24. 
 
 il III! nil II il I III ii ili'i ill [ il II 1 1 
 
 7| 8] S\ ^ 
 
 i il I nil I I I " il'i' liiil"-'' 
 
 Fig. 24.— Author's optometer. The test-type is represented as exactly 6 inches from 
 the patient's cornea. I have recently added a grooved ring at the corneal end for 
 the reception of lenses. 
 
 attached to it, we can expeditiously find and read off the 
 near-point, and approximatively the far-point. This Httle 
 apparatus is seen at Fig. 24. 
 
 Prof. V. Grafe has proposed a set of fine wires set in a frame 
 as an optometer ; when the wires are seen singly and well- 
 defined, vision is accurate. But in practice I find this and all 
 such tests fail, even with educated persons, and much more so 
 with hospital patients. 
 
 Suppose, then, you find the patient loses perfect definition 
 of No. 1 type at 6 inches, that his farthest point of critically 
 distinct vision is, roughly, about 6 inches, we should then.
 
 MYOPIA. 41 
 
 according to theory^ give him for distant objects a concave 
 glass of 5^ inclies focal length (the glass being ^ inch from 
 the cornea) ; for then we should present to the eye an image 
 of the distant object exactly (6 inches from his cornea) at his ex- 
 treme limit of distinct vision. But practice teaches us that even 
 a 6-iuch concave would be too strong a glass for distance ; he 
 will probably require a 7- or an 8-inch one : which of these^ 
 we must find out by trial,* Now, what does this apparent 
 contradiction between theory and practice arise from ? From 
 two circumstanceSj viz. : — 1. The associated action of the 
 internal recti and ciliary muscles. Parallelism of the eyeballs 
 corresponds with relaxation of these muscles, convergence with 
 their contraction. Now, when we tried our patient with the 
 test-type with his naked eyes, these converged to a point 6 
 inches off — the ciliary muscles acting consentaneously increased 
 slightly the convexity of his crystalline lenses — increased their 
 power. But when, with concave glasses on, the patient regards 
 distant objects, the consequent parallelism of the eyeballs is 
 associated with a corresponding relaxation of the ciliary 
 muscles — a diminution of power of the crystallines; the con- 
 cave glasses must, therefore, be of longer focal length than 
 the found far-point, they no longer having any accommodation 
 to neutralize. And 2. From the letters of the type not being, 
 perhaps, distinctly recognizable farther than 6 inches, on 
 account of the diminution in their angular magnitudes. f 
 
 * This trial may be facilitated by an ingenious adaptation, by Prof, 
 von Griife, of the GalUean telescope to optometric j)urposes. In this in- 
 strument, when the eye-piece and object-glass are separated by a distance 
 equal to the difference of their focal lengths, the rays emergent from the 
 system are parallel ; if this distance be diminished, divergent ; if increased, 
 convergent. On the body of this optometer the different degrees of myopia 
 or hypermetropia respectively are marked off corresponding to different 
 degrees of separation of the eye-piece and object-glass. The physical and 
 psychical fatigue and confusion involved in the repeated apposition and com- 
 parison of various trial glasses to the eye are thus avoided ; but, at the same 
 time, I must add that the results I have obtained from von Griife's instiia- 
 ment were often fallacious. 
 
 t Prof, von Griife states, that in myopics (not simultaneously affected with 
 weakness of the internal recti) the power of the internal recti increases with 
 age, so that they can reach their far-point under a considerable convergence
 
 42 OPTICAL DEFECTS OF THE EYE. 
 
 The rationale of tho action of a concave glass in myopic 
 patients was at one time a matter of extreme difficulty to me. 
 The knot I found cut by authors by two views — (1) that the 
 concave glass substitutes for a distant object its virtual image 
 at the patient's farthest point of distinct vision ; (2) that it 
 simply increases the divergence of the rays, leaving the locality 
 of the object unchanged. Both explanations are as confused, 
 as they are unsatisfactory. Of the following experiments, I 
 beheve the second one to be original j if not so, it is certainly 
 but little known : — 1° view any object, such as a candle-flame, 
 through a deep concave glass (or an inverted opera-glass), it 
 appears diminished and much farther off than it really is. 
 This is owing to our mental association of diminution of size 
 (an inherent effect of the concave glass) with distance. But 
 2° view the object with either eye in such a way as to see 
 it simultaneously both through and not through the concave 
 lens, we then at once perceive the object at its real distance 
 from the eye, its image at a distance from the lens exactly 
 equal to the focal length of this latter.* To a myopic pe 'son 
 with his proper reducing glasses all distant objects appt, r 
 in their real positions (to high myopics, who have never 
 before worn glasses, objects often at first appear further off 
 than they really are) ; but if the myopic patient with either 
 eye regards, as in 2°, an object at the same time through and 
 oiot through his reducing glass, then he at once perceives the 
 
 of the optic axes, -which convergence does not imply any exertion of the recti. 
 As far as my obser%^ation goes, I cannot agree with the learned professor on 
 thLs point, my experience aU tending to corroborate the views expounded in 
 the text. 
 
 * The simultaneous perception of the real position of the object with its 
 image destroys the mental illusion that refers this latter to the presumed 
 situation of the object. I may say that Mr. Parkinf^on, of Cambridge, to whom 
 I submitted this theory, quite coincides with my views on the subject. A 
 similar mental illusion is the " conversion " of a has- ruto a haut-reUei, accord- 
 ing to the way in which the light ftiUs on an intaglio (or cameo). — Vide 
 Brewster's " Natural Magic," p. 98, et seq. The observation " 2° " in the text 
 explains, I think, certain cases of motiocular diplopia, due probably to differ- 
 ences of refractive power in different parts of the optical constituents of tho 
 globe. I have thus recently observed a very marked case of monocular 
 diplopia ill a case of partial dislocation of the lens.
 
 MYOPIA. 43 
 
 object in its real position, but its image formed exactly at tlie 
 focal lengtli of tlie concave glass. 
 
 These simple experiments prove that the myopic eye with 
 its reducing concave really sees the virtual images of distant 
 objects at the focal length of the glass ; but mentally refers 
 them to their true 'planes of situation. Were it not for this 
 faculty of the mind, the concave glasses would confuse their 
 wearers much more than they assisted them. 
 
 Professor Donders has proposed a convenient nomenclature 
 for different degrees of myopia, viz., naming them by the 
 poivers of their respective reducing glasses. In this way we 
 speak of a person having a myopia of \, another of \, a third 
 of -^j &c. The advantage of this system is, that these frac- 
 tions, by their intrinsic value, express the degrees of the 
 myopia, which 2, 4, 12, &c. (the focal lengths of the glasses) 
 would not do. 
 
 Having accurately corrected the myopia of a patient by his 
 reducing glasses, have we practically transformed the eye into 
 a normal one ? By no means. In the first place (as we shall 
 hereafter advert to), most myopic eyes are essentially diseased 
 ones. Secondly, all concave glasses diminish more or less — 
 the vii'taal images presented by the glass to the eye appear 
 less than their respective objects. The virtual image of a 
 distant object has a less angular magnitude at the optical 
 centre of the eye, than the object itself has at the same jjoint. 
 But for retinal images (in the case before us the image of 
 an image) to convey to the mind defined impressions, they 
 must not be under a certain magnitude ; in plain language, 
 there are many objects so small that no one can see them 
 beyond a certain distance, or even at any distance, ever so 
 near the eye.* "VVTien we are dealing with low degrees of 
 myopia, such as ^-^, 3^, -j^q, &c., the diminishing property of 
 concave glasses is attended with no inconvenience ; but as 
 soon as we arrive at high degrees, such as ^, ^, i, or ^, &c., 
 we find that no glasses, at any rate, at first, however accurately 
 chosen, will enable the patient to read a moderately small 
 
 * Dr. Hook estimated the minimum visibile as comprised in an angle of 
 1 miuute, as is assumed in Giiaud-Teulon's aud Dr. Snellen's test-types.
 
 44 OPTICAL DEFECTS OF THE EYE. 
 
 type, such as 19 or 18, at any considerable distance ; 
 but that for each of the types from 23 to 18 he must ap- 
 proach nearer and nearer to the type in proportion to its 
 smallness. 
 
 Another remai'kable circumstance affecting the application 
 of reducing glasses to high myopics is, that these patients 
 invariably complain of a peculiar "dazzHng" appearance the 
 very deep glasses confer on objects. So disagreeable is this 
 sensation, that such patients invariably prefer lower glasses to 
 those which completely neutralize their myopia; in other words, 
 they prefer sacrificing definition in order to rid themselves of 
 this annoying feeling. Bonders {ArcMu f. Opiitlialmologie, 
 vol. iv. part 1, page 312) has also noticed the fact, but does 
 not appear to have further inquired either into its cause or its 
 remedy. I at fii'st thought it depended on the back (ocular) 
 aspect of the deep concave glass in its capacity of a concave 
 mirror reflecting side-Hght on to the eye ; but I found that 
 covering this back with a dead-black paper, leaving only a 
 central aperture for vision, had no remedial effect on the 
 " dazzling. ^^ I have, however, succeeded in finding the 
 remedy : it consists simply in tinting the glass.* This appears 
 to me to prove that the sensation alluded to has its origin iu 
 some hyperaesthetic condition of the retina to light. The only 
 dii-ect evidence I have to advance in favour of this hypothesis 
 is, that I have never seen any of the highest degrees of myopia 
 unaccompanied by morbid alterations of the fundus oculi (sta- 
 phyloma postic.) but once ; and in this instance, given at p. 38 
 — a myopia of \i — the patient wore a 2 -inch uncoloured concave 
 glass without any inconvenience. 
 
 On the other hand, it is but fair to say, I have seen a few 
 cases of high degrees of myopia, accompanied by extensive 
 
 * The most conyenient method of applying this principle is joining a piece 
 of plane tinted glass with Canada balsam to the back of a plano-concave lens. 
 If a double concave lens is simply manufactured out of a piece of tinted glass, 
 the varying thickness of the glass is accompanied ■with a varying degree of 
 intensity of tint, besides the unpleasant conspicuousness of the depth of colour 
 of the extreme margin of the glass. Messrs. Murray & Heath have suggested 
 that this difficulty may be obviated by grinding the unused circumference of 
 the lens down to a thin flat sui-fiice.
 
 MYOPIA. 45 
 
 stapliyloma posticum, in whicli the reducing glasses did not 
 dazzle the patient. 
 
 Case II. 
 
 Lady B., set. 63, consulted me on June 26th, 1862. Vision: left eye too 
 amblyopic to have any useful sight ; right eye, read No. 1 between 2|'' and 
 2^", so that her accommodation was about -j-V- With a 3-iuch concave she read 
 the larger of Jager's types at distance with her left eye, in which a well- 
 defined large external staphyloma posticimi was seen. She said the 3-inch 
 glass did not " dazzle " her, that tinting made the light " softer," but that she, 
 on the whole, preferred it not tinted. 
 
 I have thus succeeded in solving what I think will be 
 allowed to be an important practical problem — that of en- 
 abling high myopics to avail themselves of all the advan- 
 tages of the perfect definition their actual reducing glasses 
 can give^ without at the same time their being annoyed by 
 this disagreeable " dazzling." * 
 
 Case III. — Myojnaof ^,. — Impossihility of ivcaring the reducing 
 
 glasses on account of their '^ dazzling." — TJiis remedied by 
 
 tinting the glasses. 
 
 Jane R, set. 25, consulted me on January 15th, 1862. I found that at 
 about 15 feet she could not read No. 23, that the far-point for the right eye 
 was, for No. 1 2|", for the left eye the same ; whilst the uear-points for both 
 eyes were respectively 2j" and 2|'' ; so that she practically possessed no 
 accommodation. In each fundus oculi an extensive external staphyl. postic. 
 was seen. The globes were full and prommeut. With 35-inch concaves she 
 read Nos. 23 and 22 at distance quite distinctly, but was obliged to put up 
 with 5-inch concaves, which did not enable her to see so far off, on account of 
 the distressing dazzling sensation the 3j-iiich glasses caused her. This was 
 the first case I tried the effect of tinting the glasses in, and on seeing her tliree 
 months afterwards, she said she had constantly worn her tinted 3j-inch glasses 
 — I verified then- focal lengths — without ever tigam experiencing the " dazzling" 
 sensation they, before beiiig tinted, caused. In these cases we may some- 
 times with advantage substitute for a deep concave glass a more shallow one, 
 in combination with a stenopceic apparatus : the effects are a larger, equally 
 defined image with a diminution of the amount of light. 
 
 * My opticians, Messrs. ]\Iurray & Heath, inform me that they are 
 now constantly in the habit of adopting this plan with the most marked 
 
 success.
 
 46 OPTICAL DEFECTS OF THE EYE. 
 
 An important practical question arises, whether a short- 
 sighted person should use the same glasses for near as he does 
 for distant objects. This will in great measure depend on the 
 amount of accommodation he possesses. A young subject, 
 but moderately myopic, with an average accommodation, may 
 use the same glasses for both pm^poses. But you will almost 
 invariably find that persons who are extremely short-sighted, 
 so as {e.g.) to require concave glasses anything deeper than 
 6 inches focal length, have a very limited accommodation, or 
 none at all. Such patients imperatively require two pau's of 
 glasses — one for constant use, the other for reading, working, 
 &c. In the choice of the latter you must be guided by the 
 amount of accommodation present. How is this accurately 
 found ? 
 
 The best test-object for the determination of the near-point 
 {p) I consider to be a small type, this possessing the advantage 
 of our really knowing that the patient must see it tolerably 
 accurately to be able to read it. In the choice of the size 
 of such a type we must be guided by the condition of the eye 
 under examination — i.e., we should employ a larger type in 
 the case of a presbyopic or hypermetropic eye than we should 
 in a normal or myopic one. No. 1 of Jager's types is, gene- 
 rally sjoeaking, one that may be advantageously employed. 
 In testing the near-point with an auxiliary convex lens {vide 
 infra), great accuracy may be attained from some of the minute 
 types furnished by photo-hthography, a whole page of which 
 may be bought for the purpose in the 9th number of the 
 Popular Science Review. 
 
 The far-point (r) may be determined in one of two ways. 
 If the person with the unassisted eye reads the larger of Jiiger^s 
 types (20 — 18) at 20 feet, it may be inferred to be infinitely 
 far off (r=oo ). If, on the other hand, a lens (convex or con- 
 cave) is required to read these or larger types (there being co- 
 existent amblyopia), the focal length of the glass employed 
 gives the required far-point, the distance of the glass fi'om 
 the cornea being subtracted in the case of myopia,* added in 
 
 * Higli myopics often prefer glasses of longer focns than their really 
 neutralizing ones, (1) on account of the diminution of the size of the images 
 these cause, and (2) on account of the " dazzling " referred to aboYe.
 
 MYOPIA. 47 
 
 that of hypermetropia^ to the focal length found. A second, 
 perhaps more accurate,* method of determining the far-point 
 (or near-point) consists in placing before the eye a convex 
 glass of 10"— 12" focus, and then determining the far-point 
 (or near-point) by the aid of minute types. From the arti- 
 ficial values thus obtained, the natural positions of both these 
 points may be readily inferred from the formula given in the 
 fifty-seventh number of the British and Foreign Medico- 
 Chirurgical Review {supra citat.), which I now give with some 
 modifications of the letters employed : — - 
 
 Let the focal length of the auxiliary glass =/. The distance 
 between the glass and the crystalline lens = A:. The distance 
 between the glass and the object=Z^. The distance between 
 the glass and the point from which the rays seem to proceed, 
 or towards which they converge after refraction =/3(=p or r), 
 according as we are determining the near or far-point. 
 
 1 l_l 
 
 ^ h-f 
 
 From the found values of j[) and r, we deduce the accommoda- 
 tion, A, from the formula — 
 
 J_l_l 
 A p r. t 
 
 I may illustrate the application of this formula in my own 
 left eye, which is myopic to about ■^-^. I find using a 12" 
 
 * This method possesses the further advantages of not requirmg a long 
 room for its application, and of our being able to select the best light. 
 
 f This formula is derived from the formula expressing the relation between 
 the principal (/) and conjugate foci (a and n) of an equi-convex glass lens. 
 This formula is 
 
 1_1 1 
 f~a a 
 
 In the case before us, a=2J and is </and a = r and negative. Substituting 
 these values, we get 
 
 1_L_1 L 
 f~A~p~r 
 (as in the text).
 
 48 OPTICAL DEFECTS OF THE EYE. 
 
 convex lens^ for the far-point bzzb~", for the near-point 
 6=2|", k being in both casesr:|". Therefore — 
 
 _5|xl2_ 516 
 
 adding ^" to this, my far-point (=r) would be 10'2": the 
 reducing glass of my myopia is, as previously ascertained, y^. 
 Similarly I deduce, using the same auxiliary lens, my near- 
 point (jii) to be 3*4", and without it = 3"4". 
 
 Suppose, now, we find that a person who uses 7-inch con- 
 caves for distance has an accommodation of y^- ; to see dis- 
 tinctly at 12 inches the patient will, with his reducing glasses 
 on, have to use his entire accommodation. But such a pro- 
 cedure would soon so exhaust the ciliary muscle as to lead to 
 all those symptoms of fatigue of the eye which we shall here- 
 after fully enter into under the head of "Asthenopia/^ If we 
 give the patient for reading, working, &c., at 12 inches, a 
 pair of 17-inch concaves (i — xV=tV)j he will for 12 inches 
 have no occasion to use his accommodation at all, but will 
 have a certain useful range of vision both beyond and on this 
 side of 12 inches. I must here distinctly confess that these 
 calculations must all be controlled by actual clinical expe- 
 rience. As far as rules can be laid down, I have endeavoured 
 to do so. These are — 
 
 1. If little or no accommodation exists, give a glass which 
 entirely supplants any slight natural accommodation still 
 present. 
 
 2. If a moderate amount of accommodation exists, give a 
 glass which partially supplants this natural accommodation. 
 
 3. a perfect accommodation exists, the same glass that does 
 for distant will do for near objects. 
 
 Is it possible to diagnose a case of myopia altogether inde- 
 pendently of any subjective symptoms ? It is. Most patients 
 present unusually large eye-balls : this is rendered most 
 obvious when the person looks as far as he can to the side. 
 The great length of the antero-posterior axis * and the genei'al 
 fulness of the globe then become very apparent. 
 
 * " Nimia oculi longitudo facit myopiain." (Boerhaave's " Praelect. de Morb. 
 Ocul." Gotting. 1746, p. 174.)
 
 MYOPIA. 49 
 
 One of tlie lieirlooms of traditional Surgery is the sup- 
 posed undue prominence of tlie cornea in myopia. Helm- 
 holtz and Donders have distinctly disproved the general 
 truth of this statement by laborious measurements of the 
 cornea by the ophthalmometer. A second set of very 
 diagnostic symptoms are those furnished by the ophthalmo- 
 scope.* These are the possibility of seeing the inverted image 
 of the fundus oculi without the use of an objective — : myopic 
 refraction. This of itself is very strong presumptive evidence of 
 (a high degree of) myopia. But to obtain a sufficiently extensive 
 field of view,, it is better to employ the ordinary objective — a 
 lower one than is used for normal eyes. A very characteristic 
 appearance is then usually seen. Embracing the optic papilla 
 is a crescentic pale or white patch. It varies greatly in 
 extent, being sometimes very narrow, sometimes very broad; 
 generally situate between the optic papilla and yellow spot, 
 not unfrequently, however, entirely surrounding the papilla. 
 The retinal vessels coursing over the white patch are seen 
 unusually well by reason of the contrast between their 
 red colour and the subjacent white ground, and often pursue 
 an unusually straight course. The " crescent," or staphyloma 
 posticum, as it is called, is mostly well defined, often very 
 abruptly so, and not uncommonly bordered with a punctuated 
 line of black pigment. f 
 
 " Myopic refraction " may be considered pathognomonic of 
 myopia; a staphyloma posticum almost so. Surgeons in 
 military practice, more especially on the Continent, may be 
 
 * For further information on this point consult the admirable edition of 
 " Zander on the Ophthalmoscope," by R. B. Carter, of Stroud. 
 
 + I have notes of sixty cases of myopia which I have examined with the 
 ophthalmoscope. In forty-four of these staphyloma posticum was present ; 
 in thu-ty-five it is noted as being at that side of the papilla which is towards 
 the macula lutea, whilst in five cases only was it exclusively to the inner 
 side, in one only exclusively above. In nine cases no staphyloma posticum 
 existed. In addition to the staphyloma posticum, I have in nearly alS 
 extreme cases of myopia observed the optic entrance either ill-defined, 
 deformed, atrophied, or hypervascular. The inner tissues of the eye, too, 
 in such cases nearly invariably ]iresent an appearance of tension and thinnin.rj 
 of the choroid, the vessels of this coat appearhig much more numerous and 
 less in caUbre than natural.
 
 )0 
 
 OPTICAL DEFECTS OF THE EYE. 
 
 called upon to decide the question of tlie existence or not of 
 myopia: in the event of prevarication on tlie part of the 
 person under examination, the ophthalmoscopic tests of myopia 
 wiU prove most valuable and decisive. Even in civil practice, 
 these tests are sometimes of great utility. Hysterical simula- 
 tion or dissimulation of ametropia I have met with on more 
 than one occasion. 
 
 In one case I observed the staphyloma was fringed out- 
 wardly by a peculiar halo of regular short lines. In another 
 
 rare case I observed the sta- 
 Fi<^- 25. phyloma was excavated — was 
 
 in fact a staphyloma posticum 
 in the true sense of the word. 
 
 It occurred in a woman set. 
 38. She had a myopia of ^ in 
 each eye, and possessed an 
 acuteness of vision of about ■^. 
 1 give a woodcut (Fig. 25) of 
 the inverted ima^-e of the 
 staphyloma posticum of the 
 right eye. 
 
 Sometimes, however, the 
 crescent merges gradually in- 
 to the surrounding redness of 
 the fundus, and is in cases of 
 extreme myopia not unfrequently accompanied by other iso- 
 lated irregular white patches, intermingled with black pigment 
 spots. V. Griife considers all these appearances as due to 
 a diflFused sclerotico-choroiditis, which, according to FoUin, 
 assumes an atrophic tendency in the involved tissues. Those 
 myopic eyes which have been anatomically examined have 
 generally displayed a marked elongation of the globe, espe- 
 cially so in the posterior hemisphere — hence the other name 
 which has been applied to this peculiar pathological condition, 
 viz., staphyloma posticum.* I have met with a few cases of 
 
 S = Excavated staphyloma posticum. 
 
 a =An arten' pressinfj over the bent vein, I 
 
 b' = A second bent vein. 
 
 0= Deformed and atrophied optic papilla. 
 
 ■"■ The first anatomical dfscriptioii of staphyloma posticum is to be found 
 in Scarpa's Treatise (translated by Briggs, 2nd edit. p. 399). In one of 
 two tlissectious he found " there was a deficiency of the nervous expansion 
 of the retina within the cavity of the staphyloma ; that the choroid coat was
 
 MYOriA. Ol 
 
 myopia, ono of the highest degree I ever saw (yith) in which 
 no such staphyloma existed. In these it is fair to assume the 
 myopia was caused by some abnormal refractive condition of 
 the humoiu's. 
 
 A form of apparent myopia, of which I have seen several 
 instances, is one which I, with my friend. Dr. Giraud- 
 Teulon, consider produced by a spastic state of the ciliary 
 muscle. This " functional myopia '^ (?) yields on atropine to 
 emmetropia, or even hypermetropia ; or to a lower degree of 
 myopia, to which it is a superadded factor. This form of 
 short-sight is generally very slight and has been observed by 
 me principally in patients who were also presbyopic. 
 
 very thin and discoloured at this part, and wanted its usual vascular plexus ; 
 and that the sclerotica, particularly at the apex of the staphyloma, was 
 rendered so thin as scarcely to equal the thickness of writing paper.' 
 The predisposition to staphyloma posticum in the position it always con- 
 stantly occupies is probably due to its being that of the protuberantia 
 scleras fetalis (v. Ammon), where the sclerotic, even at birth, is thinner 
 than elsewhere.
 
 52 
 
 OPTICAL DEFECTS OF THE EYE. 
 
 CHAPTER V. 
 
 HTPERMETEOPIA. 
 
 rilHE two cliief anomalies of refraction the eye is subject to 
 -^ are myopia and hyjDermetropia.* The former we have dis- 
 cussed at some length ; the latter it will now be our task to 
 inquire into, and in doing so I shall, as in the case of myopia, 
 follow the table of diagnostic signs (p. 30) of the eye in a 
 state of rest. In Hij]jermetropia : — 
 
 (1.) " Parallel rays are focussed behind the retina.'' — 
 
 Fig. 26. 
 
 f 
 
 
 
 Parallel rays (Fig. 26) from each luminous point (j-)) of a 
 distant object (0) tend to a focus (F) behind the retina (E) ; 
 
 •^ The following quotations on the history of this defect of vision may not 
 be devoid of interest. Bonders {Archiv f. Ophtlmlm., vi. 1, 1860) says : — 
 " Hypermetroijia has, up to the present time, been very little studied : the 
 
 older oculists either knew it not or misunderstood it The 
 
 ignorance on this important defect was universal Even by 
 
 Ruete (1845) was its existence not known." I here, however, distinctly 
 assert that Mackenzie, in his "Physiology of Vision," p. 150, published in 
 1841, gives a clear definition of the disease : " but there are instances of the 
 eye being so flat as to requu-e convex glasses to converge even parallel 
 rays to the retina, which is equivalent to a power of bringing convergent 
 rays to a focus." However, no unprejudiced person can rise from the 
 perusal of Professor Bonders' admirable exposition of the anomalies of 
 refraction and accommodation of the eye without being impressed with the 
 importance of his researches, — the truly philosophical spirit in which they 
 have been conducted.
 
 HYPERMETROPIA. 53 
 
 if we imagine tlie tunics of the back of the eye removed, a 
 distinct image of (p) would be formed behind the retina, at 
 (-F). As it is, however, every luminous point (/') of the object 
 is reproduced on the retina as a circle of diffusion (C), and 
 hence no distinct image of a distant object is formed on the 
 retina. 
 
 Thus, a hypermetropic eye is deficient in its converging 
 power on the rays «of light. As an illustration, I may say 
 that after operations for cataract, the share the crystalline lens 
 takes in this converging power being lost, the eye becomes 
 hypermetropic. 
 
 Again, we may artificially convert a normal eye into a 
 hypermetropic one by placing before it a concave lens ; a 
 myopic eye into a hypermetropic one by placing before it a 
 concave lens stronger than is necessary to neutralize its myopia. 
 But, as we shall hereafter see, by far the greater number of 
 hypermetropic eyes met with in practice are the result of a 
 congenital conformation of the globe. 
 
 (2) '' The liypermetropic eye in a state of rest is ada^ptedfor 
 convergent rays.'' — Although, as we have just seen, the hyper- 
 metropic eye has too low a converging power to focus parallel 
 (and a fortiori divergent) rays on to the retina, you can readily 
 understand that it may have sufiicient to focus rays which are 
 already in a state of convergence before they strike the cornea. 
 Now, as I have before said, the only two classes of rays 
 met with in natm'e are parallel and divergent ones ; but we 
 may confer on either of these any desu'ed amount of con- 
 vergence by receiving them on a suitable convex lens. Hence 
 the remedy for hypermetropia is a convex lens of such a 
 power as to give the incident rays from objects a sufficient 
 degree of preliminary convergence before they strike the cornea 
 as to enable the eye to effect the additional amount of con- 
 vergence necessary for the production of a defined, distinct 
 image on the retina. 
 
 (3.) " The far-iooint of a hypermetropic eye in a state of 
 rest is at a finite distance [from the eye), and negative." — In 
 the normal eye in a state of rest, the focus (luminous point) 
 of rays which, striking the cornea, will be reunited to a focus 
 on the retina, is, as we have seen, positive and infinite, in the
 
 54 OPTICAL DEFECTS OF THE EYE. 
 
 myopic eye positive and finite. In the hypermetropic eye 
 rays of light to bo brought to a focus on the retina must, 
 before they strike the cornea, in the first instance (by a convex 
 lens held before the eye) be made to tend to (converge to) a 
 focus behind the retina. The higher the power of this convex 
 lens the higher the degree of hypermetropia, whilst, when this 
 power is ^o, or less, the eye closely approximates to the 
 normal one. Measuring the focus of these convergent rays 
 from the cornea backwards, we may by a figure of speech 
 (in analogy to the far-points of normal and myopic eyes), 
 say that in the hypermetropic eye the far-point is finite and 
 negative. 
 
 The amount of deficient converging power of the hyper- 
 metropic eye — the amount of hypermetropia — ^is given by the 
 convex lens necessary to produce distinct vision of distant 
 objects. The power of this lens may vary from -^^ up to |- ; 
 at least this last is the highest degree I have ever seen (in a 
 student at Utrecht),* excepting, of course, a hypermetropic 
 condition of the eye dependent on a cataract operation or a 
 congenital deficiency of the ciystalhne. 
 
 The following is the highest degree of hypermetropia I have 
 ever seen in my own practice. 
 
 Case IV. 
 
 John Edwards was 6 years old when he was brought to me at the Ophthal- 
 mic Hospital, Southwark, by liis mother, on December 12th, ISGO. Since 
 the child was three years old she had observed his sight was defective, and 
 that he always brought objects very close to his eyes when he wished to see 
 them distinctly. I found he required 5-iach convexes to read No. 22 at 
 distance ; but after the eyes had been subjected to atropine for an hour he 
 required 3j-inch convexes, with which he read No. 23 at 6 feet 9". I re- 
 examined his eyes about twelve hours later, and found 5-uich convexes were 
 the glasses with which he obtained the most distinct vision for distance, 
 reading with them No. 21. On examining his eyes ophthalmoscopically, no 
 morbid appearances were perceptible in either fimdus oculi ; but I could 
 observe the direct image in either eye as far off as 3 inches from the cornea, 
 after the full action of atropine. Both catoptric images from the lens-surfaces 
 
 * It is a case mentioned by Bonders, at p. 241 of his work on the " Ano- 
 malies of Accommodation and Refraction."
 
 HYPERMETROPIA. 55 
 
 could be distinctly seen in each eye, proving it was not a case of congenital 
 absence of the crystalline lenses, I saw the case again upwards of a year 
 afterwards ; the boy ever since I first saw liim wore 5-inch convexes. With 
 these he read at' 20 feet No. 21, without them he could not read 
 No. 23. 
 
 Tlio following are the outlines of two other cases of un- 
 usually high degrees of h5rpermetropia. 
 
 Case V. 
 
 Sarah W., set. 8, was brought to the hospital on account of her " short 
 sight " — she was always obliged to hold things very close to her eyes to see 
 them — and her inability to read, &c., for longer than a few minutes together. 
 She had also been observed to squint, a fact which I myself observed in 
 either eye momentarily, but decidedly, when she looked intently at a near or 
 distant object. The eyeballs appeared very small ; the cornea measured 
 5'" in diameter ; the palpebral ajjertures only 8"' in length ; the pupils in a 
 full light measured 1'" and acted well. With the right eye she covdd not 
 read C C (Snellen) beyond 10' ; with a 4-inch convex, LXX. at 20', and 
 the same type with the same glass after two hours' application of a strong 
 solution of atropine (gr. viii. ad 3J). The left eye was quite similar in its 
 refraction, exceptmg that after atropine she preferred a 34" convex. This 
 was, however, not a case of imcomplicated hypennetropia, as the ophthabno- 
 scope showed. The optic entrances were only recognisable by the confluence 
 of the retinal vessels, which were, at parts, much obscured. In the left 
 fundus some pale, irregular spots, with slightly pigmented deposits, were 
 observed. The direct image in each eye was well seen at 7*— 8" off without 
 any ocular ; at 1" — 2" a 12-iuch convex ocular was necessary. From the 
 above it follows that part of the amblyopia was owing to some chronic retinal 
 and choroidal processes. 
 
 Case VI. 
 
 Henry N., a^t. 10. — His sight had been defective as early as he could re- 
 member, having been unaljle to see either near or distant objects well. He had 
 been repeatedly very severely punished by his father, who thought he was 
 malingering at school ; but an inspector of schools, who believed in the 
 reahty of his defective yision, advised him to go to the hospital to have his 
 eyes examined. I found a manifest hypermetropia of -jV, and a total of j, 
 with an acuteness of vision of j. Ophthalmoscopically I detected nothing 
 morbid, but slightly egg-shaped optic entrances. He said he saw horizontal 
 lines better than vertical ones. But I found him so unintelligent as to render 
 any exact determination of his astigmatism practically impossible. 
 
 NoWj as you perfectly uudcrstaud^ the eye htis vvithiu itself
 
 56 OPTICAL DEFECTS OF THE EYE. 
 
 an intrinsic power of increasing its converging action (in 
 the crystalline). The normal eye has no occasion to exercise 
 this function for distant objects, and therefore it remains in 
 a state of rest. The myopic eye is already abnormally 
 excessive in its converging power, and would therefore, if it 
 could, rather diminish, than increase the convexity of its 
 crystalHne. 
 
 But, as you might a priori have expected, the hypermetropic 
 eye accommodates even for distance, abnormally using a func- 
 tion for distance which a normal or myopic eye only employs 
 for near objects. It in this way avails itself of its power of 
 accommodation to supplement its defective power of refraction. 
 Such an eye is then " never in a state of rest *' in the sense 
 we have used this expression. Have we any means of reducing 
 the eye to such a state, and by thus eliminating the element 
 accommodation, estimating the total amount of deficient con- 
 verging power — of hypermetropia — present ? We have : by 
 paralysing the ciliary muscle by the instillation of a solution 
 of atropine.* 
 
 The preceding will become intelligible from an actual 
 example. A young woman required a 40-inch convex glass 
 for the distinct perception of distant objects ; after instillation 
 of atropine she could, with the naked eye, hardly see distant 
 objects at all; but could so perfectly with an 8-inch glass. In 
 the first instance her hypermetropia was neutrahzed by tv»-o 
 
 * As I shall later have occasion to advert to at length, with advancing 
 age our faculty of accommodation diminishes: in old presbyopic subjects 
 this precaution is therefore unnecessary. I may, however, here state I 
 consider even in myopia, with a fau- amount of accommodation, it is ad%as- 
 able to first paralyse the accommodation by atropine before determining 
 the reducing glass, as I find patients frequently on first trial prefer too 
 high glasses, and might thus by their use acquire a hypermetropic 
 accommodation, which would necessarily lead to diminution of their normal 
 accommodation and consequent asthenopia. My observations agree with 
 those of Professor Donders, who has, in a tabular view he gives of the 
 relative range of accommodation of the myopic eye, pointed out its excessive 
 positive value for low degrees of convergence of the eyeballs (for distant 
 objects), and the total absence of negative value — so that myopic patients 
 possess no power at all of overcoming the weakest convex glasses for 
 distance. For further information on this interesting subject, vide Arch 
 f. OpJdhnlm. vi. 1.
 
 HYPEEMETROPIA. 57 
 
 factors, (1) the ^V convex lens, (2) the increased curvature she 
 herself gave her crystalline lens by contraction of the cihary 
 muscle — by her accommodation, in a word ; in the second 
 instance we destroyed this last factor by atropine, and then we 
 found she required, instead of a 40-inch, an 8-inch convex glass. 
 Therefore, she previously accommodated by a quantity equi- 
 valent to the difference between these two glasses, viz., a 
 10-inch glass: 8~4V=iV- '^^^ total hypermetropia of such a 
 case is |, the latent -j^, the apjparent or manifest Jl. There 
 are several circumstances which may lead to the suspicion of 
 hypermetropia ; by a low-powered convex glass either im- 
 proving, or, at any rate, not deteriorating vision for distance, 
 as it would in a normal or myopic eye. In applying this test 
 you must allow the patient to try the glass on for a few 
 minutes, as he has so accustomed himself to accommodate for 
 distance, that he cannot for the moment relax even the very 
 slight fraction of this effort, which the lowest powered convex 
 glass is intended to supplant.* Convergent strabismus and 
 asthenopia are commonly associated with hypermetropia {vide 
 Cap. X.). An abnormally distant position of the near point in 
 a young subject is highly indicative of hypermetropia. Such 
 cases are often misunderstood as " singular instances of pres- 
 byopia in young subjects.''^ Their rationale is evidently that the 
 patient using a great part of his accommodation to neutralize 
 his hypermetropia, has a proportionately small part remain- 
 ing over available for its legitimate purpose of neutralizing the 
 
 * Many asthenopic patients possess such powers of accommodation that they 
 can read as small a print as No. 16 at a considerable distance. This is, however, 
 no jDroof of the absence of hypennetropia ; this can only be inferred by a 
 convex glass not improving vision for distance, after the instillation of 
 atrojyine. I may here remark that in this way normal eyes may become very 
 slightly hypermetropic, probably from the atropine destroying the tonicity of 
 the cUiary muscle, and I think I have similarly observed myopia somewhat 
 decreased by the same agent. Of 86 cases of hypermetropia, 38 were males, 
 48 females ; vv'hilst of 50 cases of myopia the numbers of either sex were 
 precisely equal. This preponderance of the female sex in hypennetropia is 
 probably more apparent than real. The patients were principally hospital 
 ones, and of these the occupations of females would be more likely to cause 
 them to feel inconvenience from asthenopia (depending on hypermetropia) 
 than those of males would.
 
 58 OPTICAL DEFECTS OK THE EYE. 
 
 divergence of the rays from near objects. A liighly-interest- 
 ing test of myopia or hypermetropia is afforded by tbe unequal 
 rcfrangibility of tlie different coloured rays of the solar spec- 
 trum. Many dark violet (or cobalt-blue) stained glasses only 
 permit the blue and red rays of light to pass through them — 
 the highest and least refrangible rays of the spectrum. If a 
 moderately myopic eye regards the flame of a candle or of gas 
 through such a glass, the red rays will be nearly focussed on 
 the retina, whilst the blue ones form large circles of diffusion 
 on that structm^e ; the consequence is such a flame appears to 
 the myopic eye violet-red, surrounded by a hlue margin. To a 
 hypermetropic eye the flame appears reddish-hlae surrounded 
 hij a red margin. 
 
 How do we determine what glass is required to neutralize 
 the hypermetropia ? As far as I know, this is a mere matter 
 of trial. In the practical treatment of a case you must not at 
 once prescribe this neutralizing glass, but begin with the 
 highest that improves vision for distance, before the eye has 
 been subjected to the action of atropine.* For you must 
 remember accommodation for distance has become an instinc- 
 tive habit, not to be broken by any such inide violence. You 
 will find, if you watch a case, that after a time this habit gives 
 way, and thus by degrees you will have to increase the power 
 
 * However, I have met with cases where the hypermetropic state of the 
 eye was not demonstrable before the action of atropine, so strongly ingrained 
 had become the habit of accommodating for distance. In one case a young 
 woman read with her naked eyes No. 16 easily at 20 feet ! Yet atropine 
 revealed an hyijermetropia of -^ ! In some cases hypermetropic patients, 
 before atropine has been instUlated, even assert that their vision is improved 
 by concave glasses. These, of course, add to their hyijemietropia, but 
 apparently improve their vision by stimulating them to an extra amount 
 of accommodation, not only sufficient to overcome this superadded artificial 
 hypermetropia, but to still further correct their inherent hyjiermetropia. 
 Donders thinks the apparent improvement of vision is due to the patient's 
 preferring the smaller retinal images iaduced by the concave glasses (?). 
 As an example I may quote the case of a young woman, 16 years old, 
 who suffered from asthenopic symptoms, and declared, before atropine 
 was applied to her eyes, her vision for distance was improved by 24-inch 
 concaves ; but after atropine, actually required 18-inch convexes to read the 
 type she read before atropine. Such cases, of which we must be on our 
 guard, are cases of the so-called " spasm of acconmiodation."
 
 HYPERMETROPIA. 59 
 
 of the glasses the patient wears^ till such ones are at last 
 reached which completely neutralize the entire hypermetropia 
 present. As in myopia^ each eye must be examined sepa- 
 rately^ and then the two together. Thus, in a case I found 
 one eye had a hypermetropia of ^, the other of xV J wliilst, 
 in three cases, one eye was normal whilst the other was hyper- 
 metropic. 
 
 The question, in how far glasses for distance are available 
 for near objects, depends (1) on* the total amount of accommo- 
 dation the patient possesses, and (2) on how much of this he 
 habitually expejids in the neutralization of his hypermetropia. 
 This is practically determined thus : — Let the patient (his eyes 
 not being under the influence of atropine) have on the strongest 
 convex glasses he can bear for distant objects ; then find with 
 these glasses on the nearest point of distinct vision of a small 
 type. If this distance varies anywhere between 9 — 5, or less 
 inches, he has a fair amount of accommodation still at his dis- 
 posal for its legitimate object (the neutralization of the diver- 
 gence of the rays), and he may use the same glasses for near, 
 as for distant objects. But if the patient's near-point with 
 his distance-glasses on is, say, 20 inches, he has only -^ 
 accommodation at his disposal for the purposes of reading, 
 writing, &c. Let the distance-glasses, e.g., he-^ power ; then 
 with glasses of 7 inches focus (rs+l — 2V =t) ^^^ person will 
 be able to read, &c,, at 8 inches by using all his accommoda- 
 tion, and at such convenient distances as 10, 11, or 12 inches 
 by only a partial expenditure of that function. 
 
 No one could a priori form any conception how common a 
 disease hypermetropia is. In 166 cases of abnormal refraction 
 of the eye, I find 80 of myopia and 86 of hypermetropia. 
 The truth is, many of these latter pass unrecognized, and some 
 of the higher degrees are mistaken for enigmatical cases of 
 myopia, as the patients not uncommonly hold objects close to 
 the eye to see them distinctly, and yet are not only not im- 
 proved, but actually made worse, by a concave glass. They 
 do this, I think, to correct the defective refraction of the eye 
 by the consentaneous contraction of the pupil. If you make 
 your own eye artificially hypermetropic by a concave glass, 
 you will find your sight considerably impaired, but may at
 
 60 OPTICAL DEFECTS OF THE EYE. 
 
 once restore it by a pin-hole aperture^ tlie rationale of whicli 
 has already been explained.* Bonders thinks these patients 
 seek to increase their accommodation by increasing the con- 
 vergence of the optic axes, but seems to ignore the simulta- 
 neous, greatly-increased ratio the divergence of the rays ac- 
 quires by this approximation of the object. 
 
 Independent of the above subjective symptoms, there is a 
 diagnostic objective one of the presence of hypermetropia. 
 The direct image of the fundus oculi may, if the observer's 
 own eye be normal or myopic, be seen without any concave 
 eye-piece. In 45 cases of which I possess ophthalmoscopic 
 notes, I find in 32 the fundus ocvdi was perfectly normal ; in 
 8 next the optic entrance was a narrow black pigment line ; 
 and in 4 cases a staphyl. post, was observed, not distinguish- 
 able from that so generally met with in myopia. As the eyes 
 of myopic patients are often fuller than natural, so those of 
 hypermetropic ones are more sunken and less. 
 
 * The same holds Kood for the disturbing influence of a convex gla&s.
 
 ASTIGMATISM. 61 
 
 CHAPTER VI 
 
 ASTIGMATISM, 
 
 TFj for the sake of precision^ we call the centre of the cornea 
 -*- the anterior pole of the eyeball, the corresponding point 
 at the back of the globe its posterior pole, then a circle passing 
 through both poles is a " meridian.''^* In general, it was 
 assumed that all the meridians of the cornea were circles of 
 equal curvature, and that, consequently, the cornea refracted 
 equally in all its meridians. Such, however, is not strictly the 
 fact. The cornea has been clearly proved to be not the 
 segment of a sphere, but of an ellipsoid, with three unequal 
 axes, the long (antero-posterior) axis corresponding to the 
 axis of the globe ; whilst the other two axes are — one, the 
 longer, as a rule, horizontally, the other, the shorter axis, 
 vertically disposed. The consequence of this is that (with 
 one fixed accommodation) vertical and horizontal rays of light 
 have two different foci. To see a vertical line distinctly, all 
 the horizontal rays emanating from it must be accui'ately 
 focussed on the retina. The vertical rays have no such 
 necessity, for the circles of diffusion resulting from imperfect 
 focussing overlap one another, so as not to interfere with the 
 definition of the line, excepting at its upper and lower ends. 
 Mutatis onutandis, the same facts hold good as regards the 
 accurate perception of horizontal lines. If we now, on a piece of 
 cardboard, draw a set of vertical and a similar set of horizontal 
 lines, accm'ate observation shows that very few eyes are able to 
 perceive both sets at the same time with the same degree of 
 distinctness. This proves that few, if any, eyes possess the 
 
 * If we call a line passing through the yellow spot and the centre of the 
 eyeball the optic axis ; one passing through the centre of the cornea, and the 
 centre of the eyeball the globe's axis ; then in emmetropic eyes the optic 
 axis cuts the globe's axis to the inner side of the cornea's centre at about 
 an average angle of 5°, in myopic eyes of 2% in hypermetropic eyes of 7°.
 
 62 OPTICAL DEFECTS OF THK EYE. 
 
 same focus for horizontal and vertical pencils of light. This 
 inequality of refractive power of the different meridians of the 
 eyeball was named by Professor Whewell " Astigmatism " 
 (from a, privative^ and oTLyfxa, points focus). Astigmatism is, 
 then, a normal state of the human eyeball; but it exceptionally 
 amounts to so substantial an optical defect as to require treat- 
 ment, and in this sense only we shall hereafter employ the term 
 in question. Up to the time of Donders's recent researches, only 
 eleven cases of astigmatism had been recorded during a period 
 of nearly seventy years. Thomas Young, in 1793,* was the 
 first to discover this defect in his own eye. This, " in a state of 
 relaxation, collects to a focus on the retina those rays which 
 diverge vertically from an object at the distance of ten inches 
 from the cornea ; and the rays which diverge horizontally from 
 an object at seven inches distance.^^f Consequently, the refrac- 
 tion of his globe was greater in the horizontal than in the ver- 
 tical meridian. In 1827, Professor Airy jDublished a remarkable 
 instance of the same anomaly in his own (left) eye. J It will be 
 readily understood that astigmatism may ensue from other 
 causes than those of an inequahty of curvature of the meridians 
 of the cornea. The same inequality in the curvature of the sur- 
 faces of the lens, any abnormal position (or dislocation) of this 
 latter will produce astigmatism. Many persons who wear glasses 
 are doubtlessly constantly astigmatic from the slanting positions 
 the lenses of these glasses acquire from imperfect adaptation of 
 the frames, or from their bending, by the habit many persons 
 have of carrying their spectacles carelessly in their pockets 
 without the case. Astigmatism may be recognized by the 
 following signs : — 
 
 1. A defect in visual power of an optical nature not reme- 
 diable by ordinary spherical lenses. A patient presents him- 
 self with defective vision. With or without ordinary glasses, 
 as the case may be, he finds it impossible to read, say No. 19 
 of Jjiger, at twenty feet. This may be due to some organic 
 or functional disease of the tissues of the globe ; or it may be 
 
 * Philosophical Transactions, vol. Ixxxiii. p. 169. 
 t Philosophical Transactions, for 1801. 
 
 X Transactions of the Cambridge Philosophical Society, for 1827, vol. ii. 
 p. 267.
 
 ASTIGMATISM. 63 
 
 due to astigmatism. To which of these two categories it 
 belongs becomes^ then, a matter for further investigation. 
 
 2. For simplicity, we may regard the rays of light emitted 
 from a luminous point as consisting of two sets — vertical and 
 horizontal rays. If an astigmatic eye unites either of these two 
 sets accurately to a focus point on the retina, the other set forms 
 a linear image (produced by a series of circles of diffusion 
 linearly disposed) on the retina. Thus an astigmatic patient, 
 instead of perceiving a luminous point as a point, perceives it as 
 a Hne, vertically or horizontally disposed, according as, by his 
 accommodation, he is focussing the horizontal or vertical rays 
 respectively. But the refraction of the eye in question may be 
 such as to prevent the patient^s bringing either of these two 
 sets accurately to a focus. In such a case, where the circles of 
 diffusion are exactly of the same size, the luminous point will 
 appear as a circle of diffused light: in all other positions between 
 the two linear images as an elHpse with its major axis, vertical 
 or horizontal, according as the circles of diffusion arising from 
 the vertical or horizontal rays are the larger in area. But, as 
 will readily appear from a little reflection, we can always find a 
 lens, either convex or concave, which will unite the less or 
 more refracted of the two sets of rays. The direction of the line 
 observed by aid of the convex lens represents that of the meri- 
 dian of maximum curvatm'e (refraction) of the cornea, and vice 
 versa with the concave lens. The distance between the two 
 Hnear images is the " focal interval " of Sturm, and varies pro- 
 portionately to the astigmatism present. All these facts may 
 be beautifully illustrated by observing the forms of the images 
 thrown on a screen from a distant luminous point by a refractive 
 combination, say, of a 6-inch ordinary spherical convex lens, 
 next to which is placed, say, a 30-inch convex cylindrical lens,* 
 with its axis, say, vertical. A¥henthe screen is five inches from 
 
 * I find the most convenient source of obtaining a luminons point is from 
 a magic lantern, the light from which is allowed to pass through a perforated 
 blackened diaphragm. The cyUndi-ical lens in question (as will be hereafter 
 fidly explained) refracts all the horizontal rays of the lunmious point to a 
 focus at 30 inches, whdst all the vertical ones undergo no refraction whatever, 
 but pass through perfectly unchanged in tlieu' direction. The screen in our 
 experiment represents the retina.
 
 64 OPTICAL DEFECTS OF THE EYE. 
 
 this compound lens, the horizontal rays will be accurately united 
 to a focus, having been refracted (1) by the 6-inch spherical, and 
 (2) by the 30-inch cyUndrical lens (i-|--^=i). The vertical 
 rays, on the other hand, are in progi'css of union to a focus, but 
 are not yet united, and will, consequently by their circles of 
 diffusion, produce a vertical line on the screen. When, on the 
 other hand, the screen is six inches from the compound lens, the 
 vertical rays are united to a point, whilst the horizontal ones, 
 having been already united at five inches, as above, have 
 diverged again, and form a horizontal line on the screen. In an 
 intermediate position, between five and six inches, the image is 
 that of a circle, whilst in front of this circle it is an ellipse, with 
 its major axis vertical; behind it, one with its major axis hori- 
 zontal. If we suppose the screen to be fixed, as the retina is, say 
 in some position intermediate between either of the positions 
 necessary to exhibit either of the two linear images, a little 
 reflection will show that, by means of an appropriate convex 
 or concave lens, we may throw forwards or backwards the 
 entire system of images in such a way as to successively bring 
 the horizontal or vertical linear images on to the screen. Or 
 we may suppose the screen fixed altogether in front or alto- 
 gether behind the limits of the two lines (the focal interval). 
 In the first case (one of inequality of hypermetropia in the 
 two meridians), an appropriate convex lens will evoke a ver- 
 tical line, a still stronger one a horizontal line on the screen, 
 and vice versa in the second case (one of inequality of myopia 
 in the two meridians) ; and these experiments are most in- 
 structive, and agree very accurately with what is observed in 
 most cases of marked astigmatism. 
 
 A consideration of the above facts will explain all the 
 various appearances a luminous point may assume to an 
 astigmatic eye, with or without the aid of an auxihary lens. 
 In all these experiments it is absolutely necessary that the 
 patient should hold the head perfectly vertically, as the direc- 
 tions of the linear images vary according to the inclination of 
 the head. 
 
 3. The inability to perceive vertical and horizontal lines at 
 the same time with equal distinctness. The rationale of this 
 has been before considered, and is a mere case of linear
 
 ASTIGMATISM. G5 
 
 aggregation of luminous points. As from (2) may be inferred, 
 the indistinct set may be rendered distinct by an appropriate 
 spherical lens, when the set previously distinct becomes in- 
 distinct. 
 
 4. We may destroy the astigmatism by letting the patient 
 see through a narrow slit (about two millimetres broad) ; this 
 permits only the vertical or horizontal rays of objects to strike 
 the cornea accordingly as it is held vertically or horizontally. 
 By far the most practical way is to test the far-point in each 
 meridian by a small type, with an auxiliary convex lens, by 
 the method detailed at p. 47. I am indebted to Mr. Windsor 
 for this practical hint. We may thus investigate the refraction 
 of each meridian separately : this may be normal, myopic, or 
 hypermetropic (the latter two may be of different degrees), so 
 that we may (theoretically) have eight different varieties of 
 refraction of the cornea. The most common case is for the 
 cornea to have its maximum refraction in the vertical meridian, 
 as in Airy's case, but it may be in the horizontal meridian, as 
 in Young's, whose case is further exceptional in the astig- 
 matism having been due to some irregularity of the lens, 
 whereas it is in by far the vast majority of cases due to some 
 irregularity of curvature of the cornea. 
 
 5. Astigmatism may be remedied by cylindi'ical lenses, 
 always presupposing that any general optical defect of the eye 
 has been previously corrected by its proper spherical lens. 
 
 A spherical lens is a segment of a sphere, and refracts the 
 incident rays of light equally in all planes of the segment ; a 
 cylindi'ical lens is the segment of a cylinder, and refracts rays of 
 light most in a plane at right angles to the axis of the cylinder, 
 of which it is a segment, whilst those rays of light which strike 
 it in the plane of the axis, undergo no refraction whatever. For 
 the sake of simplicity we may therefore restrict our considera- 
 tions to these two directions — that of the axis and that of the 
 transverse diameter. A 6-inch convex cylindrical lens means 
 one which refracts a pencil of parallel rays, thus: (1) those which 
 strike it parallel to the transverse diameter of the cylinder are 
 focussed at G inches from the surface of the lens (2), those which 
 strike it parallel to the axis of the cylinder are not focussed at 
 all by the lens, but pass through it refracted not more than they 
 
 F
 
 66 orncAL defects op the eye. 
 
 would have been by passing through a piece of plane glass. 
 Of a pencil of parallel rays striking a 6-inch concave cylindrical 
 lens : (1), those which impinge in the plane of the transverse 
 diameter acquire thereby a divergence as if they had proceeded 
 from a point 6 inches from the surface of the lens, whilst (2) 
 those which impinge in the plane of the axis retain the same 
 direction they had before impaction. It is thus apparent that 
 we in cylindrical lenses have (as was first pointed out by Airy) 
 the means of correcting the refraction of one meridian of the 
 eye, leaving the other in its original state — of correcting the 
 astigmatism.* 
 
 The following cases are illustrative of the facts and prin- 
 ciples above laid down : — 
 
 Case VII. — Myopic Astigmatism {~q) due to Unduly High 
 Curvature of the Vertical Meridian, remedied by a Cylin- 
 drical Lens. 
 
 Mr. A., £et. 24, coiLsulted me for acute astlienopic symptoms, which 
 had become more esj)ecially severe during the time he had been read- 
 ing hard for a University degree. He had for many years worn concave 
 glasses, which he had found it necessary to change several times for ones suc- 
 cessively stronger, till he at last used ones of 5 inches focal length. There was 
 no marked insufficiency of the internal recti muscles. Before examining his 
 eyes with the ophthalmoscope, I expected to find extensive staphylomata 
 postica, if not more serious organic lesions^ of the fundus oculi. But to my 
 sur^Drise the appearances were altogether at variance with the apparently high 
 degree of niyopia present. In truth there were hardly any appreciable devi- 
 ations from the natural state of the fundus, nor was the indii-ect miage 
 definable without the use of an object lens. 
 
 This led me to suspect the presence of astigmatism. 
 
 Left eye. — (1) With a 6-inch concave at 8 feet vertical lines appear very 
 indistinct, horizontal ones distinct, and vice versa with a 14-inch concave. 
 
 (2) With a 4-inch concave at 8 feet a luminous point appears as a narrow 
 horizontal line, thickened at its centre with a 12-inch concave as a narrow 
 vertical line. 
 
 (3) Further investigation showed that the eye was myopic in the hori- 
 zontal meridian to a vW, in the vertical to a -^^.t 
 
 * A complete set of cylindrical trial-lenses may be procured of Murray & 
 Heath, 143, Piccadilly. 
 
 t The numerical discrepancies in observations (I), (2), and (3) are probably 
 due to variations in the patient's involuntaiy acconmiodation.
 
 ASTIGIiIATISM. 67 
 
 With a combination of a 10-inch spherical concave and a 20-inch cylin- 
 drical concave (axis horizontal) the appearances (1) and (2) disappeared, and 
 his visiial poiccr was exactly double of that with his old glasses on (from S= \ 
 to jS* = ^). For at 8 feet with a 5-mch concave on he only read XXX ; witli 
 the above combination he read XV. 
 
 The right eye was also astigmatic, though not to quite so high a degree as 
 the left. 
 
 Tliis case is interesting from its singular completeness, illustrating, as it 
 does, nearly all the preceding observations in a truly practical manner. 
 
 Case VIII. — Myojnc Astigmatism (■^) Limited to the Right 
 Eye. — Oblique Position of the Axes of Curvature. — AfpU- 
 cation of a Cylindrical Lens. 
 
 Wm. M., a painter, applied at the Ophthalmic Hospital, Southwark, in Sep- 
 tember, 1863, on account of his sight gradually failing him, especially in his 
 right eye. He had never had any symptoms of lead-poisoning, excej)ting 
 slight wrist-di-op. Notwithstanding this his failure of sight had been attri- 
 buted, by a well-known ophthahuic surgeon, to lead. 
 
 On testmg his vision I found that with a 1 1-inch concave to the right eye he 
 read No. 23 at 20 feet ; with the left eye No. 20 ; but with a 24-inch concave 
 No. 18 at the same distance. 
 
 The opththalmoscope disclosed evidence of subacute retmitis in both fundi, 
 and a singularly distinct and extensive pulsation of the retinal veins in both 
 eyes.* 
 
 After treatment of the retinitis, his vision improved in either eye ; but 
 still that of the right eye remained almost as defective as before, till at last we 
 began to suspect the presence of astigmatism, which was completely established 
 by the following observed ftxcts : — 
 
 Bight eye. — At 8 feet with a 9-inch concave sees vertical lines somewhat 
 plainer than horizontal ones ; with a 12-mch concave the reverse. With a 
 12-inch concave a luminous point assumes the appearance of a point with 
 two linear expansions directed upwards and inwards, at an angle of about 
 20° from the vertical. With a 12-inch concave spherical, plus a 20-inch 
 concave cylindrical lens, its axis inclined as above, he is enabled to read 
 No. 19 at 20 feet, although with either glass alone he cannot read No. 23 
 with any degi'ee of certainty. 
 
 If he with a 9-inch concave regards a series of lines inclined at about 20° 
 to the vertical, and a second set at right angles to these latter, both set 
 appear equally distinct to him. An oblique position of his head, or of the 
 concave glass, produces the same effect. 
 
 * Since my attention has been drawn to this fact, I have found pulsation 
 of the retinal veins a by no means vmcommon incident to the normal fundus 
 oculi.
 
 G8 
 
 OPTICAL DEFECTS OF THE EYE. 
 
 The inference to be dvavm from the above facts is that in the right eye 
 there is a general myopia of Vt; with a second superadded myopia of Vtt 
 (constituting the astigmatism), in the upward and inward meridian of 
 tlie globe. 
 
 October 24 (dull day). — Oc. d. with his cylindrical concave now only reads 
 22 — both papUlte optic?e red, stajDhyloma post, (iuwards) in left eye, less 
 in right eye ; paLsation of veius still seen. 
 
 Fig. 27. 
 
 In several otlier cases of astigmatism^ -wliicli I have inves- 
 tigated, I have found tliat the principal axes of curvature 
 deviated more or less from the vertical 
 and horizontal directions. I have also 
 observed that astigmatic patients are 
 remarkably sensitive to the slightest 
 deviation of the axis of the cylindrical 
 glass from that which most com- 
 pletely corrects the asymmetry, the 
 least change of position of the axis at 
 once lowering the acuteness of vision. 
 It is therefore not only scientifically, 
 but practically necessary to be able to 
 determine and note for the guidance 
 of the optician the precise angular 
 position of these axes. 
 
 The instrument [Astigmometer] 
 (Fig. 27) for this purpose consists 
 essentially of a circular shallow white- 
 metal cell (c), into which the cylin- 
 drical glass is adapted by a collar (/•:) . 
 The cell is graduated in half its cir- 
 cumference into degrees, commencing 
 at 0,° and extending on each side from 
 0° thi'ough aU intermediate degrees 
 to 90°. The cell is revolvable by a handle (h) at the 
 opposite point of the circle to 0°, around a fixed second brass 
 circle (c') ; this latter is fixed to the stem {8) of the instru- 
 ment, so that it can be held by the hand of the surgeon 
 in front of the patient^s eye (e) . The circle (c') has a notch 
 {n) corresponding to its uppermost point when the instrument 
 is held vertically. In using the instrument, the cell (o) is 
 
 S
 
 ASTIGMATISM. 69 
 
 first brouglit by the handle (A) into such a position that 
 0° corresponds exactly to the notch (v) ; the cylindrical 
 glass is now dropped into the cell (c), so that the axis of the 
 lens corresponds to n and 0°^ and is fixed in this position by 
 the collar (k) which, to avoid any angular displacement of the 
 lens, sUdes — not screws — into c. The instrument being now 
 held vertically before the patient's eye in the same plane aa 
 the cornea, the handle (A) is turned round till the greatest 
 definition of vision is obtained. The inclination of the axis 
 of the lens is now read off by seeing to which of the degrees 
 on c, n corresponds. 
 
 Case IX. — Mi)oi)Ig Astigmatism (Jg-) in the Vertical Meridian, 
 remedied hij Gijlmdrieal Lenses. 
 
 Hamiah M., aged 24, a tailoress, consulted me at the Ophthalmic Hospital, 
 Southwark, in September, 1861, on account of asthenopia, from which she had 
 suffered for the kist ten years. The symptoms were dimness of vision, a 
 sensation of " burning " in the eye-balls, congestion, and lachrymation, all of 
 which came on after readmg, &c., for a quarter of an hour, or after five 
 minutes, if she had previously been hard at work at her business. I found 
 that No. 22 was the highest type she could read at 15 feet distance ; and 
 finding that no glasses, either convex or concave, enabled her to read any 
 lower tyjDe, I, after a month's iueflectual treatment by tonics, eye-waters, rest, 
 &c., gave her case vip in despair. 
 
 On the 13th of February, 1862, I desired her to call on me, with a view of 
 seeing whether, perhaps, her symptoms depended on astigmatism. 
 
 Eepeated examinations yielded the following results with each eye : — 
 
 1. On requesting her to regard a luminous point of two millimetres' 
 diameter, and placmg a 20-inch convex lens before the eye, she, after a time,* 
 declared it appeared to her as a narrow vertical line. 
 
 2. On her regarding a series of black, vertical, and horizontal lines at about 
 8 feet off, she saw the vertical ones perfectly distinctly, the horizontal ones 
 very indistinctly ; a 16-inch spherical concave glass reversed the order of 
 things — the horizontal lines then became distinct, the vertical ones 
 indistmct. 
 
 * In experimenting on my own eyes with cylindrical lenses, I have observed 
 the phenomena of astigmatism of a luminous point never appeared perfect) 
 till a sufficient time was allowed for my accommodation to settle down into 
 one constant condition.
 
 70 OPTICAL DEFECTS OE THE EYE. 
 
 3. With the naked eye, at 12 feet, she could read no lower than XX of 
 Snellen's Types ; but with a cylindrical concave lens of 16 inches' focus, she 
 read XII. Thus, her " sharpness of definition " increased from -J-f (:=§ 
 nearly) to 1 — became, in fact, absolutely perfect. This only occurred, how- 
 ever, when the axis of the cylindrical lens was transversely placed : if 
 vertically, she could hardly see at all. In order to eliminate all source of 
 error, I tried the crucial experiment of placing an ordinary (spherical) 
 16-inch concave glass before the eye : this only made her see worse than 
 with the naked eye. 
 
 I then found that she, with her cylindrical glasses, could also read an 
 ordinary printed book at a usual reading distance without feeling any fatigue 
 in the eyes. 
 
 Both eyes, on careful examination, proved to be equally astigmatic. 
 
 From the above it follows that this patient's eyes possess a normal refrac- 
 tion in their horizontal meridians, but are myopic (-j-V) in the vertical cues, 
 her astigmatism equallmg -rV- 
 
 Case X. — Hypermetropic Astigmatism (Jg) of Left Eye. 
 
 Mary D., set. 49, consulted me, on November 15th, 1862, on account of 
 asthenopic symptoms. After repeated examinations, I found that she was 
 emmetropic in the right eye, but that in the left eye there existed an hyper- 
 metropia of -^ limited to its horizontal meridian, as evidenced by the follow- 
 ing facts : — 
 
 1. Horizontal lines at 8 feet ajjpear to her more distinct than vertical ones, 
 and vice versa with an 18-inch convex. An 18-inch convex, cylindrical, axis 
 vertical, makes both set appear equally distinct. 
 
 2. With an 18-inch convex a luminous point assumes an oval shape, the 
 axis being slightly out of the vertical. 
 
 3. With an 18-inch convex, cylindrical, axis vertical, her visual power (vide 
 Case VIII.) is doubled. She is also presbyopic with either eye to about 
 
 a tV- 
 
 With glasses corresponding to the above facts she reads No. 2 at 11 
 inches, without any asthenopic symptoms. 
 
 Case XI. — Hypermetropic Astigmatism. 
 
 In ISIay, 1863, I received a letter from a British physician in practice in 
 South America, giving a very lucid description of his case, which I transcribe, 
 as far as possible, in his own words : — 
 
 He is 30 years old, and had suffered from childhood from weak sight, and 
 had never been able to read by artificial light anything but the largest print ; 
 but up to ^vithin the last four years could read the Times, including adver- 
 tisements, without glasses. Being obliged to write at night, he began to use
 
 ASTIGMATISM. 71 
 
 slightly convex glasses, which enabled him to see more clearly, but always 
 magnified objects. Finding his sight became weaker, even by day, he gra- 
 dually continued the use of glasses by day, and now cannot read even a 
 leading article in the Times without them, and for not longer than twenty 
 minutes at a time. On repeating the experiments of astigmatism, he obtained 
 the following results : — 
 
 1. A liuninous point was converted into a vertical line by an 8-inch 
 convex lens. 
 
 2. At 5 yards ho saw horizontal lines plainly, vertical ones not at all, 
 merely a grey colour in their place. With a 6-inch convex lens, the defini- 
 tion of the lines was reversed. 
 
 He further on says : — " I had long known this peculiarity of my sight. , . 
 After half-an-hoar's reading I am unable to distinguish m from n, or the 
 Roman number III. from II. ; and looking at our town clock from a distance, 
 I can see the hands when they point to IX. or III., but not when they are 
 
 directed towards XII. or VI AH kinds of stuffs, cloth, &c., when 
 
 striped vertically with black appear to me gTey, unless the lines are far 
 asunder, and then they dazzle me." He then very correctly diagnoses his 
 case thus : — " From these observations I have concluded my refraction is 
 natural in the vertical plane, but presbyopic {should he ^ hypermetrojpic'') in 
 the horizontal." 
 
 Case XII. — Compound Syjpermetro'pic Astigmatism Limited to 
 Left Eye. — Hypermetro^na of Vertical Meridian=yt) of 
 Horizontal=Y- Vide similar case in Donders^ Tract, 
 p. 109. 
 
 James B., set. 13, asthenopic for three years. 
 
 1. Right Eye. — Normal ; reads 19 (Jager) at 20 feet ; glasses produce no 
 efl'ect. 
 
 2. Left Eye. — Reads no type at 20 feet ; with 16 convex 23, and at 3 
 feet horizontal lines much clearer than vertical ones. 
 
 Results after Paralysis of Accommodation by Atropine {(jr. iv. ad. 3J). — 
 Reads at 20 feet no ty|De, with 14 convex 23. Appearances of a luminous 
 point at a few feet : with 8 convex as a line pointing upwards and- inwards 
 '^'~~--^. ; with 14 convex as a line pointing upwards and outwards ^^ — -^ ; 
 with 14 convex spherical and 14 convex cylindrical (axis upwards and inwards) 
 the point appears round. With this last combination at a few feet horizontal 
 and vertical lines appear equally distinctly, and 22 is read weU at 20 feet. 
 With the ophthalmoscope the direct image is seen well without an eye-piece ; 
 the outer edge of the optic entrance is ragged and ill-defined, with a few 
 granular pigment spots. 
 
 In this case I have reason to believe that a third meridian of astigmatism 
 exists, but up to going to press I have not yet had time to investigate it.
 
 72 
 
 OPTICAL DEFECTS OP THE EYE. 
 
 Whilst these sheets were going through the press, my 
 attention was attracted to a very interesting paper on astig- 
 matism, by Javal {Annales d'Octdist'tquc, for January and 
 February, 1865). The following extract contains a new 
 method of determining the existence and amount of as- 
 tigmatism : — 
 
 " 1. Monocular Demonstration. — Draw a circle with radii 15° from each 
 other, and let the patient view this figure through a lens, which, for the sake 
 of precision, I will suppose to be a 3-inch one. Withdraw the figure gra- 
 dually, till all the lines become dim, or disappear, excepting one. Then try- 
 all your cylindrical concaves, one after the other, beginning with the lowest 
 powers, with their axes peri^endicular to the radius, which has remained 
 black, till you have found the glass which makes all the radii equally black. 
 You have then at once diagnosed astigmatism and determined the number 
 
 Fig. 28. 
 
 and position of the correcting glass. . . . For every person with binocular 
 vision the following procedure is much more preferable. 
 
 " 2. Binocular Determination. — We now add before the other eye a 
 lens and a circle, identical with the preceding ones, the cu-cle being first at 
 the focal lengths of the glasses with their centres as far apart as the distance 
 between the two eyes. The circles caimot be fixed, excepting the visual 
 lines are parallel and the head straight. Tlie relative position of the visual 
 lines being a fixed one, this sufliciently giiards against any changes of 
 accommodation. 
 
 That the patient may readily say what he sees, the radii are marked with 
 the same numbers as the hours of a watch, and these numbers not being
 
 ASTIGMATISM. 73 
 
 seen by the same eye as the radii, afford a constant test of binocular 
 vision. 
 
 " M. Nachet has constructed an apparatus realizing the above conditions. 
 By a simple mechanism the circles may be moved forwards or backwards, 
 or the lenses may be decentrated, so as to always face the two images, and 
 in less than a minute the whole series of cylindrical glasses may be submitted 
 to the eye in any desired position "
 
 74 OPTICAL DEFECTS OF THE EYE. 
 
 CHAPTEK VII. 
 
 PEESBYOPIA. 
 
 rpHE normal eye of a youBg subject can generally accommo- 
 -^ date to as close a distance as four inches, and even closer. 
 But, as every one knows, persons in advanced years are, if 
 they do not use glasses, obliged to read, work, &c., at a 
 considerable distance from the eye. In other words, their 
 near-point has receded. If you were to be guided solely by 
 the etymology of the word presbyopia, you might be led to 
 believe this recession of the near-point only occurred in old 
 persons ; but the fact is, that it commences in early Life 
 (accol'ding to Donders, from the tenth year of age), progres- 
 sively increases, but does not come to be felt as a substantial 
 annoyance till after about the fortieth year. For practical 
 purposes it is best to take presbyopia in this limited sense. 
 When shall we, then, say presbyopia commences ? Donders 
 says, when the near-point has receded beyond eight inches 
 from the eye : I think about ten inches would be a more prac- 
 tical standard. However, the chief circumstance I wish 
 you to observe is that the definition is a purely artificial one, 
 founded on the conventional exigencies of hfe. Indeed, pres- 
 byopia might be said to vary with the pursuits of a person ; 
 for, accordingly as these demand a greater or less closeness of 
 vision, will he, or will he not, feel any inconvenience from his 
 defective accommodation. In this sense, for example, a tailor 
 or a student becomes presbyopic at an earlier age than a 
 seaman or a soldier. 
 
 The estimation of the amount of presbyopia in any given 
 case will now claim our attentive consideration, as it afi'ects 
 the three classes of eyes. 
 
 1. Preshyo;pia in Normal Eyes. — Let a person be able to
 
 PRESBYOPIA. 75 
 
 see with his nuked eyes distant objects, but not near ones 
 distinctly any closer than, say, fourteen inches, what amount 
 of accommodation does he possess ? — Answer : -^ {vide 
 supra). 
 
 There is one objection that may be m'ged against this 
 method of estimation, viz., that the convergence of the eyes 
 for fourteen inches is too low to elicit all the energy of 
 accommodation the patient is actually endowed with. We 
 may, however, control our observations by putting the patient 
 on a pair of strong convex glasses — say a 10-inch pair — and 
 then seeing how much nearer he can accommodate than ten 
 inches from the glasses. This was the plan adopted by 
 Macgillaviy ; * as the learned author of an elaborate 
 article in the 57th vol. of the British and Foreign Medico- 
 Ghirurgical Review very justly observes, — " It is advisable not 
 to use too strong glasses, for, owing to the increased size 
 of the image, the circles of dissipation are not so soon 
 perceived.^' 
 
 Accoirjding to our assumed standard near-point of eight 
 inches for the commencement of presbyopia, the minimum 
 amount of accommodation a person must possess not to be 
 presbyopic is \; but in our supposed case he has only yt- There- 
 fore the differ^ice between these two fractions: -§■— -r4=TV 
 (nearly) — expresses the deficit of accommodation — the pres- 
 byopia — the patient labours under. For distinct vision at 
 eight inches he finds himself minus such an amount of accom- 
 modation as is equivalent to a 19-inch convex glass lens.f If 
 we, therefore, artificially supply him with such a lens, we at 
 once correct his presbyopia, always with the proviso that, if he 
 himself at the same time employs all his natural accommoda- 
 tion (y^), he will then be able to read, work, &c., at eight 
 inches. Few persons, however, would be able to endure such 
 a strain on the ciliary muscle for any length of time without 
 fatigue (asthenopia) ; but, on the other hand, few persons 
 
 * Onderzoekingen over de Hoegrootheid der Accommodatie. Utrecht, 
 1858, 
 
 t Convex glasses in presbyopia act in two ways : (1) they increase the 
 divergence of rays, (2) they magnify. We always find patients can read a 
 larger type with them.
 
 76 
 
 OPTICAL DEFECTS OF THE EYE. 
 
 desire to employ their eyes continuously for so close a distance 
 as eight inches. For such convenient distances as 10^ 11, or 
 12 inches, convex glasses of 19 inches focus will answer 
 admirably without overtaxing the patient's natui'al power of 
 accommodation. Thus, e.fj., for reading at 12 inches he will 
 only have to use ^2" accommodation, which is about half of the 
 entire amount he has (-3^2 = t*— 1* ^3 nearly). At 19 inches 
 off, with his presbyopic glasses of xq ^^j lie will, of course, 
 have to entirely suppress his accommodation. Beyond that 
 distance he will not be able to see at all (nor will he require 
 to do so) with his presbyopic glasses on.* If 7= the pres- 
 byopic's natural accommodation, the glass he requires is then 
 given by the formula, i — ^=-1-, where /=the focal length of 
 the requn-ed glass. This result of theory must, however, 
 always be checked by actual clinical observation. As a rule, 
 on account of the convergence of the optic axes, the patient 
 generally prefers a lower glass than the one given by the above 
 formula. I may say, generally, that in prescribing a pair of 
 glasses for a presbyopic, your object should be to reinforce his 
 defective accommodation by convex lenses, neither so strong 
 as to supersede his own remaining natural accommodation, 
 nor so weak as to tax it further than it admits of. The 
 following table from Dr. Kitchener's " Economy of the Eyes " 
 may give a general idea of the glasses required at different 
 periods of life : — 
 
 Years of age. Inches of focus. 
 
 40 36 
 
 45 30 
 
 50 24 
 
 55 20 
 
 58 18 
 
 GO 16 
 
 65 14 
 
 Years of age. Inches of focus. 
 
 70 12 
 
 75 10 
 
 80 9 
 
 85 8 
 
 90 7 
 
 100 6 
 
 Donders gives a similar table of what glasses are required 
 
 * I have, at the risk of being prolix, adduced these various illus- 
 trations of presbyopia in order that the reader, by perceiving their several 
 truths, may satisfy himself that he has thoroughly understood the points 
 at issue.
 
 PRESBYOriA. 
 
 77 
 
 at different ages in emmetropia^ with normal acuteness of 
 vision and accommodation^ for writing, and for reading or- 
 dinary type. 
 
 a 
 Age. 
 
 Glasses requireii. 
 
 d 
 
 Distance 
 
 of distinct 
 vision. 
 
 e 
 
 b 
 In present 
 
 c 
 In original 
 
 E,. 
 
 p„. 
 
 48 
 
 1 
 
 Tt'tt 
 
 14" 
 
 60" 
 
 10" 
 
 50 
 
 tV 
 
 ITT 
 
 14" 
 
 40" 
 
 12" 
 
 55 
 
 Vt^ 
 
 tV 
 
 14" 
 
 30" 
 
 12" 
 
 58 
 
 17 
 
 TJTT 
 
 13" 
 
 22" 
 
 12" 
 
 60 
 
 tV 
 
 tV 
 
 13" 
 
 18" 
 
 12" 
 
 62 
 
 TT 
 
 IT 
 
 13" 
 
 14" 
 
 12" 
 
 65 
 
 iV 
 
 tV 
 
 12" 
 
 13" 
 
 11" 
 
 70 
 
 1 
 
 1 
 7-5 
 
 10" 
 
 10" 
 
 10" 
 
 75 
 
 H 
 
 6^ 
 
 9" 
 
 9" 
 
 9" 
 
 78 
 
 h 
 
 6^ 
 
 8" 
 
 8" 
 
 8" 
 
 80 
 
 T 
 
 4-5 
 
 7" 
 
 7" 
 
 7" 
 
 " Column b gives the glasses required for E (Evimetropia), proved at the 
 moment ; c for E in youth, and therefore for H (Hy2)ermetropia), acquisita 
 at the time of observation ; in both cases the diminished acuteness of vision 
 belongmg to the time of life is taken into account ; d indicates the distance 
 which is preferred for vision with tliese glasses ; e, finally, the space through 
 which they admit of acute vision, that is from B^ with the least, to P^ with 
 the greatest convergence (5-=;^ — ;^)."— Bonders' work on Accommoda- 
 tion and Eefraction, p. 220, 1. 
 
 2. Preshyojna in Myopic Eyes.^ — You will in practice find 
 that all the higher myopic eyes, as a rule, are more or less 
 deficient in accommodation — more or less presbyopic, we might 
 almost say. But presuming we adopt our previous standard of 
 
 * Mr. White Cooper quotes several cases of the kind at p. 101 et scq. 
 of his treatise " On Near Sight, Aged Sight, &c." — a work which, although 
 not up to the knowledge of the present day, may stUl be advantageously 
 consulted on many practical details in the management of cases of defective 
 vision.
 
 78 OPTICAL DEFECTS OF THE EYE. 
 
 presbyopia, it is clear that the term is inapplicable to all degrees 
 of myopia above ^ ; for tlaeir far-point is already less than 8 
 inches, and a fortiori their near-point. But when we come to 
 deal with low degrees of myopia you will find, at a certain age, 
 presbyopia a very usual concomitant. For example, a person^s 
 eye presented this combination — (1) the far-point was 20 
 inches; (2) the near-point 12 inches. On account of (1) the 
 patient is myopic ; of (2) he is also presbyopic. He is short- 
 sighted and long-sighted at the same time. His far-point is 
 too close ; his near point too far. In fact, his range of distinct 
 vision with the unaided eye is comprised in a line of 8 inches. 
 His power of accommodation, j is J^- — ro=^-io-* What glass 
 would the eye require (abstracting oui'selves for a moment 
 from the co-existing myopia), using its entire -^ of accommo- 
 dation to read distinctly at 8 inches ? 
 
 a convex glass of 11 inches focal length, therefore, corrects the 
 presbyopia, but we must also correct the myopia (of a -^) . In 
 
 other words, from jQT^we must subtractgQwhichgives ^^nearly.f 
 
 A convex glass of 24-inches focus is the glass required to 
 correct simultaneously the presbyopia and the myopia. Gene- 
 rally, if J = the patient^ s accommodation, - his myopia, the 
 glass required to neutralize his presbyopia and myopia together 
 is given by the formula i — (r + ^) = ^, F being the focal length 
 
 * The rationale of this formula is as follows : — If the eye were a normal 
 one X would = ^. But we know the eye's refractive power is too great by 
 a quantity=a ^v (convex glass lens). Thus part of the -iV is really due to 
 this -ijV. We must, therefore, subtract this -^ from the -^, in order to esti- 
 mate the amount of active accommodation the patient ho7id fide possesses — 
 indeed, the myopic condition of sucli an eye is by some authors considered to 
 be the result of a certain portion of the patient's accommodation having 
 become, so to say, fixed by partial spasm of the ciliary muscle. Correct the 
 myopia (-jV) by a 20-inch concave glass, and the near-point is no longer at 
 12 inches, but at 30 inches. 
 
 t We may carry this out in practice by ascertaining the near-point of the 
 patient with his reducing concaves on. This, in the above case, is 30 inches. 
 From \ subtract Voj i^nd from the result -^, and the final result will be -jVi 
 a.s before.
 
 PRESBYOPIA. 
 
 79 
 
 in inches of the required convex glass. This patient required 
 a 20-inch concave for distance, a 24-inch convex for near 
 objects. If j+^ is greater than ^, i is negative. In other 
 words, the resulting fraction expresses the power of the 
 concave glass the patient will require for near objects. This, 
 of course, will always be a lower power than the one he uses 
 for distance. 
 
 To take an example. Let j in a given case = -td and ^ = 
 -^,theni- (i + ^) = i-(iV+TV) = i-|-= -^- This patient 
 requires a 14-inch concave for distant objects, a 24-inch con- 
 cave for near ones. 
 
 These results of calculation applied to practice will be mostly- 
 found correct, but must, notwithstanding, always be checked 
 by clinical experience of the case jtself. 
 
 (3) Presbyopia in Hypermetropic Eyes. — A little reflection 
 will show that in this combination the amount of latent hy- 
 permetropia is too small to require being taken into account. 
 Having ascertained the amount of the manifest hypermetropia 
 present, we may, of course, from the position of the near 
 point, readily estimate that of the presbyopia. For example, 
 a person has a hypermetropia of -j-V, but with his 10-inch 
 convex glasses cannot see nearer than 24 inches. To see at 
 8 inches he will require glasses of ^3^4- 4 — t¥= t^ nearly ; with 
 5^-inch convex glasses to read at 8 inches he will have to use 
 all of his natural accommodation (gV)? bnt does not for any 
 distances further than 8 inches. 
 
 What is the proximate cause of presbyopia? It may be 
 due to deficient power of the ciliary muscle or to increased 
 hardness of the lens. It can hardly be called a disease any 
 more than grey hairs or atrophy of the lower jaw. Donders 
 and myself have observed that with advancing age persons may 
 gi'ow not only presbyopic, but at last actually hy|3ermetropic 
 (probably from loss of tonicity of the ciliary muscle), so as to 
 requii-e convex glasses even for distance. Porterfield (vol. ii. 
 p. 69) observed the same, "that long-sighted people at last 
 cannot see at all without the assistance of spectacles.^'
 
 80 
 
 OPTICAL DEFECTS OF THE EYE. 
 
 EFFECT OF BINOCULAR VISION ON THE USE OP GLASSES, 
 
 In all that lias preceded we have regarded each eye separately 
 without taking cognizance of any modifications the application 
 of glasses might require to fulfil any additional circumstances 
 attending the exercise of the two eyes together — binocular 
 vision. This subject has been specially^ and deeply, studied by 
 my valued friend Dr, Giraud-Teulon, of Paris, of whose views 
 I now proceed to give a short abstract. 
 
 Fig. 29. 
 
 Supposing the two presbyopic eyes (in Fig. 29) at their 
 maximum accommodation to have the near-point A', and that 
 the two convex lenses produce virtual images of the point A at 
 the same distance as that of the near-point, then the eye o will 
 perceive an image of A at n, the eye o' one at on ; there will, 
 in a word, be crossed diplopia ; this is, however, overcome by 
 the eyeballs converging to A,hj which means the two images, 
 m and n, are fused at the point A'. The eyes are then con-
 
 PRESBYOPIA. 81 
 
 verged to A, but accommodated for A'. Hence tlie natural 
 harmony of convergence and accommodation is broken. By 
 usingj however^ only the external portions of the two lenses, 
 the two images, m and n, may be fused without any such un- 
 natural convergence of the globes, a physical (prismatic) action 
 being substituted for a physiological. It is evident that to 
 use the internal halves of the lenses would only be constraining 
 a still higher degree of convergence on the eyeballs. Such 
 are the theoretical considerations advanced by Dr. Giraud- 
 Teulon. These he experimentally endeavours to corroborate 
 by the fact that, if we cover the external halves of the lenses 
 of a pair of convex glasses and then regard a near object, 
 crossed double vision is at first produced, and by certain 
 arguments derived from stereoscopic considerations. He 
 hence advises the use of eccentric lenses (lentilles decentrees) 
 in the manufacture of spectacles, by which, he states, no 
 muscular effort being required, vision is rendered materially 
 easier and less fatiguing. 
 
 The same considerations reversed apply to the use of con- 
 cave glasses for myopic patients ; in them the thinner halves 
 of the glasses being mounted inwards, whilst in presbyopic 
 ones they are outwards. For the details of the theory and 
 application of these decentrated lenses, I must refer to Dr. 
 Giraud-Teulon^s own work on binocular vision, p. 394, ei seq. 
 Of the validity of the reasons there adduced for the use of 
 decentrated lenses, I am not yet in a position to offer any 
 definite opinion, as the entire problem still requires an 
 extended clinical experience for its ultimate solution.
 
 82 OPTICAL DEFECTS OF TUE EYE. 
 
 CHAPTER YII I. 
 
 PARALYSIS OP ACCOMMODATION.* 
 
 rpHE prominent diagnostic sign of Paralysis of Accommo- 
 -^ dation is an impairment of vision for near objects, whilst 
 that for distant ones remains, in the majority of cases, com- 
 paratively unaffected. This second clause refers, however, 
 only to loss of accommodation as it occurs in the normal 
 (emmetropic) eye ; for, should the refraction of the affected 
 eye have originally been defective in converging power (hyper- 
 metropic) in its conformation, with a tolerable range of accom- 
 modation, then its vision will commonly be found to be 
 diminished, not only as regards near objects, but also for 
 distant ones. This latter defect will therefore require to be 
 corrected by suitable convex glasses, before we proceed to 
 examine into the amount of accommodation available for 
 near objects. 
 
 I have not yet seen paralysis of accommodation occurring in 
 a myopic patient, but I should imagine that in such a case vision 
 would remain unimpaired for distance, whilst the near-point 
 would be removed further off from the eye. 
 
 Presuming that the eye (if necessary) has been in the first 
 instance restored to a state of normal refraction by suitable 
 glasses, the near-point will be found (taking into consideration 
 the age of the patient) f to be abnormally distant from the 
 vertex of the cornea, but may be approximated as closely as 
 we please by the appropriate convex glasses {Vide Cases XIV., 
 XV., and XVI.). 
 
 * This chapter is a reprint from a paper Mr. Robt. C. Moon, House Surgeon 
 to the Ophthalmic Hospital, Southwark, drew up under my supervision, and 
 published in the first number of the Ophthalmic Review. In it the effect of 
 the Calabar bean in such cases is esj^ecially referred to. 
 
 t For the average diminution in range of accommodation in emmetroi^ic 
 eyes at different ages, see the table given by Professor Bonders in " Ametro- 
 pia en hare Gevolgen," at p. 79, or reproduced in the British and Foreign 
 Medico-Chirurgical Rexieio for January, 1862, p. 27.
 
 PARALYSIS OP ACCOMMODATION. 83 
 
 In addition to the paralysis of the ciHary muscle, the 
 sphincter pupillas often loses its power of contractility, the 
 pupil being then more or less dilated, either perfectly fixed 
 (as in Cases XIII. and XIV.), or acting but sluggishly under 
 the stimulus of light (Case XYI.). Cases XV. and XVII., in 
 which the pupil contracted with the greatest activity, are, 
 however, exceptions to this rule. 
 
 In judging of the action of the Calabar bean in these cases, 
 two factors require distinctly separating,* — viz., 1, the im- 
 provement in vision resulting from a contraction of the pupil 
 (which, by diminishing the size of the circles of dissipation, is 
 in itself capable of accounting for a considerable share of the 
 increase of visual power) ; and 2, that produced by the action 
 of the Calabar bean on the ciliary muscle. In a case of hypei'- 
 metropia, as may be inferred from what has preceded, this 
 remark applies to distant as well as to near objects. Before, 
 therefore, applying the Calabar bean, the effect of minute 
 apertures in increasing the visual power, both for distant and 
 near objects, should be carefully noted, so as to admit of com- 
 parison with the effects subsequently obtained by the Calabar 
 bean. Without this precaution, it is impossible to separate 
 the above-mentioned two factors in its action. 
 
 For the sake of clearness and facility of comparison, the 
 annexed cases are arranged according to the following scheme, 
 which, to some extent, may be of service as a guide to the 
 order of investigating cases in practice : — 
 
 1. History. 
 
 2. Appearances of the eye (including the size of the 
 
 pupil). 
 
 3. Visual power : — 
 
 (A) For distance, before and after atropine,! with the 
 effects of glasses in both cases. 
 
 * In a series of cases reported in the Ophthalmic Hospital Repiorts for 
 October, 1863, these are not separated, neither is the diagnosis of paralysis 
 of accommodation rigidly established, or the state of refraction of the eye 
 accurately given. 
 
 + In order to avoid errors in the examination of cases, it is better to defer 
 the instillation of atropine until the previous results have been obtained with 
 glasses and the Calabar bean. 
 
 *G 2 '
 
 84 OPTICAL DEFECTS OP THE EYE. 
 
 (B) For near objects, before and after atropine^ with 
 tlie effects of glasses in both cases. 
 4. Effects of the Calabar bean : — 
 
 (A) For distance. 
 
 (B) For near objects. 
 
 Giving the comparative experiments made with 
 minute apertures previous to the apphcation of the 
 Calabar bean. 
 
 The report of Case XIII. is abridged from a detailed 
 analysis which has already appeared in the Ophihahmc Hos- 
 jntal Reports for October^ 1863, but is reproduced here for 
 the sake of comparison with Case XIV., and on account of 
 its details being brought down to a later date than in the 
 previous report. 
 
 Case XIII, — Traumatic Paralysis of tJie Circular Fibres of the 
 
 Ciliary Muscle {and Lis), occurring in a case of Hyper - 
 
 metropia {^), corrected {partially) by Convex Glasses, or 
 
 by the Calabar bean. 
 
 Fanny V., set. 28, presented herself at the Ophthalmic Hospital, Southwark, 
 on Angiist 20th, 1863. Eight clays previously, whilst opening a lemonade- 
 bottle, the cork flew out, and struck her on the left eye, causing great pain 
 and dimness of vision, which continued tiU she came to the hospital. When 
 the patient was seen, the pupil was observed to be dilated and fixed ; she 
 could read no less type than No. 16,* and that but indistinctly. The fundus 
 oculi was indistinct, as if from exudation. 
 
 1^ Potass, iodid., gr. ij, ter die. 
 August 24th. — The pain has left her ; she now reads No. 14. 
 ^ Potass, iodid. cum 
 
 Quin. disuljDh., gr. i, ter die. 
 September 1st. — From the patient remarking to-day that her vision was 
 greatly improved by lookmg through a pin-hole in a card, the optical condi- 
 tion of her eye was investigated more closely than had hitherto been done, 
 the case having previously been taken rather for one of simj^le retinitis. 
 
 * p. p. = nearest point of critically distinct vision (punctum proximum). 
 p. r.= furthest point „ „ (punctum remotum). 
 
 '=feet ; "= inches ; '"= lines, 
 -fbefore the number of a glass = convex. 
 — .. .. „ = concave.
 
 PARALYSIS OF ACCOMMODATION. 85 
 
 For the sake of conciseness I shall omit the yarious steps in the examination, 
 and group together the results of the observations made during the three 
 following days. 
 
 The pupil of the left (injured) eye measured 3'" in diameter, and was per- 
 fectly fixed, whilst that of the riglit measured but 2'" in the same light, and 
 contracted with the greatest activity. There was no evidence of any dislo- 
 cation of the lens, either in a tremulous condition of the iris or in a direct 
 objective examination. The ophthalmoscope disclosed a hyjaermetropic re- 
 fraction of the eye. It seemed probable that some injury to the iris and 
 ciliary muscle was the real nature of the case. 
 
 With the left eye at 20' she read No. 22 imperfectly, but tlu'ough an 
 aperture of ^"' diameter she could read No. 20, or with -|-16 alone No. 18 
 well Finding that with +16 she could distinguish a much smaller type 
 than with an aperture of ^" diameter, it was clear that the defect in vision 
 was rather dependent on some deficiency in the refractive power of the eye 
 than simply on the dilatation of the pupil. 
 
 At first, the right eye appeared to be emmetropic, as at 20' she could read 
 No. 18 with ease. But finding that low convex glasses did not deteriorate 
 vision for distance, and that the near-point of this (right) eye was unusually 
 distant, viz., 8^",* we were induced to paralyse her power of accommodation 
 in this eye ; after which it was found that she could not read even No. 23, 
 but with +16 she read No. 18 just as well as before any atropine had been 
 applied, the direct image being also clearly seen m this, as it was in the left 
 eye without an eye-piece. 
 
 With the injured eye she read No. 18 at about 13", but with +16 she read 
 No. 8 at the same distance. A square of the Calabar bean paper was now 
 applied to this eye. In ten minutes the pupil was contracted to a diameter of 
 about V" (a little larger) ; she now read at 20' No. 20 (which was a number 
 lower than she could read through an aperture of 1'" diameter jDrevious to 
 the application of the Calabar bean), and with +36 No. 19 well, her near- 
 point for No. 10 being 18". Although the pupil continued to contract till at 
 last it was only -g'" in diameter (where it remained fixed), no further 
 improvement m vision ensued for distance, but she could now read No. 8 at 
 4", and with +24 No. 2 at the same distance. This fact, combined with 
 the previous observation made with the stenopseic hole, proved that the Calabar 
 bean improved the patient's vision, not alone by its action on the pupd, but 
 in a far greater degree by that on the ciliary muscle."]" 
 
 The nature of the case was now clearly made out — viz., that it was one of 
 a tolerably high degree of hypermetroiDia of both eyes, and that the defect of 
 vision noticed m the left eye was due to a paralysis of the ciliary muscle, 
 
 * Erroneously printed as 3-^" in the Ophthalmic Hospital Ecports. 
 
 -f" The necessity of employing a convex glass for attaining the maximum 
 degree of acuteness of vision possible, depended probably on the ciliary 
 muscle being too much injured to acbnit of perfect contraction even under 
 a stimulus so powerful as that of the Calabar bean.
 
 86 OPTICAL DEFECTS OP THE EYE. 
 
 which prevented the patient correcting her hypermetropia by voluntary 
 accommodation, but which was entirely supplemented by convex glasses, or 
 restored to a great extent by the Calabar bean. 
 
 When last seen, on February 17th, 1864, her visual power was found to 
 have become considerably improved. From September 5th, 1863, she had 
 applied the solution of the Calabar bean regularly twice a day to the eye, 
 but had discontinued its use for the last five days. Both pupils now acted 
 readily in a full light ; the right measuring 1'", the left 1^'" in diameter. 
 The visual power of the right eye was as when she first came to the hospital. 
 
 February 17th, 1864.— Left %e:— Pupil =1|"' in diameter. 
 
 At 20' reads No. 18, and somewhat more perfectly with +24 
 p.p. for No. 8 = 8i". 
 
 „ No. 2 with +16 = 8|". 
 „ No. 1 with +8=4". 
 11.30 A.M. Calabar bean applied to left eye. 
 11.50 „ Pupil measures 1'". 
 p.p. for No. 1 = 12". 
 12. Pupil =1'". 
 
 p.p. for No. 1 = 7^". 
 12.20 P.M. Pupil =i"'. 
 
 p.p. for No. 1 = 8". 
 
 At 20' reads No. 18 perfectly. 
 
 From what precedes it is doubtful whether the Calabar bean improved 
 the vision of the left eye by its action on the iris or that on the ciliary 
 muscle. That part of her accommodation, however, which she, previous to 
 the accident, used to correct her hypermetropia of this eye appeared to have 
 become all but restored. 
 
 Case XIV. — Traumatic Paralysis of the Ciliary Muscle 
 and Iris. 
 
 Under the Care op Mr. Laurence. 
 
 Albert H., Pct. 28, presented himself at the Ophthalmic Hospital, South- 
 wark, on August 28th, with acute corneitis and haemorrhage into the anterior 
 chamber of the right eye, the results of a blow from a piece of putty. 
 Under treatment, all the inflammatory symptoms subsided, the blood in the 
 anterior chamber disappeared, and the cornea became quite transparent ; 
 but the pupil, on the 17th of September, was foimd to be widely dilated and 
 fixed, measuring 3'" in diameter, whilst that of the left (uninjured) eye 
 measured but 1"', and contracted briskly under the stimulus of light. There 
 was no evidence of a dislocation of the lens discoverable, either from a 
 tremulous iris, or from a dii-ect objective examination. The patient stated
 
 PARALYSIS OF ACCOMMODATION. 
 
 87 
 
 that previous to the accident, he had never observed the slightest difference 
 in the sight of the two eyes. 
 
 Visual Power : — 
 For distance : — 
 
 Eight (injured) eye, at 20' reads No. 23. 
 
 With +18, „ „ No. 18. 
 
 Left (soimd) eye, „ „ No. 18. 
 
 +40 „ miproves for No. 18. 
 
 For near objects : — 
 
 Right eye reads No. 10 at 8". 
 
 With +4 reads with ease No. 1 at 4". 
 „ +6 „ „ No. 1 at 6".* 
 
 Left eye p.p. for No. 1 = 4". 
 
 The ophthalmoscope disclosed a hypermetropic refraction of the right eye, 
 and rather less so of the left ; but there appeared to be nothing abnormal 
 in either fundus. 
 
 On September 18th, a square of the Calabar bean paper was applied to the 
 right eye, and a strong solution of atropine (grs. viii ad 3J) to the left. 
 In about an hour afterwards the right pupil had contracted to 2'", and the 
 left had dilated to 3"' in diameter, when, with the right eye, he was able to 
 read No. 21 at 20' and No. 2 at 8". With the left eye he could only read 
 No. 20 at 20' and required +40 to read No. 18 well. The first piece of 
 Calabar bean paper was now removed from the right eye, and a second 
 applied : but although the pupil of the right remained fixed at a diameter 
 of 2'" for about two hours, the vision continued to improve, till he was 
 able to read No. 19 distinctly at 20', and No. 1 at 4", whereas, with an 
 aperture of 2'" diameter, previous to the application of the Calabar bean, 
 he could read no lower a number than No. 22. 
 
 This case is very similar to the last, in being one of traumatic paralysis of 
 accommodation, which could either be substituted by convex glasses, or, for 
 the time, completely restored by the Calabar bean. The above results show 
 that the contraction of the pupil after the application of the Calabar bean 
 assisted considerably in improving vision ; but from the fact that he con- 
 tinued to read smaller types, although the pupil remained stationary, it is 
 evident that a still greater benefit was attributable to its action on the ciliary 
 muscle, which continued to be acted on by the Calabar bean after this had 
 ceased to affect the iris. 
 
 * It might be objected that the convex glasses act simply by their magni- 
 fying power, but that this is not in any sense adequate to account for the 
 improvement in vision will be seen from the foUowLng .table : — 
 
 To an emmetropic eye — 
 
 +4 appears to magnify No. 1 only to about No. 5. 
 +6 „ „ „ No. 3. 
 
 +8 „ „ „ No. 2.
 
 OC5 OPTICAL DEFECTS OP THE EYE. 
 
 Case XV. — Paralysis of the Ciliary Muscle following 
 sufpressed Scarlatina. 
 
 Under the Caee of Me. Laurence. 
 
 Aniy M., tet. 7, a most inteUiCTent child, was admitted on December Uth, 
 1863, as an out-patient at the Ophthalmic Hospital, South wark, complaining 
 of " dimness of sight." About six weeks previous to her admission she was 
 suifering from suppressed scarlatina, and had lately experienced severe 
 mental distress from the loss of a sister. On December 1st she complained 
 of a " pricking " in the right eyelid, followed by impaia-ment of vision for 
 near objects, that for distance remaining imaffected. In about a week after- 
 w^ards, the same defect occurred in the left eye, and subsequently she com- 
 plained of double vision. Each pupil measured 1'" in diameter, and con- 
 tracted with the greatest activity in full daylight. There was a very slight 
 insufficiency of the tntemal recti muscles, otherwise the single and combined 
 movements of the eyeballs were perfect. 
 
 Visual Power : — 
 For distance : — 
 
 Eight eye at 17' reads No. 20. 
 
 With -)-40 „ „ somewhat more clearly. 
 Left eye „ „ imperfectly. 
 
 With -f 40 „ „ • perfectly. 
 For near objects :■ — 
 
 With either eye reads No. 14 at 8|".* 
 
 (No. 14 is the smallest type she can decipher.) 
 With 4-4 reads No. 1 at 4". 
 j> +6 „ „ 6". 
 
 5) +8 „ „ 8". 
 
 At 11.35 a.m., a square of Calabar bean paper was applied to the right eye, 
 and atropine (grs. iv ad 3J) to the left. At 12.5, the Calabar bean paper was 
 removed, the right pupU having contracted to |"', and the left dUated to 2"' 
 in diameter. 
 
 VistJAL Power : — 
 For distance : — 
 
 Right eye at 20' reads No. 20. No glass appears to improve 
 
 vision. 
 Left eye at 20' reads No. 21. She now requires -|-24 to read 
 No. 20. 
 For near objects : — 
 
 Right eye p.p. for No. 12 = 2f . 
 Left eye „ „ No. 16 = 6". 
 
 * According to Donders, the near-point in the emmetropic eye at 7 years 
 is about 2^ inches (o|?. supr. cit).
 
 PARALYSIS OF ACCOMMODATION. 89 
 
 A stenopaeic hole of a smaller diameter than her pupil, applied previous to 
 the Calabar bean, did not in the least improve the vision of either eye. 
 
 From the above observations, coupled with the fact that the direct image 
 could be fairly seen without an eye-piece, it was inferred that this patient 
 had also been originally hyjjermetropic, and that she had partially lost the 
 power of voluntarily accommodating the eye for distant as well as for near 
 objects, which was, however, completely substituted by convex glasses, and 
 to some extent temporarily restored by the Calabar bean. The-mother was 
 directed to apply the Calabar bean extract to the child's eyes every morning ; 
 but about five weeks elapsed before she again made her appearance at the 
 hospital. It was then found that the power of accommodation had been 
 spontaneously and completely restored, and that she could now (January 
 26th, 1864) read No. 19 at 20', and No. 1 at 4", with either eye. This case 
 is instructive in showing how careful one should be in attributing effects 
 to therapeutic agents, as upon inquiry it was found that the Calabar bean 
 had not been applied at all to the patient's eyes since her last visit to the 
 hospital, and she had merely been staying for a short time in the coimtry, 
 where her health and sight had gradually improved. 
 
 Case XVI. — Complete Paralysis of the Cilianj Muscle and 
 partially of the Iris ; fi'om Debility. 
 
 Under the Care op Mr. Holthouse. 
 
 Elizabeth R., a very intelligent child, ?et. 1 1 years, came to the out-patients' 
 room of the Oi^hthalmic Hospital, Southwark, on Febnxary 4th, 1864. For 
 two or three weeks previously she had been suflerLng from dyspepsia and 
 general debUity, and during the last few days had complained of being 
 unable to read small print, or to distinguish distant objects clearly. Both 
 pupils measm-ed 3'" in diameter, and acted but sluggishly under the stimulus 
 of light. 
 
 Visual Power :— 
 For distance : — 
 
 With either eye at 20' reads No. 22. 
 
 With +30 „ „ No. 19. 
 
 For near objects : — 
 
 With either eye, the nearest distance she reads No. 16 = 7". 
 
 With +30 „ „ No. 12 = 9". 
 
 In addition to +30 (the neutralizing glass for her liypermetropia) she 
 requires 
 
 +4 to read No. 1 at about 4". 
 +6 „ „ 6". 
 
 +8 „ „ 8".* 
 
 * That these convex glasses did not improve simply by their magnifying 
 power, vide note to Case XIV.
 
 90 OPTICAL DEFECTS OP THE EYE. 
 
 But with +30 alone, she reads No. 12 (the least tj^e she can decipher) 
 at 9".* 
 
 After the instillation of a strong solution of atropine (grs. viij ad 3J) on 
 the following day, the visual power remained exactly the same as when she 
 came to the hospital, showing that the paralysis of the ciliary muscle was as 
 complete as it could possibly be. The refraction of the eye was hyper- 
 metropic : a narrow pigment line was present to the inner side of each optic 
 nerve entrance. 
 
 At 4.7 P.M., the Calabar bean extract was applied to both eyes, which were 
 • in the same condition as above noted. 
 
 4.37. Both pupils measure I'" in diameter. 
 
 At 20' she reads No. 19 ) ^jth either eye. 
 p.p. for No. 1 = 3" ) 
 
 With a stenopseic aperture of I'" diameter, previous to the application of 
 the Calabar bean, she could read No. 20 at 20', and No. 6 at 3". 
 
 4.47. Both pupils contracted to \"' in diameter, but the visual power 
 remains the same as at 4.37. 
 
 5.17. Both pupils measure |"' in diameter. 
 
 At 20' she reads No. 19 ^ 
 
 (No convex or concave glasses improve) > with either eye. 
 
 p.p. for No. 1=2" ) 
 
 With a stenopseic hole of ^"' diameter, previous to the application of the 
 Calabar bean, she read No. 20 at 20' and No. 1 at 3", which shows that, in 
 this case, the contraction of the pupU. was a very important element in the 
 improvement of vision both for near and distant objects. 
 51 Haust Quinte cum Ferro, 5J ter die. 
 
 When last seen, on February 1 5th, both pupils measured 3 j'" in diameter. 
 With either eye at 20' she read No. 19, and No. 2 at 11". With +30 she 
 read No. 1 at 8". 
 
 Case XVII. — Complete Paralysis of the Ciliary Muscle in a 
 
 case of Hypermetropia {-io) J yrohahly front debility. 
 
 Under the Care of Mr. Holthouse. 
 
 Eliza P., set. 30, came to the Ophthalmic Hospital, Southwark, on Feb- 
 ruary 1st, 1864, in a very feeble condition, and complaining that she was 
 unable to see near objects distinctly. For the last two years, when reading 
 a book, she had been compelled to hold it at a considerable distance off for 
 distinct vision, especially at night, when she was in the habit of placing the 
 candle between herself and the book. During the last eleven or twelve 
 
 * According to Bonders, the near-point in the emmetropic eye at 1 1 years 
 of age is about 2f inches ipi). supr. cit.) ; in our case, after correction of the 
 hypermetropia, it is seen to be three times as distant.
 
 PARALYSIS OP ACCOMMODATION. 91 
 
 months these symptoms have become greatly aggravated, and of late, after 
 reading or sewing for a short time, the eyes begin to " itch," and feel irri- 
 talile. Both pupils measure 2f" in diameter, and contract readily under the 
 stimulus of light. 
 
 Visual Power : — 
 For distance : — 
 
 With either eye at 20' she reads No. 19^ 
 -|-10 enables her to see No. 19 a little more clearly. 
 For near objects : — 
 
 With either eye reads No. 2 from 12" to 14". 
 With +40 (the neutralizing glass for her hypermetropia) reads 
 No. 1 at 10".* 
 
 But with +40 alone she cannot read either No. 2, No. 4, or No. 6 at any 
 distance less than 10" from the eye.t With the ophthalmoscoj)e nothmg 
 abnormal was discoverable in either fundus oculi. 
 
 At 4.20 P.M., the Calabar bean extract was applied to the eyes. 
 4.40. Both pupUs have contracted to 1'" diameter, 
 p.p. for No. 1 = 6". Visual power for distance not tested. 
 With a stenoppeic hole of 1'" diameter, previous to the application of 
 the Calabar bean, the nearest and only point of distmct vision for No. 1 
 was 10", from which it is evident that the mere contraction of the pupil 
 was not adequate to explain the great improvement in the visual power for 
 near objects. 
 
 4.45. Both pupils are ^'" diameter. At 20' she is now unable with either 
 eye to read any type less than No. 22, and requires 14-inch concave to 
 read No. 20. All convex glasses deteriorate vision, and no concave glass 
 enables her to read a less type than No. 20 ; p.p. for No. 1 = 5". 
 
 With an aperture of ^"', previous to the application of the Calabar bean, 
 she read No. 19 at 20' just as well as without it, and she read No. 1 in a 
 range of 5^" to 6^". 
 
 The last observation made at 4.45, talcen in conjunction with the previous 
 comparative experiment of a stenopoeic aperture, made it evident that the 
 diminution in the power of vision could not be referred to the extremely 
 contracted state of the pupil, but to the stimulating efl'ect of the Calabar 
 beau on the ciliaiy muscle, which was so great as to make the patient for the 
 time, from having been previously hjq^ermetropic, actually myopic, and that 
 in a very marked degree. 
 
 I have lately noticed the same effect produced in another case of hyper- 
 metropia with paralysis of accommodation, in which the hypermetrojiia 
 (-J^) became converted into a myopia of xV by the ai^plication of the ordeal 
 bean. 
 
 * The natural near-point for 30 years of age is about b^". 
 
 t After the full effect of instillating a strong solution of atropine had been 
 obtained, the visual power was found to remain in precisely the same condi- 
 tion as previous to its application.
 
 92 OPTICAL DEFECTS OF THE EYE. 
 
 CHAPTER IX. 
 
 ASTHENOPIA. 
 
 ASTHENOPIA^ or weak sight, is succinctly defined by- 
 Mackenzie as " that state of vision in which the eyes are 
 unable to sustain continued exercise upon near objects, although 
 the patient, on first viewing such objects, generally sees them 
 distinctly, can employ his sight for any length of time in 
 viewing distant objects, and presents no external appearance 
 of disease of the eyes.''^ Travers described the symptoms of 
 asthenopia as one of his varieties of '' functional amaurosis,^^ 
 and considered loss of adjusting power as an " occasional symp- 
 tom'^ ('^Synopsis of the Diseases of the Eye,'' 2nd ed., 1821, 
 p. 187). The above description is sufficiently accurate to give 
 you a general idea of the disease we are considering ; but it will 
 be as well to follow out the symptoms into a little further detail. 
 These are all the symptoms of muscular fatigue and nervous 
 irritation. At first, as the word " fatigue " itself implies, they 
 are not felt, but come on with a rapidity which may vary from 
 a few seconds to an hour or more. Some patients cannot 
 read two hues 'consecutively without an accession of asthenopic 
 symptoms. They declare the letters " run into one another, 
 and become misty and confused ; ■" if they persist, the eyes 
 ache and water till they are obliged to leave off from sheer 
 pain and discomfort.* A little rest generally enables them 
 to begin again, but I have met with a case in which the pain 
 lasted for many hours after. In this same (and a second) 
 case there existed an indescribable genei'al feeling of nervous- 
 ness and agitation, a sensation of nausea and vertigo, although 
 
 * The mLstiness of vision that ensues is probably partially a reflex pheno- 
 menon, as almost certainly the aching, injection (sometimes observed), and 
 suffusion are evidences of reflex-irritation of the fifth and sympathetic 
 nerves.
 
 ASTHENOPIA. 93 
 
 the two patients in themselves were anything but nervous or 
 hysterical females. 
 
 The first of the above two cases is suflBciently remarkable to 
 deserve a more extended notice here. 
 
 Case XVIII. 
 
 Marcella D. was 43 years old when, she first consulted me. From child- 
 hood she had suflFered from asthenopic symptoms ; but for the last eight or 
 nine years they had so increased in intensity as to utterly incapacitate her 
 for all occupations that demanded close work of any description. Indeed, 
 what was remarkable, she could not regard any object, whether close or ' 
 distant, with any degree of attention without experiencing the most distress- 
 ing sensations in the eyes. She had consulted, without avail, no less than 
 sixteen or seventeen oculists. I found, on examination, that at fourteen 
 feet she could not, with the naked eyes, read No. 22 with certainty ; but 
 with 14-inch convexes read No. 19 well. Repeated examinations yielded as 
 their result that she was hypermetropic (manifestly) to a -P^, and also slightly 
 presbyopic, so as to require 10-inch convexes for reading. With these 
 glasses she was enabled to read from four to five hours at a stretch without 
 experiencing any of her old asthenopic symptoms. Subsequently her mani- 
 fest hypermetropia increased to a -^, and she then required 7-inch convexes 
 for reading. Latterly she became again slightly asthenopic, owing to insuffi- 
 ciency of the internal recti, for which I recommended division of the muscles, 
 to which, however, I could not get her to consent, she being satisfied with the 
 benefit she got from her convex glasses. 
 
 Stellwag V. Carion speaks of " cases of a high degree," .... 
 .... in which " the hypereesthesia of the retina and ciliary 
 nerves renders even indefinite vision at distant objects with 
 high illumination, contrasts of Hght^ glaring colours, or strong 
 reflexions, insupportable." 
 
 A little reflection will convince you how serious a disease 
 we are considering, especially when you are dealing with 
 poor patients, such as that humble section of the female com- 
 munity who gain their livelihood by the '' needle," — 
 
 " With fingers weary and worn, 
 With eyelids heavy and red." 
 
 Mackenzie very properly remarks : '' It is an infirmity much 
 more to be dreaded than the many disorders of the eye which 
 to superficial observation present a far more formidable 
 appearance." And yet there is perhaps no one disease
 
 94 
 
 OPTICAL DEFECTS OF THE EYE. 
 
 the true nature of whicli is more generally misunderstood. 
 Mackenzie, after enumerating a great variety of causes, comes 
 nearest to by far the most common one when he suggests 
 
 that " the organs of adjustment are probably the chief 
 
 seat of the complaint.^' Donders, in calling attention to its 
 very frequent dependence on a hypermetropic condition of 
 the eye, has undoubtedly rendered a great service to practical 
 ophthalmology; but at the same time my experience has 
 taught me how greatly we should err, were we to exclusively 
 assign it to this cause. 
 
 I beheve the causes of asthenopia may, generally speaking, 
 be grouped under the following three heads : — * 
 
 I. Optical Defects of the Eye. 
 
 II. Deficient Poiver of the Internal Recti Muscles. 
 
 III. Hypercesthesia of the Retina. 
 
 I. Optical Defects of the Eye. 
 
 I must, in limine, remark that most persons we are now 
 speaking of apply to the surgeons solely for their asthenopia ; 
 they never suspect the least wi'ong with their sight ; on the 
 contrary, they declare they see remarkably well generally, and 
 that it is only when they try to read or work any length of 
 time they feel their eyes ^' weak." 
 
 The most frequent optical defect is a hypei'metropic con- 
 dition of the eye.. Part of the accommodation of such an eye 
 is expended in neutrahzing its defective refraction, and, as a 
 consequence, when the patient employs his eyes on near 
 objects, he unnaturally '^ forces " his accommodation, so that 
 the ciliary muscle is wrought up to a degree of tension that 
 soon obUges it to give in, and thus are caused all those symp- 
 toms of muscular and nervous irritation and fatigue, and 
 consequent indistinctness of vision, which we have already 
 described. You may for a few minutes sustain a heavier 
 
 * I find that the same classification has been independently adopted by 
 Stellwag von Carion (" Lehrbuch der Augenheilkimde." Vienna, 1861, 
 p. 658). He recognises two species of asthenopia : — 1, an accommodative ; 
 2, a muscular ; and with both "a hypersesthesia of the retina and ciliary 
 nerves in, close pathogenetic relation therewith."
 
 ASTHENOPIA. 95 
 
 weight than your muscles will naturally support; but try to 
 prolong the effort, and you will soon experience that sense of 
 inability and fatigue which the ciliary muscle feels in another 
 way. 
 
 Case XIX. 
 
 Robert W. W., set. 15, consulted me, on November 15th, 1862, on account 
 of asthenopic symptoms. He had been using 12-inch convex glasses for 
 about three years and a half ; but they no longer helped him. I found that 
 at 20 feet he read with the right eye No. 21, and with -\-9 No. 20, to read 
 which, after instillation of atropine, he required +7 ; with the left eye he 
 read at 20 feet No. 23, with -|-6 No. 22, to read which, after atropine, he 
 required +5. I prescribed a 9-inch convex glass for his right eye and a 
 6-inch one for his left eye. I saw him about a year afterwards, when he 
 informed me the glasses had completely cured his asthenojiia. 
 
 But I have also met several cases of asthenopia which were 
 due to myopia. This may exist in a very slight degree. A 
 woman, aged 33, applied to me for intense asthenopia. She 
 was cured, and that there and then, by the use of a p&ir of 
 40-inch concave glasses. Within a very brief period two ladies 
 consulted me for asthenopic symptoms, which I found depended 
 on presbyopia, and were remedied by suitable convex glasses. 
 Mr. White Cooper has (op. cit., p. 123) remarked the same 
 thing, and at p. 92 quotes a peculiarly acute case that occurred 
 in his own practice. In short, any optical defect of the eye 
 attended with limited power of accommodation may be the 
 cause of asthenopia, and therefore it is always absolutely neces- 
 sary in these cases to subject the eyes to a rigid optical 
 examination. 
 
 I may here attract attention to a very important class of 
 cases which I believe have hitherto escaped attention. I have 
 met with asthenopia between the ages of about 20 and 35 
 where no defect of refraction was by the most delicate tests 
 detectable, nor did the accommodation at first seem hardly 
 impaired. Yet in these cases the patient was at once relieved 
 of all asthenopic symptoms by very low-powered convex glasses 
 (4V to ■—). Such cases are, I believe, true instances of 
 " premature presbyopia." With these same glasses distant 
 objects were misty and confused.
 
 96 OPTICAL DEFECTS OP THE EYE. 
 
 II. Deficient Power of the Internal Recti Muscles. 
 
 This is tlie disease wliicli the German authors characterize 
 as " insufficiency " of the internal recti. You will find an 
 elaborate description of it at p. 242 of Alfred Griife^s " Moti- 
 litilts-Storungen des Auges/' a work which^ embodying all 
 the profound researches of his illustrious teacher. Professor v. 
 Grafe, will amply repay the serious study of those who wish 
 to have any clear idea of the action and derangements of the 
 muscles of the human eye. 
 
 Weakness of the internal recti may effect one or both 
 muscles — most commonly both. Such a case is detected by 
 observing that when we, covering one eye with the hand, 
 direct the patient to regai'd a near object with the other, the 
 covered eye deviates outwards — in a word, the patient has, 
 under these circumstances, a divergent squint of that eye. As 
 soon as we remove our hand, the divergent eye at once turns 
 inwards, and resumes its symmetrical position to the other 
 eye.* There are many cases of this infii'mity, which would 
 be, and constantly are, overlooked, from neglecting this simple 
 
 * In the first case the eye, excluded from common vision with its fellow- 
 eye, is at liberty to assume that position indicated by the relative strengths 
 of the two antagonist muscles, the iutemal and external recti. In the second 
 case the participation in the act of vision forces the weak internal rectus to 
 prematurely exert itself, to avoid the crossed double images that would 
 necessarily result from any divergence of its eye. In this way an eye affected 
 with weakness of the internal rectus assumes a divergence, if the patient is 
 in reverie, if the affected eye becomes the subject of cataract, &c. In this 
 latter case a successful operation for cataract may at the same time restore 
 the symmetry of the eyeball. The test we have just been discussing becomes 
 still more evident if the object be held, not directly on a level with, but 
 above that of the eyebaUs. We inay further add, in reference to this test, 
 that the fusion of the double images produced by holding a prism with its 
 base inwards before one eye, by the compensating action of the external 
 rectus, becomes easier the nearer the object, and vice versa ; so that we can 
 overcome the " doubling " action of stronger prisms for near objects than for 
 distant ones. If the prism Is held with its base outwards, the very reverse 
 of the above holds good, tiU the object is placed at, or within, the near- 
 point, when the internal recti appear to be the stronger. In weakness of 
 these muscles the preponderance of abduction for the usual distance of read- 
 ing, writing, &c., is, as we have seen, well marked, and, if the weakness be at 
 all excessive, even for greater distances.
 
 ASTHENOPIA. 97 
 
 method of examination. Again^ if we before a healthy eye 
 place a prism with the base outwards or inwards^ the person 
 at first sees double ; but, if the angle of the prism is not too 
 great, he soon, in order to overcome the consequent double vision, 
 respectively squints inwards or outwards with the eye before 
 which the prism is held. In cases of weakness of the internal 
 recti, you will find that this muscular power of neutralizing 
 diplopia by convergence of the eye is diminished, that by 
 divergence increased; so that, e.g., in a case mentioned by 
 Alfred Grafe, " the patient possessed the power of overcoming 
 prisms up to 18°, with the base turned inward before the left 
 eye by a corresponding degree of strabismus divergens. 
 Prisms of 2° and 4°, with the base placed outwards, he only 
 momentarily overcame by an opposite (convergent) squint." 
 If a prism with its refracting angle upwards be held before 
 one eye, any object (such as a dot of ink on a piece of paper) 
 seen with both eyes together appears double, the image through 
 the prism being directly above that seen with the other (naked) 
 eye, thus : 
 
 • The faculty of single vision with two eyes is thus 
 , almost insuperably annihilated, on account of the very 
 slight power we possess of isolating the action of either the 
 superior or inferior recti muscles, so as to fuse the double 
 images (as is readily done, as we have mentioned above, by 
 the lateral ocular muscles). Under these circumstances, the 
 accommodative action of the internal recti being no longer 
 called into play, each eyeball assumes that position indicated 
 simply by the relative strength of the internal recti. Should 
 this be normal, the dots will assume the position delineated 
 above ; but should either, or both, internal recti be preter- 
 naturally weak, the eyeball, or balls, will, from the preponde- 
 rating strength of the external rectus, pass into a state of 
 divergent squint, and thus crossed double images arise. The 
 final result is that the two dots exhibit in such a case not only 
 a difierence of height, but, in addition, a lateral crossed sepa- 
 ration. The images then appear thus : 
 
 " A second prism held with its angle outwards before 
 
 , the second eye destroys this lateral separation, leaving 
 
 the difierence of height unchanged. The strength {— number
 
 98 OPTICAL DEFECTS OP THE EYE, 
 
 of degrees) of the prism necessary for tins purpose affords a 
 direct measure of the degree of weakness of the internal rectus. 
 
 It has been thought weakness if the recti interni is confined 
 to myopic patients. I beheve it is by far most frequently met 
 with in such cases, especially if the myopia is high, in which, 
 ultimately, you will very commonly find a confirmed divergent 
 squint and consequent amblyopia. 
 
 But I have met with a case of hypermetropia of -^, in 
 which the most intense asthenopia was caused by weakness of 
 the internal recti, with a second of almost equal intensity in 
 a hypermetropia of ^i^, and several other less severe cases. 
 I may here remark that, owing to the connexion between 
 accommodation and convergence of the optic axes, two causes, 
 acting and reacting upon each other, are at play in producing 
 the asthenopic symptoms that attend weakness of the internal 
 recti. 
 
 Asthenopia from this cause, occurring in a myopic patient, 
 may be sometimes relieved by the use of glasses, which enable 
 him to read at a sufiicient distance from the eyes to moderate 
 their convergence to a degree that entails but a very slight 
 action of the affected muscles ; or by the systematic applica- 
 tion of prismatic glasses with the bases turned outwards, the 
 internal recti may be gradually strengthened in the effort they 
 will instinctively make by converging the optic axes to over- 
 come the crossed double vision the prisms would otherwise 
 cause. Prismatic glasses with the bases turned inwards have 
 no effect in radically curing the disease — on the contrary, 
 rather encom'age it, but render vision more distinct for the 
 time and relieve the asthenopia by supplementing the action 
 of the internal recti. Thus, in a case of this kind I found the 
 patient, who had a myopia of ^, read a type of about No. 6 
 easily with a prism of 11°, the base inwards applied to the 
 ' right eye, although she could hardly read the type at all with 
 the unaided eyes. The last resource is total or partial division 
 of the external rectus muscle. 
 
 III. Hypercesthesia of the Retina. 
 
 Besides the above cases of asthenopia, we every now and 
 then meet with ones which are attended by a general low state
 
 ASTHENOPIA. ■ 99 
 
 of the system, often associated with photophobia. These gene- 
 rally get well under the use of tonic medicines and regimen, 
 and therefore may not unreasonably be ascribed to hyper- 
 eesthesia of the retina, or perhaps even they may depend upon 
 some temporary asthenic state of the organs of adjustment. 
 The asthenopia often met with after exhaustive fevers, diph- 
 theria, &c., probably belongs to this category. 
 
 n 2
 
 100 OPl^ICAL DEFECTS OP THE EYE. 
 
 CHAPTER X. 
 
 THE CONNEXION BETWEEN CONVERGENT STKABISMUS AND 
 HYPERMETROPIA. 
 
 rpHE terms 'Hotal/^ ^^ manifest/^ and "latent" hyperme- 
 -^ tropia have already been explained (p. 57). Donders 
 recognizes three foi^ms of manifest hypermetropia dependent 
 on the amount of accommodation the eye possesses. 
 
 (a) Absolute manifest hypermetropia : Let the patient 
 accommodate as much as ever he can with a maximum 
 convergence of the optic axes, he still fails to neutralize all 
 his hypermetropia; a portion of it still remains uncorrected 
 by any natural effort at his command. Distinct vision is 
 impossible under any circumstances without the aid of convex 
 glasses. 
 
 (h) Relative manifest hypermetropia : The patient possesses 
 sufficient accommodating power to neutralize his manifest 
 hypermetropia, but can only avail himself of it on one con- 
 dition, viz., that he converges the eyes to a nearer point than 
 that for which he accommodates them. He is only able to 
 elicit sufficient accommodation to neutralize his hypermetropia 
 for any point by converging his eyes to a nearer one than 
 that at which the point in question is really situated. And 
 this is what such a patient may actually do — an additional 
 proof that accommodation and convergence are not indisso- 
 lubly connected acts. The patient, unaided by glasses, would 
 be thus theoretically placed in this unfortunate position : if he 
 sees an object single, he does not see it accurately : if he sees 
 it tolerably accurately, he sees it double. How the patient may 
 obviate the double vision in the latter case we shall see pre- 
 sently. Relatively to any point of convergence, with single 
 vision, then, the hypermetropia is, in truth, absolute. 
 
 (c) Facultative manifest hypermetropia : The patient pos- 
 sesses sufficient accommodation to neutrahze his hyper-
 
 STEABISMUS AND HYPBRMETROPIA. 101 
 
 metropia without the necessity of any undue convergence 
 of the eyes, as was the case in relative hypermetropia. 
 
 A little consideration will show that two factors determine the 
 kind of hypermetropia a patient experiences : firstly, the total 
 amount of hyperinetropia ; and, secondly, the total amount of 
 accommodation he possesses. If the patient's total hyper- 
 metropia exceeds his total accommodation, absolute hyper- 
 metropia must necessarily result ; if it does not exceed it, but 
 is still considerable in proportion to it, relative hypermeti'opia 
 will probably ensue ; while, if it is inconsiderable in propor- 
 tion to the accommodation, facultative hypermetropia arises. 
 As the amount of accommodation diminishes (vide p. 74) as 
 age advances, facultative hypermetropia will give way to 
 relative, and relative to absolute. 
 
 It had been long known that persons who squinted inwards 
 suffered also from defective \asion ; but this knowledge was, 
 and even now is, not so widely diffused as, in the interests of 
 humanity, it should be. A squint is, even in these days, un- 
 fortunately but too often regarded as a mere personal deformity, 
 which may, or may not, according to the patient's position 
 in life, be well left to his own taste and discretion to be cured 
 of, or not. 
 
 It had been observed by various authors that persons who 
 squinted inwards presented a peculiar form of " short sight," 
 which was remedied, not by concave, but by convex glasses. 
 I myself, some years ago, published a paper on this subject in 
 the Glasgow Medical Journal, entitled "On the Short Sight of 
 Squinters," in which I adduced several cases of this to me, at 
 that time, inexplicable contradiction of facts. After Donders 
 had recognised the widely spread diffusion of hypermetropia, 
 he further remarked that most all cases of ordinary convergent 
 squint were accompanied by this optical defect. Subsequent 
 researches have fully proved the important fact that hyper- 
 metropia is in these cases the cause of the squint, and that 
 this latter is a mere symptom and nothing more. 
 
 Convergent squint is, in most cases, caused by relative* 
 
 * As will be seen from •what follows, a person with absolute hypermetropia 
 would gain little or nothing by squinting ; whilst a person with facultative 
 hypermetropia sees acc-urately without it.
 
 102 OPTICAL DEFECTS OF THE EYE. 
 
 hypermetropia. In tliis case the patient^ if unaided by art^ 
 has^ as follows from wliat I have above said, three alternatives : 
 1°, imperfect, single, binocular vision ; 2°, more perfect, double, 
 binocular vision; 3°, perfect, single, monocular vision. The 
 second alternative would be more confusing to the patient than 
 the first. As a rule, he prefers the first. 
 
 The third alternative ensues in the act of squinting. The 
 object of the squinter is to obtain perfect, single, monocular 
 vision. In order to attain perfect vision there must be — 
 
 (a) A perfectly accurate image of the object projected on the 
 retina of at least one of the eyes. In relative hypermetropia 
 this is only possible when the lines of vision of the two eyes 
 intersect each other in a point nearer than the object. 
 
 (/>) The image must fall on the macula lutea of that eye, 
 because this portion of the retina is endowed with the greatest 
 functional perfection. 
 
 When conditions {a) and {h) are both fulfilled, the line of 
 the vision of the other eye, as it intersects that of the former 
 eye at a point nearer than the object, must of necessity be 
 directed inwards past the side of the object; and this de- 
 viation amounts to a squint. I conceive that the squint 
 serves a second [perhaps more important] purpose — namely, 
 to throw the image on an eccentric, less sensitive, part 
 of the retina.* This image the patient ignores by a pro- 
 cess of mental abstraction, in which he is foi-ther aided by 
 the circumstances under which the squint usually arises : 
 " those which diminish the value of binocular vision " — e. g., 
 a natural difierence of refraction in the two eyes, an amblyopic 
 state of one eye, spots on the cornea, &c. To these I would 
 add the general inertia of the retina, which, as will be shown 
 further on, exists more or less in all hypermetropic eyes. I 
 think myself that habit has a great deal to do with squinting 
 in these cases. Myopic persons, after they have found out 
 
 * Dr. Eainy has, iu a letter, mentioned to me a remarkable case of paresis 
 of the ocular muscles, in which, " on -moving an object in a certain direc- 
 tion, the images seen by the two eyes appeared wider and wider ajjart, uiitU 
 suddcnhj one of them disappeared. The object had not gone out of the field 
 of vision of the deviated eye, but I beheve that its image came to fall on a 
 part of the retina sufficiently eccentric to admit of its suppression."
 
 STRABISMUS AND HYPEEMETEOPIA. 103 
 
 that they see more distinctly with one than with two eyes_, often 
 get into the way of habitually reading with one eye. I have 
 done so myself. Similarly, I conceive, after a hypermetropic 
 person has once, perhaps in the first instance by mere chance, 
 discovered how much better he sees with one eye alone than 
 with the two together, he tries it again and again, till it becomes 
 a confirmed habit. I have met with several hypermetropic 
 persons who were perfectly conscious that whenever they 
 wished to see an object distinctly with one eye, they instinc- 
 tively turned the other in. Many hypermetropics, who are 
 not aware of ever having squinted in their lives, may be made 
 to do so by shading one eye ; they may then be often observed 
 to squint with the shaded eye, as soon as the patient regards 
 attentively any near object with the other eye. Again, if a 
 person with normal vision be rendered artificially hyper- 
 metropic by concave glasses, he may often be seen to squint 
 with one eye as soon as he endeavours to see any object 
 distinctly. 
 
 The squint is at first occasional, periodic, only, and may in 
 this stage be genially cured without an operation by convex 
 glasses, prisms, or JavaPs stereoscopic method of ocular gym- 
 nastics. Indeed, after the operation for convergent strabismus, 
 the tendency to squint still exists — for the operation is power- 
 less in removing the fundamental origin of the squint, the 
 hypermetropia — and may reassert itself in all its former inten- 
 sity, if the patient does not, at any rate for all close work, use 
 proper convex glasses. 
 
 Case XX. 
 
 I operated by double tenotomy on a boy, set. 11, for a concomitant con- 
 vergent squint of the right eye, both eyes being hypermetropic to a -rV, and 
 having an acuteness of vision (S) of about ^. When I saw him about three 
 weeks after the operation, I fomid the squint had recmTcd to a degree of 2'" 
 in the right eye, although I was convinced I had performed the tenotomy 
 very completely. I then ordered him to wear 16-inch convex glasses. 
 When I again saw him, three weeks later, the squint had disappeared, and 
 had not returned on a subsequent "V'isit from him three months afterwards. 
 
 In how many such cases a second operation has been un- 
 necessarily performed, from the operator's impression that the
 
 104 OPTICAL DEFECTS OP THE EYE. 
 
 muscle had not been " completely " divided in tlie fii'st opera- 
 tion, I am not in a position to say. 
 
 After a squint has existed any considerable length of time 
 it becomes confirmed. The eye turns in, more or less, always, 
 and from this time forwards its vision gradually diminislies. 
 This arises, I believe, from a secondary change occurring in 
 the retina. Hypermetropia mostly depends upon a congenital 
 smallness of the eye, and, in a restricted sense, may thus be 
 regarded as an instance of arrest of development. It is not 
 unnatural, under such circumstances, to suppose that the 
 retina participates in the general want of formation, and 
 this view is borne out by facts.' The acuity of vision is gene- 
 rally somewhat less than normal in all hypermetropic eyes 
 (altogether apart from any exceptional astigmatism), and is 
 markedly so in all the higher degrees. The following table 
 contains an analysis of the acuteness of vision in 112 hyper- 
 metropic eyes which I have carefully examined during the last 
 five years : — 
 
 Summary of the Acuity of Vision (S) in 112 Hypermetropic Eyes. 
 
 Degree of Hyper- I Degree of Hyper- 
 
 metropia. Average S. 
 
 to i 
 
 44 
 
 metropia. Average S. 
 
 •ZO " 3 6 
 
 4 '» 5 
 
 A glance at the table proves the important fact that the 
 acuity of vision [8) exhibits a progressive diminution as the 
 degree of hypermetropia rises. Donders remarks the same 
 thing, " that even after the correction of the astigmatism of 
 the hypermetropic, the acuteness of vision usually remains 
 rather considerably below the normal." Yet when we come 
 to inspect such eyes with the ophthalmoscope, we can detect 
 nothing morbid beyond the low refractive state of the eye. 
 The only explanation that I can give of the amblyopia in 
 hypermetropia is, as I have before said, that the retina itself 
 is imperfectly developed; Admitting such a predisposition to 
 imperfect visual perception, we might, a priori, have assumed,
 
 STRABISMUS AND HYPERMETROPIA. 105 
 
 what is found by experience^ tliat an eye wliicli has not partici- 
 pated in vision for perhaps many years^ at last practically 
 loses its sight. 
 
 This assumption is fully borne out by facts. The table on 
 the next page exhibits the acuity of vision {S) in 33 cases of 
 convergent squint. S^ is the hypermetropia of the primarily 
 squinting eye^ H^ that of the secondarily ; 6^^ the acuity of 
 vision of the former, 8^ that of the latter.* 
 
 In no less than 26 of the 33 cases is the acuity of vision of 
 the squinting eye less than that of the straight eye ; the ave- 
 rage ratio of the one to the other being, indeed, so low as 
 1 to 6. A comparative reference to the column " Age/^ and 
 to the table given at p. 37 of the influence of age in diminishing 
 the acuity of vision, proves that this factor may be practically 
 left out of consideration in the preceding deduction. One 
 very important fact is rendered probable by this table : that 
 the duration of the squint has a powerful influence in dimi- 
 nishing the patient's vision. The necessary practical corollary 
 to this is, that a squint is not an afiection to be temporized 
 with ; it must be cured as early as possible, whether by opera- 
 tion or otherwise. If it is not cured, the patient gradually, 
 but surely, loses all usefvJ vision with the squinting eye. A 
 loss of binocular vision is not so small a loss as may be 
 imagined. The power of judging of distance is deteriorated, 
 and thus the patient is exposed to a variety of accidents. 
 Should he meet with one to the straight eye, he runs the risk 
 of becoming blind altogether. 
 
 It would be foreign to the scope of this work to describe 
 the several methods of ocular tenotomy ; all I would here 
 remark is, that for the cure of squint I have practised both 
 the subconjunctival section of the ocular muscles and the old 
 operation of Dieffenbach ; and I must confess that, pre- 
 possessed as I was at first with the former operation, I have 
 again reverted to the older one, which I beHeve, performed 
 cautiously, with the addition of an ad libitum limiting suture, 
 surpasses the subconjunctival one — not in certainty, but in the 
 power it afibrds us of very nearly regulating our operation in 
 
 * In the table an Acuity of Vision under 1^0 I have arbitrarily designated 
 by 0.
 
 106 
 
 OPTICAL DErECTS OP THE EYE. 
 
 ACUITY OF VISION (S) IN THIRTY-THREE CASES OF 
 MONOLATERAL CONVERGENT STRABISMUS. 
 
 No. 
 
 Age. 
 
 Daration. 
 
 Degree of In- 
 version. 
 
 Hi 
 
 H, 
 
 Si 
 
 Sa 
 
 1 
 
 16 
 
 Congenital 
 
 2'" 
 
 Vt 
 
 TT 
 
 1 
 
 1 
 
 2 
 
 12 
 
 Congenital 
 
 ? 
 
 -V 
 
 1 
 "JTT 
 
 
 
 * 
 
 3 
 
 13 
 
 9 years 
 
 2'" 
 
 TU 
 
 TTT 
 
 2 
 
 f 
 
 4 
 
 20 
 
 Congenital 
 
 w 
 
 ? 
 
 Vtj- 
 
 
 
 2 
 7 
 
 5 
 
 19 
 
 ? 
 
 1 
 
 ? 
 
 ? 
 
 TTT 
 
 1 
 
 6 
 
 15 
 
 ? 
 
 3'" 
 
 TB- 
 
 ^ 
 
 2 
 
 7 
 
 1 
 
 7 
 
 9 
 
 6 years 
 
 3'" 
 
 ■sV 
 
 ■bV 
 
 1 
 
 i 
 
 8 
 
 12 
 
 8 years 
 
 2| 
 
 -iV 
 
 1 
 
 i 
 
 1 
 
 9 
 
 20 
 
 18 years 
 
 2'" 
 
 Tff 
 
 Vir 
 
 
 
 1 
 
 2^ 
 
 10 
 
 22 
 
 15 years 
 
 2'" 
 
 ? 
 
 tV 
 
 
 
 9 
 
 11 
 
 10 
 
 1 
 
 5'" 
 
 ? 
 
 ? 
 
 
 
 2 
 T 
 
 12 
 
 40 
 
 14 years 
 
 ir 
 
 1 
 
 TT 
 
 T 
 
 i" 
 
 13 
 
 boy 
 
 11 years 
 
 2'" 
 
 TTT 
 
 tV 
 
 h 
 
 1 
 2 
 
 14 
 
 19 
 
 ? 
 
 w 
 
 ? 
 
 To 
 
 
 
 i 
 
 15 
 
 20 
 
 Congenital 
 
 5"' 
 
 yT 
 
 •sV 
 
 4. 
 
 3 
 
 "3 
 
 16 
 
 14 
 
 9 years 
 
 2'" 
 
 ? 
 
 ? 
 
 
 
 :J 
 
 17 
 
 21 
 
 13 years 
 
 2'" 
 
 "TT 
 
 1 
 
 tV 
 
 1 
 
 18 
 
 20 (?) 
 
 Congenital 
 
 4"' 
 
 To 
 
 1 
 
 TTT 
 
 2 
 ■3 
 
 19 
 
 22 
 
 14 years 
 
 o/// 
 
 ? 
 
 ? 
 
 TTT 
 
 4. 
 
 20 
 
 11 (.0 
 
 ? 
 
 3"' 
 
 ^ 
 
 To 
 
 ■?■ 
 
 1 
 
 21 
 
 27 
 
 ? 
 
 i-r 
 
 ? 
 
 ? 
 
 
 
 1 
 
 22 
 
 15 
 
 12 years 
 
 1 
 
 ? 
 
 ? 
 
 
 
 1 
 
 23 
 
 10 
 
 Congenital 
 
 2'" 
 
 TTT 
 
 ■3V 
 
 tV 
 
 2. 
 
 24 
 
 17 
 
 12 years 
 
 2'" 
 
 ? 
 
 ? 
 
 
 
 ? 
 
 25 
 
 42 
 
 Many years 
 
 Very slight 
 
 ■sV 
 
 1 
 ■JTT 
 
 i 
 
 ■3 
 
 26 
 
 32 
 
 ? 
 
 V" 
 
 tV 
 
 7V 
 
 2. 
 
 h 
 
 27 
 
 14 
 
 Congenital 
 
 If" 
 
 tV 
 
 ? 
 
 ^V 
 
 f 
 
 28 
 
 17 
 
 ? 
 
 W" 
 
 tV 
 
 tV 
 
 f 
 
 2 
 3- 
 
 29 
 
 38 
 
 30 years 
 
 2'" 
 
 
 
 TT 
 
 
 
 1 
 
 30 
 
 15 
 
 11 years 
 
 2'" 
 
 
 
 
 
 
 
 i 
 
 31 
 
 17 
 
 3 years 
 
 3'" 
 
 i 
 
 1 
 
 i 
 
 i 
 
 32 
 
 15 
 
 13^ years 
 
 5'" 
 
 -rs 
 
 TTJ 
 
 TT7 
 
 i 
 
 33 
 
 15 
 
 2 years 
 
 3'" 
 
 1 
 
 TTJ- 
 
 tV 
 
 TTT 
 
 i
 
 STRABISMUS AND HYPEEMETROPIA. 
 
 107 
 
 Fig. 30. 
 
 proportion to the amount of effect we wisli to produce. This 
 may be estimated by means of an instrument, of wliicli the 
 annexed description is from the second number of the Opli- 
 thalmic Review : — 
 
 "A Strabismometer. 
 
 " The object of this instrument is to measure precisely and readily the 
 linear amount of deviation of a squinting eye. The method that has hitherto 
 been adopted for this purpose is described by Dr. Alfred Grafe in the foUow- 
 ing words : — ' To express the amount of displacement of a deviated eye, it is 
 better in practice to make such determinations, as von Grafe does, by linear 
 than by angular measurements, the estimation of the former being much 
 more certain than that of the latter. Mark 
 on the lower eyelid' the point which corresponds 
 to the [vertical] diameter of the pupil of the 
 de\'iating eye ; then [by excluding the other 
 eye from vision] cause the patient to fix an 
 object with this same eye, and again mark the 
 pomt corresponding to the [vertical] diameter 
 of the pupU : the distance of these two points 
 from one another is the linear expression of 
 the deviation in question.' * There are several 
 objections to this method. It is tedious : it 
 involves four distinct acts — two on the part of 
 the patient and two on that of the surgeon — 
 both inaccurate in their character. The devi- 
 ated (squinting eye) is not at aU uncommonly 
 amblyopic ; and hence its power of fixation 
 uncertain. The 'marking' of the lower eye- 
 lid involves several inaccuracies. The marking 
 often in-itates the eyelid sufficiently to prevent 
 the patient keeping it perfectly fixed in one 
 position. The mark itself, made by pen and 
 ink, embraces three soiurces of error : its diffi- 
 culty of execution, its necessary irregularity 
 and thickness, and an inconvenient method of 
 sighting the diameter of the pupil. This 
 method of von Grafe's contains, I think, so 
 
 many elements of error as to cast serious doubts on the numerical precision 
 of the data it affords. I, therefore, some years ago had an instrument 
 (Fig. 30) constructed, which I conceived might obviate these various 
 objections. It consists of an ivory plate (P), moulded (from actual experi- 
 ments on the living subject) to the conformation of the lower eyelid, the free 
 
 * " Klinische Analyse der Motilitatsstorungen des Auges," von Dr. Alfred 
 Grafe, Berlin, 1858, p. 27.
 
 108 OPTICAL DEFECTS OP THE EYE. 
 
 border corresponding to that of the lid. This border is graduated in such a 
 manner, that, while its centre is designated by 0, Paris lines and half-lines 
 are marked off on each side of 0, in the manner indicated in the aimexed 
 diagram. Attached to the plate is a handle {H). The aiii^lication of this 
 strabismometer is obvious. The ivory plate is applied to the lower eyelid, 
 the borders of the two corresponding. If the cornea is central, the vertical 
 diameter of the pupil corresponds to ; if inverted, to a graduation on 
 the inner side of ; if everted, to one on the oiiter side of 0. These 
 remarks hold good when the patient is regarding a distant object. 
 It may be objected to this method, that as the plate of the instrument 
 necessarily cannot exactly correspond in size and conformation to every 
 eyelid to which it is applied, and secondly, as the non-deviated eye itself 
 may not be exactly central, the resulting determinations must be inexact. 
 After some years' experience of its use, I (and others who have employed the 
 instrument) find these objections are practically of no moment. Moreover, 
 our object in practice is to estimate rather the relative differences of position 
 of the two eyes than their individual absolute positions, and the data afforded 
 by the strabismometer are always exact in this point of view. At the time 
 of my constructing this instrument, I sent one to my friend Dr. Snellen, of 
 Utrecht, and one to Dr. Liebreich, then of Berlin, now of Paris. The only 
 literary notice of the strabismometer I published is contained in a foot-note 
 to page 8 of my address on ' The Progress of Ophthabnic Surgery ' (read 
 before the North London Medical Society on February 11, 1863), where 
 I say, * The author may here perhaps allude to a very simple little instru- 
 ment he has constructed for the linear measurement of the exact amounts of 
 deviation of the eyeballs in cases of strabismus. It may be had of Messrs. 
 Weiss.' 
 
 " It will be readily seen that by the strabismometer the relative positions of 
 the comejB may be ascertained for all positions of the eyes ; and, further, 
 that it is equally applicable to cases of divergent strabismus. 
 
 " I have found it a convenient notation to designate by 2}lus (-{-) all positions 
 of the centre of the cornea to the inner (nasal) side of ; by minus ( — ) those 
 to the outer side. Thus, if the patient regards a distant object in the middle 
 line, a convergent squint of the right eye of 2 lines would be briefly desig- 
 nated as : — 1 
 
 1. e. : o 
 
 r. e. : +2 ; 
 
 a divergent squint of 3 lines of the left eye as : 
 
 r. e. : 
 1. e. : -3; 
 
 a convergent squint of both eyes, in the right of 2j lines, in the left of 
 1 line as : r. e. : +2^ 
 
 1. e. : +1, &c. &c. ; 
 
 and the same method is applicable to aU Literal positions of the eye, whether 
 squint exists or not."
 
 STEABISMUS AND HYPERMETROPIA. 109 
 
 I would finally remark^ that of all cases of strabismic am- 
 blyopia which I have examined with the ophthalmoscope, I 
 cannot find a single one in my notes in which anything morbid 
 was observed in the eye beyond a hypermetropic refraction. 
 The amblyopia is, then, probably the result of a species of 
 chronic paralysis, or inertia of sensibility of the retina, pre- 
 disposed to in the first instance by imperfection of de- 
 velopment, directly caused by the practical exclusion of the 
 squinting eye from the act of vision. 
 
 THE END.
 
 INDEX. . 
 
 Abnormal refraction, general deter- 
 mination of, 38. 
 
 Accommodation of tlie eye, 19 ; pro- 
 duced by change of form of the 
 crystalline, 19 ; Cramer's experi- 
 ments on, 19 ; explanation of, 20 ; 
 paralysis of, 21, 82, cases of, 84 ; 
 effect of atropine on, 21 ; measure- 
 ment of, 22 ; mechanism of, 22 ; 
 Mannhardt's theory of, 22 ; Becker's 
 experiments on, 22 ; Mackenzie's 
 views on, 23 ; effect of pupil on, 
 23 ; V. Griife's case, 24 ; theories of, 
 25 ; lost after cataract-operations, 25 ; 
 effect of convergence of eyes on, 26; 
 determination of, 46 ; its influence 
 on glasses in myopia, 48 ; in hyper- 
 metropia, 56 and 59 ; in presbyopia, 
 75. 
 
 Acuteness of vision {S), 37 ; influence 
 of age on, 37. 
 
 Asthenopia, 92 ; its symptoms, 92 ; its 
 causes, 94 ; from optical defects, 94 ; 
 from hypermetropia, 94 ; from de- 
 ficient power of the internal recti 
 muscles, 96 ; from hypersesthesia of 
 the retina, 98. 
 
 Astigmatism, 61 ; its seat and cause, 61 ; 
 its diagnosis, 62 ; diffusion-lines in, 
 63 ; treatment by cylindrical lenses, 
 65 ; myopic, 66 ; cases of, 66 ; in- 
 strument for determination of axes 
 of, 68 ; hypermetropic, 70 ; Javal's 
 method of determining, 72. 
 
 Astigmometer, 68. 
 
 Axes of a convex lens, 6. 
 
 Binocular parallax, 26. 
 
 Binocular vision, 26 ; its influence on 
 
 the use of glasses, 80. 
 Brewster's theory of the stereoscope, 
 
 29. 
 
 Calabar Bean, action of, 83. 
 
 Classification of eyes, 30. 
 
 Concave lenses, 32 ; rationale of their 
 
 action, 42 ; diminish, 43. 
 Convergent pencils, 3. 
 
 Convex lenses, 5 ; their effect on parallel 
 rays, 7 ; on divergent rays, 9 ; mag- 
 nify, 15. 
 
 Cramer on accommodation, 19. 
 
 Cylindrical lenses, 65. 
 
 " Dazzling " of high concave lenses, 
 theory and treatment of, 44 ; illus- 
 trative case, 45. 
 
 Diameter of a convex lens, 6. 
 
 Diffusion-lines in astigmatism, 63. 
 
 Divergent pencils, 3 ; focus of, in con- 
 vex lenses, 9 ; focus of, in concave 
 lenses, 34. 
 
 Divergent rays impinging on a concave 
 lens, 33. 
 
 Eye compared to a camera obscura, 
 
 17; accommodation of, 19. 
 Eyes, classified table of, 30. 
 
 Focal lengths of lenses, methods of 
 
 determining, 34. 
 "Focus," definition of, 4. 
 Focus, principal, of a convex lens, 7. 
 " Focal interval " of Sturm, 63. 
 
 Giraud-Teulon, his test-types, 37; 
 on binocular vision in the use of 
 glasses, 80. 
 
 Haan's, Dr. de, table of the influence 
 of age on the acuteness of vision, 37. 
 
 Hypermetropia, 52 ; its diagnostics, 52 ; 
 cases of, 54 ; explanation of the terms 
 "total," "latent," and "manifest," 
 
 57 ; colour-test for, 58 ; its correction, 
 
 58 ; effect of accommodation in, 59 ; 
 its ophthalmoscopic signs, 60 ; acute- 
 ness of vision lowered in, 104. 
 
 Hypermetropic accommodation, 56. 
 
 Images, formation of, by a minute 
 aperture, 12 ; by convex lenses, 13 ; 
 by concave lenses, 34 ; real images, 
 12 ; virtual ones, 15. 
 
 Iris, supplementary to accommoda- 
 ' tion, 24.
 
 112 
 
 INDEX. 
 
 Javal's method of determining astig- 
 matism, 72. 
 
 Lenses, optical, 5 ; convex, 5 ; focus 
 of, 7 ; parallel rays on, 7 ; nomen- 
 clature of, 8 ; effect of combinations 
 of, 9 ; divergent rays on convex, 9 ; 
 formation of images by convex, 
 10 ; magnifying power of convex, 
 15; "power" of, 15; of the eye, 
 17 ; concave, 32 ; on concave, 
 33 ; determination of focal lengths 
 of, 34 ; opticians' " numbers " of, 
 36; rationale of concave, 42 ; dimi- 
 nishing power of concave, 43 ; 
 " dazzling " of high concave, 44 ; 
 cylindrical, 65. •• 
 
 Light, nature of, 2 ; rays of, 2 ; pencils 
 of, 3 ; corpuscular and undulatory 
 theories of, 5. 
 
 Magnifying power, 15. 
 
 " Minimum visibile," 43. 
 
 Minute apertures, effects of, on vision, 
 23. 
 
 Monocular relief, 28. 
 
 Myopia, 32 ; characteristics of, 36 ; 
 cases of, 38, 45 ; effect of converg- 
 ence of the eyes on its determination, 
 41 ; nomenclature of, 43 ; effect of 
 accommodation, 48 ; ophthalmoscopic 
 signsof, 49 ; "crescent" (staphyloma 
 posticum) in, 49 ; "functional," 51. 
 
 "Myopic refraction," 49. 
 
 "Numbers" (opticians') of lenses, 
 table of, 36. 
 
 Optic centre of a convex lens, 6. 
 Optical considerations, 1. 
 
 Optometer, author's, 40 ; v. Grafe's, 
 40, 41. 
 
 Parallel pencils, 3 ; focus of, in convex ' 
 lenses, 7 ; in concave lenses, 32. 
 
 Pathological optics, 30. 
 
 Phakoidoscope, the, 19. 
 
 Physiological optics, 17. 
 
 "Positive" and "negative," explana- 
 tion of the terms, 33. 
 
 "Power " of lenses, 15. 
 
 Preface, v. 
 
 Presbyopia, 74 ; its diagnosis, 74 ; in 
 normal eyes, 74 ; its estimation, 75 ; 
 its correction, 75 ; in myopic eyes, 
 77 ; in hj^permetropic eyes, 79 ; its 
 cause, 79. 
 
 Presbyopic glasses, Kitchener's table of, 
 76 ; Donders's t.able of, 77. 
 
 Radius of curvature of a convex lens, 
 
 6. 
 Relief, binocular, 27 ; monocular, 28. 
 
 Sensoridm, its share in vision, 18. 
 
 Snellen's test-types, 37. 
 
 Staphyloma posticum, 49. 
 
 Stereoscope, theory of, 27 ; Brewster's 
 theory of, 29. 
 
 Strabismus convergens, 100 ; its de; 
 pendence on hypermetropia, 101 ; its 
 treatment, when periodic, 103 ; bj' 
 tenotomy, 105 ; acuteuess of vision 
 lowered in, 106. 
 
 Strabismometer, the, 107. 
 
 Table of refractive powers of eyes, 30 ; 
 of presbyopic glasses, Kitchener's, 
 76 ; Donders's, 77. 
 
 Unequal refraction of the two eyes, 39. 
 
 cox AND WTMAN, PRINTERS, GRE.VT QUEEN STREET, LONDON, W.C.
 
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