THE OPHTHALMOSCOPE By the same Author. THE REFRACTION OF THE EYE. Eighth Edition, with 100 illustrations, crown 8vo., Qs. THE OPHTHALMOSCOPE A MANUAL FOR STUDENTS BY aiTSTAVTJS HARTRIDaE, F.R.C.S. SURGEON TO TUE HOVAL WKSTMIXSTKR OFHTUALMIC UdSPITAL; OPHTHALMIC SfllGEOX AND LECTURER ON OPUTUAl.MIC SL'KGKRY TO THK WESTMINSTER HOSPITAL; OPHTHALMIC 3UBGEO.N TO ST. BARTHOLOMEW'S HOSPITAL, CHATHAM; COXSfLTIXG OPHTHALMIC SURGEON TO ST. GEOUGK's DISPENSARY, HANOVF.R sqUABK, ETC. WITH SIXTY.EIGHI ILLUSTR.\.TI0X3 AND FOUR PLA.TES THIRD EDITION LONDON J. & A. CHURCHILL 7, GREAT MARLBOROUGH STRKET 1897 OPTO- -TRY UB.xARY PREFACE TO THIRD EDITION In preparing a third edition of ' The Ophthalmo- scope ^ for the press_, every page has been carefully revised^ and a few additions made which will, I trust, enhance the value of the book as a guide to ophthalmoscopic work. G. H. 12, WlMPOLE StEEET, \V.; Jul I/, 1897. M675i:i8 PREFACE The subject of this volume is one that has made great progress during the last few years, not only in the mechanism of the instrument, but also in the methods of using it to the best advantage. In the present day the ophthalmoscope is almost as neces- sary to the physician as it is to the ophthalmic sur- geon, since many serious general diseases may first be detected by changes taking place in the fundus, frequently without any subjective symptoms; thus the importance and usefulness of the instrument is greatly extended. In introducing this small work to the profession, I do so in the hope that it may be found useful not only to the ophthalmic students who, in London and other large medical schools, have the advantage of practical demonstrations on the subject, but also to the large class of practitioners whose opportunities of seeing cases are few and far between, and who may desire to learn the use of the ophthalmoscope when practical instruction is out of their reach. Vlll PHEFACK While hoping that the description given will be found sufficiently clear and elementary to enable the most inexperienced to understand it^ I trust that even the advanced student may find some help and instruction. The arrangement of the book is simple and syste- matic, and an endeavour has been made to keep it small, so that it may be conveniently carried in the pocket for reference in the out-patient room ; and this, perhaps, constitutes one of its chief advantages. The work is profusely illustrated with woodcuts — a matter of some importance, as it is almost impossible to make the subject clear without them, especially to those of my readers who may not have access to an instructor ; illustrations, although in many instances conveying only a somewhat imperfect idea, certainly impress the subject on the studeut^s mind. A slight knowledge of optics is essential, and therefore the first chapter from my work on the ' Refraction of the Eye ^ is reproduced, and several of the woodcuts from that book are doing duty again. a. H. 65, Geeen Street, Park Lane, W.; August, 1891. ] I ^-pvy nf the Alame(Ta ^i optometrists CONTENTS CHAPTER I PAGE Optical Principles involved in the Use of the Ophthalmoscope . . . . 1 CHAPTER II The Ophthalmoscope . . . .21 CHAPTER III Methods of Examination . . .32 CHAPTER IV The Appearances of the Normal Fundus . 81 CHAPTER V The Cornea, Anterior Chamber, Iris, and Lens . 92 h CONTENTS CHAPTER VI PAGE The Vitreous . . . .99 CHAPTER VII The Choroid ..... 105 CHAPTER VIII The Retina . . . . .116 CHAPTER IX The Optic Nerve .... 136 Appendix . . . . .151 LIST OF ILLUSTRATIONS No. PAGE 1. Reflection by a plane surface . . . .2 2. Virtual image formed by a plane mirror . . 3 'S. Reflection by a concave surface . . .4 4. Ditto ditto . . . .5 5. Reflection by a convex surface . . .6 6. Refraction by a plane surface . . .7 7. Refraction by a prism . . .8 8. Ditto ditto . . . .8 9. Refraction by a spherical surface . . 9 10. Ditto ditto . .10 11. Formation of a convex lens . . . .11 12. Different forms of lenses . . . .12 13. Refraction of secondary axes by a biconvex lens . . 12 14. Refraction of parallel rays by a convex lens . . 13 15. Ditto ditto . . .14 16. Properties of a biconvex lens . . . .14 17. Ditto ditto . . . .15 18. Properties of a biconcave lens . , .16 19. Refraction of parallel rays by a concave lens . . 16 20. Formation of an inverted image . . .17 21. Real inverted image formed by a convex lens . . 18 22. Virtual image formed by a convex lens , . .19 23. Virtual image formed by a concave lens . . 19 24. Light entering the eye and returning by tlie same path . 21 xu LIST OF ILLUSTRATIONS No. 25. Light returning from hypermetropic eye 20. Liglit returning from inyo})ic eye 27. Hehnholtz's first ophthalmoscope 28. Tlie ophthalmoscope 29. Morton's refracting ophthalmoscope 30. Concave mirror for retinoscopy 31. Frost's artificial eye 32. The ohlique illumination 33. Oblique illumination with magnifying glass 34. Rays coming from emmetropic eye 35. Rays coming from hypermetropic eye . 30. Rays coming from myopic eye 37. Position for the indirect examination . 38. Formation of image in indii*ect examination 39. Indirect examination in emmetropia 40. Indirect examination in hypermetropia . 41. Indirect examination in myopia 42. Size of the image in emmetropia 43. Size of the image in hypermetropia 44. Size of the image in hypermetropia 45. Diagram showing the real size^of the disc and the images in the indirect and direct methods 46. Position for the direct examination 47. Formation of image in direct examination 48. Size of image on retina in emmetropia . 49. Determination of size of image by coin . 50. Method of estimating enlargement 51. Direct examination in emmetropia 52. Direct examination in hypermetropia . 53. Direct examination in myopia 54. Method of retinoscopy 55. Shadow in retinoscopy 56. Movement of shadow 57. Formation of image in myopia 58. Formation of image in hypermetropia . 59. Oblique shadows 60. Method of recording the chief meridians 61. Diagram of the normal disc and nerve . PAGE 22 23 24 25 29 31 34 30 37 38 39 39 42 43 46 46 47 48 48 49 the size of LIST OF ILLUSTRATIONS xni No. 62. Position of opacities 63. Movements of vitreous opacit 64. Physiological cup 65. Atrophic cup 66. Glaucoma cup 67. Parallax . 68. Papillitis . es PAGE 97 102 140 141 142 143 145 Coloured Plates (between pages 82 and 83) : I. — Fig. 1. Fundus of a child of medium complexion. Left eye. Direct. Fig. 2. Albino. Right eye. Indirect. II. — Fig. 1. Dark fundus. Left eye. Direct. Fig. 2. Choroid tigre. Right eye. Indirect. III. — Fig. 1. Choroiditis in the exudative stage' Fie:. 2. Disseminated choroiditis . ( opposite Fig. 3. Senile choroidal change Fig. 4. Coloboma of the choroid IV. — Fig. 1. Retinitis pigmentosa . Fig. 2. Thrombosis of central vein Fig. 3. Atrophy of the optic disc Fig. 4. Albuminuric retinitis . page 106 opposite page 118 THE OPHTHALMOSCOPE CHAPTER I OPTICAL PRINCIPLES INVOLVED IN THE USE OF THE OPHTHALMOSCOPE Light is propagated from a luminous point in every plane and in all directions in straight lines ; these lines of direction 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 diver- gence 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. 1 )i THE OrHTHALMOSCOPE A ray of liglit meeting with a body, may be absorbed; reflected ; or if it is able to pass tlirough this body, it may be refracted. Reflection Reflection by 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 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 ; and 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 c', a distance behind the EEFLECTION mirror equal to the distance of the candle in front of it, and an observer's eye placed at E would receive the rays from c as if they came from c'. Fio. 2. M. The mirror, c. The candle, c'. The virtual image of the candle. E. The eye of the observer receiving rays from mirror. The image of the candle so formed by a plane mirror is called a virtual image. Reflection by 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 reflected as convergent rays, which meet on the axis at a point (f, Fig. 3} called the princijml focus j about equally distant from the mirror and its centre c. The distance of the principal focus from the mirror is called the focal length of the mirror. 4 TIIR OPITTHALMOSCOPE 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 point is at the centre of the concavity of the mirror (c), the rays return along the same lines, so that the point is its own image. If the point be at a the focus will be at a, and it Fro. 3. will be obvious that if the 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 conjugate focus is between the principal focus and the centre. If the luminous point is between the principal focus and the centre, then its conjugate is beyond the centre; so that the nearer the luminous point approaches the principal focus, the greater is the distance at which the reflected rays meet. If the point be nearer the mirror than (f) the prin- cipal focus, the rays will be reflected as divergent, RKFLKCTION and will therefore never meet; if, however, we con- tinue these diverging rays backwards, they will unite at a point (h) behind the mirror; this point is called Fia. 4. the virtual focus, and an observer situated in 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 oneself in a concave mirror. If the mirror be placed nearer than its principal focus, then one sees an enlarged erect virtual image, which increases slightly in size as the mirror is made to recede ; this image becomes confused and disappears as the principal focus of the mirror is reached ; on moving the mirror still further away (that is, beyond its focus) one obtains an enlarged inverted image, which diminishes as the mirror is still further withdrawn. 6 THE OPHTHALMOSCOPE Reflection hi/ a Coiivejc Surface Parallel rays falling on sucli a surface become divergent, hence never meet; but if the diverging rays thus formed are carried backwards by Hups, then an imaginary image is formed which is called negative, and at a point called the principal focus (f). Foci of convex mirrors are virtual; and the image, whatever the position of the object, is always virtual, erect, and smaller than the object. Fig. 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 medium into another of a different density is refracted, unless the ray fall perpendicular to the surface separating' the two media, when it continues its course without undergoing any refraction (Fig. 6, h k). KE FRACTION 7 A ray is called incident before passing into the second medium, emergent after it has penetrated 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 perpen- dicular 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). Reflection accompanies refraction, the ray dividing itself at the point of incidence into a refracted portion (b c) and a reflected portion (b e). FiG.G. 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 has an index 8 Tiiy OrUTHALMOSCOPli; 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 refracted twice, as at b and c (Fig. G), 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 perpeudi. cular to more than one surface at a time. Therefore every ray falling on a prism must undergo refraction, and the deviation is always towards the base of the prism. The rehitive 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 REFRACTION 9 at H, is called the angle of deviation, and is about half the size of the i^incipal angle formed at A by the two sides of the prism. Refraction hy a Spherical Surface Parallel rays passing through such a surface, sepa- rating media of different densities, 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 Fig. 9. angle, and will therefore be refracted towards the perpendicular c J, meeting the ray d at F ; ray E will be refracted in the same way, likewise all rays parallel in medium M. The point f where these rays meet is the principal focus, and the distance between the principal focus and the curved surface is spoken of as the principal focal distance. 10 THE OPHTHALMOSCOPE Rays proceeding from f, will be parallel in m after passing throngli the refracting surface. Kays parallel in medium n will focus at f', which is called the anterior 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. Fro. 10. and will therefore never meet ; by continuing these rays backwards they will meet at L, so that the conju- gate focus of will be at l, on the same side as the focus ; and the conjugate focus will in this case be spoken of as negative. 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 Fio. 11. opposite surfaces, one or both of which may be segments of a sphere ; they are then called spherical lenses, of which there are six varieties : 1. Plano-convex, the segments 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 air) through a lens, is deviated towards the thickest part, therefore the first three lenses, which are 12 THK OPHTHALMOSCOPE thickest at the centre, are called converging j and the others, which are thickest at the borders, diverging. Fio. 12. A line passing througli the centre of the lens (called the optical centre) at right angles to the surfaces of the lens is termed the principal accis, 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 de- viation, but emerge in the same direction as they entered ; the deviation in thin lenses is so slight that they are usually assumed to pass through in a straight line; these are called secondary axes (Fig. 13). Fig. 13. Lenses with secondary axes undergoing slight deviation. BICONVEX LENSES 13 Parallel rays falling on a biconvex lens are rendered convergent ; thus in Fig. 14 the rays A, b^ c, strike the surface of the lens (l) at the points d, b, f; the centre Fig. 14. ray (b) falls on the lens at E perpendicular to its sur- face, and therefore passes through in a straight line; it also emerges from the lens at right 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) which is of greater den- sity, 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 lens, striking its opposite surface obliquely at o, and as it leaves the lens, enters the rarer medium (air), being deflected from the per- pendicular NO; it 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 (a) beyond the lens ; this point is called the principal 14 THE OrHTIlALMOSCOPE focus, and the distance of this focus from the lens is called the focal length of the lens. Fio. 15. 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 principal focus (f. Fig. 16) meet at a distance beyond (p') the Fig. 1(3. 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). If the luminous point (d) be between the lens and BICONVEX LENSES 15 the principal focus (f), then the rays will issue from the lens divergent, though less so than before enter- ing ; and if we prolong them backwards they will meet at a point (h) further from the lens than the point D ; H will therefore be the virtual focus of b, and the con- jugate focus of D may be spoken of as negative. Biconvex lenses have therefore two principal foci, p 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) pass- ing through the optical centre (c) of the lens (r.) is a Fig. 17. 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 always that the inclination of this secondary axis is not too great, when the focus would become imperfect from much spherical aberration. 16 THE OriTTTTALMOSCOPE In biconcave lenses tlie 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). Fro. IS. 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- FiG. 19. site side of the lens perpendicular to its surface, it continues in a straight line; the ray A strikes the lens obliquely at d and is refracted towards the perpendicular, shown by the dotted line a H; the ray after deviation passes through the lens to k, where, FORMATION OV IMAGKS 17 on entering the medium of less density obliquely, it is refracted from the perpendicular o p, in the direc- tion K M ; the same takes place with 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. Place on one side of a screen having a small perforation, a candle, and on the other side a sheet of white cardboard at some distance from the screen, to receive the image formed; rays diverge from the candle in all directions, most of those falling on the screen are intercepted by it, but some few rays pass through the perforation and form an image of the candle on the cardboard, the image being inverted because the rays cross each other at the orifice ; it can further be shown that when the candle Fro. 20. -e) and cardboard are equally distant from the perfo- rated screen, the candle flame and its image will be of the same size. If the cardboard be moved further from the perforation the image is enlarged, if it be moved nearer it is diminished ; if we make a dozen more perforations in the screen, a dozen more images will be formed on the cardboard, if a hundred, then 2 18 THE OPHTHALMOSCOPE a liuiidred ; but if the apertures come 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 Fig. 21. Ktiil inverted image formed by convex lens. two lines passing through the optical centre (c) of a biconvex lens, the image of A will be formed some- where on this line A c 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 5 a is a small inverted image 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 image would be formed at the same point on the opposite side of the lens, of the same size as the object, and in- verted. FORMATION OP IMAGES ]9 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 the focus, then the rays which diverge from the candle will, after passing through the convex lens, be still divergent, so that no image is formed ; an eye placed at e would receive the rays from a b as if they Fig. 22. Virtual image formed by convex lens. came from ah; a 6 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 Fig. 23. Virtual image formed by concave lens. virtual, erect, and smaller than the object; let A b be a candle, and f the principal focus of a biconcave .20 THE OPHTHALMOSCOPE lens ; draw from A b two Hues 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 b, 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. TFIR OPHTHAOIOSCOPE 21 CHAPTER II THE OPHTHALMOSCOPE When an eye is looked at, tlie pupil appears black altliough the media are perfectly transparent ; this is because the rays entering the eye return to the point from which they emanate, and therefore, unless the observing eye can be placed in the path of returning rays, none of them will pass through the observer's pupil, and so no illumination will be seen. Fig 24. In Fig. 24 rays will be seen entering the eye from candle a, and since the refractive system of the eye is exactly adjusted for the candle flame, the rays returning from the eye will traverse the same path as those entering it; if the eye be not adapted for the candle flame, then the rays will return towards the source of light. 22 THE OrHTflALMOSCOPE In the albino, as in the white rabbit, the pupil may appear illuminated ; this is due to the transparency of the iris, so that the returning rays cover a larger area than is the case when passing through an ordinary pupil, then some of the outer rays may pass through the observer's pupil, if placed nearly in the line of light from which the observed eye receives its rays : that this is the correct explanation can easily be proved by covering up the part corresponding to the iris by an opaque diaphragm, when the pupil will at once appear black, as in the normal eye. In hyper- m'^tropia and myopia with a dilated pupil, one fre- quently gets a slight fundus illumination. This is illustrated in the following figures. In hypermetropin, Fig, 25, the returning rays, Fio. 25. instead of being parallel as in emmetropia, diverge somewhat, so that the observing eye placed at b would receive some of the returning rays. In myopia. Fig. 26, the returning rays converge, cross, and diverge, so that the eye placed at b would receive some illumination. The ophthalmoscope is a contrivance which enables the observing eye to be placed in the path of the re- THE OPHTHALMOSCOPE 23 turning rays, and consists of a reflector with a hole in the centre. Fio. 20. Although the inveutiou of the o])litlialmoscope is of recent date, it had long been known that the eye was illuminated by rays entering the globe, and it was thought that these rays were entirely absorbed by the pigment contained in the retina and choroid, but it was afterwards pointed out that some parts, such as the disc, must even then reflect some light, and that the apparent blackness of the pupil must be due to the optical law "that rays of light leaving the eye, return towards the source of illumi' nation'^ so that unless the eye of the observer can be placed in the path of the returning rays, no illumina- tion can be obtained. Briicke was very nearly solving the question by placing a tube through the flame of a candle, which enabled him to catch some of the re- turning rays of light; but it remained for Helmholtz to overcome the difficulty by producing his first oph- thalmoscope. Helmholtz's ophthalmoscope, which he introduced in 1851, was composed of three pieces of plane glass. Fig. 27. Rays reflected from a light A, are projected into the eye b, by the mirror m, the light returning from 24 THE OrnTHALMOSCOrE the observed eye by the same path will fall on the glass Mj a part is reflected to a, and a part passes Via. 27. through the glass towards c ; an observing eye placed behind the mirror will, therefore, receive some of the returning rays. Ophthalmoscopes have undergone numerous modi- fications, and the iustrument with which we now work consists of a silvered concave glass mirror, with a central perforation. In Fig. 28, divergent rays from a candle c, falling on the mirror m, are rendered convergent, and when reflected into the eye e, cross in the vitreous and light up the fundus between the points G and d ; if point H of this illuminated area be taken, the rays will (in the emmetropic eye) issue parallel, and pass- ing through the sight-hole of the mirror, will enter the observing eye A, forming on the retina at h' an image of h. The amount of fundus illumination obtained will necessarily depend upon the source and intensity of •VUK OrnTITALMOSCOPE 25 the liglit^ the concavity of tlic mirror used, tlie dis- tance from the eye at which the examination is made, and the size of the pupil in the observed eye. Modern ophthalmoscopes are fitted with a series of 26 THE OrnTHALMOSCOrE lenses, which can bo revolved in front of the sight- hole ; these are known as refracting ophthalmoscopes. Many good ones have been devised, varying but slightly in some or other minor particulars. The essential points of a thoroughly complete ophthalmoscope are, that it should be supplied with three mirrors, — a small concave, a large concave, and a plane one ; together with a fairly complete set of lenses, which can be brought in front of the sight-hole of the instrument as occasion requires. (1) The small concave mirror is for the direct examination; it should have a focus of about 7*5 cm., so that light reflected from it will enter the eye as convergent rays. The sight-hole should not be larger than 2| mm., because only that part of the mirror which immediately surrounds the aperture is available in the direct examination, and should the sight-hole be larger than tlie pupil, then no fundus illumination will be obtained. The small mirror may be conveniently tilted about 25° ; this allows the ophthalmoscope to be held perfectly straight while the light is reflected into the observed eye, enabling one to look through the lens which may be behind the sight-hole at right angles to its surfaces. With the old-fashioned mirror the ophthal- moscope itself had to be tilted towards the light, and with it, of course, the lenses, so that they were looked through obliquely, and thus the strength of the lens was increased and some astigmatism pro- duced ; hence the estimation of the refraction by the direct method was liable to be inaccurate. The THE OPHTHALMOSCOPE 27 disadvantage of some tilted mirrors is the distance that intervenes between the two sides of the sight- hole, technically called "tunnelling;" this tunnelling somewhat diminishes the size of the field seen, hut, what is more important, prevents the eye of tlie examiner from approaching sufficiently near that of the person examined ; and it may be stated as an axiom, that the nearer the observing eye can approach the ohservedj the more accurate will he the estimation of the refraction. (2) The large concave mirror is for the indirect method of examination and for retinoscopy ; it should have a focal length of 25 cm., so that rays from a light situated 25 cm. from the mirror will be reflected parallel ; when the light is further off than 25 cm. then the rays will be slightly convergent ; this mirror may possess an aperture of 3 or 3 J mm. (3) The plane mirror is useful for the examination of the vitreous, and in some cases of high mjopia; with this mirror rays of light coming from a lamp at a finite distance, will be reflected into the eye as divergent rays. The plane mirror may also be used for retinoscopy. Tlie ophthalmoscope should be supplied with a set of lenses, which can in turn be brought behind the sight-hole by means of a finger wheel ; this wheel should be so made and placed, that it may be rotated easily while the instrument is in position without losing sight of the fundus. The lenses may be some- what as follows: a convex series -f5 D., +1 D., increasing by one dioptre up to +10 D. ; and a 28 TFIE OPTTTHALMOSCOPE concave series —'5 D., —1 D., increasing by one dioptre up to —12 D., and then by two dioptres up to — 20 D. Sometimes a higher glass may be re- quired; these may be supplied on a separate disc; the lenses in this disc may, by combination with the other lenses, form a very large series. The lenses should not be less than 6 mm. in diameter, otherwisv_ they are difficult to centre pro- perly, and cannot be easily cleaned — a point of some importance ; they may occasionally be used for the subjective test of estimating the visual acuteness, should the box of trial lenses not be at hand. It will be sufficient here to describe and to figure one of the ophthalmoscopes in general use, though numerous other good instruments will be found in this country and abroad. Morton's ophthalmoscope, shown in Fig. 29, is a modification of an instrument introduced by Mr Couper; it contains a series of twenty-nine lenses in metal rings, and one metal ring without a glass ; these run round a continuous channel, and are so arranged that each can be brought successively in front of the sight-hole by means of a driving wheel. When no lens is required, then the empty ring occupies the sight-hole ; these lenses touch each other sideways, but are not fixed in any way ; on the spindle that carries the driving wheel is another wheel with teeth, which propel the lenses round the instrument ; a spring and notch attached to the driving wheel centres each lens as it arrives at the sight-hole. The strength of the glass before the THE OrilTHALMOSCOrE 29 sight-hole is recorded by an index wheel, which, being geared to the driving wheel, keeps pace with it, and therefore with the lens series. The minus glasses are contained in white rings, and are in- dicated by white numbers ; the convex glasses are in red rings and have red numbers. Fio. 29. This series of lenses is usually sufficient for most ordinary purposes, but occasionally other lenses are required and are provided on a special disc. Some- times a strong convex glass is required for the examination of the cornea or lens; at other times a strong concave lens is necessary for a case of high myopia. This separate disc, therefore, has a + 20 D. and a — 50 D. so placed that they can be instantly put in front of, or removed away from, the sight-hole 30 THE OPHTHALMOSCOPE without rotating the whole series of lenses. On this same disc are also a + '5 D. and a — 10 D. : the former of these enables one to estimate to within half a dioptre in special cases ; and the latter, by use in conjunction with other concave lenses contained in the series, gives us from — ID. to— 20D., with in- tervals of one dioptre, and — 20 D. to — 30 D., with intervals of two dioptres. This disc is well shown in the illustration. The instrument is supplied with three mirrors ; a large concave one of 25 cm. focus, and a small tilted one of 7' 5 cm. focus ; these are fixed by a pivot, so that either can be turned in front of the sight-hole as occasion requires : the large concave mirror can be replaced, when necessary, by a plane one. The movement in this ophthalmoscope is a great improvement over the method formerly employed, of placing the lenses in a revolving wheel. The credit of this ingenious invention is due to Mr Couper, who was much assisted by Mr Paxton, of the firm of Curry and Paxton. M. Parent, of Paris, has recently brought out a very beautiful instrument, combining the advantages of all the recent ophthalmoscopes, and fitted with a series of cylindrical lenses in addition to the ordinary spherical series. M. Parent strongly advocates the use of cylindrical lenses for the estimation of the refraction in cases of astigmatism by the direct method. Lang's ophthalmoscope is also used a good deal in this country, and is a very convenient instrument. THE OPHTHALMOSCOPE 31 A useful mirror to carry in the waistcoat pocket is that known by the name of Galizowski, Fig. 30; it is convenient for the indirect examination and for retinoscopy; its focal length is 25 cm. If the handle be made to double over the face, no case will be required to protect it. Fig. 30. The student also requires a large biconvex lens (+13 D.), which is necessary for focal illumination and the indirect method of examination. A magnifying glass made of flint and crown glass (achromatic), and having a focus of 1*5 cm., is very convenient for examining the cornea, iris, and lens. Demonstrating ophthalmoscopes are also made, but need not be described. 32 THE OrilTIFALMOSCOPE CHAPTER III METHODS OP EXAMINATION The incandescent electric li"lit of sixteen-candle power contained in a ground-glass sliade, and the ordinary gas Argand burner, are both suitable lights for ophthahnoscope work ; they should be arranged on a bracket which is capable of up and down as well as lateral movement^ while the arm of the bracket should be sufficiently long to allow the light to be placed on either side of the patient without his having to move. When these lights are not to be obtained_, an ordinary oil lamp may be used, or even a candle. The patient should be seated on a chair, while the observer may conveniently use a music stool, the height of which can be altered as occasion requires; some observers prefer to stand when making an examination. A dark room is also an advantage. In many cases it is necessary to dilate the pupil with a mydriatic; one that acts quickly and fully, and the effect of which soon passes off, is to be pre- ferred. The most convenient combination is — 1^ Homatropinse Hydrobromatis, gr. iv. Cocainse Hydrocliloratis, gr. x. Acidi Salicjlici, gr. ij. Aquae Destillatse, ^j. Gr. guttse. METHODS OF EXAMINATION 33 A drop of this solution produces full dilatation in a very sliorfc time, and the effect passes off in two or three hours. Of course in many cases no mydriatic is required, but when a thorough examination is necessary it is a great advantage to examine the eye through a well-dilated pupil ; this is especially the case when some changes have been detected in the fundus, and further information is required by a more searching examination, carried out under the most favorable conditions. But the student should learn not to rely too much upon these minor aids, but accustom himself to examine the fundus in various positions and uuder different surroundings, with or without a mydriatic. It is recommended that every opportunity be taken to make repeated ophthalmoscopic examinations, and where a large number of patients is not to be met with. Frost's artificial eye (Fig. 31 ) will be of great service in enabling the student to acquire the neces- sary associated movements, as well as to understand and appreciate many points of importance with regard to the size and formation of the images in the various conditions of refraction. The first thing for a beginner to do is to familiarise himself with the variations of the normal fundus. Very great differences are met with ; as numerous almost as the various shades of hair found in tlie human race. Besides, even in those cases where the visual acuteness is normal, and no symptouis indicative of disease are present, gross changes may be found, or 3 34 THE OrnxnALMOSCOPE congenital peculiarities may exist wliicli ought not to be missed. Fig. 31. Generally, young ophthalmologists disdain to look at a normal fundus, caring only for pathological con- ditions ; it would be much better for every one to look at a certain number of normal fundi before pass- ing on to the various diseases. Usually the reverse is the method of procedure ; abnormal cases are looked at first, varied only very occasionally by a normal case. METHODS OP EXAMINATION 35 In undertaking an ophthalmoscopic examination, it should be conducted quietly and without hurry; a number of students standing round anxious to look at the same case is not conducive to a thorough exa- mination. A regular routine is absolutely necessaiy ; accuracy and confidence are thereby attained. First, the cornea, iris, and lens must be examined hy focal ilhimination ; then the large concave mirror is used at a distance of about two thirds of a metre, and may give an indication of the refraction of the eye, and allow the condition of the vitreous to be ascertained ; this should be followed by the indirect method of exa- mination, which enables that part of the fundus which is within reach of the ophthalmoscope to be readily scanned ; the examination is completed by the direct method, which gives an image magnified some eighteen diameters, and allows minute changes, not visible by the indirect method, to be detected, while at the same time it allows of an estimate of the refraction beins* made. When any change has been detected in the cornea, iris, or lens, this may conveniently be examined with the oblique mirror, having a +20 D. behind the sight-hole of the ophthalmoscope, and approaching close to the patient as with the direct method ; when the iris and lens are examined in this way, a some- what weaker glass is necessary, + IG D. 36 THE OPnTITALMOSCOrE Focal Illumination Tlie patient being seated opposite a good artificial light, the observer takes up the large biconvex lens of 13 D. between the thumb and forefinger of one hand, and concentrates the light obliquely on the cornea, iris, and crystalline lens successiv^ely. By this means opacities and irregularities of the cornea, affections of the iris, opacities of the lens, and even disease involving the anterior part of the vitreous may be detected. To examine every part of the lens and the vitreous, ifc is absolutely necessary that the pupil be dilated with a mydriatic. Then, tumours of the ciliary region, vitreous opacities, sparkling synchj^sis, and even detached retina, may be seen. Fig. 32. By varying the position of the light and of the eye ider examination, every part can be thoroughly in- METHODS OF EXAMINATION 37 spected. Wlieu the deeper parts of tlie lens and the vitreous are examined, the light must be thrown into the eye in an almost perpendicular manner, as shown in Fig. 32. Fio. 33. One great advantage of the focal illumination is that everything is seen of its real colour and in its true position. This examination may with advantage be supple- mented by using a second lens before the eye as a magnifying glass, oi", still better, the small achro- matic glass made for that purpose and referred to on p. 31. (Fig. 33.) 38 THE OPHTHALMOSCOPE The Large Concave Mirror at a Distance The light shoukl now bo pbicod on one side of the patient, on a level with the head and slightly behind, so that no direct light falls on his face ; the observer, sitting opposite^ places before his eye the large concave mirror, and at a distance of about two thirds of a metre reflects the light into the eye he wishes to exa- mine; usually a red fundus reflex is obtained, but no details will be visible; should any of the vessels or a part of the disc be seen, then we shall know that the eye under examination is ametropic. Because in emmetropia (Fig. 34), the rajs which come from the two extremities of the disc (a b) emerge as two sets of parallel Fig. 34. rajs in the same direction as the rajs a c, b d, which, having passed through the nodal point, undergo no refraction. These two sets of rajs soon diverge, leaving a space between them, so that an observer, unless he be quite close to the observed eje, is unable to bring these raj's to a focus on his retina, and therefore at a distance from the eje the observer sees onlj a diffused and blurred image. In lijpermetropiu (Fig. 35) the rays from the two points (a b) emerge from the eje in two sets of diverging rajs, in the same direction as the rajs a c, b d, which undergo no refraction. These THE CONCAVE MIRROR AT A DISTANCE 39 divergin<^ rays liave tlie appearance oE coining from two points {a b) behind tlie eye, where an erect imaginary image is formed (a 6). Fig. 35. Here the observer at a distance sees a clear, erect image, which is formed behind the eye. In myopia (Fig. 36), the rays from the two points (a b) emerge Fig. 36. as two converging sets of rays, whicli meet at a h on tlieir secon- dary 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 higlier the myopia the nearer to the e^'e will tliis 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 tlie same direction, tlie case would be one of hypermetropia, the image formed being an erect one. 40 THE OrHTOALMOSCOPE Had tlic vessels moved in the opposite direction to the observer's head the case would be one of myopia, the image being an inverted one formed in the air in front of the eye. If the vessels of one meridian only are visible, then we have a case of astigmatism, h^^permetropic if moving in the same, and myopic if moving in tlie 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. Should no f Lindas reflex be obtained when the light is thus properly reflected into the eye^ the case may be one of haemorrhage into the vitreous, or other serious lesion : but the reflex may be good, and yet it may appear irregular by the presence of black spots here or there; in this case probably some opacity exists in the cornea, lens, or vitreous, which interferes with the returning rays of light, and so appears black, whatever the real colour of the opacity may be; and if nothing was seen by careful inspection with focal illumination, the opacity is in all proba- bility situated in the vitreous ; this is certainly the case if the opacity is floating. The movements of these floating opacities will be more conspicuous if the patient be directed to first look upwards, then downwards, and finally straight in front of him ; the rate of movement will be a guide as to the consis- tency of the vitreous, and the direction of their move- ments will depend upon their position in the vitreous, whether they are in front of or behind the centre of rotation of the eyeball — a point situated in the normal THE INDIRECT METHOD 41 emmetropic eye 9*8 mm. in front of the retina (Fig. 63). Sometimes the vitreous opacities may be so thin that some of the returning liglit may pass through them ; they will then appear more or less white or pink ; occasionally light may be reflected from the surface of the opacity, and then it will appear white and more or less glistening, this is the case when cholesterine or ty rosin crystals are present. The Indirect Method The examination with the large concave mirror at a distance, which has taken some time to describe, occupies only a very short time, and we pass on without a break to the indirect examination. With the large concave mirror still before the observer's eye, and lighting up the eye under examination, the biconvex lens which was used for the focal illumina- tion is held up between the mirror and the patient's eye at about its focal distance from the latter ; an inverted image of the fundus will thus be obtained, magnified about five diameters : the amount of magnification depends upon the strength of the objective used ; the stronger the lens, the less is the image magnified, and therefore the greater the field that comes into view. The size of the field that can be seen at once will depend upon the strengtli and size of the object-glass : thus, with a dilated ])upil and a lens of + 13 D., having a diameter of about 5 cm..j the size of the field will be about 8 mm., or four 42 THE OPHTHALMOSCOPE times as large a field as will be seen by the direct method. The image formed by the lens will in the case of emmetropia be at the focus of the convex glass, between it and the observing eye, so that the learner has to remember to accommodate for the image at this distance. It is convenient to use the mirror before the right eye with the right hand when examining the patient's Fig. 37. right eye, and before the left eye with the left hand when examining the patient's left eye; then the objective will be held between the finger and thumb of the opposite hand. This may be steadied if neces- sary by resting the little finger against the forehead. By adopting this procedure the hand holding the THE INDIRECT METHOD 43 c « o d >< ^" 2 — Tn ^ ® " '5 ?j p o - H ^;, -, = o :: '- § g "-^ ^ « « S ii ^j 1* ^ -^ «♦-< *-> O > Ta -^ '''' a> ^ s 5 5 2 5 S .2 u o •^::3 -3 ^ •- « — o o 03 ii -i<*.S OS O 44 THE OPHTHALMOSCOPE objective is not over the patient's face. Some ob- servers, however, always use the same eye for the indirect examination ; thus if the right be the one preferred, tlic observer will always hold the ophthal- moscope in his right hand, using the objective with the left : this is simply a matter of individual con- venience. The eye of the observer not in use may with advantage be kept open. Although not necessary it is an advantage to use a + 4 D. behind the ophthalmoscope ; one thus obtains a somewhat larger image, which can be seen without accommodatiug, at the focus of the biconvex lens used. Thus when examining the fundus of an emmetrope, the aerial inverted image will be formed at the focus of the objective, which in the case of a + 13 D. lens will be a little less than 8 cm., the observer (with a + 4 D. behind his mirror) situated at 25 cm. from this image will see it clearly and well defined at this distance without any accommodation : the advantage of this plan is that the observer sits nearer the patient than when the examination is made without the -f 4 D., and does not have to stretch his arm out so far when holding up the objective. The first part to which attention should be directed is the disc. If one is examining the right eye, the patient should be told to look towards one's right ear, or, what is perhaps better, at the upheld little finger of the right hand which is holding the mirror; the fundus reflex being obtained, it will be noticed to be somewhat whiter when coming from the optic disc ; the large convex glass is held between the THE INDIRECT METHOD 45 thumb and finger of the left hand, about 3 or 4 cm. directly in front of the observed eye; this will form an inverted image of the disc, which should be clearly seen by the observer. The beginner will find some difficulty at first in performing these asso- ciated movements of lighting up the fundus with the mirror, and keeping the light steadily on the eye while the objective is held in the other hand and moved about backwards and forwards, and from side to side. The next part to examine is the periphery ; this must be gone over systematically by directing the patient to look up, then down, then to the right, and finally to the left. By this means the posterior hemisphere of the eyeball may be thoroughly in- spected, especially when the pupil has been dilated with a mydriatic. A still more peripheral part of the fundus can be seen by using an objective glass which is composed of two elements, a biconvex lens and a prism ; such a glass is called a prismosphere. Finally the macula region demands attention. This part is difficult to see, as the pupil contracts vigorously when the light is directed on this, the most sensitive part of tlie fundus; besides, the corneal reflex comes directly in the line of vision. The patient should be directed to look at the sight-hole of the mirror, or slightly to one side of it ; then with a little manoeuvring with the mirror and lens, and a certain amount of practice, a fairly good view of this part may be obtained. 46 THE OPnTKALMOSCOPE Tlie reflections formed by the cornea and by the objective are always somewhat troublesome to the Le<>^inner ; by tiltini^ the lens slightly these images will bo thrown out of the line of vision. Variations in the Size of the Image in Ametropia. — The contlition of tlie refraction of the eye under examination will cause some variation in the size of the images obtained ; thus in emmetropia, rajs coming from a, Fig. 39, emerge from the eye parallel, and are focussed by the biconvex lens at a, and rays Fig. 39. coming from B are focussed at 6 ; so also vs^ith rays coming from every part of A B, forming an inverted image of A b at h a, situated in the air at the principal focus of the biconvex lens. In hypermetropia (Fig. 40) the rays from A emerge divergent, Fig. 40. so also, of course, those from b ; if these rays are continued back- ward, they will meet behind the eye, and there form an enlarged THE INDIRECT METHOD 47 upright image (a /3) 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. In myopia (Fig. 41) the rays from A and b emerge from the eye Fig. 41. convergent, forming an inverted aerial image in front of the eye at /3 a, its punctum remotum. It is of this image we obtain, with a biconvex lens placed between it and the eye, a final image (b a) situated within the focus of the biconvex lens. Tiie 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. The lens should next be held close to the patient's eye, and then gradually with- drawn, while the aerial image of the disc is steadily kept in view; if any increase or decrease take place in the size of this image, we shall know that the eye is ametropic. If no change take place in the size of the image on thus with- drawing the objective the case is one of emmetropia, because rays issue from such an 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 (Fig. 42). 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. This change in size may be explained by remembering that in 48 THE OPHTHALMOSCOPE hypernictropia the imajj^e of the disc formed by the object-glass is situated beyond its principal focus, owing to the rays issuing from Fig. 42. E. Emmetropic eye. Kays issuing parallel, image formed at the principal focus of the lens, no matter at what distance the lens is from the eye. the eye being divergent ; the relative size of the final image (3 a to the object a b will therefore vary directly as the length c a, and inversel}" as the length c a ; so that on withdrawing the lens c from the observed eye, c a diminishes and c a increases ; there- fore the ratio of a h io a 13 diminishes, i.e. the size of the image diminishes. The two diagrams 43 and 44 show images formed by Fig. 43. Lens at 4 cm. from the curuea. 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. THE INDIRECT METHOD 49 Tliis increase in tlie size of the image can also be explained with the help of mathematics, remembering that, in myopia, an Via. 44, Lens at 12 cm. from the cornea. H. Hypermetropic eye. c. The centre of the lens. A b. Image on the retina, a h. Projected image. /3 a. The final image formed by the objective. inverted image is formed in front of the eye (Fig. 36), and it is of this we obtain a final 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 one meridian decrease in size, while the other remain stationary as the objective is withdrawn, it is a case of simple hypermetropic astigmatism. If the whole disc decrease in size, one meridian diminishing more than the other, it is compound hypermetropic astigmatism, the meridian being most hypermetropic which diminishes most. Increase in one meridian, the other remaining stationary, indi- cates simple m^'opic 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 the case is one of mixed astigmatism. It must be remembered that by the indirect method everything is inverted ; thus the apparent position of 4 50 THE OPHTHALMOSCOPE Fig. 45. B The above figure is intended to represent, by means of A, the real size of the optic disc; by means of b, the size of the image formed by the indirect method; and by c, the size of the image formed by the direct method. It also shows in the case b the effect of the inversion ; this effect is rendered more apparent by the patch of choroiditis shown in the figure. THE INDIEECT METHOD 51 the macula is to the inner side, when of course its real position is outside, the apparent upper edge of the disc is the lower, and so on. Fig. 45 b represents this inversion. Magnification of the image seen by the indirect method. — To estimate the degree of enlargement obtained by the inverted image we must recall to our minds the fact, that the image we see in this method of examina- tiou is a real image formed in the air by the union of the rays coming from the eye and passing through the convex lens. This image can be received upon a screen and measured exactly ; therefore to estimate the degree of enlargement, we have only to measure the size of the inverted image of the disc and com- pare it with the size of the disc itself. We are at once met by the difficulty that the size of the disc varies in different individuals, so that we do not know its exact size in any particular case. If the biconvex lens with which we obtain the inverted image be placed at such a distance from the eye that its focus coincides with the nodal point of the observed eye, a point situated in the emmetropic eye 15 mm. in front of the retina; then the enlarge- ment may be expressed by the simple formula — n of which X equals the amount of enlargement of the image ; / equals the focal distance of the lens 52 THE OPHTHALMOSCOPE used to produce the inverted image ; while u is the distance between the nodal point and the retina. Suppose, for example, we wish to know the amount of cnlarcrement of the ima":e in the case of an emme- trope with a ghiss of 13 D. Then /equals the focal distance of the lens 13 D., which is 77 mm. ; and u equals 15 mm., that is the distance between the nodal point and the retina; we shall then have for our formula — 77 X = -- = ^ tmies. 15 Therefore the enlargement will be five times, and if on measuring the image thus obtained we find it to be 7*5 mm., then we shall know that the real size of the disc was 1*5 mm. In hypermetropia we must find the amount of shortening of the eyeball (3 D. = l mm.), and deduct this from the 15 mm. which is the distance between the nodal point and the retina, and then proceed as before. In myopia the increase in length of the eyeball must be added on to the 15 mm. To carry out this experiment practically a demon- strating ophthalmoscope such as Beale's is necessary, provided with a screen marked out in millimetre squares on which the image is received. With Frost's artificial eye the experiment can be easily demonstrated. The large lens used as a condenser and magnifier. — The indirect examination being completed, before TFIE DIRECT METHOD 53 laying down the mirror and lens a further method of examination may be briefly mentioned. The ob- server approaching to a short distance from the patient (about 15 or 20 cm.), reflects the light into the eye by means of the mirror, and having illumi- nated the eye he then interposes the convex lens, which is now used not to obtain an image, but to condense the light from the mirror, while at the same time the cornea, iris, and lens are seen under its magnifying influence ; opacities of the cornea, in- juries or affections of the iris or lens are often con- veniently examined in this way ; by slightly moving the lens backwards and forwards the light is focussed on the different planes. This method of examination, though very useful in many cases, must never be used to the exclusion of the focal illumination. The Direct Method The direct method of examination is next employed. This method has the advantage of enabling us to see the parts in their true position, and gives us an image magnified some 16 to 18 diameters, though, of course, a much smaller part of the fundus is seen at once. The amount of fundus which will be visible depends chiefly upon the size of the pupil, but partly also upon .the size of the light used ; with a pupil of 4 mm. and a large gas flame, one gets a field little bigger than 2 mm. ; although only this small part can be seen at once, yet by varying the position of the head and ophthalmoscope one is able to look over a 54 THE OPHTHALMOSCOPE consitlorable part of tlie posterior hemisphere of the eye. A strong prism placed behind the ophthalmo- scope will allow a still more peripheral part of the fundus to be seen. Fig. 46. For the direct examination the tilted short focnssed mirror is used ; it may be quite small, as only those rays reflected from the part immediately around the sight-hole enter the pupil, — this is especially the case when the pupil is small ; the sight-hole should not be larger than about 2J mm., for if the sight-hole be larger than the pupil, then no rays may enter the eye, and we shall fail to get any illuinination. The ob- server first corrects 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 THE DIRECT METHOD 55 which corrects the refraction of the patient and himself in the manner to be presently described ; he •S s ^ s ^ o ,• o "- ^ ^ o:^ 2 oj ^ c: o -»^ ^ 'fcC « a> _ 3 c tc o 2 I -^ g > O CI "Vrf r5 aj o aJ .Sr ai gj o. X ... ^ S ^ tc^ M J K o J >> - 2 "< •= p^ fee's ;» C -^-^ '' ^ OJ .X "^ > "* C (1 > '^ o « O '^ w 2 »; « ee <; c^ ft j; « 'S ^ tc 'w 2 2 ?* t^ cs 03 *" +^ C> ji* -*J 2 G -^ .C o vr O « then sits or stands as he may prefer on the same side as the eye he is about to examine, so that the observer 50 THE OPHTHALMOSCOPE uses his right oyo 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; the examinee's head may with advantage be inclined slightly towards the observer, while the observer inclines his own head slightlj in the reverse direction ; 1. e. in examining the right eye the patient iticlines his head slightly to the right, while the observer inclines his slightly to his own right, so that the two eyes may come very close together, the brows even may touch, while the respiratory orifices of patient and observer are away from each other. The patient is directed to look straight in front of him, and take as little notice as possible of the examiner,, the surgeon resting the edge of the ophthalmoscope against his brow, reflects the light into the eye, and approaching close to the patient, first looks for the disc; then scans the periphery by directing the patient to look in different directions; and finally examines the macula region. The great difficulty which the beginner finds with this method is to keep his accommodation passive ; usually some practice is required before this can be managed, so that a concave glass has to be used before a clear view of the fundus can be obtained. By using a weaker concave glass eadi time, the accommo- dation will be gradually relaxed. Should the disc, when first seen, appear quite clear and distinct, one must not at once assume that the patient is emme- tropic, bnt onl}^ on finding that the weakest convex THE DIRECT METHOD 57 glass behind the oplitlialmoscopc impairs the clear- ness of the image. Another difficult}^ the beginner has, is to disregard the corneal reflex, which is most troublesome when the macula region is inspected. Magnification of the image seen by the direct method. — The estimation of the amount of enlargement by means of the direct method of examination is more difficult and less exact than with the indirect method. The size of the image of the disc of an emmetrope formed on the retina of the emmetropic observer will be exactly the same size as the disc itself, clearly shown in Fig. 48. This is KiG. -JS. The image is therefore of considerable size, and covers a good many retinal elements ; and to find out the magnification, we have only to consider some external object which^ placed at a certain distance from the eye, forms a retinal image of the same size, viz. 1*75 mm.; half-a-crown held 25 cm. from the eye will produce a retinal image of about this size. If we divide the distance nt which the coin is 58 THE OrnTnALMOSCOPE placed (25 cm.) by tlie distance from the retina to the nodal point, 15 mm., we shall arrive at the amount of enlargement : 25 cm. 250 ram. 15 mm. = 17. 15 mm. The diameter of our coin should therefore be 17 times larger than its retinal image ; we know the size of the retinal image is 1*75 mm., therefore the diameter of the half-crown should be about 30 mm. ; on measuring it this will be found to be the case. Fig. 49. It is obvious that the disc or any part of the fundus will appear more magnified, the greater the distance to which its image is projected. Hence the ophthalmoscopic image of the same disc does not alwa^^s appear of the same size to different observers, owing to the varying distance to which the image is mentally projected by them. To still further elucidate the subject, the following explana- tion may be of service. Every student is familiar with the plan sometimes THE DIRECT METHOD 59 adopted in the case of the microsco}3e ; when making a drawing of a specimen, the observer looks with one eye, we will assume the left, down the tube of the microscope, while with the right he looks by the side of the tube on to apiece of drawing-paper placed on a level with the stage of the instrument. The specimen, seen by the left eye, is projected upon the paper with the right, and can there be drawn ; or if it is wished to estimate the magnifying power of the microscope, a scale divided into hundredths of a millimetre is placed under the eye-piece, while beside it on the stage, or on the same level, is placed a scale divided into millimetres, so that the image seen in this case with the left eye is projected on to the scale placed at the side of the instrument ; thus the two scales are superimposed, and a comparison can be made ; if the squares on the two scales exactly cover each other, then we should know that the microscope mag- nified one hundred times. This experiment will also enable anyone to under- stand what is intended by the expression ^^ projecting a retinal image/' A somewhat similar plan is adopted with the oph- thalmoscope, but it must be slightly modified since the conditions are different. For with the ophthalmoscope, using the direct method, we approach so near to the observed eye that it would be impossible to see it at the same dis- tance with the eye alone ; it is just the same as looking through a strong convex lens. For this reason it is obvious that we must place the standard scale at a 60 THE OPHTHALMOSCOPE greater distance from the eye, and since the apparent size of the image is larger in proportion to the dis- tance to which it is projected, it is necessary to fix some distance at whicli tlie measuring scale shall be placed so as to render this method of any value; 33 centimetres has been decided upon, a distance at whicli we ordinarily look at near objects. Therefore, to estimate the amount of enlargement by the direct method of examination, we look with one eye through the ophthalmoscope at the disc of the observed eye, while with the other we look at Fig. 50. a sheet of paper on which is ruled a millimetre scale placed 33 cm. away ; with a little practice the student will be able to project upon this scale the image of the disc which he sees with the other e^^e, and, by counting the number of squares on the scale THE DIRECT MKTIIOD 61 that is covered by the iaiage^ the amount of enlarge- ment can be estimated. We encounter here the same difficulty that was re- ferred to on page 51, that the size of the disc may vary in different individuals. If the observer be emmetropic, then it is necessary for him to put on —3D. behind his ophthalmoscope, so that he may accommodate 3 D. with each eye ; he wouki then see clearly the disc of the eye under observation, while the other eye will be adapted for the distance at which the scale is placed, 33 cm. ; were this proceeding not adopted, the eyes would have to accommodate in unequal degrees in order that both the disc and the scale may be seen clearly. Another plan of estimating the enlargement by the direct method is to place behind the sight-hole of the ophthalmoscope a plain mirror from which a good deal of the silvering has been scratched away, to receive an image of the scale which is placed behind the observer's head and a little to one side, and thus the image of the disc and the scale will be super- imposed, and the two will be seen by the one eye. This plan is useful when the observer's eyes are not of equal value. Both the indirect and direct examituitions should always be employed, each method has its own special advantages ; thus the indirect method gives us a large field and allows us quickly to scan over the whole of the posterior part of the fundus, while the patient's re- fraction need not be corrected, and the observer may disregard his own ametropia provided he can adapt his 62 THE OPHTHALMOSCOPE eye for the distance at which the aerial image will be formed. The direct method gives a smaller field but greatly magnified, so that minute changes which are not visible by the indirect method can be detected ; it also gives us more accurate information of any lesion with regard to its level, &c., being an upright image every- thing is seen in its proper position, whereas with the indirect method the image is inverted ; and finally with the direct, the refraction of the observer and observed must be corrected. To the experienced ophthalmoscopist this becomes an advantage, as an estimate of the patient's refrac- tion can thus be made. One really wishes to estimate the refraction at the macula, but this region is not suit- able, partly because there are no convenient vessels, and partly because it is very sensitive to light, and therefore causes the pupil to contract vigorously, hence one usually selects the disc as the most favorable part for our purpose ; occasionally the refraction at the macula differs considerably from that at the disc, but usually little difference exists. The estimation of the refraction by the direct method. — To estimate the refraction of the patient 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 atropine 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 ; because rays coming from an THE DIRECT METHOD 63 emmetropic eye (Fig. 51, e) issue parallel, and the observing eye receiving these rays will, if emmetropic with its accommodation suspended, be adapted for Fig. 51. parallel rays, so that a clear image of a in the ob- served eye will be formed at h on the retina of the observing eye. Supposing the image does not appear clear and distinct without an effort of the accommodation, then we turn on convex glasses behind the sight-hole of the ophthalmoscope. The strongest positive glass with which we are able to get a perfectly clear image is a measure of the hypermetropia, because rays coming from a (Fig. 52) Ftq. 52. in the hypermetropic eye (h) issue in a divergent direction as though coming from u, the punctum re- motum behind the eye. The convex lens (l) renders 64 TirK OrilTIIALMOSCOPE tliein parallel, and they then focus at h^ on the retina of the observing emmetropic eye (e). If, however, the image of the disc appear indistinct, and the convex glass, instead of rendering the image clearer, have the opposite effect, we mnst turn the wheel of the ophthalmoscope in the other direction, and so bring forward the concave glasses. The weakest with which we can see the details of the fundus clearly is a measure of the myopia, because any stronger glass merely brings into play the accommodation of the observer. Rays from a (Fig. 53) leave the myopic eye (m) so convergent, tliat they would meet at (li) the punctum remotum. The concave lens (l) renders them parallel before falling on the relaxed eye (e) of the observer. Fto. .^3. 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 in- strument in hypermetropia, adding them together in myopia. If astigmatism exist the proceeding is more difficult, because one wishes to find out not only the refraction of the two chief meridians, but also the axis of these meridians. To discover the meridian of greatest re- THE DIRECT METHOD 65 fraction, and to estimate it by the direct method, we keep in view the disc, then if the case be one of hyper- metropic or mixed astigmatism, we find the strongest convex lens thronsrh which one of the vessels still remains distinct (vessels going in other directions will be indistinct) ; this lens will be the measure of the refraction of that chief meridian which is at right angles to the vessel. To estimate the other chief meridian we select a vessel whose course is at right angles to that first chosen, the strongest convex glass through which this vessel is seen distinctly will give us the measure we require. Had the case been one of myopic astigmatism, then the weakest concave glass with which any vessel is first clearly defined will indicate the strength of one meridian ; to estimate the other meridian we must as before select a vessel which is at right angles to that first clearly seen, then the weakest concave lens which allows us to see it well defined, will indicate the refrac- tion of the other chief meridian. The estimate is more easy to make, when the chief meridians are vertical and horizontal ; but unfortunately many cases occur in which they are more or less oblique, and it is not always easy to find a vessel whose course exactly coincides with these oblique meridians. The essential point to remember in estimating astigmatism by this method is, that the glass with which the vessels in one direction are clearly seen, is the measure of the refraction of that meridian which is at right angles to the vessels ; the student needs only to recall to his mind the principles of the per- 5 66 THE OPHTHALMOSCOPE ception of a lino by an astigmatic eye to understand this. Tlie subject may possibly be made clear by quoting a few examples ; we will take a case in whicli the vertical vessels and lateral sides of the disc appear distinct without any lens, and which the weakest convex glass renders indistinct, then the horizontal 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 discs, require a convex or concave 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 astigma- tism. If both the vertical and horizontal vessels can be seen through a convex glass, but a stronger one is required for the horizontal than for the vertical, then the case is one of compound hypermetropic astig- matism, the vertical meridian being the more hyper- metropic. 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 clearly defined through a convex glass, while the horizontal vessels require a concave glass to render them distinct, the case is one of mixed astigmatism, the horizontal meridian being hypermetropic, the vertical meridian myopic. The estimation of the refraction by the direct THE DIRECT METHOD. G7 method is exceedingly valuable, ])nt requires great practice. lu cases of hypermetropia and low myopia, one is able to estimate the amount of error within half a dioptre, and in cases of astigmatism where the chief meridians are horizontal and ver- tical, one can come very near the exact correction, and without subjecting the patient to the inconveni- ence of having his accommodation paralysed with atropine. The comparison of the direct and indirect methods of examination is also very useful in astigmatism. If, for instance, the disc is elongated horizontally in the erect, and oval vertically in the inverted image, we know that the curvature of the cornea is greater in the horizontal than in the vertical meridian. 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 ophthalmoscope ; he is then in the position of an emmetrope : but, if he prefer it, he may subtract the amount of his own hypermetropia or myopia from the glass with which he sees clearly the patient's discs. Thus, if the observer have 2 D. of hypermetropia and require +3 D. to see the fundus clearly, ( + 3 D.) - ( + 2 D.) = -I- 1 D., the patient would have 1 D. of hypermetropia. Had he required -2D. then (— 2 D.) - ( + 2D.) =: ( — 4D.) the observed would have 4 D. of myopia. The same with the myopic observer; if his myopia amount to 3 D., then he will require — 3 D. to see 68 THE OPHTHALMOSCOPE clearly tlio emmetropic fundus ; if he sees well with- out a glass, then the eye under examination has 3 D. of hypermetropia ; if he require a + 2 D., then the hj^permetropia will be 5 D., and so on. Retinoscopy Retinoscopy, or the shadow test, may conclude our ophthalmoscopic examination ; this is especially neces- sary when the vision has been found defective, and nothing has been detected by the indirect and direct methods. Retinoscopy is carried out by means of the large concave mirror used in the indirect method, or by a plane mirror ; the following description refers to the method carried out with the concave mirror. The light is placed over the patient's head, and the observer sitting at one and a quarter metres away, reflects the light into the eye he wishes to examine ; the converging rays of light reflected from the mirror focus in front of it, cross and diverge. Some of these rays passing through the pupil of the eye under observation, form a cone of light, which in the case of emmetropia would come to an exact focus on the retina. When the observer looks through the sight-hole of the mirror, he will obtain 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 KETINOSCOPY 69 tliis illiuuinated area may appear) on tlio same .side as the rotation or in the opposite direction, according to the refraction of the eye under observation ; 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 the mirror, the case is one of myopia, whereas if the shadow had moved in the opposite direction to the mirror, the case would be one of hypermetropia. If we place before a screen a convex lens, at such a distance from it that converging rays from a con- cave mirror, having crossed and become divergent, are brought to an exact focus, a small, erect, well- defined image will be formed on the screen of the lamp from which the concave mirror received its rays ; erect, because it has suffered two inversions. Fig. 54. a. The concave mirror, h. The candle, c. The lens. tf. The screen, d. Small image of candle formed on tl>c screen. f. Dense shadow around. This image of the lamp is surrounded by a sharply defined and dark shadow. If we move the lens nearer to, or further from, the 70 TFIE OrFITlTALMOSCOPE screeu, a circle of diffusion and not an accurate image is formed, as shown in Fig. 55. Fig. 55. At e a suiiill image of the candle is formed; at d and/", circles of diffusion. The mirror being rotated on its vertical axis, the imnge of the candle, with the surrounding shadow, will always be found to move in the opposite direction to the mirror, whatever be the distance of the lens from the screen. Fig. 56. 31. The mirror, ai'. The mirror after rotation. The ex- tremities of the dotted line have moved iu the opposite direction to the rotation of the mirror. This is exactly what takes place in the eye, of which our screen and lens are a representation. Therefore the illumination and shadows which we RETINOSCOPY 71 see iu retinoscopy are the enlarged image of the lamp with the surrounding shadow, brought more or less to a focus on the retina according to the refraction of the eye. They always move against the mirror, but as these movements are seen through the transparent media of the eye, and thereby undergo refraction, the " apparent " may differ from the "real '* movements. The image we see of the lamp, and its surrounding shadows, are formed in the same manner as all other images. In emmetropia the image is formed at infinity; and therefore, at a distance from the eye, the observer sees only a diffused and blurred image (Fig. 34). In hypermetropia the final image of the candle and its surrounding shadow, produced by the concave mirror, is an erect one formed behind the e^^e, and as it is viewed through the dioptric system of the eye, it therefore moves against the mirror (Fig. 35). In myopia the final image is an inverted one, pro- jected forwards. This, therefore, moves with the mirror, it having undergone one more inversion (Fig. 30). Therefore, if the image move with the mirror, the case is certainly one of myopia. If it move against the mirror, it is most likely one of hypermetropia ; but it may be emmetropia, or a low degree of myopia. 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 brightiiess of its edge, enable us to judge approxi- mately the amount of ametropia to be corrected ; 72 THE OPHTHALMOSCOPE some practice, liowcver, is required befoie we can form au opinion with anything like accuracy. The extent and rate of movement is always in inverse proportion to the ametropia ; the greater the error of refraction, the less the movement and the slower does it take 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 througli the nodal point B to c. Now, if the case be one of myopia (Fig. 57), an inverted projected image Fio. 57. of A is formed somewhere on this line, say at c. The higher the myopia, the nearer to the nodal point will this image be; and hence we may suppose it formed as near as d. If the mirror be now rotated, so that it takes up the position of the dotted line m', c will have moved to c, and J) to d ; hence it is clear that c has made a greater movement than d. Had the case been one of hypermetropia (Fig. 58), the image would have been projected backwards, and as in myopia, the higher the degree of hypermetropia, the nearer to the nodal point is the image formed. KETINOSCOPY 73 In this case, the line from the nodal point b to A is prolonged backwards, and the image of the luminous point in a low degree of hypermetropia is formed, say at c, and in a higher degree, say at i). On moving the mirror into the position of the dotted line m', c moves to c and b to d ; whence it is clear that c has made a greater movement than D. Fig. 58. Therefore, as the ametropia increases, the extent of the movement of the imaofe decreases. The clearness of the image and the brightness of its edge decrease as the ametropia increases. It was shown in Fig. 55, that on placing before a screen a convex lens at such a distance that converg- ing rays from a concave mirror cross and become divergent, they are brought to an exact focus, forming a small, erect, well-defined image on the screen of the lamp from which the concave mirror received its rays. On moving the lens nearer to or further from the screen, the larger becomes the area of light, and the feebler the illumination, owing to the circles of diffu- sion formed on the screen. Therefore, in the case of the eye, the greater the 74 THE Ol'HTHALMOSCOrE ametropia, the larger is the circle of diffusion and the weaker the illumination, so that the image we see is less bright and its edge less distinct. It is, therefore, in the lower degrees of ametropia that we get the brightest and best-defined shadows; and when we thus see them, we may assume that we are approaching the stage of correction. The patient, then, being seated in the dark room, the pupils dilated, and the lamp over his head, as before described, we take up our position 120 cm. in front, with a concave mirror of 25 cm. focus. The patient is then directed to look at the centre of the mirror, so that the light from the lamp may be re- flected along the visual axis. On looking through the perforation of the mirror, we get the ordinary fundus reflex, bright if the patient be emmetropic, less so if he be ametropic ; 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 direction as the circle of light on the patient's face^ the case is one of myopia. Should the edge of the image appear well defined and move quickly, in addition to a bright fundus reflex, we infer that the myopia is of low degree and proceed to correct it. Each eye must of course be tried separately. The patient having put on a pair of trial spectacle- frames, we place a weak concave glass, say —ID. RETINOSCOPY 75 before the eye we are about to correct. If the image still move with the mirror, we place in the frame — TS D, then —2 D., and so on, until we find the point at which no distinct shadow can be seen. Sup- posing this to be —2 D. and that on trying — 2'5 D. the image move against the mirror, — 2 D. is assumed to be the correcting-glass. This, however, will be found not to be the full correction of the myopia, because, being situated at 120 cm. from the patient, when his far point approaches that distance, we are unable to distinguish the movements of the shadow ; and when the far point of the observed, though not situated at infinity, is still at a greater distance than the observer, we get a shadow moving in the opposite direction. Hence it is customary in cases of myopia to add on —'5 U. to the correcting-glass, aud this would give us —2-5 D. as the proper glass for our case. In correcting myopia, it is a convenient and reli- able plan to stop at the weakest concave glass which makes the image move against the mirror^ and put that down as the correcting-glass. When the myopia is of high degree, and a strong concave glass has to be used for its correction, the light reflected from the mirror is so spread out by the concave ghiss, that fewer rays pass into the eye, and therefore the illumination is not so good as in other states of refraction. Had we obtained a reverse sliaduw, we should then try convex glasses, when, if -f *5 D. neutralised it, we should assume the case to have been one oi low 70 TIIK Ol'irrilALMOSCOI'K myopia. Had it required + 1 D. then it would be cue of emmetropia ; above this, bypermetropia. We pro- ceed exactly as before, putting up stronger and stronger glasses, until we are unable to make out the movements of the image. This is assumed to be the correcting-glass, and just as in the above case the myopia was under-corrected, so in this, the byperme- tropia is slightly over-corrected ; and hence it is usual to deduct from this glass + 1 D., or we may stop at the strongest convex glass with which we still get a reverse shadow. To sum up, therefore, if the shadow move with the mirror, it is a case of " myopia ; ^^ if against, it may be weak myopia if +*5 D. cause tlie image to move with the mirror ; emmetropia if + 1 D. neutralise it ; bypermetropia if a stronger glass is required. 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 reflex ; 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 bypermetropia or myopia simply. If, however, the case be one of astigmatism, then the refraction of the two chief meridians will differ. In astigmatism, the diffusion patch on the retina is more or less of an oval, instead of being cither a small well-defined image of the candle, or a circle, RETINOSCOPY 77 according to whether the eye be emmetropic, myopic, or hypermetropic. This oval may have its edges horizontal and vertical ; frequently, however, they are more or less oblique. The oblique movements of the shadow are inde- pendent of the direction in which the mirror is rotated. This obliquity is produced thus : (Fig. 59) if behind a circular opening, which is to represent the pupil, we place obliquely an oval piece of card, which is to represent the image on the retina ; on moving the card across in the direction o d, it has the appear- ance of moving in the direction o c, at right angles to the edge of the card. Hence the direction of the shadow's movement is deceiving, and its 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 Fig. 59. apparent movements are always at right angles to the edgfe of the shadow. The same takes place in astigmatism, the two chief meridians of which are parallel and perpendicular to 78 THE OPHTHALMOSCOPE the shadows in retinoscopy ; therefore when the edge of the image is oblique, we know at once that the case is one of astigmatism. If, however, it should be liorizontal or vertical, we judge if one shadow be more distinct or quicker in its movements than the other, though we are not always able to say at once that astigQiatism exists. We therefore proceed to correct one meridian. If the shadow move against in all meridians, we first take the vertical, and put up in front of the patient, in a spectacle-frame, convex spherical glasses, until we find the strongest with which the shadow still moves against the mirror. We put this down as the correcting-glass for the vertical meridian, and let us suppose that glass to be -f 2 D. We next take notice of the horizontal meri- dian, and if -f-2D. is also the highest glass with which we still get a reverse shadow, then, of course, we know the case is one of siuiple hypermetropia. But sup- posing the highest convex glass had been + 4 D., we indicate it conveniently thus : + 2 D. + 4D. The case is one of compound hypermetropic astigma- tism, and will require for its correction + 2 D. sphere combined with -f 2 D. cylinder axis vertical. We will take another case — that in which the vertical meridian requires —2D. to give a reverse shadow, and the horizontal + 2 D., this being the highest glass with which we still obtain a reverse shadow. Here we have a case of mixed astigmatism RETINOSCOPY 79 which can be corrected by a +2 D. sphere combined with — 4 D. cylinder axis horizontal. Supposing the axis of the shadow to be oblique, 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 hori- zontally and vertically. We judge of the amount of obliquity by the eye, and can frequently 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 be therefore at 110°) require — 3D., we express it as in Fig. GO, and correct it with sphere —2D. combined with cylinder — ID. axis 20°, the case being one of compound myopic astigmatism. Fig. 60. Often one is 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 only a slightly reversed shadow in every direction, the glasses are assumed to be the right ones, and we proceed to confirm it by 80 THE OPHTHALMOSCOPE tryinpf the patient at the distant test typo, making any slight alterations that may be required. In most cases it is necessary to dilate the pupils for retinoscopy, the refraction at the macula can then be obtained ; without a mydriatic one would only be able to estimate the refraction at the disc. When the plane mirror is used for retinoscopy, then the movements of the shadow are the reverse of those obtained with the concave mirror. ArPEARANCES OF THE NORMAL FUNDUS 81 CHAPTER IV THE APPEARANCES OF THE NORMAL FUNDUS It is essential that the learner should become familiar with the different varieties of the normal fundus before passing on to the various pathological conditions. The beginner may thiuk this a very easy matter, but he will soon discover that it is far from being so ; for instance, in cases of slight indis- tinctness of tlie margin of the disc, it may sometimes be exceedingly difficult for even the most experienced and skilful ophthalmoscopist to know exactly when this slight blurring has passed the border line of health and become pathological. As the complexion and the colour of the hair varies greatly in the human race, it is not to be wondered at, that the colour and appearance of the back of the eye, which depend in great measure on the amount of pigment contained in the tissues, should also show great variations. Plates I and II are intended to illustrate some of the types of the normal fundus ; and when we consider that no distinct line separates these different varieties, but that one type passes imperceptibly into another, it will be realised what great differences may be met with. 6 82 THE OrHTHALMOSCOl'E Tlieso variations depend in great measure upon tlie auiouut of pigment eontained in the hexagonal cells of the epithelial layer of the retina, and upon the stellate pigment present in the tissue of the choroid. The pigmentation varies greatly in different people, as a rule the lighter the complexion the less pigment is found in the retina and choroid; the albino may be taken as the specimen at one end of the scale, in which the least pigment is found, while the negro represents the other end of the scale,, in which there is the greatest amount" of pigment. Plate I, fig. 2, represents the right fundus of an albino, as seen by the indirect examination; Plate II, fig. 1, the left eye of a very dark English child, seen by the direct method. The ordinary red fundus reflex is due to the highly vascular choroid, modified more or less according to the amount of pigment present; the colour and amount of light used will also have much influence on it ; and some variations will be found in different parts of the same eye ; thus the macula region is somewhat darker than the rest of the fundus, shading off gradually into the colour of the other parts. The periphery is usually lighter and may possibly show some of the details of the deeper parts of the choroid ; the colour immediately round the disc may also be somewhat lighter than the general tint of the fundus. The retina. — The retina is a membrane of consider- able thickness, being "4 mm. at its posterior and thickest part, where it is four times as thick as the PLATE I. Fig. 1. — Fundus of a child aged 10 years; of medium complexion, light brown hair, grey irides, fair skin. Erect image. Left eye. The disc is not so shsirply defined as in many cases ; there is slight pigmentation of the outer edge of the disc, \vhich indicates the edge of the choroidal ring, and may be looked upon as physiological. There is no physiological cup. Fig. 2. — Fundus of an albino aged 24; white hair, eyebrows, and eyelashes; irides light grey and translucent. Inverted image. Right eye. The absence of pigment is here well shown, each vessel of the deep layer of the choroid can be seen. The disc appears darker than normal, in contrast with the lightness of the rest of the fundus. The light-coloured interspaces between the choroidal vessels are due to the white of the sclerotic. On the outer side of the disc is a slight crescent, caused by the choroidal opening being rather larger than the sclerotic ring, so that a small portion of the sclerotic is here exposed. Plate!. A. W Head del. Bale