\/ ^'\. %^^ THE REFRACTION OP THE EYE A MANUAL FOB STUDENTS BY GUSTAVUS HARTRIDGE, F.R.C.S. SENIOR SURGEON TO THE ROYAL WESTMINSTER OPHTHALMIC HOSPITAL OPHTHALMIC SURGEON AND LECTURER ON OPHTHALMIC SURGERY TO THE WESTMINSTER HOSPITAL ; CONSULTING OPHTHALMIC SURGEON TO ST. Bartholomew's hospital, Chatham, and to st. George's dispensary, hanoyer square, etc. WITH ONE HUNDRED AND NINE ILLUSTRATIONS FOURTEENTH EDITION PHILADELPHIA P. BLAKISTON^S SON & CO. 1012, WALNUT STEEET 1907 OPTO First Edition, Jan., 1884 Second „ „ 1886 Third » 1888 Fourth „ Nov., 1889 Fifth „ July, 1891 Sixth „ Oct., 1892 Seventh „ Sept., 1894 Eighth June, 1896 Ninth „ Aug., 1898 Tenth March, 1900 Eleventh „ July, 1901 Twelfth „ Oct., 1903 Thirteenth „ June, 1905 Fourteenth,, Jan., 1907 Total 28,000 copies. PRINTED IN GREAT BRITAIN. PREFACE QpTO FOURTEENTH EDITION # In preparing tlie f ourteentli edition of ' Refraction of the Eye ' for publication, tlie original plan of tlie book has been maintained, and no effort has been spared to make the work more worthy of the favour with which it has been received in this country and abroad. Although but a short time has elapsed since the last edition of 3000 was published, the book has been carefully revised, and alterations made in accordance with our increasing knowledge of the subject. a. H. 12, WiMPOLE Street, W. January, 1907. PREFACE FIRST EDITION I HAVE endeavoured in the. following pages to state "briefly and clearly the main facts with which practi- tioners and students should be acquainted, in order to enable them to diagnose errors of refraction accu- rately, and to prescribe suitable glasses for their correction. Those who would do this with facility can only acquire the requisite amount of dexterity by prac- tically working out a large number of cases of refrac- tion. No book, or even the knowledge gained by watching others who are thus employed, can take the place of this, the practical part of the subject. To many of my readers the chapter on Optics may appear unnecessary. I have added it for the benefit of those whose school education did not include this subject, since its elementary details so completely underlie the whole subject of refraction, that every VI PREFACE student should understand tliem thoroughly before passing on to the real subject in hand. I have found it necessary in several instances to repeat important matters, and this I have done to obviate the necessity of continual reference to other parts of the book, as well as in some cases to impress the importance of the subject upon the student. The woodcuts are numerous in proportion to the size of the work, but I consider that they are a very great help to the thorough understanding of the subject. The old measurements have been purposely omitted in favour of the almost universally adopted metrical system. It is confusing to the learner to have two distinct sets of measurements to deal with, and no possible good can accrue from perpetuating the old system of feet a.nd inches. At the end of the work I have given a list of those authors to whom I have been indebted for much valuable information ; and in conclusion, I take this opportunity of thanking my numerous friends for their help and suggestions. G. H. January, 1884. CONTENTS CHAPTER I PAGE Optics . . . ... .1 Reflection . . . . .2 Refraction . . . . .6 Formation of Images . . . .17 CHAPTER II Refraction of the Eye . . . .22 Accommodation . . . .32 Convergence . . . . .41 CHAPTER III Methods of Determining the Refraction . 53 Acuteness of Yision . . . .55 Sclieiner's Method . . . .64 CHAPTER lY The Ophthalmoscope . . . .66 The Indirect Method . . . .66 The Direct Method . . . .73 Vlll CONTENTS CHAPTER Y PAGE Retinoscopy . . . . .82 CHAPTER YI Hypermetropia ..... 117 Aphakia ...... 132 CHAPTER YII Myopia ...... 134 CHAPTER YIII Astigmatism ..... 156 Anisometropia ..... 184 CHAPTER IX Presbyopia . - . . . . 187 Paralysis of the Accommodation . . . 196 Spasm of the Accommodation . . .197 CHAPTER X Strabismus ..... 200 CONTENTS IX CHAPTER XI PAGE Asthenopia . - . . . . 224 Accommodative .... 226 Muscular ..... 228 Eetinal ..... 234 CHAPTER XII Spectacles ..... 237 Cases . . . . . . 244 Appendix ..... 259 Regulations fok Army, Navy, &c. . . 261 Test Types ..... 265 LIST OF ILLUSTRATIONS No. 1. Eeflection by a plane surface 2. Virtual image formed by a plane mirror 3. Reflection by a concave surface 4. Ditto ditto 5. Eeflection by a convex surface 6. Eefraction by a plane surface 7. Eefraction by a prism 8. Ditto ditto . 9. Eefraction by a spherical surface 10. Ditto ditto 11. Formation of convex lenses . 12. Different forms of lenses 13. Eefraction of rays (secondary axes) by a convex lens 14. Eefraction of parallel rays by a convex lens 15. Ditto ditto 16. Properties of a biconvex lens 17. Ditto ditto 18. Properties of a biconcave lens 19. Eefraction of parallel rays by a concave lens 20. Formation of an inverted image 21. Eeal inverted image formed by a convex lens . 22. Virtual image formed by a convex lens 23. Virtual image formed by a concave lens 24. Diagram of eye showing the cardinal points 25. Formation of inverted image on the retina 26. Emmetropic, hypermetropic, and myopic eyeballs 27. Eye represented by a biconvex lens . 28. Formation of visual angle . 29. Diagram of accommodation . 30. Scheiner's method of finding the punctum proximum 31. Amovint of accommodation at different ages 32. Diagram representing the convergence 33. Landolt's ophthalmo-dynamometer 34. Diagi-am of the relative accommodation 35. Angle subtended at nodal point by test type . LIST OF ILLUSTEATIONS XI No. PAGE 36. Sclieiner's method .... 37. Image formed in emmetropia by the indirect ophthalmo seopic method .... 38. Image formed in hypermetropia 39. Image formed in myopia ... 40. Size of the image in emmetropia for different distances of the objective .... 41 & 42. Decrease of the image in hypermetropia on with drawing the objective 43. Image formed in emmetropia 44. Image formed in hypermetropia 45. Image formed in myopia 46. Direct ophthalmoscopic examination in emmetropia 47. Estimation of hypermetropia by the ophthahnoscope 48. Estimation of myopia by the ophthalmoscope . 49. Kays coming from the hypermetropic eye 50. Eays coming from the myopic eye 51. Position of light for retinoscopy 52. Light with diaphragm 53. Plane mirror 54. Shadows in retinoscopy 55. Eetinoscopy with the plane mirror 56. Image formed in myopia 57. Image formed in hypermetropia 58. Oblique shadows in astigmatism 59. Cause of oblique shadows 60. Band-like shadows . 61. Band-like shadows . 62. Eecording the astigmatism . 63. Retinoscopy with the concave mirror 64. Shadows with the concave mirror 65. Movements of the shadow with the concave mirror 66. Refraction of a hypermetropic eye 67. Refraction increased by changes in the lens 68. Correction by a biconvex lens 69. Accommodation at different ages in hypermetrope of 3 D 70. Refraction of a myopic eye . 71. Ditto ditto 72. Correction by a biconcave lens Xll LIST OF ILLUSTEATIONS No. PAGE 73. Section of a myopic eyeball . . . 139 74. Accommodation at different ages in a myope of 2 D. . 140 75. Size of retinal image in myopia . . . 145 76. Section of cone of light after passing through an astig- matic cornea. . . . . .159 77. Diffusion patches when the cone is divided at right angles . , „ . . . . 159 78. Interval of Sturm ^ . . . . 160 79. Simple hypermetropic astigmatism . . . 161 80. Compound hypermetropic astigmatism . . 162 81. Simple myopic astigmatism ... 162 82. Compound myopic astigmatism . . . . 162 83. Mixed astigmatism . . . . 163 84. Astigmatic clock face .... 170 85. Astigmatic fan ..... 171 86. Erect image of a disc seen through an astigmatic cornea. 172 87. Same disc seen by the indirect method . . 172 88. Tweedy's optometer .... 179 89. Diagram of the accommodation . . .188 90. Angle a in emmetropia .... 201 91. Angle a in hypermetropia . . .. . 202 92. Angle a in myopia .... 202 93. Diagram of primary and secondary deviation . . 204 94. Strabismometer ..... 206 95. Method of measuring the angle of the strabismus . 208 96. Diagram representing convergent strabismus . . 210 97. Diagram representing divergent strabismus . . 216 98. Worth's amblyoscope .... 220 99. Stereoscopic slide ..... 222 100. Graefe's test for insufficiency of internal recti miiscles . 232 101. Scale for testing latent deviation at the reading distance. 233 102. Convex and concave glasses acting as prisms . . 234 103. Bifocal lens . . . . .241 104. Bifocal lens . . . . .241 105. Invisible bifocal lens .... 242 Lithographic Plate opposite page 147 ; 1, 2, and 3. Drawn from myopic patients. 4. Copied from Atlas of Wecker and Jaeger. Test types .... 168, 265 THE REFEACTION OF THE EYE CHAPTER I OPTICS Light is propagated from a luminous point in every plane and in every direction in straight lines ; these lines of directions are called rays. Rays travel with the same rapidity so long as they remain in the same medium. The denser the medium^ the less rapidly does the ray of light pass through it. Rays of light diverge, and the amount of diverg- ence is proportionate to the distance of the point from which they come ; the nearer the source of the rays, the more they diverge. When rays proceed from a distant point such as the sun, it is impossible to show that they are not parallel ; and in dealing with rays which enter the eye, it will be sufficiently accurate to assume them to be parallel when they proceed from a point at a greater distance than 6 metres. A ray of light meeting with a body may be absorbed, 1 2 THE REPEACTION OF THE EYE reflected, or if it is able to pass through this body it may be refracted. Reflection Reflection hy a Plane Surface Reflection takes place from any polished surface, and according to two laws. 1st. The angle of reflection is equal to the angle of incidence. 2nd. The reflected and incident rays are both in the same plane, which is perpendicular to the reflect- ing surface. Fig. 1. Thus, if A B be the ray incident at b, on the mirror c D, and B E be the ray reflected, the perpendicular p B will divide the angle a b e into two equal parts, the angle a b p is equal to the angle p b e ; while A B, p B, and e b lie in the same plane. When reflection takes place from a plane surface, the image is projected backwards to a distance behind the mirror equal to the distance of the object in front of it, the image being of the same size as the object. Thus in Fig. 2 the image of the candle c is formed behind the mirror m, at &, a distance behind the REFLECTION mirror equal to the distance of the candle in front of it; an observer's eye placed at e would receive the rays from c as if they came from c\ Fig. 2. M. The mirror, c. The candle, c'. The virtual image of the candle. E. The eye of the observer receiving rays from the mirror. The image of the candle so formed by a plane mirror is called a virhial image. ^ Reflection hy a Concave Surface A concave surface may be looked upon as made up of a number of planes inclined to each other. Parallel rays falling on a concave mirror are re- flected as convergent rays^ which meet on the axis at a point (f, Fig. 3) called the priiicipal focus, midway between the mirror and its optical centre c. The dis- tance of the principal focus from the mirror is called the focal length of the mirror. 4 THE REFRACTION OF THE EYE If the luminous point be situated at f, then the diverging rays would be reflected as parallel to each other and to the axis. If the luminous point is at the centre of the con- cavity of the mirror (c), the rays return along the same lines, so that the point is its own image. If the luminous point be at a the focus will be at a, Fig. 3. and it is obvious that if the luminous point be moved to a, its focus will be at a; these two points therefore, A and a, bear a reciprocal relation to each other, and are called conjugate foci. If the luminous point is beyond the centre, its con- jugate focus is between the principal focus and the centre. If the luminous point is betAveen the principal focus and the centre, then its conjugate is beyond the centre ; so that the nearer the luminous point ap- proaches the principal focus, the greater is the dis- tance at which the reflected rays meet. If the luminous point be nearer the mirror than the EEFLECTION 5 principal focus (f), the rays will be reflected as diver- gent, and will therefore never meet : if, however, we continue these diverging rays backwards, they will unite at a point (h) behind the mirror ; this point is called the virtual focus, and an observer situated in Fig. 4. the path of reflected rays will receive them as if they came from this point. Thus it follows that — Concave mirrors produce two kinds of images or none at all, according to the distance of the object, as may be seen by looking at one^s self in a concave mirror. If the mirror is placed nearer than its principal focus, then one sees an enlarged virtual image, which in- creases slightly in size as the concave mirror is made to recede; this image becomes confused and disap- pears as the principal focus of the mirror is reached : on moving the mirror still farther away (that is be- yond its focus) one obtains an enlarged inverted image, which diminishes as the mirror is still further withdrawn. b THE REFRACTION OF THE EYE Reflection hy a Convex Surface Parallel rays falling on a convex surface are reflected as divergent, hence never meet ; but if tlie diverging rays thus formed are carried backwards by lines, then an imaginary image is formed Avhich is called negative, and at a point called the principal focus (r). Foci of convex mirrors are therefore virtual; and the image, whatever the position of the object, is always virtual, erect, and smaller than the object. Fia. 5. The radius of the mirror is double the principal focus. Refraction Refraction hy a Plane Surface A ray of light passing through a transparent me- dium into another of a different density is refracted, unless the ray fall perpendicular to the surface sepa- rating the two media, when it continues its course without undergoing any refraction (Fig. G, H k). REFRACTION A ray is called incident before entering the second medium, emergent after leaving it. A ray passing from a rarer to a denser medium is refracted towards the perpendicular; as shown in Fig. 6, the ray A b is refracted at b, towards the per- pendicular p p. In passing from the denser to the rarer medium the ray is refracted from the perpendicular; b d is refracted at c, from p p (Fig. 6). Fig. 6. Keflection accompanies refraction, the ray dividing itself at the point of incidence into a refracted portion B c, and a reflected portion b e. The amount of refraction is the same for any medium at the same obliquity, and is called the index of refraction; air is taken as the standard, and is called 1 ; the index of refraction of water is 1*3, that of glass 1*5. The diamond has almost the highest refractive power of any transparent substance, and 8 THE REFRACTION OP THE EYE has an index of refraction of 2'4. The cornea has an index of refraction of 1"3^ and the lens 1*4. The refractive power of a transparent substance is not always in proportion to its density. If the sides of the medium are parallel_, then all rays except those perpendicular to the surface which pass through without altering their course are re- fracted twice, as at b and c (Fig. 6), and continue in the same direction after passing through the medium as they had before entering it. If the two sides of the refracting medium are not parallel, as in a prism, the rays cannot be perpen- dicular to more than one surface at a time. Therefore every ray falling on a prism must un- dergo refraction, and the deviation is always towards the base of the prism. The relative direction of the rays is unaltered (Fig. 7). Fig. 7. Fia. 8. If D M (Fig. 8) be a ray falling on a prism (a b c) at M, it is bent towards the base of the prism, assuming the direction m n; on emergence it is again bent at n; an observer placed at e would receive the ray as if it came from k; the angle k h d formed by the two lines REFEACTION 9 at H is called the angle of deviation, and is about half the size of the 'principal angle formed at A by the two sides of the prism. Refraction by a Spherical Surface. Parallel rays passing through a spherical surface separating media of different density do not continue parallel^ but are refracted, so that they meet at a point called the principal focus. If parallel rays k, d, e, fall on A b, a spherical sur- face separating the media m and n of which n is the denser; ray d, which strikes the surface of a b at right angles, passes through without refraction, and is called the principal axis ; ray K will strike the surface at an angle, and will therefore be refracted towards the perpendicular c J, meeting the ray d at P; so also with ray E, and all rays parallel in medium m. The point F where these rays meet is the principal focus, and the Fig. 9. distance between the principal focus and the curved surface is spoken of as the principal focal distance. Rays proceeding from f will be parallel in m after 10 THE EP]FRACT10N OF THE EYE passing througli the refracting surface. Rays parallel in medium n will focus at f', which is called the ante- rior focus. Had the rays in medium m been more or less diver- gent^ they would focus on the principal axis at a greater distance than the principal focus, say at H; and conversely rays coming from h would focus at G; these two points are then conjugate foci. When the divergent rays focus at a point on the axis twice the distance of the principal focus, then its conjugate will be at an equal distance on the other side of the curved surface. If rays proceed from a point o, nearer the surface than its principal focus, they will still be divergent after passing through A b, though less so than before, and will therefore never meet; by continuing these Fig. 10. rays backwards they will meet at L, so that the conju- gate focus of o will be at l, on the same side as the focus; and the conjugate focus will in this case be spoken of as negative. U LENSES 11 Refraction hy Lenses Refraction by lenses is somewhat more complicated. A lens is an optical contrivance usually made of glass, and consists of a refracting medium with two opposite surfaces, one or both of which may be seg- ments of a sphere; they are then called spherical lenseSj of which there are six varieties. Fig. 11. 1. Plano-convex, the segment of one sphere (Fig. 11, B). 2. Biconvex, segments of two spheres (Fig. 11, a). 3. Converging concavo-convex, also called a con- verging meniscus. 4. Plano-concave. 5. Biconcave. 6. Diverging concavo-convex, called also a diverg- ing meniscus. Lenses may be looked upon as made up of a number of prisms with different refracting angles — convex lenses, of prisms placed with their bases together; concave lenses, of prisms with their edges together. A ray passing from a less refracting medium (as 12 THE EEFEACTION OP THE EYE air) througli a lens_, is deviated towards the thickest part, therefore the first three lenses, which are thickest at the centre, are called converging ; and the others, which are thickest at the borders, diverging. Fig. 12. A line passing through the centre of the lens (called the optical centre), at right angles to the sur- faces of the lens, is termed the principal axis, and any ray passing through that axis is not refracted. All other rays undergo more or less refraction. Rays passing through the optical centre of a lens, but not through the principal axis, suffer slight devia- tion, but emerge in the same direction as they entered. These are called secondary axes (Fig. ]3). The deviation in thin lenses is so slight that they are usually assumed to pass through in a straight line. Parallel rays falling on a biconvex lens are ren- dered convergent ; thus in Fig. 14 the rays A, B, c, strike the surface of the lens (l) at the points d, e, f; the centre ray (b) falls on the lens at e perpendicular to its surface, and therefore passes through in a straight line ; it also emerges from the lens at right BICONVEX LENSES 13 angles to its opposite surface^ and so continues its course without deviation ; but the ray a strikes the surface of the lens obliquely at d, and as the ray is passing from one medium (air) to another (glass) Fig. 13. Lens with secondaryaxes undergoing slight deviation. which is of greater density, it is bent towards the perpendicular of the surface of the lens^ shown by the dotted line m k ; the ray after deviation passes through the lenSj striking its opposite surface obliquely at o, Fig. 14. and as it leaves the lens^ enters the rarer medium (air), being deflected from the perpendicular n o ; it 14 THE REFRACTION OF THE EYE is now directed to h^ where it meets the central ray B H ; ray c^ after undergoing similar refractions, meets the other rays at h, and so also all parallel rays falling on the biconvex lens (l). Parallel rays, therefore, passing through a convex lens (l) are brought to a focus at a certain fixed point Fig. 15. (a) beyond the lens; this point is the ^principal focus, and the distance of this focus from the lens is called the focal length of the lens. Rays from a luminous point placed at the principal focus (a) emerge as parallel after passing through the lens. Divergent rays from a point (b) outside the princi- pal focus (f, Fig. 16) meet at a distance beyond (f') the principal focus on the other side of the lens (l), and if the distance of the luminous point (b) is equal to twice the focal length of the lens, the rays will focus at a point (c) the same distance on the opposite side of the lens ; rays coming from c would also focus at B : they are therefore called conjugate foci, for we can indifferently replace the image (c) by the object (b), and the object (b) by the image (c). BICONVEX LENSES 15 If the luminous point (d) be between the lens and the principal focus (p), then the rays will issue from the lens divergent^ though less so than before enter- ing it ; and if we prolong them backwards they will Fig. 16. meet at a point (h) further from the lens than the point D ; H will therefore be the virtual focus of d, and the conjugate focus of d may be spoken of as negative. Biconvex lenses have therefore two principal foci, r and f'^ one on either side^ at an equal distance from the centre. In ordinary lenses^ and those in which the radii of the two surfaces are nearly equal^ the principal focus closely coincides with the centre of curvature. We have assumed the luminous point to be situated on the principal axis; supposing, however, it be to one side of it as at e (Fig. 17), then the line (e f) passing through the optical centre (c) of the lens (l) is a secondary axis, and the focus of the point e will be found somewhere on this line, say at f, so that what has been said respecting the focus of a luminous point on the principal axis (a b) is equally true for points on a secondary axis, provided ahvays that the inclina- 16 THE REFRACTION OF THE EYE tion of this secondary axis is not too great, when the focus will become imperfect on account of the spherical aberration which will be produced. Fig. 17. In biconcave lenses the foci are always virtual, whatever the distance of the object. Rays of light parallel to the axis diverge after refraction, and if their direction be continued back- ward they will meet at a point termed the principal focus (Fig. 18, f). Fig. 18. Fig. 19 shows the refraction of parallel rays by a biconcave lens (l) ; the centre ray B strikes the lens at E perpendicular to its surface, passing through without refraction, and as it emerges from the oppo- site side of the lens perpendicular to its surface, it FORMATION OF IMAGES 17 continues in a straight line; the ray A strikes the lens obliquely at d and is refracted towards the perpen- dicular, shown by the dotted line G h ; the ray after Fig. 19. deviation passes through the lens to k, where, on entering the medium of less density obliquely, it is refracted from the perpendicular o p, in the direction K M ; the same takes place witli ray c at f and n ; so also with all intermediate parallel rays. Formation of Images. — To illustrate the formation of images the following simple experiment may be carried out: — Take a screen with a small perforation and place on one side of it a candle, and on the other side a sheet of white cardboard at a suitable distance to receive any image : rays diverge from the candle in all directions, most of those falling on the screen are intercepted by it; but some few pass through the perforation and form an image of the candle on the cardboard, the image being in- verted because the rays cross each other at the orifice. It can further be shown that when the candle and cardboard are equally distant from the 2 18 THE REFRACTION OF THE EYE perforated screen, the candle flame and its image will be of the same size. If the cardboard be moved further from the screen the image is enlarged, if it be moved nearer it is diminished; if we make a Fig. 20. dozen perforations in the screen, a dozen images will be found on the cardboard, if a hundred then a hundred; but if the apertures are so close together that the images overlap, then instead of so many distinct images we get a general illumination of the cardboard. The image of an object is the collection of the foci of its several points ; the images formed by lenses are, as in the case of the foci, real or virtual. Images formed, therefore, by convex lenses may be real or virtual. In Fig. 21, let A b be a candle situated at an infinite distance ; from the extremities of A b draw two lines passing through the optical centre (c) of a biconvex lens, then the image of a will be formed somewhere on the line AC a (termed a secondary axis), say at a; the image of b will be formed on the line B c 6, say at h; therefore 6 a is a small inverted image FOEMATION OF IMAGES 19 of the candle a b, formed at the principal focus of the convex lens. Had the candle been placed at twice the focal distance of the lens, then its inverted image Fig. 21. Eeal inverted image formed by convex lens. would be formed at a corresponding point on the opposite side of the lens, and would be of the same size as the object. If the candle be at the principal focus (f), then the image is at an infinite distance, the rays after refrac- tion being parallel. If, however, the candle (a b) be nearer the lens than Fig. 22. Virtual image formed by convex lens. the focus, then the rays which diverge from the candle will, after passing through the convex lens, be still 20 THE REFRACTION OF THE EYE divergent, so that no image is formed ; an eye placed at E would receive the rays from a b as if they came from ah; a h is therefore a virtual image of A b, erect and larger than the object, and formed on the same side of the lens as the object. Images formed by biconcave lenses are always virtual, erect, and smaller than the object. Let A b Fig. 23. Virtual image formed by concave lens. be a candle, and f the principal focus of a biconcave lens ; draw from a and b two lines through c, the optical centre of the lens, and lines also from a and b parallel to the axis ; after passing through the lens they diverge and have the appearance of coming from a h, which is therefore the virtual image of A b. A real image can be projected on to a screen, but a virtual one can only be seen by looking through the lens. EE FRACTION 21 CHAPTER II REFRACTION. ACCOMMODATION. CONVERGENCE The eye may be looked upon as an optical instru- ment, a sort of photographic camera, designed to pro- duce by means of its refracting system a small and inverted picture of surrounding objects upon the retina; the stimulation produced by this picture on the retina is conveyed through the optic nerve to the ganglion cells of the cortex, that part of the brain known as the optical area ; this excitation of the gan- glion cells of this centre becomes sensation, and thus it is that the retinal picture comes within the domain of consciousness, and the brain interprets the im- pressions transmitted to it from the retina. Immediately behind the transparent retina is a layer of pigment, which absorbs some of the rays of light as soon as the image is formed ; were this not so the rays would be reflected to other parts of the retina, and cause much dazzling, considerably interfering with vision; this is the case in those persons who have a congenital absence of pigment, and who are known as albinos. The refracting system of the eye is so arranged, that very little spherical or chromatic aberration takes place, as is the case with ordinary optical instruments. For distinct and accurate binocular vision the fol- lowing conditions are necessary : 22 THE EEPRACTION OF THE EYE 1. That a well-defined inverted image be formed on tlie layer of rods and cones at tlie yellow spot of each eye. 2. That the impression here received be conveyed to the brain. In a work of this character the first of these condi- tions alone concerns us^ and for the carrying out of this — the media being transparent — three important factors call for a separate description, viz. : Refraction. Accommodation. Convergence. Refraction. — This term is used to express the optical condition of the eye in a state of rest. There are three refracting surfaces in the eye — the anterior surface of the cornea, the anterior surface of the lens, and the anterior surface of the vitreous ; and three refracting* media — the aqueous, the lens, and the vitreous. These together make up the dioptric system, and may for the sake of simplicity be looked upon as equal to a convex lens of about 23 mm. focus. What was said about convex lenses applies equally to the eye as an optical instrument. A ray of light falling on the cornea does not, how- ever, follow the simple direction we might imagine when considering the eye merely as a lens of 23 mm. focus : the eye must be looked upon as a compound refracting system, composed of a spherical surface and a biconvex lens. To enable us to understand the course of a ray of light through the eye, it is neces- EEFEACTION 23 sary to be acquainted with tlie cardinal points of this compound system. Too much space would be occu- pied to explain how the position of these points is arrived at^ but it suffices to say that, having first found the cardinal points of the cornea and then those of the lens, the cardinal points of the eye will be the result of these two systems together. The cardinal points of the eye are six in number, two prmcipal points, two nodal points, and two prin- cipal foci. Fig. 24. In the above diagram of the emmetropic eye the cardinal points of this compound system are shown, all situated on the optic axis (f a) : at b are two prin- cipal points situated so closely together in the anterior chamber that they may conveniently be looked upon as one point; at N are two nodal points, also close together, — for simplicity we shall consider them as one point ; at f is the first principal focus, at a the second principal focus. We then have the following: 24 THE REFRACTION OF THE EYE c, the cornea ; l, the lens ; m, the macula ; o, the optic nerve; f a, the optic axis; b, the principal point; n, the nodal point ; h^ the centre of rotation of the eye, 9*8 mm. in front of the retina; a, the second principal focus ; and F, the first principal focus. The nodal points correspond nearly to the optical centre of the refracting system, the axis ray passing through these points is not refracted ; every ray directed to the first nodal point appears after refrac- tion to come from the second point, and continues in the same direction to that which it first had : the nodal points in the eye are situated about 7 mm. behind the cornea (Fig. 24, n). The princijml points. — When an incident ray passes through the first principal point, the corresponding emergent ray passes through the second principal point, but the incident and emergent rays are not parallel; the principal points are situated about 2 mm. behind the cornea (Fig. 24, b). The first principal focus is that point on the axis Avhere rays parallel in the vitreous meet ; this point is about 13-7 mm. in front of the cornea (Fig. 24, f). A vertical line passing through this point is called the first principal plane. The second principal focus is that point on the axis where parallel rays meet after passing through the eye, 22*8 mm. behind the cornea (Fig. 24, a). A vertical line passing through this point is called the second principal plane. A luminous point placed above the principal axis has its image formed on the retina below this axis; and EEFEACTION 25 inversely^ the image of a point below the principal axis will be formed above it. If we replace these two points by an object the same thing occurs, and we get an inverted image (Fig. 25) : it is essential that the method of formation of these inverted images be thoroughly understood. From every point of an object abc proceed diverg- ent rays. Some of those rays coming from A, pass through the pupil, and being refracted by the dioptric system, come to a focus on the retina at a; some coming from b, focus at h, and some from c at c. In Fig. 25. the same way rays come from every part of the object as divergent rays, and are brought to a focus on the retina ; so that the retina, being exactly at the focal distance of the refracting system, receives a well- defined inverted image. Much has been said and written as to why images which are formed in an inverted position on the retina should be seen upright, and all sorts of ingenious explanations have from time to time been given. The whole thing is entirely a matter of education and ex- perience, which is supplemented and corroborated by 26 THE REFRACTION OP THE EYE the sense of touch. We have no direct cognizance of the image on the retina, nor of the position of its different parts, but only of the stimulation of the retina produced by the image ; this stimulation is conducted by the optic nerve to the brain, producing there certain molecular changes. We do not actually see the retinal image, but the eye receives the rays emanating from the object looked at, and we refer the sensation in the direction of these rays; thus, if an image is formed on the upper part of our retina, we refer the sensation downwards from which the rays must have come. The great advantage of inverted images is, that for a given-sized pupil a much larger retinal picture can be formed than would be the case if no inversion took place; for in the latter case all images must necessarily occupy a smaller space on the retina than the size of the pupil. The refraction of the eye is said to be normal when parallel rays are united exactly on the layer of rods and cones of the retina; in other words, when the retina is situated exactly at the second principal focus of the eye. This condition is called emmetropia {Ifi, within; juirpov, measure; w^, the eye (Fig. 26, a). If parallel rays are focussed behind or in front of the retina, then the term ametropia (a, priv. ; jucrpov, measure ; wxp, the eye) is used, and of this there are two opposite varieties : Hypermetropia, when the eyeball is so short that parallel rays are brought to a focus behind the retina (Fig. 26, B). REFRACTION 27 Myopia, when the eyeball is too long, so that parallel rays focus in front of the retina (Fig. 26, c) . Fig. 26. A. Emmetropic eye. b. Hypermetropic eye. c. Myopic eye. Emmetropia in a strict mathematical sense is very rare. If we represent the eye by a biconvex lens, and the retina by a screen ; then it will correspond to emme- tropia when the screen is situated at the principal focus of the lens, as e. Fig. 27 ; we represent hyper- metropia (h) by bringing forward the screen, and myopia (m) by moving it further away from the lens. In all eyes, vision ranges from the far point or 28 THE EEFRACTION OF THE EYE punctum remotum (which in the emmetropic eye is at infinity) to the near point or punctum proximum. Fig. 27. i Convex lens of 23 mm. focus. Parallel rays focus at e (emmetropia) on the screen, forming a well-defined image of the object from which rays come ; at h (hypermetropia) they form a diffusion patch instead of an image, m (my- opia), also a diffusion patch, the rays having crossed and become divergent. The near point varies in the normal eye according to the amount of the accommodation, receding gradu- ally as age advances; when it has receded beyond 22 cm. (which usually occurs in the emmetropic eye about the age of forty-five) the condition is spoken of as presbyopia. Infinity is any distance beyond six metres, the rays coming from a point at or beyond that distance being parallel or almost so. The emmetropic eye, therefore, has its far point, or punctum remotum, situated at infinity ; the hyperme- tropic eye has its punctum remotum beyond infinity, and the myopic eye has its punctum remotum at a finite distance. Generally the two eyes are similar in their refrac- VISUAL ANGLE 29 tion, though sometimes there is a very great difference. One eye may be hypermetropic, the other myopic; or one emmetropic, the other ametropic. Anisome- tropia is the term used when the two eyes thus vary in their refraction. There may be differences also in the refraction of the different meridians of the same eye — astigmatism. In all forms of ametropia the acuteness of vision is liable to be diminished. The visual acuteness usually decreases slightly as age advances, without any dis- ease. The visual acuteness refers always to central vision. The yellow spot is the most sensitive part of the retina, and the sensibility diminishes rapidly towards the periphery. The acuteness is measured by the size of the visual angle, that is the angle formed at the pos- terior nodal point, which point closely coincides with the posterior surface of the lens, and is about 15 mm. in front of the yellow spot. In Fig. 28, let c d be an object for which the eye is Fig. 28. accommodated. The lines c c, a d, drawn from the extremities of the object, cross at the nodal point n. The angle c n d will be the visual angle under which 30 THE REFEACTION OF THE EYE the object c d is seen. The size of the angle depends upon the distance of the object as well as upon its magnitude, and the size of the image thus formed on the retina will also depend upon the antero-posterior diameter of the eyeball. Thus an object a b, which is as large as c d, but nearer to the eye, will be seen under a larger angle, the angle an b being greater than the angle c N d. It is also clear that the image formed on the retina will be smaller at 1, when the antero-posterior diameter of the eye is less, as in hypermetropia, than it is at 2 in emmetropia, ^nd that it will be larger in myopia, as at 3, where the eyeball is elongated. It is, therefore, easy to understand that a patient may be able to read the smallest type and still have some defect of refrac- tion, unless the type be read at its proper distance (see Fig. 35). It is by the unconscious comparison of things of known size, and the amount of accommodation brought into play, that we are able to estimate the distance of objects, and not by the visual angle alone. Objects must therefore be of a certain size, and it has been proved that to enable us to see a complex figure like a letter distinctly, each part of the figure must be separated from the other parts by an interval equal to not less than the arc subtending an angle of r at the nodal point. It has been shown (Fig. 26, b) that in the hyper- metropic eye in a state of rest, parallel rays are brought to a focus behind the retina, so that instead of a clear, well-defined image, we get a circle of dif- LENSES 31 fusion. Convex glasses render parallel rays passing through them convergent, so that by placing a lens of the right strength in front of the hypermetropic eye, we bring forward its focus on to the retiua. In myopia (Fig. 26, c) the focus for parallel rays is in front of the retina; concave glasses render parallel rays passing through them divergent, so that the proper concave glass will carry back the focus on to the retina. Lenses. — The lenses used for estimating the visual acuteness consist of two kinds, spherical and cylin- drical. Spherical lenses were until recently numbered according to their radii of curvature, which was considered as equal to their focal length in inches, a" glass of 1-inch focus being taken as a standard. To this plan there were several objections. The standard glass being a strong one, weaker glasses had to be ex- pressed in fractions. Thus a glass of 4-inch focus was one fourth the strength of the standard 1 inch, and was expressed as J. In addition to the trouble and incon- venience of working with fractions, the intervals between the lenses were most irregular, and, moreover, the inches of different countries vary. At the Oph- thalmological Congress in 1872 it was decided to adopt a metrical scale of measurement. A lens of 1-metre focus is taken as the unit, and is called a dioptre; a weak instead of a strong glass therefore becoming the unit, a lens of two dioptres is twice the strength of the former, and has a focal length of half a metre. Thus each lens is numbered according to its refracting power, and not, as in the old system, according to its 32 THE KEFEACTION 0¥ THE EYE focal length; so that we have a series composed of equidistant terms. The numbers 1 to 20 indicate the uniformly increasing power of the glasses. The focal length of a lens is not expressed in the dioptric measurement; we have only to remember that it is the inverse of the refracting power^ so that by dividing 100 cm. by the number of the lens we obtain its focal length in centimetres : for example^ if the strength of the lens be 2 D._, then the focal length will be 50 cm. ; if 10 D., then 10 cm. The intervals between dioptres is somewhat large, so that decimals, "25, '50, "75 of a dioptre, are intro- duced ; these work easily. On looking through a convex glass objects look larger, through a concave glass they look smaller. The cylindrical lens still remains to be mentioned; it consists of a lens one surface of which is usually plane while the other is the segment of a cylinder, and may be either convex or concave : if a convex cylinder be held vertically, the vertical meridian will be plane, exercising no influence on rays passing through it in that meridian; while the horizontal meridian will be convex, and will act as such on rays passing through it. The axis of the cylinders is usually indicated by a portion of the lens on each side being ground parallel to its axis. Accommodation. — In the normal eye, in a condition of complete repose, parallel rays come to a focus exactly on the rods and cones of the retina, and the object from which the rays comes is therefore seen distinctly. ACCOMMODATION 33 Eays from a near object proceed in a divergent direction^ and come to a focus behind the retina ; the object would not then be clearly seen unless the eye possessed within itself the power of bringing rays which are more or less divergent into union on the retina. This power of altering the focus of the eye is called accommodation, and is due to an alteration in the form of the lens. That the eye possesses this power can easily be proved in many ways, apart from the con- scious muscular eifort ; perhaps as simple a way as any to demonstrate it to one's self is to look through a net held a short distance off at some distant object. Either the net or the object can be seen distinctly, but not both at once. If the meshes of the net be looked at, then the distant object becomes indistinct, and on looking at the object the meshes become con- fused. Accommodation, therefore, increases the refraction of the eye, and adapts it to near objects. The changes which take place in the lens during accommodation are: 1st. The anterior surface becomes more convex and approaches the cornea. 2nd. The posterior surface becomes slightly more convex, but remains the same distance from the cornea. That these changes take place may be proved in the following manner: — A lighted candle, or other con- venient object, being held on one side of the eye, so as to form an angle of 30° with its visual axis, an observer looking into the eye from a corresponding 3 34 THE REFRACTION OP THE EYE position on the other side, will see three images of the flame : the first upright, formed by the cornea ; the second larger, upright, and formed by the anterior surface of the lens; the third smaller and inverted, formed by the posterior surface of the lens. When the accommodation is put in force, images one and three remain unchanged in shape and position ; image two, which is that formed by the anterior surface of the lens, becomes smaller, more distinct, and approaches image one, proving that this surface of the lens has become more convex and has approached the cornea. In an emmetropic eye adapted for infinity, it has been proved that the radius of curvature of the anterior surface of the lens is 10 mm. ; when accommodated for an object 13'5 cm. off it is changed to 6 mm. During accommodation the pupil becomes smaller, the central part of the iris advances, while the peri- pheral part slightly recedes. The alteration in the shape of the lens is due to the contraction of the ciliary muscle, which draws forward the choroid, thereby relaxing the suspensory ligament, and allowing the elasticity of the lens to come into play. This elasticity is due to the peculiar watch-spring arrangement of the lens fibres. When the ciliary muscle is relaxed, the suspensory ligament returns to its former state of tension, and so tightens the anterior part of the capsule, flattening the front surface of the lens."^ * Another theory of accommodation is Tscherning's, whose ex- periments have led him to believe that when the ciliary muscle contracts it increases the tension of the zonula, and alters the lens surface from a spherical to a hyperboloid form. PUNCTUM PROXIMUM 35 When the muscle is relaxed to its uttermost, the lens has assumed its least convexity, and the eye is then adapted for its far point {punctum remotum) (r). In this condition the eye is spoken of as being in a state of complete repose. In the emmetropic eye the punctum remotum is situated at infinity. Fig. 29. Diagram of lens, cornea, &c. The right half is represented as in a state of accommodation, the left half at rest. A, The anterior chamber, c. The cornea, l. The lens. V. The vitreous humour, i. The iris. m. Ciliary muscle. When the ciliary muscle has contracted as much as it can, the lens has assumed its greatest convexity, and its maximum amount of accommodation is in force. The eye is now adapted for the nearest point which can be seen distinctly; this is called the punctum proximum (p). The position of the punctum proximum can be determined in several ways ; the ordinary plan is to place in the patient's hand the small test type, and note the shortest distance at which he can read No. 1 with each eye separately. Or we may measure its posi- tion with the wire optometer, which consists of a steel 36 THE REFRACTION OF THE EYE frame crossed by tliin vertical wires; this is supported in a liandle to which a tape measure is attached; the tape is placed against the temple, and held there while the frame is made gradually to recede from the patient's eye we are examining, stopping as soon as the wires become distinct, and reading off the number of centimetres on the measure. Another excellent plan by which to find the position of the punctum proximum is that of Scheiner : close in front of the eye we wish to examine is placed a card pierced with two small pinholes, which must not be further apart than the diameter of the pupil; through these two holes the patient is directed to look at a pin held about one metre away (the other eye is of course excluded from vision during the ex- periment) ; the pin will be clearly and distinctly seen. We then gradually approach it to the eye : at a certain place it Avill become double : the point at which the pin ceases to appear single will be the punctum proximum. In Fig. 30 the biconvex lens L represents the eye. Fig. 30. D the perforated card, p the pin, e e' the two sets of rays from p, which focus exactly at b, the retina. If, AMPLITUDE OF ACCOMMODATION 37 however, the pin be brought nearer, so that the accom- modation is unable to focus the two sets of rays, they will form, instead of one, two images of the pin on the retina as at a. These will be projected outwards as crossed images. The space between the punctum remotum and the punctum proximum is called the range of accommoda- tion. The force necessary to change the eye from its punctum remotum to its punctum proximum is styled the amplitude of accommodation. The amplitude of accommodation, therefore, is equal to the diiference- between the refracting power of the eye when in a state of complete repose, and when its maximum amount of accommodation is in force, and may be ex- pressed by the formula a ■= p — r. A convex glass placed in front of the eye produces the same effect as accommodation, i.e. it increases its refraction and adapts the eye for nearer objects. We can easily understand that the lens which enables an eye to see at its near point without accommodating is a measure of the amplitude of accommodation, giving to rays which come from the near point a direction as if they came from the far point. The amplitude of accommodation is much the same in every kind of refraction. If we wish to measure it in an emmetrope, we have merely to find the nearest point at which the patient can read small print. A lens whose focal distance corresponds to this is a measure of the amplitude of accommodation. Thus, 38 THE EEFEACTION OF THE EYE supposing 20 cm. the nearest distance at whicli he is able to read small prints we divide this into 100 cm. to find the focal distance of the lens (-^-^ = 5 D.) ; and in this case a lens of 5 D. is the measure of the amplitude of accommodation. Or we can measure it in an inverse manner by looking at a distant object through a concave glass ; the strongest lens with which we can see this distant object distinctly is the amplitude of accommodation, the concave lens giving to parallel rays coming from the distant object such an amount of divergence as if they came from a point situated at the principal focal distance of this glass. Therefore the amplitude of accommodation in emmetropia is equal to the refraction when adapted to its punctum proximum, and may be expressed by the formula a = p — GO "^ or a = j) — or a = p. The Accommodation of Hypermetropes. — A hyper- metrope requires some of his accommodation for dis- tant objects ; we must therefore, in order to find the amplitude of accommodation in his case, add on to the lens whose focal length equals the distance of the near point, that convex lens which enables him to see distant objects without his accommodation, and we express it by the formula a = p — (— r) =p + r. Thus^ to take an example, we will assume the * X is the sign for expressing infinity. AMPLITUDE OF ACCOMMODATION 39 patient^s near point to be 25 cm. (^^ = 4 J).), and that lie has to use 4 D. of accommodation for distant objects ; then the amplitude of his accommodation would be 4 D. + 4 D. = 8 D. a = 4 D. - {- 4^ D.) = 8 D. The Accommodation of Myopes. — In a myope we have to subtract the glass which enables him to see clearly distant objects^ from that lens whose focal length equals the distance of the near point. The formula will then be a = p — r. Thus, to find the amplitude of accommodation in a myope of 2 D., the near point being at 10 cm., we subtract from (-y^ = 10) 10 D. the amount of the myopia, 2 D., and the resulting 8 D. is therefore the amplitude of accommodation. a = 10 D. - 2 D. = 8 D. Hence it is obvious that, with the same amplitude of accommodation, the near point is further away in hypermetropia than in emmetropia, and further in emmetropia than in myopia. Thus an emmetrope, with an amplitude of accommodation of 5 D., would have his near point at (-^-|-^ = 20) 20 cm. ; a hyper- metrope of 2 D., whose amplitude equalled 5 D., would require to use 2 D. of his accommodation for distance, leaving him 3D., which would bring his near point to (i^ = 33) 33 cm. ; and a myope of 2 D., who would require a concave glass of this strength to enable him to see at a distance, would have a near point of 14 cm. (-L^ =14) with the same amplitude. 40 THE REPEACTION OF THE EYE Accommodation is spoken of as absolute, hmocular, and relative. Absolute is the amount of accommodation wliicli one eye can exert wlien the other is excluded from vision. Binocular, that which the two eyes can exert to- gether, being allowed at the same time to converge. Relative, that which the two eyes can exert together for any given convergence of the visual lines. Fig. 31. Dioptres. Diagram showing by the number of squares through which the thick lines pass, the amplitude of accommodation at different ages in emmetropia. The figures above represent the amount of accommodation ; those below, the near point ; and those on the left, the age of the individual. Fig. 31 diagrammatically represents the amplitude of accommodation in ennnetropia. As age advances the elasticity of the lens dimin- ishes, the accommodation therefore becomes less, and the near point gradually recedes. These changes commence at a very early age, long before the indi- vidual has come to maturity. CONVERGENCE 41 The following table gives the amplitude of accom- modation at different ages as shown in Fig. 89, p. 188. Years. Amplitude of accommodation. 10 14 D. 15 12 D. 20 10 D. 30 7 D. 40 4-5 D. 50 2-5 D. 60 ID. 75 Convergence. — This is the remaining element of dis- tinct binocular vision, and with this function the accommodation is very intimately linked, so that usually for every increase of the convergence a certain increase in the accommodation takes place. Convergence is the power of directing the visual axes of the two eyes to a point nearer than infinity, and is brought about by the action of the internal recti muscles. When the eyes are completely at rest, the optic axes are either parallel, or more usually slightly divergent. The anoxic thus formed between the visual and the optic axis is called the angle a, and varies according 'to the refraction of the eye. In emmetropia the angle is usually about 5°; in hypermetropia it is greater, sometimes as much as 7^ or 8°, giving to the eyes an appearance of divergence ; in myopia the angle is less, often about 2°, or the optic axis may, even in extreme cases, fall on the inside of the visual axis, 42 THE REFEACTION OF THE EYE when the angle a is spoken of as negative (p. 203) ; so that in myopia there is frequently an appearance of convergence^ giving one the idea of a convergent squint ; hence the mere look of the patient^s eyes with regard to their axes is not always quite reliable. The object of convergence is the directing of the yellow spot in each eye towards the same point, so as to produce singleness of vision ; diplopia, or double vision, at once results when the image of an object is formed on parts of the retina which do not exactly correspond in the two eyes. To test the power of convergence prisms are held with their bases outwards. The strongest prism which it is possible to overcome, that is the prism which does not produce diplopia on looking through it at a distant object, is the measure of the converg- ence, and varies in different persons, usually between prisms of 20° and 30°, divided between the two eyes. This is the relative convergence for infinity. In considering convergence we have not only to bear in mind the condition of the internal recti muscles, but also the state of equilibrium produced by them and the action of their antagonists — the external recti. The nearer the object looked at, the more we have to converge, and the. greater the amount of accom- modation brought into play. Hence, on converging to any particular point, we usually also involuntarily accommodate for that point, the internal recti and ciliary muscles acting in unison. Nagel has proposed a very simple and convenient CONVEEGENCE Fig. 32. 43 44 THE REFRACT] ON OF THE EYE plan, by wliicli we may express tlie convergence in metres, calling the angle formed by the visual and median lines, as at m', the metrical angle. In Fig. 32 E, e' represent the centres of rotation for the two eyes ; E H e' is the base line between the centres. When the eyes are fixed on some distant object, the visual lines are parallel or almost so, as e a, e' a' ; the angle of convergence is then at its minimum, and the convergence is said to be adapted for its pionctum remotum ; this then, being at infinity, is expressed C ^ = oo . If the eyes be directed to an object one metre away, the metrical angle E m' h equals one, i. e.C = J . If the object is 50 cm. olf, then = 2; if 10 cm., then (Y/ = 10; C = 10. If the object had been be- yond 1 metre (our unit), but not at infinity, say 4 metres, then C = ^. When the visual lines, instead of being parallel, diverge, then the punctum remotum is found by con- tinuing these lines backwards till they meet at c, behind the eye ; the convergence is then spoken of as negative. When the eyes are directed to the nearest point at which they can see distinctly, say at m'", the angle of convergence is at its maximum, and it is said to be adapted for its ininctum lyroAmiim. The distance between the punctum proximum and the punctum remotum is the range of convergence. The amplitude of convergence is the whole converg- * C is the sign for convergence. CONVEEGENCE 45 ence that can be put in force, and we express it by the formula. c = p — r. The functum remotum of convergence is seldom situated at a finite distance : sometimes it is exactly at infinity, but in the majority of cases it is situated beyond infinity, i. e. the visual lines diverge slightly. In order to measure this divergence, and so obtain the punctum remotum of convergence, we place before the eyes prisms with their bases inwards (abducting prisms), and the strongest prism through which the person is still able to see singly is the measure of the negative convergence. Prisms are numbered in degrees according to the angle of the prism. The deviation produced by a prism is equal to half its angle ; thus prism 8 will produce a deviation of the eye of 4°, and prism 20 a deviation of 10°. When a prism is placed before one eye, its action is equally divided between the two eyes. To take an example : if an abducting prism of 8° placed before one eye (or what is the same thing, 4° before each eye) be found to be the strongest through which a distant object can be seen singly, then each eye in our example has made a movement of diverg- ence equal to 2°, and the punctum remotum of con- vergence in this case is therefore negative, and is ex- pressed — 2°. By referring to the table on page 49 it will be seen that when the centres of rotation of the eyes are 6*4 cm. apart, then the metre angle 46 THE REFRACTION OP THE EYE equals 1° 50', so we reduce the 2° to metre angles, thus : 2^ 120 ^ p^,=: j-^^= 1-09 m a; or it is sufficient to remember to divide the prism placed before one eye by seven ; thus in our example we should divide prism 8° by seven, and this would give us approximately the same result. Another excellent plan for finding the punctum remotum of convergence is by Maddox^s test, which consists of a small glass rod placed behind a stenopaic slit ; when this is held horizontally before the right eye, and the flame of a candle viewed from a distance of 6 metres with both eyes open, the left eye receives the image of the flame, while the right receives the image which is drawn out by the rod into a long vertical strip of light ; and since the image received by the two eyes is very different, there is no tendency to fusion, and the eyes take up their position of rest. A suitable scale placed behind the candle will give us the amount of convergence or divergence in metre angles, according to the position occupied by the streak of light on the scale. Should the patient be a myope or a hypermetrope he should wear his correc- tion when this test is applied. To find the punctum proximum of convergence, hold a prism, base outwards (adducting prism), before one eye, and the strongest prism which can be so employed without producing diplopia, divided between the two eyes, gives the punctum proximum of convergence in degrees. But the accommodation must be stimulated CONVERGENCE 47 at the same time by means of a concave glass, other- wise we obtain only the relative punctum proximum. This can be reduced to metre angles as before. Or a simpler plan is to measure it with Landolt's ophthalmo-dynamometer, which is a small instrument consisting of a black metallic cylinder, a, made so as to fit upon a candle, b. The cylinder has a vertical slit '3 mm. in breadth, covered by ground glass : the candle being lighted, this slit forms a luminous line, Fig. 33. and will serve as a fixation object. A tape measure is conveniently attached, being graduated in centi- metres on one side, and on the other in the corre- sponding numbers of metre angles. To find the punctum proximum of convergence, the measure is drawn out to about 70 cm., its case being held beside one of the eyes of the patient, while the object of fixation is placed in the median line. If the illuminated line is seen singly, by pressing the knob 48 THE REFRACTION OF THE EYE of the case the spring rolls up the tape^ and the fixa- tion object is brought nearer the eye. So soon as the patient commences to see double, the nearest point of convergence is obtained, and the maximum of con- vergence is read off the tape in metre angles. This experiment should be repeated several times. In a normal case the minimum of convergence is usually about — 1 7n a, the maximum 9"5 m a; so that the amplitude of convergence equals 10*5 m a. We know that the accommodation increases the nearer the object approaches, hence we see that both the convergence and accommodation increase and decrease together ; and in recording the convergence in the manner proposed by Nagel, it will be seen that in the emmetropic eye the number which expresses the metrical angle of convergence expresses also the state of refraction for the same point — this is a great advantage. Thus, when looking at a distant object, the angle of convergence is at infinity, C. = oo ; and the refraction is also at infinity, A = go . When the object is at 1 metre, the angle of convergence = 1, and the amount of accommodation put into play = 1 D. When the object is at 25 cm., then the angle of convergence = 4, and the amount of accommoda- tion = 4 D. The amplitude of convergence is somewhat greater than the amplitude of accommodation, passing it both at its punctum remotum and its punctum proximum. The following table shows the angle of convergence in degrees, for different distances of the object, when the eyes are 6*4 cm. apart: CONVERGENCE •istance of the object from the eyes. The metrical angle. 1 metre 1 50 cm. 2 33 „ 3 25 „ 4 20 „ 5 16 „ 6 14 „ 7 12 „ 8 11 „ 9 10 „ 10 9 „ 11 8 „ 12 7-5„ 13 7 „ 14 6-5„ 15 6 „ 16 5-5„ 18 5 „ 20 49 Value expressed in degrees. 1=50' 3° 40' 5° 30' 7° 20' 9° 10' 11° 12" 50' 14° 40' 16° 30' 18° 20' 20= 10' 22° 23° 50' 25° 40' 27° 30' 29° 20' 33° 36° 40' Althougli accommodation and convergence are thus intimately linked together, it can very easily be proved that they may have a separate and indepen- dent action. If we suspend the accommodation with atropine, the convergence is not interfered with ; or an object at a certain distance being seen clearly without a glass, it can still be seen distinctly with weak concave and convex glasses, without any altera- tion of the convergence. It may, therefore, be stated that although the accommodation and convergence are intimately asso- ciated, they may be independent of each other to a certain degree, so as to meet ordinary requirements ; thus for instance, as age advances changes take place 4 50 THE KEFRACTION OF THE EYE in the lens wliicli necessitate a stronger contraction of the ciliary muscle to produce the requisite change in the accommodation^ while the convergence remains the same. It is obvious also that the relations between accom- modation and convergence must necessarily be very different in ametropia^ and this relation will be agrain referred to when treatino- the various errors of refraction in detail. The following diagram (Fig. 34) shows the relative amount of accommodation for different points of con- FiG. 34. vergence in an ennnetrope aged fifteen. The amount of accommodation in excess of the metrical angle of RELATIVE ACCOMMODATION 51 convergence is called iiositire, and tlie amount below negative. The diagonal d d represents the convergence from infinity to 5 cm. ; it is also a record of the accommo- dation. The line p p' p" indicates the maximum ac- commodation for each point of convergence^ and the line r v the minimum. The numbers on the left and below the diagram are dioptres and metrical angles of convergence ; thus, when the visual lines are parallel, it will be seen that 3'5 D. of positive accommodation can be put into play, i. e. the object can still be seen distinctly with a concave glass of that strength ; 3'5 D. is therefore the relative amplitude of accommodation for convergence adapted to infinity ; or the metrical angle C being 5, which is a distance of 20 cm. away, the accommodation for that point would equal 5 D. ; the positive amount that can be put in force while the angle of C remains the same would be 3 D., and the negative also 3 D., the object being seen clearly with a concave or convex glass of 3 D., therefore the relative amplitude of accommoda- tion for C 5 is 6 D. When the angle C = 10 or any larger angle, the accommodation that can be put in force will be seen to be entirely on the negative side. Thus, the convergence being fixed, the amount of accommodation which can be brought into play for that particular point is the sum of the diiference between the strongest concave and convex glass employed. The eye being accommodated for an object at a certain distance, the amount of convergence for that 52 THE REFRACTION OP THE EYE particular point may be measured by placing in front of the eyes prisms, bases outwards; the strongest prism through which the object is still seen singly is the measure of the positive part of the amplitude of convergence. Prisms, bases inwards, give us the negative part — the sum of these is the amplitude of relative convergence. METHODS OF DETERMINING THE EEFRACTION 53 CHAPTER III METHODS OF DETERMINING THE REFRACTION In entering upon the practical part of the subject the following subjective and objective methods present themselves for consideration. 1. The acuteness of vision. 2. Scheiner's method. 3. The ophthalmoscope. {a) The indirect method. (b) The direct method. (c) Retinoscopy. In every case we must proceed in a systematic manner^ and before commencing to take the patient^s visual acuteness^ something may be gained by noticing the general appearance of the patient, the form of the face, head, etc. ; thus a flat-looking face is sometimes an indication of hypermetropia ; a head elongated in its antero-posterior diameter, with a long face and prominent nose, may indicate myopia. If the two sides of the face are not symmetrical, or if there be some lateral displacement of the nose from the median line, astigmatism may be suspected. We should also notice the shape of the eyes themselves, if large and prominent, or small; in the former case we may sus- pect myopia, in the latter hypermetropia. Large pupils are suggestive of myopia, and small pupils of 54 THE REFRACTION OF THE EYE liypermetropia. In liigli degrees of astigmatism it can sometimes be seen that tlie curvature of tlie cornea in one meridian exceeds that of the other. The distance between the eyes should also be noted^ as well as the direction of their visual axes. We next listen to the patient's own statement of the troubles from which he suffers ; he may say that he sees distant objects well but has difficulty in reading, especially in the evenings, or that after reading for some time the type becomes indistinct, so that he must rest awhile, — here we suspect hypernietropia ; or he may be able to read and do near work, but sees badly at a distance, — then we suspect myopia; or both near and distant vision may be defective, — in this case our first object must be to decide whether the imperfect vision is due to some error of refraction or to some structural change in the eyes themselves ; and we possess an extremely simple method by which to differentiate between them, and this method is called the Pin-hole test. Pin-hole Test. — A black diaphragm having a small perforation in its centre (the box of trial glasses usually .contains such a diaphragm) is placed quite close to the eye under examination. This perforation gives passage to a small pencil of rays which passes through the axis of the refracting system of the eye, so that the image formed is clearly defined for all distances : if then the pin-hole improve vision, the refractive system is at fault; but if, on the contrary, vision is not improved, then we suspect that the transparency of the media or that the retinal sensi- ACUTENESS OF VISION 55 bility is defective ; thus we possess a very simple and reliable plan, which if used systematically, may save much loss of time. The points to notice when apply- ing this test are, that the illumination is good, and that the pin-hole is immediately in front of the centre of the pupil. Having then found out that the patient's refraction is defective, we proceed to the first method, the acuteness of vision. The Acuteness of Vision. — This must not be confused with the refraction, and it is necessary clearly to understand the difference between these two terms. The acuteness of vision is the function of the nervous apparatus of the eye, while the refraction is the func- tion of the dioptric system; so that the acuteness of vision may be normal, even if the refraction be very defective, provided it has been corrected by glasses. The refraction, on the other hand, may be normal, even though the eye is unable to see, as in cases of optic atrophy, etc. We may define the acuteness of vision as that degree of sight which an eye possesses after any error of its refraction has been corrected, and the glasses neces- sary for this correction are therefore a measure of the error of refraction. In order to find out the acuteness of vision, it is necessary to determine the smallest retinal image the form of which can be distinguished in the normal eye; it has been discovered by experiments that the smallest distance between two points on the retina which can be separately perceived is 0"00436 mm., about twice 56 THE REFRACTION OF THE EYE the diameter of a single cone; but it is only at the macula and the part immediately around it^ Avhich is the most sensitive part of the retina_, that the cones are so close together as "002 mm. ; in the periphery of the field of vision the two points must be further apart to appear distinct. It is probable that rays from two points must fall upon two diiferent cones in order to be visible as two distinct objects. The same thing may be expressed in another way ; the smallest retinal image which can be perceived at the macula corresponds to a visual angle of V, so that two stars separated by an angular interval of less than r would produce upon the eye the eifect of one star only. The visual angle has been shown to be an angle included between two lines drawn from the two oppo- site edges of the object through the nodal point (Figs. 28 and 35). Fig. 35. Test-types have been constructed upon these prin- ciples for determining the acuteness of vision, Snel- ling's being those ordinarily used. Every letter is so made that when at its proper distance, each part of it ACUTENESS OF VISION 57 is separated from the other parts by an interval equal to not less than the arc subtending an angle of 1' at the nodal pointy while the whole letter subtends an angle of 5'. In order to estimate the refraction by the acuteness of vision, the test object must be placed in a good light, and so far away as to exclude as much as pos- sible the accommodation, — 6 metres has been found to be a sufficient distance ; rays coming from an object so far off may be assumed to be parallel, and falling on an emmetropic eye at rest, come to a focus on the retina. The smallest letter which can be seen distinctly at this distance will represent the patient^s vision. Snellen^s type consists of rows of letters, each row being marked above with the distance in metres at which it should be read. The top letter should be distinct at 60 metres, the next at 36, and each succeed- ing row at 24, 18, 12, 9, and 6 metres respectively."^ The patient placed at six metres should, without any accommodation, be able to read the bottom line with either eye. This is expressed in the form of a fraction, in which the numerator indicates the distance at which it is read, and the denominator the number of the line. We note down the result found for each eye sepa- rately : if the bottom line is read, |- expresses it ; if the next, |-; the top, -^, etc. If our patient is not able even to see the large letter at the top, we allow him to approach the board, telling * The set of test-types at the end of the book has two additional lines, marked 5 and 4, so that a greater amount of visual acute- ness than f can be estimated, and is, of course, recorded | and f . 58 THE REFRACTION OF THE EYE him to stop when he recognises the letter. Supposing he stop at two metres from the board, we express that as -^^^ ; if he is not able to read it when quite close to the eye, we see how far off he can count fingers. If unable to do this, the hand may be passed quickly in front of the eye, and if these movements are seen the vision is expressed "hand movements^' (H. M.). Should these movements not be seen we throw light into the eye with a convex lens ; if the light is per- ceived, it is called perception of light (P. L.). When the patient fails to distinguish the difference between light and darkness the eye is quite blind, there is no perception of light (no P. L.). Although the capability of reading the bottom line at 6 metres is the average acuteness of vision, yet it is not the maximum, since many young people will be found who are able to read line six at 7 metres, or even further, in which case their acute- ness is I". Savages often have an acuteness of vision much in excess of the normal. Thus we have a standard of normal vision, and a con- venient method of expressing it in a numerical manner. We put our patient then, with his back to the light, in front of the test-types, which must hang well illu- mined at 6 metres distance, and having armed him with a pair of trial frames, we exclude the left eye from vision by placing in front of it an opaque disc, and proceed to test the right eye by asking him how much of the type he is able to read ; if he read the line marked 6, then his vision is ^ or 1, that is to ACUTENESS OF VISION 59 say, his distant vision is normal ; we may, therefore, assume the absence of myopia or astigmatism; but he may have hypermetropia, and only be able to read -| by using his accommodation ; this we decide by hold- ing a weak convex glass (+ '5 D.) in front of the eye, when if he still be able to read the same line f, he has hypermetropia, and the strongest convex glass with which # can be read is the measure of the mani- fest hypermetropia ; supposing + 1 D. the strongest glass with which -J can be read, then we record it thus : R. V. f Hm. 1 D. = f . I say manifest hypermetropia, because in all cases occurring in young people this is not the total hyper- metropia; for in these a great part of the error is latent, and can only be discovered by using atropine, or by estimating the refraction by the direct opthalmo- scopic method. Many cases will come before us having two or three dioptres of hypermetropia, who complain that the weakest convex glass impairs distant vision ; in these cases the hypermetropia is wholly latent. We may say, therefore, that a patient w^ho is able to read |- with one eye, must be — Emmetropic or Hypermetropic in that eye. If hypermetropic, a part of the defect is usually manifest ; the strongest convex glass which does not impair distant vision gives us the amount. If the hypermetropia is wholly latent, then it is necessary to atropise the patient before it can be demonstrated. 60 THE REFRACTION OF THE EYE Supposing^ however, our patient^s vision is below the normal, and, instead of reading |-, he is only able to read, say the third line (2^^), and that this is blurred with a weak convex glass, he may have — Myopia, Astigmatism^ or Spasm of the accommodation (see p. 197). We try if a weak concave glass helps him ; if it does so^ the case is one of myopia ; and we find the iveakest concave glass with which he sees best ; thus to take an example in which the patient is a myope and sees only YT^ but with — 2D. reads f ; we repeat the exami- nation with the second eye, and record it — R.V.J__2D. =f. L.V.^\-2D. =|. If the patient is not improved with concave glasses, then we assume that some astigmatism is present, presupposing of course that there is no other cause for bad vision. To estimate this astigmatism we must call to our aid some of the methods described in the chapter on astigmatism, p. 156, or first find out the spherical glass with which he is able to see best, then rotate in front of it a weak convex cylindrical glass, starting with its axis vertical ; no improvement occurring, do the same with a weak concave cylinder, starting with its axis horizontal ; finding by this plan the glass and its particular axis which gives the best result. It is necessary that the eye be thoroughly under the influence of atropine, in order to enable us to arrive ACUTENESS OE VISION 61 at definite and reliable results by this method. With practice, one is able in this way to work out simple cases of astigmatism accurately and quickly. The object in view is always to bring up the vision of each eye as nearly to the normal standard (-f) as possible. Frequently, however, we have to be satisfied with |- or -j\. But should the case appear to be a difficult one, it is better perhaps for the student not to waste time, but proceed at once to retinoscopy. When trying the patient at the distant type it is convenient to have two or more sets of letters, so that the type may be changed when the patient gets accus- tomed to one set. The near type is chiefly used to estimate the accommodation, by finding out the far and near point at which any particular line is read. Snellen^s and Jaeger^s are the types most commonly in use, many preferring Jaeger's, owing to the letters being of the ordinary shapes; but they have the disadvantage that they are not arranged on any scientific plan, but are simply printer's types of various sizes : the set of reading type at the end of the book is so arranged that when held at the distance for which it is marked, each letter subtends an angle of 5' at the nodal point. It must, however, be remembered that sentences are an inferior test to letters, many people recognising the words by their general appearance, whereas they may be unable to see distinctly every letter of which each word is composed. 62 THE REFEACTION OF THE EYE Having tested our patient's vision at the distant type and recorded the result, we place in his hand the reading type, and note the smallest print he is able to read and the distance at which he reads it ; first w4th each eye separately, then with the two together. In cases of myopia we may thus get a valuable hint as to the amount of the defect : take for an ex- ample a case in which the patient can read -J^-^ with the right eye; we give him the near type, and if he can read the smallest only by holding it at a nearer point than the distance for which it is marked, note the greatest distance at which he is able to read it; if the type marked for 1 metre cannot be read further off than 25 cm., our patient has then most likely myopia of 4 D., because 25 cm. is probably his far point. In this case a glass — 4 D. will give to rays coming from a distant point the same amount of divergence as if they came from 25 cm. (-y^ = 4). We try the patient at the distant type with — 4 D.; if he now read -| the myopia is confirmed, and the weakest glass with which he reads it is the measure of his myopia. If the patient read -J, but be unable to read the near type except it be held at a further distance than that for which it is marked, the case is one of paralysis of the accommodation, or presbyopia; and as the latter only commences in emmetropia about the age of forty- five, it will be clear according to the age of the patient to which division the case belongs. As objects seen through a convex lens appear en- larged, and through a concave lens diminished, it ACUTENESS OF VISION 63 follows thatj when placed before the eye, they will produce the same effect. Now the hypermetropic eye sees objects smaller and the myopic eye larger than the emmetrope, and if glasses which are to correct the ametropia be placed in the anterior focal plane, i. e. about 13' 7 mm. in front of the cornea, the retinal image of the ametrope will be of the same size as that of the emmetrope. Before leaving this subject of the acuteness of vision the following directions may be given : 1st. Thd test-type must be in a good light; the advantage of artificial illumination is that it is uni- form. 2nd. Commence with the right eye, or that Avhich has the best vision, covering up the other with an opaque disc placed in a spectacle frame ; do not be contented to allow the patient to close one eye, as he may not do so completely, or he will probably uncon- sciously slightly diminish the palpebral aperture of the eye under examination, whereby the circles of diffusion may be somewhat diminished and so give misleading results. Neither should he close the eye with his hand, he may look between the fingers, or exercise some pressure, however slight, on the eye- ball, which may interfere temporarily with the func- tion of the retina and so cause delay. 3rd. Having noticed what each eye sees without glasses, always begin the examination with convex ones, so as to avoid calling the accommodation into action. 4th. Having recorded the result found for each eye separately, try the two together, the binocular 64 THE REFEACTION OF THE EYE visual acuteness being usually slightly greater than that for one eye. 5th. Test the patient with the reading type, noting the nearest and farthest point at which the smallest type can be read. Schemer's Method. — Although this plan for detecting ametropia is now but little used, it is necessary the student should understand the principles upon which it is based. A diaphragm having two small perfora- tions is placed in front of the eye we wish to examine ; the perforations must be so near together that rays passing through them will enter the pupil (Fig. 86). The patient is directed to look at a small flame 6 metres off; rays emanate from this flame in all direc- tions, some fall on the diaphragm, the greater number are thus cut off, but a few rays pass through the two openings, and if the eye be adapted for the flame, ^. e. if it is emmetropic, these two sets of rays will Fig. 36.* meet exactly on the retina, forming there one image of the flame (b. Fig. 36) ; if, however, the eye be * In the above diagram, p is represented as a near object -with rays diverging from it ; it should bo a distant object with parallel rays. 65 hypermetropic (with suspended accommodation), then the two sets of rays will reach the retina before meeting, each set forming an image of the flame (a_, Fig. 36). The greater the hypermetropia the further apart will the images be formed ; these are projected outwards as crossed images, and the patient has therefore crossed diplopia. That convex glass (from our trial box) which, held behind the diaphragm, causes the flame to be seen singly, is a measure of the hyperme- tropia. If the eye be myopic, then the two sets of rays will have crossed and are diverging when they reach the retina, where two images of the flame are therefore formed (c. Fig. 36). These images are crossed again as they are projected outwards, and having twice crossed, homonymous images are the result. To find the amount of myopia, we have only to find the concave glass which, placed behind the diaphragm, brings the two images into one. To enable us to tell if the images are crossed or homonymous, it is usual to have in front of one of the perforations a piece of coloured glass. We will sup- pose the diaphragm held so that the two openings are horizontal, that to the patient^s right having in front of it a piece of red glass : if only one flame is seen the case is one of emmetropia ; if two images of it appear, one white, the other red, with the red to the left of the other, the images are crossed, and the case is one of hypermetropia. If the red appear on the right, then the case is one of myopia. The further apart the images are, the greater is the ametropia. 66 THE REFRACTION OF THE EYE CHAPTER IV THE OPHTHALMOSCOPE The Ophthalmoscope furnishes us with several methods for determining the refraction of the eyes. a. The indirect method. h. The direct method. c. Retinoscopy. The Indirect Method. — By the indirect method we obtain an inverted image of the disc by means of a bi- convex lens placed in front of the eye. In emmet ropia (Fig. 37) rays coming from a emerge from the eye Fig. 37. parallel, and are focussed by the convex lens at a, rays coming from b are focussed at h, so also with rays coming from every part of a b; an inverted image of A B is therefore formed in the air at h a, the princij^tal focus of the biconvex lens. THE OPHTHALMOSCOPE 67 In hypermetropia (Fig. 38) the rays from A emerge Fig. 38. divergent^ so also of course those from b ; if these rays are continued backwards, they will meet behind the eye (at the punctum remotum), and there form an enlarged upright image (a j3) of a b ; it is of this imaginary projected image that we obtain by the help of the biconvex lens a final inverted image (b a), situated in front of the lens beyond its principal focus. Fig. 39. In myopia (Fig. 39) the rays from A and b emerge from the eye convergent, forming an inverted aerial image in front of the eye at j3 a, its punctum remotum. It is of this image we obtain, with a biconvex lens 68 THE REFRACTION OF THE EYE placed between it and the eye, a final image {h a) situated within the focus of the biconvex lens. By this method we are able to detect the form of ametropia by the changes which take place in the size and shape of the optic disc, always remembering that the inverted image of the disc, produced by a convex lens at a certain fixed distance from the cornea, is larger in hypermetropia, and smaller in myopia, than in emmetropia. When the lens is held close to the patient^s eye, and then gradually withdrawn, while the aerial image of the disc is steadily kept in view ; the rapidity with which any increase or decrease takes place in the size of this image gives an indica- tion of the amount of the refractive error. If no change take place in the size of the image on thus withdrawing the objective the case is one of emmetropia, because the rays issue from such an eye Fig 40. E. Emmetropic eye. Rays issuing parallel, image formed at the principal focus of the lens, no matter at what distance the lens is from the eye. parallel, and the image formed by the object-glass will always be situated at its principal focus, no matter at what distance the glass is from the observed eye INDIEECT EXAMINATION 69 (Fig. 40). As the distance of the image from the object-lens is always the same^ the size of the image will also be the same. If diminution take place in the size of the image, the case is one of hypermetropia, and the greater the diminution the higher is the hypermetropia. Fig. 41. Lens at 4 cm. from the cornea. Fig. 42. Lens at 12 cm. from the cornea. H. Hypermetropic eye. c. The centre of the lens. ab. Image on the retina, ah. Projected image. /3 a. The final image formed by the objective. This change in size may be explained by remem- bering that in hypermetropia the image of the disc formed by the object-glass is situated beyond its 70 THE EEFRACTION OF THE EYE principal focus_, owing to the rays issuing from the eye being divergent ; the relative size of the final image j3 a to the object a h will therefore vary directly as the length c a, and inversely as the length C a so that on withdrawing the lens c from the ob- served eye c a diminishes and c a increases ; there- fore the ratio oi a j3 to a h diminishes^ i. e. the size of the image diminishes. The two diagrams Figs. 41 and 42 show images formed by the object-glass when held at 4 cm. and at 12 cm. from the cornea, the latter image being the smaller. If the image become larger on withdrawing the object-glass, the case is one of myopia ; the greater the increase of the image, the higher the myopia. This increase in the size of the image can also be explained with the help of mathematics, remembering that in myopia an inverted image is formed in front of the eye (Fig. 45), and it is of this we obtain an image with a convex glass placed between the eye and the inverted image, which we must regard as the object ; the object and its image being both on the same side of the lens. In astigmatism, the disc, instead of appearing round, is frequently oval. If the image of the disc decrease in size in one meridian, while the other remain sta- tionary as the objective is withdrawn from the eye, it is a case of simple hypermetropic astigmatism. If the whole disc decrease in size, one meridian diminish- ing more than the other, it is compound hypermetropic astigmatism, the meridian being most hypermetropic which diminishes most. CONCAVE MIREOE AT A DISTANCE 71 Increase in one meridian, tlie other remaining stationary, indicates simple myopic astigmatism. Increase in the size of the disc, but one meridian increasing more than the other, indicates compound myopic astigmatism, that meridian being most myopic which increases most. If one meridian increase while the other decrease, mixed astigmatism is our diagnosis. The Large Concave Mirror at a Distance. — If the observer be able to see the disc or some of the vessels with the mirror alone at a distance from the patient, the case is one of hypermetropia or myopia. The explanation of this is, that in emmetropia (Fig. 43) the rays which come from the two extremities of the disc (a b) emerge as two sets of parallel rays in the Fig. 43. same direction as the rays A c, b d, which, having passed through the nodal point, undergo no refraction. These two sets of rays soon diverge, leaving a space between them, so that an observer (unless he be quite close to the observed eye) is able only to bring rays from one point to a focus on his retina; and there- fore, at a distance from the eye, the observer sees only a general illumination. 72 THE EEFRACTION OF THE EYE In hypermetropia (Fig. 44) the rays from the two points A B emerge from the eye in two sets of diverg- ing raySj in the same direction as the rays A c_, B d, which undergo no refraction. These diverging rays have the appearance of coming from two points {a h) behind the eye^ where an erect imaginary image is formed. Fig. 44. The more the rays diverge on exit, the sooner they will meet when prolonged backwards ; and hence the greater the hypermetropia, the nearer will the image be to the nodal point. Fig. 45. The observer at a distance sees a clear, erect image which is formed behind the eye. In myopia (Fig. 45) the rays from the two points DIEECT EXAMINATION 73 (a b) emerge as two converging sets of rays, which meet at a h on their secondary axes, thus forming an inverted image in front of the eye. This image can be distinctly seen by the observer if he be at a sufficient distance from the point, and accommodating for the particular spot at which the aerial image is formed. The higher the myopia, the nearer to the eye will this image be formed. From the above observations it will be understood that if the observer now move his head from side to side, and the vessels of the disc are seen to move in the same direction, the case will be one of hyperme- tropia, the image formed being an erect one. Should the vessels move in the opposite direction to the observer's head the case will be one of myopia, the image being an inverted one formed in the air in front of the eye. If the vessels of only one meridian are visible, then we have a case of astigmatism, hypermetropic if moving in the same, and myopic if moving in the opposite direction to the observer's head, that meridian being ametropic which is at right angles to the vessels seen. In mixed astigmatism the vessels of one meridian move against the observer's movements, and those of the other meridian with them : this is difficult to see. Thus we have obtained an indication of the form of ametropia. We may, however, estimate the amount of error by means of a refracting ophthalmoscope, of which there are many. The Direct Method. — By the direct examination we 74 THE REFRACTION OP THE EYE obtain mucli more important information, not only of a qualitative, but also of a quantitative character. In endeavouring thus to estimate the refraction, it is essential that the accommodation of both the patient and observer be suspended. The observer first cor- rects any ametropia that he may have, either by having the proper correction in a suitable clip behind the sight-hole of his ophthalmoscope, or he may deduct his own ametropia from the glass which corrects the refraction of the patient and himself in the manner to be presently described. He then sits or stands as he may prefer on the same side as the eye he is about to examine, so as to use his right eye for the patient^s right, and his left for the patient^s left. The light is placed on the side to be examined, a little behind and on a level with the patient's ear ; then with the mirror held close to the eye to be examined, so that the ophthalmoscope may occupy as nearly as possible the position of the spectacle glass, the observer looks for the disc. We really wish to estimate the refraction at the macula, but to this there are several obstacles : the light falling on this, the most sensitive part of the retina, has a very dazzling unpleasant effect for the patient, and causes the pupil to contract vigorously, the reflex from the cornea and the lens is exactly in the line of vision, and further there are no convenient vessels in this part which we may fix as test objects ; whereas the disc is but little sensible to light, and the vessels of this part, as well as the margins of the disc itself, are very convenient for our purpose, and although occasionally the refrac- DIRECT EXAMINATION 75 tion of the macula and disc are not exactly the same, practically it is sufficiently accurate to take that of the latter. To estimate the refraction by the direct method, it is necessary that the patient's accommodation should be relaxed; this will generally be the case when the examination is made in a dark room ; or a mydriatic may be used ; then, if the observer's own accommodation be suspended, and the image of the disc appear quite clear and distinct, the case is one of emmetropia. This we know,- because rays coming from an emmetropic eye (Fig. 46, e) in a state of repose Fig. 46. will issue parallel, and the observing eye receiving these rays will, if emmetropic with its accommodation suspended (which often requires great practice), be adapted for parallel rays, so that a clear image of a in the observed eye will be formed at h on the retina of the observing eye. Supposing the image does not appear clear and dis- tinct without an effort of accommodation, then we turn the wheel of the ophthalmoscope so as to bring for- ward convex glasses in front of the observing eye. The strongest positive glass with which we are able to 76 THE REFRACTION OF THE EYE get a perfectly clear image of the disc is a measure of the hypermetropia, because rays coming from a (Fig. 47) in the hypermetropic eye (h) issue in a divergent direction as though coming from R^ the punctum remotum behind the eye. The convex lens L renders these rays parallel, and they then focus at h, on the retina of the observing emmetropic eye (e). Fig. 47. In practice many observers find it difficult or im- possible to tell if their own accommodation be com- pletely relaxed, so that if they see clearly the disc of the patient under examination, they do not at once assume that he is emmetropic, but only do so on find- ing that the weakest convex glass behind the ophthal- moscope impairs the clearness of the image. If, however, the image of the disc appear indistinct, and the convex glass, instead of rendering the image clearer, have the opposite effect, we must turn the wheel of the ophthalmoscope in the other direction, and so bring forward the concave glasses. The weakest with which the details of the fundus can be clearly seen is a measure of the myopia, because any stronger glass merely brings into play the accommodation of the observer. Rays from a (Fig. 48) leave the myopic DIRECT EXAMINATION 77 eye (m) so convergent, that they would meet at (r) the punctum remotum. The concave lens l renders these rays parallel before falling on the relaxed eye (e) of the observer. Fig. 48. ^^ If the ophthalmoscope is not held very close to the eye, we must deduct from the focal distance of the lens the distance between the cornea and the instru- ment in hypermetropia, adding them together in myopia (p. 118). If astigmatism exist, the plan is to find the glass which enables the vertical vessels and lateral sides of the disc to be seen distinctly, and then the glass with which the vessels at right angles are best seen. Suppose the vertical vessels and lateral sides of the disc appear distinct without any glass, then the hori- zontal meridian, i. e. the meridian at right angles to the vessels clearly seen, is emmetropic ; and suppose also that the horizontal vessels with the upper and lower borders of the disc require a convex or con- cave glass to render them clear and distinct, then the vertical meridian is hypermetropic or myopic, and the case is one of simple hypermetropic or myopic astigmatism. If both the vertical and horizontal vessels can be 78 THE EEFRACTION OP THE EYE seen with convex glasses, a stronger one being required for the vertical than for the horizontal vessels_, then the case is one of compound hypermetropic astigmatism, the horizontal meridian being the more hypermetropic. If both meridians had required concave glasses, but of different strengths, then the case would be one of compound myopic astigmatism. If the vertical vessels and the lateral sides of the disc can be seen distinctly through a convex glass, while the horizontal vessels require a concave glass, the case is one of mixed astigmatism, the horizontal meridian being hypermetropic, the vertical meridian myopic. The essential point to remember is, that the glass with which the vessels in one direction are seen is a measure of the refraction of the meridian at right angles to these vessels. The estimation of the refraction by the direct oph- thalmoscopic method is exceedingly valuable, but requires great practice; some observers find con- siderable difficulty in relaxing their accommodation completely, even after long practice. The student should take every opportunity to become thoroughly proficient in estimating the re- fraction by this method; in fact, every case that is not of an inflammatory character should be examined with the ophthalmoscope, and the refraction as esti- mated by the direct method recorded as a matter of routine ; the ophthalmoscopic examination may con- veniently follow the testing of the patient's visual acuity. DIRECT EXAMINATION 79 In hypermetropia and myopia one is able to esti- mate the amount of error accurately, and in cases of astigmatism where the chief meridians are hori- zontal and vertical one can come very near the exact correction, and without necessarily subjecting the patient to the inconvenience of a mydriatic : when the meridians are oblique the estimation is more difficult, because we may find no vessel whose course exactly corresponds with the chief meridians. Still the more this method is practised the more accurate will be the results obtained. The correction must always be confirmed by trying the patient at the test types with lenses, making any slight alteration that may be necessary. It is also an additional advantage that one can esti- mate the refraction at the same time that one makes an examination of the fundus. The comparison of the direct and indirect methods of examination is very useful in astigmatism. If, for instance, the disc is elongated vertically in the erect, and oval horizontally in the inverted image, we know that the curvature of the cornea is greater in the vertical than in the horizontal meridian (see Figs. 86 and 87). The ametropic observer must always remember, when using the direct method for the estimation of errors of refraction, that he must correct his own defect either by wearing spectacles or by having a suitable glass in a clip behind his opthalmoscope ; he is then in the position of an emmetrope ; but, if he prefer it, he may subtract the amount of his own 80 THE REFRACTION OF THE EYE hyper me tropia or myopia from the glass with which he sees clearly the patient's discs. Thus, if the ob- server have 2 D. of hypermetropia^ and require + 3 D. to see the fundus clearly ( + 3 D.) - ( + 2 D.) = + 1 D., the patient would have 1 D. of hypermetropia ; had he required — 2 D., then the patient would have 4 D. of myopia, because (—2 D.) — (+ 2 D.) = — 4 D. The same with the myopic observer ; if his myopia amount to 3 D._, then he will require — 3 D. to see clearly the emmetropic fundus ; if he see the disc well without a glass, then the eye under examination has 3 D. of hypermetropia; if he require a + 2 D., then the hypermetropia will be 5 D._, and so on. Ametropia may also be easily recognised in the following manner : — The fundus being illuminated by a mirror about one metre from the patient, if the eye be emmetropic the rays of light will return parallel to one another, and a red reflex can only be obtained when the observing eye is in the path of these rays, that is behind the perforation of the mirror. If hyperme- tropic the returning rays will diverge (Fig. 49), and Fig. 49. the observer will notice, as he moves his eye (b) from behind the mirror at l (and at right angles to the WITH THE MIREOR 81 visual axis of the patient, who should fix on the centre of the mirror), that the last ray of light {a h') is seen, or, in other words, the red reflex disappears, on the same side of the pupil as that to which the observer moves his head. If the eye be myopic the rays will converge, cross, and diverge (Fig. 50) ; when the error is 1 D. Fig. 50. or more, the last ray of light is seen, or the red reflex disappears, on the opposite side of the pupil. A single trial of this will prove its correctness. The endeavour to estimate the amount of myopia or hypermetropia by measuring the distance betAveen the perforation of the mirror, and the point at which the last ray was seen, has been unsuccessful, owing to the varying size of the pupil. The ophthalmometer of Javal and Schiotz, and Tweedy's optometer, can, I think, be more conveniently considered when treating of astigmatism. 82 THE REFRACTION OF THE EYE CHAPTER V RETINOSCOPY Retinoscopy, or the shadow test^ is deservedly one of the most popular objective methods of estimating the refraction of the eye. It has the great advantage of being easily learnt, and can be carried out quickly, saving much time in difficult cases of astigmatism ; it is especially useful in young children, in amblyopic patients, and in malingerers ; besides, it enables very small degrees of astigmatism to be detected which but for this method would probably escape notice. Retinoscopy may be carried out either with a plane or a concave mirror. The plane mirror is now almost universally used ; the shadows are better seen, and the results obtained are more exact than with the concave mirror. The observer who has not good vision, f, is at a disadvantage in employing retinoscopy, as he will see the shadows less clearly ; he should therefore wear his own correcting glasses if he has any error of refraction. Light. — A good light for retinoscopy is the incan- descent focal lamp of 16 candle-power. These lamps are made with one side of ground glass, the EETINOSCOPY 83 other clear, and the filament in the shape of a gridiron so as to give a greater light surface. For Fig. 51. retinoscopy the transparent side of the lamp is turned towards the observer, the ground side being re- served for the ordinary ophthalmoscopic examination Fig. 52. Where the electric light is not available a gas A rgand burner or an oil lamp may be used : the 84 THE REFRACTION OP THE EYE flame should be of fair size with well-defined edges. The light should be on a bracket which can be moved in any direction. The light is usually placed over the patient's head and slightly behind, so as to leave the eyes in shade (Fig. 51). Some observers prefer a bright screened light, coming from a diaphragm opening of 5 to 10 mm., placed 2 cm. from the mirror (Fig. 52) ; the illumina- tion is brighter the nearer the light is to the mirror. A dark room is an advantage, otherwise the curtains of the room must be drawn ; for the darker the room the easier is retinoscopy. Retinoscopy with the Plane Mirror. Most modern ophthalmoscopes are provided with a suitable plane mirror. The drawing below shows a simple and convenient one for the pocket. Fig. 53. If light from the ophthalmoscope lamp be reflected into the eye by means of a plane mirror, at a dis- RETINOSCOPY 85 tance of a metre or so, an observer looking througli the sight-hole of the mirror will notice the ordinary red fundus reflex ; on slightly rotating the mirror, the illuminated area of the pupil may disappear (or, what may be more easily seen, the edge of the shadow bounding this illuminated area may appear), on the same side as the rotation or in the opposite direction, according to the refraction of the eye under observa- tion : thus if the mirror be rotated to the right, and the edge of the shadow move across the pupil also to the right, i. e. in the same direction as the rotation of Fig. 54. the mirror, the case is one of hypermetropia ; whereas if the shadow had moved in the opposite direction to the mirror, the case would be one of myopia. This method of employing retinoscopy is so simple that a few practical trials will suffice to make it understood, although, of course, as in all other mani- pulations, some little practice is required in giving to the mirror the necessary movements, and in enabling one to appreciate what is seen. The illumination and shadows we see are an image of the lamp with the surrounding shadow formed on the retina of the observed eye. 86 THE REFRACTION OF THE EYE When diverging rays of light fall on a plane mirror they are reflected as divergent rays, as if coming from a point behind the mirror ; so that the image of the plane mirror is a virtual one : the diverging rays passing through the pupil of the eye under observation are brought more or less to a focus according to the refractive condition of the eye ; on rotating the mirror in one direction the retinal image will move in the same direction. The following diagram will help to make this clear. Fig. 55. Rays of light from l fall on the plane mirror A, and are reflected as divergent rays into the eye as if coming from point G behind the mirror ; these rays focus on the retina at c. On tilting the mirror into position B, the rays from l diverge from the mirror (as if coming from h) and focus at d ; therefore the real movement of the light on the retina is with the mirror. Hence the movement of the retinal image is always with the mirror ; but as these movements RETINOSCOPY 87 are seen tlirongli the dioptri3 system of the eye^ and thereby undergo refraction^ the apparent may differ from the real movements. The retinal image the observer sees of the lamp and its surrounding shadow are formed in the same manner as all other images. In hypermetropia the final image of the lamp and its surrounding shadow, produced by the plane mirror, is an erect one formed behind the eye, and as it is viewed through the dioptric system of the eye, it therefore moves with the mirror (Fig. 44). In myopia the final image is an inverted one, pro- jected forwards. This, therefore, moves against the mirror (Fig. 45). If, however, the observer be nearer than the patient's far point the image will move with the mirror. This is the case in low degrees of myopia, where the patient's far point is beyond 120 cm. Therefore, if the image move against the mirror, the case is certainly one of myopia. If it move with the mirror, it is most likely one of hypermetropia ; but it may be emmetropia, or a very low degree of myopia ( — '50 D.). The movements tell us the form of ametropia we have to deal with. The extent of the movements on rotation of the mirror, the clearness of the image, and the brightness of its edge, enable us to judge approximately the amount of ametropia to be cor- rected; some practice, however, is required before we can form an opinion with anything like accuracy. The extent and rate of movement is always in 88 THE REFRACTION OF THE EYE inverse proportion to the ametropia ; the greater the error of refraction^ the less the movement, and the slower it takes place. This may be explained in the following way : Suppose A to be the image of a luminous point formed on the retina, and that a line be drawn from A, through the nodal point b to c. Now, if the case be one of myopia (Fig. 56), an inverted projected image of a is formed somewhere on this line, say at c. The higher the myopia, the nearer to the nodal Fig. 56. point will this image be; and hence in a higher degree of myopia we may suppose it formed as near as D. If the mirror be now rotated, so that it take up the position of the dotted line m', c will have moved to c, and i> to d ; hence it is clear that c has made a greater movement than d. Had the case been one of hypermetropia (Fig. 57), the image would have been projected backwards, and, as in myopia, the higher the degree of error the nearer to the nodal point is the image formed. In this case the line from the nodal point b to a is EETINOSCOPY 89 prolonged backwards^ and tlie image of the luminous point in a low degree of liypermetropia is formed say at c, and in a higher degree say at d. On moving the mirror into the position of the dotted line m', c moves to c, and d to cZ ; hence it is clear that c has made a greater movement than d. Fig. 57. Therefore, as the ametropia increases, the extent of the movement of the image decreases. The clear- ness of the image and the brightness of its edge decrease as the ametropia increases. Aetificial Cycloplegia. — A mydriatic is not abso- lutely essential; still, when we have to examine young people and children, the use of atropine is certainly advisable. In the first place, the dilatation of the pupil renders our examination so much easier; and secondly, atropine enables us to arrive at a more accurate estimation by thoroughly paralysing the accommodation : for although the examination take place in a dark room, and with the patient looking into distance, it must be remembered that there is often (especially in the case of children) some accom- 90 THE REFRACTION OP THE EYE modation still in force ; or there may be spasm of tlie ciliary muscle. In persons over the age of twenty, atropine is not usually employed, owing to the great discomfort entailed by its paralysing effect on the ciliary muscles, which lasts for seven or eight days, and because the use of this drug has occasionally produced glaucoma in people beyond middle life ; we may, however, wish to dilate the pupils with a mydriatic that acts fully and quickly, the effects of which last but a short time. The most convenient combination for this purpose is — P> Homitropinse Hydrobrotnatis, ^r. iv ; Cocainse Hydrochloratis, gr. x ; Acidi Salicylic!, gr. j ; Aq Destillatse, 3j. Ft. guttse. One drop of this solution applied two or three times at intervals of ten minutes produces rapidly a maximum dilatation of the pupil, which passes off in about twelve hours. Another great advantage of a mydriatic is that the refraction at the macula can be measured, whereas when the pupil is not dilated we have to be satisfied with the refraction at the optic discs, which may occasionally vary considerably from that found at the macula : the estimation of the refraction at the macula constitutes one of the chief advantages that retino- scopy possesses over the measurement obtained by the direct method. Therefore atropine should be used either in the form of drops or ointment — RETINOSCOPY 91 1. In all cases of concomitant squint. 2. In hypermetropes under twenty. 3. In cases of defective vision under eighteen, due to myopia or astigmatism. Homatropine and cocaine may be used witli ad- vantage — 1. In all cases of astigmatism over twenty. 2. Where the correction by glasses has failed to relieve a patient's discomfort. In all persons over thirty, care should be taken to estimate the tension of the eye before applying a mydriatic, and when it has been applied a drop of a solution of eserine (gr. j to 5j) should be instilled into the eye after the examination is completed. In patients above the age of twenty-five a mydriatic is not always necessary ; many simple cases of astig- matism can be worked out rapidly and accurately without one. Remember the patient must not look directly at the mirror, but slightly inwards when using retinoscopy without a mydriatic. To proceed now to the estimation of the refraction by retinoscopy. The patient being seated in the dark room, the pupils dilated, and the lamp over his head, we place a pair of trial frames on his face and take up our position 120 cm. or further (I usually sit 150 cm. from the patient) in front, with a plane mirror. The patient is then directed to look at the centre of the mirror, so that the light from the lamp may be reflected along the visual axis of the right eye. On 92 THK EEFEACTION OF THE EYE looking through the perforation of the mirror we get the ordinary fundus reflex, bright if the patient be emmetropic, less so if he be ametropia; and the greater the ametropia, the less bright will the fundus reflex be. We now rotate the mirror on its vertical axis to the right; if a vertical shadow come across the pupil from the patient^s right, i. e. in the same direction as the movement of the mirror, or, what is the same thing, if the shadow move in the same direc- tion as the circle of light on the patient^s face, the case is one of hypermetropia. Should the edge of the image appear well defined and move quickly, in addition to a bright fundus reflex, we infer that the hypermetropia is of low degree, and proceed to cor- rect it. First place a weak convex glass, say + *50 D., before the eye in the spectacle frame ; if the shadow still move with the mirror we change the glass for + 1 D., then -|- 1'5 D., and so on, until we find the glass with which no distinct shadow can be seen. Supposing this to be + 2 D., and that on trying + 2'5 D. the shadow move against the mirror, -f 2 D. is put down as the correcting glass. Had we obtained a reverse shadow concave glasses would be employed, proceeding exactly as before, commencing with a weak concave glass, putting up stronger and stronger concave lenses until we have neutralised the shadow. This is put down as the correcting glass. These glasses are not the exact estimate of the refraction, because the observer is not sitting at infinity, but at 120 cm. from the eye, so that when EETINOSCOPY 93 no shadow is obtained we are sitting practically at the patient's far point. Therefore the hypermetropia is over-corrected 1 D. and the myopia is under-estimated ID.; so in hyper- metropia we deduct 1 D. and in myopia we add 1 D. to the correcting glass. To sum up^ if the shadow move with the mirror^ it may be weak myopia if + '50 D. obliterates the shadow; emmetropia if + 1 D. neutralise it; hyper- metropia if a stronger glass is required : when the shadow moves against the mirror it is a case of myopia. In high myopia a strong concave glass has to be used for the correction ; the light from the mirror is so spread out in passing through this lens that fewer rays pass into the eye^ therefore the illumination is not so good as in other states of refraction, and the examination becomes more difficult. The points to be observed are — (1) the direction of the movement of the image, as indicating the kind of ametropia : (2) the rate and amount of movement, (3) the brightness of the edge of the image, and (4) the amount of fundus rerflex all indicate the degree of ametropia. We have taken notice only of the horizontal axis, but any other meridian will, of course, do equally well, if the case be one of hj^permetropia or myopia simply. If, however, the case be one of astigmatism, then the refraction of the two chief meridians will differ. In astigmatism, the image of the flame of the lamp formed on the retina is distorted so as to be more or 94 THE EEFKACTION OF THE EYE less of an oval form^ according to the position of the retina and the maximum and minimum curvatures of the cornea (Fig. 76). So that^ in astigmatism, the image on the retina may be more or less of an oval, instead of being either a small well-defined image of the lamp, or a circle of diffusion as in the case of emmetropia, myopia, or hypermetropia. This oval may have its edges hori- zontal and vertical ; frequently, however, they are more or less oblique, as shown in the following figures (Fig. 58). Fig. 58. Oblique shadows in astigmatism. The oblique movements of the shadows are inde- pendent of the direction in which the mirror is rotated. This obliquity is produced thus (Fig. 59) : — If we cut a circular opening in a piece of cardboard to represent the pupil, and then place behind it an oval piece of card which is to represent the shadow, so that that part of its edge which occupies the pupil has an oblique position, then on moving the card across in the direction o d, it has the appearance of moving in the direction o c, at right ai^gles to the edge of the card. Hence the direction of the shadow's movement EETINOSCOPY 95 is deceiving, and the oblique edge is due to the fact that only that edge which coincides in direction with one of the principal meridians is seen well defined by the observer. Therefore the apparent movenients correspond with the edge of the shadow. Fig. 59. The same takes place in astigmatism, the two chief meridians of which are parallel and perpendicular to the edge of the shadow. In retinoscopy, therefore, when the edge of the image is oblique, we know at once that the case is one of astigmatism. Another characteristic appearance that will be some- times met with in astigmatism is, that the fundus illumination may assume a band-like shape something Fig. 60. Fig. 61. like Fig. 60; and on tilting the mirror on an axis parallel to this band, a dark shadow will appear to 96 THE REFRACTION OF THE EYE come from both edges of the pupil at once, uniting in the centre to form a black band_, leaving the upper and lower part of the fundus illuminated as shown in Fig. 61. This band is parallel with one of the chief meridians, and indicates the point of exact neutralisa- tion of the meridian at right angles to it. The effect is due to the retinal images being now in the form of a line (see Figs. 76 and 77, ii and vi). This variety of movement of the shadow is some- times spoken of as "the scissor movement.^^ Supposing we take a case in which the meridians are horizontal and vertical, we judge if one shadow be more distinct or quicker in its movements than the other, though it is not always easy to recognise the presence of astigmatism at once, so that it may be necessary to correct one meridian before we can be certain. If the shadow move with the mirror in all meridians, we first take notice of the vertical one, and put up in front of the patient, in the spectacle frame, convex spherical glasses, until the glass which neutralises the shadow has been found. This is put down as the correcting glass for the vertical meridian; let us suppose that glass to be + 2 D. We next take notice of the horizontal meridian, and if + 2 D. is also the glass which neutralises the shadow, then of course we know ^the case is one of simple hypermetropia. But supposing the convex glass necessary to correct this meridian is + 4 D., we indicate it conveniently thus : + 2 D. [ --r 4D. EETINOSCOPY 97 The case is one of compound hypermetropic astigma- tism, and will require for its correction + 2 D. sphere combined with + 2 D. cylinder axis vertical. We will take another case — that in which the vertical shadow is neutralised by a + 2 J)., while for the horizontal shadow — 2 D. is required. 2D. — + 2D. Here we have a case of mixed astigmatism, which can be corrected in either of the three following ways : 1st. — 2 D. cylinder axis horizontal combined with + 2 D. cylinder axis vertical ; this is a plan seldom used, and is not so easy to work with as a sphere and a cylinder. 2nd. —2D. sphere combined with -h 4 D. cylinder axis vertical, or 3rd. + 2 D. sphere combined with — 4 D. cylinder axis horizontal. The last is the preferable plan. Supposing the axis of the shadow to be oblique (Fig. 58), we know at once that astigmatism exists, and we proceed to correct each meridian separately, moving the mirror at right angles to the edge of the shadow, not horizontally and vertically. We judge of the amount of obliquity by the eye, and can fre- quently tell within a few degrees. If the vertical meridian be 20° out, and require for its correction — 2 D., and the axis at right angles to this (which will therefore be at 110°) require — 3 D., we express it as 7 98 THE REFRACTION OF THE EYE in Fig. 62, and correct it with sphere — 2D. com- bined with cylinder —ID. axis 20° : this is a case of compound myopic astigmatism. One is often able to put up the cylinder in the spectacle frame with the exact degree of obliquity. Having found the glasses which correct the two meridians_, we put up the combination in a spectacle trial frame, and if we now get no shadow the glasses are assumed to be the right ones, and we proceed to confirm it by trying the patient at the distant type, making any slight alteration that may be necessary. When employing retinoscopy the sectors of the lens often show up very plainly (when no opacity is Fig. 62. to be seen) ; this may be the earliest sign of a patho- logical process taking place in the lens. In irregular astigmatism retinoscopy does not give satisfactory results, the shadows being indefinite and irregular. In conical cornea the shadow may appear circular, occupying an intermediate position between the centre and the edge of the cornea ; on tilting the mirror the shadow appears to run round the base of the cone in a circular direction. RETINOSCOPY 99 A further modification of retinoscopy wliicli may sometimes be useful is that proposed by Dr. Jackson, of Philadelphia. The object is to find out the point of reversal of the image. Thus, if a patient be a myope of 2 D., the observer, at a distance of a metre, sees the shadow moving against the mirror ; on coming near he will find the image disappear at 50 cm., and on coming still closer the image will move with the mirror; the point of reversal is therefore at 50 cm. By dividing the distance of the point of reversal into 100 cm. we arrive at the patient's error. If the point of reversal is at 25 cm., then the myopia will be 4 D. In order to use this method satisfactorily, one or two points require attention. In Simple Myopia. — When the observer's eye has come quite close to the patient's, say one eighth of a metre, and the inverted image is still seen, it is best to place a concave lens (— 8 D.) before the patient's eye and then estimate the amount of myopia uncor- rected; by adding this to the amount which the lens used has corrected we determine the total myopia present. It is evident that if the point of reversal is close to the eye the error of a few centimetres as to its position entails an error of some dioptres in the amount of myopia present. Therefore we place before the patient^s eye a concave glass strong enough to remove the point of reversal a metre or so from the eye. In Hypermetropia. — Place before the patient's eye a convex glass strong enough to over-correct the 100 THE EEFEACTION OF THE EYE hypermetropia. Then by the method given above, determine the degree of myopia so produced. Deduct this amount of myopia from the strength of the convex glass used ; this will give the amount of hyper- metropia present. Suppose, for example, the hyper- metropia amounts to four dioptres : place before the eye + 5 D., it is found that one dioptre of myopia is produced; the point of reversal being at one metre. Therefore to estimate hypermetropia by this method a convex lens must always be used. Emmetropia. — A weak convex lens being placed before the eye, the point of reversal will be found to equal the strength of the lens used. Regular Astigmatism. — We find the point of rever- sal for each of the principal meridians. Retinoscopy with the Concave Mieeor. When the concave mirror is used, then the move- ments of the shadows are in the opposite direction Fig. 63. to those obtained with the plane mirror. Divergiug rays of light falling on the concave mirror converge RETINOSCOPY 101 and form an inverted image of tlie lamp between the observer and tbe patient^ and this image becomes the object. From this image rays diverge. Some of them enter the pupil of the observed eye, and are brought more or less to a focus on the retina, accord- ing to the refraction of the eye. Fig. 64. Hence, with the concave mirror, the image suffers another inversion, therefore the image on the retina always moves against the movement of the mirror, which is the reverse of that obtained Avith the plane mirror. Fig. A few cases from my note-book will do more than any description to elucidate the subject of retino- scopy with the plane mirror. 102 THE EEFRACTION OF THE EYE Case 1. Spasm of the Ciliary Muscle. — Boy, get. 10 years, is brought because lie is unable to see the bla- J^?ard at school. B.V,-i%-lD. = f. L.V.-ia-lD. = f. Retmoscopy. — Bright fundus reflex, shadows move against the mirror. Direct method, the discs clearly seen without a glass. Guttse atropise sulphatis, gr. iv to 5j- Ter in die. On the third day the pupils are found well dilated. Eetinoscopy gave + 3 D. in both eyes. On trying the patient with the test-type, E.V. + 2D.=f. L.V. + 2 D. = J. o This, therefore, was a case of hypermetropia with spasm of the ciliary muscle simulating myopia. Such cases are very common, and one should always be on the look-out for them. Case 2. Hypermetropia. — A young woman a3t. 15, suffering from blepharitis. R.V. I Hm. 1 D. = f. L.V. ^ Hm. 1 D. 6 6 Guttse atropise sulphatis, gr. iv to ^j. Ter in die. On the fourth day the patient returns for examina- tion. With the mirror the fundus reflex is moderate ; the shadow moves slowly with the mirror. On trying + 2 D. the illumination improves, and the shadows RETINOSCOPY 103 are more distinct and move quicker; + 4 D. neu- tralises the shadow. E.V. + 3-5D. = f. L.V. + 3-5D.=S^. o This patient has^ therefore^ a total hypermetropia of + 3*5, taking off + 1 D. for the atropine. + 2'5 D. ordered for constant use. Case 3. Myopia. — A young man eet. 18 complains that he is unable to see distant objects well. K.V.-Aj-3-5D. = |. L.V.-Ay-8-5D. = f- After using atropine for three days the eyes are again examined. The shadows move against the movements of the mirror. —2D. neutralises the shadow in both eyes. R.V.-3 D. 6 L.V.-3D. = |. — 3D. ordered for constant use. Case 4. Myopia. — Man a3t. 30 complains of seeing distant objects badly, but has no difficulty with near work. E.V.g%-2-5D. = f. L.V.-8V2-5D=|- After three applications of homatropine and cocaine the pupils are found to be well dilated. Retinoscopy : 1-5 -1-5 E. 1-5 L.- E.V.-2-25D. = f L.V.-2-25D.-|, 1-5 Order for distance — 2*25 D. 104 TFE REFRACTION OF THE EYE Case 5. Compound Hypermetropic Astigmatism. — A man set. 20 : R.V.^4-Hm.4D. = f. Under atropine, right eye at distant types sees only -f^. Fundus reflex very dull, movements of shadow slow and with the mirror. On putting up + 5 D. the reflex is much brighter, the edge of the shadow distinct, and its movements quicker. We try + 6, 7, 8, 9, and the last lens gives a shadow against the mirror. On trying the eye at the distant type with + 8 D. |- and four letters of f are at once read. No alteration in the glass improves sight. Left eye : the fundus reflex and movements are the same as in the right. We commence by trying + 8 D., which we found the other eye required. In the vertical meridian the movement is with the mirror, while + 9 D. causes it to move against. In the horizontal meridian with + 8 D. the shadow moves against the mirror, and + 7 D. causes it to move with. We express it thus : + 8D. - + 7D.; and on trying the combination at the distant type, + 7 D. sp. + 1 D. cy. axis horizontal, the patient is able to read |-; on decreasing the sphere from 7 D. to 6'5 D., ^ is read, so that the proper correction for this eye is — -t^6_5 R sp. + 1 D. cy. axis horizontal ; EETINOSCOPY 105 in this case^ therefore, hypermetropia was present in one eye, compound hypermetropic astigmatism in the other. Case 6. Astigmatism. — Young woman set. 17 sees with each eye -f^ — 1 B. = j\. Retinoscopy with- out atropine : ,-3-5 D. \-/~^"^* ID. I^./\ / \+lD. Ordered guttle atropise sulphatis, gr. iv to 5j, three times a day in each eye, for three days ; then with retinoscopy the result is : 2-5 D E. Em. =--50^ilP-_ . — 2-5 D. cy. axis horizontal, = ^. _\'/ = +'50^ = 9. ^'~ \ - 3D. cy. axis 130° \ + 1 D. After recovering from atropine the result was con- firmed, and the following correction ordered to be worn constantly : E. -_l_I^-sp^ — 2*5 D. cy. axis horizontal. • L. -3 D. cy. axis 130°. Case 7. Mixed Astigmatism. — Mary E — , £et. 15, pupil-teacher, brought up from Cardiff about her eyes; suffers much from headache and pain in the eyeballs, especially the right, worse in the evenings. Has tried many opticians to get spectacles to suit 106 THE REFRACTION 0¥ THE EYE lier^ but has always been unable to do so. R. V. 3^ slightly improved with — ID. L. V. -3% also slightly improved with —ID. On placing the patient in the dark room, retinoscopy at once shows the case to be one of mixed astigmatism, with the chief meridians horizontal and vertical; we proceed to correct each meridian, and the result is : R. 5D. [-5D. -+1D. L.— — + 1-5D. On trying this combination before the right eye, -^-^ is read. We express the vision of right eye thus : R.-^ + '50 D. sp.O — 6 D. cy. axis horizontal = ^%. With the left eye the combination gives, with the cylinder not quite horizontal, but slightly outwards and downwards, |-. L. 3^g + 1 D. sp.O -6 D. cy. axis 170° =|^. The patient remarked that she had never seen things so clearly before. The result was very satisfactory, and was arrived at in about ten minutes, thus saving an infinite amount of time and trouble, which would have been required to work out such a case by any of the older methods. Ordered guttse atropioe sulphatis, gr. iv to 3J^ three times a day for four days, when the result was : |-4D. E.— — +2D. -4 D. — + 2D. E.V.-^Q +1-5 D. sp.O -6 D. cy. axis l75°=f. L.V. -g^Q + 15 D. sp.C^-e D. cy. axis 170°=-|. KETINOSCOPY 107 In this case tlie glasses were again tried after the atropine was recovered from, and the following glasses ordered, which were of course to be worn constantly : ■^ + -75 D. sp . j^ + 1 D. sp . * — 6 D. cy. axis 175°. * — 55 D. cy. axis 170' >7no Case 8. Astigmatism. — Mr. C — , £et. 24, has noticed that for the past few years the eyes become very tired at night, especially when much writing or reading has been done ; he thinks he sees distant objects less clearly than formerly. R. y. 2T ^^^ improved with convex or concave spheres ; with pin-hole test -^. L. Y. -f-g not improved with convex or concave spheres; with pin-hole test y^. After using atropine for four days, retinoscopy gave the following results : 1+3 D. R.— I— +oD. L.— I + 2-5 D. — +5 D. R.V. + 2 5 D. cy. axis 160' = f. L.V. + 1-5 D. axis cy. 165°- f. We direct the patient to return after the eifects of the atropine have passed off, which he does in ten days; we then try our correction, deducting + 1 D. sphere for the atropine. ^^ -ID.sp. 6 j^y -^^D. 6 + 2-5 D. cy. axis 160° ~ 6 • + 1*5 D. cy. axis 165° 5 This correction was accordingly ordered to be worn constantly. 108 THE EEFEACTION OF THE EYE Case 9. Astigmatism. — Sarah K — , set. 21^ com- plains that her eyes have of late been very painful, and she has also suffered much from headaches, which have sometimes ended with an attack of sick- ness. E.V.-,% -1^=T^2- L.V.-,V2D.=^3-. After atropine, retinoscopy gave : E.— -1-25 D. — + 2D. \ : / + 2 D. /\-2-5D. E.V. +1^!?: -2-25 D.c y. axis horiz. ^ ■ -4D.cy.axis 125°' 6 = 9 When the effects of the atropine had passed off, the correction which gave the best results was : E.V. -2-25 D. cy. axis horiz. =^ j^y +-25D.sp. =6 J b- — 4D.cy. axis 125 9 These spectacles were ordered to be worn con- stantly. Case 10. Simple Hypermetropic Astigmatism. — Jane Q — , set. 11, has always seen near objects badly; she turns her head to one side instead of looking directly at the object. K. V. 2^ not improved with spheres, with pin- hole 3%-. I^- ^' -2T ^^^ improved with spheres, with pin- hole ^. Eetinoscopy after atropine gives : j+2D. I + 1-75D. E.— — + 6D. L.— — + 5D. RETINOSCOPY 109 R.V. + ^ ^- ^P- =_6_ L.V. + '"^^ P- S P- =_6_ + 4 D, cy. axis vert. 1 2 * ' ' + 3-5 D. cy. axis vert. ^ ^ ' Ordered for constant use : E. V. + 4 D. cy . axis vert. = -j^. L.V. + 3-5 D. cy. axis vert. = -^. Case 11. Myopic Astigmatism. — Jane P — , ast. 23, always seen rather badly, and has had a good deal of pain and discomfort in the eyes for the past six months, especially when using them by gas-light. About a week ago she noticed, on closing the left eye, that the vision of the right was almost gone, though she admitted never having tried them separately before; occasionally the right eye turns outwards. E.V.-ij-4D.=^. L.V.^-1 D. = -,%. Homatropine was applied once, and at the end of half an hour retinoscopy gave : 5-5 D. E. 3 D. ID. -Em. With glasses : E.V. L.V. -3-5 D. ^6 -2-5 D.cy. axis 175° 12"- -• 50 D. sp. ^6^ - 1 D. cy. axis 5° 6' This correction was ordered for constant use. Case 12. Concomitant Convergent Strabismus. — Mabel C — , set. 9, commenced to squint with the right eye about the age of four and a half. Has never worn glasses. E V -^- Ti V -^- 110 THE REFRACTION OF THE EYE On taking the patient into the dark room retino- scopy at once reveals the presence of hypermetropic astigmatism. Grutta3 atropinae sulpha.tis, gr. iv to ^j, was pre- scribed, one drop to be applied to each eye three times a day for three days ; then with retinoscopy : E ,+3 + 1D. J- .5 L._— +4D 1 -tS. = _6_ axis vert. 1 2 • ^■^- 't!^' = 6. axis vert. 6(4) Prescribed for constant use, deducting -|- *50 D. for atropine : + 3 D. cy. axis vert. L. + 3 D. cy. axis vert. Case 13. Right Constant Convergent Concomitant Strabismus. — George A — _, aet. 5. Has squinted with the right eye for the past two years. The angle of the squint is 30°. He does not know his letters, so is ordered guttae atropinae sulphatis for four days. On his return the eyes appear almost straight. Retino- scopy gives : + 5 +4 E. + 6 L. + 5 We deduct + 1 D. for the over-correction by retino- scopy, and + 1 D. for the atropine, and order him for constant use : RETINOSCOPY 111 + 3 D. sp. + 2 D. sp. ^' + 1 D. cy. axis | + 1 D. cy. axis | The spectacles are made with twisted wire tem- porals to go round the ear. Case 14. Compound Myopic Astigmatism. — Miss M. E — _, sdt. 22, has always been short-sighted, but thinks the sight has lately got worse. Retinoscopy gives without a mydriatic : -6 -7 E. R. L. ■6 — 55 D. sp. ^6 — ID. cy. axis horiz. 9 * -6-5 D. sp. ^_Q — ID. cy. axis horiz. 1 2 • These glasses were ordered for constant use. Case 15. Mixed Astigmatism. — Miss L. U — , ast. 54, has worn glasses constantly for the past twenty years. R.V 2\(2)N.I.S. L.V.-2\(2)N.I.S. inoscopy without a mydriatic : E. ^-3-5D. /-3-5D. + -25 D. sp. _ ^'^' _4 D. cy. axis 170° g • + -25D.SP. _6 ^•^- -4D.cy.axisl70° 6(4). 112 THE REFRACTION OF THE EYE Ordered these glasses for distance ; the patient also requires glasses for near work, and reads best with this correction : + 2-5 D. sp. ^' -41). cy. axis 170°. + 2-5 D. sp. • -4 D. cy.axis 170°. These were therefore prescribed for this purpose. Case 16. — G-eorge M — , ast. 23, complains of diffi- culty in reading and aching of the eyes. E.V. -j^2- N.I.S.* L.V. -j^g- N.I.S. After three applications of homatropine and co- caine, retinoscopy gives : i-iD. -ID. E.— l—E. L.— — E. I I - 1 D. cy. axis horiz. 6* • • — 1 D. cy.axis horiz. 6* Ordered these glasses for constant use. Case 17. — James C — , mt. 48, has a difficulty in reading at night. E.V.-j^g--l D. cy.axis horiz. |. L.V.-j^g— 1 D. cy. axis horiz. |. Retinoscopy : E.— E. + 1 L.— E. + 1 Deducting + 1 D. sphere from this result, which we obtained with retinoscopy, gives us : — ID. cy. axis horiz. for distance. * N.I.S. signifies not improved with spherical lenses. RETINOSCOPY 113 We wish to add + 1 D. sphere to this correction for reading thus : + 1 D. sp. + 1 D. sp . — 1 D. cy. axis horiz. ' —ID. cy. axis horiz. But this is not the simplest expression of the glass ; it should be : + 1 D. cy. axis vertical. This patient therefore requires two pairs of glasses : — ID. cy. axis horizontal for distance. + 1 D. cy. axis vertical for reading. Case 18. Myopia with Divergent Strabismus. — Jane M — J set. 22, has always been short-sighted since she can remember, and is now wearing — 3' 5 J)., which she has worn constantly for the past three years. The left eye is weaker than the right, and turns out some- what, especially when she is tired. E.V..^-6D. = f. L.V,/„-6D.=-4-. Drops of homatropine and cocaine were applied three times at intervals of ten minutes. In half an hour the pupils being well dilated, the patient was taken into the dark room for retinoscopy, with the following result : -5D. -6 E.— ! 4-5 D. L. 5 D. ' — •50 D. cy. ax. horiz. ^' j^^-5D. sp. ^ ^^ — ID. cy. ax. horiz. ^ V 3 j • 114 THE REFRACTION OF THE EYE These glasses were prescribed for constant use, and with these the divergence of the left eye was corrected. Case 19. Mixed Astigmatism. — James B — _, set. 24, has always seen badly, and is very subject to head- aches, which affect the occipital region chiefly. These headaches are always made worse by reading, and frequently come on after a long spell of near work. Has worn glasses constantly for the past six years, but the present ones are not comfortable. E.V.6%N.LS. L.V.-e^N.I.S. Retinoscopy without a mydriatic : + 1D. -3D. + 1D. -3D. -P y + 1 D. sp. ^ 6_ ■ *-4D. cy. ax. 165° 12- L.V. "^ ^ ^- ^P- — 4 D. cy. ax. horiz. 6 1 2 Homatropine and cocaine to both eyes ; after three applications the pupils are found fully dilated. Retinoscopy : E. + 3 .2D. ^ + 3 D. 1-5D. y +2D. sp. ^6^ ^ -5D.cy. ax.l65° 6(3). j^y -t- 2 D . sp. ^6^ * — 4-5 D. cy. ax. horiz. ^' prescribed for constant use. RETINOSCOPY 1 1 5 T, + 15 D. sp. ' — 5 D. cy. ax. 165''. T +15 D. sp. — 4*5 D. cy. ax.horiz. Case 20. — Mrs H. — , aet. 50, has worn glasses for near work for the past six years. During the last few months the distant vision has deteriorated, and the present reading glasses are not satisfactory. R.V.-,%Hm.lD. = f ,g L.V.-j^g-Hm. ID. = f ) ^ Beads best with + 3 D. Ordered + 1 D. for dis- tance, and + 3 D. for near work. Case 21. Astigmatism with Presbyopia. — Mr. N — , aet. 48, sees badly at all distances : B,.Y.-^^+15 D. cy. ax. vert. = f. L.V.-j^g- + 1-5 D. cy. ax. vert. = f . Reads best with : + 1 D. sp. + 15 D. cy. ax. vert. Ordered, therefore, two pairs of glasses, one for dis- tance, the other pair for near work. Case 22. Myopic Astigmatism with Presbyopia. — Miss K — , set. 60, has difficulty in doing near work. R.V.-j^g— 1 D. cy. ax. lioriz. = |^. L.V.^g--! D. cy. ax. horiz. = |-. Retinoscopy without a mydriatic : R. E. L.— — E. Ordered two pairs of glasses. Distance — ID. cy. ax. horizontal. 116 THE EEFRAOTION OP THE EYE Reading and near work : + 2 D. sp^ + 1 D. cy, ax. vertical. In most cases thus worked out tlie glasses may be ordered at once, without waiting for the effects of the atropine to pass off — in fact, experience teaches that in children, it is a good plan to continue the atropine until the spectacles have been made ; re- membering when ordering the correction that in hypermetropia and hypermetropic astigmatism the spherical glass will require slightly diminishing, usually about "50 D. ; in myopia and myopic astigmatism the spherical glass has to be slightly increased. HYPEEMETROPIA 117 CHAPTER VI HYPEEMETROPIA Hypermetropia (H.) {'Yirlp, in excess; juiTpov, mea- sure ; and wxp, eye) may be defined as a condition in which the antero-posterior axis of the eyeball is so shorty or the refracting power so low, that parallel rays are brought to a focus behind the retina (the accommodation being at rest). In other words, the focal length of the refracting media is greater than the length of the eyeball. Fig. 66. Parallel rays focus at h behind the retina ; those coming from the retina emerge as diverging rays, d, e. In the passive hypermetropic eye, therefore, paral- lel rays c and g come to a focus behind the eye at 6, forming on the retina at a a circle of diffusion instead of a point. Rays coming from the retina of such an 118 THE REFRACTION OF THE EYE eye emerge having a divergent direction (d and e) ; these rays^ if prolonged backwards, will meet at k, which is the punctum remotum, and this point being situated behind the eye is called negative. The distance of the punctum remotum behind the eye will equal the focus of the convex lens which corrects the hypermetropia ; thus, supposing the p. r. situated 20 cm. behind the retina (^^'^ = 5), 5 D. will be the convex glass which will render parallel rays so convergent that they will focus on the retina, or cause rays from the retina to be parallel after passing through it ; to be mathematically correct, allowance Fig. 67. Parallel rays fociissed on the retina by accommodation. The dotted line shows the lens more convex as a result of the contraction of the ciliary muscle. must be made for the distance between the cornea and the convex lens ; thus, for instance, if the lens be placed 20 mm. from the cornea, then the exact amount of hypermetropia which the -f 5 D. glass will correct will be : 1000 ^ 1000 ^ K... 200 - 20 180 In low degrees of hypermetropia the difference is HYPERMETROPIA 119 SO slight as to be unimportant ; in the higher degrees the difference is considerable. The hypermetropic eye at rest is only able to bring convergent rays to a focus on the retina. All rays in nature are divergent, some so slightly so, that when coming from a distant object they are assumed to be parallel. Rays can be made convergent by passing them through a convex lens placed in front of the eye ; or the refraction of the dioptric system may be increased by the accommodation, so that parallel rays may then focus on the retina of a hyper- metropic eye. Therefore a hypermetrope with relaxed accommo- dation sees all objects indistinctly. The hypermetropic eye has to use some of its accommodation for distance, so starts with a deficit for all other requirements, equal to the amount of hypermetropia. Fig. 68. Parallel rays rendered so convergent by passing through a convex lens that they focus on the retina. Thus, supposing an individual hypermetropic to the extent of four dioptres, and possessing 6 D. of accommodation, he will, by the exercise of this power 120 THE EEFEACTTON OF THE EYE to the extent of 4 D.^ be able to bring parallel rays to a focus on the retina^ and so see distant objects clearly; this leaves him 2 D. of accommodation for near objects^ which will bring his near point to 50 cm., a distance at which he will be unable to read comfortably. Besides, it must be remembered that only a part of the accommodation can be used for sustained vision, fatigue soon resulting when the whole of the accom- modation has to be put in force. ^J'he following diagram is intended to show the amount of accommodation possessed by a hyperme- trope of 3D,; each space represents a dioptre, and the thick white lines drawn through the spaces give Fig. 69. Dioptres. HHHB the amplitude of accommodation for different ages as given on the left of the diagram. The figures above indicate the number of dioptres, and those below, the near point for each increasing dioptre of accommoda- tion. HYPERMETROPIA 121 The amount of hypermetropia is calculated and expressed by that convex glass whicli makes parallel rays so convergent that they meet on the rods and cones of the retina, the accommodation being sus- pended. The commonest amount of error is about 2 D. Small degrees may require some trouble to discover, and sometimes can only be found out after the eye has been atropized. Hypermetropia is divided into latent and manifest. The manifest, Bonders subdivides into absolute, rela- tive, and facultative : Absolute, when by the strongest convergence of the visual lines accommodation for parallel rays is not attained — in other words, when distant vision is impaired; this variety is seldom met with in young people. Relative, when it is possible to accommodate for a near point, by converging to a point still nearer, — in fact, by squinting. Facultative, when objects can be clearly seen with or without convex glasses. In young people the hypermetropia may be facul- tative, or relative, becoming in later life absolute. Causes of Hypermetropia : ]. The antero-posterior diameter of the eyeball is too short (axial hypermetropia). This is by far the most common cause, and is con- genital. 122 THE REFRACTION OF THE EYE 2. A flattened condition of the cornea, the result of disease or occurring congenitally. 3. Absence of the lens (aphakia). 4. Detachment or protrusion of the retina, owing to a tumour or exudation behind it. 5. A diminution in the index of refraction of the aqueous, lens, or vitreous. Hypermetropia, therefore, is usually due to shorten- ing of the axis of the eyeball. The following table shows the amount of shortening for each dioptre of hypermetropia, the axial line in emmetropia being estimated at 22*824 mm. or -5 of D. of H. there is a diminution in the axial line of -16 mm ID )» }> } •31 „ 1-5 ,, „ . , •47 „ 2- „ „ , •62 „ 2-5 j> >f } •77 „ 3- ,, }> J •92 „ 3-5 „ 1-06 „ 4- ,, „ J 1-22 „ 4-5 ,, „ J 1^4 „ 5- >» » i 1-6 „ 6- „ 1^9 „ 7- „ „ , 22 „ 8- „ , 2-6 „ 9- „ „ J 29 „ 10- TTvnprT riAfvn-niji, i« \\\t -far fliA mn<2f, fvp 3-2 „ tion of the refraction. It may be looked upon as a congenital defect; frequently also it is hereditary, several members of the same family suffering from it. Hypermetropia is usually due to an arrest of deve- HYPERMETROPIA 123 lopment, which varies from the slightest degree to the extreme condition known as " microphthalmos." The following are some of the chief points in which the hypermetropic differs from the emmetropic eye : — the eye looks small, being less than the normal in all its dimensions, especially the antero-posterior ; the sclerotic is flat, and makes a strong curve backwards in the region of the equator, which can easily be seen on extreme convergence, or can be felt by the finger. The lens and iris are more forward, the anterior chamber is shallow, and the pupil small ; the centre of rotation of the eye is relatively further back, while the angle a, which is formed between the visual and optic axis, is invariably greater, averaging about 7° (see p. 202). The result of the large angle a in hypermetropia is that the eyes often have an appear- ance of divergence, which has sometimes been mis- taken for real divergence; whereas in myopia the small angle gives to the eyes an appearance of con- vergence. The ciliary muscle, upon the action of which the accommodation depends, is much larger than in em- metropia, the anterior portion, which consists chiefly of circular fibres, being especially developed, no doubt hypertrophied by the constant state of con- traction in which it is kept. This contraction is called into action by the instinctive desire for clear images which all eyes possess, the accommodation having to be used for distant as well as for near objects. Another result of the constant and exces- sive accommodation, is that its linked function — the 124 THE REPEACTION OF THE EYE convergence — is liable also to be used in excess ; in this case an object at a certain distance being accom- modated for^ one eye will be directed to the object, while the other, taking up the excessive convergence, will be directed inwards, and so a convergent stra- bismus will be produced. To fully understand how this convergent strabismus becomes developed, I must refer the reader to the chapter on that subject (Chap. X). When the hypermetropia is of high degree the optic nerve is smaller, and contains fewer fibres, so that the visual acuteness in these cases may be below the normal. Sometimes the face also has a characteristic appear- ance, being flat-looking, with depressed nose, the orbits being shallow, and the eyes set far apart. Frequently, however, there is no distinctive physiognomy. The hypermetropic eye is very liable to asymmetry, as will be shown when speaking of astigmatism. Symptoms of Hypermetropia. — The patient usually sees well at a distance, but has difficulty in main- taining clear vision for near objects ; and since the hypermetropia can be more or less corrected by ac- commodation, if the error be of a low degree (as 2 or 3 D.), no ill effects may for some time be noticed; at length, however, a point is reached when the accom- modation is not equal to long-sustained efforts of reading and near work, then accommodative asthe- nopia is the result (p. 226). This is especially liable to show itself after an illness, or if the patient's health has deteriorated from over-work, anxiety, or HYPERMETKOPIA 125 other causes. He then complains that after working or reading for some time, especially during the evenings, the type becomes indistinct, and the letters run together ; after resting awhile the work can be resumed, to be again shortly laid aside from a repetition of the dimness : the eyes ache, feel weak, water, etc., frequently headache supervenes ; there is a feeling of weight about the eyelids, and a difficulty in opening them in the morning. When the hyper- metropia is of high degree, the patient may be said by his friends to be short-sighted, because when reading he holds the book close to his eyes ; by doing this he increases the size of his visual angle, and thus gets larger retinal images ; this is counter- balanced by an increase in the circles of diffusion, but as the pupils also contract by approaching the book to his eyes, some of these are cut off; so that the advan- tage is in favour of holding the book close, especially as the patient is probably not accustomed to clear, well-defined images. In some cases the ciliary muscle contracts in excess of the hypermetropia, so that parallel rays focus in front of the retina, and the patient therefore presents many of the symptoms of myopia : we should always be on our guard against such cases. The manner in which the patient reads the distant type is often a guide to us in hyperme- tropia; he takes a considerable time to make out each line, and yet, if not hurried, eventually reads the whole correctly. On looking at the eyes one notices that they are red and weak, the lids look irritable, and on eversion the conjunctiva is hyperaemic, espe- 126 THE REFRACTION OF THE EYE cially that of the lower lids, while the papillae are frequently enlarged ; the edges of the lids sometimes become inflamed and thickened. All these symptoms are probably the commencement of troubles which, if allowed to go on, may develop into conjunctivitis, blepharitis, derangements of the lacrymal apparatus, etc., — this much we can see ; how much more injurious must be the changes which are liable to take place in the interior of the eyeball from prolonged hyperasmia ! It cannot be too forcibly insisted on, that in all oph- thalmic cases, except those of an acute character, the refraction should be taken and recorded as a matter of routine, since complaints which prove very intract- able are often easily and quickly cured when the proper glasses have been prescribed. As the patient advances in age he will become pre- maturely presbyopic, so that at thirty-five he may sufPer from the same discomforts as an emmetrope of fifty. To test the hypermetropia and measure the amount; we commence by taking the patient^s visual acute- ness, each eye separately ; having found that they are alike in their refraction, we try the two together ; stronger glasses being often borne when both eyes are used, than when one is excluded from vision. The strongest convex glass with which he is able to read f , or with which he gets the greatest acuteness of vision, is the measure of the manifest hyperme- tropia (Hm.). This is not, however, the total hyper- metropia, for if the accommodation be paralysed by applying a solution of atropige sulphatis, gr. iv to 3j, HYPEEMETROPFA 127 three times a day for four days (when we may feel sure that not the least vestige of accommodation remains), a much stronger glass can be tolerated, and will be required to enable the patient to read ^. This strong glass represents the total hypermetropia, the additional amount to that found as Hm. being called late7it (HI.). The following plan is an excellent one for measur- ing the manifest hypermetropia. Place in spectacle- frames before the eyes such convex lenses as over- correct the Hm. ( + 4 D. will usually do this) ; then hold in front of these, weak concave glasses, until we find the weakest, which thus held in front of + 4 D. enables f to be read; the difference between the glasses is then the measure of the Hm. By this plan the ciliary muscle is encouraged to relax, and we get out a larger amount of manifest hypermetropia than is obtained by the ordinary method. Thus, supposing — 2D. the weakest glass which, held in front of the convex 4 D., enables the patient to read f, -+ 2D. is the measure of the Hm. ( + 4 D.) + (— 2 D.) = + 2D. As age advances the accommodation diminishes, and the latent hypermetropia becomes gradually manifest. Thus a person may have 6 D. of hypermetropia latent at ten years of age, three of which may have become manifest at thirty-five, and the whole of it at about sixty-five or seventy, when the total hypermetropia is represented by the manifest. With the advance of age certain changes take place in the structure of the crystalline lens, by 128 THE REFRACTION OF THE EYE which its refraction becomes diminished. This change takes place in all eyes, and at a regular rate ; thus at fifty-five the refraction has diminished '25 D._, at sixty-five '75 J)., at seventy 1 D._, and at eighty as much as 2*5 D. Hypermetropia when thus occurring in eyes previously emmetropic is styled acquired hyper- metropia, in contradistinction to the congenital form, which is called original hypermetropia. The normal refraction of the eye in early child- hood is hypermetropic ; some remain so, a con- siderable number become emmetropic as they get older, and a certain percentage of these pass on to myopia. In the diagnosis and estimation of hypermetropia several methods are useful. We first estimate the acuteness of vision, remembering that being able to read |^ does not exclude hypermetropia, and that we must in all cases try convex glasses ; and if the same letters can be seen with as without them, then the patient certainly has hypermetropia, and the strongest convex glass with which he sees them is the measure of his Hm. We next proceed to retinoscopy ; with the plane mirror we get a shadow moving with the mirror : the quicker the movement and the brighter its edge, the lower is the degree of hypermetropia (see p. 87). With the ophthalmoscope by the indirect method of examination, the image of the disc is larger than in emmetropia, and diminishes on withdrawing the objective from the eye (p. 69). With the mirror alone at a distance, an erect image HYPERMETROPIA 129 of the disc is seen, which moves in the same direction as the observer's head (p. 72). By the direct method the accommodation of the observer and observed being relaxed, a convex glass is necessary behind the ophthalmoscope, to enable the observer to bring the diverging rays from the observed eye to a focus on his retina; the strongest convex glass with which it is possible to see the details of the fundus clearly, is the measure of the total hyperme- tropia (Fig. 47). The treatment of hypermetropia consists obviously in prescribing such convex glasses as will give to rays passing through them an amount of convergence, so that they will meet on the retina without undue accom- modation. It might be thought that, having obtained the measure of the total hypermetropia, nothing re- mained but to give such positive glasses as exactly neutralise the defect, and that we should then have placed the eye in the condition of an emmetropic one. Such at first was thought to be the case, though it is by no means so, because persons who have been accustomed to use their accommodation so constantly, both for near and distant objects, as hypermetropes have to, possess very large ciliary muscles which they cannot suddenly completely relax ; possibly also the elasticity of the lens capsule is somewhat im- paired. In children and patients under twenty years of age it is much better to atropize them at the first, and so measure once and for all the amount of total hyper- metropia ; otherwise it will frequently be found that 130 THE REFRACTION OF THE EYE the spectacles have to be constantly changed^ the asthenopia is unrelieved, and probably the patient has to be atropized after all, or becomes dissatisfied and goes off to some one else. Another reason in favour of atropine is, that with it we cannot possibly mistake cases of spasm of the ciliary muscle in hypermetropia for myopia, which might otherwise happen, since the spasm causes the lens to become so convex that parallel rays are even made to focus in front of the retina, thus simulating myopia. It must always be borne in mind that it is dangerous to atropize patients above the age of thirty-five, many well-marked cases of " glaucoma " having been traced to the use of this drug ; moreover as age advances the latent hypermetropia gradually becomes manifest, so that the necessity for paralysing the accommodation becomes less. There exists some difference of opinion among ophthalmic surgeons as to the amount of the total hypermetropia we ought to correct ; some give such glasses as neutralise the manifest hypermetropia only, while others, after estimating the total, deduct perhaps 1 D. from this. It will be found that patients vary much as to the amount of correction which is most comfortable for them. A good practical rule is to prescribe such glasses for reading as correct the manifest and one third of the latent hypermetropia. For example, a child having 5 D. of hyperme- tropia of which 2 only are manifest, will require + 3 D. for reading. At the age of twenty, about HYPEEMETROPIA 131 3 D. will have become manifest^ and tlie patient will then want + 3-75 D. ; at forty, 4 D. will be manifest, and he may then be able to use his full correction. Hence it will be seen that, as age advances, the spectacles will have occasionally to be changed for stronger ones, as the latent hypermetropia gradually becomes manifest. The question arises, should spectacles be worn con- stantly or only for near work ? So long as distant objects (|-) can be seen comfortably without them, their use is unnecessary except for reading and near work; this is generally the case in young persons where the hypermetropia does not exceed 3 or 4 D. When a convex glass improves distant vision, then such can be constantly worn ; somewhat stronger glasses will be required for reading after the age of forty-five. The disadvantage of using spectacles constantly is, that after wearing them for some time the patient finds he is unable to see without them, which is a serious inconvenience ; so that the plan is not to give spectacles for constant use until the hypermetropia has become relative or absolute. In cases of concomitant squint, spectacles which correct the hypermetropia are to be worn constantly, and here our object must be to give as near the full correction as is consistent with the patient\s comfort ; this we can only find out by experiment in each case. The best plan is to measure under atropine the total hypermetropia, deduct 1 D., and give this correction for constant use : the reason for making this deduction 132 THE REFEACTION OF THE EYE is that the ciliary muscle is never so completely relaxed as when under atropine. Convergent strabismus and asthenopia, two of the most frequent results of hypermetropia, will be treated of in Chapters X and XI. See Cases 1 and 2, p. 102 ; also 23, 25, and 30, p. 244. Aphakia Aphakia {' A, priv ; (JyaKOQ, lens) is the name given to that condition of the eye in which the lens is absent. - There are several causes, by far the most frequent being some form of cataract operation. Besides this aphakia may be caused by dislocation of the lens from injury, or dislocation may occur spontaneously, and this is probably the cause of those congenital cases where no lens can be seen. Aphakia necessarily converts the eye into a very hypermetropic one. The length of the eyeball which would be required (the curvature of the cornea being normal and the lens absent) to bring parallel rays to a focus on the retina is 30 mm., whereas normally the antero-posterior diameter of the eyeballs is only about 22-8 mm. To test aphakia : when a bright flame is held in front of and a little to one side of a normal eye, three images of the flame are formed, one erect on the cornea, another erect on the anterior surface of the lens, and a third inverted, formed on the posterior surface of the lens. On moving the flame up and down, the erect images move with it, and the in- APHAKIA - 133 verted one in tlie opposite direction. In aphakia two of tliese images are absent^ viz., those formed on the two surfaces of the lens. Treatment. — Strong convex glasses will be required to take the place of the absent lens, the previous re- fraction of the eye of course influencing their strength. In hypermetropia, stronger glasses will be required; in myopia, weaker. The convex glass usually required by an eye pre- viously emmetropic, to bring parallel rays to a focus on the retina is from 10 to 13 D. As every trace of accommodation is lost with the lens, stronger glasses will be required for reading or near work, and to find out the necessary glass for a certain distance, we have only to add to the distance glass one whose focal length equals the distance at which we wish our patient to see. Thus, if he require + 10 D. for distance, and wish to see to read at 25 cm., we add + 4 D. to his other glass, and the result- ing + 14 D. will adapt the e3'e for 25 cm. The patient may be taught a sort of artificial accommodation by moving the spectacles along the nose, nearer or further from the eyes, his working point being thereby moved away or brought nearer to him. In correcting aphakia it will often be found that the vision is below the normal. Frequently also there is some astigmatism, especially in cases after cataract extraction. See Case 36, p. 257. 134 THE REFRACTION OF THE EYE CHAPTER VII Myopia (m.) Myopia (Muw, I close : wi//, the eye), or sliort-siglit, is the opposite condition to hypermetropia. We saw that the hypermetropic eyeball was too short, so that parallel rays focussed behind the retina; it is therefore not adapted to any real distance, be- cause in order to see any object clearly, it is necessary that the defect should be corrected either by the accommodation or by means of a convex glass. Now in myopia, although the eyeball is too long to allow of distant objects being seen clearly, it is perfectly adapted for near vision, so that a low degree of myopia may not be a very serious disadvantage. We spoke of hypermetropia as congenital, due to an arrest of development; myopia is an acquired defect, and may be looked upon as an effort of nature to adapt the eye to near objects, as a result of civili- sation and its incessant demands on near vision. Myopia is peculiar to the human race, and is met with much more frequently in civilised than in un- civilised races. Low degrees, such as 1 D., may have no very serious MYOPIA 135 drawbacks, because although the full visual acuteness can only be obtained by the help of concave glasses, many people go half through life, playing cricket, tennis, shooting, etc., without finding out the defect ; their near vision is really better than that of the emmetrope, for they obtain larger retinal images, and they have to accommodate less ; against these advan- tages it may be stated that many myopes suffer from asthenopia, the result of disturbance of the harmony between the two functions, accommodation and con- vergence, though this disturbance will, of course, be more marked in the higher degrees of ametropia. Medium degrees of myopia, from 2 to 6 D., are ex- ceedingly common ; the visual defects are more pro- nounced, and it becomes necessary to use glasses for many things : often they have to be worn constantly. Such patients are liable to suffer from asthenopia, or from divergent strabismus and its accompanying evil — loss of binocular vision. The higher degrees of myopia which increase steadily and constantly from an early stage, reaching often a very high degree, and carrying in its wake damage and destruction to important ocular tissues, must be looked upon as a serious disease ; it is desig- nated by the name progressive myopia. We must now refer to the optical condition of the myopic eye. Parallel rays, falling on a myopic eye, focus in front of the retina, cross and form a circle of diffusion (Fig. 70), in place of a clear image. Only divergent rays focus on the retina, and hence 136 THE EEFRACTION OF THE EYE it is necessary that the object looked at be brought so near^ that rays coming from it are suiBciently divergent (Fig. 71)^ or they must be rendered so by passing them through a concave lens (Fig. 12), before they fall upon the cornea. Fig. 70. Fig. 71. Fig. 72. We may say^ then^ that in myopia the retina is at the conjugate focus of an object^ situated at a finite distance. The accommodation being at rest^ an MYOPIA 137 object situated at this point will be distinctly seen ; further off it will be indistinct^ nearer it can still be seen clearly by putting in force the accommodation. The greatest distance at which objects can be seen clearly is called the far point (punctum remotum), and is always at a definite distance. The higher the myopia the nearer to the eye is its punctum remotum (p. r.). The nearest point of distinct vision is the punctum proximum (p. p.)^ and is determined by the amount of the accommodation. To find out the punctum proximum, we place in the patient's hand the near type, and note the shortest distance for each eye separately at which the smallest type can be read_, or we measure it by the wire optometer in the manner before described. The amplitude of accommodation in low myopia is usually equal to that in emme- tropia, but in the higher degrees it becomes consider- ably diminished. The greatest distance at which an object can be clearly seen is the exact measure of the myopia ; for instance, if the far point be at one metre, a concave glass of that strength ( — ID.) w^ould render parallel rays as divergent as if they came from a distance of one metre, and with a glass of this focus the person would be able to see distant objects clearly. Myopia was for a long time thought to be due to an increase in the convexity of the cornea, but as a matter of fact the cornea is usually less convex, and, as a rule, the greater the myopia the less the convexity. 138 THE REFRACTION OF THE EYE 2. 3. Causes of Myopia : 1. Too great length of the antero-posterior dia- meter of the eyeball (axial myopia). This is the common cause of myopia. Increase of the index of refraction of the lens. This occasionally occurs in the development of cataract. Conical cornea : this disease simulates myopia at its commencement. It may therefore be stated that myopia almost invariably depends upon the lengthening of the visual axis, accompanied in many cases by the formation of a posterior staphyloma which further increases the antero-posterior diameter of the eyeball. This bulg- ing, when it occurs, takes place at the outer side of the optic nerve towards the macula, and consists of an extension backwards with thinning of the sclerotic and choroid, and more or less atrophy of the latter. So constant is this lengthening of the visual axis, that from the number of dioptres of myopia can be calculated the increase in the length of the eye- ball. The following table gives the calculation up to 10 D. Degree of Distance of the p. r. Increase in length of the myopia. in millimetres. myopic eye in millimetres •5D. 2000 •16 1- 1000 •32 1-5 666-6 •49 2- 500 •66 2-5 400 •83 3- 333-3 1^ MYOPIA 1 Degree of Distance of the p. r. Increase in length of the myopia. in millimetres. myopic eye m millimetres 3-5 285-7 1-19 4- 250 1-37 4-5 222-2 1-55 5- 200 1-74 6- 166-6 213 7- 142-8 2-52 8- 125 2-93 9- 111-1 3-35 10- 100 3-80 139 It will be remembered that the emmetropic eye measures in the antero-posterior diameter 22*824 mm. Fig. 73 shows a section of a myopic eye, in which the outside measurements were — antero-posterior diameter, 30^ mm.; vertical diameter, 25 mm.; trans- verse diameter, 25 mm. Fig. 73. In Fig. 74 the amount of accommodation is indi- cated in a myope of 2 D. by the number of spaces through which the thick lines pass; thus at the age of thirty the accommodation is equal to 7 D., and the near point will be ] 1 cm. ; the distance of the punc- tum proximum is given for each dioptre at the bottom of the diagram. 140 THE liEEKACTlON OF THE EYE As the punctum remotum in myopia is situated at a finite distance, therefore^ for the same amplitude of accommodation, the punctum proximum is nearer the eye in myopia than in emmetropia. The near point Fig, 74. Dioptres. Diagram showing the amount of accommodation at different ages in a case of myopia of 2 D. gradually recedes with advancing age at the same rate, whatever the refractive condition of the eye ; it is clear, then, that the near point in myopia Avill be longer in reaching that point (22 cm.) at which pres- byopia is arbitrarily stated to commence than in emmetropia, so that in prescribing glasses for pres- byopia, the amount of myopia has to be deducted from the glass which the emmetrope would require at any given age. If the myopia amount to 4*5 D., then the patient can never become presbyopic, because his punctum remotum is only 22 cm. away, so that he will always be able to see at that distance. Most people imagine that those who do not require MYOPIA 141 glasses with advancing age have very strong eyes ; how frequently does one hear the remark, when in- inquiring of a patient's family history, " Oh, my father had excellent sight, he was able to read at sixty without glasses." This is proof positive that he had myopia, though probably you will be unable to convince the patient of this fact. In hypermetropia it was shown that the power of accommodation had to be used in excess of the con- vergence. In myopia we have the opposite defect, the patient having to converge in excess of his accom- modation ; thus if he be myopic 4 D,, his far point will be at 25 cm. ; when looking at an object at this distance, it is necessary for him to converge to this particular point, his angle of convergence being 4, while his accommodation remains passive. Determining^ Causes. — The chief factors in the pro- duction of myopia are : the constant use of the eyes for near work, especially at an early age, when these organs are developing ; disturbances of nutrition in the tissues of the eye, together in some cases with a peculiar conformation of the skull. In a large majority of cases myopia is acquired, but in a small proportion of cases it may be congenital ; this latter form is frequently of high degree in early life, may occur in one or both eyes, and bears no relation to the occupation of the patient. Though seldom congenital it not infrequently happens that one or other of the parents has suffered from myopia. There is little doubt that in many cases there is an hereditary tendency to it, which, transmitted through 142 THE REFEACTION OF THE EYE several generations under favourable conditions for its development, becomes very decided. As in the greater number of cases of myopia the factor whicb tends to produce it, is the proloji^ed use of the j eyeS: -on near ob|ects, especially while young ; we may set '.down myopia as one of the results of civilisation and education, and in these days of high pressure and competitive examinations it is constantly on the increase. The result of the very numerous statistics that have been collected, especially by German ophthalmologists (myopia in Germany is exceedingly common), points to the production of myopia in direct proportion to the amount of edu- cation. The amount of myopia was found to be much greater in town than in country schools, no doubt because the general health was better amongst those living in the country. Erismann has come to the pleasant conclusion that, if myopia increase in the same ratio as it had done during the last fifty years, in a few generations the whole population will have become " myopic." The normal refraction of the eye in childhood is hypermetropic ; some few remain so, a great number becoming emmetropic as they get older, and a large percentage of these pass on to myopia. In proof of this hereditary tendency to myopia. Dr. Cohn has summarised the statistics of various German writers on this subject. In public shools, myopia was found to exist without predisposition in 8 per cent., with predisposition in 19 per cent. In the higher schools the result was — without MYOPIA 143 predisposition 17 per cent., with predisposition 26 per cent. Residence in towns is also conducive to short- sight by causing people to gaze constantly at near objects. The cause why myopia when once established is very liable to increase, is that the extreme converg- ence, which is necessary to enable the patient to see at the limited distance to which he is confined, causes the weakest part of the globe (that part, in fact, which is least supported) to bulge, forming a posterior staphyloma. In support of this method of the production of myopia may be stated the well- known fact, that people such as watchmakers and jewellers who habitually use a strong convex lens before one eye, and work at the focal distance of that lens, are not especially liable to myopia, proving that close work without convergence does not tend to pro- duce it. As the eyeball becomes elongated, its move- ments become more difficult, and the pressure produced by the muscles during prolonged convergence tends still further to increase the myopia. The stooping position which so many myopes take up causes an accumulation of blood in the eyeball which tends to raise the tension as well as materially to interfere with its nutrition. Hence results a state of congestion, softening, and extension, leading to a further increase of the myopia. The more advanced these changes, the more difficult is it for the myopia to become stationary. In addition to these two causes, extreme con- 144 THE EEFRACTION OF THE EYE vergence and the stooping position, it is possible tliat, as a result of the constant convergence, the optic nerves may be somewhat pulled upon, and thus further assist in producing myopia. Cases of nebulae, cataract, and other causes of imperfect sight in children may give rise to myopia by causing them to hold objects they wish to see close to the eyes. Symptoms. — The patient sees distant objects badly and near objects well. The eyes look prominent ; the pupils are usually large in young people; as age advances they contract, thus diminishing the circles of diffusion, and so slightly improving vision. Eserine acts in the same manner, so does the nipping together of the eyelids, which is so characteristic of patients suffering from myopia, and to which the defect owes its name. The acuteness of vision is frequently below the normal, though objects within the patient's far point appear larger than they do to the emme- trope, the distance between the nodal point and the retina being greater in myopia (Fig. 75). This, however, may be partly counterbalanced by the stretching of the retina, so that, although the image may be somewhat larger, it may not cover a greater number of cones than would be the case in an emme- tropic eye. If the myopia be progressive, frequent limitations in the field of vision occur, in the form of scotomata due to patches of choroidal and retinal atrophy. Besides seeing distant objects badly, the patient complains of pain, fatigue, and intolerance of light, MYOPIA 145 with a state of irritation^ especially after using the eyes by artificial light. There may be hyper^emia Fig. 75. A. The retina in an emmetropic eye. b. The retina in a myopic eye. c. The visual angle, n. The nodal point. The distance from n b is greater than n a, and the image of o'p is greater at b than at a. of the eyes and lids^ spasm of the accommodation (which increases the apparent amount of myopia) pain in the eyeballs on pressure^ photopsia, an appearance of convergence due to the small size of the angle a (p. 202), together with " muscge volitantes." Muscse are often a source of great anxiety ; the patient may, however, be assured that, although they cannot be removed, there is no cause for uneasiness ; these muscas are probably the remains of vitreous cells, which, being situated a considerable distance in front of the retina, throw shadows on it and are projected outwards as much larger images than would be the case in an emmetropic eye ; they appear to the patient as black spots, rings, or lines floating about. The ciliary muscle is smaller than in emmetropia, the circular fibres (which are so hypertrophied in hypermetropia) being almost absent. 10 146 THE REFRACTION OF THE EYE The internal recti muscles often act badly, so that convergence becomes painful and difficult, often going on to divergent strabismus. In myopia the convergence has to be used in excess of the accommodation ; some patients as they become myopic learn to use these two functions in unequal degrees, while others are unable thus to dis- sociate them; so that on looking at an object situated at the myope's far point, no accommodation and no convergence take place, it becomes necessary then that the two eyes shall make a conjugate movement in one direction, so that one eye may receive the image of the object on its macula, the other eye, as a result of the conjugate movement has deviated outwards, — in other words, divergent strabismus has occurred. When the myopia is of high degree, the patient often uses one eye only for reading, then of course he does not require to converge. The refraction diminishes slightly with advancing age (see p. 128) ; the pupils also become smaller, thus cutting off some of the patient's circles of diffusion ; so that frequently a marked improvement takes place in the vision of myopes as they get older. Ophthalmoscopic Appearances. — With the ophthalmo- scope a crescentic-shaped patch of atrophy is fre- quently seen on the outer side of the optic disc, embracing it by its concave edge ; this is called the '^myopic crescent." In an early stage the crescent looks somewhat white, the large choroidal vessels often appear more ^^ ^^ Bale . = 10 D.) — 10 D. would be the measure of the myopia. With retinoscopy the shadows move against the movements of the plane mirror so long as the observer is beyond the jDatient^s far point (p. 87). With the ophthalmoscope, by the indirect examina- tion, the disc looks smaller than in emmetropia, and becomes larger on withdrawing the objective further from the eye (p. 70). With the mirror alone at a distance, an inverted magnified image of the disc can be clearly seen, provided always that the observer be not nearer the aerial image than his own near point (Fig. 45). The lower the myopia the greater the image, because the longer is the distance between the image and the myopic eye. On moving the head from side to side the image of the disc will always move in the opposite direction, showing that it is an inverted one. By the direct method of examination the fundus can- not be clearly seen until a concave glass is placed in 150 THE REFEACTION OF THE EYE front of the observing eye. The weakest concave glass with which the details of the macula and disc can be clearly seen (the observer's eye being emmetropic and the accommodation relaxed) is a measure of the myopia (Fig. 48). This test may be relied upon for the lower, but not for the higher degrees of myopia. The treatment of myopia. — The chief indications are : 1st. To prevent the increase of the myopia. 2nd. To enable the patient to see well. 3rd. To prevent the various troubles from which myopes are so liable to suif er, as asthenopia, divergent strabismus, etc. To carry out the first of these indications, strong convergence and the stooping position, which play so important a part in the production of myopia, must be avoided, the patient being directed never to read in a train or carriage, where every movement requires a change in the accommodation ; he should not look at near objects for too long together : the natural tendency for a myope who is excluded in great mea- sure from seeing distant objects is to devote himself to near ones. In reading, writing, or working, he must keep 35 cm. away from the book or paper, use books printed in good bold type, and not write too small, while the desk and seat should be conveniently arranged so as to avoid stooping. He should do as little as possible by artificial light ; when necessary, it is best to use a reading lamp, so placed that it throws the light down upon the work, leaving the remainder of the room in comparative darkness, so MYOPIA 151 that when the eyes become tired they may be rested by turning them from the light. The stooping position must be strictly avoided, as it causes an increased flow of blood to the interior of the eyeball, and at the same time, by compressing the veins in the neck, obstructs the returning blood, and so produces hyperaemia with symptoms of irritation, and possibly some slight increase of tension. When reading or writing the patient should sit with his back to the window, so that the light may fall on the book or paper over his left shoulder, the shadow of his pen being thus thrown to the right, enabling him to see plainly the letters he is forming. Attention must be paid to the general health : iron internally often being especially useful, combined with regular outdoor exercise and good nutritious food. When symptoms of irritation show themselves, with a rapid increase in the myopia, complete rest must be given to the eyes, and in no way can this be so conveniently carried out as by dropping into the eyes a solution of atropine (gr. j to 3J) three times a day, for some two or three weeks ; counter-irrita- tion may be applied to the temples and behind the ears in the shape of small blisters, or by a solution of iodine : no spectacles must be allowed but smoke- coloured protectors. Sometimes, where there are symptoms of congestion present the artificial leech applied to the temple once a week for a few weeks does good. As the irritation gradually subsides, the patient may be allowed to do a little reading daily in a good light, the eyes all the time being 152 THE REFEACTION OF THE EYE kept under atropine ; he may require glasses to enable him to do this. Thus if he have myopia of 3 D. he will not require them, his far point being at 33 cm.; if he has — 1*5 D. he will require 4- I'o J)., to enable him to read at about 33 cm. (+ 3 D.) + (— 1*5 D.) = + I'D D. ; if the myopia is 6 D. he will require — 3 D. to put back his far point from 16 to 33 cm. (+ 3 D.) + (- 6 D.) = - 3 D. So long as the myopia is progressive it must always be a source of anxiety to us. To enable the patient to see well both near and distant objects, as well as to prevent extreme converg- ence, we must correct the myopia. In young people with good accommodation and with a low degree of myopia the full correction may be well borne, the patient wearing such glasses constantly; and it has been observed that in those who from their youth have worn their full correction constantly, for both near and distant objects, the myopia has in some cases remained stationary. There are two exceptions to this general rule of the full correction of myopes : 1st. Where the myopia is of high degree, and the acuteness of vision is reduced, then the concave glasses so much diminish the size of the retinal images, that the individual is induced to make these images larger by bringing the object closer. 2nd. When the myopia is of high degree, and the patient has learnt, from long practice, to exercise the function of convergence in excess of his accommodation, the full correction, which gives him MYOPTA 153 perhaps excellent distant vision, causes him pain when used for near objects. Here we must give two pairs of spectacles, one for distant vision and the other for near objects ; the latter may be gradually increased in strength as the patient becomes accus- tomed to them, so that after a time, possibly a year or so, the full correction may be comfortable for constant use. In those cases where the myopia is of high degree, and the patient is unable to bear the full correction for reading, we find out the necessary glass by sub- tracting from the lens which gives the best acuteness of vision that glass whose focus represents the dis- tance at which the patient wishes to read or work. Thus, for example, — 9 D. gives the best distant vision; the patient wishes for glasses with which to readat33cm.(- 9D.) + (+3D.) = (_6D.);-6D. will be the glass required, and will enable the patient to read at 33 cm. without using his accommoda- tion. Glasses may also be required for music. When the myopia is of low degree, and we are certain that the disease is stationary, folders may be allowed for dis- tance, no glass being used for near work. Single glasses are occasionally allowed in low degrees of myopia for looking at distant objects; they have the disadvantage that they encourage the patient to give up binocular vision, and may so assist in the development of a divergent squint. When muscular asthenopia is present, prisms with their bases inwards (which diminish the necessity for 154 THE REFRACTION OF THE EYE convergence), with or without concave glasses, are of great value. When photophobia is a prominent symptom tinted spectacles may be comfortable (p. 243). It is important to impress on the patient that the glasses for reading are not given to enable him to see better, but to increase the distance at which near work can be done. When the myopia has been estimated under atro- pine, it is often necessary to add on to the glass so found — '5 D., as the full correction under the mydriatic is usually this much weaker than the cor- rection found without it, the reason being that the ciliary muscle is never so completely relaxed as it is by atropine. I am of course aware that the above optical treat- ment of myopia is at variance with the teaching of French authorities. Landolt considers that the action of the ciliary muscle may have a tendency to increase the myopia, and therefore states that myopes should never wear glasses which require the patient to use his accommo- dation : so that in low degrees of myopia glasses are only allowed for distant objects ; in medium degrees, glasses which under-correct the myopia are given for near objects, so as to enable the wearer to see at a given distance without accommodation. My own opinion is, that every case requires treating on its own merits ; very many myopes wear their full correction constantly with comfort, and if not with benefit to the eyes most certainly without injury ; MYOPIA iO& while other myopes will occasionally be found who suffer from asthenopia when using their full correction for near vision. In extreme degrees of myopia, and in those where the disease is increasing rapidly, rest for the eyes, and not spectacles, is the essential treatment. In cases of high myopia (over 15 D.) the lens may be removed by a needle operation followed by curet- ting, and thus the eye may be brought nearly to the point of emmetropia, the patient getting good distant vision without glasses. Many of these cases have given most gratifying results, while in others com- plications have arisen leading later to detachment of the retina or other troubles. Usually only one eye is operated upon. See Cases 24, 31, and 32, pp. 247 and 255. 156 THE REFRACTION OP THE EYE CHAPTER VIII ASTIGMATISM AND ANISOMETROPIA Astigmatism (^ A, priv ; aTL^jia, a point) . Hitherto we have seen that the cornea usually takes but little part in the defects we have been con- sidering. It has been shown that hypermetropia is almost invariably due to the eyeball being too short, and myopia to its being too long. We now come to a defect in which the curvature of the cornea plays a very important part, with or without some decrease or increase (from the emmetropic standard) in the antero-posterior diameter of the eyeball; I refer, of course, to astigmatism, which is the commonest of all the refractive errors, few cases of hypermetropia being entirely free from it, and still fewer cases of myopia. Astigmatism may be defined as that state in which the refraction of the several meridians of the same eye is different : for instance, the vertical meridian may be emmetropic, the horizontal hypermetropic. Astigmatism is usually congenital, but may be acquired; frequently there is some hereditary ten- dency. ASTIGMATISM 157 Astigmatism was first discovered by Thomas Young in 179oj who was himself astigmatic. Astigmatism is divided into two chief varie- ties : 1. Irregular. 2. Regular. Irregular astigmatism consists in a difference of refraction in the different parts of the same meridian, and may be further subdivided into normal and abnormal, (a) Normal irregular astigmatism is due in great measure to irregularities in the refracting power of the different sectors of the lens; it causes a luminous point to appear stellate, as in the case of a star, which is, in reality, round. (b) The abnormal variety may arise from the condition of the lens or of the cornea : when the lens is at fault, it may be a con- genital defect, it may be acquired from changes taking place in the lens itself, or it may result from partial displacement. The changes in the cornea which may produce it are, conical cornea, nebulae, and ulcers. Little can be done in the way of glasses towards correcting this form of astigmatism, though improve- ment of vision sometimes occurs when stenopaic spec- tacles are worn, the opening being made to suit the peculiarity of each case. We now pass on to the much more common variety, which can frequently be exactly corrected by the help of cylindrical lenses. Regular astigmatism is due to the curvature of the cornea being different in the two meridians, that of maximum and minimum refraction ; these are called 158 THE REFRACTION OF THE EYE the chief meridians, and are always at right angles to each other. In the normal eye the cornea is the segment of an ellipsoid and not of a sphere, so that there is a slight difference in the refraction of the two chief meridians, the focus of the vertical meridian being slightly shorter than that of the horizontal. This can easily be proved by looking at a card on which is drawn two lines crossing each other at right angles; the card is held close to the eye and gradually made to recede ; both lines cannot be seen at the same time with equal clearness, the horizontal being seen clearly at a shorter distance than the vertical line. So long, however, as the acuteness of vision is not impaired it goes by the name of normal astig- matism, or regular astigmatism of the normal eye. Parallel rays passing through a convex spherical glass come to a focus at a point. If the cone of light thus formed be divided perpendicular to its axis, at any point between the lens and its focus, or beyond the focus after the rays have crossed and are diverg- ing, a circle is formed. In astigmatism the case is diiferent : if parallel rays pass through a convex lens which is more curved in the vertical than in the horizontal meridian, those rays which pass through the vertical meridian come to a focus sooner than those which pass through the horizontal; and the resulting cone, instead of being circular as in the pre- vious case, will be more or less of an oval, forming a circle only at one point (4, Figs. 76 and 77). Let us now divide this cone at different points at right angles ASTIGMATISM 159 to its axis, and notice the shape of the diffusion patches thus produced. At 1, an oblate oval is formed ; at 2, a horizontal straight line, the rays passing through the vertical meridian having come to a focus ; at 3, 4, 5, the rays Fig. 7G. Fig. 77. Section of cone of light at 1, 2, S, 4, 5, 6, 7, Fig. 76. passing through the vertical meridian have crossed and are diverging, and the rays passing through the horizontal meridian are approaching ; at 4 a circle is formed ; at 6 a vertical straight line, the rays passing through the horizontal meridian have met, while those passing through the vertical meridian are still diverg- ing ; a large prolate ellipse is formed at 7. The space between h and v, h being the point at 160 THE REFRACTION OF THE EYE which the rays passing through the horizontal meri- dian focus^ and v the point at which the rays passing- through the vertical meridian meet, is called the interval of Sturm (i, Fig. 78). Regular astigmatism was at one time thought to be due to defects in the curvature of the lens, but it has since been proved to depend almost entirely on Fig. 78. asymmetry of the cornea. The lens may, however, influence it in two ways : — 1st. Its two chief meri- dians may not correspond to those of the cornea. 2nd. Owing to the position of the eye the lens may be situated obliquely. It has been experimentally proved that slight amounts of corneal astigmatism may be corrected or disguised by the unequal contraction of the ciliary muscle (one segment of the muscle acting Avhile the rest of the circle remains passive) ; the curvature of the lens is thus increased in the direction of the ciliary contraction only. In astigmatism the vertical meridian of the cornea has usually the maximum, and the horizontal meri- dian the minimum of curvature, corresponding to the astigmatism of the normal eye, when this is so, we ASTIGMATISM 161 speak of it as astigmatism according to the rule. To tliis^ liowever, there are numerous exceptions. Thus the chief meridians may occupy an intermediate position, or the vertical meridian may have the mini- mum, and the horizontal the maximum of curvature, then Ave have astigmatism against the rule. What- ever the direction of the two chief meridians, they are always at right angles to each other. There are five varieties of regular astigmatism : 1. Simple hypermetropic astigmatism. 2. Compound hypermetropic astigmatism. 3. Simple myopic astigmatism. 4. Compound myopic astigmatism. 5. Mixed astigmatism. In the first variety, one set of rays (we will assume the vertical, v) have come to a focus on the retina, while those at right angles, the horizontal (h), focus behind the eye. Thus, instead of a point, as in emme- tropia, a horizontal straight line is formed on the retina, Fig. 79. Fig. 79. In the second variety, both sets of rays focus behind the retina, forming an oblate oval on the retina (Fig. 80). In the third variety, one set of rays (we will assume 11 162 THE REFEACTION OF THE EYE the vertical) focus in front of the retina, the other set on the retina, thus forming a vertical straight line instead of a point (Fig. 81). In the fourth variety, both sets of rays focus in Fig. 80. Fig. 81. front of the retina, forming an upright oval on the retina (Fig. 82). Fig. 82. In the fifth variety, one set of rays has its focus in front, and the other set behind the retina (Fig. 83). ASTIGMATISM 163 In these five figures the focus of the rays which have passed through the vertical meridian has been Fig. placed in front of the focus of the rays which have passed through the horizontal meridian ; of course, it will be understood that the position of these two foci are frequently reversed. From what has been said it will easily be seen, that when an astigmatic eye looks at a spot, it sees not a spot, but a line, an oval, or a circle ; hence its name (a and aTiyfia). It is necessary that it should be thoroughly under- stood how the image of a line is formed on the retina : the clear perception of a line depends upon the dis- tinctness of its edge, and to gain a clear image of this line it is necessary that the rays coming from a suc- cession of points which make up this line (they of course emerge in every direction) should be brought to a focus on the retina, having passed through the cornea at right angles to the axis of the line. Should they not do so circles of diffusion are formed, which overlap each other and so render the edges ill-defined. The rays which diverge from the line parallel with its 164 THE REFEACTION OF THE EYE axis^ overlap each other on the retinal image, in- creasing its clearness, except at the extremities, where they overlap and cause some slight indistinct- ness. Thus a person with simple astigmatism, myopic in the vertical meridian and emmetropic in the hori- zontal, sees distinctly vertical lines, because the rays coming from the edges of the vertical line pass through the horizontal or emmetropic meridian, while those which come from the line parallel with its axis pass through the myopic meridian and overlap jeach other without causing any indistinctness of its edges. There- fore a patient with simple astigmatism sees clearly the line which is parallel with his ametropic meridian, and indistinctly the line parallel with his emmetropic meridian. Causes : 1. Congenita! malformation of the cornea, which may in astigmatism of high degree be part of a general malformation of the face and skull. This variety of astigmatism usually remains unchanged throughout life. 2. Operations involving the cornea or sclerotic, such as cataract extractions, iridectomy, etc. ; these operations often cause by their cicatri- sation a high degree of astigmatism, which changes considerably with time. Symptoms. — There is frequently a want of sym- metry about the patient's head or face. If young, and the astigmatism hypermetropic and of low degree, few symptoms may be present ; usually, however, the patient complains of defective vision, with asthenopia. ASTIGMATISM 165 especially if his work be such that his accommodation is in constant use; sometimes headache is a very marked symptom, either frontal or occipital; he has probably tried all sorts of spectacles, and can find none to suit him. On trying him at the distant type, his acuteness of vision is ahvays below the normal, the mixed variety of astigmatism affecting it most, and next the myopic varieties. The way the patient reads the type may be an indication of the defect : he may be able to read certain letters better than others ; thus he may decipher some letters of -^ only, and yet be able to read some of -^j and even some of -|. We sometimes notice, when trying the acuteness of vision, that the patient sees much better if allowed to hold his head on one side ; by doing this he places his nose somewhat in the line of vision of the eye he is using, and so cuts off some of the rays which would otherwise enter his pupils, thus diminishing his circles of diffusion. It is possible that if his chief meridians are oblique, by thus tilting them he brings them to correspond with the meridians of the object looked at. Whether this explanation be the correct one I know not, but we may generally feel pretty confident, when we see the patient looking at the test-type with his head on one side, that astigmatism is present. One frequently hears it said that images formed on the retina in astigmatism are distorted; this, however, is not the case, as can readily be proved by making one^s own eye astigmatic, by placing in front of it a cylindrical glass : a certain amount of blurring and indistinctness is produced, but no actual 166 THE REEEACTION OF THE EYE distortion^ tlie distance between the cornea and retina being insufficient. Usually both eyes are affected^ sometimes quite symmetrically. Frequently, however, there is a great difference, one eye being almost emmetropic, the other very astigmatic. In astigmatism, when the chief meridians of one eye are at right angles to the chief meridians of the other, binocular may be much better than monocular vision : we will illustrate this by a simple example. The right eye we will assume to be hypermetropic 2 D. in the vertical meridian, emmetropic in the hori- zontal; the left emmetropic in the vertical, hyper- metropic in the horizontal 2 D. We know that the patient, looking at the fan of radiating lines with the right eye only, will see the vertical lines distinctly, the horizontal ones only by accommodating ; with the left eye the horizontal lines will be clearly seen, the vertical ones indistinctly ; with the two eyes all the lines will appear fairly distinct, the image in one eye overlapping that of the other. We seldom find a case in which the correction is so complete as in our example, but we meet with cases where partial cor- rection takes place. In my experience vision is less impaired when the chief meridians are vertical and horizontal than when they are oblique. As hypermetropia is more common than myopia, so also is hypermetropic astigmatism of more frequent occurrence than the myopic variety, though few myopes will be found who are quite free from astig- 168 THE REFRACTION OF THE EYE PRAY^S TEST TYPES FOR ASTIGMATISM. Horizontal. 15° 30° 45° 60° 76° "O ^T^ \C^' 90° 105° 120° ll'lil'' f% T/A llliull kjf //u/j 135° 150° 165° % % ^ ^ ASTIGMATISM 169 matism. Mixed astigmatism is the least frequently met with. If, after trying the patient at the distant type, we are not satisfied with the result, though perhaps Ave have some improvement with either convex or con- cave spheres, w^e may suspect astigmatism and pass on to some of the special tests by which it may be diagnosed and estimated. If astigmatism exist, our first object must be to find out the direction of the two principal meridians, viz. those of maximum and minimum refraction. Most of the subjective tests for astigmatism are based upon the principles of the perception of a line. An astigmatic eye looking at a test object composed of lines radiating from a centre, and numbered for convenience like the face of a clock, is unable to see all the lines equally clearly. The line seen most dis- tinctly indicates the direction of one of the two chief meridians ; the other chief meridian being of course at right angles to the one most clearly seen. The fan of radiating lines now ver}" commonl}^ used, as well as the clock face with movable hand, are all convenient test objects. The striped letters of Dr. Pray are useful for indicating one of the chief meridians. To test and measure the astigmatism, we place our patient at a distance of six metres in front of the clock. Fig. 84, covering up one eye with a ground- glass disc. Supposing he sees plainly the three lines from 12 to 6, all the other lines being more or less indistinct, those from 3 to 9 most so ; and further, if 170 THE EEFRACTION OF THE EYE on placing before the eye a weak positive glass we find that lines from 12 to 6 are blurred_, we know then Fig. 84. that the horizontal meridian — that is, the meridian at right angles to the clearly defined line — is emme- tropic, as well as being one of the principal meridians. We now direct him to look steadily at the lines from 3 to 9, {. e. those at right angles to the lines first seen, and try what spherical glass enables him to see these lines distinctly and clearly ; this glass is the measure of the refraction of the vertical meridian, and there- fore also of the astigmatism. To obtain reliable results, the eye omist be tho- roughly under the influence of atropine. ASTIGMATISM 171 Supposing lines from 12 to 6 be clearly seen, but that with a weak convex glass they are blurred ; and that on looking at lines 3 to 9 no convex glass im Fig. 85. proves their clearness, while —ID. renders them quite distinct, the case is one of simple myopic astig- matism. With the ophthalmoscope the astigmatism may also be recognised. 1st. With the indirect method we find that the shape of the disc, instead of being cir- cular, is more or less oval, changing its shape as the objective, which must be held exactly perpendicular, is withdrawn. 2nd. With the direct method we find that the disc appears oval, the long axis of the oval corresponding to the meridian of greatest refraction. Figs. 86 and 87 show the same disc as seen by the direct and indirect examination. It is, however, the diiference in degree of the clear- ness of the retinal vessels that is to be taken as the 172 THE REFRACTION OF THE EYE guide, not only of tlie cliief meridians^ but also of the kind and amount of error. To detect this, assuming that the chief meridians are vei;tical and horizontal, we take notice first of the lateral margins of the disc, Fig. 86.* Fig. 87. Erect image. Inverted image. and of a vessel running in the vertical direction, and find out the b-b'ongest positive_, or the icecikest negative glass, with which these are distinctly seen, using a re- fracting ophthalmoscope. We then take a horizontal vessel with the upper and lower margins of the disc, and estimate their refraction in the same manner. Thus a vessel gomg upwards is first taken ; it is seen well with convex 1, the horizontal meridian therefore is hypermetropic 1 D. A horizontal vessel is now looked at, and can be best seen with concave 1, show- ing that the vertical meridian is myopic one dioptre ; * I have to thank Mr. Nettleship for these woodcuts from his work on * Diseases of the Eye.' ASTIGMATISM 173 the case is, therefore, one of mixed astigmatism. When the chief meridians are not vertical and hori- zontal, we must endeavour to find a vessel which coincides with one of the chief meridians, and having estimated this, we look for a vessel at right angles to that first chosen, and find out its refraction in the same way; this gives us the other chief meri- dian. 3rd. Retinoscopy. This is the easiest and most trust- worthy of all the objective methods. The patient being fully atropised, the principal axes can be seen at a glance, and the proper glasses for correcting the error easily found by anyone who has taken the trouble to familiarise himself with this method of examination. For a full description of retinoscopy the reader must refer to Chap. V, p. 82. Astigmatism requires for its correction a cylindrical glass, and reference has already been made to such a lens on p. 32. This cylindrical glass is the segment of a cylinder ; whereas a spherical glass is the segment of a sphere. The cylinder may be either concave or convex, and is numbered according to the refraction of the meridian of greatest curvature; the result upon rays which pass through it is, that those which pass through parallel to its axis undergo no refraction ; all other rays are refracted, those most so which pass at right angles to the cylinder. A cylinder thus possesses the power of exactly neutralising the astigmatism. On referring back to Fig. 79, which represents a case of simple hypermetropic astigmatism, the vertical 174 THE EEFRACTION OF THE EYE meridian being emmetropic and the horizontal meri- dian hypermetropic, it will be seen that a convex cylinder can be found, which with its axis vertical will increase the refraction of rays passing through the horizontal meridian, so that they meet exactly on the retina. Suppose the glass required be + 1 D. cylinder, this not only corrects, but is itself a measure of the astigmatism. If a patient with astigmatism of 1 D. be able to read -f-^ of the distant type and with the cylinder + 1 D. axis vertical -J, it may be ex- pressed in the following manner : y^2' + -'^ -^* ^J' ^^^^ vert. = f . Fig. 80 represents compound hypermetropic astig- matism. We find out the refraction of each chief meridian by retinoscopy or the clock face. Assuming, then, the vertical meridian to be +1 D., and the horizontal + 2 D., if we place our positive cylinder + 1 D. with its axis vertical, we shall have corrected the astigmatism, and the error will be reduced to one of simple hypermetropia, requiring for its correction 4- 1 D. sphere. This combination of sphere -|- 1 D. with cylinder + 1 D. axis vertical is made in one glass, by the optician grinding upon one side the sphere + 1 D. and on the other the cylinder + 1 D. The lens thus formed is called a spherico-cylindrical lens. Fig. 81 represents simple myopic astigmatism, in which the vertical meridian is myopic and the hori- zontal emmetropic. To correct this error it is neces- sary to cause the rays which pass through the vertical meridian to be so refracted that they meet at instead ASTIGMATISM 175 of in front of the retina. Here it is obvious that a negative cylinder with its axis horizontal will accom- plish this object. Fig. 82 represents compound myopic astigmatism. Both sets o£ rays focus in front of the retina, one set in advance of the other. This is corrected by carrying the focus back by a negative sphere, and so reducing the case to one of simple myopic astig- matism, which is corrected by a negative cylinder. This glass is called a concave spherico-cylindrical lens. Fig. 83 represents mixed astigmatism. One set of rays focus in front of the retina, the other set behind it. The difference between these is the amount of astigmatism, and may be corrected in three different ways. Thus supposing the vertical meridian myopic 1 D., and the horizontal hypermetropic 1 D., the cor- rection may be made by — 1 D. cylinder axis, horizontal, which puts back the vertical rays so as to focus on the retina, combined with a + 1 D. cylinder axis vertical, which brings forward the horizontal rays to the retina. This compound lens is called a concavo-convex cylinder or crossed cylinders. There are, however, some difficulties in using this method of correction ; the axes of the cylinders have to be arranged with such exactness, that the slightest variation may upset the whole result. Besides, it is difficult, when using such a combination at the distant type, to make altera- tions with the same facility with which one does other combinations. Moreover, during the grinding, very great care is required of the optician ; so that either of the following plans seems preferable : by 176 THE EEFRACTION OF THE EYE a concave spherical glass of 1 D., combined with a convex cylinder of 2 D. axis vertical; or by a + 1 D. sphere^ combined with — 2D. cylinder axis horizontal. Treatment. — Having found out by one of these numerous methods the refraction of the two chief meridians, we confirm the result by trying the patient at the distant type with the combination so found, making any slight alterations that may be necessary. These glasses may be ordered at once, remembering, when atropine has been used, that in hypermetropic astigmatism we must reduce the convex sphere about ID., while in the myopic variety the concave sphere must be slightly increased by about '50 D. We frequently have to be satisfied with glasses which do not raise the vision to f, and if such have been carefully chosen, we often find that after they have been worn for some time the vision improves, due no doubt to the retina becoming more sensitive to well-defined images, a condition of things to which it was previously unaccustomed. In ordering glasses for astigmatism, we must be careful to give the exact axis of each cylinder ; con- venient forms may be had with the diagram of a frame marked in degrees ; the axis is indicated by drawing a line through this diagram. The * Ophthalmometer of Javal and Schiotz is an instrument for measuring the amount of cornea . astigmatism. Scientifically it may be of much value, as by it we are enabled to separate astigmatism due to the cornea from that due to the lens; but the ASTIGMATISM 177 price will prevent its coming into general use^ especially as we possess so many other methods by which astigmatism may be estimated; the separa- tion of the two forms of astigmatism is a disadvant- age practically, when we are seeking to correct the defect. With the ophthalmometer two objects are reflected on to the cornea of the observed eye ; these objects are of white enamel, one quadrilateral in shape, the other of the same size except that on one side it is cut out into five steps : these two objects slide on a semicircular arm, which rotates round the tube through which the observer looks, one object on either side of the tube ; the observer looking through this tube, which contains a combination of convex glasses and a bi-refracting prism, sees four magnified images in a line on the cornea under examination. First find out the meridian of least refraction ; this we are able to do by finding the position of the semi- circular arm, in which the two central images (one quadrilateral, the other with steps) are furthest apart. We slide the two objects together until we see the two central images on the observed cornea just touch, the lowest step of the one with the side of the other ; this, then, is the meridian of least refraction, and we note it down as such. Now turn the arm at right angles to this meridian, and notice the amount of overlapping of the tAvo central images ; each step in the one figure that is overlapped by the quadri- lateral one is equal to one dioptre. Thus if it over- lap three steps, there is a difference of 3 D. between 12 178 THE EEiliACTlON OF THE EYE tlie meridians of least and greatest refraction ; we know this to be the meridian of greatest refraction, because it is at right angles to the one first found. As there are only five steps, when there is a differ- ence of 5 D. between the two meridians, the one figure will exactly overlap the other : for higher degrees we have to calculate how much the figure with the steps projects beyond the quadrilateral figure ; or we may place in the tube a stronger bi- refracting prism, then each step may be counted as two dioptres instead of one. Nordenson has obtained some interesting statistics with this ophthalmometer. He examined with it 226 school children. As a result of these statistics he is of opinion — 1st. That the correction of corneal astigmatism by means of the lens in young persons is the rule. 2nd. That corneal astigmatism amounting to one and a half dioptres is incompatible with normal acuteness of vision. Nordenson^s observations agree with the opinion expressed by Javal that astigmatism predisposes to myopia. Tweedy^s Optometer affords an easy method of estimating the refraction in astigmatism. It consists essentially of a plate carrying the figure of, a dial marked with fine dark radiating lines at angles of 15° with each other; the plate is attached to a horizontal bar half a metre long, divided into centimetres, on which it may be made to slide : at the proximal end of the bar is a semicircular clip, marked with degrees ASTIGMATISM 179 corresponding to those on the dial, and intended to hold the cylindrical lens. In order to use the instru- ment properly, the following instructions must be strictly complied with : 1st. The eye about to be examined having pre- viously been placed completely under atropine, and made artificially myopic to about 4 D. by means of a Fig. 88. strong convex lens placed in a spectacle frame, and the opposite eye excluded by an opaque disc, the patient should sit down before the instrument, place the eye with the lens before it close to the clip, and with the head erect should look straight in front at the radiating lines of the dial. 2nd. The dial having first been removed beyond the point of distinct vision, should then be gradually 180 THE EEFRACTION OF THE EYE approximated along tlie bar, until at least one of the lines is clearly and distinctly seen; after this the dial should on no account be moved, but its distance from the eye accurately noted. If all the radiating lines come into view with equal clearness at the same time there is but slight astig- matism ; but if whilst one line is clearly seen, that at right angles to it is blurred, there is astigmatism, which may be corrected by placing in the semicir- cular clip a concave cylindrical lens with its axis parallel to the blurred line, or at right angles to that first distinctly seen. From the result of (2) we learn {a) the direction of the two principal meridians, of maximum and mini- mum refraction ; (b) the presence or absence of hypermetropia or myopia, and the degree; (c) the presence or absence of abnormal regular astigmatism, including its direction and degree, (a) The meridian of greatest refraction is parallel to the line seen at the greatest distance of distinct vision, while the meridian of least refraction is always at right angles to it. (b) The presence or absence of ametropia is determined by the distance at which the radiating lines are clearly seen. If there be emmetropia, the lines will be seen exactly at the distance of the focal length of the lens employed to produce the artificial myopia : if there be hypermetropia, the lines will be seen beyond that point; if myopia, within. The degree of ametropia may be estimated by the following cal- culation. The greatest distance of distinct vision, minus the focal length of the lens, divided by the ASTIGMATISM 181 multiple of these numbers, equals tlie degree of ametropia. (c) If, however, there be astigmatism, the above calculation will give the refraction for the meridian of least refraction only; the degree of astigmatism will be represented by the focal length of the weakest concave cylinder, which, placed with its axis parallel to the blurred line, makes this line as clear and distinct as that first seen. The whole ametropia may then be corrected by combining the spherical lens required for the correction of the meridian of least refraction, with the weakest cylindrical lens which by actual experimentation has been found sufficient to correct the astigmatism. Placido's disc consists of a circular sheet of tin on which is painted concentric circles of black and white; it enables one to detect the chief meridians of the cornea at a glance. The patient being placed with his back to the light is directed to look at the centre of the disc, while the observer, holding the instru- ment close to his own eye and at a convenient dis- tance from the patient^s, looks through the hole in its centre ; he sees an image of the concentric circles reflected on the cornea : if astigmatism exist, the rings will appear elliptical, with the long axis corresponding with the meridian of least curvature. Cases of irregular astigmatism and conical cornea are easily detected by this method. The stenopaic slit, which consists of a metal disc having an oblong opening in it about 2 mm. broad, is used by some observers for working out cases of 182 THE EEFRACTION OF THE EYE astigmatism. The disc is placed in a trial frame in front of the eye we wish to examine ; and while the patient looks steadily at the distant type the disc is slowly rotated, so that the slit is brought successively in front of each meridian ; the position in which the best vision is obtained is noted ; we then try convex and concave spheres in front of the slit, to see if any improvement take place. The slit is now in line with one of the chief meridians ; let us turn the disc round 90°, so that the slit may occupy the position of the other chief meridian, and find out what sphere most improves vision. Thus, supposing with the slit in the vertical direction the patient reads -|, while convex glasses in front of the slit make it indistinct, the vertical meridian is emmetropic ; and on turning the slit so that it is horizontal, the patient reads -fj, but with + 2 D. sphere the vision equals |-, the horizontal meridian is then hypermetropic, and the case is there- fore one of simple hypermetropic astigmatism, requir- ing for its correction + 2 D. cylinder axis vertical. On looking through the slit, placed between the principal meridians, circles of diffusion are formed, and the object has the appearance of being drawn out in the direction of the slit. Dr. Tempest Anderson, of York, has invented an ingenious instrument by which astigmatism may be estimated in a subjective manner; the image of an illuminated radiating screen is thrown on the retina, and is visible to the observer; the position of the screen on a graduated bar shows the refraction. ASTIGMATISM 183 Tlie inventor claims for his instrument the follow- ing advantages : 1. The observations and measurements are made by the observer, and are entirely independent of the patient's sensations, though these may be used as an adjunct if wished. 2. An image thrown on the retina being used as an object, the error arising from the vessels or optic nerve being before or behind the retina is avoided. 3. The refraction and accommodation of the ob- server does not affect the result. It is only necessary that he should be able to see whether certain lines are sharply defined. A beautiful modification of this instrument has been brought out by Chambers Inskeep & Co., and has been named " Stigmatometer.^' In addition to the methods already described for estimating astigmatism, many others are known. See Cases 5, 6, 7, 8, 9, 10 and 11, p. 104, etc.; also 33 and 34, p. 256. 184 THE EEFRACTION OF THE EYE Anisometeopia Anisometropia (a, priv. ; Icrog, equal; //tVpov^ mea- sure ; w;//, the eye) is the term applied to cases Avhich frequently occur, where the two eyes vary in their refraction. The defect is usually congenital, but it may be acquired, as in aphakia or loss of accommoda- tion in one eye. Every possible combination may exist : one eye may be emmetropic, the other myopic or hypermetropic; or one more myopic, hyperme- tropic, or astigmatic than the other. Anisometropia may be met with under three chief forms : 1. Cases where binocular vision is present. 2. When the eyes are used alternately. 3. One eye is permanently excluded from vision. 1. In the first variety the difference in refraction is usually not very great ; and if it were possible for the patient to accommodate unequally in the two eyes, he might be able to obtain clear images on each retina ; but it is probable that the two ciliary muscles make the same effort, with the result that in one eye the image is well defined, in the other indistinct. 2. When the eyes are used alternately, then one eye is usually emmetropic or hypermetropic, and is employed for distant vision ; while the other is myopic and is used for near work. 3. When the difference between the two eyes is very great the best eye may be used exclusively, while the vision in the other is very bad, and frequently ANISOMETROPIA 185 deviates outwards or inwards ; in many of these cases one eye is emmetropic or slightly hypermetropic, the other highly myopic. Treatment. — When the difference is not very great (1 or 1*5 D.), and vision in both eyes is good, we may give each eye its correction for constant use : for so long as the eye wliose refraction is the more defective still co-operates in binocular vision, sight is improved thereby. Especially is this full correction useful in cases of myopia with divergent strabismus, the in- creased stimulus to binocular vision being sometimes sufficient to prevent the squint. Many cases do not stand their full correction for each eye with comfort; they complain of strain, dis- comfort, and headache, though the younger the patient the less liable is he to suffer from these symptoms. The asthenopia which often results from giving each eye its exact correction may possibly be due to the different prismatic effect which must result when the patient looks obliquely through his two glasses which have a different refractive power, and it has been suggested by Mr. W. A. Dixey to overcome this difficulty by using a bifocal lens before the eye whose refraction is more defective. This can be done by grinding a small central portion of the glass — that, in fact, which is immediately in front of the pupil — of such a focus as to fully correct the error, while the other part of the lens will be ground of the same focus as the glass in front of the less defective eye. Thus, to take an example, a patient has 4 D. of myopia in the right eye and 2 D. in the left ; for the right 186 THE EEFRACTION OF THE EYE eye lie would require a glass whicli was — 4 D. in the centre^ — 2 D. at the margin, while the left eye would be supplied with — 2D. When one eye is emmetropic and the other myopic, no glass will probably be required, the emmetropic eye being used for distance, the myopic eye for reading, etc. When the difference in the refraction is greater than 1'6 D. we may have to be satisfied with partially correcting the difference, and this result can only be arrived at by trying each case, some people tolerating a much fuller correction than others, our object being to give as near as possible the full correction for each eye. In children the full correction for each eye can frequently be given even when the difference between them is very great. When binocular vision does not exist then often no attempt can be made to correct the two eyes, and we generally give glasses that suit the better eye. In cases of aphakia, &c., where one eye is used almost entirely, while the other, though defective, still possesses vision, it is an excellent plan to insist on the latter being daily exercised with a suitable glass, the good eye being at the same time covered ; by this means the bad eye is prevented from becoming worse, and can at any time be utilised should occasion require. See Cases 27 and 28, pp. 250 and 253. PEESBYOPIA 187 CHAPTER IX PEESBYOPIA. Pr. {wQtcrPvg, old ; wi//, eye) With advancing age many changes take place in the eye. The acuteness of vision becomes less^ owing j^artly to a slight loss of transparency in the media, and partly to a diminution in the perceptive and con- ductive powers of the retina and the optic nerve. At the age of forty the acuteness of vision is almost unaltered, the bottom line of the distant type being read at a little over 6 metres ; at fifty it can still be read at 6 metres, but after this time it diminishes regularly, so that by the eightieth year vision may have decreased to |- or -^■^. In addition to these changes, the accommodation gradually diminishes from a very early period, the near point slowly but steadily receding. This change in the accommo- dation occurs in all eyes, whatever their refraction, and is due to an increased firmness of the lens, whereby its elasticity is lessened; and perhaps also in some slight degree to loss of power in the ciliary muscle due to advancing age. The lens also approaches the cornea, increases in size, and becomes somewhat flatter. This failure of the accommodation begins as early as the tenth year, at an age when all the functions of the body are still developing. 188 THE EEFRACTION OF THE EYE Presbyopia must therefore be looked upon as a physiological condition. When the binocular near point has receded beyond the distance at which we are accustomed to read Fig. 89. Diagram showing the course of accommodation in an emmetropic eye. The figures at the top of the diagi-am indicate the age ; those at the side the amount of accommodation and the p. p. in centimetres ; the oblique line represents the course of the punctum proximum, and the horizontal line that of the punctiim remotum ; the space between the two lines gives the amplitude of accommodation. From this diagram we can calculate the amplitude of accommodation possessed at any and write with comfort, we become restricted in our work. Bonders has fixed this point at 22 cm. PEESBYOPIA 189 Presbyopia^ therefore, may be arbitrarily stated to exist when the binocular near point has receded to 22 cm., and this occurs usually in the emmetrope about the age of forty-five. Because in order to see at 22 cm., a positive refractive power of 4*5 is necessary (-y^O. = 4*5) ; at the age of forty, the eye possesses just this amount of refractive power; but if accom- modation is less, then we must give such a convex glass which, added to the amplitude of accommoda- tion, brings up the positive refraction to 4"5 D. : for example, at the age of fifty-five, when the eye possesses only 1*5 D. of accommodation, we give a convex glass of 3 D., because 1*5 D. -f- 3 D. = 4*5 D. (see table, p. 191). To find the punctum proximum of an emmetrope, we have only to divide the number of dioptres of accommodation which he possesses into 100 cm. Thus at twenty there are 10 D. of accommodation; this would give us 10 cm. as the near point. At forty there are 4'5 D., in which case the near point is 22 cm. When the punctum proximum has receded to 22 cm., the point at which it is convenient to read is considerably further away, since for sustained vision only about half of the accommodation can be used. Thus a person with 4 D. of accommodation would have his near point at 25 cm. with the maximum contraction of his ciliary muscle, and if he can only comfortably use about half this for continuous work, his reading point would be 50 cm. ; this is too great a distance. We bring back the near point by convex 190 THE REFRACTION OF THE EYE glasses, which is practically the same as increasing the accommodation. Although we have said that only about one half of the accommodation can be used for sustained vision, this is not absolutely correct : the amount which must be in reserve varies much with different individuals ; thus in one case with a surplus of 1 D. much work can be done, whereas in another a surplus of 3 or 4 D. is necessary. Symptoms. — The presbyope sees well at a distance, but has difficulty in maintaining clear vision for near objects ; the chief symptoms are a feeling of weari- ness in the eyes after reading, especially in the evenings, small objects being less easily seen than formerly, because, having to be held further from the eyes, they subtend a smaller visual angle. The patient seeks a strong light, or places the lamp he is using between his eye and the book; by doing this he causes his pupils to contract, and so lessens his circles of diffusion ; he avoids small print, and holds the book or work further away. These symptoms are due to a recession of the near point, and if asthe- nopia occur, this may be dependent upon a disturbance of the balance between accommodation and conver- gence ; the convergence being the same for any given point, a much greater accommodative effort is neces- sary than was formerly the case. The treatment of presbyopia consists in prescribing convex spectacles for reading and near work, so as to bring back the near point to a convenient distance. The best reading distance for a person with normal PRESBYOPIA 191 visual acuity is from 30 to 40 cm. ; most emmetropes will, therefore, require a convex glass for near work soon after the age of forty-five ; we have only to re- member to add on + 1 D. for every five years until we have reached + 3*5 D. An emmetrope with good visual acuity will never re- quire a stronger glass than + 3*5 D. even when sixty or seventy years of age, because these glasses adapt him for a distance of 30 cm. without any accommodation. Should the patient have defective vision, then it may be necessary for him to hold near objects much closer to the eyes than 30 cm. in order that he may get larger retinal images; here we should be justified in prescribing much stronger glasses. The following table gives approximately the strength of glasses required by emmetropes at dif- ferent ages to bring back their punctum proximum to 22 cm: Age. 45 Amount of accommodat ' possessed at that age. 3-5 D. ion The near point. 28 cm. Glass required to bring back p. p. to 22 cm. + 1D. 50 2-5 D. 40 cm. + 2D. 55 1-5 D. 67 cm. + 3D. 60 •5D. 200 cm. + 4D. vo •OD. infinity- + 4-5 D. It must be understood that this table indicates the glass that brings the p. p. back to 22 cm., and if we wish the patient to read at 33 or 40 cm. these glasses will in practice be found too strong. To find the glass required in presbyopia, we sub- tract the glass which represents the receded near point from the glass whose focus represents the point 192 THE EEFKACTION OF THE EYE we wish to make the near point. Thus the near point has receded to 50 cm, ; the glass representing this point is + 2 D. {-^-^ = 2). We wish to bring the near point to 25 cm.; this would be + 4 D. {y~ = 4) ; hence + 2 D. from + 4 D. gives -f 2 D. as the glass required. Although glasses can be frequently thus ordered by a sort of rule of thumb^ it is always well to bear in mind that the definition given of presbyopia with reference to its near point is entirely an arbitrary one, and that we must take into account the distance at which the individual has been accustomed to read and work. In this there is great variety. Many small people work and read at 25 cm., Avhereas very tall people may be uncomfortable unless the book they are reading is 35 or 40 cm. away. The distance for which the presbyope requires spectacles will also vary much according to the occupation for which he requires them ; thus a carpenter sixty years old with emme- tropia may require to work at his bench, which may be at one metre away; to enable him to see at this dis- tance he will require -\- 1 D., while for reading at 33 cm. he will require + ^ !)• There exists a popular prejudice against the use of strong glasses, all sorts of maladies having been attri- buted to their use. This prejudice is quite unfounded; if the lenses are too strong they may bring the reading point inconveniently near, and so produce asthenopia by causing the patient to converge excessively. Before ordering glasses for presbyopia, always try the patient's distant vision, so that any hyper- PRESBYOPIA 193 metropia or myopia may be recognised. If hyper- metropia exist, the amount must be added to the presbyopic glass; if myopia, it must be sub- tracted. Thus a patient with hypermetropia requir- ing + 2 D. for its correction, at the age of forty-five will require + 3 D. for reading (H. 2 D. + Pr. 1 D. = -f 3D.). A myope of 1 D. will require no glass at the age of forty-five (M. 1 D. + Pr. 1 D. = 0). If the myopia be 3*5 D., then the patient can never require a glass for presbyopia, his far point being at 30 cm. His near point may recede to this distance when all accommodation is lost, but he will still be able to read at that distance, though at that distance only. Many people with a low degree of astigmatism have no discomfort and see fairly well, therefore they never wear a correction ; when glasses become neces- sary for near work such persons may prefer a simple sphere rather than a sphere to which has been added their astigmatic correction. Each case must be treated on its merits. But it must be borne in mind that as age advances the refraction of the eye diminishes ; in other words, the eye if emmetropic becomes hypermetropic (called acquired hypermetropia). The myopic eye becomes less myopic, so that a real improvement in vision takes place. The hypermetropic eye becomes more hypermetropic. This change takes place at a regular rate in all eyes ; at fif tj^-five the refraction has diminished "25 D., at sixty-five -75 D., at sixty-eight 1 D., and at eighty as much as 2*5 D. Thus at 13 194 THE EEFEACTION OF THE EYE eighty an emmetrope will have acquired 2*5 D. of hypermetropia, and will therefore require a convex glass -H 2'5 D. for distant objects to be seen clearly. A myope of 2*5 D. would at eighty have become emmetropic, and require no glass for distance. A hypermetrope of 2'5 D. will add on to his defect 2*5 D., and will require a + 5 D. for distance. This change is due to sclerosis and enlargement of the crystalline lens, by which its refractive power is diminished. Dr. Scheffler some years ago proposed the use of orthoscopic lenses, that is, lenses with two elements, a sphere and a prism, so proportioned that the amount of accommodation and convergence should exactly correspond. Thus in the case of a pres- byope aged fifty, requiring + 2 D. to make him read comfortably at 25 cm., it would be combined with a prism of 2 m.a. base inwards, the result being that the patient would .then have to put on 2 D. of his accommodation and 2 m.a. of convergence, and thus these two functions would be used in equal degrees. The results, however, are not so good as might have been hoped; the glasses are too heavy, and on looking at a flat surface some distortion is produced. Nevertheless cases do occur in which, though the presbyopia is corrected, the patient after reading a short time complains of asthenopia. Such cases are frequently at once and completely relieved by combining with the spheres, prisms of 2° or 3° with their bases inwards; or by having the lenses decentred, forming convex prismo-spheres (Fig. 102). A lens of 1 D. must be decentred 8*7 mm. to pro- PRESBYOPIA 195 duce a prismatic effect of 1°. Thus in order to find out the amount that a lens should be decentred to produce a given prismatic effect^ it is necessary to multiply 8*7 by the number of the prisms we wish to use, and divide the result by the number of the lens. Thus, to take an example, with a + 6 D. we wish to produce a prismatic effect of 2°, then 8-7 X 2 _ 17-4 _ ^ _ ^, , , , ^ —^—— 2*9 mm.; the glass would reqmre to be decentred inwards 2"9 mm. Care must be taken to see that all glasses are properly centred, unless they have been ordered otherwise ; for if the frames are too broad or too narrow, the prismatic effect produced is very apt to give rise to asthenopia, by disturbing the relations between convergence and accommodation. In cases where the convex glasses have frequently to be changed for stronger ones, " glaucoma" should be carefully looked for; and if any symptoms of it appear, no near work must be allowed, as it is important to avoid all possible causes of tension. The commencement of cataract may in some cases hasten presbyopia, but it more frequently produces myopia, so that the presbyope requires his glasses diminished in strength. In each case of presbyopia first test the patient^s distant vision, so as to detect any hypermetropia, myopia, or astigmatism ; and having recorded this, we add the glass which he requires for his presbyopia and try him with the reading type : should they suit, we direct the patient to read with them for half an 196 THE EEFEACTION OF THE EYE hour or so ; if found satisfactory we order spectacles of this strength. See cases 25, 29, 30, and 31, p. 249, etc. Paealysis oe the Accommodation Paralysis of the accommodation, either partial or complete, arises from loss of power in the ciliarj^ muscle (cycloplegia), and is due to paralysis of the third nerve, or of that branch of it which supplies the muscle of accommodation and the circular fibres of the iris. Cases do occasionally occur, though very rarely, of paralysis of the ciliary muscle not involving the constrictor pupillas. Both eyes may be affected, or only one. When the paralysis is confined to the ciliary muscle and iris, it goes by the name of ophthalmoplegia interna. Causes. — Atropine is the most common cp.use, but it may be due to diphtheria, rheumatism, fever, any complaint of a lowering character, cerebral trouble, syphilis, diabetes, or some reflex irritation, e. g. decayed teeth, &c. ; the cause may, however, not be apparent. When the whole third nerve is involved, ptosis, ex- ternal strabismus, &c., occur ; but in those cases where the branch supplying the ciliary muscle and the circular fibres of the iris is alone implicated, the indicating symptoms are, asthenopia, dilatation of the pupil, and loss of the power of accommodation, whereby the patient, though able to see distant objects well (if emmetropic), is unable to read or do SPASM OF THE ACCOMMODATION 197 any near work. In liypermetropia, both near and distant vision will be impaired ; in myopia^ the patient is able to see only at his far point. We try the patient at the distant type, and if he is able to see f and yet is not able to read near type, the diagnosis is obvious. Treatment consists in giving such convex glasses as enable him to read. In order to bring the emme- trope's far point from infinity to 33 cm., + 3 D. is required {-^J~ = 3). We must bear in mind that by encouraging the action of the ciliary muscle we hasten the patient^ s recovery ; we therefore order the weakest convex glasses with which he is able to read, changing them for weaker ones occasionally as the ciliary muscle gains strength. Sulphate of eserine in solution, gr. j to ^j, causes contraction of the ciliary muscle as well as of the iris, and temporarily relieves the symptoms. I think much good sometimes results from its use once every other day for some weeks ; the ciliary muscle being made to contract, relaxing again as the elfect of the myotic passes off : some- times the local application of electricity is useful. Attention must be paid to the general health, iodide of potassium or nerve-tonics being given when indi- cated by the cause. See Case 26, p. 250. Spasm of the Accommodation Spasm of the accommodation may be of two kinds. Tonic and Clonic. 198 THE REFRACTION OF THE EYE Clonic spasm occurs only when tlie eye is in use, ceasing as soon as it is in a condition of repose. Tonic spasm is more permanent, requiring atropine or one of the other mydriatics for its relief ; the ex- pression spasm of accommodation usually refers to this variety of the disorder. Tonic spasm of the ciliary muscle may be occasion- ally met with in eyes whatever their refraction, though most commonly in cases of hypermetropia and low myopia ; it has the effect of increasing the re- fraction of the eye, and is found most frequently in children. Causes. — It may occur as a result of uncorrected ametropia, or in emmetropia from overwork, espe- cially when such work has been done in a bad light ; or it may follow contusion of the eyeball, and sometimes it occurs with cyclitis. Symptoms. — It usually affects both eyes, giving rise to symptoms of asthenopia with a feeling of con- striction and discomfort in the eyes themselves ; there may be an increased secretion of tears with or without blepharospasm ; the acuteness of vision may be diminished and is very variable, while the size of the pupil usually remains unaffected. In emmetropia we may get symptoms of myopia, owing to the parallel rays coming to a focus in front of the retina. In hypermetropia the symptoms may also simulate myopia, and for this we should always be on our guard. I have on several occasions seen liyper- metropes going about wearing concave glasses to correct their supposed short-sightedness. Some SPASM OV THE ACCOMMODATION 199 time ago I saw a young man who had worn — 7 D. constantly for years, though his refraction was really emmetropic. In myopia the real defect is apparently increased by the spasm, and we might be in danger of ordering too strong concave glasses, &c. For these reasons the systematic use of atropine in young people (whereby one is enabled to estimate and record the exact state of the refraction) cannot be too strongly insisted upon. The treatment, where spasm of the ciliary muscle is suspected, is to drop into the eyes three times a day a solution of sulphate of atropine, grs. iv to 5J; foi' two or three weeks ; this quickly relieves the spasm, and gives the eyes complete rest : any ametropia that may exist must be corrected, and the patient's general health attended to, tonics being administered if necessary. A few cases of acute spasm of the accommodation have been recorded which resisted the treatment by atropine ; the spasm, though relaxed by this means, returned as soon as the atropine was discontinued. See Case 1, p. 102. 200 THE KEFKACTION OF THK EYR CHAPTER X Strabismus [cTTpirpw, I turn aside] Strabismus exists when tliere is a deviation in the direction of the eyes^ so that the visual axes are not directed to the same object. Strabismus may be divided into two classes — Concomitant. Paralytic. Concomitant strabismus is a frequent result and complication of refractive errors_, and has already been referred to on pages 124 and 146. The deviation of one eye from its correct position is the result of disturbed muscular equilibrium^ and may be due to — 1. Defective anatomical conditions, or 2. Abnormal innervation causing contraction of the muscles not in accordance with the re- quirements of binocular vision. Most cases of concomitant strabismus are due to this second cause. The points to note when a case of strabismus presents itself are — 1. Is the strabismus real or only apparent ? STEABISMUS 201 2. If real, to which variety does it belong ? 3. Which is the deviating eye ? 4. In which direction is the deviation ? 5. What is the degree of the deviation ? 6. What is the cause of the strabismus ? The first of these questions may seem unnecessary, but it is not always easy to say if squint is really present or not, because one judges of the direction of the eyes by the position of the centres of the corneae, or rather by the optic axes, and these may diverge slightly while the visual axes are really parallel. This requires some explanation. The visual axis is the line passing from the macula through the nodal point to the object looked at, shown in the following figures as v m. Fig. 90. The optic axis is the line passing through the nodal point and the centre of the cornea to the inner side of the macula, o h in figures. It will thus be seen that these two axes form an angle at the nodal point which in emmetropia amounts usually to 5°^ (Fig. 90). * This angle is by some authors called the angle y. 202 THE EEFEACTION OF THE EYE This is called the angle a, and when thus formed by the crossing of the visual and optic axes it is said to be ijositive. In hypermetropia (Fig. 91) the angle a increases Fig. 91. with the degree of hypermetropia^ and if it be high may attain 7°, 8°, or even more ; this large angle gives to the eye an appearance of divergence. Fig. 92. M. The maciila. n. The nodal point, b. Optic nerv^e. v. The object. V M. The visual axis, o h. The optic axis. a. The angle alpha formed between the visual and optic axes. c. The centre of rotation of the eyeball situated on the optic axis. y. The angle gamma (Fig. 90) is formed at the centre of rotation of the eyeball, by the optic axis and a line drawn from the centre to the object looked at. In myopia (Fig. 92) the angle a decreases^ and in STEABISMUS 203 high myopia the visual axis may approach the optic axis, so that the angle a is very small, or it may coincide with it when no angle is formed ; or even be altogether on the outer side o£ it, when the angle is said to be negative. This small angle a gives to the eyes an appearance of convergence."^ In order to find out the variety to which our case of strabismus belongs, as well as to decide which is the deviating eye, we direct the patient to look at an object held about a metre in front of him, then gradually bring this object nearer to him, so as to call into action the accommodation : if both visual axes continue to be directed steadily towards the object as it is made to approach the eyes, the case is one of apparent strabismus ; but if one eye fix the object, while the other, after following it up to a certain dis- tance, suddenly deviates inwards or outwards, the condition is spoken of as concomitant strabismus (convergent or divergent) ; or both eyes may follow the object up to a certain point, when one eye stops, after making a few jerking oscillating movements; it then belongs to the paralytic variety of strabismus. Again, direct the patient to look at some object held midway between the two eyes and about a metre away; if the right eye fix the object while the left deviates inward, we mark upon the edge of the lower lids with a pen the position of the external margin of * Another angle sometimes mentioned is the angle y, which is the angle formed at the centre of rotation of the eye by the optic axis and a line drawn from this centre to the object looked at, shown in Fig. 90. 204 THE REFRACTION OP THE EYE the cornea in eacli eye, s and p : on covering the right eye with a card, the left will at once make a Fig. 93. P D L The two upper diagrams, r and l, show the primary position of the eyes, the right being the fixing eye, while the left is deviating inwards. On covering the right eye with a card, as shown by the dotted lines, the left eye fixes while the right deviates inwards, p d therefore indicates the primary deviation, s d the secondary deviation. movement outwards to fix the object, and we make a second ink mark, d, on the lids corresponding with the outer edges of the cornea in this position. The distance p d gives us the amount of lyrlmary deviation-, it may further be seen that on covering the right eye so as to cause the left eye to fix the object looked at, the right eye has made a movement inwards behind the screen, a s'econdarij deviation has taken place, this is recorded l)y an ink mark on the right STRABISMUS 205 lower lid at d ; we have thus found the linear measure- ment of the secondary deviation, s d. The primary deviation (p d) will be found to equal the secondary deviation (s d), a characteristic of concomitant squint. In paralytic squint the secondary deviation is always greater than the primary. This variety of squint is known as concomitant, because the squinting eye follows the fixing one in all its movements, the amount of squint is the same in whichever direction the eyes are turned ; therefore the range of movement in concomitant squint is as great as in cases where no squint exists; it is simply displaced. In the paralytic variety, the movements of the squinting eye are usually much curtailed ; this we detect by holding up the finger about 50 cm. in front of the patient, and directing him, while keeping the head still, to follow the movements of the finger, which is moved to either side, then up and down. So that in concomitant strabismus the squinting eye will almost exactly accompany the other, the visual lines being at the same angle, except perhaps in the extreme periphery, whereas in paralytic squint one eye will stop at a certain point, while the other eye continues to follow the finger. Concomitant squint is characterised by the fact that the primary and secondary deviations are equal ; in paralytic strabismus the secondary deviation ex- ceeds the primary ; in paralytic squint the diplopia is the most prominent subjective symptom, while in the concomitant variety it is seldom complained of. 206 THE EEFRACTION OF THE EYE When either eye fixes indifferently, the vision being equally good in both, the strabismus is alternat- ing ; when the same eye always squints, it is mono- lateral or constant. The vision in the squinting eye is often below that in the fixing one. Periodic is the name applied to the squint when it only comes on occasionally, as after looking for some time at near objects. With judicious treatment this variety can be cured without operation ; if neglected it generally passes on into one of the constant forms. There are several ways by which the amount of the deviation may be estimated. Thus we may record it in the form of a diagram (Fig. 93). The strabismometer (Fig. 94) consists of a handle Fig. 94. Strabismometer. STEABISMUS 207 supporting a small ivory plate, shaped to the lower lid, and having on it a scale by which Ave measure the amount of deviation of the centre of the pupil. This is an easy method of measuring the strabismus, but is not to be depended upon. The measurement of the angle of the strabismus is the best method of recording the amount of squint. The angle of the strabismus may be defined as that angle which the visual axis makes with the direction it should have in a normal state. For this measurement we require a perimeter, in front of which we seat the patient, with the quadrant placed according to the kind of squint we are about to measure ; if it be convergent or divergent, then the quadrant is placed horizontally. The patient being seated so that his deviating eye is in the centre of the instrument, we direct him to fix with both eyes some distant object (o. Fig. 95) placed in a line with the centre of the perimeter ; a lighted candle is moved gradually along the inside of the quadrant from the centre of the instrument outwards ; the observer, following the movement of the candle with his head, stops as soon as the reflection of the candle on the cornea of the squinting eye occupies the centre of its pupil, this gives the direction of the optic axis ; what we really wanted was the direction of the visual axis, but for all practical purposes the former is sufficient. The degree is read off the quadrant at the point where the candle was stopped, and this result recorded. The angle of deviation for near vision should next be taken; this is done by requesting the patient to look 208 THE EEFEACTION OF THE EYE at the centre of the perimeter, proceeding with the candle as before, and recording the result. Fig. 95.* Concomitant strabismasis intimately connected with hypermetropia and myopia ; it may be — Convergent or Divergent. Convergent Concomitant Strabismus. — On looking at any object, one eye only is directed to it ; the other, as the name implies, turns inward so that the angle * In the above diagram, o is intended to represent a distant object ; it is placed near the perimeter in order to take up less room. CONVERGENT STRABISMUS 209 of convergence is much greater in the deviating than in the fixing eye. This variety of squint is usually due to hyper- metropia^ at least 80 per cent, of the cases are due to this cause ; its method of production depends upon the intimate connection that exists between accom- modation and convergence. The convergence is most marked when looking at near objects ; sometimes there may be no squint when distant objects are viewed. A person who is hypermetropic requires to use some of his accommodation for distance; for near objects he must, of course, use still more, and for every increase in the accommodation there is a desire for an equal increase in the degree of convergence. Thus an emmetropic individual, accommodating for an object at 30 cm., would at the same time converge for that particular point. If the person were hypermetropic to the extent of 4 D., and supposing the amplitude of his accommo- dation amounted to 8 D. ; then he would require to use half this (4 D.) to enable him to bring parallel rays to a focus on the retina ; and he would have the tendency at the same time to use 4 metre-angles of convergence. Thus for distant objects he would have an inclination to converge, his internal recti acting ; and it is only by the increased tension of the external recti, called into action by the desire which all eyes possess for singleness of vision, that convergence is prevented. The more we accommodate the greater is the stim^ulus to converge, so that on looking at near 14 210 THE EEFEACTION OF THE EYE objects — wliicli necessitates an increase of the accom- modation — an increased tendency to convergence is produced. Fig. 96. R. Right eye directed to object o. l. Left eye deviating inwards, m. Macnla. Now, if the hypermetropia be of such a degree that for any given point of convergence it exceeds the positive part of the relative accommodation (Fig. 34, p. 50), one of two things must occur; either the patient must see indistinctly by not accommodating sufficiently, or one eye must be allowecl to converge. CONVERGENT STRABISMUS 211 Some patients will prefer binocular indistinct vision, others, monocular clear vision with squint. One occasionally finds an individual who can thus choose which he will do ; we are trying his acuteness of vision at the distant type perhaps; he stops at some place, we will suppose -f^, and says that he is unable to read the next two lines unless he squint. The accommodation necessary to read -| makes a heavier call on the convergence than can be borne ; such a case forms a good illustration of the manner in which convergent strabismus is produced in a hypermetrope. Hence, if the impulse to see distinctly is greater than the desire to retain binocular vision, one eye yields, and squint occurs ; at first diplopia follows the convergence, and is always in the opposite direc- tion to the deviation. Possibly the convergence of the deviating eye is increased by the desire that the weaker image may be made still weaker, by falling on a more peripheral part of the retina. In most cases squint begins before binocular vision is acquired, so that the fusion faculty is imperfectly developed, the squinting eye is but little used, and the eye quickly becomes amblyopic. There are two varieties of amblyopia met with in squint — a rare form which is put down as congenital. In this variety the vision of the defective eye is usually extremely defective, so that the patient may only be able to distinguish the movement of the hand before the face. No improvement is to be expected in these cases; the cause of the amblyopia is un- 212 THE EEFRACTION OF THE EYE known. The second variety of amblyopia is very common and is tlie result of non-use ; it consists partly in an awkwardness and difficulty in using the eye^ but more especially to a real loss of function, consisting of a weakness of the fusion faculty. Some- times the patient says he is unable to see with the eye at all, and yet when encouraged to read with it and with the proper optical correction before the eye -^ or even -f-g may be read; here practice is essential, and the earlier the age at which exercises are started the greater the chance of improving the vision and establishing binocular vision. In amblyopia it will often be found that the vision is best on the temporal part of the field, that part which is most used in peripheral vision. In high degrees of hypermetropia, when no amount of accommodation can make vision distinct, squint is less likely to occur. It is usually, therefore, in cases of from 2 to 4 D, that convergent strabismus is most frequently met with, and it generally makes its appearance about the fourth or fifth year, — as soon, in fact, as the child begins to look much at near objects. Anxious parents frequently have all sorts of excel- lent reasons to which they attribute the defect ; they say that the child has been imitating its playmate, or that the nurse did something which caused it to squint, by making the child look too much or too constantly in one direction. Any cause which by rendering the image in one eye less distinct than that in the other, as a nebula CON\^ERGENT STRABISMUS 213 an ulcer of the cornea, a difference in the refraction of the two eyes, or even wearing* a shade for a few days for some trivial complaint, may, where hyper- metropia is present, combine to produce strabismus by weakening the power of fusion; the impulse for binocular vision is lessened, and the eye in which the fainter image is formed converges. It is thus seen that convergent strabismus gra- dually destroys binocular vision. In cases of hyper- metropia, where binocular vision does not exist owing to great difference in the refraction of the two eyes, divergent strabismus may occasionally occur. The intimate connection betAveen accommodation and convergence, together with the method of the production of strabismus, will be more easily under- stood by carrying out some such simple experiments as the following. We will assume the observer to be emmetropic : the strongest concave glass with which he, having binocular vision and being at a distance of six metres, can still read f, is the measure of the relative accommodation. The absolute accommodation is measured by the strongest concave glass with which each eye separately can read -|. In my own case, with — 4 D. before each eye, |- can be seen singly and distinctly, —4'5 D. renders it indistinct, and each increase in the glass increases the indistinctness, but produces no diplopia. Separately each eye can over- come — 7 D. Armed Avith — 4 D. before each eye, I am able to see |- distinctly, using, of course, 4 D. of my accommodation; if a coloured glass be placed before one eye, homonymous diplopia at once appears. 214 THE EEFEACTION OF THE EYE proving that one eye has deviated inwards; with — 3D. and the coloured glass^ squint was produced, but with no weaker concave glass. On repeating the experiment in an individual with '5 D. of myopic astigmatism in the right eye, and emmetropia in the left, — 2D. before each eye was the strongest glass with which -J could be seen clearly and singly, — 2'5 D. did not render it indis- tinct, but produced diplopia. The absolute accom- modation for each eye amounted to 6 D. With — 2D. before each eye, the coloured glass was placed before the astigmatic one, and diplopia was pro- duced. With — ID. and the coloured glass the result was the same except that the two images were nearer together. With — '6 D. actual diplopia was not produced. These experiments require but little explanation. In my own case, when using 4 D. of accommodation, there is a tendency also to use a corresponding- amount of convergence ; I am conscious of this muscular disturbance by the effort I make, and by a feeling amounting almost to giddiness, produced when first looking through the — 4 D. The instinc- tive desire to see clearly and singly is so great, that the external recti contract, thereby balancing the increased contraction of the internal recti. Any increase of my accommodation above 4 D. when looking at -J causes the letters to become indistinct, the desire to maintain binocular vision being greater than that for clear images. On placing the coloured glass before one eye, we diminish the retinal impres- DIVERGENT STRABISMUS 215 sion in tliat eye ; the demand for binocular vision is lessened, the external recti cease to act, and as a result of the increased action of the internal recti squint occurs. In the second experiment the retinal impression in one eye, even with so slight an amount of astigma- tism, is reduced so that with 2 D. of accommodation the desire for clear images is greater than that for binocular vision, and diplopia, the symptom of squint, appears. A certain number of cases of convergent strabismus get well about the age of seventeen ; this is most likely to take place when the vision in both eyes is good, though it sometimes occurs even where a high degree of amblyopia is present and binocular vision cannot exist. An unsatisfactory explanation some- times given is, that as the accommodation diminishes, the time at length arrives when the amount of accom- modation at the patient\s disposal is not sufficient to produce clear images ; he therefore relaxes his accom- modation, and with it extreme convergence. Divergent Concomitant Strabismus exists when one e^^e only fixes the object looked at, the other deviat- ing outwards (Fig. 97) ; it is usually dependent on myopia, a state of refraction in which the converg- ence has to be used in excess of the accommodation if an image is to be formed on the macula of each eye ; but divergent strabismus may occur in any eye in which binocular vision does not exist, as in some cases of high hypermetropia or astigmatism; or it may result from a too free division of the internal 216 THE REFEACTION OF THE EYE rectus muscle, in attempting to cure a case of con- vergent strabismus. Divergent strabismus is also occasionally met witli in emmetropia and liyperme- tropia, and is then due to congenital insufficiency of the convergence. Fig. 97. Divergent concomitant strabismus is much less common than the convergent variety. In myopia the antero-posterior diameter of the eyeball is elongated, the range of movement is dimin- ished, and the extreme convergence Avhich is neces- sary to enable the patient to see objects within his far point tires out the internal recti muscles, giving rise DIVEEGENT STKABISMUS 217 to muscular asthenopia ; to relieve this^ one of the in- ternal recti gives way, and the eye deviates outwards. Sometimes the deviation only takes place after the patient has been working some time and the eyes feel fatigued; in others it is only noticed when looking at objects beyond their far point. Soon, however, the squint becomes constant, and a divergent strabismus once established usually increases. In high myopia which is uncorrected by glasses, the patient has to hold objects so close to enable him to see them, that the necessary convergence becomes impossible, and binocular vision is therefore sacrificed. In the treatment of concomitant squint our objects are, to cure the deviation of the eye, and to retain or re-establish binocular vision ; the subject may con- veniently be divided into three parts — 1. Optical. 2. Training the vision. 3. Operative. The optical treatment consists in prescribing glasses which correct any error of refraction ; by doing this we prevent excessive accommodation, and equalise the two functions of accommodation and convergence. It is essential that in all patients over the age of one year, the refractive condition of the eyes should be accurately estimated under atropine by retino- scopy, and the proper correcting glasses ordered for constant use. In a new-born child squint is often present, the eyes move independently of each other and without purpose, the vision being no doubt very imperfect. 218 THE KEFEACTION OF THE EYE but gradually as the brain-centres develop, the two eyes move together and binocular vision becomes gradually acquired_, probably between the sixth and eighth month, so that squint before this age is not of much importance ; but at the age of one year squint is abnormal and should receive immediate attention. In order that the squinting eye may be used in young children and not become amblyopic it is an excellent plan to put one drop of a h per cent, solution of atropine into the fixing eye twice a week ; by doing this we prevent this eye being used for near objects, compelling the deviating eye to be employed for this purpose. In some cases, when the squint has only just com- menced and arises only under the influence of ex- cessive accommodation necessary to enable the child to see near objects (periodic squint), this treatment may at once correct the deviation of the eye, and the glasses may only be necessary for constant use for a year or two, but must, of course, be continued for near work for a much longer time. When, however, the squint has already become per- manent, the glasses may have to be worn constantly for many years. Young children should always be encouraged to look at distant objects rather than near ones. It will frequently be noticed that after the use of atropine the deviation may be diminished, or in slight cases it may have disappeared ; this is due to accommodation being rendered impossible : these are the cases that are usually curable by glasses. STRABISMUS 219 If the case belong to the less common variety of squint — divergent with myopia — we endeavour to give the patient as near as possible his full correction for constant use. 2. Training the vision. — In most cases it will be found that even when the deviation of the eye has been corrected binocular vision is not obtained, and the cure of the case cannot be considered complete as long as binocular vision is absent; great patience and care will be required in carrying out the necessary exercises. The development and cultivation of the fusion faculty should be carried out by the use of one of the many stereoscopes, of which Worth^s amblyoscope is the best ; it consists of two halves joined together by a hinge; each half consists of a short brass tube joined to a longer tube at 120°; at the angle of junc- tion of the tubes is an oval mirror, protected on the outside by an oval plate of brass. Each half of the instrument has at its distal end an object slide carrier, and at its proximal end a convex lens, having a focal length of 5 inches — the distance of the reflected image of the object slide ; in front of each lens is a slot into which a prism axis vertical may be inserted if required; the diameter of the tubes is 1| inch. A brass arc connects the two parts of the instru- ment, being clamped on one side by a binding screw set in a long slot and on the other by a binding screw set in a short slot. When the screw of the long slot is loosened the two parts of the instrument can be brought together to suit a convergence of the visual 220 THE REFEACTION OF THE EYE axis up to 60° or separated to suit a divergence of as much as 30°. Fig. 98. When this screw is tightened and the screw in the short slot loosened an amplitude of movement of about 10° only is permitted. The convex lenses, of course, render unnecessary any adjustment of the instrument for the patient^s interpupillary width. Each object slide is illuminated by a separate electric light so arranged that the illumination of either of the object slides may be separately increased or diminished by bringing its lamp nearer or pushing it further away. The object slides used with this instrument are of three kinds — (a) Those which require only simultaneous vision. (b) Objects which require true fusion. (c) Devices which can only be appreciated by persons having a sense of perspective. For a full description of the use of the amblyoscope I must refer the reader to Worth's recent work on * Squint.' STEABISMUS 22 1 A very cheap and convenient stereoscope for home use has been introduced by my colleague, Mr. Brooksbank James. Box stereoscopes are also useful. They are made without prisms, but fitted with a clip at each sight- hole >3apable of taking the lenses of the ordinary trial box. The patient being emmetropic he will require in the clip a convex glass whose focal length cor- responds with the length of the stereoscope ; thus if it be 16 cm. long, he will require + 6 D. ; should the patient be hypermetropic, 3 D. ; then he will require + 9 D. ; if myopic 3 D., then + 3 D. would be the glass required : the object is to enable him to see the slide at the end of the stereoscope without accom- modating. A convenient slide may be made, composed of two vertical lines, one above and the other below the same horizontal line, so arranged that the two lines can be made to recede or approach each other : this test object is placed in the box instead of the ordinary views. The two lines being now separated to a dis- tance equal to that between the two eyes, and the clips containing the necessary convex glasses, the patient will see the lines without accommodation or convergence, and should succeed in fusing the two lines into one. When this is done binocular vision is obtained with parallelism of the lines of fixation. We endeavour at future trials to obtain fusion with an equal amount of convergence and accommodation. This is done by sliding the two lines towards each other about 1 cm. ; this will call into play something 222 THE REFEACTION OF THE EYE like 1 m. a. of convergence; we then diminisli tlie convex glass 1 T)., so that the amount of accommoda- tion provoked (1 D.) may correspond to the amount of Fig. 99. convergence. In this way we slide the lines nearer and nearer together, diminishing the + glasses at the same time, until the two form one vertical line, then binocular vision is obtained with 6 m. a. of convergence and with 6 D. of accommodation ; when this point has been reached, stereoscopic pictures may be used as slides. When the deviating eye is amblyopic and has lost central fixation, the fixing eye should be covered up for an hour twice a day by means of a pad or shade or a small black metal disc may be made to slip over the spectacle glass of this eye, great care being taken that the eye is completely covered or the child will try to dodge the shade. Another most useful exercise is the reading bar, which may be used for a short time twice a day — a pencil, the finger, or a small strip of metal will answer the purpose ; this is merely to be held about 6 or 7 cm. in front of the book when reading; if only one eye is used the patient stops when he comes to that part of the line covered by the bar ; when both STEABISMUS 223 eyes are in use^ then lie goes on reading without any interruption. Herring^s drop test is good for finding out the presence of binocular vision with a sense of perspec- tive, but is not absolutely reliable. This test is seldom used clinically. Operative treatment. — In many cases, after the glasses have been worn for some months, the stra- bismus may still exist, and it may then be necessary to supplement the treatment by tenotomy. A free division of one muscle may cure a deviation of 15° : when a greater effect is required an advancement of one of the muscles may be necessary, with or without a tenotomy of its antagonist. After the operation for convergent strabismus there should still remain a slight tendency to convergence when the glasses are removed ; because as the child approaches the age of maturity the excessive in- nervation of the internal recti may subside, and then there may be danger of a deviation of one eye out- wards. Paralytic strabismus does not come within the province of this work. See Case 35, page 257. 224 THE REFRACTION OP THE EYE CHAPTER XI ASTHENOPIA Asthenopia (^A, priv. ; aOtvo^, strength ; io\p, the eye), or weak sight, is a term used to designate a group of symptoms caused by fatigue or strain of some part of the eye or its muscles. Asthenopia frequently accompanies hypermetropia, myopia, and astigmatism, and reference has often been made to it when speaking of these errors of refraction. It is also met with in cases where no ametropia exists, and may then be caused by over- fatigue or diminished power of the ciliary muscles, weakness of the internal recti, or exhaustion of an over-sensitive retina. Asthenopia shows itself by the inability to sustain long and continuous near work, and is accompanied w^ith more or less pain ; the condition is a very common one, and it may be stated with confidence that pain in the eyes, unconnected with inflammation, is almost invariably due to asthenopia, and but seldom to any deep-seated disease. The more acute the pain, the more does it point to asthenopia ; as a rule, however, the pain is not very severe : it may be situated in the eyes themselves, or around the orbits, and is always ASTHENOPIA 225 increased when the eyes are used for near objects ; in some cases no pain is felt^ but after reading for a while the type becomes indistinct or double, so that the patient has to stop and look about the room, or rub his eyes, after which he may be able to resume reading for a short time, to be again quickly inter- rupted by a repetition of the same symptoms. If the work be still persisted in, the pain around the eyes increases, there is photophobia and lachrymation, a sensation of dazzling and dimness, more or less con- junctival congestion, the eyes and lids becoming red and irritable; all these symptoms are liable to be worse in the evening after a day^s work, when there is the additional disadvantage of an artificial light, which may be hot and irritating. Headache is often a prominent symptom of asthe- nopia j it may take the form of heaviness or pain over the brow (which may or may not be combined with general headache) ; it is often periodic in character, and is always made worse by reading ; frequently there is a tender spot on the top of the head, or pain in the occipital region, occasionally also there is pain in the back of the neck. These symptoms may be associated with dyspepsia, palpitation, and vomiting, and in some cases with obstinate insomnia. This train of symptoms has occasionally been so severe as to lead to the diagnosis of brain disease, hence it is a good rule to test the refraction under atropine in all cases of persistent headache not giving way to ordinary medical treatment, and it must be remembered that a very slight amount of astigmatism 15 226 THE EEFEACTION OP THE EYE left uncorrected^ even though the chief portion of it may be corrected, will be sufficient in some cases to keep up the headache. There is little doubt that frequently reflex nervous disorders are caused by asthenopia. Asthenopia may be divided into — 1. Accommodative. 2. Muscular. 3. Retinal. Accommodative Asthenopia is exceedingly common, and is due to fatigue of the ciliary muscle ; and may be met with in emmetropia or ametropia. It makes itself known by an inability to maintain the necessary accommodation, and may arise (a) from a weak con- dition of the ciliary muscle, (h) from excessive use, as in hypermetropia, (c) from unequal demand, as in astigmatism, [d) from unequal demand in the two eyes, as in anisometropia, (e) from diminished elasticity of the lens, as in presbyopia. When Bonders discovered the common occurrence of hypermetropia, he soon became aware of the very intimate connection which existed between it and asthenopia, and was at first inclined to attribute every case to this cause. Where no manifest hyperme- tropia was present he gave a solution of atropine to paralyse the accommodation, feeling confident that some latent hypermetropia would then display itself ; such cases were usually completely cured by proper convex glasses. Accommodative asthenopia is due in great measure to the constant and excessive action of the ciliary muscle, but partly also to the ACCOMMODATIVE ASTHENOPIA 227 abnormal relations existing between tlie two func- tions^ accommodation and convergence; this state- ment is supported by the fact that hypermetropes who squint seldom suffer from asthenopia. An emmetrope looking at distant objects does so without any accommodation, the ciliary muscle being passive ; but the hypermetrope has to use his ciliary muscle even for distant objects, and therefore much more for reading or near work ; so that the ciliary muscle practically gets no rest. A young and vigorous person whose hypermetropia is not very high may resist asthenopia for a long time, but as he gets older, or if his health suffer from any cause, symptoms of this disorder are apt to appear, and when once established they may continue, notwithstanding im- provement in the patient's general condition. In women asthenopia is very liable to come on during lactation. Treatment. — We order such glasses as are neces- sary to correct the refraction according to the rules given. In some cases where convex glasses do not produce the desired relief, prisms of 2° bases iuAvards combined with the spherical correction are of great use, or in slight cases we place the convex glasses somewhat near together, so that the patient looks through the outer part of them (Fig. 102). This plan is frequently very useful in presbyopia. Here the asthenopia is due to a greater muscular effort being required to produce the necessary change in the shape of the less elastic lens, and perhaps, also, in part to the difficulty of maintaining an exact state of 228 THE REFRACTION OF THE EYE equilibrium between tlie internal and external recti muscles. In the hypermetrope there is a want of harmony between the accommodation and the convergence^ the two functions having to be used in unequal degrees; and when we correct his refraction with glasses he will have to use these two functions equally, or at least in different proportions from that to which he has been accustomed. Many people are able at once to adapt themselves to this new state of aifairs ; but there are others in whom the force of habit is so strong that they cannot thus throw off the acquired one of using the accommodation in excess of the convergence. You must not, therefore, be dis- couraged if occasionally your patient is not at once and completely relieved of his asthenopia, as soon as you have given him convex spectacles. A fort- night's trial should be made before we can decide that sugh spectacles will not relieve the patient of his asthenopia. Asthenopia depends much upon the nervous system of the individual; in some, a very slight amount of astigmatism will produce accommodative asthe- nopia ; one hypermetrope will have no uncomfortable feelings, while another, whose condition seems exactly similar, will suffer much, so that it is essential to attend to the patient's general health. Muscular Asthenopia is due to fatigue of some of the external muscles of the eyeballs. The two eyes should be in a condition of perfect muscular equilibrium in every position of the eyes; MUSCULAR ASTHENOPIA 229 wlien this is not the case, it may be detected by covering one eye witli a screen, while the other is made to fix a small object, such as the point of a pen, held about 30 cm. away ; the covered eye should be accurately adjusted for the object, although it no longer sees it : when this is not the case the covered eye may deviate inwards or outwards ; and if the screen now be withdrawn a movement of readjust- ment takes place. Maddox has pointed out that in practice it will be found that on using the two functions of accom- modation and convergence, the convergence has a tendency to lag behind the accommodation and re- quires the further stimulus of fusion to ensure the exact direction of the visual axes, so that Avhen one eye is covered the other may deviate outwards a few degrees and still be considered within normal limits. When the deviation of the covered eye is greater than 5°, then there is disturbance of the muscular equilibrium, and the condition is spoken of as in- sufficiency or latent convergence or divergence. When insufficiency exists the results which follow vary. 1. Slight degrees may be corrected by increased innervation of the weak muscles, and so give rise to no symptoms. 2. Muscular asthenopia may result from excessive innervation ; this may at once be relieved by closing one eye. 3. Or concomitant squint may develop. Heterophoria is the term now generally employed 230 THE REFEACTION OF THE EYE to express a disturbance of the equilibrium of the muscles of the eyeball, and may be divided into — 1. Exophoria. 2. Esophoria. 3. Hyperphoria. 4. Insufficiency of the oblique muscles. a. Hyperesophoria. j3. Hyperexophoria. Exoj)horia, or latent divergence ; one eye tends to turn out, and is only prevented from doing so by increased innervation of the internal recti muscles, there is, in fact, insufficiency of these muscles, resulting in a strain of the convergence. Exophoria is the commonest of these defects, and is most frequently associated with myopia, though it occasionally occurs in emmetropia or even hyperme- tropia ; it is characterised by inability to maintain prolonged convergence. The patient complains that the eyes become tired, especially during the evenings, reading or writing cannot be continued for any length of time ; he has pain in and around the eyes, with headache ; objects look dim and indis- tinct, and there is a tendency to see things double ; sometimes the patient experiences a sensation as if one eye had turned outwards, which may really be the case ; frequently the patient finds relief by closing- one eye. In myopia the disturbance of the two functions accommodation and convergence, may in some mea- sure tend to the production of this form of asthenopia. Thus a patient with 4 D. of myopia has his punctum HETEEOPHOEIA 231 remotum at 25 cm. ; to see an object at that distance he must converge to that pointy maintaining at the same time a passive condition of his accommodation. Strain of the internal recti muscles is essentially dependent upon binocular vision; no convergent strain can exist where binocular vision is not present. Esophoria, or latent convergence^ is due to insuffi- ciency of the external recti, and is seldom seen, for it quickly passes on to convergent squint with loss of binocular vision ; here, as a rule, we are dealing with excessive innervation of the internal recti muscles rather than with an insufficiency of their opponents. Hyperphoria, one eye tends to deviate upwards ; this is usually associated with esophoria, but may exist alone ; this is not a very uncommon defect. Insufficiency of the obliques I have never seen, and no cases have been recorded in this country, though they seem to occur in America. To test and record the amount of latent deviation, the glass-rod test described on page 46 may be em- ployed. The patient is directed to look at the flame of a candle 6 metres away ; immediately behind the flame is a scale for measuring the amount of devia- tion ; the glass rod is then placed horizontally before the right eye- in testing for exophoria or esophoria ; if the streak of light appear on the right of the candle homonymous diplopia is present, and the con- dition of the eyes is one of convergence : whereas if the streak has its position on the left side of the candle, crossed diplopia is present, and the eyes are divergent ; the number on the scale corresponding 232 THE EEFEACTION OF THE EYE Fig. 100. witli tlie position of tlie streak of light indicates the amount of convergence or divergence. When em- ployed for hyperphoria the rod must be placed before the eye vertically. Another test for detecting insufficiency of the con- vergence is sometimes employed. Place a prism of about 12°, with its base down- wards, in a spectacle frame before one eye : by this means we cause a displace- ment of the eye upwards which produces vertical diplopia. The patient is now directed to look steadily at a card, on which is drawn a line with a dot in its centre, placed at the patient's ordinary reading distance (Fig. TOO). Naturally he will see two dots. If he see one line only with two dots on it, his muscles are assumed to be of the normal strength ; if, however, two lines are seen with a dot on each, then insufficiency exists, and the strength of the prism which is neces- sary with its base inwards to produce fusion is the measure of the insufficiency. The most satisfactory test for muscular insufficiency is, however, the rod test ; and having recorded the amount of lateMt con- vergence or divergence for distance, we next ascertain if there is any latent deviation in near vision. A prism of 12° base upwards being placed before the right eye in a spectacle frame, the scale is held \ metre from the eyes. The scale used by Maddox CO — OJ < CM CO MUSCULAR ASTHENOPIA 233 consists of a horizontal line in tlie centre of which is an arrow pointing upwards (Fig. 101). The line is divided into metre angles which are marked by figures, black on the right of the arrow, red on the left. The prism of course causes two arrows and two lines to be seen : the patient is directed to fix the fine print just below the arrow, and if there is no deviation the two arrows will be seen, one immediately below the other; if the lower one point to the right (black figures) there is latent convergence, but if to the left (red figures) latent divergence for this distance the amount of deviation is read off the scale and duly recorded. Treatment. — In cases of myopia we give such glasses as correct the refraction to be worn con- stantly ; these frequently succeed in relieving the asthenopia. When this is not the case, weak prisms bases inwards, by which we diminish the amount of convergence necessary, often give instant relief. It is sometimes useful to combine the prisms with concave glasses, or by separating the glasses some- what widely we may produce the same result. Fig. 102 shows concave spectacles, which act as prisms by being slightly separated; convex glasses have the same effect when placed so close together that the patient looks through the outer part of the lenses. Or the lenses, instead of being thus displaced, may be decentred by so grinding the glass that its optical centre is not the centre of the glass. These de- centred glasses are spoken of as frismosjpheres, and when ordered the amount of decentration should be 234 THE EEFRACTION OF THE EYE stated in millimetres on the order card ; the more the glasses are decentred the greater will be the pris- matic effect produced (page 194). When actual divergence of one eye takes place, advancement of one internal rectus, with or without division of its antagonist, may be necessary. Retinal Asthenopia is due to fatigue of the retina. In addition to those cases of asthenopia occurring with hypermetropia, myopia, and astigmatism, which should be relieved by the proper optical correction EETINAL ASTHENOPIA 235 restoring the balance of the extra- and intra-ocular muscular systems, every one will occasionally meet with cases where there is intense discomfort and inability to read or do near work for any length of time, but where no ametropia exists, as proved by placing the patient under atropine and then testing the refraction. The visual acuteness is often very good, frequently rising to |- or higher. The pain complained of is usually at the back of the eyes, with more or less headache, photophobia, lachrymation, a feeling of tension and heat, together with itching and pricking of the eyelids. Sometimes the chief symptom complained of is the conjunctival irritation accompanied with increased secretion. These cases are always exceedingly troublesome and difficult to cure; they occur most frequently among young unmarried girls of an hysterical or nervous temperament. Less frequently men are affected, and then it is chiefly amongst those who are feeble, hypochondriacal, and nervous. With the ophthalmoscope the eyes may appear quite normal, or the retinal veins may be full with or without some slight haziness of the edges of the discs; the perimeter may reveal a spiral contraction of the visual fields due to progressive exhaustion of the retina, which probably follows reflex contraction of the retinal vessels. Retinal asthenopia may be attributed to long hours of near work which has been done by artificial light, especially in those who have been previously reduced by some lowering illness. I have met with several 236 THE REFRACTION OF THE EYE cases amongst those making gold lace^ and no doubt the briofht materials here worked with had somethins: to do with the production of the retinal hyper^es- thesia. It seems generally accepted by all authorities on this subject that in most cases the nervous system is exceedingly sensitive and unstable. Sometimes the asthenopia is distinctly of reflex origin^ produced by disturbance of the internal organs; when leucorrhoea exists in young unmarried women, with troublesome asthenopia, masturbation may be suspected. Irritation of the fifth nerve from carious teeth may also act as the exciting cause. Treatment. — Complete abstinence from near work does not always give relief, nor is this abstinence to be encouraged. A slight amount of regular work should be done every day, with rest for the eyes during the evening. Usually some form of nerve tonic is indicated, with plenty of outdoor exercise and the avoidance of strong light or places where there is a great glare, such as the sea-side in summer. Tinted glasses are to be avoided under ordinary conditions of light, since they merely tend to increase the hyper^esthesia of the retina. SPECTACLES 237 CHAPTER XII SPECTACLES Having referred to the subject of spectacles when considering the correction of the different forms of ametropia^ I will noAV briefly recapitulate what was then said^ even at the risk of being accused of un- necessary repetition. Hypermetropia. — So long as -J can be read with each eye^ no glass is necessary for distant vision ; for reading and near work we give such glasses as correct the manifest and 4- of the latent hyperme- tropia. If distant vision be improved by convex glasses^ then these may be prescribed. In hypermetropia complicated with strabismus Ave estimate the total hypermetropia under atropine^ then give the full correction to be worn constantly. Myopia. — In cases of low degree in people over twenty years of age we may prescribe folders for distance, allowing the patient to read and write without glasses if only he keep a sufficient distance (30 cm.) from his book and suffers no inconvenience ; under the age of twenty a full correction should be worn constantly. In medium degrees the best results often ensue when the full correction is always worn. Where the myopia is of high degree the full cor- 238 THE EEFRACTION OF THE EYE rection may be satisfactory for distance^ but uncom- fortable or impossible for reading, owing to the accommodation being insufficient. These glasses also have the disadvantage of diminishing the size of objects ; here we give two pairs of spectacles, one for distance, and a weaker pair for reading. Astigmatism. — Our object is to give as near as possible the full correction (found by atropizing the patient) ; these glasses should be worn constantly. Atropine is seldom necessary in patients over twenty years of age. Homatropine and cocaine is usually sufficient in older people. Convex glasses render parallel rays which pass through them convergent : they add therefore to the refraction of the dioptric system, and are called positive. Concave glasses render parallel rays divergent ; they therefore diminish the refraction of the dioptric system, and are called negative. Convex glasses add to the quantity of light entering the eye, while concave glasses diminish it. The size of the image is modified : thus positive glasses bring forward the nodal point, and so increase the size of the image ; while negative glasses carry the nodal point backwards, and so diminish the size of the image. Glasses may be made of rock crystal (commonly called pebbles) or crown glass. Those made from the former material have the advantage of being harder, and are therefore less likely to be scratched than glass; the weight is much the same in both cases. SPECTACLES 239 Pebbles absorb more heat, and unless cut exactly at right angles to their optic axis tliey are apt to refract unequally; besides, it is difficult to get rock crystal free from strise, so that lenses made from good crown glass are quite equal to the best pebbles, very much cheaper, and almost universally used. The method of mounting spectacle glasses is of the greatest importance ; they must be accurately centred in frames that are light, strong, and fit well, other- wise the good effect of the most carefully chosen correction may be entirely frustrated by a faulty position of the glasses, or even a fresh source of eye- strain may be introduced. Gllasses for constant use should be in the same plane and centred for distance; those intended for near work only should converge slightly, be centred for the reading distance, and be inclined downwards 15° to 20°. Each lens should be in the first focal plane of the eye, that is 13*7 mm. in front of the cornea. When this is the case, the images formed on the retina will be of the same size as in emmetropia. The bridge of the frame should be moulded to suit the shape of the nose, resting on it by a broadish surface so as not to cut or indent the skin, while the glasses should be a sufficient distance from the eyes to just clear the lashes. The sides of the frames should pass back immediately above the ears, and in many cases, especially where glasses are required to be worn constantly, they may with advantage bend directly round the ears, fitting the posterior part of the concha; these ear-pieces may be made of twisted 240 THE EEFEACTION OF THE EYE wire^ wliicli gives them considerable elasticity and strength. The frames may be made of gold, rolled gold, or steel ; the latter material has the great dis- advantage of rusting easily. When glasses are worn for myopia or hyper- metropia they should not be further from the eye than 13*7 mm. For presbyopia the person may be allowed to suit his own convenience and comfort, 2h cm. being an ordinary distance. Single glasses may occasionally be allowed in low degrees of myopia for looking at distant objects. They have the disadvantage of encouraging mon- ocular vision, and sometimes one eye is used so entirely that the sight in the other may deteriorate from want of sufficient use. Folders (pince-nez), of which there are many varie- ties, may be used in some cases of hypermetropia and myopia; many presbyopes find them very con- venient for reading. The frameless pince-nez are very neat and comfortable. Spectacles are as a rule to be preferred in children, since they are more accurately centred and fit better. In cases of astigmatism it was formerly the custom to order spectacles and not folders, as in the latter it is difficult to be certain that the cylindrical glasses are always in their proper axis ; but several ingenious pince-nez have been brought out which are free from this objection. That form in which the glasses slide on a horizontal bar is so arranged that they fit on the nose very easily, and are extremely comfort- able, they may be recommended in many cases. SPECTACLES 241 In addition to concave, convex, and cylindrical glasses, others are sometimes used. Periscopic glasses are an advantage in many cases. They give a large field, and there is less spherical aberration. Bifocal glasses are often convenient for presbyopes Fig. 103. who are also ametropic. The upper part of the glass is then used for distance, the lower part for reading. There are several varieties — Fig. 104. (a) Made in two halves separately (Fig. 103). (b) Ground on one glass by a special tool (Fig. 104). (c) The invisible bifocal, which consists of two thin layers of glass, between which is placed at its lower 16 242 THE EEPRACTION OF THE EYE part a thin circular lens having a high refractive index (Fig. 105). Fig. 105. Stenojpaic spectacles consist of an opaque screen with a small central aperture which may be of any shape to suit a particular case^ so that all the peri- pheral rays are cut oif^ only those that are in the visual axis being allowed to pass through. They can be combined with convex or concave glasses^ and are sometimes exceedingly useful in cases of leucomata, nebulae, irregular astigmatism, conical cornea, etc., where the vision is much disturbed. Prismatic spectacles may consist of prisms alone, or they may be in combination with concave or convex lenses. It is not convenient to use prisms much stronger than 3° or 4°, owing to their weight and the chromatic aberration they produce. They are use- ful in certain cases of paralysis of muscles, to correct the diplopia, and in some cases of hypermetropia, myopia, and astigmatism which are not relieved by their proper correction ; prisms are also used for testing the ocular muscles and for detecting malin- gerers. When ordered in cases of asthenopia with SPECTACLES 243 errors of refraction, they may be combined with the glasses which correct these errors (p. 194). Tinted glasses are often required for diminishing excessive light, especially where there is irritation or inflammation of the retina; they are also useful in some cases of photophobia, arising from various causes, as myopia, etc. Where the aim is to relieve the retina without injuring the distinctness of vision the light blue glasses are the best, as they cut off the orange rays; where the object is to act on the quantity and not the quality of the light, smoke- coloured glasses are to be preferred. Tinted glasses sometimes do real harm, as in cases of asthenopia, by increasing the sensitiveness of the retina; they are always somewhat heating to the eyes, in proportion to the amount of rays they absorb. We sometimes combine them with convex or concave glasses. There are also various forms of protector^s ; those hollowed out like a watch-glass, so as to fit closely, are to be preferred to those with wire sides called goggles, or those with sides of glass, which have the disadvantage of being heavy. Workmen wear diife- rent sorts of protectors to keep off dust, fragments of stone, etc., which may be made of glass, talc, wire gauze, or other material. Shot-proof spectacles are made for sportsmen. It is sometimes necessary to find out and record the strength of glasses that are being worn ; this is easily done. If convex, we take a concave glass out of the trial box, hold it against the glass we are trying, and look through them at a line, e. g. the bar of 244 THE EEFEACTION OF THE EYE a window or any similar object. We move the glasses to and fro in front of the eye ; if the line remains immovable the neutralisation is complete ; if it move in the same direction the concave glass is too strong ; if in the opposite direction it is not strong enough. The Greneva lens measure is a simple instrument for estimating the strength of lenses. Cases Commence the examination in a systematic manner. First, notice the general appearance of the patient, then the shape of the head and face; next the eyes, as to whether they are large and prominent, or small and sunken-looking. Listen patiently to the sufferer's complaints, and having submitted to this ordeal, test the acuteness of vision of each eye separately, and afterwards together, writing down the result, remembering always to commence with convex glasses. Then place the near type in the patient's hand, noting the number of the type and the distance at which it can be read. Next pass on to the ophthalmoscope, first applying the " retino- scopic test,^^ then the "indirect examination," and finally the " direct method," first at a distance, and then close to the eye. If any ametropia exist, the advisability of paralysing the accommodation with a mydriatic must be considered. In order to illustrate this method of examination, I will give a few additional cases. Case 23. Hypermetropia. — E. M — , a young woman, a book-folder, aet. 17, suffering from tinea tarsi, CASES 245 complains tliat her eyes get very tired at niglit, so mucli so, in fact, tliat she is unable to read for any length of time. Her general appearance is healthy, and nothing special is noticed about her face, except that the eyes are small. The acuteness of vision for both eyes is normal. On placing + 1 D. in front of the right eye, |- is seen more distinctly than without, with + 2 D. -| is still read, with + 2*5 D. vision is not so good j the same result is obtained with the left eye. + 2 D. for each eye is the strongest convex glass with which |- can be read, and is therefore the measure of her Hm. ; on trying the two eyes together + 2"5 D. still gives |-. We record it thus : R.V. fHm. 2D. = |^) « 6 b ^Hrn. 2oD. = f. L.V. |Hm. 2D. = |) On placing the patient in the dark room, and directing her to look at some distant object or at a black wall, so as to relax as much as possible the accommodation, Avith the plane mirror the shadow we perceive moves slowly with the mirror. We put + 2 D. in a spectacle frame, in front of the eye ; the shadow is more distinct, and moves more quickly. We try stronger glasses, and then find that + 3*5 D. is the highest with which we still get a shadow moving with the mirror. Both eyes are alike. Next examine Avith the ophthalmoscope. By the indirect method the disc becomes smaller on with- drawing the objective from the eye. With the mirror alone at a distance, we see an image of the disc which moves with the observer's head, proving 246 THE REFRACTION OF THE EYE the image to be an erect one. With the direct method the disc is not seen well, unless we put in force our own accommodation; with our accommodation suspended, we turn the wheel of the ophthalmoscope so as to bring forward convex glasses ; the clearness of the fundus is improved; + 4 D. is the strongest convex glass with which the details can be distinctly and clearly seen by myself. We might be satisfied with this result, assuming 4 D. to be the amount of total hypermetropia, but in young people it is much more satisfactory to be able to record once and for all the total hypermetropia beyond doubt. Atropine (grs. iv to 5J) is therefore ordered, one or two drops to be placed in both eyes three times a day for four days, warning her that she will be unable to see well, and that the pupils will be dilated during their use. We also recommend a shade to be worn to protect the eyes from the light. Gn her return she reads only g-^ with each eye, and she now requires + 5 D. to enable her to read -J. We also find with retinoscopy that + 5 D. is the strongest glass with which we get a shadow moving with the mirror. Our patient, therefore, has a total hypermetropia of 5 D., two dioptres of which were manifest, and three latent. For work and reading we order her spectacles + 3 D. At present she will not require them for distance. About thirty she will probably require her glasses increased to + 4 D. ; about forty she may be able to bear her full correction, and may then begin to wear them constantly. CASES 247 We must remember that when atropine has been used it is necessary to take off '50 D. from the measure- ments thus found. Case 24. Myopia. — A young man, set. 20, next presents himself. He has a long intellectual face with prominent nose ; the palpebral apertures are wide ; and on directing him to look inwards as much as possible, the eyeballs seem elongated in the antero- posterior diameter. His eyes, he says, are excellent, but he is unable to recognise people as well as formerly. We test the acuteness of vision, and find that he reads -^^ with each eye. Convex glasses make even that line in- distinct, our patient is probably myopic. We place in his hand the near type, and he reads No. 1 at once and easily. The farthest point at which he can read it is 25 cm. (-y/ = 4 D.) ; - 4 D. should be the measure of his myopia. We try — 4 D., directing him again to look at the distant type. He reads with each eye |- ; we reduce the glass to find the weakest with which he reads the same, and with — 3*5 D. he reads it, though hardly so well ; with — 3D. he reads only f; - 3*5 D. is therefore the measure of his myopia, and we record it thus : E Y _6_-3-5D.= 6 L.V.3%-3-5D. = f. If we employ retinoscopy — 3*5 D. is the concave glass which neutralises the shadow. We next subject the eye to the indirect ophthalmo- 248 THE REFRACTION OF THE EYE scopic examination. The image of the disc becomes larger on placing the objective near the eye and gradually withdrawing it^ and in addition we see also a slight myopic crescent on the apparent inner side of the disc. From this case disc No. 1 was drawn (p. 147). With the mirror alone at a distance from the eye the disc cannot be well seen, because in our case the aerial image will be formed about 25 cm. in front of the patient's eye. To enable us to see this aerial image it is necessary we should be some 30 cm. away from it : so that we should require to be 25 + 30 = 55 cm. from the observed eye, and at that distance the illumination will be very weak. With the direct method the details appear blurred until we put on a concave glass by turning the wheel of our refracting ophthalmoscope. The weakest con- cave glass with which we are able to see the details of the fundus clearly is the measure of the myopia. Thus we have four distinct plans of measuring our case of myopia : 1st. The farthest distance at which the near type is read, 25 cm. {-^^ = 4 D.). 2nd. The weakest concave glass which gives the greatest acuteness of vision. 3rd. The weakest concave glass with which we get a retinoscopic shadow moving with the plane mirror. 4th. The weakest concave glass Avith which the details of the fundus can be distinctly seen by the direct method. Should any of these results vary much, we should suspect that the myopia is increased by spasm of the CASES 249 accommodation, and we atropize the patient in tlie manner before described, and at the end of fonr days go over tlie ground again, remembering that when atropine has been used, it is necessary to add on about — '5 D. to the glass found, because the ciliary muscle is probably never so completely relaxed as when it is under the influence of atropine. Having found, then, that our patient's myopia amounts to — 3*5 D., we give spectacles of that focus for constant use. In addition to ordering spectacles, we give him also some very important general direc- tions : he must always hold his book or work 35 cm. away, bring the work to his eyes, and not his eyes to the work ; writing should be done at a sloping desk, he should sit with his back to the window, so that the light comes over his left shoulder on to his work, and do as little near work as possible by artificial light. Case 25. Hypermetropia and Presbyopia. — A gentle- man, 83t. 56, comes with the complaint that he cannot see to read as comfortably as formerly, though he sees distant objects well. We try his acuteness of vision, and find that he reads f badly. With + 1 D. he sees much better, reading some of the letters of -f. We then try + 1"5 D., and these he rejects. Hence we conclude that he has Hm. 1 D. We know from his age that he will also be presbyopic 3 D., and we add on to this + 1 D. for his hypermetropia, directing him to read the newspaper with + 4 D. for half an hour. He thinks these rather strong for him, as they make his eyes ache. With + 3*5 D. he feels quite comfortable, and we therefore give him + 3*5 D., 250 THE REFRACTION OP THE EYE telling him that lie may require tliem clianged for slightly stronger ones in about five years. Case 26. Paralysis of the Accommodation. — Kate L — , £et. 12, has been very ill from diphtheria, but is now much better. She complains that she is unable to read or work, though able to see distant objects well. The pupils are very large, and act badly to light. Hence we suspect paralysis of the accommo- dation. We test her acuteness of vision, and she sees |- with each eye ; on trying convex glasses '5 D. she still reads f, but 1 D. she rejects. Our diagnosis is therefore confirmed. We next find the weakest glass with which she is able to read, weakest because we are anxious to encourage the ciliary muscle to act, since by replacing it entirely we should prolong the patient^s recovery. The glasses must be changed for weaker ones as the ciliary muscles recover tone. We saw that she had a slight amount of hyperme- tropia, and also that there was some accommodation left, enough at least to correct this, otherwise she could not have read f without + '5. A tonic con- taining iron and strychnine was also prescribed. Case 27. Anisometropia. — A young woman, ^t. 20, has never seen well, either at a distance or near at hand ; has tried spectacles of all sorts, but never been able to find any that suited her. The eyes look somewhat irritable, but there is nothing conspicuous about their size or shape. There is some want of symmetry about the face, the nose being deviated from the median line slightly to the left. CASES 251 We first try the acuteness of vision of tlie riglit eye. She reads y^, and with + 1 D. vision is somewhat improved; with + 1*5 D. it is made worse. Still armed with + 1 D. we direct the patient to look at the fan of radiating lines (Fig. 85). She sees plainly the horizontal lines, whilst all the others are more or less indistinct, the vertical line most so ; still looking at the horizontal line, we alternately hold in front of + 1 D., which is before the eye under examination, + '25 D., which makes it worse, then — '25 D., which she says at once makes it perfectly clear and distinct. We therefore put down + '75 as the correction for the vertical meridian, and pass on to the horizontal. Our patient is directed to look steadily at the vertical line. We try convex glasses, these improve it, + 3 D. making it quite clear ; a stronger glass than this renders it slightly indistinct. It is evident, there- fore, that her horizontal meridian is hypermetropic + 3 D. We put up the correction found, + '75 D. sp., + 2*25 D. cylinder axis vertical, and direct her again to look at the distant type ; -| is read, though with some difficulty. This result is not, however, reliable, and we proceed to confirm it by retinoscopy, obtaining + 2 D. for the vertical, and + 4 D. for the horizontal meridians. On trying this correction, however, the vision is not so good. We now test the acuteness of vision in the left eye ; she sees -g^, and neither convex nor concave glasses improve it. On looking at the fan of radiating lines, all seem indis- tinct, and having thus far no data to go upon, we, instead of wasting time, at once pass on to retino- 252 THE REFEACTION OP THE EYE scopy. We get oblique shadows^ the horizontal moving against the mirror and the vertical with it : here^ then^ is a case of mixed astigmatism, We find out that — 2 D. is the correction for the horizontal meridian and + 3 D. for the vertical^ the degree of obliquity being about 25°. This result is noted down thus : 3D. 2D. We therefore place in a spectacle frame + 3 D. spherical^ combined with - 5 D. cylinder^ axis devi- ating outwards from the vertical 25°. With this cor- rection the patient at once reads yg"- ^^ ^^® ^^^^ ^^ be satisfied with this result^ but give the patient a solution of sulphate of atropine^ grs. iv to 3J> "^^ith directions to come again in four days. At the end of that time she returns^ and we find with retinoscopy — R. 2-5 D. + 4-5 D. The right eye with this correction reads ■§- readily, and the left also -J, but rather slowly. This result is very satisfactory. At the end of a week, when the patient has recovered from the eifects of the atropine, the results were confirmed before ordering spectacles; then for the right eye the best vision was obtained with CASES 253 + 1*5 sp. c:; + 2 D. cy, axis vertical (|-) ; and for the left + 3 D. spherical o - 5 D. cylind. axis 70° (f). Spectacles of this strength were therefore ordered, and the patient directed to wear them constantly. Case 28. Anisometro'pia. — Jane W — _, set. 30, pre- sents herself, complaining that the sight in her left eye has been gradually getting dim for some months. She is a small, healthy-looking woman, with nothing characteristic in her appearance. We test the acute- ness of vision : Right f Hm. 1 D. = f . Left -yej ^^^^ improved with spherical glasses. We try retinoscopy, but the pupils are so small that the result is not very satisfactory. One can, however, make out in the left eye a shadow moving with the mirror in the horizontal meridian, which + 2 D. over-corrects, + TS D. being the highest glass with which we get a shadow moving with the mirror ; the vertical meridian appears emme- tropic. There is, therefore, no doubt that the de- fective vision in this eye is due to astigmatism. The patient complains that the examination has made her eyes ache, so we do not proceed further, but order a solution of hydrobromate of homatropine (2 grs. to the 5J) to be used every three hours, and direct her to come again on the following day. Then the result with retinoscopy is — + 1-5 D. + -SOD. E. + 1-5 D. L. + 2 D. 254 THE EEFEACTION OF THE EYE We try this at the test type. R. 2^ + 1-5 D. = f. I^ 6 g + -SD. sp. ^6 • 3 6 + 1-5 D. cy. axis vert. 6 * We make a slight deduction from the sphere in each case for the homatropine^ and order for constant use — E. + 1 D. sph. L. + 1*5 D. cy. axis vert. Case 29. Presbyopia. — John G — , set. 50, has always enjoyed good sight ; he still sees distant objects well, but finds some difficulty in reading, especially during the evenings. R.V.f, noHm. L.y.f, noHm. We try him with + 2 D. for reading, and with these he sees perfectly ; this, therefore, is a simple case of presbyopia, requiring a pair of folders + 2 D. for reading, writing, etc. Case 30. Hypermetropia and Presbyopia. — Mr. K — , set. 60, sees badly both near and distant objects ; he wears + 4 D. for reading, but they are not com- fortable. E.V.g^Hm. 3D. = f . L.V.3%Hm.3D.=f. He therefore wants + 3 D. for distance ; and to find the glass he will require for reading, it is neces- sary to add on to this distance lens the glass he would require for presbyopia if he were an emme- trope, viz. + 4 D. We therefore try him with + 7 D., CASES 255 but these make his eyes ache; we next try + 6*5 D., and with these he sees comfortably. This patient_, then^ requires two pairs of spec- tacles^ — + 3 D. for distance ; + 6"5 D. for reading, &c. Case 31. Myopia and Presbyopia. — Mrs. C — , aet. 55, complains that her eyes become tired at night; she has tried several pairs of spectacles, but without finding any that exactly suit her. L.V.-A--2D. 6 36 ^^-"g- Our patient requires, therefore, this correction for distance, but she also wants spectacles for reading and near work ; an emmetrope of fifty-five requires presbyopic glasses + 3 D. ; she is, however, a myope of 2 D., so we have to deduct this from the presbyopic glass ( (+ 3D.) + (- 2D.)= + 1 D.), and try the + 1 D. for reading. With these she is able to read the smallest type comfortably ; we therefore pre- scribe two pairs of spectacles, — - 2 D. for distance ; + 1 D. for reading. Case 32. Myopia. — Annie C — , aet. 19, came because she was unable to see distant objects. E.V. -2^-3-5 D. = f. L.Y. #i--2-5D. = #. After using atropine — B.V./g-8D.=f, L.V.-,^-2D. = f. 256 THE REFRACTION OF THE EYE Ordered spectacles for distance R. — 3 D.^ L. — 2 ID., with directions to present herself again in six months,' when, should the myopia have increased, or should she complain of asthenopia, it may be necessary to prescribe spectacles for constant use. Case 33. Sim'ple Myoinc Astigmatism. — Thomas J — , ast. 18, sees rather badly both near and distant objects. E.V. -j^2^ not improved with spheres ; with pin-hole = ^. L.V. -j^2'' ^ot improved with spheres ; with pin-hole = -I". After atropine had been used for four days retino- scopy gave — , + 1 D. [+ 1 D. R. ^ Em. L. ^ -Em. R. + 1 D. cy. axis horiz.=^. L. + 1 D. cy. axis horiz. = ^. After the atropine has passed off — R. — l D. cy. axis vert. = ^- L. — 1 D. cy. axis vert. = ^. This correction was given for constant use. Case 34. Gompoiond Myopic Astigmatism. — Miss W — , set. 13, has seemed short-sighted for the last year or two. Mother and father both have good sight. The pupils are large, so that retinoscopy can be easily carried out. 10 D. ! - 10 D. 1 R. 6 D. -6D. CASES T>y gZJ^J^L^- . , . =.-,%- and 2 letters of -,%, -K" V • c _ 4 D. cy. axis lioriz. i 8 1-2 .-YD. sp. 6 257 - 6 D. sp c L.V -3D. cy. axis horiz. 1 2 • On examination of the eyes with the ophthal- moscope the choroid is found to be exceedingly thin, there is a large crescent in both eyes, and in the right are three or four patches of choroiditis, with one haemorrhage near the macula. The patient was ordered the full correction for distance, and advised to do no reading, writing, or near work for six months, then to return for inspec- tion; she was also recommended to spend as much of her time as possible in the open air, and a mixture containing syrup of the iodide of iron was pre- scribed. Case 35. Concomitant Squint. — George W — , £et. 5, has squinted inwards for the last three months. On covering the non-squinting eye and directing the little boy to look at the finger held a short distance from him, the deviating eye immediately righted itself and fixed the finger, the covered eye at the same time turning in. We prescribed a solution of sulphate of atropine to be applied to both eyes, and at the end of a week the patient is brought back : the squint is now much less apparent, and with retino- scopy we find 3'5 D. of hypermetropia in each eye. The direct examination gives the same result. We order our patient spectacles + 3 D. to be worn con- stantly. Case 36. Aphakia. — Thomas B — , ast. 50, game- 17 258 THE EEFRACTION OF THE EYE keeper. Had the right lens removed for cataract nine months ago, and last week the opaque capsule remaining was needled. E.V. c + 11 D. = f, and with + 14 D. No. 1 of the near type was read with comfort; the patient was therefore ordered the following spectacles : + 11 D. for distance; + 14 D. for near Avork. These were arranged in a reversible frame, so that either glass could be brought in front of the right eye as occasion required. APPENDIX 259 APPENDIX In tlie metrical system tlie unit of length is a metre^ equal to 100 centimetres^ 1000 millimetres, or 40 English inches ; so that 1 inch is equal to 2 J cen- timetres. A lens of 1 metre focus is called a dioptre, a lens of ^ a metre (50 cm.) is 2 D., -^-^ of a metre (10 cm.), 10 J)., etc. In the old system the lenses were numbered according to their focal length in inches, a lens of 1-inch focus being the unit ; a lens of 2-inch focus was expressed by the fraction ^, one of 10-incli focus yij, and so on. If we wish to convert a dioptric measurement into the corresponding inch measure- ment of the old system, we have only to remember that the unit 1 metre = 40 English inches, so that a glass of 1 D. = 4^5- in the old system, 2 D. ='^V — ~2V^ 5 D. = -^^ = "I", and so on. The table on the next page gives approximately the equivalent of each dioptre or part of a dioptre in English and French inches, and their focal length in centimetres. 260 THE REFRACTION OF THE EYE Dioptres. English inches. French inches. Centimetres. •25 160 146 400 •50 80 73 200 •75 52 50 130 !• 40 36 100 1-25 31 29 77 1-50 26 24 65 1-75 22 21 55 2- 20 18 50 2-25 17 16 43 2-50 16 15 40 2-75 14 13 35 3- 13 12 33 3-50 11 10 27 4- 10 9 25 4-50 9 8 22 5- 8 7 20 5-50 7 6.^ 17 c- 6i 6 16 7- 6 5 .15 8- 5 4| 12^^ 9- 4| 4 11 lo- 4 31 10 ll- 3.^ H 9 12- H 3 8 ]3- 3 2f 7^ 14- 2| 2^ 7 15- 2* 2i eh 16- 2i 2i 6 18- 2i 2 5i 20- 2 n 5 APPENDIX 261 Regulations for Candidates for Commissions in the Army A candidate must be able to read at least -f-^ with each eye separately without glasses, and this must be capable of correction with glasses up to |^ in one eye and -^-^ in the other ; he must also be able to read No, 1 of the near type with each eye without the aid of glasses. Squint, colour-blindness, or any serious disease of the eye renders the candidate ineligible. Navy A candidate must be able to read |^ with each eye, and the near type at the distance for which it is marked, without glasses. Colour-blindness, squint, or any disease of the eye disqualifies. Indian Civil Service A candidate must be able to read |- with one eye and |- with the other, with or without correcting lenses. Any disease of the fundus renders the candidate ineligible. Myopia, however, with a posterior sta- phyloma, may be passed if the ametropia do not exceed 2*5 D., and the candidate has a visual acute- ness equal to that stated above. Indian Medical Service The candidate must have a visual acuteness of -| in one eye and jV "^ ^^^ other. Hypermetropia and 262 THE REFEACTION OF THE EYE myopia must not exceed 5 D., and then witli the proper correction the vision must come up to the above standard. Astigmatism does not disqualify a candidate, pro- vided the combined spherical and cylindrical glass does not exceed 5 D., and the visual acuteness equals |- in one eye and -^^ in the other. Colour-blindness, ocular paralysis, or any active disease of the fundus renders the candidate ineligible. A nebula of the cornea will not disqualify the candidate if he is able to read -f-^ with this eye and -| with the other. Public Worlcs Candidates for the departments of Public Works, Survey, Forest, Telegraph, Railways, Factories, and Police of India must pass the following eyesight tests. If myopic, the defect must not exceed 2*5 D., and with this glass the candidate must read |- in one eye and f in the other. If myopic astigmatism is present, the vision must reach the above standard with correcting glasses, and the combined spherical and cylindrical glass must not exceed 2-5 D. In hypermetropia and hypermetropic astigmatism an error of 4 D. is permissible provided that with this glass |- is read with one eye, and -g- with the other. A corneal nebula with vision of j~ ^^^^ tt ^^^ ^^^® other eye will not disqualify the candidate. Colour-blindness, any disease of the eye, or paralysis of one of the muscles of the globe, will disqualify. APPENDIX 263 English Railways There is^ unfortunately, no uniform standard for our railways ; each company has its own standard, in many cases a very low one; every engine-driver should have at least jj in each eye without glasses, and normal colour vision. TEST TYPES 265 TEST TYPES No. 1. 25cm.*= /(f.. 1 danger of breaking do' We passed that Act because we thought , was not then sofficicntly secured. Yet icnvv) than it will be if this House takes No. 2. 1 J. 33cm. = /J^ on itself to be the supreme criminal judicature in political cases." Warm eulogies were pronounced on the ancient national mode of trial by twelve good men and true ; and indeed the advantages of that mode of trial in political cases are obvious. The prisoner is allowed to challenge any number of jurors with cause, and a considerable number without cause. The twelve, from the moment at which they are invested with their short magistracy, till the moment when they lay it down, are kept separate from the rest of the community. Every precaution is taken to prevent any agent of power from soliciting or corrupting them. Every one of them No. 3. 2 J., '50 Sn.t 50cm. x^^^ must hear uvery word of the evidence and every argument used on either side. The case is then summed up by a judge who knows that, if he is guilty of partiality, he may be called to account by the great inquest of the nation. In the trial of Fenwick at the bar of the House of Commons all these securities were wanting. Some hundreds of gentlemen, every one of whom had much more than half made up his mind before the case was opened, performed the functions both of judge and jury. They were not restrained, as a judge is restrained, * The number indicates the distance at which the type should be seen by a normal eye. t Corresponding Jaeger and Snellen type. 266 TEST TYPES No. 4. 5 J., -75 Sn. 75cm. =:J(\ I>y the sense of responsibility ; for who was to punish a Parliament ? They were not selected, as a jury is selected, in a manner which enables the culprit to exclude his personal and political enemies. The arbiters of his fate came in and went out as they chose. They heard a fragment here and there of what was said against him, No. 5. 6 J., 1 Sn. lm.:3ff^ and a fragment here and there of what was said in his favour. During the progress of the bill they were exposed to every species of influence. One member was threatened by the electors of his borough with the loss of his seat : another might obtain a frigate for his brother from No. 6. 8 J., 1-25 Sn. l-25m.r^^ Russell ; the vote of a third might be secured by the caresses and burgundy of Wharton. In the debates arts were practised and passions excited which are unknown to well- constituted tribunals, but from which no great No. 7. 10 J., 1-5 Sn. l-5m.- ^ popular assembly divided into parties ever was or ever will be free. The rhetoric of one orator called TEST TYPES 267 No. 8. 12 J., 2 Sn. 2m. O o UJ < N Z Q. O D o > Q. Z o It. X llJ Q -I < INDEX Abducting' pi-isms, 45 Accommodation, 32, 187 absolute, 40, 213 amplitude of, 37 at diiJerent ages, 41 binocular, 40 diminution of, 40, 187 of emmetropes, 37 of h.^-permetropes, 38 of myopes, 39 paralysis of, 196 produced by, 33 range of, 37 relative, 40, 50 spasm of, 197 Accommodative asthenopia, 124, 132, 226 Acquired hj'permetropia, 128 Acuteness of vision, 29, 55 in hjqaermetropia, 126 in mj^opia, 148 in astigmatism, 165 diminishes with age, 187 Adducting prism, 46 Aerial image, 73, 245 Alternating- strabismus, 206 Amblyopia, 211 Amblyoscope, 220 Ametropia, 26 Amplitude of accommodation, 37 of convergence, 44 Anderson, Dr. Tempest, 182 Angle a, 41, 123, 145, 201 V, 203 metrical, of convergence, 44 of deviation, 9 of strabismus, 207 principal, 9 visual, 29, 56 Anisometropia, 29, 184 correction of, 185 treatment of, 185 Anterior focal point, 24 focus, 10 Aphakia, 132 case of, 256 test for, 132 treatment of, 133 *i.pparent strabismus, 123, 200 Appendix, 259 Army, regulations for, 261 Asthenopia, 124, 224 accommodative, 124, 132, 226 muscular, 146, 228 retinal, 234 retinal veins in, 235 Astigmatism, 29, 156 causes of, 164 compound hypermetropic, 161 compound myopic, 161 estimation of, 167 irregular, 157 mixed, 161 principal meridians in, 158, 168 regular, 157 shape of disc in, 171 simple hjqjermetropic, 161 simple myopic, 161 svmptoms of, 164 treatment of, 176, 238 Astigmatic clock-face, 170 fan, 171 Asymmetry of cornea, 157 Atropine, 60, 89, 126, 151, 170, 196 in astigmatism, 170 in myopia, 151 in retinoscopy, 89 in hypermetropia, 126 Axial line, 121, 138 hypermetropia, 121 mj'opia, 138 Axis, optic, 201 principal, 9, 12 secondary, 12, 15 visual, 201 B Bar reading, 222 Biconcave lenses, 11, 16, 20, 238 Biconvex lenses, 11, 19, 238 Bifocal lens, 241 Binocular accommodation, 40 vision, 211, 219 Capsule of lens, 34 Cardinal points, 23 Cataract, 132, 148, 164 in myopia, 148 ■'-fc^^-'-. '^:3fi'> m $ INDEX Abducting- prisms, 45 Accommodation, 82, 187 absolute, 40, 213 amplitude of, 37 at different ages, 41 binocular, 40 diminution of, 40, 187 of emmetropes, 37 of hypermetropes, 38 of myojies, 39 paralysis of, 196 produced by, 33 range of, 37 relative, 40, 50 spasm of, 197 Accommodative asthenopia, 124, 132, 226 Acquired hypermetropia, 128 Acuteness of vision, 29, 55 in hj^permetropia, 126 in myopia, 148 in astigmatism, 165 diminishes with age, 187 Adducting prism, 46 Aerial image, 73, 245 Altei'nating strabismus, 206 Amblj^opia, 211 Amblj^oscope, 220 Ametropia, 26 Amplitude of accommodation, 37 of convergence, 44 Anderson, Dr. Tempest, 182 Angle a, 41, 123, 145, 201 y, 203 metrical, of convergence, 44 of deviation, 9 of strabismus, 207 principal, 9 visual, 29, 56 Anisometropia, 29, 184 correction of, 185 treatment of, 185 Anterior focal point, 24 focus, 10 Aphakia, 132 case of, 256 test for, 132 treatment of, 133 ^i-pparent strabismus, 123, 200 Appendix, 259 Army, regulations for, 261 Asthenopia, 124, 224 accommodative, 124, 132, 226 muscular, 146, 228 retinal, 234 retinal veins in, 235 Astigmatism, 29, 156 causes of, 164 compound hypermetropic, 161 compound myopic, 161 estimation of, 167 irregular, 157 mixed, 161 principal meridians in, 158, 168 regular, 157 shape of disc in, 171 simple hypermetropic, 161 simple mj'opic, 161 svmptoms of, 164 treatment of, 176, 238 Astigmatic clock-face, 170 fan, 171 Asymmetry of cornea, 157 Atropine, 60, 89, 126, 151, 170, 196 in astigmatism, 170 in mj^opia, 151 in retinoscopy, 89 in hypermetropia, 126 Axial line, 121, 138 hypermetropia, 121 myopia, 138 Axis, optic, 201 principal, 9, 12 secondary, 12, 15 visual, 201 B Bar reading, 222 Biconcave lenses, 11, 16, 20, 238 Biconvex lenses, 11, 19, 238 Bifocal lens, 241 Binocular accommodation, 40 vision, 211, 219 Capsule of lens, 34 Cardinal points, 23 Cataract, 132, 148, 164 in myopia, 148 270 INDEX Cases, retinoscopy, 102, 244 others, 244 Centre of motion of the eye, 24 optical, 12 Choroid, thinning of, in mj^opia, 147 Ciliary muscle, fvinction of, 34 in hypermetropia, 123 in myopia, 145 body, 34 Civil Service, regulations for, 261 Cohn, 142 Cocaine, 90 Compound hyi)ermetropic astig- matism, 161 myopic astigmatism, 161 system, points of, 23 Concave lenses, 11, 16, 20, 238 mirror, in retinoscopy, 100 Concomitant squint, 200 Conjugate focus, 4, 14, 136 Conjunctiva, 126, 225 Convergence, 41 amplitude of, 44 insuflBciency of, 229 latent, 229 metrical angle of, 44 punctum proximum of, 44 punctum remotum of, 44 range of, 44 relative, 50 Convergent strabismus, 124, 208 Cone, 56, 144 of Ught, 158 Convex lenses, 11, 19, 238 Cornea, 22 image formed on, 34 Crescent, myopic, 146 Crystalline lens, 33, 122, 148, 157 Cylindrical glasses, 32, 157, 173 Decentering lenseSj 194, 233 Detachment of retina in myopia, 148, 155 Deviation, angle of, 9 primary, 204 secondary, 204 Dioptre, 31, 259 Dioptric system, 31 Diplopia, 42, 213 Direct ophthalmoscopic examination, 53,73 Disc, Placido's, 181 shape of, in astigmatism, 172 Distant type, 56 Divergent strabismus, 146, 208, 215 Divergence, appearance of, 123, 203 latent, 230 Donders, 121, 188, 226 Educational treatment of squint, 219 Elasticity of capsule, 34 of lens. 34, 40 Elasticitj' of lens, diminution with age, 40, 187 Elongation of eyeball in myopia, 138 Emergent ray, 7 Emmetropia,' 26 punctum proximum in, 28, 35 punctum remotum in, 28, 35 Erect image, 53, 73 Erismann, 142 Eserine, 144, 197 Esophoria, 230 Exercises, orthoptic, 220 Exophoria, 230 Eye, 21 refracting media of, 22 refracting surfaces of, 22 Face, asymmetry of, in astigmatism, 53, 164 in hjqiermetropia, 53, 124 in myopia, 53 Far point, see punctum remotum, 28, 35 Focal length, 31, 259 interval, 160 points, 24 Focus, anterior, 10 conjugate, 4, 14, 136 principal, 3, 5, 9, 14, 24 Formation of images, 17 by the eye, 25 Fundus, 146 Glaucoma, 130, 195 Glasses, 237 biconcave, 11, 16, 20, 237 biconvex, 11, 19, 238 cylindrical, 32, 157, 173 orthoscopic, 194 periscopic, 241 prismatic, 241 spherical, 31 stenopaic, 157, 242 tinted, 243 Goggles, 243 H Hereditary tendency in myopia, 141 Herring's drop test, 223 Heterophoria, 229 Homatropine, 90 Homonymous images, 231 * Hypermetropia, 26, 59, 117 al)solute, 121 acquired, 128 amount of, 126 angle a in, 41, 123, 150, 201 axial, 122 causes of, 121 diagnosis of, 126 estimation of, 59, 126 facultative, 121 INDEX 271 Hvpermetropia, latent, 59, 127 length of eyeball in, 122 manifest 59, 126 oriorinal, 128 relative, 121 spectacles for, 129, 237 symptoms of, 124 tests for, 126 treatment for, 129, 237 Hjqjerme tropic astigmatism, simple, 161 compound, 161 Hyperphoria, 230 Images, crossed, 65 formation of, 17 homonymous, 65 in astigmatism, 76 in emmetropia, 75 in hypermetropia, 76 in myopia, 77 on cornea, 34 on lens, 34 projected, 67 real, 18 virtual, 3, 19 Indian Services, regulations for, 261 Indirect ophthalmoscopic examina- tion, 53, 66 Insufficiency of convergence, 229 test for, 232 Internal recti, 146, 217 Interval of Sturm, 160 focal, 160 Inverted image, 25 ophthalmoscopic images, 53, 66 Inversion of images by lenses, 18 by the eye, 25 Iris in accommodation, 34 in hypermetropia, 123 in myopia, 146 Irregular astigmatism, 157 Jackson, Dr., 99 Jaeger, test type, 61 Javal and Schiotz ophthalmometer, 176 Lachrymal apparatus, 126 Landolt, 154. Latent convergence, 229 deviation, 231 divergence, 230 hypermetropia, 59, 127 Length of eyeljall, 22 focal, 31 in hypermetropia, 122 in myopia, 138 Lens, crystalline, 35, 122, 148, 157 Lenses, 11, 31, 237, 260 Lenses, biconcave, 11, 16, 20, 238 Ijiconvex, 11, 19, 238 bifocal, 241 conjugate focus, 4, 14, 136 converging, 12 cvlindrical, 32, 157, 171 decentred, 194, 233 diverging, 12 foci of, 14, 16 images formed by, 17, 19 influence of, on the size of the image, 238 principal focus, 5, 14, 24 refraction by, 11 spherical, 31 table for presbyopia, 191 Light, artificial, 82, 150 Long sight, see presbyopia, 28, 1S7 M Macula, 29,'56, 147 Haddock's rod test, 46, 229 Manifest hyiJermetropia, 59, 126 Medium, refraction by, 7 Meniscus, 11 Metrical angle, 44 system of lenses, 31, 260 Microphthalmos, 123 Mirror, concave, for retinoscopy, 100 plane, for retinoscopy, 84 reflection by a plane, 2 from a concave, 3 from a convex, 6 Mixed astigmatism, 161 Monolateral strabismus, 206 Movements of mirror in retinoscopy, 85 Musca?. volitantes, 145 Muscle, ciliary, 34, 123, 145 iris, 34, 123, 146 Muscular asthenopia, 146, 228 Myopia, 27, 134 axial, 138 causes of, 138 determining causes, 141 diagnosis of, 148 estimation of degree, 148 formation of image in, 136 length of eyeball in, 138 ophthalmoscopic appearances in, 146 posterior staphyloma in, 138 progressive, 135 stationary, 152 statistics in, 142 symptoms of, 144 treatment for, 150 Mj^opic astigmatism, 161 crescent, 146 N Nagel on convergence, 42 Navy, regulations for, 261 Near point (punctum proximum), 28, 35 272 INDEX Negative, angle a, 203 Nerve, optic, in hypermetropia, 124! in myopia, 147 Nodal points, 24 Nordenson, statistics of, 178 Objective examination, 53 Operative treatment of squint, 223 Optics, Chap. I Optic axis, 201 disc in myopia, 146 nerve in hypermetropia, 124 in myopia, 147 Optical centre, 12 Ophthalmo-dynamometer, 47 Ophthalmological Congress, 31 Ophthalmometer of Javal and Schiotz, 176 Ophthalmoscope, 53, 66 direct examination, 66, 73 indirect examination, 66 Ophthalmoscopic appearances, 146 Optometer of Tweedy, 178 wire, 35 Oris'inal hjqjermetropia, 128 Orthoptic exercises, 219 Orthoscopic lenses, 194 Paralysis of accommodation, 196 causes of, 196 treatment of, 197 Pantoscopic spectacles, 241 Perimeter, 207 Periodic strabismus, 206 Periscopic lenses, 241 Pin-hole test, 54 Placido's disc, 181 Plane mirror, 84 Points, cardinal, 23 nodal, 23 principal, 23 Position in myopia, 150 Posterior staphyloma, 138 Pray, test letters of, 168 Presbyopia, 28, 187 age at which it commences, 189 definition of, 188 glasses for, 190 symptoms of, 190 table for, 191 treatment of, 190 Principal angle, 9 focus, 3, 5, 9, 14, 24 points, 24 Prismatic, spectacles, 242 Prisms, 8, 42, 240 abducting, 45, 242 adducting, 46, 242 to test convergence, 42 PriKmos])heres, 233 Progressive myopia, 135 Protectors, 243 Public works, regulations for, 262 Punctum proximum, 28, 35 in emmetropia, 35 in hypermetropia, 38 in myopia, 39, 137 remotum, 28, 35, 137 m emmetropia, 35 in hji)ermetropia, 38 in myopia, 137 Pupil in accommodation, 34 in hypermetropia, 123 in myopia, 146 R Railways, regulations for, 263 Range of accommodation, 37 convei-gence, 44 Rays, 1 incident, 7 emergent, 7 Recti, internal, 146, 217 Reflection, 2 by concave surface, 3 l)y convex surface, 6 by plane surface, 2 Refraction, 6, 22 diminution of, 128, 146 estimation of, 53 index of, 7 by lenses, 11 by i)]ane surface, 6 by prisms, 8 by spherical surface, 9 liy the eye, 22 Regulations for army, 261 for civil service, 261 for English railways, 263 for Indian medical service, 261 for navy, 261 for ijul/lic works, 262 Regular astigmatism, 157 Relative accommodation, 40, 50 convergence, 50 Remotum punctum, 2S, 35, 137 in emmetropia, 35 in hypermetropia, 38 in myopia, 137 Retina, 21, 25, 147 Retinal asthenopia, 234 Retinal image, size of, in hyper- metroi)ia, 69 in myopia, 70 Retinoscopy, 52, 66, 82 cases of, 102 in astigmatism, 96, 173 in liypernietropia, 128 in niy()])ia, 149 mirror for, 84 o])li(iue movements in, 94 plane mirror in, 81 rate of movement in, 93 Rods and cones, 56, Iti Rod test, 16, 229 INDEX 273 Scale for testing deviation, 232 Scheffler, 194 Scheiner, 36, 64 Scotomata, 144 Secondaiy changes in myopia, 147 Shadows in retinoscopy, 82 Shadow test, 82 Short sight (myopia), 27, 134 Snellen, 56 Spasm of accommodation, 197 causes of, 198 symptoms of, 198 treatment for, 199 Spectacles (see also glasses), 237 for aphakia, 133 for astigmatism, 176, 238 for hypermetropia, 129, 237 for myopia, 152, 237 for presbyopia, 190 for strabismus, 217 Simple hypermetropic astigmatism, 161 myopic astigmatism, 161 Sqmnt, see strabismus, 200 Staphyloma, posterior, 138 Stationary myopia, 152 Statistics in myopia, 142 Stenopaic slit, 181 glasses, 157, 242 Stereoscope, 221 Strabismometer, 206 Strabismus, 200 alternating, 203 angle of, 207 apparent, 200 concomitant, 205 constant, 206 convergent, 124, 208 divergent, 146, 208, 215 monolateral, 20G paralytic, 200 periodic, 206 real, 201 treatment of, 217 Sturm, interval of, 160 Surfaces, refracting, of the eye, 22 Sj^mptoms of astheiiopia, 225" astigmatism, 164 hypermetropia, 124 Symptoms of myopia, 144 presbyopia, 190 Table for presbj^opia, 191 of amplitude of accommodation, 41 of angles of convergence, 49 of inches and dioptres, 260 of length of axial line in hj^per- metropia, 122 in myopia, 138 Tenotomy, 223 Test for aphakia, 132 clock-face, 170 fan, 171 letters. Fray's, 168 pin-hole, 54 tj^jes, for near vision, 265 Jaeger, 61 Snellen, 56 Treatment of asthenopia, 227, 233, 236 astigmatism, 176, 238 hyiDermetropia, 129, 237 myopia, 150, 237 paralysis of accommodation, 197 presbyopia, 190 spasm of accommodation, 199 strabismus, 217 Tweedy's optometer, 178 Virtual focus, 5 images, 3, 19 Vision, acuteness of, 29, 54 binocular, 211, 219 in astigmatism, 165 in hypermetropia, 126 in myopia, 148 Visual angle, 29, 56 axis, 201 Vitreous, 148 w Worth's amblyoscope, 220 Yellow spot, 29, 56, 147 Young, 157 PRINTED BY ADLARD AND SON, LONDON AND DORKING. 18 1 a 4 4 1 g .^ i^S£UUk lETURN OPTOMETRY LIBRARY rO— #^ 490 Minor Hall 642-1020 _OAN PERIOD 1 NOM.P li.S 2 3 1 5 ( 5 ALL BOOKS MAY BE RECALLED AFTER 7 DAYS RENEWALS MAY BE REQUESTED BY PHONE DUE AS STAMPED BELOW UNIVERSITY OF CALIFORNIA, BERKELEY ORM NO. DD 23, 2.5m 12/80 BERKELEY, CA 94720 (g)s M (Ft: %-^ BERKELEY LIBRARIES A 'a. %