THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA LOS ANGELES REFRACTION HOW TO REFRACT THORINGTON BY THE SAME AUTHOR. Rctinoscopy (The Shadow Test) in the Determination of Refraction at One Meter Distance with the Plane Mirror. 54 Illustrations, a number of which are in Colors. Fifth Edition. Cloth, net, $1.00 From The Medical Record, New York. " It presents a clear, terse, and thorough exposition of an objective method of determining refraction errors which is de- servedly increasing in popularity. In our opinion the author is amply justified in declaring that its great value in nystagmus, young children, amblyopia, aphakia, and in examining illiterates and the feeble minded, cannot be overestimated, and we agree with him in reminding those who attempt retinoscopy, fail, and ridicule it, that the fault is behind and not in front of the mirror. The book is well printed and usefully illustrated." The Ophthalmoscope and How To Use It. With De- scriptions and Treatment of the Principal Diseases of the Fundus. With 12 Colored Plates and 73 other Illustrations. Cloth, net, $2.50 HORIZONTAL SECTION OF THB RIGHT ~E\K.(Landois.) Cornea, b. Conjunctiva, c. Sclerotic, d. Anterior chamber containing the aqueous humor. Of ('.'; i/l V '' I OPTICS. 1 3 proportion of absorption and reflection of rays of light depends greatly upon the quality of the suiface whether light colored or polished, or dark colored or rough. ^) Reflection. From the Latin rcflectcrc, "to rebound." This is the sending back of rays of light by the surface on which they fall into the medium through which they came. \Yhile most of the rays falling upon the surface of a trans- parent substance pass through it, with or without change 'in their direction, yet some of the rays are reflected, and it is by these reflected rays that surfaces are made visible. D FK;. 5. FIG. 6. !A substance that could transmit or absorb all the rays of light coming to it (if such a substance existed) would be invisible. ( Reflection, therefore, always accompanies refrac- tion, and, if one of these disappear, the other will disappear also. ) Laws of Reflection. (i) The angle of reflection is equal to the angle of incidence. (2) The reflected and in- cident rays are in the same plane with the perpendicular to the surface. (See Fig. 5.) If A B represent a polished surface and I the incident ray, then P D I is the angle of incidence ; R being the re- REFRACTION AND HOW TO REFRACT. R E F LEG TION 1 ~ fl T 3 03J HOIT fleeted ray, then P D R, equal to it, is the angle of reflection. I D, P D, and R D lie in the same plane. A reflecting surface is usually a polished surface (a mirror), and may be plane, concave, or convex. Reflection from a Plane Mirror. {Rays of light are reflected from a plane mirror in the same direction in which they fall upon it : if parallel, convergent, or divergent be- fore reflection, then they are parallel, convergent, or diver- gent after reflection. An object placed in front of a plane mirror appears just as far back in the mirror as the object is in front of it. (See Fig. 6.]} A B represents a plane mirror with E F, rays from the extremes of the object I, reflected from the mirror A B, and meet- ing at the observer's eye as if they came from the object I in the mirror. (See Visual Angle, p. 62.) The apparent dis- -7 tance of the object I from the observer is equal Jo the combined length of the incident and reflected rays. The appearance of an image in a plane mirror is not exactly the same as that of the object facing the mirror ; it undergoes what is known as lateraljm^sion. This is best understood by holding a printed page in front of a plane mirror, when the words or letters will read from right to left (See Fig. 7.) An observer facing a plane mirror and raising his right hand, his image apparently raises the left hand. Tilting a plane mirror gives an object the appearance of FIG. 7. Lateral Inversion. moving in the opposite direction to that in which the mirror is tilted. Spheric Mirrors. A spheric mirror is a portion of a reflecting spheric surface ; its center of curvature is therefore the center of the sphere of which it is a part. (' Spheric mir- rors are of two kinds concave and convex.) Reflection from a Concave Mirror (Fig. 8). Parallel rays are reflected from a concave mirror, and are brought to a focus in front of it. This point is called the principal focus (P.P.). The^principal axis of a concave mirror is a straight line drawn from the mirror through the principal focus and v; /' ~ < ~^~- ^.P'F':-^..J ' ^- -- . FIG. 8. the center of curvatureYi i), and(a secondary axis (2', 2', 2', 2') is any other straight line passing from the mirror to the center of curvature (C.C.).J Rays which diverge from 'any point beyond the principal focus are reflected con- vergently (G J). Rays which diverge from any point closer than the principal focus are reflected divergently (V V). Images Formed by a Concave Mirror.-j^To find the position of an image as formed by a concave mirror, two rays may be used/ one drawn from a given point on the object to the mirror, and parallel to its principal axis, and reflected through the principal focus (P.P., Figs. 9 and 10); the other, the secondary axis, from the same point, passing /S ^ Ft^ /y c* +* 1 6 REFRACTION AND HOW TO REFRACT. through the center of curvature. The place where the secondary axis and the reflected ray or their projections in- tersect gives the position of the image. Unlike the plane mirror, which produces images at all times and at all dis- jtances, the concave mirror produces either an erect, virtual, 'and enlarged image, if an object is placed closer than its principal focus, or an enlarged inverted image if the object is between the principal focus and the center of curvature. By withdrawing the mirror in the former instance the erect image increases slightly in size, and in the latter the inverted image diminishes in size. /'At the principal focus there is no image formed.) FIG. 9. Figure 9 shows an erect, virtual, and enlarged image of A R which is closer to the mirror than the principal focus. Parallel rays from A and R are reflected to the principal focus, P.P. Lines drawn from the center of curvature through A and R to the mirror are secondary axes ; these lines and those reflected to the principal focus do not inter- sect in front of the mirror, but if projected, will meet at a and r behind the mirror, forming a magnified image of A R. If the mirror is withdrawn from the object, the erect magnified image will increase in size, but at the prin- cipal focus no image will be formed, as the rays are reflected parallel. Figure 10 shows a real inverted image of A R at a r ; A R situated beyond the principal focus. Lines drawn from A and R through C.C. are secondary axes. Parallel rays from A and R converge and cross at the principal focus (P.F.). Where D P and F E intersect the secondary axes, the in- verted image a r of A R is situated. ^Vhen the object, as in this instance, is situated beyond the center of curvature, the image is smaller than the object. , As the image and object are conjugate to each other, they are interchange- able, and in such a case the image would be larger than the object and inverted. /This is always true when the FIG. 10. object is situated between the center of curvature and the principal focus. j(vVhen an object is situated at the center of curvature, its image is equally distant and of the same size, but inverted. Tilting a concave mirror gives an object placed inside of / r i its principal focus the appearance of moving as the mirror is tilted ; but if the object is situated beyond the principal focus, the object appears to move in the opposite direction. Reflection from a Convex Mirror. All rays are re- flected divergently from a convex mirror, and parallel rays diverge as if they came from the principal focus situated behind the mirror at a distance equal to one-half its radius 18 REFRACTION AND HOW TO REFRACT. of curvature. The principal focus of a convex mirror is therefore negative. The foci of convex mirrors are virtual. are Images Formed by a Convex Mirror. These ^always virtual, erect, and smaller than the object^) The closer the object, the larger the image ; and the more distant the object, the smaller the image. (Tilting a convex mirror, ,the image does not appear to change position.. In figure 1 1 parallel rays from the object A R are reflected from the mirror as if they came from the principal focus situ- ated at one-half the distance of the center of curvature, C.C. Lines drawn from the extremes of the object to C.C. are C.C. P.F. FIG. ii. secondary axes, and f' the image is situated at the point of intersection of the secondary axes and the rays from the principal focus^ and as these meet behind the mirror, the jimage is virtual and erect. Refraction. From the Latin refrangere, meaning " to bend back " /. e., to deviate from a straight course. /Refrac- tion may be defined as the deviation which takes place in the direction of rays of light as they pass from one medium into another of different density)* (* As ordinarily understood in ophthalmology, refraction has come to mean the optic condition of an eye in a state of repose or under the physiologic effect of a cycloplegic. OPTICS. 19 Two laws govern the refraction of rays of light : 1. A ray of light passing from a rare into a denser medium is deviated or refracted toward the perpendicular. 2. A ray of light passing from a dense into a rarer medium is deviated or refracted away from the perpen- dicular. Aside from these laws, there are other facts in regard to rays of light that should have consideration. A ray of light will continue its straight course through any / number of different transparent media, no matter what their I densities, so long as it forms right angles with the surface ICE \ / FLINT GLASS > CROWN " 3 PLATE " $ > FIG. 12 or surfaces. (Such a ray is spoken of as the normal or per- pendicular} {such surfaces are plane, the surfaces and per- pendicular forming right angles.) (See Fig. 12.) In any lease of refraction the incident and refracted rays may be [supposed to change places. Figure 1 3 shows the perpendicular (P P) to a piece of plate glass with plane surfaces. The ray in air incident at O on the surface S F is bent in the glass toward the per- pendicular, P P. The dotted line shows the direction the ray would have taken had it not been refracted. As the ray in the glass comes to the second surface at R, and 2O REFRACTION AND HOW TO REFRACT. passes into a rarer medium, it is deviated from the perpen- dicular, P P. The ray now continues its original direction, but has been deviated from its course ;(it has undergone lateral displacement) Critical Angle or Limiting Angle of Refraction. This is /the angle of incidence which just permits a ray of light in a dense medium to pass out into a rare medium) The ^ J (size of the critical angle depends upon the index of refrac- tion of different substances. \ Figure 14 shows an electric light suspended in water. The ray from this light which forms an angle of 48 35' with the surface of the water FIG. 14. Critical Angle. will be refracted and pass out of the water, grazing its sur- face ; but those rays which form an angle greater than 48 35' will not pass out of the water, but will be reflected back into it. /The surface separating the two media be- comes a reflecting surface and acts as a plane mirror.^ The critical angle for crown glass is 40 49'. Index of Refraction. By this is (meant the relative density of a substance or the comparative length of time required for light to travel a definite distance in different substances. The ('absolute index <>f n-fraction is the density or refractive power of any substance as compared OPTICS. 21 with a vacuum.) According to the first law of refraction, a ray of light passing from a rare into a dense medium is refracted toward the perpendicular ; (in other words, the angle of refraction is smaller, under these circumstances, than the angle of incidence. J) In the study of the compara- tive density of any substance it fwill be seen that the angle of refraction is usually smaller the more dense the substance^ this is well illustrated in figures 15 and 1 6. I The greater the density, the /slower the velocity or the more effort apparently for the wave or ray to pass through the substance. This is illustrated in figure 17, where a ray or wave of light is seen passing at right angles through different media. A ray passes through a vacuum without apparent resistance, but in its course through air it is slightly impeded, so that/air has an index of refraction of FIG. 15. FIG. 16. / / / / / / Ice 'X_/'X_/-\_x Glass ^\^^^ Diamond Vacuum Air FIG. 17. 1.00029+ when compared with a vacuum); but as this is so slight, air and a vacuum are considered as one for all pur- poses in refraction. To find the index of refraction of any substance as compared witrravacuun^orjur^tis neces- I sary to divide the sine of the angle of incidence by the sine I of the angle of refraction. 22 REFRACTION AND HOW TO REFRACT. In figure 18 the angle of incidence P C I is the angle formed by the incident ray I with the perpendicular, P P. The angle of refraction P C R is the angle formed by the refracted ray with the perpendicular, P P. Drawing the circle P H P O around the point of incidence C, and therx p* drawing the sines^TD X. and B F, perpen- diculars to the per- pendicular P P.raivide the sine D X of the angle of incidence by the sine F B of the angle of refraction to obtain the index of refraction) in this in- stance, water as com- pared with air. D X equaling 4 and F B equaling 3, then 4 di- vided by 3 will equal , or 1.33 -J-, the index of refraction of water as compared with air. (To find the index of refraction of a rare as compared with a dense substance, divide the sine of the angle of refraction by the sine of the angle of incidence /. c., air as compared with water would be ^, or 0.75. \ INDEXES OF REFRACTION. Water . . . . . .177 Cornea . . . . .7777 , Crown plass. .C Flint elass .58 Crystalline lens, nucleus, .4.7 " " intermediate layer, .41 " " cortical layer. , 3Q OPTICS. 23 A prism is a wedge-shaped portion of a refracting medium contained between two plane surfaces. The sides of a prism are the inclined surfaces. The apex is where the two plane surfaces meet. The base of the prism is the thickest part of the prism. The refracting angle is the angle at which the sides come together. Position of a Prism. When a prism is placed in front of an eye, its position is indicated or described by the direc- tion in which its base is situated : base down means that the thick part of the prism is toward the cheek ; base up means that the thick part of the prism is toward the brow ; base in means that the thick part of the prism is toward the IMG. 19. FIG. 20. nose ; and base out means that the thick part of the prism is toward the temple. Prismatic Action. Rays of light passing through a f \ prism are^a.hvays'i-efracted toward the basejbf the prism. If an incident ray is perpendicular to the surface of a prism, there will be only one refraction) and that takes place at the point of emergence. ^The angle of incidence in this instance will equal the angle of the prism, and the maximum deviation takes place, as all the refraction is done at one surface./ In figure 19 the incident ray (I) is perpendicular to the surface A B, and is not refracted until it comes to the sur- 24 REFRACTION AND HOW TO REFRACT. face A C at E, when it is bent toward the base B C, all the refraction taking place at the surface A C. \ If an incident ray forms an angle other than a right 1 ingle with the first surface of the prism, then it will be re- iiracted twice as it enters and as it leaves the prism. ^In figure 20 X N is the perpendicular to the surface A B The ray (I) incident at N is refracted toward this perpen- dicular and follows the course N E inside of the prism. On emergence it is refracted from the perpendicular E P of J;he surface A C, and in the direction of the base of the prism. ) / If the incident ray (I) so falls upon the surface A B that the refracted ray (N E) is parallel to the base (B C), and the emergent ray is such that the angle of emergence equals the angle of incidence (I N X), as in this instance, then the angles of incidence and of emer- gence are equal, and the deviation is at a minimum, or the least possible. J Angle of Deviation (Fig. 21). This is the angle formed between the directions of the incident and emergent rays, and measures the total devia- FlG 2I tion. In all prisms of ten degrees or less the angle of deviation is equal ,7\ to half the angle of the prism, but in prisms of more than ten degrees the angle of deviation increases. Summary. Prisms do not cause rays of light to con- ^ verge or to diverge ; rays that are parallel before refraction are parallel after refraction. (Therefore, prisms do not form images ; prisms have no foci.y Effect of a Prism. An object viewed through a prism has the appearance of being displaced, and in a direction opposite to the base /. e., toward the apex. Rays from the object (X, Fig. 22) strike the prism at C, OPTICS. undergo double refraction, and, falling upon the retina of the eye, are projected back in the direction in which they were received, and the apparent po- sition of X is changed to X', away from the base of the prism and toward the apex. Numbering of Prisms. Form- ytrly, prisms were numbered by their refracting angles ; now, however, two other methods are in use : jDennet's method, known as the centrad '/method, known as the prism- diopter. Dennett's Method (Fig. 23). The unit, or centrad (ab- breviated V ), is a prism that will deviate a -ray of light the Y^-jj- part of the arc of the radian. This is calculated as follows : As much of the circumference of a circle is taken as will equal the length of its radius of curvature ; this is ^called the arc of the radian, and equals 57.295 degrees.; The arc of the radian is then divided into 100 parts. /A FIG. 22. and Prentice's Radius FIG. 23. I Meter FIG. 24. prism, base clown, at the center of curvature that will devi- /[> ate a ray of light downward just T ^-$ part of the arc of the radian is a one centrad, and equals T ^g- of 57.295 degrees, or 0.57295 of a degree. ) 26 REFRACTION AND HOW TO REFRACT. 9876543210 Ten centrads will deviate a ray of light ten times as much as one centrad, or 10 X 0.57295 = 5.7295 degrees, etc. Prentice's Method (Fig. 24). The unit, or prism-diopter (abbreviated P.D.,or/\),is a prism that will deviate a ray of light > just i cm. for each meter of dis- tance that is, the -^ part of the radius measured on the tan- gent. The deviation always be- ing i cm. for each meter of dis- tance, i P. D. will deviate a ray of light 2 cm. for 2 meters of dis- tance ; 3 cm. for 3 meters, etc. The comparative values of cen- trads and prism-diopters is quite uniform up to 20, but above 20 the centrad is the stronger. Neutralization of Prisms. Knowing that rays of light are de- viated by centrads and prism- diopters up to 20, in the ratio of i cm. for each meter of distance, then tc^ find the numeric strength of any prism all that is necessary is to hold the prism over a series of numbered parallel lines, sepa- rated by an interval of i cm. or fraction thereof, and note the amount of displacement) For example, figure 25 shows a series of vertical lines */j of a cm. apart, and numbered from o to 9; an X is placed at the foot of the o line. Holding a prism, base to the right, at a distance of ^ of a meter (as 98765 I 3 2 : : D X FIG. 25. OPTICS. the lines are ^ of a cm. apart) and looking through the prism at the X on the o line, it will be seen that the X has been displaced to the line to the left corresponding to the number of centrads or prism-diopters in the prism ; in this instance three. TABLE SHOWING THE EQUIVALENCE OF CENTRADS IN PRISM-DIOPTERS AND IN DEGREES OF THE REFRACTING ANGLE (INDEX OF REFRACTION 1.54). 'TO CENTRADS. PRISM-DIOPTERS. REFRACTING ANGLE. I. I. I.oo 2. 2.0001 2. 12 3- 3.0013 3 .i8 4- 4.OO28 4-23 5- 5-0045 5. 28 6. 6.0063 6. 3 2 7- 7.0II5 7-35 8. 8.0172 8. 3 8 9- 9.0244 9-39 10. 10.033 10. 39 ii. II.O44 ii. 3 7 12. 12.057 i2-34 13- I3-074 I3.29 14. 14.092 14. 23 15- 15.114 I5.i6 16. 16.138 i6.o8 17- 17.164 i6.98 1 8. 18.196 i 7 .8 5 19. 19.230 i8.68 20. 20.270 i9 -45 25- 25-55 23-43 30- 30.934 26.8i 35- 36.50 29. 72 40. 42.28 32.i8 45- 48.30 34. 20 50. 54-514 35-94 60. 68.43 38-3i 70. 84.22 39-73 80. 102.96 40. 29 90. I26.OI 40-49 100. 155-75 39. H >rism may be ncutralix.cd by placing another prism in apposition to it, with their bases oppositejso that in look- /V""^ -> L-^- 4yt**4&**. V*y /4 kinds of lenses spheric and cylindric. 3O REFRACTION AND HOW TO REFRACT. Spheric Lenses. Abbreviated S. or sph. Spheric lenses are so named because their curved surfaces are sec- tions of spheres. A spheric lens is one which refracts -rays of light equally in all meridians or planes. Spheric lenses are of two kinds convex and concave. A convex spheric lens is thick at the center and thin at the edge. (Figs. 27, 28, 29.) The following are synony- mt>us terms for a convex lens : (i) Plus ; (2) positive ; (3) collective ; (4) magnifying. A convex lens is denoted by the sign of plus ( + ). Varieties or Kinds of Convex Lenses. I. Planoconvex, meaning one surface flat and the other convex. It is a section of a \ A V sphere. (See Fig. 27.) \ / \ \\ 2. Biconvex, also called con- vexoconvex or bispheric, for the reason that it is equal to two planoconvex lenses with their / \l I/ plane surfaces together. (Fig. FIG. 27. FIG. 28. FIG. 29. 28.) 3. Concavoconvex. This lens has one surface concave and the other convex, the convex surface having the shortest radius of curvature. (Fig. 29.) ^ The following are synonymous terms for a concavoconvex lens : (i) Periscopic ; (2) convex meniscus ; (3) converging meniscus (meniscus meaning a small moon). (See Fig. 29.) . / A periscopic lens enlarges the field of vision, and is of / especial service in presbyopia. A Concave Spheric Lens. Such a lens is thick at the edge and thin at the center. (Figs. 30, 31, 32.) The following are synonymous terms for a concave lens: (i) Minus ; (2) negative ; (3) dispersive ; (4) minifying. A concave lens is denoted by the sign of minus ( ). . ^^ ' OI'TICS. Varieties or Kinds of Concave Lenses. 1. J^anoconcai'C, meaning one surface flat and the other concave. (Fig. 30.) 2. Biconcave, also called concavoconcave or biconcave spheric, for the reason that it is equal to t\vo planoconcave lenses with their plane surfaces to- gether. (Fig. 31.) 3. Conyexoconcavf. This lens has one surface convex and the other concave, the concave sur- face having the shortest radius of curvature. (Fig. 32.) The following are synonymous terms for a concavoconvex lens : (i) Concave meniscus ; (2) diverging meniscus ; (3) peri- scopic. 17 V7 A FIG. 30. FIG. 31. FIG. 32. FIG. 33. FIG. 34. (A. spheric lens may be considered as made up of a series of prisms which gradually increase in strength from the center to the periphery, no matter whether the lens be con- cave or convex. / 32 REFRACTION AND HOW TO REFRACT. In the convex sphere the bases of the prisms are toward the center of the lens, whereas in the concave the bases of the prisms are toward the edge. (See Figs. 33, 34.) Knowing that a prism refracts rays of light toward its base, it may be stated as a rule that every lens bends rays of light more sharply as the periphery is approached i. c., at the periphery the strongest prismatic effect takes place. Lens Action. --As a ray of light will travel in a straight line so long as it continues to form right angles with sur- faces) then the ray A in figure 35 passes through the bicon- vex lens unrefracted, or without any deviation from its course whatsoever, for at its points of entrance and emer- FIG. 35. gence the surfaces of the lens are plane to each other. This ray is called the axial ray, and /the line joining the centers of curvature of the two surfaces i The axis of a planoconvex or planoconcave lens is the liiu drawn through the center of curvature perpendicular to the plane surface. The ray B in figure 35, though parallel to the ray A, forms a small angle of incidence, and must, therefore, be refracted toward the perpendicular to the surfaces of the lens, and, passing through the lens, will meet the axial ray at P.P. The rays C, D, and K, also parallel to A and B, form progressively larger angles with the surface of the lens, OPTICS. 33 and finally meet the axial ray at P.P. < It will be seen at once that the rays all meet at P.P., showing the progres- sively stronger prismatic action that takes place as the per- iphery of the lens is approached..' In figure 36 we have similar rays, A, B, C, D, and E, passing through a con- cave lens. The axial ray A passes through the centers of curvature unrefracted, but the rays B, C, D, and E are pro- gressively refracted more and more as the periph- FIG. 36. ery is approached. The ray E in each instance is refracted the most. The action of a convex lens is similar to that of a concave mirror, and the action of a concave lens is similar to that of a convex mirror. Principal Focus. The principal focus of a lens may be defined (i) as the point where parallel rays, after refrac- tion, come together on the axial ray ; or (2) as the shortest focus ; or (3) as the focal point for parallel rays. Focal Length. This is (the distance measured from the optic center to the principal focus. The principal focus of an equally biconvex or biconcave lens of crown glass is situated at about the center of curvature for either surface of the lens.)^A. lens has two principal foci, an anterior and a posterior, according to the direction from which the par- allel rays come, or as to which radius of curvature is re- ferred to.) (Seep. 60.) Figure 35 shows parallel rays, B, C, 1), and E, passing through a convex lens and coming to a focus on the axial ray (A) at P. F. ; and as the path of a ray passing from one point to another is the same, no 34 REFRACTION AND HOW TO REFRACT. matter what its direction, then if a point of light be placed at the principal focus of a lens, its rays will be parallel after passing back through the lens. This is equivalent to what ' takes place in the standard or emmetropic eye. An eye, in other words, which has its fovea situated just at the princi- pal focus of its dioptric media, such an eye in a state of rest receives parallel rays exactly at a focus upon its fovea, and therefore is in a condition to project parallel rays outward. Conjugate Foci. Conjugate meaning "yoked to- gether." ^The point from which rays of light diverge (called the radiant) and the point to which they converge (called the focus) are conjugate foci or points. ) For in- stance, in figure 37 the rays diverging from A and passing FIG. 37. through the lens converge to the point B ; then the points A and B are conjugate foci. They are interchangeable, for if rays diverged from B, they would follow the same path back again and meet at A. The path of the ray C C' is the same whether it passes from A to B or from B to A : there is no difference. It is by the affinity of these points for each other, with respect to their positions, that they are called conjugate. The conjugate foci are equal when the point of diver- Igence is at twice the distance of the principal focus. The v equivalent to conjugate foci is found in the long or myopic eye ; an eye, in other words, which has its fovea situated further back than the principal focus of its dioptric media, the result being that rays of light from the fovea of such an OPTICS. 35 eye would be projected convergently after passing out of the eye, and would meet at some point inside of infinity. In other words, only those rays which have diverged from some / point insidq of six meters will focus upon the fovea of this / long eye. ( The fovea of the myopic eye represents a con- I jugate focus./ A myopic eye is in a condition to receive divergent rays of light at a focus on its retina and to emit convergent rays. Ordinary Foci. When rays of light diverge from some point inside of infinity (six meters) they will be brought to a focus at some point on the other side of a convex lens, beyond its principal focus ; this point is called an ordinary focus. (A lens may have many foci, but only two principal FIG. 38. fociJ The further away from a lens the divergent rays pro- ceed, the nearer to the principal focus on the other side of the lens will they converge. As the divergent rays are brought closer to the lens they reach a point where they will not focus, but will pass parallel after refraction. c_This point is the principal focusj^ (See Fig. 38.) (^A lens, there- fore, has as many foci as there are imaginary points on the axial ray between the principal focus and infinity/) (When rays of light diverge from some point closer to a lens than its principal focus, they do not converge, but, after refraction, continue divergently ; their focus now is negative or virtual, and is found by projecting these diver- gent rays back upon themselves) to a point on the same 30 REFRACTION AND HOW TO REFRACT. side of the lens from which they appeared to come. (See Fig- 39-) This is the equivalent of what takes place in a short or hyperopic eye, an eye which has its macula closer to its dioptric media than its principal focus. In a state of rest FIG. 39. the fovea of such an eye would project outward divergent rays, and would be in a position to receive only convergent rays of light at a focus upon its fovea. Secondary Axes. In the study of the direction of a ray of light passing through a dense medium with plane sur- FIG. 40. faces, it was found that it underwent lateral displacement (see Fig. 1 3), and(so in lenses there is a place where rays undergo lateral displacement) Figure 40 shows a convex lens of considerable thickness, and on each side is drawn a radius of curvature (C C). The ray indicated by the arrow 7\ OPTICS. 37 passed through the two surfaces, has undergone lateral displacement, but continues in its original direction ; such rays are called secondary rays or axes. The incident ray is projected toward N 1 in the lens on the axial ray, and the emergent ray, if projected backward, would meet the axial ray at N 2 . (These points on the axial ray are such that a ray directed to one before refraction, is directed to the other after refraction. The points N 1 and N 2 are ^ spoken of as nodafpoints. /Every lens, therefore, has two nodal points, but in thin lenses the deviation of the second- ary rays is so slight that, for all practical purposes, only FIG. 41. ~>^tt^^ Jr>*-W^ / c ^ "^ ^one nodal point is recognized. \ It is spoken of as the optic /V^. ' center. When writing prescriptions for glasses, this point of having the lenses or glasses -made as thin as possible, must be borne in mind. / Optic Center. This term is used synonymously with nodal point, and/ is the point where the secondary rays (sia.\\\ Fig. 4 1 ) cross the axial ray. It is^not always the geometric center) /Rays of light crossing the optic center in thin lenses are not considered as undergoing refraction^) (See Fig. 41.) Action of Concave Lenses. Rays of light passing through a concave lens, no matter from what distance, are ^always refracted divergently, and its focus is, therefore, always negative" or virtuah)and is found by projecting these 35 REFRACTION AND HOW TO REFRACT. divergent rays backward in the direction from which they appear to come until they meet at a point on the axial ray. I The principal focus and conjugate foci of concave lenses I are found in the same way as in convex lenses. (See Figs. ' 36, 44-) Images Formed by Lenses. ^An image formed by a lens is composed of foci, each one of which corresponds to a point in the object. Images are of two kinds real and virtual. A Real Image. This is^an image formed by the actual meeting of rays) such images can always be projected on to a screen. A Virtual Image. This is one that is formed by the prolongation backward of rays of light to a point. FIG. 42. /To find the position and size of an image it is necegsary to obtain the conjugate foci of the extremes of the object, as the image of an object is equal to the sum of its inter- mediate points. iOnly two rays are required for this pUr- pose, one parallel to the axial ray, and one secondary ray passing through the optic center ; the image of the extreme point of the object will be located at the point of inter- section of these rays. In figure 42 A B is an object in front of a convex lens, o is the optic center and P. F. the principal focus. A ray drawn from A parallel to the axial ray o, and a secondary ray from the same point drawn OPTICS. through the optic center, will give at their point of inter- section the conjugate focus of the luminous point A, which will be at A'. In the same way the conjugate focus of B and points intermediate in the object may be obtained. A' B' is a real inverted image of A B ; /the size of the image of A B depends upon the distance of the object from the lens. The relative sizes of image and object are as , their respective distances from the optic center of the lens.) For example, if an object ten millimeters high is three meters (3000 mm.) from the optic center of a lens, and its image is sixty millimeters from the lens, the image will be ToMhr or To" f tne s ' ze f tne object ; that is, the image will be -^ of ten millimeters (the height of the object) namely, I of a millimeter high. As conjugate foci are interchangeable, then in figure 42 if A! B' was the object, the image A B would be the image of A' B', and, therefore, larger than the object. Three facts should be borne in mind in the study of real images formed by a convex Inis : ^~\^JjuC \ 1. The object and image are interchangeable. 2. The'object and the real image are on opposite sides of "T) the lens, and, 0> ^ / 3. As the rays which pass through the optic center cross each other at this point, the real image must be in- verted. A&*ruS\>W^ ^MMH^^M* ' *f L3 Rays of light from an object situated at the distance of Q^ J / the principal focus would proceed parallel after refraction, r~ r > 1 - t ' P- and no image of the object would be obtained. / If an object is situated just beyond the principal focus, / then the image would be larger than the object, real and / inverted. (See Fig. 42, reversing image for object.) If an object is situated at twice__the distance of the prin- cipal focus, then its image would be of the same size, real, 4O REFRACTION AND HOW TO REFRACT. inverted, and at a corresponding distance, as these conju- gate foci are equal.y If an object is situated at a greater distance than twice the principal focus, and nearer than infinity, its image will be real, inverted, and smaller than the object. FIG. 43. Rays of light from an object situated closer to a lens than its principal focus would be divergent after refraction, and could only meet by being projected backward ; the image would, therefore, be larger than the object, erect, and FIG. 44. virtual. Such an image is only seen by looking through the lens ; the lens in this instance being a magnifying glass. (Fig. 43.) Images Formed by Concave Lenses. These images are always erect, virtual, and smaller than the object. (See Fig. 44.) A concave lens is, therefore, a minifying lens. &AC& OPTICS. 41 Parallel rays from the extremes of the object A ix form the divergent ray A' and R' after refraction. Secondary rays pass through the optic center o unrefracted, A" and R". At the points of intersection where these rays meet after being projected backward, the image of A R is found, erect, virtual, and diminished in size. This image is only ;een by looking through the lens. Numeration of Lenses. -/-Formerly, lenses were num- bered according to their radii of curvature in Paris inches (27.07 mm.). The unit was a lens that focused parallel rays of light at the distance of one English inch (25.4 mm.) from its optic center,/ As lenses for purposes of refraction were never as strong as the unit, they were numbered by fractions, thus showing their relative strength as compared to this unit ; for instance, a lens that was one-fourth the strength of the unit was expressed by the fraction ^, or a lens that was one- sixteenth the strength of the unit was expressed as ^g-, etc., [the denominator of the fraction indicating the focal length 'of the lens in Paris inches. There are three objections to this nomenclature : (i) The difference in length of the inch in different countries ; (2) the inconvenience of adding two or more lenses num- bered in fractions with different denominators ylj -f -% -f- ^; (3) the want of uniform intervals between num- bers. In the new nomenclature, and the one that is now quite universal, known as the metric or dioptric system (diopter, abbreviated D.),/^f lens has been taken as the unit which has its principal focus at one meter distance (39.37 English inches), commonly recognized as 40 inches. c."> / rv ^Lenses in the dioptric system are numbered according to their refractive power and not according to their radii of 42 REFRACTION AND HOW TO REFRACT. curvature.") (The strength or refractive power of a dioptric lens is, tKerefore, the inverse of its focal distance.) To find jthe focal distance of any dioptric lens in inches or centi- Imeters, the number of diopters expressed must be divided linto the unit of 40 inches or 100 cm. For example, a 2 D. lens has a focal distance of 40 -4- 2 equals 20 inches ; or 100 cm. -4- 2 equals 50 cm. A -(-4 D. has a focal dis- tance of 40 -4- 4, equaling 10 inches, or 100 -4- 4, equaling 25 cm. /Lenses that have a refractive power less than the unit are not expressed in the form of fractions, but in the form of decimals ; for example, a lens which is only one-fourth, one- half, or three-fourths the strength of the unit is written 0.25, 0.50, 0.75, respectively, and their focal distances are found in the same way as in dealing with unitsj) 0.25 D. has a focal distance of 40 H- 0.25 or 100 -4- 0.25, equaling 160 inches or 400 cm. ; 0.50 D. has a focal length of 40 -4- 0.50 or 100 -4- 0.5-0, equaling 80 inches or 200 cm. ; 0.75 D. has a focal length of 40 -4- 0.75 or 100 -4- 0.75 equaling 53 inches or 133 cm. Unfortunately, 0.25 D., 0.50 D., and 0.75 D. are frequently spoken of as twenty- five, fifty, and seventy-five, which occasionally leads to confusion in the consideration of the strength and focal dis- tance. The student should learn as soon as possible to change the old nomenclature into the new, as he will have to make these changes in reading other text-books. To change the old " focal length " or inch system of numbering lenses into diopters, divide the unit (40 in.) by the denominator of the fraction, and the result will be an approximation in diopters ;/ for example, y^- equals y$ or 4 D. ; -^V equals |^ or 2" D. The following table, from Landolt, gives the equivalents in the old and new systems : OPTIQS. 43 OLD SYSTEM. NEW SYSTEM. I. II. III. IV. V. VI. VII. VIII. No. No. No. of the Focal Focal of the Focal Focal Cories- Lens, Distance Distance Equiva- Lens, Distance Distance poiKlinir Old in English in Milli- lent in New in Milli- in English of the Old System. Inches. meters. Diopters. System. meters. Indies. System. 72 67.9 1724 0.58 0.25 4000 IS7 48 166.94 60 56.6 '437 0.695 ' 0.5 2OOO 7874 8346 48 45-3 1150 0.87 o.75 1333 52-5 5563 42 39-6 1005 0.99 i 1000 3937 41-73 36 34 863 I 16 '25 800 3i 5 3339 3 28.3 718 i 39 i-5 666 26.22 2779 24 22.6 574 1-74 i-75 571 22.48 23-83 20 18.8 477 2.09 2 500 19.69 20.87 18 I? 431 2-31 2.25 444 1748 1853 16 15 38i 2.6 2-5 400 15-75 16.69 15 I4.I 358 2.79 3 333 "3 i? "3-9 14 13-2 335 2.98 3-5 286 ii 26 n-94 13 12.2 312 3-20 4 250 9.84 10.43 12 II. 2 287 3-48 45 222 8-74 9 26 II I3 261 3.82 5 2OO 7.87 8-35 10 9-4 239 4.18 5-5 182 7 16 76 9 8-5 216 4.63 6 166 654 693 8 7-5 190 5-2.S 7 '43 563 597 7 6.6 167 5-96 8 125 4.92 5-22 (>K 6.13 155 642 9 III 4-37 4-63 6 5-6 142 70 10 IOO 394 4 '7 5% 52 132 7-57 ii 91 3-58 38 5 f 4-7 119 8.4 12 83 3-27 3-46 4% 42 106 9-4 13 77 3-3 3-21 4 f 3-8 96 10.4 14 7i 2.8 2.96 3% 3-3 84 11.9 15 67 2.64 2.8 3Y4 3-1 79 12.7 16 62 244 2.59 3 2.8 71 14.0 17 59 2.32 2 46 2K 2.6 66 15-1 18 55 2 17 2.29 *S 2.36 60 17.7 20 5 1.97 2.09 2* 2.1 53 18.7 2 1.88 48 20.94 Cylindric Lenses. Abbreviated cyl., c, or C. A cylin- dric lens, usually called a " cylinder," receives its name from being a segment of a cylinder parallel to its axis. (See Fig. 45. T Occasionally cylinders are made with both sur- faces curved, and are then equivalent to two planocylinders with their plane surfaces together. A cylinder may be defined as a lens u'hich refracts rays of light opposite to its axis. This definition should be carefully borne in mind in contradistinction to a spheric lens, which refracts rays of light equally in all meridians. /A cylindric lens has no one 44 REFRACTION AND HOW TO REFRACT. common focus or focal point, but a line of foci, which is parallel to its axis.) Axis of a Cylinder. That dimension of a cylindric lens which is parallel to the axis of the original cylinder of' FIG. 45. FIG. 46. FIG. 47- which it is a part is spoken of as the axis, and is indicated on the lens of the trial-case by a short diamond scratch on the lens at its periphery, or by having a small portion of FIG. 48. its surface corresponding to the axis ground at the edges, or it may be marked in both ways. (See Fig. 47.) Cylin- ders are of two kinds convex and concave. (Figs. 45, 46.) Cylinder Action. /A convex cylinder converges parallel OPTICS. 45 FIG. 49. rays of light so that after refraction they are brought into a straight line which corresponds to the axis of the cylin- der j/for instance, a +5 cyl. will converge parallel rays so that they come together in a straight line at the dis- tance of eight inches, or twenty centimeters, and this straight line will be parallel to the axis of the cylinder. (Fig. 48.) A concave cylin- der diverges rays of light opposite to its axis, as if they had diverged from a straight line on the opposite side of the lens. (Fig. 49.) Spherocylinders. A spherocylinder is a combination of a sphere and a cylinder, and is/Therefore a lens which has one surface ground with a spheric curve and the other sur- face cylindric/ A spherocylindric lens is/also spoken of as an astigmatic lensj 1 (See Fig. 100.) A spnerocylindric lens is{pne which has two focal planes.j Spherocylinders have different curves : the spheric curve may be convex, with the ! cylindric surface convex ; or the spheric surface may be concave, with the cylindric surface concave ; or the spheric surface may be convex, with the cylindric surface concave ; or the spheric surface may be concave, with the cylindric surface convex. The Trial-case (see Fig. 50). This case contains pairs of plus and minus spheres and pairs of plus and minus cylinders ; also prisms numbered from ^ or ^ to 20 A. The spheres are numbered in intervals of o. 12 up to 2 S.; and from 2 S. up to 5 S. the interval is 0.25 S.; and from 5 S. to 8 S. the interval is 0.50 S.; and from 8 S. to 22 S. 46 REFRACTION AND HOW TO REFRACT. the interval is i S. The /cylinders have similar intervals, but seldom go higher than 6 or 8 cyy The trial-case also contains a trial -frame, which is used to place lenses in front of the patient's eyes. (See Fig. 51.) The eye -pieces of such a frame are numbered on the periphery in degrees of half a circle, so that the axis of a cylinder can be seen during refraction. The left of the FIG. 50. horizontal line in each eye-piece is recognized as the start- ing-place, or zero (o), and the degrees are marked from left to right on the lower half, counting around to the horizontal meridian, which at the right hand is numbered 1 80; this horizontal meridian is, therefore, spoken of as horizontal, zero (O), or 1 80 degrees. The meridian midway between zero and 1 80 is spoken of as vertical, or 90 degrees. In some countries the meridians are differently num- OPTICS. 47 bered (see Fig. 52); for example, the vertical meridian is called zero, and the degrees are marked on each side of zero up to 90 degrees. Only the upper half of the eye-piece is thus numbered, so that when a cylinder has the upper end of its axis inclined toward the nose, the record would be so many degrees of inclination to the nasal side ; or if the upper end of the cylinder was inclined toward the temple, the record would be so many degrees to the temporal side. For example, in the right eye 1 5 degrees nasal would mean axis 75 on the ordinary trial-frame, and 15 degrees temporal would mean 105 degrees. The trial-case also contains other accessories, such as blanks or blinders, a stenopeic slit, pin-hole disc, etc., all of which are referred to in the text. Combination of Lenses. The sign of combination is ^. Combining Spheres. Any number of spheric lenses placed with their optic centers over each other, and sur- 4 8 REFRACTION AND HOW TO REFRACT. faces together, will equal one lens the value of their sum : for example, -f-2 S. Q -f I S. Q -(-3 S. will equal -j-6 S.; or a 2 S. ^ I S. (^ 3 S. will equal a 6 S. If a plus and minus sphere, each of the same strength, be placed with their optic centers together, the refraction will be nothing, for the one will neutralize the effect of the other ; for instance, +48. and 4 S. will be equivalent to a piece of plane glass, as the 4 S. will diverge rays of light as much as the -(-4 S. will converge them, and the result FIG. 52. is, rays of light parallel before refraction are parallel after passing through such a combination. (If, however, a plus and a minus sphere of different strengths are placed together, the value of the resulting lens will equal their difference, in favor of the higher numberj for instance, -{-48. and 2 S. will equal a -\-2 S., the 2 S. neutralizing 2 S. of the +48., leaving +2 S. Combining Cylindric Lenses. -(-Any number of cylin- dric lenses placed together, with their axes in the same OPTICS. 49 meridian, are equal to a cylinder the value of their sum ; for example : -j-2 cyl. axis 90 degrees and +3 cyl. axis 90 degrees will equal a -f 5 cyl. axis 90 degrees ; or 2 cyl. axis 1 80 degrees and 3 cyl. axis 180 degrees will equal a 5 cyl. axis 180 degrees ; or 2 cyl. axis 180 degrees and +i cyl. axis 180 degrees will equal a I cyl. axis 1 80 degrees. As a cylinder refracts rays of light only in the meridian opposite to its axis, this opposite meridian can always be found by the following simple rule : " Add 90 u'hcn the given axis is yo or less than po, and subtract go when the given axis is more than 90." j For example : -f- 3 cyl. axis 90 refracts rays of light in the 1 80 degree meridian (90 + 90= 180); or +3 cyl. axis 75 refracts rays of light in the 165 meridian (75+90 165). A 3 cyl. axis 135 refracts rays of light in the 45 meridian (135 less 90 =45). A 2 cyl. axis 180 re- fracts rays of light in the 90 meridian (180 less 90 = 90). Combining two cylinders of the same strength and same denomination, with their axes at right angles to each other, will equal a sphere of the same strength and same denomination. For instance, +3 cyl. axis 90 and +3 cyl. axis 180, placed together, will equal a +3 S. /. e. y the +3 cyl. at axis 90 will converge parallel rays in the 1 80 meridian, while the -f- 3 cyl. axis 1 80 will converge parallel rays in the 90 meridian, producing a principal focus /therefore any sphere is also equal to two cylinders of its same strength and same denomination with their axes at right angles to each otherj (Combining cylinders of different strength, but of the same denomination, with their axes at right angles to each other, such a combination will equal a sphere and a cylinder of the same denomination. ) For example : REFRACTION AND HOW TO REFRACT. -\-2 cyl. axis 75 O +3 cyl. axis 165 will equal -f-2 S. O ~\-i cyl. axis 165. The -\-2 cyl. axis 75 takes -f 2 of the -j~3 cyl. axis 165 and makes a + 2 S., leaving -f- 1 cyl. axis at 165 ; the result is then -f-2 S. O + I cyl. axis 165. Or 3-5O cyl. axis 15 O - 4.50 cyl. axis 105, will equal 3.50 S. O I cyl. axis 105. The 3.50 axis 15 takes 3.50 of the 4.50 and makes a 3.50 S., leaving I cyl. axis 105; this I cyl. axis 105 is now joined to the 3-5O sphere, making 3-5O S. O I cyl. axis 105. Combining a sphere and a cylinder of the same strength, but of differ- ent denomination, will 'equal a cylinder of the opposite sign and opposite axis from the cylinder given. For example : -f- 1 sphere O I cyl. axis 1 80 will equal -f- I cyl. axis 90. The -f- i S. equals two -f- I cylinders, one at axis 90 and one at axis 180, and the I cyl. at axis 180 is neutralized by the -f- 1 cyl. at the same axis, leaving -f- I cyl. axis 90. This may be better understood by the diagram (Fig. -49). Or 3 S. O +3 cyl. axis 90 equals 3 cyl. axis 180. The 3 S. is equal to two 3 cylinders, one at axis 90 and one at axis 180 ; the one at axis 90 is neutralized by the +3 cyl. at the same axis, leaving 3 cyl. axis 180. Combining a Sphere with a Weaker Cylinder of Dif- ferent Denomination. Such a combination should be 1.00 FIG. 53. OPTICS. 5 1 changed to its simplest form of expression, and will equal a sphere of the same denomination, of the value of their difference, combined with a cylinder of the same strength as the cylinder given, but of opposite sign and axis. For example: -j~4 S. O i cyl. axis 180. The minus one cylinder is refracting in the 90 degree meridian, therefore it reduces the strength of the +4 S. in this axis, making it a plus 3. The horizontal or 1 80 degree meridian of the plus 4 S. has not been altered, but still remains -(-4, and the re- sult is, plus 3 in the vertical meridian and plus 4 in the horizontal meridian, equaling, therefore, -j~3 S. ^ -f I cyl. axis go. The following rule will be of service in making this change, and, in fact, this rule will apply in any instance where the sphere and cylinder are of different denomina- /s^ tion, no matter what their respective strengths may be : |(/^ Rule. Subtract the less from the greater, and to the res2tlh. { prefix the sign of the greater ; continue isith this the same\ ^',(Y^ strength cylinder, using the opposite sign and opposite axis.\ ^T^* \ Example : +2.25 S. O 0.75 cyl. axis 75 degrees ; sub- /**' tracting the less from the greater ( 0.75 from +2.25), and\ prefixing the sign of the greater (+), will leave -}- 1.50 S. ; and combining with this the same strength cylinder (0.75), with opposite sign and axis (+ and 165), will be +0.75 cyl. axis 165. Result, +I-5O S. O +0.75 cyl. axis 165. Combining a Sphere and Cylinder of the Same De- nomination. This is recognized as the minimum or ^ simplest form of expression, and is seldom changed. For example : 2 S. O 6 cyl. axis 180 is considered as the thinnest lens and the one with the least weight that can be made by such a combination. It may be changed, how- ever, by the reverse of the rule above given, and will equal 8 S. O -f 6 cyl. axis 90. REFRACTION AND HOW TO REFRACT. Combining Two Cylinders of Different Denominations with Opposite Axes. Commonly called crossed cylinders. Such combinations can be written in three ways : 1. -)-Cyl. ^ cyl. axes opposite. 2. -f Sphere O cyl. (cylinder stronger than sphere). 3. Sphere +cyl. (cylinder stronger than sphere). y For example : i.oo cyl. axis 180^ +2.50 cyl. axis 90 may be changed to one of the following : I.oo S. O +3-5 cyl. axis 90 ; or ^+2.50 S. C 3-50 cyl. axis 180.' \J| ' \ \ i7, L^ t*T. /K/Y^ n~V^^ J t^" -j The first formula shows that the vertical meridian must always be I and the horizontal or 1 80 meridian must always be +2.50, and with this clearly in mind, the second /^?4rfkf '&ird formulas will be understood. In the second &** ' formula ( i.oo S. O +3.50 cyl. axis 90) the +3.50 cyl. is only equal to +2.50, as it has I D. neutralized by I of the I sphere. In the third formula (+2.50 S. O " ^^/3/5 cyl. ax i s !8o) the 3-5 cylinder is only equal to HT.OO cylinder, as it has 2.50 neutralized by +2.50 of the sphere. In any spherocylindric combination the meridian in which the axis of the cylinder lies has the strength of one lens, and the meridian opposite to the axis of the cylinder has the com- bined values of sphere and cylinder /. c., i.oo S. O -(-3.50 cyl. axis 90 means i.oo on the axis (90) of the cylinder, and opposite to the axis therefore at 1 80, it equals + 2.50 (1 and 4-3-50). Crossed cylinders in themselves are seldom ordered in a prescription, preference being given to a spherocylindric combination, ('when to order a plus sphere with a minus cylinder, and when to order a minus sphere with a plus cylinder, depends upon the individual lenses.) For example : -(-0.50 cyl. axis 90 O 5.00 cyl. axis 180 equals +0.50 S. O 5.50 cyl. axis iSoor 5 S. O +5. 50 cyl. axis 90.' Preference would be given to the plus sphere combina- tion, on account of thinness and lesser weight of the lens. The following formula, i cyl. axis 180 degrees ^ 3 cyl. axis 90 degrees, equals i.oo S. ^ +4 cyl. axis 90, or +3 S. ^ 4 cyl. axis 180, and for similar reasons preference would be given to the minus sphere combination. Whichever combination makes the thinnest and lightest weight of glass is the one to be ordered, as a rule. The student should practise these combinations at the trial-case, and be able at a glance to change one formula into another without diagram or rule. Prescription Writing. In writing prescriptions for lenses the right eye is indicated by one of three signs R, Rt, or O. D., the latter from the Latin for right eye, OCH/HS Dexter. The left eye is also indicated in one of three ways L, Lt, or O. S., the latter from the Latin for left eye, Oculus Sinister. A prescription may call for any one of the following : -(-Sphere, written 4 4 D. or -(-4.00 D. S. or -f 4 S. or -} 4 sph. Sphere, written 2 D. or 2.00 D. S. or 2 S. or 2 sph. -(-Cylinder, written -f 4.00 D. C. or -f 4 C. or -(-4 cyl. (axis as indicated). Cylinder, written 2.00 D. C. or 2 C. or 2 cyl. (axis as indicated), -f Sphere and -f- cylinder, written +2.00 S. ^ -\-2.oo cyl. axis 90 degrees. Sphere and cylinder, written 2.00 S. ^ 2.00 cyl. axis 180 degrees. -f Sphere and cylinder (cylinder stronger than sphere), 4 2 - S. O 3-OO cyl. axis 180 degrees. C Sphere and -f cylinder (cylinder stronger than sphere), 2.00 S. ^ 43- ) cyl. axis 90 degrees. A plus cylinder and minus cylinder may be prescribed, and, if so, their axes must be at right angles to each other. An occasional exception to this may be found in irregular astigmatism. Or a prism with its base indicated may be ^ REFRACTION AND HOW TO REFRACT. &! \j "Nadded in any one of the foregoing formulas ; for example : * 2 S. O 2.OO cyl. axis 1 80 O 2 A base in ; or the direc- >tion of the base may be abbreviated as follows : B. I., ^ meaning base in ; B. O., meaning base out ; B. U., meaning base up ; and B. D., meaning base down. Prescriptions are never written for two spheres. Prescriptions are never written for two cylinders at the ame axis. Prescriptions are never written for two cylinders at axes Bother than those at right angles to each other, except, as just noted, in irregular astigmatism. For obvious reasons prescriptions are never written for a sphere and two cylinders^ except in irregular astigmatism. Recognition of Lenses? A convex sphere is thick at the center and thin at the r\j scribed as the power of the eye to focus rays of light upon its retina from different distances at different times. In other words, the eye can not focus rays of light upon its retina from different points at one and the same time. For example, the point of a pencil held six inches in front of the eye is not seen clearly (is hazy) as the eye looks at a printed page thirteen inches beyond ; and, vice versa, the printed page is not seen distinctly if the point of the pencil is looked at. In the study of convex lenses it was noticed that when an object was brought closer than infinity, the focus of the lens was correspondingly lengthened ; and so, in the photographer's camera, to keep the focus on the ground-glass or sensitive plate as the object is brought toward the camera, it is 6 66 REFRACTION AND HOW TO REFRACT. necessary to push the lens forward by means of the accor- dion plaits ; but the human eye does not lengthen or shorten in this way. Normally, the eyeball is inextensible, and to accomplish this same purpose the ciliary muscle must contract, causing the crystalline lens to become more convex, and thus keep the rays of light entering the eye at a focus upon the fovea. The Mechanism of Accommodation. To appreciate this, it is necessary to understand something of the anatomy of the ciliary body, of which the ciliary muscle is a part. The ciliary body is circular in form and occupies a small . (3 mm.) area in the eye, just beneath the sclera, at its cor- neal junction. (See Fig. 61.) In section the ciliary body is triangular in shape, the base of the triangle measuring about 0.8 mm. and facing toward the anterior chamber, the apex of the triangle extending backward beneath the sclerotic. The ciliary body lies in apposition to the sclera, but has only a very minute attachment to it, at the sclerocorneal junction, called the ligamentum annulare, or pectinatum. That portion of the ciliary body lying next to the hyaloid membrane of the vitreous humor is composed of folds, known as the ciliary processes, seventy or more in number. A portion of the ciliary body is composed of muscular fibers disposed in flat bundles, which interlace with each other, forming a sort of plexus, and called the ciliary mus- cle. This muscle, by the character of its fibers, has been subdivided into three parts : (i) Meridional (2) radiating ; and (3) circular or sphincter fibers. The (meridional are the longest, lie next to the sclerotic in lamellae, parallel with it, and pass back to join the choroid coat of the eye,) (forming what is known as the tensor choroidea?, or muscle |of Brucke or Bowman. The radiating fibers are fan-shaped, few in number, and scattered through the ciliary body. THE MECHANISM OF ACCOMMODATION. 6/ The circular or sphincter fibers also called annular are sometimes referred to as the muscle of Miiller, or com- pressor lentis, and are the most important fibers in the consideration of accommodation ; they form a sphincter ring concentric with the equator of the lens. Attached to the ciliary body, well forward on its inner side, near the ' base of the triangle, is the ligament of the lens (zonule of Zinn), and it in turn sends fibers to the anterior and poste- rior capsule of the lens. This ligament of the lens occu- pies an interval of about 0.5 mm. between the ciliary body and the periphery of the lens, and is a constant factor in all conditions of the healthy eye. During the act of accommodation the following changes take place in the eye : 1. The ciliary^ muscle contracts. 2. The ciliary muscle (sphincter), by contracting, makes a smaller circle. 3. The tensor choroideae draws slightly upon the choroid (compressing somewhat the vitreous body), and these two sets of fibers, sphincter and meridional, acting together, i relax the ligament of the lens, with the result that 4. The lens fibers, no longer held in check, become re- laxed, and by their own inherent quality (elasticity) allow the lens to become more convex, especially on its anterior surface. 5. The anterior surface of the lens being made more convex, approaches the cornea. | 6. The posterior surface of the lens becomes slightly /more convex, but retains its position at the pole. 7. The lens axis is lengthened, but the equatorial diame- ter diminishes, thus keeping up the uniform interval between the equator of the lens and the ciliary body, as previously referred to. The lens docs not increase in volume. 68 REFRACTION AND HOW TO REFRACT. 8. The anterior chamber becomes slightly shallower at the center and deeper in the periphery. 9. That portion of the iris resting upon the anterior cap- sule of the lens is pushed forward, espe- cially at its pupillary edge. 10. The iris contracts, producing a smaller pupil ; but it must be remembered that contraction of the iris is not an essen- tial condition in accommodation. The shape of the cornea is not changed during con- traction of the ciliary muscle. The following table shows the compara- tive measurements of a lens at rest and during the height of accommodation in a healthy emmetropic eye of ten years. The dotted lines in figure 69 indicate the changes in the shape of the lens at the height of accommodation. FIG. 69. AT REST. Radius of curvature of anterior surface of lens, . 10 mm. " " posterior " " . 6 " Distance from anterior surface of cornea to ante- rior surface of lens, 3.6 " Anteroposterior diameter, on axis, 3.6 " Distance from anterior surface of cornea to poste- rior surface of lens, 7- 2 " Equatorial diameter, 8.7 " HEIGHT OF ACCOMMODATION. 6 mm. 5-5 " 3-2 4 7.2 8.2 Far Point. Latin, punctum rcmotum ; abbreviated p. r. ^or r. The far point may be (defined as the greatest distance at which an eye has maximum sharpness of sight, or the most remote point at which the eye, in a state of rest, has maximum acuity of vision^) Infinity (sign of infinity, oo )^ is the far point of an emmetropic eye. The standard or emmetropic eye, when looking at distant' objects, receives parallel rays of light at a focus upon its AMPLITUDE OF ACCOMMODATION. 69 fovea (Fig. 70), and also emits parallel rays ; under these conditions the ciliary muscle is not acting, the eye is in a condition of complete repose, of rest, of minimum refrac- tion, and is adapted for its far point. Near Point. Latin, punctual proximum ; abbreviated p. p. or p. This may be .defined as the nearest point at which an eye has maximum sharpness of sight^ or the nearest point to the eye at which it has distinct vision, the lens is in the condition of greatest convexity, of maximum refraction. Amplitude of Accommodation. This is also called the range * or power f of accommodation, and may be^e- fined as the difference between the refraction of the eye in a state of rest (or adapted for its far point) and in a condi- tion of maximum refraction, or adapted for its near pointy For example, an emmetropic eye has infinity for its far point, and if 10 cm. distance is its near point, then the dif- ference between the lens adapted for infinity and locm. will be 10 D., as 10 cm. represents the focal length of 10 D. In other words, there is no accom- modation used for infinity, but there is an accommodation of 10 D. for the near point, which is the amplitude or power of accommodation. The emme- tropic eye in a state of accom- FIG. 70. modation adds on to the ante- rior surface of its lens what is equivalent to a convex meniscus. Figure 70 shows an emmetropic eye at rest receiving parallel rays of light at a focus upon its retina, * Range applies to the space between the far and near points, f Power applies to the force or strength or diopters necessary to change the refraction from the far to the near point 7O REFRACTION AM) HOW and it also shows the same eye in its maximum state of accommodation for a point 10 cm. distant; the broken line representing what is equivalent to a convex meniscus, added to the anterior surface of its lens. When the distance of the near point is known in inches or centimeters, the equivalent in diopters is found by divid- ing 40 by the near point in inches, or by dividing roo by the near point in centimeters. The near point being 10 cm., ! or 4 inches (10 into 100 or 4 into 40) the amount of accom- modation will be 10 D. In the study of healthy emmetropic eyes it has been found that the power of accommodation gradually dimin- ishes as the eye passes from youth to old age. This is the result of one or more changes : the lens fibers lose their elasticity, becoming sclerosed, or the ciliary muscle grows weak, or both of these changes may exist together. Rarely the cornea may flatten. A knowledge of the power of accommodation~~isabsolutely essential, so that any vari- ations from the standard condition may be noted. The fol- lowing table gives the ages from ten to seventy-five years, respectively, with five-year intervals, and the near point consistent with each, as also the amplitude of accommoda- tion for each period. I AMPLITUDE AMPLITUDE YEAR. /NEAR POINT. IN DIOPTERS. YEAR. NEAR POINT. IN DIOPTERS. 10 > 50 40 " / ^ 2.5 20 4io " 10 55 55 ">/ 3/> 0.75 35 18 Jf> S-S 7 400 " 0.25 40 My'TV 4 ' 5 75 I > ' / This table of near points applies only to emmetropic eyes /or those eyes which are made emmetropic by the adjust- -C. ^ / ' ^ , FAR POINT. 7 1 ment of suitable correcting lenses. The table of amplitudes, however, is the same, with a few exceptions, for all eyes of whatever degree or amount of ametropia. For a better appreciation of the amplitude of accom- modation it is necessary to understand the two forms of eyes already referred to in figure 68. First, the eye which has its retina closer to its refractive media than the principal focus ; such an eye is spoken of /as a short or hyperopic eye. (H in Fig. 68.) (Hyperopia : ^/ \ Greek, o-^p, over; and & point of a myopic eye are conjugate foci. (See Fig. 72.) The myopic far point is equivalent to just that much refrac- tion in excess of the emmetropic eye. An emmetropic eye under the influence of atropin would require a -f- 2 S. placed in front of it to make rays of light focus upon its retina from a distance of 50 cm., and rays of light from the retina of this eye with a -j-2 S. in front of it would focus at 50 cm. This eye, then, equals a myopic eye of 2 D. This myopic eye would have a far point of 50 cm. Where ^ /the rays of light meet as they come from a myopic eye in a I state of rest is its far point. The Near Point of a Myopic Eye. This is always ^>| closer than in the emmetropic eye for a corresponding age, and depends upon the distance of its far point. For exam- ple, an eye at twenty-five years has 8.5 D. amplitude of accommodation, but if it has a far point of 70 cm., then its near point will be represented by 8.5 D. and 70 cm., /. c., 1.5 D., which would equal 10 D., or a near point of 10 cm. . ^C-^W^ The following table gives the comparative near points jn an emmetropic eye, a hyperopic eye of 2 D., and a myopic eye of 2 D. : AGE. 10 15 20 25 30 35 40 45 5 55 60 65 70 75 F.tnnu-tropia, p.p. 7 8.3 10 12 14 18 22 28 40 5S 100 UT 400 at 2 1 ). 11 vperopia, " 8.3 10 125 16 20 28.5 40 66 200 00 2 D. Myopia, " 6 7 -3 10 11 13 15-3 " 22 25 33 36 44 50 Determining the Vision. This may be considered as the method of finding out what an eye can see without any lenses placed in front of it ; in other words, determining the vision may be defined as ascertaining the seeing quality of the 7 74 REFRACTION AND HOW TO REFRACT. _ L 6 A T G"Y D FAVHU DLOECN CPAR'CEOU unrefracted eye. The refraction of an eye should never be confounded with the visual quality, as refraction applies to the refractive media ; for example, an emmetropic eye with a hemorrhage at the fovea would be practically without visual quality, and yet its refraction or refractive condition would be standard. The most acute vision is at the fovea and the region immediately surrounding it, but this sensi- bility diminishes as the fovea is departed from and the per- ipheral portion of the retina approached ; this is due to the fact that the cones are as close as 0.002 mm. at the macula, and not so close or C_ f>^ numerous in the forepart of ^^ *^ the eye-ground. Test-type or Test-let- terS f r Distant vision -- To determine the vision we employ cards on which are engraved test-type or letters of various sizes, constructed so that each letter subtends an angle of five minutes, as suggested by Snellen, and described on page 64. Figure 73 shows such cards of test-letters, reduced in size. The Roman characters just over the top of the letters indi- cate the distance in meters that the letters should be seen by the standard eye, and the little figures at the left of the letters indicate the equivalent distance in English feet. The top letter should be seen at 60 meters, and the bottom letters at 3 meters ; the intervening letters are to be seen at the respective distances indicated. As it is not unusual to find eyes that have a seeing quality better than that obtained O A L G Y D D T n VFHUA LE C~H D O GEORLCPA FIG. 73. Randall's Test-letters. Block letters in black on cream-colored cards. TEST-TYPE OR TEST-LETTERS FOR DISTANT VISION. 75 with Snellen's type constructed on the angle of five min- utes, Dr. James Wallace has constructed letters which sub- tend an angle of only four minutes. Such a card is shown in figure 74 and has a large field of usefulness. While test-cards are usually white or cream-colored, with black letters, Gould has white letters constructed on black cards. (See Fig. 77.) As white stimu- . Cj lates the retina and black does not, it * will be recognized at once that in one -Q ^3 instance the card, and in the other the letters, produce the retinal stimulation. The white letters seem to stand out from the black card al- most as if they were embossed, giving a clear-cut edge and most soothing effect to the eye under ex- amination, and can be recognized when subtending a much smaller angle than the black letters. To this card should be -o o o -X X Y V a a 8 H M -1 1 O O A R V V T 3 S X *AOO8JT FIG. 74. Four Min- ute Letters of Dr. J. Wallace. This card is constructed principally for re- flection purposes. (See Fig. 7.) ->javoid reflection, hung at an angle. For aliens who do not know the English letters, and for illiterates, a special card has been made, known as the illiterate or "dummy" card, with characters consisting of lines shaped 111 3 LU E 111 3 E 3 ui m ui 3 E FIG. 75. like the capital letter " E," and made to conform to the five- minute angle. As these letters are variously placed, the patient is asked to tell, or indicate with his finger or fingers, 76 REFRACTION AND HOW TO REFRACT. the direction in which the prongs of the "E" point: up, down, to the right or left. This illiterate card (see Fig. 75) is much to be preferred to the German, Hebrew, and " figure " cards occasionally displayed in clinics. Kindergarten Card. To keep the attention of children who do not know the alphabet, or who attend a Kinder- garten school, the author has prepared a card of test objects as shown in figure 76. These objects have been drawn up to the angle of five minutes, j . though some of their com- ponent parts do not measure Um up to one minute. ^^ Selection of Test-cards. ^J f iMt ^ e surgeon should have several of these in duplicate with the order of the letters changed (Figs. 73, 77), as pa- tients not infrequently and unintentionally commit them to memory. Care should be exercised in the selection of test-cards, to see that each letter on the card measures O t o a * A a a T o & FIG. 76. Author's Test Objects or Kindergarten Card for Children and Illiterates. up to the standard square of five minutes, as many of the A's and R's and N's, etc., on the old cards as seen in the shops measure six and seven minutes horizontally. It is a matter of choice with the surgeon whether to use test- cards with the block or Gothic letters. It is well to have both. THE RECORD OF THE VISUAL ACUITY. 77 Method of Procedure. The test-card should be hung on the wall with its ^ line five or six inches below the level of the patient's eyes,, and illuminated by means of reflected artificial light. This is always a certain quantity, whereas daylight is too variable and not to be depended upon. The patient should be placed with his back toward any bright light, and at a distance of six meters from the card. FIG. 77. Gould's Test-letters. Gothic letters in white on black cards. Sometimes the surgeon's office is not six meters long, and this distance must be obtained by using diagonal corners of the room or by using a plane plate-glass mirror and a specially prepared test-card with reversed letters (see Fig. 74), the card being hung as many meters in front of the mirror as will make six meters when added to the length 78 REFRACTION AND HOW TO REFRACT. of the office. While a distance of six meters is always to jbe preferred, yet if this can not be obtained, the surgeon I may use a distance of four meters, but never less than this. Each eye should be tested separately, the fellow-eye being shielded or covered by a card or opaque disc held in front of it or placed in the trial-frame. The eye should never be d shut, and any pressure upon the eyeball must be avoided. The record of the visual acuity is usually made in the form of fractions, using Arabic or Roman notation ; figures usually indicate feet, and Roman letters usually sig- meters^ though there is no fixed rule for this. How- ever expressed, the denominator indicates the size of the I frjt^typjs which the eye reads, at (The distance indicated by the y N (0 0) h* (0 0) ^+>*~' O J x u L D Q. I m J O i Z K S o < 1 a cr 2 i a. ? T o > ^- O DC I Q X H H J i O o < cu B CO - a 1 n 1 OJ cn CO - *^ i 2 [ O ^ N Q " u J X * z o o L. J Q. K U X v z O o a at z u IT J H n X u 1 I Q O h Z a a * * n a. o z x z a. \v J z j V, z o K " o h U m - n 82 REFRACTION AND HOW TO REFRACT. 0. h K O & -1 K X ft P > O rv co 01 H O O H J H O w n D H J J U E-i ^ o ^ K H > * 00 en F* 00 o> W K > H P H H H H O M N n " K O Q O O * en * IA w p H > U X H H PH rt CM CO iH CM H J H u o o X p H P. > a i4 H p. o as O j J H 0. H K X " ** a O H a |l o o Q | > o 0. K ^ j H j J H O ft. P > u H jj 1 H O * in * IA 1C PC a o H H K D j> X X 0. O U J u p. o H O X H u J > j K Pi j " X f> ** M CONVERGENCE. 83 .'another way is to hold the card close to the patient's eye 'and gradually withdraw it until he can just recognize the : letters ; when this point is reached, the distance from the eye to tlie card is measured with the meter stick, and this dis- tance, as also the size of the type which was read, is care- fully recorded. For example, the patient selecting the type marked 0.50 D. and is able to read it as close as 8 cm. and no closer, the record will be " near point equals type 0.50 D. at 8 cm."; or abbreviated, would be "type 0.50 D. = 8 cm." In some instances the patient may not be able to read any of the near type without the aid of a glass, and if so, it will be necessary to place a plus sphere in front of the eye to assist in finding the near point ; for example, if a + 28. was employed, then the record might be "near /' .- - - point equals type 0.50 D. at 12 cm. with -}-2 S.," or " -{-2 /V^ S. = type 0.50 D. at 12 cm." Convergence. Con, " together," and vergere, " to turn " ; literally, turning together. This is the power of the internal recti muscles (especially) to turn the eyes toward the median line ; to " fix" an object closer than infinity. Standard eyes, when looking at an object at a distance of six meters or more, are not supposed to converge ; the visual lines are spoken of as parallel and the power of con- vergence is in a state of repose. The angle which the visual line makes in turning from infinity ( oc) to a near point is called -th~e"angle of convergence, and the angle which is formed at one meter distance by the visual axis with the median line is called tlTerneter angle, or the unit of the angle of convergence. (See I, in Fig. 80.) If the visual line meets the median plane at ^ of a meter, it has then two-meter angles of convergence ; at 1^ of a meter, four-meter angles of convergence, etc. Or 8 4 REFRACTION AND HOW TO REFRACT. five-meter angles means that the eye is converging to a point ^ of a meter distant. The size of the meter angle varies ; it is not the same in all individuals ; in fact, the meter angle is smaller in children than in adults, as a rule, on account of the shorter inter- ocular distance. In children thisliistance is about 50 mm., whereas in adults it is, on the average, 60 or 64 mm. While standard eyes, to see a point one meter distant would converge just one meter angle, they would also accommodate just one diopter ; to see a point at ^ of a FIG. 80. meter they would converge just three meter angles, and at the same time would accommodate three diopters, etc., thus showing how intimately the powers of convergence and accommodation are linked together, though it is possible to converge without accommodation (see Presbyopia) or to accommodate without convergence (paralysis of the in- terni). Far and Near Points of Convergence. Just as we have a far and a near point of accommodation, we also have a far and a near point of convergence. The far point of conver- gence is the point to which the visual lines are directed when convergence is at rest, or at a minimum. The near ANGLE GAMMA. 85 point of convergence is the point to which the visual lines are directed when the eyes are turned inward to their utmost degree. Infinity, or parallelism, is the position of the visual lines in the standard eyes in^a state of rest (E o>, in Fig. 80). Visual lines that diverge in a state of rest can only meet by being projected backward, and, therefore, meet at an imag- inary point behind the eyes (N, in Fig. 80) ; convergence is then spoken of as negative, or minus ( ). If the visual lines meet in a state of rest, then conver- gence is spoken of as positive (-f ). The amplitude of convergence is the distance measured from the far point to the near point of convergence, and is FIG. 81. represented by the greatest number of meter angles of con- vergence which the eyes can exert. Angle Gamma. An understanding of what is known as the angle gamma is important, that the observer may understand and appreciate the real or apparent position of the eyes when looking at a near or distant point. Figure 8 1 shows the line O A (optic axis) and the optic center, or nodal point (N), is situated on this line in the posterior part of the crystalline lens. The line V M is really a secondary axis to this dioptric system of the eye, and unites the object (V) with the fovea centralis at M ; this line is known as the tst fc^X. f m tt*^tte/ ?^&9 S3 -a. ->^^*^ 86 REFRACTION /v V- tf AND HOW TO REFRACT. "^H&* 4-^ #7w< ' ^ '^^ S~ ' visual line. ' The angle formed by the visual line with 'the axis at the nodal point may be considered as the angle gamma.* . If the fovea centralis at M was situated on the optic axis ~*r~& A, then the visual line and optic axis would coincide, and there would not be any angle gamma. In hyperopic and emmetropic eyes the outer extremity of the visual line lies $, 7, 'or, in some instances, as much as 10 degrees to the nasal side of the optic axis (averaging about $ de- grees), and is spoken of as positive, and given the plus sign. In some long myopic eyes, however, the outer extremity of the visual line may lie to the outer side of the optic axis, when it is spoken of as negative, and given the minus sign. To demonstrate the an- gle gamma, the patient is told to look at the point of a pencil or pen held in the hand of the surgeon, about 1 3 inches distant (A in Figs. 82 and 83). If the angle gamma is positive, the eyes will appear divergent to the observer, who looks at the position * This is not a perfectly correct statement, as the real angle gamma is the angle formed by the line of fixation V R with the optic axis, R being the center of rotation. The angle V N O and the angle V R O being so nearly equal, are, for all intents and purposes, considered as the same. FIG. 82. ANGLE ALPHA. of the poles of the corneas or centers of the pupils. (See Fig. 82.) If the angle gamma is negative the eyes will appear convergent that is, they appear to converge to a point in front of the pencil. (See Fig. 83.) The amount of the angle gamma can be measured by using the arc of the perimeter held horizontally, the patient being placed in the same position as when having 1 his field taken. To do this, while the eye fixes the central point, the surgeon passes a candle- flame along the arc until the catoptric image of the flame is seen at the center of the pupil ; this F g position of the candle- flame on the arc is noted in degrees, which is the size of the angle gamma. Angle Alpha. This is the angle formed by the long axis of the corneal ellipse with the visual axis. In the consideration of this angle it must be remembered that the cornea resembles, in its central area, at least, an ellipsoid of revolution, with the shortest radius usually in the verti- cal meridian. The angle alpha is spoken of as positive when the outer extremity of the long axis of the cornea is to the outer side of the visual line, and negative when it is to the nasal side. < CHAPTER III. OPHTHALMOSCOPE. DIRECT AND INDIRECT METHOD. Ophthalmoscope. From oyffa^x;, "eye," and axo-siv, " to observe " or " view "; literally, " to view an eye." An instrument used for studying the media and interior of the eye. The pupil of an eye in health appears to an observer as black ; this is due to the fact that the observer's eye does not ordinarilyintercept any of the rays of light which return from the eye. Rays of light entering an eye are returned toward their immediate source, and, therefore, if an observer wishes to see into or study the interior of an eye, he must have his own eye in the path of the returning rays. To accomplish this, the observer places a mirror in front of his eye, so that the reflected rays entering the eye are returned toward the mirror. There is an infinite variety of these in- struments in the market, but for the general student the modified instrument of Loring appears to meet with most favor. (See Fig. 84.) This has a concave mirror with a radius of curvature of 40 cm., giving a principal focus, therefore, at 20 cm. The sight-hole is round and about 3 ^ mm. in diameter, cut through the glass ; this mirror can be tilted to an angle of 25 degrees. As an improvement over such a mirror, and to take its place, the writer would recommend the mirror used on his own ophthalmoscope, which has a radius of curvature of 15 cm.; and the sight-hole, 2^ mm. in diameter, is not cut through the glass, but is made by removing the quick- 88 OPHTHALMOSCOPE. 8 9 silver. The glass at the sight-hole gives additional reflect- ing surface, and at the same time does away with much annoying aberration which results when the glass is per- forated. FRONT BACK FIG. 84. The small sight-hole is an advantage, also, in looking into small pupils. The mirror, oblong in shape, 18 by 33 mm., is secured at the center of its ends, by two elevated screws, to a hollow disc 4^ cm. in diameter, in which is a revolving milled wheel, containing small spheres, each about 6 mm. in diameter. The series of spheres ranges 8 9O REFRACTION AND HOW TO REFRACT. from I D. to 8 D., and from -f I D. to +7 D. The central aperture does not contain a lens, but is left open. When it is desirable to use any lens stronger than 8 D. or + 7 D., there is an additional quadrant, which can be superimposed and turned into place at the sight-hole ; it contains four lenses, 0.50 D. and 16 D., also +0.50 D. and -{-16 D. With this quadrant and the spheres in the milled wheel, any spheric combination can be made from zero to 24 D. or to + 23 D. An index below the sight-hole of the instrument records the strength of lens FIG. 85. that may be in use ; minus lens.es are usually marked in red and plus lenses in white. pKwU^i V.\ How to Use the Ophthalmoscope. There are two ways or methods by which the ophthalmoscope may be used the direct and the indirect. The Direct Method (see Fig. 85). Proficiency with the ophthalmoscope does not come except from long and constant practice, and several important matters should receive very careful attention before the student attempts to study the interior of an eye. OPHTHALMOSCOPE. 9! The Room. This should be darkened by drawing the shades or closing the blinds ; the darker the room, the better. The Light. This should be steady, clear, and bright ; a good lamp is suitable, but an Argand burner gives more intense light, and is to be preferred, especially if it is placed on an extension bracket that can be raised or lowered and is capable of lateral movement. Position of Light and Patient. The light should be several inches to one side and back of the patient, and on a level with the patient's ear, so as to illuminate the outer half of the eyelashes of the eye to be examined ; it may even be well to have the tip of the patient's nose illuminated. The patient should be seated in a comfortable chair (without arms), and is instructed to look straight ahead into vacancy, or at a fixed object if necessary, and is only to change the direction of his vision when told to do so. Under no circumstances should the patient be allowed to look at a light, as this will contract the pupil. For the beginner, it may be well to dilate the patient's pupil with a solution of cocain or homatropin. The student, however, should learn as soon as possible to see into an eye without the aid of a mydriatic, as many patients seriously object to the slight inconvenience that results from the drugs mentioned. The Observer. If the observer has any decided refrac- tive error, he should wear his correcting glasses ; the reason for this will be explained later. The observer should be seated at the side of the patient corresponding to the eye he is to examine. Examining the right eye, the observer should be on the patient's right ; if the left eye, then on the patient's left. "When examining the right eye, the ophthalmoscope is held in the right hand, before the right eye ; and in the left 92 REFRACTION AND HOW TO REFRACT. hand, and before the left eye, when examining the left eye. The silrgeon's eye should be a little higher than the patient's. and observer should keep both eyes open. The exception to this is when the patient has a squint, when it will be necessary for him to cover the eye not being examined, and in this way the eye under observation will look straight ahead. The surgeon holds the ophthalmoscope perpendicularly, so that the sight-hole in the mirror is directly opposite to his pupil and close to his eye. The side of the instrument rests on the side of his nose or the upper margin is in the hollow of the brow. The mirror is tilted toward the light, surgeon's elbow should be at his side, and not form an angle with his body. With these several details carefully executed, the surgeon" ^begins his examination at a distance of about 25 or 30 cm., never closer ; and at this distance he reflects the light from the mirror into the eye, and observes a " red glare," which occupies the previously black pupil. This is called the " reflex," and is due to the reflection from the choroidal coat of the eye. The color of the reflex varies with the size of the pupil, transparency of the media, the refraction, and the amount of pigment in the eye-ground. Having obtained the " reflex," it will be well for the be- ginner to practise keeping the reflected light upon the pupil by changing his distance, approaching the eye as close as an inch or two ; this must be done slowly, and not with a rush. What the Observer Sees. Having learned to keep the light on the pupil, the next thing is to study the transparency of the media i. e., to find out if there is any interference with the free entrance and exit of the reflected rays, such as would be caused by opacities in the cornea, lens, lens OPHTHALMOSCOPE. 93 capsule, or vitreous ; and, if present, to note their character and exact location, whether on the visual axis or to one side, etc. The next objective points will be mentioned individually, and with the idea of systematizing the study. The Optic Nerve. Also called the disc or nerve head or papilla. Color of the Optic Disc. This has been described as resembling in color the marrow of a healthy bone, or the pink of a shell, etc. ; yet this is not by any means a true statement or description, as the apparent color of the nerve controlled in great part by the surrounding eye -ground whether this is heavily pigmented or but slightly so, or whether there is an absence of pigment, as in the albino. The student should be ready to make allowances for these contrasts. The shape of the disc varies : it may appear round, oval, or even irregular in outline. Usually it is a vertical oval.- The vessels on the disc which carry the blood to and from the retina are not of the same caliber, nor do they have the same curves and branches in all eyes or in the same pair of eyes. The central artery may be single or double (if it has branched in the nerve before entering the eye), and enters the eye at the nasal side of the center of the disc. Approximating the central artery on its temporal side is the retinal vein, which may also be double. The relative normal proportion in size between arteries and veins is nerally recognized as about two to three. The veins are usually recognized by their larger size and darker color. *- At or near the center of the disc is often seen a depression, I known as the physiologic cup ; this may be shallow or 1 deep ; it may have shelving or abrupt edges ; it may even ibe funnel-shaped. 94 REFRACTION AND HOW TO REFRACT. At the bottom of the cupping is frequently seen a gray stippling, the membrana cribrosa ; openings in the sclera for the passage of the transparent optic nerve-fibers which go to form the retina. Surrounding the disc proper is often seen a narrow white ring ; this is sclera, and is known as the ^scleral ring. Just outside of this ring is frequently seen a ring of pigment ; this is called the choroidal ring. In many cases the choroidal ring is not complete, the pigment being quite irregular, or possibly there may be just one large >mass of pigment .to one side of the disc ; this is not patho- logic. The retinal arteries and veins, while possessing many anomalies, and while occasionally an artery and vein are seen to twine around each other, usually pursue a uniform course up and down from the disc, and are named accord- ingly i. c., upper nasal vein and artery ; upper temporal ' vein and artery ; lower, nasal artery and vein ; lower tem- poral artery and vein. T> The retina itself, in health being transparent, is not seen. The fovea centralis, occupying the center of the macular region, is about two discs' diameter to the temporal side of the disc and slightly below the horizontal meridian. The fovea is recognized because it is a depression, and its edges give a reflex ; it is very small, and appears as a bright J spot one or two mm. in diameter. The " macular region " is the part of the eye-ground immediately surrounding the fovea ; it contains minute capillaries, but it is impossible, in Wealthy eyes, to recognize them with the ophthalmoscope. The Choroid. This is distinguished by the character of its circulation, the vessels being large, numerous, and flat- ^>tened, and without the light streak which characterizes the retinal vessels. (iPigment areas between the vessels are also diagnostic of this tunic) The choroidal circulation is best OPHTH A LMOSCOPE. 95 studied in the blond or albino, and may be seen in many eyes toward the periphery of the eye-ground. In the foregoing description of the use of the ophthal- moscope, etc., it is presumed that the instrument has been used without any lens in position, and that the observer's eye and the eye under examination are healthy emmetropic eyes with the accommodation at rest. Figure 86 shows the position of the light, L, the ophthalmoscope, the examiner's and the examined eye under these conditions. The divergent rays from the light (L) are reflected con- FIG. 86. vergently from the concave mirror, and focusing in the vitre- ous, they cross' and form an area of illumination on the retina at I I'. The retina, situated at the principal focus of the dioptric media, naturally projects out from its indi- vidual points rays of light which are parallel as they leave the eye ; some of these pass through the sight-hole of the mirror and meet upon the retina of the observer's emme- tropic eye. There are twt> very important points which must be 9 6 REFRACTION AND HOW TO REFRACT. considered when using the ophthalmoscope in the direct method : one is the direction which the rays of light take > as they leave the eye under examination, and the other is for the observer to keep his own eye emmetropic ; in other words, the observer wearing his correcting glasses should ji ot accommodate. Figure 87 shows that rays of light passing out of an eye divergently must be made parallel, so as to focus upon the surgeon's own retina (emmetropic), and to do this it is FIG. 87. T B indicate points at the edge of the disc from which rays pass out of the eye divergently in the direction T x B', 1"' B', T' B', and being received by the observer's eye, are projected backward, forming an erect magnified image at T" W. This image is not so large as that seen when looking into a myopic eye. (Fig. 88.) necessary to turn a plus lens in front of the sight-hole of the ophthalmoscope ; the strength of the convex lens thus employed, other things being normal, is the amount of the refractive error of the eye being examined. Figure 88 shows rays of light passing out of an eye convergently, and to have them parallel, so as to focus upon his own retina (emmetropic), it is necessary to turn a concave lens in front of the sight-hole of the ophthalmo- OPHTHALMOSCOPE. 97 scope ; the strength of the concave lens thus employed, other things being normal, is the amount of the refractive error of the eye under examination. The Observer's Accommodation. It has already been stated that, when using the ophthalmoscope, the observer should wear any necessary correcting lenses. If the ob- server has a refractive error and does not wear his glasses, he must deduct this amount from the lens used in the ophthalmoscope. If he has two diopters of hyperopia --B" FlG. 88. T B indicate points at the edge of the disc from which rays pass out of the eye convergently in the direction T' W , and, being received by the observer's eye, are projected backward, forming an erect magnified image at T" B" '. This image is much larger than that seen when look- ing into the hyperopic eye. (Fig. 87.) himself, and the lens used in the ophthalmoscope is plus four diopters, then the eye under examination has only two | diopters. It is not unusual for beginners to see the eye- Aground (disc) in hyperopic eyes with a strong concave lens ; this is due to the fact that they accommodate. Prac- tice will overcome this habit, and it should be mastered as - soon as possible. There are several ways of doing this : one is to begin the examination at a distance of 30 or 40 9 98 REFRACTION AND HOW TO REFRACT. cm. from the eye, with both eyes open, and to gradually I approach the eye as close as 3 cm., imagining all the time / that one is looking for some remote point ; otherwise, if one begins the examination close to the eye, and imagines he is going to see an object about an inch away, he will most invariably accommodate several diopters, with the result ^> that he turns a strong concave lens in front of the sight- hole of the ophthalmoscope to neutralize his accommodation. / This explains how so. many beginners diagnose all cases /of hyperopia as myopia. An excellent way to learn to relax the accommodation is to practise reading fine print at a distance of about thirteen inches through a pair of plus three lenses, placed before the surgeon's emmetropic eyes. Another good way to learn to relax the accommo- dation is to practise on one of the many schematic eyes found in the shops. (Fig. 143.) Size of the Image of the Eye-ground (Figs. 87 and 88). In concluding the subject of the direct method of examination it may be interesting to note the apparent size of the image of the eye-ground, which, it must be remem- ^bered, is virtual, erect, and enlarged ; in fact, it seems to be at some distance behind the eye, and if the student has paid close attention to the study of images as formed by convex lenses, detailed in chapter i, he need not have any difficulty in appreciating these facts. The optic disc of an emmetropic eye, as seen through ophthalmoscope, appears to be about 25 mm. in diam- iter, and about 250 mm. away. The retina of the emme- tropic eye is about 1 5 mm. from its nodal point ; then the actual size of the emmetropic disc is -/-$ of 25, or -f , or 1.5 mm. ; then 15 is to 250 as 1.5 is to 25, or 16.6 the magnification, in other words, when the emmetropic disc is observed, it appears about 16.6 times larger than it actually is. ]u~ I OPHTHALMOSCOPE. 99 The Indirect Method (see Fig. 89). Practising this method, the observer sees a larger part of the eye-ground FIG. 89. at one time, but it is not so perfect in detail nor is it mag- nified to the same extent as in the direct method. The observer does not have to get so close to his patient, which is a decided advantage in some clinical cases. Unfortun- ately, as a preliminary step, it is often neces- sary to dilate the pu- pil. In addition to the ophthalmoscope, there is also required a convex lens of known strength and large aperture ; the one which comes in the case with the scope is usually too small and too strong for general use. The writer prefers his plus 13 D. with metal rim and conve- nient handle, shown in figure 90 (reduced one-third in size). FIG. 90. IOO REFRACTION AND HOW /to KKFKACT. _r* X This is held at about three inches in front of the eye under examination, the observer resting his little and ring fingers on the temple of the patient. The light may be 7 over the patient's head, or to the side corresponding to the eye under examination, the patient being instructed to look with both eyes open toward the surgeon's right ear when ithe right eye is being examined, and toward the surgeon's left ear when the left eye is examined. With a +4 D. in the ophthalmoscope held close to his eye, the surgeon seats himself in front of the patient at .*> about sixteen inches distant, and reflects the light through the condensing lens into the patient's eye, and then approaches or moves away from the eye until he recog- nizes clearly a retinal vessel or the disc ; he must remem- ber, however, that he is not looking into the eye, but is (viewing an aerial image formed between the convex lens land the ophthalmoscope ; this image is not only inverted, but undergoes lateral inversion, so that the right side of the disc becomes the left side of the image, and vice versa ; the upper side of the disc becomes the lower side of the image, and vice versa. As the direct method gives Ian erect, virtual, and enlarged image, the indirect method \produces an inverted, real, and small image. The principle pf the direct method is similar to a simple microscope, and the indirect to a compound microscope. I The size of the image depends upon the refraction of ( the eye and the distance of the convex lens from the eye under examination. Mn the standard eye this is always the same, no matter how far away from the eye the convex lens is held. )To estimate the size of the image in the standard eye, all that is necessary to know is the principal focal dis- tance of the lens employed ; if a +13 D., then the image is formed at 75 mm. (three inches), and remembering that the " OPHTHALMOSCOPE. IOI retina in the eye is 1 5 mm. back of the nodal point, the size of the image will be to the size of the disc (if that is what is looked at) as their respective distances, or as 15 is to 75 which equals 5, the magnification. The purpose of the +4 D. in the scope is to take the place of the eve -piece in the microscope, and, therefore, to magnify the image at the same time it relieves the observer's (accommodation. In high myopia the -(-4 D. may be dis- pensed with. The Luminous Ophthalmoscope (Figs. 91 and 92). DcZeng Patent. This instrument is a combination of the Loring ophthalmoscope just described, with the addition of an electric light attachment. The mirror is somewhat dif- ferent from the mirror on the Loring instrument. It is f plane, circular in form, 14 millimeters in diameterfwith one millimeter of its upper area cut off horizontally^- The flat top edge of the mirror is on a level with the lower edge of the sight-hole in the ophthalmoscope. The observer in looking through the sight-hole always looks over the mirror and never through it. J The mirror is placed at an angle of 43 degrees. The handle of the instrument carries the elec- tric wires to a small lamp, and between the lamp and the mirror is placed a very strong convex lens. The rays of light from the filament falling upon the convex lens are refracted very coiivergently, and after reflection from the mirror converge to a point one inch distant. (See Fig. 92.) This instrument is ideal for both the direct and indirect method and has the following points of merit: The mirror and light arc stationary, thus giving the observer any liberty of movement necessary without any loss of reflec- tion from the mirror; the mirror never requires any tilting; the brilliancy or intensity of the illumination at the ftindus, by virtue of the light being so close to the mirror, far ex- IO2 REFRACTION AND HOW TO REFRACT. ceeds that of the nonluminous instrument; for the same reason the size of the retinal illumination is made about five FIG. 91. FIG. 92. FIGS. 91 and 92. DeZeng Luminous Ophthalmoscope. Two-thirds size. times larger than that by the old style instrument. The heat from the electric lamp (2^4 volts, ^ ampere) is infinitesimal. CHAPTER IV. EMMETROPIA. HYPEROPIA. MYOPIA. KMMETROPIA. Emmetropia ('>, "in"; plrpw, "measure"; fy 1 , " eye ") literally means an eye in measure, or an eye which has reached that stage of development where parallel rays of light will be focused on its retina without any effort of accommodation. As the emmetropic eye is the ophthal- mologist's ideal unit of measurement or goal in refraction, the beginner should know this form of eye thoroughly, so that he may recognize any departure from this standard condition. The emmetropic eye may be described in vari- ous ways, and while these descriptions may appear like repetitions, they are given for purposes of illustration : , The standard or schematic eye: Authorities differ some- what in the exact measurements of a schematic eye, but the one suggested by Helm- holtz is certainly worthy of careful consideration. (See P- 59-) An emmetropic eye is one which, in a state of rest (without an}- effort of accom- FIG. 93. modation whatever), receives parallel rays of light exactly at a focus upon its fovea. (See Fig. 93.) An emmetropic eye, therefore, is one which, in state of rest, emits parallel rays of light. (See Fig. 93.) 103 104 REFRACTION AND HOW TO REFRACT. An emmetropic eye is one whose fovea is situated exactly at the principal focus of its re- fractive system. (See Fig. 93.) *l An emmetropic eye is one the vision of which, in a state of rest, is adapted for infinity. \ ' An emmetropic eye is one [which has its near point consist- ent with its age. (See p. 70.) An emmetropic eye is one which does riot develop pres- 7byopic symptoms until forty- five or fifty years of age. (See p. 272.) \ An emmetropic eye, in con- tradistinction to a myopic eye (see p. 115), is spoken of as ia healtKy eye, or one which A shows the least amount of irri- tation in its choroid and retina. Because we refer to Hclm- holtz's schematic eye as an em- metropic eye, it will not do to say that all eyes that measure just 23 mm. in their antero- posterior diameter are emme- tropic (Fig. 94); for while an FIG. 94. I. Emmetropia. 2. Myopia due to a strong lens. 3. Hyperopia due to a weak lens. 4. Myopia due to a short radius of curvature of cornea. 5. Hyperopia due to a long radius of curvature of cornea. The anteropos- terior diameter of all these eyes is just 23 mm. AMETROPIA. IO5 eye maybe just 23 mm. in length, it may have its refractive system stronger or weaker than is consistent \vith its length, making it,. if stronger, a myopic or long eye, and, if weaker, a short or hyperopic eye. An eye, to be emmetropic, therefore, no matter what its length, must have its refractive apparatus of just such strength that, in a state of rest, the principal focus will coincide exactly with the cones at the fovea. (Fig. 93.) cu AMETROPIA. Ametropia (a priv. ; /^r/>..>, "a measure" ; os, "sight") literally means " an eye out of measure." An ametropic eye is one which, in a state of rest, docs not form a distinct image of distant objects upon its retina. An ametropic eye may be defined as one which, in a state of rest, does not focus parallel rays of light upon its fovea. fAn eye which is not emmetropic is ametropic.J) There are two of ametropia axial and curvature ametropia. Axial ametropia is the condition in which the dioptric apparatus refracts equally in all meridians, but the retina of the eye, when at rest, is either closer to, or further away from, the nodal point than the principal focus. (See Figs. 95 and 97.) The refraction is measured on the length of the anteropostcrior axis of the eye ; hence its name, axial metropia. Curvature ametropia, in contradistinction to axial ame- tropia, is the condition in which the dioptric apparatus does not refract equally in all meridians, and with the result that there is no focusing of all the rays at any one point ; or curvature ametropia may be considered as that condition in which parallel rays of light entering an eye have two focal planes for two principal meridians at right angles to each other. Curvature ametropia is commonly spoken of as astigmatism. (See Chap, v.) | ____ IO6 REFRACTION AND HOW TO REFRACT. Varieties of Axial Ametropia. Axial ametropia is of two forms : one in which the eye has its fovea closer to the Idioptric apparatus than its principal focus (see Fig. 95), | known as the hyperopic, short, or flat eye ; and the other form of the eye in which the fovea is further away than its principal focus, known as the myopic or long eye. (See Fig. 97.) HYPEROPIA OR HYPERMETROPIA. Hyperopia (t>-lp, "over"; &?, "eye") literally means an eye which does not equal the standard condition, or an eye which is less than the standard measurement. Hyper- opia is often abbreviated H. About twenty per cent, of all eyes have simple hyperopiaj) The hyperopic eye is spoken of as far-sighted, and the condition as one of far- sightedness. The hyperopic eye may be described in many different ways : 1. The " natural eye," or "the eye of nature." tf> X^n^*** 2. The "short eye." This term is used on account of its fovea lying closer to the dioptric apparatus than the principal focus. 3. Parallel rays of light passing into a hyperopic eye in P a state of rest fall upon its retina or fovea before they focus. I (See Fig. 7 1 .) 4. Rays of light from the fovea of a hyperopic eye in a state of rest pass out divergently (see Fig. 95), and the condition is equivalent to a convex lens refracting rays of 1 light which proceed from a point close'r to the lens than its principal focus. (See Fig. 39.) 5. A hyperopic eye is one which, in a state of rest, can receive only convergent rays of light at a focus upon its fovea (Fig. 95) ; therefore, to repeat : the hyperopic eye, in a state of rest, emits divergent rays and receives convergent rays at a focus upon its fovea. HYPEROPIA. IO7 6. As convergent rays are not found in nature, and are, therefore, artificial, a hyperopic eye is one which, in a state of rest, requires a convex lens to focus parallel rays of light on its fovea. (See Fig. 96.) 7. A hyperopic eye is one which must^accommodate for infinity, and, in fact, for all distances ; in other words, a hyperopic eye in use is in a constant state of accom- modation. 8. A hyperopic eye having to use some of its accommo- dative power for infinity, must, in consequence, have its near point removed beyond that of an emmetropic eye of corresponding age. (See p. 72.) 9. From the description contained in 3, it follows that FIG. 95. FIG. 96. the far point of a hyperopic eye in a state of rest is negative ( ), and is found by projecting the divergent rays back- ward to a point behind the retina. (See Fig. 95.) 10. From the description contained in 6, and the descrip- tion of accommodation on page 67, it is natural to find the ir. retina and choroid of many hyperopic eyes in a state of irritation. 11. From the description contained in 6 and 7, and on page 273, it follows that symptoms of presbyopia manifest themselves earlier in hyperopic than in any other form of eyes. 12. From the description contained in 5, and this may IO8 REFRACTION AND HOW TO REFRACT. appear like repetition), it follows that a hyperopic eye will '"* accept a plus glass for distant vision. (See Fig. 96.) 13. From 6 it is evident that the circular fibers of the ciliary muscle must become highly developed ; much more so than the longitudinal fibers. Microscopically, a section of the ciliary muscle on this account will bear evidence of >iS ,/^th(f character of the eye from which it came. Causes of Hyperopia. It is a well-known fact that the ^eyes of the new-born are, with comparatively few excep- tions, hyperopic ; such eyes are supposed to grow in their anteroposterior diameter, and at adolescence to reach that stage of development called emmetropia. It is also a well- known fact that this ideal condition of emmetropia is very rarely attained, the length of the eyeball not increasing in proportion to the strength of its refractive system. Eyes may approximate the emmetropic condition, but very seldom remain so, passing into the condition in which the fovea lies beyond the principal focus, becoming what is known as long, or myopic. A standard eye may be made hyperopic by removing its lens ; the condition following cataract extraction. (See Fig. 174.) / An eye may possibly become hyperopic in old age, from flattening of the lens due to sclerosis of its fibers. Any disease which will cause a flattening of the cornea in a standard eye will produce hyperopia. ^>A. diminution in the index of refraction of the media of the standard eye will produce hypcropial^^Aist/Tfy Subdivisions of Hyperopia. For purposes of study hyperopia has been divided into six classes or forms : I. Facultative hyperopia (abbreviated Hf.) is a condi- tion of the eye in which the patient can overcome the error by using his accommodation. It is a condition of early H\TEROPIA. IO9 t life, and is voluntary. The patient can see clearly, with or without a convex glass. 2. Absolute hyperopia (abbreviated Ha.). This is hy- Iperopia that can not be overcome by the accommodative I effort. It is generally a condition of old age, and is invol- untary ; facultative hyperopia in youth becomes absolute in old age. ^ Old age, in fact, may develop each variety except latent hyperopia. Absolute hyperopia exists whenever the defect is of so high a degree that it can not be overcome by the accommodation or when the accommodative power itself is gone. 3. Relative hyperopia (abbreviated Hr.) is where ac- commodation is assisted in its efforts by the internal recti muscles ; in other words, the eyes squint inward. 4. Manifest hyperopia (abbreviated Hm.) is repre- sented by the strongest convex lens through which an eye can maintain distinct distant vision. Manifest hyperopia, therefore, includes facultative and absolute. 5. Latent hyperopia (abbreviated HI.) is the amount \of hyperopia which an eye retains when a plus lens is '' (placed in front of it. /Or latent hyperopia is the difference between the manifest hyperopia and that lens which an eye would select if its accommodation was put at rest with a cycloplegic (atropin)J For example, an eye accepts a -f- 1.25 S. as its manifest H., and, when atropin is instilled, would accept +2.75 S. for the same distant vision ; then the difference between the manifest +1-25 S. and +2.75 S. (the total) is +1.50 S., which is the latent hyper- opia. 6. Total hyperopia (abbreviated Ht.) is the full amount --> of the hyperopia ; or is represented by the strongest glass which an eye will accept, and have clear, distinct vision when in a state of rest. &/v IIO REFRACTION AND HOW TO REFRACT. Symptoms and Signs of Hyperopia.- These are many and various ; the principal one, however, and the one that ^generally causes the patient to seek relief, is headache. Headache caused by the eyes is usually frontal, and is denominated " brow ache " ; it may be frontotemporal ; the pain or discomfort starting in or back of the eyes may extend to the occiput or all over the head, and be accom- panied with all kinds of nervous manifestations. The most characteristic distinguishing feature of ocular headache is that it comes on while using the eyes, and gradually grows worse as the use of the eyes is persisted in ; and, likewise, the headache gradually ceases after a few minutes' or hours' rest of the eyes. Vertex headache, or a feeling of weight on the top of the head, has been preempted by the gyne- cologist, and is not usually classed as ocular. The ciliary muscle being the prime factor in causing the headach the writer feels justified in calling it the " headache mus- cle." " Sick headaches " are largely due to eye-strain. Various functional disorders, such as dyspepsia, constipa- tion, biliousness, lithemia, chorea, convulsions, epileptoid diseases, hysteria, melancholia, etc., are, according to some few authorities, attributable to this condition. See Asthen- opia, page 219. Blepharitis marginalis, styes, and conjunctivitis are frequently present, and in truth the hyperopic eye on ^ this account can often be diagnosed in public outside of the surgeon's office. A feeling as of sand in the eyes, ocular pains or postocular discomfort, a dryness of the iids, as if they would stick to the eyeballs, are common complaints, and part of the conjunctivitis. Other patients have their eyes filling with tears (epiphora) as soon as they -jr begin reading, etc. A drowsiness or desire to sleep often comes on after or during forced accommodation. r / HYPEROPIA. I I I Congestion of the choroid and retina, as evidenced by the ophthalmoscope, often go together with the blepharitis and conjunctivitis. The patient complains that the print blurs or becomes dim after reading, and this is especially apt to occur by artificial light. When the "blur" comes on, he has to stop and rub his eyes or bathe them ; and then, with additional light, he is able to continue the reading for a short time longer, when the blur again returns and the effort must be given up. Strong light stimulates the accommodation. The " hyper- > opic blur" is nothing more or less than a relaxation of the accommodation. In children hyperopia sometimes simulates myopia, from the fact that the child in reading holds the print very close to the eyes. He does this in order to get a larger retinal image and to relieve his accommodation ; the retinal image is not clear, and the child has to read slowly ; the retinal image is composed mostly of diffusion circles. The child holds the print close to his eyes to avoid using his total accommodation, which he might have to do if he held the print at a respectable distance. He also calls into play the orbicularis palpebrarum, and narrows the palpebral fissure, looking through a stenopeic slit, as it were. These cases of simulated myopia can be 7 jquickly diagnosed by : I. The narrow r palpebral fissure during the act of reading, and reading very slowly, as each letter has to be studied. 7 2. The fact that very few children have myopia. 3. The comparatively good distant vision, as a rule, which myopes never have, unless the myopia is of very small amount. 4. The ophthalmoscope. The beginner in ophthalmology should be on his guard I 1 2 REFRACTION AND HOW TO REFRACT. for these "pseudo-myopias," and not be guilty of putting concave lenses on hyperopic eyes. Diagnosis of Hyperopia. This form of ametropia may be recognized in many ways : 1. Blepharitis marginalis, if present, is generally due to hyperopia. 2. Hyperopic eyes are said to be small, and to have . small pupils, which facts are generally confirmed ; but myopic eyes sometimes appear small, and have small pupils also. 3. A narrow face and short interpupillary distance are quite indicative of hyperopia, but these indexes are not infallible. 4. A child with one eye turned inward toward the nose , ' (convergent squint) has hyperopic eyes, as a rule ; the hyperopia generally not being of the same amount in the two eyes, the squinting eye usually being the more hyperopic. 5. It has been authoritatively stated that light-colored irises are seen in hyperopic eyes and dark irises are to be \found in myopic eyes, and yet this is not always correct. German students, with their blue irises, will average from 50 per cent, to 60 per cent, of myopia. I 6. Hyperopic eyes, with few exceptions, have excellent / distant vision : often ~, or even better. The student should be on his guard for this, and not imagine, because a patient has vision, that he is emmetropic ; on the con- trary, hyperopic eyes accommodate for distance, and obtain this acute vision by effort. 7. The patient gives a history of accommodative asthe- nopia, with or without headaches coming on during or after the use of the eyes. 8. The distant vision of a hyperopic eye may remain MYOPIA. -II3 unchanged or may be improved with the addition of a convex lens, which latter would be impossible in emme- / i ">/ A^^Lti^i* ->t^ &^r tropia and myopia. ' '^C~^'t^ r*4- -7^^ t-*c* * 9. The near point of a hyperopic eye without glasses lies beyond that of an emmetropic eye for a corresponding age. 10. A hyperopic eye can see fine print clearly through a convex lens at a greater distance than its principal focus, which would not be the case in any other form of eye. Other tests for determining hyperopia are with (i i) the ophthalmoscope, (12) the retinoscope, (13) Scheiner's test, (14) Thomson's ametromcter, and (15) the cobalt-blue glass test, commonly spoken of as the chromo-aberration test. These tests are described in the text' MYOPIA. ..,, Myopia (v^b, "to close"; a><}>, "eye") means, liter- ally, "to close the eye," and this origin of the name has arisen from the fact that many long eyes (myopic) squint the eyelids together when they endeavor to see beyond their far pointNBrachymetropia is another name occasionally mentioned for the same kind of eye. Myopia is abbreviated About 1.5 per cent, of all eyes have simple myopia. The myopic eye is spoken of as near-sighted, and the condition as one of near- sightedness. The myopic eye may be described in many different ways : 1. The long eve. The - r IG. 97- origin of this name is purely anatomic, thoffovea lying beyond the principal focus of the refracting system. (See Fig. 97.) 2. Parallel rays of light entering a myopic eye focus in 114 REFRACTION AND HOW TO REFRACT. the vitreous humor before they can reacli the fovea. (See Fig- 97-) 3. Rays of light from the fovea of a myopic eye pass out of the eye convergently (see Figs. 72 and 98), focusing at FIG. 98. some point inside of infinity. The refractive condition of ja myopic eye is similar or equivalent to a convex lens -^ Irefracting rays of light which proceed from some point (further away than its principal focus. (See Fig. 37.) The /nearer the emergent rays of light focus to the eye (in a state of repose), the longer the eye ; and the further away the emergent rays focus from the eye, the nearer the eye approaches to emmetropia, or normal length. 4. A myopic eye is one which receives rays of light which diverge from some point closer than six meters at a focus on its fovea and which emits convergent rays. (See Fig- 37> ar >d also description of conjugate foci.) 5. As parallel rays can not focus on the fovea of a myopic eye, it is necessary to give parallel rays entering the eye a certain amount of divergence, so as to place the focus at the fovea ; and to accomplish this, a concave lens must be used. (See Fig. 99.) A myopic eye, therefore, is one FIG. 99. MYOPIA. 115 which requires a concave lens to improve distant vision. (See Fig. 99.) /6. A myopic eye is one whose distant vision is made worse by the addition of a convex lens. 7. A myopic eye is one which does not accommodate for ,. t distance. 8. A myopic eye having a refracting system stronger than is consistent with its length, or vice versa, greater length than is consistent with its dioptric system, naturally does not use any accommodation except for points inside of its punctum remotum, and with the result that its ampli- tude of accommodation is used near by ^consequently, a myopic eye is one which has a near point closer than an , emmetropic eye of corresponding age. ) (See p. 73.) 9. From the description contained in 3 it follows that the far point of a myopic eye is positive (jj^). j^i vry( 10. From the description contained in 3 and 7, it also follows that the myopic eye does not develop presbyopic symptoms until late in life. 1 1. From 6 and 9 it follows that the circular fibers of the ciliary muscle are not used to the same extent in a myopic eye as in the emmetropic and especially in the hyperopic eye. Microscopically, a section of a ciliary muscle on this account will bear evidence of the character/ | of the eye from which it came, and have the longitudinal ' fibers more in evidence. In some very long myopic eyes there may not be any circular fibers recognized. i 2. I -AX-S in which the myopia is progressive are spoken of as " sick eyes."/ "r Ln^*+*-^*-*-<~ >lE>2<*-*<^- P*J*-*-~*^' " ' ~ ^ Causes of Myopia. Any disease or injury which will / so alter the refracting system of an eye that parallel rays / must focus in front of the fovea will produce the form of <^ r *-**^-^ eye known as long or myopic. This may be brought about > in different ways : A shortening in the radius of curvature of the cornea, such as comes with conic cornea and staphy- loma of the cornea ; an increase in the refractive power ^ &* of the lens from swelling, as often precedes cataract, and is spoken of as "false second sight ; /cyclitis and irido- cyclitis, which diseases cause- a relaxation of the lens ligament, allowing the lens to assume a greater convexitvj or ciliary spasm may produce temporarily the same con- dition. Technically, however, myopia is quite universally under- stood to mean a permanent elongation of the visual axis of the eye beyond the principal focus of its refracting system. Heredity is certainly a predisposing factor to myopia, but this does not mean that the babe is necessarily born with long eyes. On the contrary, the eye is very likely hyperopic at birth, and/what the child may inherit is weak (eye tunics,) Such eyes, when placed under strain or what xfto them is overuse, soon become elongated. This may also be brought about or assisted by poor hygienic surround- ings, poor health, or develop after an attack of typhoid or one of the~eruptive fevers. .T^Three causes for the elongation of eyes have been brought forward by able authorities and expounded as theories, any one of which, or all three, may appear conspicuously in in- dividual cases. 1. Anatomically, the size of the orbit and the broad face give a long interpupillary distance and cause excessive convergence (turning inward of the eyes) when the eyes fix at the near point. 2. Mechanically, when the eyes are far apart and attempt to converge, the external recti muscles press upon the outer side of the globes, flattening the eyes laterally, MYOPIA. 1 1 7 with the result that the point of least resistance for the compressed contents of the globes is at the posterior pole of the eye, and here it is that the pressure shows itself, by an elongation of the eye backward in its anteroposterior diameter. ^This combination of the anatomic and mechanic theories may explain in great part the presence of myopia in the average German student or any broad-faced indi- vidual.^ 3. The inflammatory theory is that a low grade of inflammation attacks the tunics of the eye, especially at the posterior pole,, and is spoken of as macular chor- (oiditis ; this is brought about by faulty use of the eyes, in the school or in the home, in a poor light or too glaring a light improperly placed, or by using the eyes with the head bent over the work so that the return circu- lation from the retina and choroid is interfered with./ This inflammation or congestion of the tunics of the eye may be primary in itself or secondary to the anatomic and mechanic causes. Be this as it may, the conditions exist, and go to show more and more that myopia is actually acquired and not per sc congenital. " The inherited con- genital anomalies of refraction, particularly astigmatism, are responsible for the myopic eye, by virtue of the pathologic changes they occasion in hard-worked eyes rather than any inherited predisposition to disease." (Risley, "School 1 Ivgiene.") Symptoms and Signs of Myopia. While the myope may complain of headache and symptoms of accommodative /asthenopia, yet the principal visual complaint will be the , inability to see objects distinctly which lie beyond the far point. The myope's world of clear vision is limited to the distance of the far point, where the rays of light leaving his eye come to a focus. Every object situated beyond the far Il8 REFRACTION AND HOW TO REFRACT. point is blurred and indistinct, and the further the object from the far point, the more indistinct it becomes. The myopic child at school soon ranks high in the class, is fond of study, of books, music, or needlework, according to the sex. The myope, in other words, is usually literary in taste. Myopes avoid out-of-door sports, such as foot-ball, base-ball, golf, etc. Diagnosis of Myopia. This form of ametropia may be recognized in various ways : 1. The prominent eyeball. This is not a positive sign of myopia, though this and other signs are mentioned for the reason that they are often present in the myopic condition. 2. The broaa face and (3) long interpupillary distance are quite significant of myopia, and yet the broadest face with longest interpupillary distance the writer ever saw was in a hyperopic subject. 4. Divergent squint usually indicates myopia, and this condition is often brought about by an inability to converge, ' or one eye may be more myopic than its fellow, with the result that the more myopic eye turns out and soon be- comes amblyopic. 5. (It has been stated that myopic eyes usually have dark-colored irises>( but this is often a fallacy, as is only too evident in the German student with his blue iris. The foregoing are but signs of myopia, and are recog- nized by inspection ; they should be looked for and care- fully estimated, and each given its due consideration. 'Subjective and objective symptoms are the true tests of myopia, and are as follows : 6^>Poor distant vision ; inability to see numbers on the houses across the street or on the same side of the street ; history of passing friends without speaking to them. The myope enjoys close work and takes little or no interest in MYOPIA. I 19 sports. A history, in other words, that is in keeping with a vision of short range. 7.-^Good near vision ; ability to see the finest print or to thread the finest needle or do the finest embroidery. 8. The near point is closer than that of an emmetropic eye of corresponding age. (See p. 73.) I 9. Distant vision is made worse by the addition of a convex lens. The writer prefers to teach the diagnosis of myopia in this way, and not to say that a concave lens will improve distant vision ; of course it will, but he does not want the student to put concave lenses before the eye of the young "pseudo-myope," referred to under Hyperopia. io.(ihe far point is brought nearer by the addition of a convex lens./ Objective methods of determining myopia are by means of the 11. Ophthalmoscope. 12. Retinoscope. 13. Schemer's method. 14. Thomson's ametrometer. 15. Chromo-aberration test. Direct Ophthalmoscopy in Axial Ametropia. Pro- ficiency in this method comes only by perseverance and long practice. It should not be employed to the exclusion of other and more exact methods. To estimate with the ophthalmoscope which lens is. required to give an eye emmetropic vision, three very important facts should receive careful attention : 1. The distance between the surgeon's and patient's eye. 2. The surgeon's and patient's accommodation. 3. The surgeon's own refractive error. First, the surgeon should have his eye as close to the patient's eye as possible, usually at 13 mm. ; this is the I2O REFRACTION AND HOW TO REFRACT. /anterior principal focus of the eye, and is the distance at '/which the patient will wear his glasses. Second, as already explained, the observer's and patient's accommodation should be in repose. The most difficult part for the student to learn is to relax his accommoda- tion. The ambitious student strains his accommodation ; (ciliary muscle) in his haste, and with the result that he ' thinks all eyes myopic and all eye-grounds as affected with " retinitis." Third, the surgeon, if not emmetropic, must wear any necessary correcting lenses ; otherwise, the lens in the ophthalmoscope will record his and the patient's error together, and deductions must be made accordingly. For instance, if the surgeon is hyperopic -f 2 S., and does not wear his glasses, and the ophthalmoscope records the fundus as seen clearly with -j- 5 S., this would mean that the patient had +3 S. (2 of the 5 S. being the surgeon's error); or if the fundus is seen without any lens in the ophthalmoscope, then the patient's error would be 2 S. (the surgeon's + 2 S. from o leaving 2 S.) ; or if the ophthalmoscope showed 2 S., then the patient's error would be 4 S. ; or if the ophthalmoscope registered -\-2 S., then the patient would be emmetropic, and this -j-2 S. is the sur- geon's error. Rules. i. When the surgeon and patient are both hy- peropic or both myopic, the surgeon must subtract his cor- rection from the lens which shows at the sight-hole in the ) J ophthalmoscope. 2. When the surgeon's eye is hyperopic or myopic, and the eye of the patient is the opposite, he must add his cor- ! rection to the lens at the sight-hole in the ophthalmoscope. With the foregoing details clearly in mind and carefully executed, the surgeon selects small vessels near the macula frt4A*X MYOPIA. 121 for his observations. If it .is impossible to see these on account of the small pupil, then he will have to observe the larger vessels at the disc (nerve-head, or papilla). \Yhenever the vessels in the macular region are seen clearly with one and the same glass in the ophthalmo- scope, the refractive error can be approximated as one of axial ametropia, and/every three diopters, plus or minus, or any multiple of three diopters, represent very closely one millimeter of lengthening or shortening of the anteropos- terior diameter of the eye.\ For example, any eye that takes a plus 3 S. to make it emmetropic is just I mm. too short ; any eye that takes a minus 3 S. to make it emme- tropic is about i mm. too long. It will be observed, however, under the head of curvature ametropia (astigmatism), that every 6 D. cylinder represents about I mm. in length, as measured on the radius of curvature of the cornea. The following table, from Nettleship, gives the exact equiva- lents in millimeters for axial ametropia : H ..... i D. = o.3 mm. M ..... I D.=o.3 mm. 61). 2 " 6 D.= 1.75 " 9D.= 3 " 9D.= 2.6 " 12 D.=4 " I2D.= 3.5 " i8D.= 5 Indirect Method. See page 99 for a full description of this method. Slowly withdrawing the objective lens, and the [disc remaining unchanged in size, signifies emmetropia ; if the disc grows uniformly smaller, it means H., and if it grows uniformly larger, it means M. (See Fig. 142.) This is merely a method of diagnosis, and is never used for definite measurements. ii * CHAPTER V. ASTIGMATISM, OR CURVATURE AMETRO- PIA. TESTS FOR ASTIGMATISM. Astigmatism (from the Greek, , priv. ; a-l-fiia, "a point"). Optically, astigmatism may be defined as the re- fractive condition in which rays of light from a point, passing through a lens or series of lenses, do not focus at a point* In ophthalmology astigmatism is recognized as that con- dition of the refractive system of an eye in which rays of light are not refracted equally in all meridians, and the resulting image of a point becomes (an oval, a line, or a circle.^ (See Fig. 100.) Or astigmatism is that condition of an eye in which there are two principal meridians, of greatest and least ametropia, each having a different focus. In the standard eye the cornea is represented as a section of a sphere; anatomically, however, the cornea is generally found to be an ellipsoid of revolution, with its shortest radius of curvature (normally 7.8 mm.) in the vertical meridian. In the study of astigmatism the meridians of minimum and maximum refraction alone are considered ; they are spoken of as the principal meridians, and are at right angles to each other. With very few exceptions most eyes have some degree of astigmatism. The standard or emmetropic eye is an *In an article published by Dr. Swan M. Burnett, in "The American Journal of Ophthalmology," for December, 1903, entitled ' Astigmia or Astig- matism," he draws attention to the fact that astigmatism is an erroneous word, and gives the true origin of the word from oriyfir]-r)^, meaning a mathematic point, whereas "anyfia" really means a "blemish" or "brand." He there- fore urges the change from astigmatism to astigmia, with the word "astigmic'' as the adjective. 122 I2 3 extremely rare condition, and plain myopic eyes (long eyes) i(.4t1iout any astigmatism are almost as rare as the emme- tropic condition ; and while plain hyperopic eyes are seen, --yet statistics show that fully eighty per cent, of hyperopic eyes have astigmatism. .. Astigmatism is located in the cornea or lens, or it may be a condition of both structures in one and the same eye. Astigmatism of the lens may increase, diminish, or neutral- ize the corneal astigmatism. Astigmatism, however, is more ? often a condition of the cornea than of the lens. , 7 / Figure 100 shows parallel rays of light passing through an astigmatic lens in which the vertical meridian has the H' H -^ ^i i V V FlG. IOO. shortest radius of curvature, with the result that those rays which pass through the vertical meridian V V come to a focus before those in the horizontal meridian H H', which has the longest radius. Intercepting the refracted rays at I, 2, 3, 4, 5, and 6, the image would be at I a horizontal oval, at 2 a horizontal line, at 3 a circle, at 4 a vertical oval, at 5 a vertical line, and at 6 a vertical oval. The space between the points of foci of the two meridians (2 and 5) is known as Sturm's interval. The importance of this space or interval is that 124 REFRACTION AND HOW TO REFRACT. it represents astigmatism. Sturm's interval is the quantity which must be found in correcting astigmatism. Causes of Astigmatism. Most cases of astigmatism are congenital, and some can be traced to heredity. Ac- quired astigmatism may result from conic cornea, cicatrices following ulcers or wounds of the cornea, or be a tempo- rary condition from pressure of a chalazion or other growth ; and, in fact, astigmatism may develop from any disease or injury that will cause a lengthening or shortening / .. 'or inequality in one or more of the meridians of the cornea or lens. Swelling of the different sectors of the lens will cause astigmatism. The visual line not passing through the center of the cornea is a cause of astigmatism, and astigmatism is the usual result following extraction of the \ xdens. Tenotomy of one or more of the extraocular mus- cles will often change the corneal curvature. Irregular Lenticular Astigmatism.-This is a normal y^ condition of all clear lenses.) It is often infinitesimal in amount, and on this account does not interfere with vision. It is caused by the different sectors of the lens or by the individual lens-fibers themselves not being uniform in their refracting power. /In this form of astigmatism a light does not appear to have a distinct edge, but, on the contrary, the edge has radiations passing from it, giving the light a stellate qppparanrp. ^ There is no known glass that vj -ilL 5 correct this variety of astigmatism, ^r******* **" <*!* "**j* Physiologic Astigmatism. This is due to lid pressure, or temporarily to extreme pulling or contraction of the extra- ocular muscles. It is a voluntary astigmatism, and therefore not constant. It is not a condition of all eyes. The writer has demonstrated with the retinoscope and ophthalmometer that the condition can be produced in eyes not otherwise astigmatic. Drawing the lids together in the act of squint- ASTIGMATISM. 125 ing or frowning, the patient can press the cornea from above and below, and give the horizontal meridian of the cornea a longer radius of curvature and the vertical meri- dian a shorter radius ; or with the eye looking into the telescope of the ophthalmometer, no overlapping of the mires is noted, but in some instances when told to open the eye widely and "stare" into the instrument, as much as lj or ^ of a diopter of astigmatism may be recorded. This "transient" astigmatism should never be corrected with a glass. Subdivisions of Astigmatism. In addition to the astigmatisms just described, curvature ametropia has been further considered as : 1. Irregular. 6. Astigmatism against the 2. Regular. rule. 3. Symmetric. 7. Homonymous. 4. Asymmetric. 8. Heteronymous. 5. Astigmatism with the 9. Homologous. rule. 10. Heterologous. 1. Irregular Astigmatism. This is usually located in ^the cornea, and is due primarily to some breach in the continuity of one or more of its meridians ; for example, the vertical meridian may appear regular, but the hori- zontal meridian is not a uniform curve, but is irregular at some point or points. Such meridians can not produce clear retinal images, but, on the contrary, the resulting retinal image is hazy or irregular. 2. Regular Astigmatism. In this variety the cornea and lens are regular in their curvatures, from the maximum to the minimum radius, and the retinal image can be made clear with correcting glasses. Before entering upon the study of the various forms of regular astigmatism, the student's attention is called to two 126 REFRACTION AND HOW TO REFRACT. important facts : (a) That, as a rule, the shortest radius of curvature of the cornea is in the vertical meridian that is to say, the vertical meridian has a stronger refracting power than the horizontal. (ti) The student should bear in mind that in the measure- ment of curvature ametropia each millimeter of lengthening or shortening of the radius of curvature is equivalent to a 6 D. cylinder. For instance, an eye which requires a -\-6 D. cylinder axis 90 degrees has the horizontal radius of curva- ture about one millimeter longer than the vertical radius ; or an eye that requires a 6 D. cylinder axis 180 degrees has its vertical radius of curvature about one millimeter shorter than the horizontal. In axial ametropia, however, it was shown that every three diopter sphere represented about one millimeter in length, as measured on the axis. Varieties of Regular Astigmatism. There are five different forms of regular astigmatism : (a) Simple hyperopic. (r) Compound hyperopic. (fi) Simple myopic. ( I y 2 per cent, of all eyes have this form of astigmatism. /This is a condition where one meridian of the eye is emme- 1 tropic, and the meridian at right angles to it is myopic (see Fig. 102); the horizontal meridian focuses parallel rays on the retina, and the vertical meridian focuses parallel rays in front of the retina (in the vitreous), with the result that they cross before reaching the retina. /The retinal image of a point is a line, usually vertical. (See Fig. 100.) The correcting lens is a minus cylinder with its axis at 180 degrees, or within 45 degrees of 180 degrees. Example, 2.50 cylinder axis 180 degrees. (r) Compound Hyperopic Astigmatism. Abbreviated H. As. Co., or Comp. Has., or H-|-Ah (hyperopia com- bined with astigmatism hyperopic). This condition repre- ~>sents nearly forty-four per cent, of all eyes ; it is the most common of all forms of re- fraction. The retinal image of a point is an oval ; never a line and never a circle. (See I/ in Fig. 100.) F, G . I03 , The correcting lenses are a plus sphere and a plus cylin- + 3.00 cylinder axis 90 de- der. Example, -}-2.oo S. grees. Compound hyperopic astigmatism is a combination of axial ametropia (short eye) and simple hyperopic astig- 128 REFRACTION AND HOW TO REFRACT. matism (curvature ametropia). In this form of astigmatism both meridians have their foci back of the retina one further back than the other. The retina intercepts the rays before they can focus. Figure 103 shows this condition. / Usually the vertical meridian focuses nearer the retina than the horizontal. (d) Compound Myopic Astigmatism. Abbreviated M. As. Co., or Comp. Mas., or M. H-Am. (myopia combined with astigmatism myopic). This is by far the most com- mon condition of all myopic eyes, and represents about eight per cent, of all eyes. The retinal image of a point is always an oval ; never a line or a circle. (See 6, in Fig. 100.) The correcting lenses are a minus sphere and a minus cylin- der. Example, i sph. O - cylinder axis 180 degrees. A combination of axial ametropia FIG. 104. (long eye) and simple myopic astigmatism. Figure 104 shows that parallel rays have t\vo points of foci in front of the retina one further front than the other. (c) Mixed Astigmatism. This form of refraction is found in about 6^ per cent, of all eyes, and is abbreviated in th/ee different ways : I/ Ah-fAm. (astigmatism hyperopic with astigmatism myopic). 2. H-f-Am. (hyperopia with astigmatism myopic). / 3. M-j-Ah. (myopia with astigmatism hyperopic). / The retinal image of a point is an oval or a circle ; never line. (See 3 and 4 in Fig. 100.) j^-i/'-'A *~^L*{ ^*- . ASTIGMATISM. 129 The correcting lenses are one of three combinations, and spoken of as crossed cylinders. Examples : 1. +1.00 cyl. axis 90 degrees ^ 2.00 cyl. axis 180 degrees. 2. -j-i S. O 3 c )'l- ax ' s I 8 degrees (cylinder always^ stronger than the sphere). 3. 2 S. ^ -f~3 cyl. ax ' 5 9 degrees (cylinder always stronger than the sphere) . ' The condition of mixed astigmatism is one of simple 7hyperopic astigmatism, with simple myopic astigmatism : one meridian focuses parallel rays in front of the retina and the other meridian (at right angles) focuses parallel rays FIG. 105. FIG. 106. back of the retina. Figures 105 and 106 show this arrange- ment. The remaining subdivisions of astigmatism are merely classifications of the different forms already described, and arise from a study of the axis of shortest radius of curva- ture. 3. Symmetric Astigmatism. \Yhen the combined values, in degrees, of the meridians of shortest or longest radii of curvature in both eyes equal 180 degrees (no more and no less), then the astigmatism in the two eyes is spoken of as symmetric. For example, if the cylinder in the right eye is at axis 75 degrees, and in the left eye at 105 de- grees ; 75 degrees and 105 degrees added together will 130 REFRACTION AND HOW TO REFRACT. make 180 degrees. (See Fig. 107.) Or if each eye takes a cylinder axis at 90 degrees, they are also symmetric, 90 degrees and 90 degrees making 180 degrees. If both eyes have axes 1 80 degrees, they are symmetric also, one meridian being considered as zero (o). 4. Asymmetric astigmatism is the reverse of sym H.JL/' metric, and is, therefore, the condition in which the com- bined values, in degrees, of the cylinder axes do not make 1 80 degrees. For instance, if the right eye has a cylinder aj^fxis 75 degrees and the left at 120 degrees, these ' ' / t/l .". 93' 13 180 135 FIG. 107. Illustrating Symmetric Astigmatism. 135 75 90 120 FIG. 108. Illustrating Asymmetric Astigmatism. added together would not make 1 80 degrees, but more than 1 80 degrees. (See Fig. 108.) Or, if the astigmatism in the right eye was at 35 degrees, and the left at 90 degrees, these added together would not make 180 degrees. Symmetric astigmatism generally accompanies a regular physiognomy, the center of each pupil being at an equal distance from the median line of the face. Asymmetric astigmatism usually accompanies an asymmetric physiog- nomy, the center of one pupil being further from the median line of the face than the other. ASTIGMATISM. 13 1 Muscular insufficiency, hereafter to be described, is much more common, and, in fact, should be looked for or antici- pated in cases of asymmetric astigmatism. ^^*^ *flf 5 and 6. Astigmatism with the Rule and Astigmatism Against the Rule. Astigmatism with the rule and astig- matism against the rule refer to the condition already ) described as that in which the vertical meridian of the eye, as a general rule, has the shortest radius of curvature. Statistic tables on astigmatism show that most eyes a plus cylinder at axis 90 degrees, or within 45 135, 45 FIG. 109. Illustrating Astigmatism with the Rule. 135 FIG. no. Illustrating Astigmatism aguinst the Rule. degrees (inclusive) either side of 90 degrees (see Fig. 109); or a minus cylinder at axis 180 degrees, or within 45 degrees (inclusive) either side of 180 degrees. For example, if an eye requires a plus cylinder at 45 degrees, or at any axis from 45 degrees up to 135 degrees (inclu- sive), taking axis 90 as the median line, then the astig- --jnatism is u'ith the rule. But if an eye should require a plus cylinder within 45 degrees either side of 180 degrees, then the condition is one of astigmatism against the rn/e. (See Fig. 1 10.) A plus or minus cylinder at 45 degrees 132 REFRACTION AND HOW TO REFRACT. or 135 degrees is recognized as astigmatism with the , lC ' &~-*9 >* * *V~, - ** ^ f 7. Homonymous astigmatism is the condition in which / the cylinder axis in each eye is the same.) t ^ ' * _ f-C^_ - -*- 1 8. Heteronymous astigmatism is the condition in which the astigmatism in one eye is with the rule, and in the other eye against the rule. For example : * O. D. 4-2 cyl. axis 90 degrees, and O. S. 4 2.00 cyl. axis 180 degrees. ?" Q. Homologous astigmatism is symmetric astigmatism **with the rule /. e. : O. D. 4 l.oo cyl. axis 60 degrees, O. S. -j-l-oo cyl. axis 120 degrees. 10. Heterologous astigmatism is symmetric astigma- tism against the rule /. e. : O. D. 4-I-OO cyl. axis 15 degrees, O. S. -\-i.O3 cyl. axis 165 degrees. Meridians of the Eye. The various axes or meridians of the eye are indicated by degree markings on the peri- phery of the trial-frame, and by corresponding imaginary lines drawn around the eyeball from the anterior pole or apex of the cornea to the posterior pole. Either eye (right or left) is exactly like its fellcnv, and is numbered by starting from zero (o) on the left-hand side of the horizontal meridian and counting downward to the right- hand side until this same line is again reached. This makes half a circle (hemisphere) of 180 degrees. (See Fig. 109.) As the degrees in this half-circle are all carried across the eye, they maintain their individual numbering, so that axes 5, 10, 15, etc., are the same whether above or below the horizontal meridian. Hence there is no reason for having a complete circle of 360 degrees. Some trial -frames have the upper, while others have the lower, half numbered ; this makes no difference in the exact numbering ; in one in- stance the count is made from left to right, and in the other ASTIGMATISM. 133 the count is made from right to left. The foreign trial-frame, as represented on page 48, may be confusing if not studied. Symptoms of Astigmatism. More aggravated symp- toms of accommodative asthenopia are apt to be detailed by the patient, but there are, in truth, no definite symptoms whereby the presence of astigmatism can be positively, dif- ferentiated from axial ametropia. The diagnosis of astig- matism by the physiognomy is confirmed only because most eyes are astigmatic ; the simple hyperopic eye squints the eyelids together just the same as the eye that is astig- matic, so that the writer would not diagnose astigmatism by the patient's individual history of his eyes. How to Diagnose Astigmatism. This is one of the very early questions of the beginner in ophthalmology. Astigmatism being the prominent factor in almost all refrac- tive work, the writer feels justified in giving this part of refraction extensive explanation. Of the various methods of diagnosing astigmatism the writer would mention the following : 1. Corneal reflex. IO. Chromo-aberration or cobalt-blue 2. Confusion letters. glass test. 3. Placido's disc. II. Thomson's ametrometer. 4. Stenopeic slit. 12. The ophthalmometer. 5. Astigmatic chart. 13. Direct ophthalmoscopy 6. The pointed line test. 14. Indirect ophthalmoscopy. 7. Perforated chart or disc. 15- Cylindric lenses. 8. Fray's letters. 1 6. Retinoscope. 9. Schemer's Test. I. The Corneal Reflex Test. The cornea and under- lying aqueous representing a spheric mirror, naturally fur- nish a small image of surrounding objects. If the cornea is astigmatic, the catoptric image must be correspondingly distorted. To make the examination, the patient stands facing a window, and the surgeon at one side observes the 134 REFRACTION AND HOW TO REFRACT. image of the window-panes in the conical mirror ; these will be broadened or lengthened, or they may appear in- clined to one side, according to the axis and character of the astigmatism. This test is not commonly used, is often overlooked ; in fact, unless the astigmatism is of consid- erable degree, is not a valuable test. 2. Confusion Letters. Letters on the card which is used for testing distant vision are arranged in such order that those which have a resemblance are placed next to each other. (Fig. 74.) For example, X and K, Z and E, O and D, C and G, P and F, S and B, V and Y, H and N, A and R, etc. The patient, in deciphering these letters in the line corresponding to his best vision, often miscalls them, and can not tell an X from a K, or a Z from an E, etc. These letters are, therefore, spoken of as confusion letters. (This is a very good general test, but is not infallible, as a patient with opacities in the media will make similar mistakes./ 3. Placido's Disc or Keratometer (see Fig. 1 1 1). To a wooden handle is secured a round piece of thin sheet-iron eight inches in diameter, and at its center is a small, round 5 mm. opening. On one side the disc is painted in alternate concentric circles or bands in black and white ; these circles are not equidistant, the radii of the several circles being cal- culated according to the law of tangents, so that when re- flected on a cornea of spheric curvature they appear equi- distant in the image. On the reverse side is placed a slot to hold a convex lens for magnifying purposes. To use this disc, the patient is placed with his back to a strong light from a window, or an artificial light may be placed over his head. The surgeon holds the disc with the sight-hole close in front of his own eye, and with the light illuminating the disc, the patient is instructed to look into the perforation. The sur- ASTIGMATISM. 135 geon then approaches the eye until the corneal image of the outer edge of the instrument corresponds to the outer edge of the patient's cornea. When this distance is reached, a convex 2, 3, or 4 D. sphere may be placed in the slot of the disc so as to magnify the corneal image. (If the cornea is not astigmatic, then the black and white circles will appear uni- form throughout ; but if there is astigmatism, the circles will appear more or less oval. If irregu- lar astigmatism or conic cornea is present, the cir- cles will appear broken or distorted in certain parts. This test has be- come almost obsol FIG. in. FIG. 112. 4. Stenopeic Slit (see Fig. 1 12). This is a round metal disc of the size of the trial-lens, and contains a central slit or opening about 25 mm. long and I or 2 mm. wide. The stenopeic slits sold in the shops have various breadths of openings, from ^ to 2 mm. ; that with the I mm. opening 136 REFRACTION AND HOW TO REFRACT. is the one recommended. The purpose of the slit is to cut off or exclude all rays of light at right angles to its position in front of the eye. When placed at axis 90, all rays in the horizontal meridian are excluded ; when placed at axis 1 80, all rays in the vertical meridian are cut off, etc. jTo use the stenopeic slit, place it in the trial-frame in front I of the eye to be examined, the fellow-eye being covered. The patient is instructed to read the letters on the distant test-card, and as he does so, the slit is slowly turned through the different meridians. If the vision remains the same, no matter through which meridian the patient reads, astigmatism may be absent ; but if the patient selects one meridian in which he sees best, and another meridian at right angles in which he does not see so well, astigmatism is usually present. For instance, if the slit is at axis 75 degrees and the patient reads ^, and at axis 165 he reads ~^, then he is astigmatic in the 165 meridian. The amount of the astigmatism can be calculated by placing spheric lenses back of the slit and finding the difference in strength of the spheres which bring the vision up to the normal. For example, when the slit is at axis 75 and the patient reads f^, if a -f 1.50 S. is used, and the vision becomes yj, then 1.50 corrects axis 75. Turning the slit to axis 165, and proceeding in the same way, if -f- 2. 50 S. brings the vision from ^ to ^}, then +2.50 corrects axis 165, the differ- ence between the +1.50 and +2.50 being i D., and the formula would be +1.50 sph. ^ -f i.oo cyl. axis 75 degrees. This test is not often used, and when resorted to, the eyes should be under the influence of a cyclo- plegic. This test is of special service in some cases of mixed astigmatism, irregular astigmatism, presbyopia, and aphakia. ASTIGMATISM. 137 5. Astigmatic Chart. There is an infinite variety of these cards (see Fig. 113), and the student is puzzled FIG. 113. Astigmatic Charts of Dr. John Green. which one to select. Ordinarily, the " clock-dial " will answer every purpose. (Fig- H4-) This is a white 138 REFRACTION AND HOW TO REFRACT. card * with peripheral Roman characters corresponding to the characters on the clock-face, hence its name. From these figures a series of three parallel and uniformly black lines, with interspaces of the same width as the lines, cross from XII to VI, III to IX, IIII to X, V to XI, VII to I, and VIII to II. This chart should be so calculated that FIG. 114. the lines and interspaces will form an angle of 5 minutes in width consistent with the distance at which the test is to be /made : if at six meters, 8.7 mm. ; if at four meters, 5.7 mm. In most charts the lines subtend an angle much greater than 5 minutes for the distance at which they are used, and in this way the true delicacy of the test for small errors or * A black card with white lines is also used. (See Fig. 115.) ASTIGMATISM. 139 amounts of astigmatism is sacrificed. The purpose of the chart is to detect, by the patient's answer, whether astigma- tism is present, and, if so, in which meridian. The chart, illuminated by reflection from a steady artificial light, is placed on a horizontal line perpendicular to the patient's eyes ; it should never be hung at an angle, and must always be perfectly flat. Each eye is to be tested separately. Looking at such a chart, if all the lines appear FIG. 115. equally black, astigmatism of any considerable degree or amount may often be excluded ; but if the patient selects one series of lines as darker than others, then the presence of astigmatism may be diagnosed. 'If the astigmatism is of a very high degree, the patient may see the three lines as one solid black line without interspaces.J RULE i. The meridian of the. eye which corresponds to the dark lines selected is the meridian of astigmatism. Example. If the horizontal lines (from III to IX) appear I4O REFRACTION AND HOW TO REFRACT. darker than all the others, then it is the horizontal meridian (o or 1 80 degrees) of the eye which is astigmatic. Or if the lines from VI to XII are darkest, then the vertical mer- idian of the eye is astigmatic. In other words, the series of darkest lines indicates the meridian of greatest ametropia. RULE 2. The axis of the cylinder in the prescription will be opposite to the meridian of the dark lines. Example. A patient who requires a plus cylinder at axis 90 degrees sees the horizontal lines (from III to IX) as very dark, and the lines from VI to XII not so dark, and the axis of the cylinder in the prescription will be opposite to 180 degrees i. e., at 90 degrees. ^According to the definition of "astigmatism with the rule " and " astigmatism against the rule," it follows that, with few exceptions, those patients who select a series of lines at 180 degrees, or within 45 degrees either side of 1 80 degrees, as darker than other lines, have hyperopic astigmatism, whereas those who select a series of lines at 90 degrees, or within 45 degrees either side of 90 degrees, have myopic astigmatism. J According to the definition of symmetric astigmatism, a patient's right eye selecting the lines at 90 or 180 as darker than those at right angles, will select the same series of dark lines in the left eye. If the series of dark lines with the right eye is from II to VIII, then the left eye selects the dark lines from X to IV, etc. The clock-dial is the form of chart in common use, and as a test for astigmatism . is not without considerable merit. When the astigmatism is of small amount, it may not be recognized by means of the clock-dial until after the spheric correction has been placed before the eye or after a cyclo- plegic has been instilled. ASTIGMATISM. 14! 6. The writer's pointed line test, as shown in figure 1 16, is a series of one-minute black squares, in three parallel lines at right angles to each other, on a cream-colored card ; the squares and adjoining spaces making a five-minute angle for six meters. By means of a clockwork and bat- tery, this dial may be revolved by pressing a button. The principle of the test is the same as the perforated disc. FIG. 1 1 6. 7. The Perforated Disc (Fig. 117). This is a modifica- tion of the astigmatic chart. A piece of white cardboard or metal, about ten inches square, has small, round perfora- tions made in it of certain definite size. ^Each perforation is separated from its neighbor by the distance of its diameter.^) These openings are arranged in series of one, two, or three parallel lines, exactly as in the pointed line test. This chart or disc is hung on the window-pane, or an illumina- tion is placed behind it. The patient, looking at the disc, 142 REFRACTION AND HOW TO REFRACT. signifies which scries of perforations appear to coalesce and form lines. This test is not commonly known or used. It FIG. 117. might be a valuable test if there was any convenient way of uniformly illuminating it from behind. 8. Fray's Letters (Fig. 118).- ^zsJT ^^ a ?^ These letters are of the Old English ~ ^rz 5 5 55^^^ =~ "^? 5^8 type, and composed of strokes which Ilium //i ///// run ^ n different meridians. The pa- flliiill %/r J/ ft ^ ent > looking at these letters, selects that letter which appears darker than all the rest. The direction of the lines j n the letter selected corresponds to the meridian of greatest ametropia. (This ^" test is very confusing to the patient, FIG. 118. wno sees first one letter and then an- other as darker than its fellows.^/ Schemer's Test. This is an old test for ametropia, \\ \A\V ASTIGMATISM. 143 good in theory, but really not sufficiently accurate for prac- tical purposes. It is explained for the student's information, and not with the idea that he will ever take time to use it. The test is made with a small piece of metal (Fig. 119) the size of the trial-lens, which contains two pin-point round openings at its center, separated by an interval of two or three millimeters. (One of these openings is covered with a red glass, as suggested by Dr. Wm. Thomson.) This disc is placed close to the eye, so that light may pass through both openings into the eye at one and the same time. The eye, if not presbyopic, should be under the influence of a Fie. 119. FIG. 120. cycloplegic. The eye looks at a distant point of light The principle of the test depends upon which part of the retina is stimulated by the rays entering the eye through these openings, all other rays being excluded. The student must remember that rays which fall upon the temporal side of the retina are referred to the nasal side ; those which fall upon the nasal side of the retina are referred to the temporal side ; those which fall upon the lower portion of the retina appear to come from above ; and those which fall upon the upper portion of the retina appear to come from below. Diagnosis of Hyperopia ( Fig. 1 20). The disc is placed with the red glass (R) above. The patient then sees a red 144 REFRACTION AND HOW TO REFRACT. FlG. 121. and a white light (W). The red appears below the white. Gradually revolving the disc, the two lights move, and keep the relative positions and distance apart. The greater the distance between the two lights, the higher the refraction or amount of the hyperopia. That plus sphere placed in front of the disc which unites the two flames into one (pink) flame is the approximate amount of the hyperopia. Diagnosis of Myopia (Fig. 121). Placing the disc before the eye as before, with the red glass (R) above, the patient sees the red flame above the white (W). Gradu- ally revolving the disc, these two lights keep their rela- tive positions and distance. That minus sphere placed before the disc which makes the two lights appear as one (pink) light is the approximate amount of the myopia. Diagnosis of Emmetropia. This condition would give but one light (pink in color), and unchanged by rotating the disc. Diagnosis of Simple Hyperopic Astigmatism. One meridian appears the same as in emmetropia, and the meri- dian opposite to the emmetropic meridian would show a separation of the two lights, as in hyperopia. The plus cyl- inder placed before the disc which unites the two lights in the ametropic meridian represents the amount of the astigma- tism. Diagnosis of Simple Myopic Astigmatism. The lights are red and white in one meridian, as in simple hyper- opic astigmatism, but the red light is seen in the direction of the red glass, and when the disc is rotated to the oppo- ASTIGMATISM. 145 site meridian, only one light appears, and of a pink color. The amount of the astigmatism is represented by the strength of minus cylinder which brings the two lights together in the ametropic meridian. Diagnosis of Compound Hyperopic Astigmatism. All meridians show two lights, the red light being in the direc- tion of the clear opening in the disc, but one meridian will show a greater separation of the lights than in the meridian at right angles. To find the correction and the amount of the astigmatism, proceed as in simple hyperopia, correcting each meridian separately with a sphere. FIG. 122. FIG. 123. Diagnosis of Compound Myopic Astigmatism. This is the same as in compound hyperopic astigmatism, with a reversal of the position of the lights, and the amount of the ametropia is obtained with minus spheres. Diagnosis of Mixed Astigmatism. One meridian appears as in simple hyperopic astigmatism, and the meri- dian opposite to it appears as in simple myopic astigma- tism. The amount of the astigmatism is calculated as in these two conditions. 10. Chromo-aberration Test. This is also known as the cobalt-blue glass test. Cobalt is a mineral, and is 13 146 REFRACTION AND HOW TO REFRACT. used as a coloring-matter by glass-blowers. Cobalt-blue glass comes in two forms : one in which the glass is colored throughout, and the other in which it is colored on only one surface, known as " flashed." To the eye, cobalt -blue glass appears dark blue, but contains a great deal of red. For purposes of testing ametropia, a dark shade of blue should be selected, or two or three pieces of a light shade may be cemented together so as to give the desired dark shade. This glass is cut round and fitted into a trial-cell. (See Figs. 122 and 123.) The power of cobalt-blue glass to exclude all but blue FIG. 124. and red rays gives this test its principle. Blue rays being /more refrangible than red, naturally focus sooner than red. ~-\ Red rays will focus back of the blue. (See Fig. 1 24.) There are several important details in the use of this test which must be carefully executed if definite results are to be obtained : 1 . The eye should be under the influence of a cydoplcgic. 2. By means of a light-screen, a small round area of steady white light should be looked at from a distance of four or six meters. 3. Each eye is to be tested separately. ASTIGMATISM. 147 FIG. 125 FIG. 126 FIG. 127. FIG. 128. FIG. 129. FIG. 130. FIG. 131. FIG. 132. FIG. 133. FIG. 134. FIG. 135. FIG. 136. 125. High hyperopia. 126. High myopia. 127. Low simple hyperopic as- tigmatism. 128. High simple hyperopic astigmatism. 129. Low simple myopic astigmatism. 130. High simple myopic astigmatism. 131. Low compound hyperopic astigmatism. 132. High compound hyperopic astig- matism. 133. Low compound myopic astigmatism. 134. High com- pound myopic astigmatism. 135. Low mixed astigmatism. 136. High mixed astigmatism. 148 REFRACTION AND HOW TO REFRACT. 4. The cobalt glass may be placed near the flame or, better still, close in front of the patient's eye ; in every instance it must be perpendicular to the front of the eye, iand never at an angle. 5. All other lights except the one in use should be excluded. Diagnosis of Emmetropia. Patient sees a small circle composed of two colors equally mixed ; purple. (See E in Fig. 124.) Diagnosis of Hyperopia (see H in Fig. 124). The patient describes a red ring of light with a blue center. Diagnosis of Myopia. The patient describes a blue ring with a red center. (See M in Fig. 124.) Diagnosis of Astigmatism. If astigmatic, then he will describe one of the conditions as shown on page 147. If the test is made as suggested, it will have three points of recommendation : 1. The character of the refraction is quickly diagnosed. 2. It may lead to an early diagnosis of red-blindness, a condition often overlooked. 3. Likewise it will show a central scotoma for red in advanced toxic amblyopia, if the eye is made myopic with a plus sphere. ii. Thomson's Ametrometer (Fig. 137). This instru- ment has two small gas-flames about five millimeters in diam- eter, one stationary and the other movable on a metal arm, which can be changed or revolved to any meridian. Each eye is tested separately at a distance of twenty feet, and preferably under a cycloplegic. The method of the test is to move one flame along the metal arm until the two lights appear to fuse. The scale, as marked on the arm, gives the approximate strength of lens necessary to correct the ametropia. By raising or lowering the arm any meridian ASTIGMATISM. 149 may be tested. It is a most ingenious test, but not in common use. {/ 12. The Ophthalmometer (see Figs. 138 and 139). This name literally means an " eye measure," but as the in- strument measures only the different radii of corneal curva- ture, a much better name would be keratometer, or measure of the corneal radii v The object of the ophthalmometer is h #- . FIG. 137. the measurement of corneal curves by means of catoptric images viewed through a telescope. The ophthalmometer consists of a telescope which con- tains a Wollaston birefrangent prism placed between two bi- convex lenses. Attached to the telescope is a graduated arc, upon which are placed two white enameled objects called mires (targets). (See Figs. 139, 140, 141.) The left mire is stationary, and is made up of two_^cm. squares, separated 150 REFRACTION AND HOW TO REFRACT. by a black line 2 mm. wide ; the right mire is movable and graduated into steps, each 5 mm. wide ; a black line passes through the middle of these steps. For purposes of focus- ing, the telescope is mounted on a movable tripod. The patient is seated with his chin and forehead resting in a FIG. 138. frame. At the side of the frame, and attached to it, are two or four electric lights or Argand burners, which illuminate the mires. The surgeon, looking through the eye-piece of the telescope, focuses the center of the patient's cornea until he sees two images of each mire clearly ; then he selects the two central images for further study and ignores ASTIGMATISM. I 5 I the peripheral images. The next step is to move the right-hand mire until these two images of the mires occupy the center or pole of the cornea, so that their inner edges just touch and the black line in each makes one continu- ous black line through both (see Fig. 140) ; and to do the FIG. 139. latter, the barrel of the telescope may have to be gradually revolved from left to right or right to left, but never more than 45 degrees either way. When this position is ob- tained, the axis or meridian is noted by the arrow, which points to the figure on the dial at the back of the arc, or, as in some old instruments, on the front of the dial. This 152 REFRACTION AND HOW TO REFRACT. position of the mires is spoken of as the primary posi- tion. Revolving the telescope to the opposite meridian (mer- idian at right angles), which is called the secondary posi- tion, the observer notes any change which may have taken place in the relative positions of the mires. If they have not changed, but still maintain their edges in apposition, as in the primary position, then the cornea has a uniform cur- vature thoughout, and there is no astigmatism of the cornea present. If, however, when the secondary position is reached and the catoptric image of the mires with the steps has encroached upon the catoptric image of the sta- tionary mire, then the astigmatism is calculated by the amount of this overlapping. (See Fig. 141.) Each step representing one diopter of astigmatism, one- half a step of overlapping FIG. 140. FIG. 141. would represent half a diopter, etc. If, in making the change from the primary to the secondary position, the mires should separate, then the surgeon would know that his secondary position should have been his primary position, and he will have to make a corresponding change. As already stated, lenticular astigmatism is not a condi- tion to be ignored, as only too often it will increase, diminish, or even neutralize corneal astigmatism, so that in point of fact the ophthalmometric findings are more often useless than of real value in estimating the total refractive error. Cylinders should never be prescribed from the ophthalmometric findings until carefully confirmed by other and much more reliable tests. As a keratometer, the instrument can not be excelled, and, therefore, it has a ASTIGMATISM. 153 place in testing the refraction in cases of aphakia. The ophthalmometer as a means of diagnosis is suggestive rather than positive. 13. Estimation of Curvature Ametropia (Astigma- tism) with the Ophthalmoscope, Direct Method. The presence of astigmatism is diagnosed by the direct method from the fact that the vessels or details of the fundus are not all seen clearly with one and the same glass in the ophthalmoscope ; in other words, the vessels passing up and down on the disc are seen clearly with a different lens in the ophthalmoscope than is required to see the vessels passing laterally or at right angles. (The amount of the astigmatism is the difference in the strength of the respective lenses used for this purpose) for instance, if the vertical vessels are seen best with a -f-4 S., and the horizontal vessels with a -f-2 S., then the amount of the astigmatism would be -\-2 D. When using the ophthalmoscope for making refractive estimates in astigmatic eyes, /the student should remember that the glass with which a vessel is seen distinctly in one meridian represents the amount of the refraction in the meridian at right angles to this vesseL) (In other words, / each vessel in the eye-ground of an astigmatic eye is seen clearest through the meridian at right angles to its course.J This is a puzzle to the beginner, but he must remember that cylinders refract opposite to their axes. In estimating the refraction with the ophthalmoscope, the observer looks first (/? * at the shape of the disc ; if it appears oval, this would be an evidence of astigmatism ; secondly, if the upper and lower edges of the disc are seen clearly with a different strength glass than that required to see the inner and outer margins, then this would be a further evidence of the presence of astigmatism ; Xut the third and confirmatory test of the presence of astigmatism should be the different strength 154 REFRACTION AND HOW TO REFRACT. glasses required to see the vessels distinctly in the neigh- borhood of the maculay An eye having an oval nerve, whose edges can all be seen clearly with one and the same glass in the ophthalmoscope is not usually astigmatic. Examples of estimated refraction by the direct method. Simple Hyperopic Astigmatism. Vertical vessels seen with a -j- I S. and horizontal vessels seen without any lens would equal + 1 .00 cyl. axis 90 degrees. Simple Myopic Astigmatism. Vertical vessels seen without any lens and horizontal vessels seen with 3 S. would equal 3 cyl. axis 180 degrees. Compound Hyperopic Astigmatism. Vertical vessels seen with -{-4 S. and horizontal vessels seen with +38. would equal +3.00 S. O + i.oocyl. axis 90 degrees. Compound Myopic Astigmatism. Vertical vessels seen with 2 S. and horizontal vessels seen with 5 S. would equal 2.00 S. O 3.00 cyl. axis 180 degrees. Mixed Astigmatism. Vertical vessels seen with -(-28. and horizontal vessels seen with 3 S. would equal 3.00 S. O +5.00 cyl. axis 90 degrees. 14. Diagnosis of the Character of the Refraction by the Indirect Method (see Fig. 142 and p. 99). There is nothing exact about this method, and the refractive error, to be recognized, must be considerable. 1. Gradually /withdrawing the lens (objective) from in front of the eye, if the aerial image of the disc retains its uniform size in one meridian, it signifies emmetropia for that meridian ; but if it grows smaller in one meridian, that meridian is hyperopic ; or if larger, then that meridian is myopic.) 2. Ifthe image grows smaller, but more so in one meridian than the other, it signifies compound hyperopia. If the image grows larger, but more so in one meridian than the other, then the condition is one of compound myopia. The ASTIGMATISM. 155 image growing smaller in one meridian, while in the other it grows larger, indicates mixed astigmatism. FIG. 142. Companion picture to figure 89. Illustrating the indirect method. Rays from (lie lamp (L) are reflected convergently from the mirror of the ophthalmoscope, and, passing through the convex lens and into the eye, produce a large retinal illumination, extending from I to I. TB are rays from the edge of the disc, and, leaving the eye parallel, pass through the convex lens and form an inverted aerial image of the disc at T/ B'. The -f-4 S. in the ophthalmoscope magnifies the image T' B'. 15. The cylinder lens test for astigmatism is described under Applied Refraction, page 251. 16. Retinoscopy is described in chapter vi. . A^< ' %k G/-&< v-tfU-W ^ ~\\ t f Cf CHAPTER VI. RETINOSCOPY. Retinoscopy, or the Shadow Test. This may be de- FIG. 143. The Author's Schematic Eye for Studying Retinoscopy. fined as the method of estimating the refraction of an eye by reflecting into it rays of light from ^a plane or concave I 5 6 RETINOSCOPY. 157 mirror, and observing the movement which the retinal illumination makes by rotating the mirror. Suggestion. Before attempting to practise retinoscopy upon the human eye, the beginner is advised to study the method upon one of the many schematic eyes to be found in the market. The principle of retinoscopy is the finding of the point of reversal, or myopic far point ; and when an eye is emme- tropic or hyperopic, it must be given a myopic far point by means of a convex sphere. (Fig. 144.) i METER. FIG. 144. Advantages of Retinoscopy. 1. The character of the refraction is quickly diagnosed. 2. No expensive apparatus is necessarily required. 3. The refraction is estimated without the verbal assist- ance of the patient. 4. The correction is quickly obtained. 5. The value of retinoscopy can never be overestimated in the young, in the feeble-minded, the illiterate ; in cases :?of nystagmus, amblyopia, and aphakia. Axiom. \Yith an eye otherwise normal except for its optic error, and under the influence of a reliable cycloplegic, there is no more exact objective method of obtaining its refraction than by retinoscopy. 158 REFRACTION AND HOW TO REFRACT. The surgeon should wear any necessary correcting glasses and have a vision of more than ; otherwise he can never get satisfaction from this method. The surgeon should keep his eyes wide open and not hesitate to use his accommodation, as it does not have any effect on the result, as in estimating the refraction with the ophthalmoscope. FIG. 145. FIG. 146. Author's Mirror with Folding Handle. FIG. 145. Showing central light C, on small mirror B. This is the light the patient sees when looking into the mirror, and corresponds in size to the one-centimeter opening in screen. D is the folding cap handle to pro- tect B when not in use. A is the metal disc. FIG. 146. Shows the light moved to one side as a result of tilting the mirror. The patient must have his accommodation under the in- fluence of a reliable cychplegic ; this is imperative. Each eye is tested separately, and if the patient has a squint, then ^^one eye should be covered while its fellow is being refracted. The patient must be comfortably seated and told to look at RETINOSCOPY. 159 the metal disc of the mirror or the observer's forehead ' above the mirror, and never into the mirror. The Retinoscope, or Mirror. The plane mirror is 2 cm. in diameter on a round 4 cm. metal disc, with a 2 mm. sight-hole at the center, made by removing the silvering, and not by cutting a hole through the glass. (See Figs. 145 and 146.) The concave mirror recommended has a 25 cm. focus (ten inches) and is 3 y z cm. in diameter on a metal disc of the same size as the plane mirror. The sight-hole is simi- lar in size and made /n the skme \\ ax- as that of the plane, mirror. The light should be steady, clear, and \vhite, and secured to a movable bracket. For general use, the Argand burner is best. The Light-screen, or Cover-chim- ney. For the purpose of intercepting the heat this is made of thin asbestos, and the iris diaphragm attached to it regulates the amount of light desired. (See Fig. 147.) The room for retinoscopy should be darkened and all sources of light except the one in use should be excluded. Position of Light and Plane Mirror. These may be as close together as 6 inches or as far apart as 6 meters. It is a matter of choice with the surgeon himself where he prefers to have them. /The writer recommends, however, having the rays of light come from the 10 mm. opening in the light-screen, at about 6 inches to the left and front of the surgeon, so that the rays pass in front of the left eye FIG. 147. Author's Iris Diaphragm Chimney. I6O REFRACTION AND HOW TO REFRACT. and fall upon the mirror held before the right eye. Some surgeons prefer having the light, with the 3 cm. opening in the screen, placed over the patient's head or to one side of it. (Fig. 148.) TJic distance between the light and mirror will not alter the direction of the rays of light which comt from the patient's eye. Position of the Light and the Concave Mirror (Figs. 148, 149, 150). As the purpose of the concave mirror in retinoscopy is to focus rays of light before they enter the I METER FIG. 148. Light over Patient's Head, and the Observer with Mirror at One Meter Distance. patient's eye, it is always necessary to have the light and mirror widely separated. Usually, the light with the 3 cm. opening in the screen is placed to one side or over the pa- tient's head, and the surgeon with the mirror is seated about one meter from the patient. This will place the focus of the 25 cm. mirror about 33 cm. in front of the mirror. Distance of Surgeon from Patient. With the plane mirror he may approach within a few inches of the patient's eye to find the point of reversal, but with the concave mirror RETIXOSCOPY. 161 he must remain at a sufficient distance to have the focus of the mirror in front of the patient's eye. How to Use the Mirror. It should be held firmly in FIG. 149. Illustrating High Myopia with a Concave Mirror. Rays of light from the lamp (L) are reflected by the mirror (//'), and form a conjugate focus at L', and the rays from this focal point illuminate the retina at L 1 . Corresponding effects result when reflection takes place from the mirror at m". The eye (E) behind the mirror recognizes points of reversal between the eye and mirror, moving in the same direction to that in which the mirror is tilted. FIG. 150. Illustrating Hyperopia with the Concave Mirror. The eye (E) recognizes a virtual image behind the eye under examination, so that when the mirror (/') is focusing the rays from the lamp (L) at I/, the upper portion of the retina is illuminated, and vice versa, when the mirror i m" \ is focusing the rays at L,, the lower portion of the retina is illuminated. The retinal illumination moves opposite to that of the mirror. the right hand before the right eye, so that the sight-hole is opposite to the observer's pupil. The movements im- 1 62 REFRACTION AND HOW TO REFRACT. parted to the mirror must be limited, though they may be quick or slow, but never at any time should the mirror be tilted more than 2 or 3 mm., otherwise the light will be lost from the eye. What the. Observer Sees, or the general appearance of the reflection from the eye. The reflex from the pupil varies in different patients, and is subject to many changes as the refraction is altered by correcting glasses, by the turning of the patient's eyes, by increasing or diminishing the distance between patient and surgeon or the distance between the light and mirror, or the strength of the light. The amount of pigment in the eye-ground will change the general appearance of the reflex, being dim in some mulattoes, and very light in the blonde or albino. If the refractive error is a high one, the reflex will appear dull ; or if a low error, it will appear very bright. If the >. media are not clear, the reflex will be altered accordingly. The bright pin-point catoptric images seen on the cornea and lens are not parts of the test, and should be avoided or ignored. The I mm. bright ring of light sometimes seen at the edge of the pupil should be avoided by the beginner in retinoscopy, as it is an indication of spheric aberration, which he will have to consider after mastering other details of the method. Facial Illumination. The rays of light reflected from the mirror illuminate a portion of the patient's face, and always move in the same direction as that in which the mirror is tilted, no matter whether the mirror is plane or concave. Retinal Illumination, This corresponds to the portion of the retina which receives the rays of light reflected from the mirror. The retinal illumination is also called "the image," "the light area," etc. RETINOSCOPY. 163 The Shadow. This is the non-illuminated portion of the retina immediately surrounding the illumination. The illumination and shadow are, therefore, in contact ; if the illumination changes its place upon the retina by a move- ment of the mirror, then the shadow will move also. By this change of illumination and shadow we speak of a movement of the shadow. Where to Look and What to Look for. Rotating the mirror through the various meridians of the eye, the observer makes a note of the (i)form, (2) direction, and ; (3) rate of movement of the retinal illumination as he watches for them through a four or five millimeter area at the apex of the cornea, as this is the portion of the refract- ive media in the normal eye that the patient will use when the effects of the cycloplegic pass away and the pupil regains its normal size. Point of Reversal. To find the point of reversal is the underlying principle of retinoscopy. For example, having determined with the plane mirror at a distance of one meter that the retinal illumination moves with the movement of the mirror and a -f~ 2 -5O S. stops all apparent movement (no movement of the illumination being seen and the shadow having disappeared), the observer knows that his eye is at the point of reversal. Or with the concave mirror the retinal illumination will move opposite to the movement of the mirror and will stop with +2.50 S. before the eye. The point at which all movement of the retinal illumina- tion appears to have ceased is the point of reversal. The real movement of the retinal illumination de- pends upon the mirror whether it is concave or plane. With the plane mirror the retinal illumination always moves with the mirror and the light on the face ; whereas with the concave mirror (focusing rays before they enter the eye), 164 REFRACTION AND HOW TO REFRACT. the real movement of the retinal illumination is always opposite to that of the mirror. The student should not get the real and apparent movements confused, but pay close attention to the apparent movement. Direction of the Apparent Movement of the Retinal Illumination. With the plane mirror, the apparent move- ment of the retinal illumination will be with the mirror and with the light on the face as long as the observer is within the point of reversal ; but just as soon as the observer is i beyond the point of reversal, the retinal illumination \\ill appear to move opposite to the movement of the mirror and opposite to the movement of the facial illumination. F" FIG. 151. With the concave mirror the apparent movement of the retinal illumination will be with the movement of the mir- ror and the light on the face as long as the observer is be- yond the point of reversal (Fig. 149) ; but just as soon as the observer's eye is within the point of reversal, the retinal illumination will appear to move against the movement of the mirror and against the light on the face. (See Fig. 1 50.) Rate of Movement of the Retinal Illumination. This is influenced by several factors, but practice will teach the observer that when the retinal illumination appears to move slowly, the refractive error is a high one, and when it moves fast, the refractive error is a low one. Figure 151 represents a myopic eye with its far point RETINOSCOPY. i6 S (point of reversal) at R/, and when rotating the mirror, this point moves to R" ; but if the eye had its far point at F', and the mirror was rotated to F", then the illumination at R', having to move through a smaller arc in the same time, appears to move slowly as compared with F', which ap- peared to move fast. The same condition is shown in figure 152, in which the observer appears to see an erect virtual image back of the retina, and R' appears to move slowly as compared with F', which appears to move fast. Form of Illumination.-eA large, round illumination may signify emmetropia, hyperopia, or myopia, with or with- out astigmatism in combination.) Astigmatism is recognized FIG. 152. by the presence of a band of light, and this band of light may be seen before any correcting lens has been placed be- fore the eye if the astigmatic error is high ; or it will be recognized during the process of neutralization if the error is small /. e., if the astigmatism is of low degree. )The pres- ence of astigmatism is known, therefore, by the band of light or when the illumination appears to move faster in one meridian than in the meridian at a right angle. The astig- matism is in the meridian of slow movement. The apparent difference between the plane and con- cave mirror in the direction of movement of the retinal illumination. \Yith the plane mirror the rays of light are reflected as if they came from a point just as far back of 1 66 REFRACTION AND HOW TO REFRACT. the mirror as the original source of light is in front of it. The surgeon's eye behind a plane mirror is, therefore, in the path of these rays, and sees that portion of the pupil- lary area illuminated to which these rays are directed. (See Fig. 153.) With the concave mirror the reflected rays come to a focus, forming an inverted image of the flame, which be- comes the immediate source of light in front of the observer's eye. When the concave mirror is tilted, the immediate source of light goes in the same direction, but with the result that the opposite portion of the pupillary area is illuminated. (See Fig. 143.) This shows the FIG. 153. immediate source of light at L' and mirror tilted downward ; the rays proceeding from L' diverge and illuminate the upper portion of the pupillary area. Tilting the mirror upward, the immediate source of light at L' moves upward also (L 2 ), and the lower portion of the pupillary area be- comes illuminated. (See also Fig. 149.) Rule for Neutralizing Lenses with the Plane Mirror. When the retinal illumination appears to move in the same direction as that of the mirror, the observer is within the point of reversal and a plus lens must be placed before the eye to stop all apparent movement. When the retinal illumination appears to move in the opposite direction to RETINOSCOPY. 1 67 that in which the mirror is tilted, the observer is beyond the point of reversal, and a minus lens must be placed before the eye to stop all apparent movement. Rule for Neutralizing Lenses with the Concave Mirror. \Yhen the retinal illumination appears to move in the same direction as that in which the mirror is tilted, the observer is beyond the point of reversal, and a minus lens must be placed before the eye to stop all apparent move- ment. When the retinal illumination appears to move in the opposite direction to that in which the mirror is tilted, the observer is within the point of reversal, and a plus lens must be placed before the eye to stop all apparent move- ment. Rule for neutralizing lenses, no matter whether the mirror is plane or concave. When within the point of reversal, use a plus lens, and when beyond the point of reversal, use a minus lens. Application of Retinoscopy in Ernmetropia (Fig. 153). Rays of light proceed parallel from an emmetropic eye under the influence of a cycloplegic, and if a -f- 1 S. is placed in front of such an eye, the rays will converge and form a point of reversal at I meter distance, and the observer at this point will not be able to see any move- ment of the retinal illumination. The same result would have been obtained at ^ of a meter if a -+- 3 S. had been used, or at 4 meters if a -(-0.25 S., or at y 2 of a meter if a +28. had been used, etc. In taking the patient from the dark-room to test his vision at 6 meters, an allowance must always be made for the distance from the patient's eye at which the point of reversal was found. If at ^ of a meter, 3 S. must be de- ducted from the lens used ; if at ^ of a meter, 48.; if at 6 meters, nothing, or o. ij2._ i68 REFRACTION AND HOW TO REFRACT. Application of Retinoscopy in Hyperopia. The x'same conditions hold good in hyperopia as in emmetropia. If a +4 S. gives a point of reversal at one meter, then I S. must be taken from the 4 S. to give the eye parallel rays of light, or infinity vision. If a +48. gave a point of reversal at 2 meters, then 0.50 S. would have to be deducted from the 4 S. for the infinity correction, which would be -+-3.50 S. Application of Retinoscopy in Myopia. Rays of light from a myopic eye come to a focus at some point inside of infinity, and if the surgeon so desires, he may approach such an eye from a distance of six. meters, until he finds a point where the retinal illumination ceases to move (where it does not appear to move) ; and then, measuring this distance from the eye under examination, he can quickly calculate the amount of the myopia. This can not be done with the concave mirror if the myopia is more than 2 S. If the reversal point is at 4 meters, 3 meters, 2 meters, I meter, *4 of a meter, ^ of a meter, or ^ of a meter, then the myopia would be 0.25 S., 0.33 S., 0.50 S., I S., 2 S., 3 S., 4 S., respectively. If the surgeon will always refract the patient's eyes so that he gets the point of reversal at I meter distance, he will have the following rule to guide him /. c. : ., To add a I sphere to the dark-room correction, no matter what that may be. For example : Dark-room, o.oo 40.258. 40.508. -(-0.758. 4- I -S. -(-1.25 S. Add, . . . i.ooS. 1.008. 1.008. i.ooS. i.ooS. i.ooS. Infinity, . . i.ooD. 0.750. 0.50 D. 0.25!). o.oo 40.25 D. Application of Retinoscopy in Astigmatism. If the surgeon has mastered retinoscopy in hyperopia and myopia, he should not have any difficulty in pursuing exactly the RETINOSCOPY. 169 same course in cases of astigmatism. As already stated, the presence of astigmatism is diagnosed by the presence of a band or ribbon-like streak of bright illumination which extends across the pupillary area. (See Fig. 154.) This band of light may be seen before any neutralizing lens is placed in front of the eye, if the astigmatism is in excess of the spheric correction, as in the following formula : I L +0.75 sph. C +4-50 cyl. axis 105 degrees. Fl . G - . I 54-~ Band of Light. Astigmatism Axis 90 de- i.oo sph. ^j 5-OO cyl. axis 165 degrees. aree Or the presence of astigmatism may not be recognized until after a sphere has been placed in front of the eye, as in one of the following formulas : ,.-,, + 4.50 sph. C +0.75 cyl- axis 75 degrees. 5.00 sph. ^ i-oo cyl. axis 180 degrees. In refracting cases of astigmatism with the retinoscope, [all the surgeon has to do is to refract the meridian of least-'' ametropia first, and then the meridian of greatest ametropia. ^*^ Taking the following formula : -(-2.50 sph. ;3 -f i.oo cyl. axis 90 degrees ; in the dark-room a +3-5O S. would make all movement cease in the vertical meridian, at one meter distant ; but when the mirror is tilted in the horizontal meridian, there would be seen a band of light extending across the pupil on axis 90 degrees. Then, substituting +4.50 S. for the 3.50 S.,all movement will cease in the horizontal meridian, a +4.50 S. neutralizing the horizontal meridian. The difference between these two spheres is I D., which is the amount of the astigmatism. In neutralizing astigmatism ppT-the writer advises using spheres, and after each meridian I/O REFRACTION AND HOW TO REFRACT. has been refracted, to make the cylindric correction, and prove it, if so desired. Axonometer. To find the exact axis subtended by the band of light while studying the retinal illumination, when the meridian of least ametropia has been corrected, the writer has suggested a small instrument, which, for want of a better name, he has called an axonometer. This is a black metal disc, with a milled edge, I ^ mm. in thickness, of the diameter of the ordinary trial-lens, and mounted in a cell of the trial-set. It has a central round opening, 12 FIG. 155. mm. in diameter the diameter of the average cornea at its base. Two heavy white lines, one on each side, pass from the circumference across to the central opening, bisecting the disc. To use the axonometer, place it in the front opening of the trial -frame, and with the patient seated erect and frame accurately adjusted, so that the cornea of the eye to be refracted occupies the central opening. As soon as that lens is found which corrects the meridian of least ametropia, and the band of light appears distinct, turn the h. RETINOSCOPV. I/I axonometer slowly until the two heavy white lines accu- rately coincide, or appear to make one continuous line with the band of light. (See Fig. 155.) The degree mark on the trial-frame to which the arrow- ead at the end of the white line then points is the exact axis for the cylinder. Application of Retinoscopy in Mixed Astigmatism. Here the dark-room correction (after making deductions for the distance of the point of reversal) will show one meridian myopic" and the other, at right angles to it, as hyperopic. If the astigmatism is more than one diopter, in each meridian, the surgeon will diagnose in the dark-room the condition of mixed astigmatism by opposite movements in the me- ridians of minimum and maximum ametropia. Application in Irregular Astigmatism. This condi- tion is either in the lens or cornea, usually in the latter. The reflex is more or less obscured by areas of darkness, which make it extremely difficult to study the refraction, and the observer will have to change his distance repeat- dly to find clear spaces as close to the center of the pupil as possible, as it is this portion of the pupillary area that the patient will see through when the mydriatic effect passes away. The kaleidoscopic picture obtained by moving the mirror so as to describe a circle at the periphery of the pupillary space is quite diagnostic of the corneal condition. \Yhatever correction is obtained should be kept for reference in a postcycloplegic manifest refraction, as it will not always do to order the glasses while the eye has its pupil dilated. The patient may choose a slightly different correction in such cases, after the pupil regains its accustomed size. Irregular Lenticular Astigmatism. This is often more uniform than the corneal variety, and is characterized by faint striae in the lens, pointing in toward the center. If REFRACTION AND HOW TO REFRACT. the striae are not very faint, they may be recognized with the ophthalmoscope, even before any cycloplegic has been used, issor Movement (see Fig. 156). This is a condi- i^/*tion in which two bands of light are present, usually in the horizontal meridian or inclined a few degrees therefrom. ^>v^* Tilting the mirror in the vertical meridian, a band of light ^^0. 3 is seen to come from above and to meet another band, which comes from below ; while these two bands are approaching, the dark space between them gradually disappears, until the two bands unite and form one band across the pupil in or approximat- ing the horizontal meridian. This movement of the bands is likened to the action of the blades of a pair of scissors, and hence the name. To refract a case of this character, the observer must proceed slowly and endeavor to neutralize the horizontal meridian first, and then add minus cylinders with the avi^ ^ojp^ponrting tr> fhp FIG. 156. Scissor Move- ment axis of two The resulting prescription should also be a plus sphere with a minus cylinder, the cylinder of less strength than the sphere, asthe condition is^notjme of mixed astigmatism, the patient preferring this combina- tion, as a rule. Conic Cornea. In this condition the observer is im- pressed at once with the' bright central illumination, which usually moves opposite to the movement of the peripheral illumination. ^The best way to neutralize a case of this character is to proceed as in a case of irregular astigmatism. The observer should also be on the lookout for a band of light in this central illumination, as most of these cases are astigmatic. RETINOSCOPY. 173 Spheric Aberration. This is of two kinds positive and negative. (See Figs. 157 and 158.) In the positive form the peripheral refraction (A, A, that at the edge of the pupil) is stronger than the central (B, B) ; the reverse of is condition, negative aberration, is seen in conic cornea. These two varieties of refraction should not worry the observer, as most of the peripheral aberration is covered up by the iris when mydriasis passes away, and, therefore, is not of any great moment, except in conic cornea. FIG. 157. Positive Aberration. Fie. 158. Negative Aberration. he Reisner Retinoscope (Figs. 159 and 160). This instrument combines the mirror with an axis finder. The metal disc, on which the mirror is secured by means of a coiled spring at one point only, has a milled edge like that of an ophthalmoscope. (See Fig. 159.) At the junction of the metal disc with the handle is a small push-button which has a short metal pointer which extends upward and beneath the mirror. Figure 160 is a back view of the in- strument, and shows the degree-marks of half a circle and an index or pointer. When beginning the examination, 1/4 REFRACTION AND HOW TO REFRACT. this instrument may be used just the same as any other retinoscope, but as soon as the surgeon sees a band of light tli en he presses the push-button to see if the mirror rotates with its axis corresponding to the long measurement of the band of light. If this is not so, then the mirror must be turned by means of the milled edge, using the index-finger as in turning the disc of an ophthalmoscope. When the axis of rotation of the mirror corresponds with the long FIG. 159. (One-half size.) FIG. 1 60. (One-half size.) axis of the band of light, then the pointer on the back of the instrument indicates the axis of the astigmatism, and now all the surgeon has to do as he makes the changes in the lenses is to press the button only. The handle of the retinoscope is now held perfectly still and the upper portion of the disc rests firmly against the observer's brow. The merits of this ingenious instrument are as follows : The RETINOSCOPY. 1/5 handle is held perfectly still, the mirror alone does the moving; the sight-hole is always in front of the pupil; the axis of the astigmatism is carefully recorded. FIG. 161. (Two-thirds size. ) FIG. 162. (Two-thirds size.) rer Th e Luminous Retinoscope (Figs. 161 and 162). DcZcng Patent. The latest improvement in retinoscopes is 1/6 REFRACTION AND HOW TO REFRACT. the luminous instrument here described. This instrument is the author's plane mirror with the electric light attach- ment. (Fig. 1 6 1.) The filament is contained in a tube placed at an angle of 45 degrees with the handle and the mirror is correspondingly tilted at an angle of 22 degrees. The light from the filament passes divergently to a strong convex lens which renders the rays less divergent as they fall upon the mirror, and from the mirror the rays pass divergently to the patient's eye. (See Fig. 162.) This in- strument has innumerable points of merit: It does away with any use of gas or lamp or cover chimney; the ob- server is not annoyed with the heat from the gas or lamp ; the observer does not have to move the light or bracket when changing from one distance to another as when work- ing with the gaslight close to the mirror ; the electric wires (cords) carrying the current to the filament are of sufficient length to give the observer two meters of space in which to practise the method; the brilliancy of the illumination can be made most intense or diminished very materially with a convenient rheostat; the size of the divergent pencil may also be controlled by adjusting the condensing lens at the end of the tube. CHAPTER VII. MUSCLES. EXAMINATION OF THE EXTERNAL EYE MUSCLES. General Considerations. When the retinal image of an object is situated exactly on the fovea, the eye is said to "fix" the object. Normally, when b^oth eyes "fix " the object, each eye has an image of the object on its fovea, and these foveal images or impressions are transmitted to the brain and fused as one image in the visual centers. This condition is spoken of as equipoise, or orthophoria, and the eyes are said to be in equilibrium, or to balance. Whenever one eye alone fixes an object, and the fellow-eye receives the image of the same object on a part of its retina distant from the fovea, then the brain takes note of two separate impressions, and this condition is spoken of as double vision (diplopia). (rt) The image of an object formed upon the retina above the fovea is projected downward /. c. t objects situated below the horizontal line of vision are recognized by that portion of the retina above the fovea. (&) The image of an object formed upon the retina below the fovea is projected upward i. e., objects situated above the horizontal line of vision are recognized by that portion of the retina below the fovea. (r) The image of an object formed on the retina to the nasal side of the fovea is projected toward the temporal side /. c., objects to the temporal side have their images formed upon the nasal portion of the retina. 177 REFRACTION AND HOW TO REFRACT. (d) The image of an object formed on the retina to the temporal side of the fovea is projected toward the nasal side /'. c., objects to the nasal side have their images formed upon the temporal portion of the retina. Homonymous Diplopia (Greek, ^xovurw-; from /*?, same, and Zwefv, to tend). In the con- sideration of vertical diplopia, which is always a condition of crossed diplopia, never homonymous diplopia, h the eye which is deviated up- ward is spoken of as the hyperphoric eye, x > and necessarily its image must be lower than its fellow. For instance, if the left eye fixes an object and the right eye is turned upward, the rays of light from the object would fall upon the upper part of the retina of the right eye, and would be projected downward below the true object ; and this position of the right eye is spoken of as right hypcrphoria. Or if the right eye fixes an object and O'M FIG. 164. ISO REFRACTION AND HOW TO REFRACT. the left eye sees a false object below, then the position of the left eye is spoken of as left hyperphoria. Unfortunately, in hyperphoria (unless from paralysis) the position of the eyes does not tell whether the right superior rectus is too strong and the left inferior rectus too weak, or the left supe- rior rectus too weak and the right inferior rectus too strong. Muscle Phorometry. Testing the power of the ocular muscles. Abduction. The power of the external recti muscles to turn the eyes outward. The patient is comfortably seated and told to look at a point of steady light at a distance of about 6 meters, slightly below the level of his eyes, never above the level. In this position prisms with their bases in- ! ward are placed in front of one or both eyes until the patient says he sees two lights very close together. The strength of the prism or prisms thus placed before the eyes which will just permit the eyes to see one object and if increased would produce diplopia, represents the power of the external recti muscles. This is spoken of as the power of abduction, and is abbreviated Abd. For example, if with 7 centrads, base in, before the eyes there are two lights, and with 6 centrads there is only one light, then 6 centrads would represent the amount of the abduc- tion. In other words, in the case supposed, as long as there is less than 7 centrads before the eyes, base inward, the external recti muscles can overcome their effect, but as soon as a prism stronger than 6 centrads is used, then the external recti muscles can not counteract the effect, and diplopia is the result. Adduction. The power of the internal recti muscles to turn the eyes inward. The power of the internal recti is tested in the same way as the external, except that the prism is placed base outward. This is spoken of as adduc- T' I I j, 1^ '\S-*- ) \S^\ X V C/i . ' ) MUSCLES. iSl tion, and is abbreviated Add. For example, if with 19 centrads, base out, before the eyes two lights are seen, and with 1 8 centrads only one light, then 18 centrads represent the power of adduction. In other words, as long as there is a prism of 18 or less than 18 centrads before the eyes, base outward in this case, the internal recti muscles can over- come the effect ; but as soon as a prism stronger than 1 8 centrads is used, then the internal recti muscles can not counteract the effect, and diplopia is the result. It must be remembered that the internal and external recti are antagonistic, and that the muscles of the two eyes are tested together. The relative power of adduction to abduc- tion has been variously estimated, but most authorities are agreed that^adduction is about three times that of abduc- tion, or about 3 to I that is to say, in eyes with normal muscle balance, if adduction is represented by 18 centrads, then abduction should be 6 ; or if adduction is represented by 24 centrads, then abduction should be 8 centrads ; or if adduction is 1 2 centrads, then abduction should be 4 cen- trads, etc^ Sursumduction. This is the power of the eyes to fuse AVO images when one eye has a prism placed base up or down before it. For example, if a 3^ centrad prism is placed base up or down before either eye and diplopia results and persists, and then a 3 centrad is substituted and there is no diplopia, then the eyes have overcome the effect of the prism and the amount of the sursumduction is said ' to be 3 centrads. This test for sursumduction is made at the same distance as in testing the lateral muscles. In health the power of the superior and inferior recti muscles is, as a rule, the same that is to say, they antagonize each other equally. The power of the superior recti is spoken of I as supraduction, snrsninvergencc ; and that of the inferior ; recti, as infraduction, dc.i>rsnnii'crgcncc. 1 82 REFRACTION" AND HOW TO REFRACT. Muscular Imbalance. Whenever there is any disturb- ance in the power, strength, or force of the ocular muscles, the condition is no longer one of equipoise, or equilibrium, or muscle balance, but is spoken of as muscular imbalance (heterophoria). From this statement it must not be sup- posed that the two eyes can not simultaneously " fix " an object, any more than it must be supposed that a hyperoptc eye can not see or have ..- vision ' without correcting glasses. Just as in hyperopia distant vision may be made clear by the effort of accommodation, so in muscular imbalance the visual axes can be directed to one point of fixation by increased innervation. Muscular imbalance is subdivided into two classes insufficiency and strabismus. The following nomenclature of muscular anomalies, sug- gested by Stevens, of New York, is in common use : OrtJiopJioria, perfect muscle balance, equipoise, or binocular equilibrium. Orthotropia, perfect binocular fixation. Hctcroplwria, imperfect binocular balance, or imperfect bin- ocular equilibrium. Hcterotropia, a squint or decided deviation or turning from parallelism. Hypcrplioria, ^tendency of one eye to deviate upward. Hypertropia, a deviation of one eye upward. Esophoria, a tendency of the visual axes to deviate inward. Esotropia, a deviation of the visual axes inward. Exophoria, a tendency of the visual axes to deviate outward. Exotropia, a deviation of the visual axes outward. Hypercsophoria, a tendency of the visual axis of one eye to deviate upward and inward. Hypercsotropia, a deviation of the visual axis of one eye upward and inward. MUSCLES. 183 Hypercxophoria, a tendency of the visual axis of one eye to deviate upward and outward. Hypcrexotropia t a deviation of the visual axis of one eye upward and outward. Insufficiency. Also called latent deviation, hetero- phoria, or latent squint. This may be defined as the con- dition in which there is a tending or tendency of the visual axes to deviate from the point of fixation ; this may be slight or transitory. Causes of Insufficiency. The chief cause of insuffi- ciency is some form of ametropia. Another cause may be an anatomic defect of one or more of the ocular muscles themselves, or a weakness of the muscle or muscles indi- vidually, or as a result of some systemic weakness. The ocular muscles often sympathize with the economy. Symptoms of Insufficiency, or Muscular Asthenopia. Accommodative and muscular asthenopia are intimately associated, and the latter is so often the companion of the former that they produce symptoms which are identical in both and make it difficult to draw any sharp line of demar- cation between the two. In muscular asthenopia, how- ever, the patient complains that the eyes "become weak" or "tired" after any prolonged use, and that this is espe- cially apt to occur by artificial light ; that nearby objects (reading, writing, or sewing) grow dim ; that the words "seem to jump," or the "letters run together," and in some cases occasionally, and in others more frequently, objects appear double for a moment. Sometimes one of the eyes feels as if it \vas turning outward or inward. There are innumerable reflex symptoms, dizziness, nausea, vomiting, fainting, and, in some instances, " all becomes dark for a minute." Such patients often become very anxious, fearing sudden blindness, etc. 184 REFRACTION AND HOW TO REFRACT. Diagnosis or Tests for Insufficiency (Heterophoria). Before taking up the individual tests for insufficiencies, it is well for the observer to study the movements or excursions of the eyes ; and to do this the patient, with his head erect and steady in one position, fixes with his eyes the point of a pencil held in the hand of the observer at about thirteen inches distant. The pencil is moved from left to right and from right to left, and upward and downward ; as this is done, the surgeon should watch closely to see that each eye has a normal mobility and the two eyes move together. From a central point of fixation the eyes should move inward about 45 degrees, outward 45 or 50 degrees, upward about 40 degrees, and downward about 60 degrees. The tropometer of Stevens will estimate the limit of motion of each eye separately, but if there is a defect in mobility, the surgeon may recognize it by comparing the distance of the corneal edge in each eye from a certain definite fixed point ; for instance, whether the lid margins encroach equally upon the cornea or have equal intervals between cornea and lid edges. The Cover Test. The patient is told to look at the point of a pencil held in the hand of the surgeon on a level with the patient's eyes in the median line, and distant about eighteen inches, or at an object six meters distant. While the eyes fix the point of the pencil or distant object, the surgeon covers one eye with a small card, and a moment later quickly withdraws it and observes the position and movement of the eye which he has just uncovered ; if it moved inward toward the nose to fix the point of the pen- cil, then there must have been an outward tendency of that eye when under cover ; in other words, the external muscles must have been strong or the internal weak. If the eye thus released from the cover had moved outward toward MUSCLES. 185 the temple to fix the point of the pencil, then the external recti must have been weak or the internal strong. If the eye released from cover goes up to fix. then the fellow-eye deviates upward, and vice versa. This test is not always reliable, and yet it may be a guide to further study. The Fixation Test. Instead of covering one eye, as in the previous test, the patient " fixes " the point of the pencil as it is slowly advanced in the median line toward the nose, 'up to within four inches, if necessary. During this advance of the pencil, if there is a weakness of the interni, the eye with the weaker internus is the one which will usually deviate outward. To Determine Lateral Insufficiency. The condition in which there is either a tendency for the visual axes to deviate outward (exophoria), or a tendency for the visual axes to deviate inward (esophoria). Proceed by producing " vertical diplopia. Place a ten centrad prism base down before one eye, for instance, the right eye, and have the patient look at a point of light on a level with his eyes at a distance of six meters. He will see two lights, one above the other ; the upper light must belong to the right eye, because the prism before the right eye bent the rays down- ward. If one light is directly above the other, then the condition is presumably one of equilibrium or equipoise. If the upper light, however, is to the right, then the visual axes deviate inward (esophoria). The amount of the esophoria (insufficiency of the external recti) is repre- sented by that prism placed base outward before the left eye which will bring one light directly above the other. If the upper light had been to the left, then there would have been a tendency of the visual axes outward (exophoria, insufficiency of the internal recti), and the amount of the exophoria is represented by the strength of prism placed 1 j bh $ fl&Sif**^*** 1 * ~T3 &- 1 86 REFRACTION AND HOW TO REFRACT. base inward which will bring one light directly above the other. To Determine Vertical Insufficiency (Hyperphoria). Proceed by producing lateral diplopia. Place a ten centrad -j prism base inward before the right eye, and have the patient look at a point of light, as in testing for lateral insufficiency. (It is always well to have the point of light just in front of a large piece of black felt cloth tacked upon the wall.) , If the two lights which the patient sees are on a horizontal 'I line, then the condition is presumably one of equipoise. But (if the right light is lower than the left, there is a tendency of the visual axis of the right eye to be higher than its fellow. ) As to which muscle is at fault, this test will not tell, and Stevens' tropometer will have to be used. The amount of the deviation is represented by the strength of prism placed base down before the right, or upward before the left eye which will bring the two lights into a horizontal line /. c., on a level. To Determine Lateral Insufficiency at the Reading Distance. Have the patient look at a black dot, with a black line two or three inches long running perpendicularly through it, at a distance of about thirteen inches. This is > known as the line-and-dot test of von Graefe, and on a larger scale may also be used in the previous tests. A prism of seven or eight centrads is placed, with its base down, in front of the right eye. If the patient sees two dots exactly one above the other on one line, there is not supposed to be any insufficiency. If, however, there are two lines and two dots, and the upper dot is on the right, there is in- sufficiency of the externi (esophoria) for near. The amount of the insufficiency is represented by the strength of prism, placed base outward, before the left eye which will bring the two dots exactly on one line. If the upper dot is to MUSCLES. the left, then there is insufficiency of the intern! (exophoria) for near, and the amount of the insufficiency is represented by the strength of prism placed base inward over the left eye which will bring the two dots, one above the other, on one line. Another method for testing lateral insufficiency at the reading distance of 1 3 inches is to have a card about 6 inches square, and on this card to draw a heavy black line about 3 inches long; this line is to be horizontal. At the middle of the horizontal line draw a heavy black line, one-half an inch long, extending vertically from the hori- zontal line ; this short vertical line to be capped with an arrow-point. The horizontal line is divided off into equal spaces, each 3 l /$ millimeters apart and .numbered from I to 1 5 each side of the arrow ; those to the left of the arrow are marked "esophoria," and those to the right of the arrow are marked "exophoria." (See Fig. 165.) To use this method, a prism of 8 centrads is placed base down before the right eye ; this doubles the scale vertically ; the upper scale belongs to the right eye. The number and the word in the upper scale to which the arrow in the lower scale points, is the approximation in centrads of the amount of the esophoria or exophoria. For instance, if the lower arrow points to figure 9 in the upper scale to the right of the upper arrow, that is, the word "exophoria," 1 88 REFRACTION AND HOW TO REFRACT. then there is approximately 9 degrees of "exophoria" at this distance of 1 3 inches, the distance at which this scale is intended to be used. These tests for insufficiencies should always be made be- s. fore estimating the refraction, and also after the correcting lenses are carefully placed, with their optic centers, before the eyes. / -To avoid confusion in making these tests, when a point ^ of light is used as the fixing object, it is customary to place a piece of plane dark red glass before one eye, so that the FIG. 166. Maddox Rod. FIG. 167. > | red light always corresponds to the eye with the red glass. Or a Maddox rod (Fig. 166), white or red, may be used for the same purpose. This may be a series of rods (see Fig. 167) placed in a metal cell of the trial -case, and the eye, looking through it at the light, will see the image of the flame distorted into a streak of broken light. A strong -f- cylinder from the trial-case will answer the same pur- pose. (As the rod refracts rays of light opposite to its axis, ; the eye will see a streak of light in the reverse meridian to MUSCLES. 189 that in which its axis is placed. To expedite the deter- minations, the rotary prism of Cretes or the revolving prisms of Risley may be employed. This latter apparatus (see Fig. 168) is composed of two superimposed prisms of I 5 centrads each, and mounted in a milled-edged cell of the size of the trial-lens. By means of a milled-edged screw these prisms are made to revolve so that in the position of zero they neutral- ize each other, and when rotated over each other the prism j. 1( . l68 strength gradually increases un- til the bases of the prisms come together and equal 30 centrads. The strength of the prism employed is indicated by an index on the periphery of the cell. Stevens' Phorometer. This is a very convenient appa- ratus, composed of two 4-degree prisms placed in a frame 3 l / 2 inches from the eyes, which with an attached lever can be rotated so as to test the strength of the vertical and lateral muscles. Indexes and letters at the periphery of the frame record the character and degree of the insuffi- ciency. (See Fig. 169.) Treatment of Insufficiencies. As ametropia is the most common cause of insufficiency, the" first consideration must be to select the proper correcting glasses. After this has been accomplished, if the insufficiency still persists and the patient is not comfortable, then the muscles should re- ceive careful attention, and their condition be studied from every point of view. The patient's general health should be looked after, and if at all defective, must have remedies prescribed for its improvement. In some instances the 190 REFRACTION AND HOW TO REFRACT. patient may have to give up any close application of the eyes for a time and pursue an out-door life. Operative interference (tenotomy) must not be entertained until all known means for the relief of the muscular asthenopia have been exhausted. The prescribing of prisms, as a fixed rule, for use, which correct insufficiency, except in vertical erro/s, is often a serious mistake on the part of the surgeon, as in most instances they often do more harm than good by in- creasing the difficulty. Internally, sedatives will frequently -- Phorometer Revolving Maddox Rod Revolving Trial Frame Revolving Rotary Prism FIG. 169. give great satisfaction and permanent relief. The writer is partial to the use of bromids with small doses of the iodid of potash three or four times a day. The modus operand' is not clear. The only guide that can be suggested is to use sedative treatment and rest of the eyes whenever there is a congestion of the choroid and retina and when the ophthalmoscope shows the nerve edges hazy, the retina woolly, etc. - "In another class of patients the internal use of nux vomica is the treatment par excellence, and it acts best in those cases where the nerve edges and the eye- ground in general appear clear and free from irritation. *s*r '>jL*fr. f \^^^ '^"Wt-rtAJM*- LA oftimes at each sitting. 2. Prism Exercises. The patient is placed, standing ^ about a foot or two from a point of steady light, on a lev or slightly below the level of the eyes, and told to look at v > F92 REFRACTION AND HOW *O REFRACT. it., and at ^nothing else. In this position a pair of weak / 4ff 4 Ufa fir l prrsms DasefouT, in a trial-frame are placed in front of his eyes. * Then he is told to walk slowly backward as he keeps his eyes fixed on the point of light. Should diplopia de- velop at any distance short of 20 feet, then he is to raise the prisms, go back to his original position, and start over again. Repeating this a number of times in the surgeon's office, it will be found, in most instances, that at the first practice a pair of 5 A or 10 A can be overcome at a distance of 20 feet. When the distance of 20 feet from the light is reached without developing diplopia, the patient is instructed to slowly count 20 or 30 (keeping the light single during this time), then raise the prisms (gazing at the light), and to slowly count 20 or 30 ag If the symptoms of muscular asthenopia persist after pre- fa ' *+ "' V scribing the ametropic correction, then prisms, bases out, ^n > may be prescribed as hook fronts to be worn over the con- stant correction when using the eyes for distance. It has been the writer's experience that esophoria of two, three, or four degrees seldom gives the possessor any discomfort whatever, i Prism exercises for esophoria give very little benefit, and are often a waste of time ; yet they should be ^^T tried thoroughly if the case appears to demand it. y 7 194 REFRACTION AND HOW TO REFRACT. Treatment of the Insufficiency of the Superior and Inferior Recti. Having prescribed the ametropic correc- tion, an attempt should be made to strengthen the weak muscles by prism exercises prism base down before one eye, and base up before the other eye. While this does not ; often give satisfactory results, yet it should be tried in each instance. If prism exercises do not correct the difficult}-, then prisms which overcome most of the insufficiency should be prescribed for constant use. Failing in this second attempt with prisms or with a full prismatic correction, then tenotomy of the overacting muscle or muscles must have consideration. Tenotomy. As previously stated, tenotomy should never be resorted to until every other known means of relief has been tried, and even then no hard-and-fast rule can be given for the amount of the insufficiency in degrees which will prompt such a procedure. Some patients with as much as four or six degrees of esophoria may never suffer the least annoyance ; and yet other patients with the same amount will estimate their sufferings as almost beyond en- durance. And the same statement holds good in other forms of insufficiency, especially exophoria. The question of personal equation, the patient's nervous system, hysteric tendencies, etc., must all be considered before undertaking a tenotomy that may result in nothing but discourage- ment. If an operation has been deemed best, then it is for the surgeon to decide whether he will divide the tendon of the strong muscle or advance the weak muscle, or both. What- ever operation or operations are performed, the amount of the deviation should be estimated immediately before, as well as during and after, the operation. When a simple tenot- omy is performed, the eye is usually left open (unband- J . ^oo. e^j^-^ ' ' ^ * I I** - Y.'hen the esop ar.ts to 6,8,r 1C Octrees, then the lent rill usually have narfte.^ s such as ecul ar hdacheE,^ runring bfclrtwen the neck,& seme tires the shoulders. They donot have am* cor.fort when readn' A 8'r 8 t T f' flWg and in fact eft en have t abandon any prolonged use of the eyes at '. hear work . "hen the patiert h--s several frees of esop f ~ :-i stance he r.ust use his din- ce correct itn which usaally corrects any furthe ^ctmfort in the use of /his eyes for distance; but he may continue to have disc at any near work h disc as ocular pains, oc hdaches, pains running o the neck end sorretimes frit between the shoul s. Such pts do not have any corcf from thier n rding or rriting or any close work vhich requires the eyes to nove instead of chaining fix For instance f a sewing werr.an will corr.e with the story that she ca n sew with comfort with her glssces on, but that she cannot read with cor.fort & she crnnot undrrctand why. A stenographer who rur the typewriter curing the day suffers from the s s just described but she cm sit and sew in the ever ing srd not get a hdache. THe trc-vble lies in w v-eak aBducting i . ower. The treatment is net to v;eal:en but t strenghten abduction. The w iter ' ethod vhich he believes to be o: iginalis to pract aba or tu 1 ning outward of khz each eye seprrat that is to tuf-n each eye outward while its fell -s covered. The pt fixes his head in one pos tion j?.nd not to turn it while practiseng ts follow to cover the eye with a card ?s shown and in such a rranner that the covered eye^ cannot see what th other eye is io ing; then /ho Id ing the index finger p ;oint an a level with the eye, the finger is gradu- ally r.ad eto describe a Tusrter circle to the sair.c thre e being exercised; the eye fixes the point of the finger to the Unit of external rotation. Fi? st one eye and then t minutes after each meal. Marked in and the ?rr ean testify to remirkab] ghting abduction. Occasionally t pt rill also hav eto have pri .After partial tenotomies TI95 r after a tenotorr.y if any annoying ir ways be successfully relievee rade in the correcting glasses. After par Ii page ?3I add - Cummar. . The f ol .owing sice is given ^s the \ tried after the correction is si patient sees 6/6 rith the f !!?. in is not to be ordered until the var5 this coorection*- to mak e sure that on. _/ 0.2 r sphere O.?r sphere. -fC.^ ^cl;: 2txfchfi5pm*xixiz. *. -f" O I ^ at the opposite axis. ^,L^ st the same axis ^ , VT *PP axis crosse r cyl lens /^fter fecus p??0 4th line ac their use is of advantage to pr ^uire lens of leesjff . v .eight anT als formerly a plusI5.COS.B.contine vri cyl next to the eye and the convex and nweildy lens, so that nor vith next to the eye and the outer aurfac an and plus 8 in the hor.rr.er. with very lone: laches appreciate to The teric Q! so allow a r.uch neater cemented on to the concave surface. ther is exercired this v ay for 5 or ten ovrrert rill follow this tr i: ew days results by this very si~ le method of stren sctice alone will not suffuce and the s (bases in) t is an interstir - rever the leas that ^ems-ins, it can Usually but not al- cribing the necessary correcting prism to be ios steps in refraction one the lens to br d to be eorrect. For ir.sta- nce,if the lus I. CO cer.b vrith rlus T.CC cyl . axisOO, this s lens have been successively placed before he eye will not accept some other coir.binsti- he same axis ic 1 ere ,-These have been described( p54)but r^ and cases of aphakia,as these cases re- ens which will enlarge the field of vision. llurr.Cl at axis I6C ras mede v:ith the 2 ras placed outward, this r.sde a very thick e toric "lens this vrould equal a plusT .r. rould have a plus 10 c;-l in the vrrt.mefcidi- I res en o; toric bee of enl. field. I eo-ple ^ ere than the old cphero-r; :r lens, focrl by alloving the scale e- segment to be Vc le with sen Itive r etir!aa do no t as a rule ftnjty t tries as they per- ' ' pheral rays to fall upon j ortions of the ret which do not tolerate the inc eased stirulatii n, I rest as a rule are not sir.ilarly disturbed. I age?94 add . Tused bifocal s(Kryptok) *-This variety is also known as "invisible" . It is not unlke the bifocal shown in Fig. 185. It is made by taking a small circular piece tf flint glass nd by greet heat fusing ne surface t a crtwr glass- piece; the cr^n glsss is square in she this is the form in vhich the Krypttk Sales C supply the opticians, who then grind the v curves to meet the conditions of the oculist prescr: ptions. After trifocals : age 295 add. One-ptece bifocl s.-This is rot unlike the solid or ground bifo- cals but it nor ncde in the toric rurve and neater and more delicate than the previaus col- id biffccal. This one-piece bifocal is frequent- ly also called"invisible tf . Close insp I Tle( ted light will generally sho^r where the two ed ge-s come together. Its think they will av seeing where the upper and lower corr come to- gether, lut they will see^/it in any bifocal arc the term invisible applies t the friends of U petient who cannot always see that '.if teals are being wor n. latients of unknown age wearing bifocals of the "inv" variety en . oy the fact that their friends still think they are youngt' MUSCLES. 195 aged) so that visual fixation is maintained, and the muscle balance tested frequently to see that, by subsequent con- traction, the insufficiency does not return. To avoid such a misfortune it may be necessary to use prism exercises dur- ing the healing process. The writer is not an advocate of partial tenotomies. V" *Z^-4Ji 'jfty- 'Vt**' * , Strabismus (arpls, "a circle," /. c., the ciliary ring, and ^tyry, " a stroke ") is a drug which will temporarily paralyze the action of the ciliary muscle. A mydriatic (from the Greek, nuSptaats, " enlargement of the pupil ") is a drug which will temporarily dilate the pupil. Atropin will dilate the pupil and also cause a paralysis of the ciliary muscle. Cocain will cause a dilatation of the pupil, but will not paralyze the action of the ciliary muscle. A cycloplegic is also a mydriatic, but a mydriatic is riot necessarily a cycloplegic. The Uses of a Cycloplegic. (i) To temporarily sus- pend the action of the ciliary muscle, or to put the eye in such a state of rest that all accommodative effort is for a time suspended while the static refraction is being estimated. (2) The retina and choroid are given an opportunity to recover from irritation and congestion incident to eye-strain (" eye-stretching "). There are many different cycloplegics employed for estimating the static refraction, and each has particular qualifications for individual cases. Cycloplegics may be classed as of three kinds : (i) those the effect of which passes away slowly ; (2) those the effect of which passes away moderately fast; and (3) those the effect of which is very brief. :/The first effect of a cycloplegic is its mydriatic quality, 208 > after which the accommodative effort is suspended. The paralysis is not permanent. The following table, from Jack- son, shows the length of time paralysis persists and the time it takes for the ciliary muscle to fully recover : J^Atropirt, effect begins to dimi Daturin, ' ' 3 " 3 " 2 " 12 hours. Hyoscyamin, Duboisin, Scopolamin, 7 Homatropin, ish in 4 days ; complete recovery, 15 days> lo 8 8 6 2 >\ If a solution of one of the above-mentioned cycloplegics be instilled into the conjunctival sac of a healthy eye, it will be carried by the blood- and lymph-vessels at the sclerocorneal junction into the ciliary muscle and iris, where it acts directly upon the nerves and ganglia of these structures, and the aqueous humor also receives some portion of the .drug. If cautiously used, the action will be limited to one I eye, showing that the drug does not pass through the car- diac circulation ; otherwise, the pupil and ciliary muscle of the fellow-eye would be similarly affected. Some conjunctivas are very sensitive to any of these drugs, and develop an inflammation so severe in individual instances as to resemble ivy poisoning of the lids. f Duboisin especially, and hyoscyamin, by absorption, may develop hallucinations and even a loss of coordination./ Any cycloplegic, in fact, when carelessly used, may pro- duce very unpleasant symptoms, such as dizziness, dry throat, flushed face and body (mistaken for scarlatina), rapid pulse, a slight rise of temperature, and delirium. To avoid such an annoyance, which is apt to reflect discredit upon the physician and upon the profession in general, the patient should always be given definite instructions how to use the drug in each instance. Stopping the use of the drug and 18 2IO REFRACTION AND HOW TO REFRACT, applying cold compresses will relieve the conjunctivitis, and if constitutional symptoms manifest themselves, a dose of paregoric, cooling drinks, a darkened room, and stoppin /the use of the ,drug will soon restore the patie ~CSy fc^-v , FORM OF Name, MR. BROWN. K. Atropin. sulphatis, ........... gr. j Aquae dest., .............. ^S 1 }' ' M. Ft. sol. Label, poison drops ! ! SlG. One drop in each eye three times a day, as directed. R. Dropper. DR. Date, Tuesday, March 14, 1899. The reason for labeling this prescription "poison drops" is not to frighten the patient, but to caution him against leaving the medicine where children may get hold of it, and at the same time to let him understand that it is to be used and handled with care. Mr. Brown is told to have one drop put. in each eye ' jthree times a day, after meals, and to report at the office on Thursday (the prescription is given on Tuesday in this case). The reason for using the drug for this length of time is to )? insure complete paralysis, and also to give the eyes a physi- ologic rest. In having these " drops" put in the eyes, the patient should tip his head backward and turn his eyes down- ward, and as the upper lid is drawn up, one drop (from the dropper) is placed '(not dropped) on the sclera at the upper and outer part. After the drops are placed in the eyes, as far away s from the puncta lachrymalia as possible, the patient holds the canaliculi closed by gently pressing with the ends of the index fingers on the sides of the nose at the inner canthi for a minute or two. If more than one drop enters the eye, / it will run over on to the cheek, and should be wiped off. With children, these instructions are not so easy of exe- CYCLOPLEGICS. 2 1 I cution, and the writer has seen a few such clinical subjects flushed and delirious from gross carelessness on the part of parents in dropping the medicine into the inner canthi, where it soon passed into the nose, or else the drug is allowed to flow over the cheek and into the child's open mouth. } Ordinarily, there need never be any discomfort from the use of these drugs beyond a slight dryness of the fauces. Caution. Cycloplegics should never be used when ~~^ Jp*' there is the least suspicion of glaucoma in one or both (j/y' eyes. Cycloplegics should not be used in the eyes of nursing women; such patients are peculiarly suscep-"' tible to the action of these drugs, and the mammary secretion may thereby be diminished in amount. After the ./ age of forty-five or fifty years, or in the condition known I as presbyopia, it is seldom necessary to use a cycloplegic. If a cycloplegic is necessary in presbyopia, one of the weaker drugs is generally employed. ^In the selection of a cycloplegic the surgeon must be guided by the patient's occupation, age, the character of the eyes, and the refraction,' From the foregoing table it will be seen that atropin and daturin are slow in passing from the eye, making their employment on this account very objectionable in many instances. The accommodation returns sooner after the use of hyoscyamin and duboisin than from atropin, but not so promptly as from scopolamin and homatropin. The effect of the latter is very brief. A patient who might lose his business position if he remained away from work for more than a week could not afford to have atropin or daturin used in his eyes, whereas a school child might accept atropin as a luxury. The man of busi- ness, the cashier in a bank, the storekeeper, and others must, in many instances, have their eyes refracted in at 212 REFRACTION AND HOW TO REFRACT. > least two days ; and this latter time means, of course, the use of homatropin. The nearer the age to forty years, the jless need for one of the stronger cycloplegics, as the power of accommodation has markedly diminished at this period of life, so that hyoscyamin or scopolamin will answer every purpose. After thirty-five years homatropin can, as a rule, be relied upon as a cycloplegic. In hyperopic eyes of young subjects it is useless to em- ploy homatropin, as the active ciliary muscle requires a strongly acting cycloplegic to stay the accommodative power. In myopic eyes one of the stronger cycloplegics may be used to advantage, for the following reasons : Myopic eyes have large pupils, as a rule, and do not mind the mydriasis ; myopic eyes are often in a state of irritation, and the drug gives them a much-needed rest ; the myope's distant vision is not disturbed by the cycloplegic, as in the case of the hyperope. Whenever a cycloplegic is prescribed, the patient should be ordered a pair of smoked-glass spectacles to wear dur- ing the mydria^sis. Of the two forms of smoked glasses, r coquilles and*' plane, the latter should always be preferred t as they are without any refractive quality, whereas coquilles have some form of refraction that may act very injuriously. Another reason for ordering the plane glass is that the patient will often wish to wear them with his prescription glasses, which he could not do so well if they were coquilles. Dark glasses are of four shades of " London smoked " A B, C, and D, A being the lightest shade and D the darkest. The prescription would be : For MR. BROWN : ^ > R . One pair plane London smoked " D." SlG. For temporary use. DR. March 14, 1899. CYCLOPLEGICS. 2 I 3 The cycloplegics above mentioned for purposes of re- fraction are ordered in the following strengths : Atropin. sulphatis, gr. j to aq. dest., . . f^ij. Duboisin. sulphatis, gr. ss " " " . . f 3 ij. Hyoscyamin. sulphatis, gr. ss " " " . . f 3 iss. Daturin. sulphatis, gr. ss " " " . . f 5 ij. Scopolamin. hydrochlor., . . . All these, except scopolamin, are ordered to be used '"7 three times a day, preferably after meals ; but scopolamin being a very powerful drug, the surgeon should ^place it in the patient's eyes himself in the office, and not give a pre- scription for it. Only two drops are necessary, and are instilled a half-hour apart, the static refraction being esti- mated one hour after using the second drop. How to Use Homatropin. This drug is expensive, and j it is never necessary to prescribe more than one grain for any one patient. Personally, the writer has found the fol- lowing most satisfactory, though the strength of the homa- tropin may be increased if desired : For Miss ROBINSON : *~1 .> t li . Homatropin hydrobromate, gr. j Aq. dest., n\, xl. M. Ft. sol. Label, poison drops ! ! SlG. One drop in each eye, as directed. R. Dropper. DR. March 14, 1899. One drop of this solution instilled into a healthy eye will produce mydriasis in a few minutes, but its action on the ciliary muscle is so trifling that the near point will be but slightly changed. It is thus shown that this drug is a decided^fnydriatic, and only becomes a cycloplegic under definite usage. To produce cycloplegia with homatropin, the patient is 214 REFRACTION AND HOW TO REFRACT. given the above prescription and told to use it as fol- lows : *, ! To place one drop in each eye at bedtime the first night. "This one drop dilates the pupil and establishes a change in the circulation of the blood-supply to the iris and ciliary body a very important matter for the patient's comfort, and at the* same time preventing a tendency to spasm of the ciliary muscle. The next morning one drop is to be placed in the eye every hour, from the time of rising until leaving ( ; home to go to the surgeon's office. At the office one drop is placed in each eye about every five minutes, until six drops have been used ; then, after waiting half an hour! ' (for the cycloplegic effect, which will last for one hour), the| refraction is carefully estimated. After a short interval the cycloplegic effect will begin to rapidly disappear, so that the /patient will be able to read within forty-eight hours' time !with his correcting glasses. Occasionally, a busy patient will insist upon having his eyes refracted during his first visit, and can not take time to use the drops in the manner above suggested. The surgeon must, therefore, start and use the drops in his office. This is forcing the ciliary muscle into a state of paralysis that does not always give ultimately satisfactory results. " Forcing " homatropin into an eye in this way will always produce a " bloot-shot " eye (hyperemia of the conjunctiva, etc.) that does not improve a patient's appear- ance ; and it often produces severe neuralgic headache that may result in nausea or vomiting in occasional instances. 'Furthermore, it is possible, with a drug like homatropin, if 7 not properly used, to have some of the sphincter-fibers become paralyzed while others may remain free to act. In this way a spasm of the ciliary muscle may be produced that will give a false astigmatism. Personally, the writer CVCLOPLEGICS. 215 is not partial to this method of forcing the ciliary muscle into repose. To somewhat obviate the "blood-shot" condition of the eye, and also to assist the action of the " forcing" process, one drop of a two or four per cent, solution of cocain maybe instilled while the homatropin is being used. This ' also diminishes the danger of spasm. But cocain is objec- tionable...^ that it will, in some cases, "haze" the cornea. The retinoscope will show this, and the patient will state that, u;hji|e he can see the letters on the test-card, yet they have a "mist" over them. Instead of using the hom- atropin alone, a small amount of cocain may be added to the solution for the purpose mentioned. Or, homatropin may be combined with cocain and chlorid of sodium in the form of a disc/andone of these, placed in the conjunctival sac, is allowed to dissolve, and in this way paralyze the accommo- dation. Or, homatropin may be used in a solution of dis- "'tilled castor oil. It is claimed that when the drug is used in this form, it remains in contact with the tissues and acts more energetically. Homatropin as a cycloplcgic should be held in reserve for individual cases, and not used as a routine practice. It >is a good, reliable paralyzer of the accommodation in many eyes at the age of thirty-five, or thereabouts ; but in a young hyperopic eye it is a waste of time to attempt successful paralysis with it, and the danger of producing a false "astig- matism should certainly deprecate its use in these cases. Another very serious objection to its use is that before the eyes can become accustomed to the prescription glasses, the ciliary muscle recovers and begins to accommodate, with the result that the patient says he can see better at a distance without his glasses than he can with them, and has no small amount of mistrust of the surgeon's ability, *k REFRACTION AND HOW TO REFRAC as he will have to wear his glasses a long time before his ciliary muscle will relax its accustomed accommodative efforts. This is not nearly so likely to occur if one of the slowly acting cycloplegics is used. X The method of refracting with one of the slowly acting cycloplegics, and then endeavoring to counteract the effect with a solution of eserin, is not recommended. Temporarily, eserin may overcome the cycloplegic ; but as its action is only transitory, the paralysis reasserts itself and will not disappear until the specified time. Refracting one eye at a time with a cycloplegic while the patient pursues his occupation with the other eye is not a method to be considered. This means a great amount of discomfort, headaches, eye-strain, and even diplopia at times, during so prolonged a treatment. If a hyperopic patient must occasionally use his eyes for near work while he has drops in them, a pair of +3 or +4 spheres may be given for temporary use. CYCLOPLEGIA. Cycloplegia is a paralysis or paresis of the ciliary muscle. This condition may be monocular or binocular ; it may be partial or complete. /Mydriasis may or may not accompany the cycloplegia, though the two conditions usually occur together ; and when they both exist, the paresis is spoken of as ophthalmoplegia interna} Th,e ciliary muscle and sphincter of the iris are controlled by branches from the third nerve ; but these branches are from independent cen- ters ; the fibers going to the ciliary muscle arise beneath the floor of the third ventricle, in front of the fibers which go to control the sphincter of the iris. Causes. Temporary paralysis of the ciliary muscle and iris, as already stated, will result from the external or internal CYCLOPLEGIA. 2 I 7 administration of a cycloplegic. It is interesting, in many cases, to find the cause and relieve the patient's anxiety when the paresis is due to one of the cycloplegics. Aside from the use of eye-drops, the question of external ' medication (liniments, ointments, and plasters) should be inquired into, as also whether rectal or vaginal supposi- tories containing a cycloplegic have been used. Other causes of this form of paralysis are tonsillitis, quinsy, diphtheria, Bright's disease, rheumatism, gout, exhausting diseases, blows upon the eye, etc. Other and more serious causes, as controlling a guarded prognosis, are intracranial hemorrhage, meningitis, syphilis, brain -tumor, etc. In some instances the cause can not be definitely ascertained. Symptoms and Diagnosis. Photophobia, dilatation of the pupil, and loss of accommodative power consistent with the optic condition of the eye. A myopic eye retains its vision at the far point only ; an emmetropic eye or a hyperopic eye wearing correcting glasses has good distant vision and absence of a near point ; an uncorrected hyperopic eye has poor distant and near vision. Prognosis. This depends upon the cause. Treatment. This must be symptomatic and expectant, with a removal of the exciting cause, if possible. As many cases of cycloplegia are the result of, or follow, an attack of diphtheria, or a disease which has reduced the system below par, tonics, fresh air, etc., must be ordered. When brought on by syphilis, mercury and iodid of potash must be prescribed. Dark glasses for the photophobia should always be ordered, and lenses for near-work may be worn as a temporary expedient. The use of eserin locally will occasionally do good work, but is not advised for constant use or for every case. Faradism may be used if the cyclo- 19 2l8 REFRACTION AND HOW TO REFRACT. plegia is very persistent, but the best results may be ex- pected from systemic treatment. The use of strychnin or nux vomica are recommended in certain instances. OF THE ClLIARY MUSCLE. 'Cramp of th ciliary muscle is the opposite condition to that of cycloplegia, just described. Ciliary cramp may occur in one or both eyes, usually in both ; it may occur in any form of ametropia or in emmetropia. Ciliary cramp is of two kinds clonic and tonic. Clonic cramp is an occasional and temporary condition which comes on while the eyes are in use, and passes away soon after the eyes have had an opportunity to rest, and may not occur again for several days. - Tonic cramp, also calledf ' spasm " of the accommodation^) is a permanent condition as compared with the clonic form, and occurs whenever the eyes are used for distant or near vision. The patient can not use his eyes for any length of time, or with any considerable concentration, without suffer- ing as a consequence. Causes. Clonic cramp may occur as one of the early symptoms of presbyopia. Ametropia is a very common cause, and especially in cases of low amounts of hyperopia or myopia. Emmetropia, or eyes made emmetropic with glasses, may develop clonic or even tonic cramp if the eyes are used to excess or in a bad light. Such cases have been -called " hyperesthesia of the retina." Tonic cramp may develop from the same causes which bring on the clonic I form, and is usually seen among young hyperopic children, or the "pseudo-myope " already described. It also occurs occasionally in hysteric patients or those recovering from some severe or long illness. The writer has seen this form CRAMP OF THE CILIARY MUSCLE. 2IQ of cramp precede or antedate by several weeks a collapse of the nervous system /. c., nervous prostration. Symptoms. Naturally, ciliary cramp means ocular pains and headaches. Opera headache, " train headache," " shopper's headache," " bargain-counter headache," etc., are some of the many names given to cramp of the ciliary muscle, and are, no doubt, the result of accommodative effort in a bright light or watching moving objects, these symp- toms being a part of the history of accommodative astheno- pia (already described) and accompanying insufficiency of the muscles. Symptoms of myopia are very evident during the cramp.^ In the tonic cramp the ocular pains and headache may be so excruciating in individual cases as to make the family physician and patient dread cerebral disease until the immediate cause is found out. Treatment. As the cause is usually one of ametropia, this must be corrected by the careful selection of glasses \while the eyes are undergoing a prolonged rest with a t cycloplegic and dark glasses. Later on the patient must be cautioned against any overuse of the eyes. The general health* should have any necessary attention. Sedatives, alteratives, and tonics have their place in individual cases. Reflex causes must be looked for and, as far as possible, re- moved. Insufficiencies should always be carefully searched Ifor, and frequently prism exercises to develop the strength lof the weak muscles may give marvelous results. Un- fortunately, there are occasional instances of tonic cramp that persist in spite of any treatment, and such cases obtain relief only when presbyopia definitely asserts itself. Asthenopia (from the Greek, a. priv. ; aOlw;, " strength "; an}> ) " eye ") means a weakness or fatigue of the eye, applying especially to the retina, the ciliary muscle, the extra-ocular muscles, or a general weakness of any one or two or all 2O 2 2O REFRACTION AND HOW TO REFRACT. of these structures in one and the same eye. Asthenopia^ is a disease, and is often spoken of as "weak sight," " eye- jtrain." or " eye-stretching/' Varieties. For purposes of study, differential diagnosis, and treatment, asthenopia, or eye-strain, has been divided into the following varieties : Retinal, muscular, accommo- dative, and asthenopia due to a combination of any two or all three varieties. Retinal Asthenopia. This is the rarest form of asthen- opia, and usually occurs in females. It is brought about by overuse of the eyes in too dim or too bright a light, and may result from a too prolonged use of the eyes at any kind of work or in any kind of light. It may result from exposure to the sun's rays, to electric lights, or to light- ning, or by reflection from bright objects, such as snow, etc. Retinal asthenopia may occur as a symptom of hys- teria, or in a patient whose nervous system is peculiarly susceptible to vibrations, sounds, and lights ; in a patient whose nervous system is an uncertain quantity. Such pa- tients are very unsatisfactory to treat or even to examine ; they often imagine that the reflected light from the ophthal- moscope or retinoscope is "very hot," etc. Symptoms. The chief symptom is a dread of light "/ (photophobia), or photophobia and lacrimation together. Treatment. The first thing to do is to remove the cause, if this can be found ; otherwise the treatment should be very conservative. Ametropia must be corrected and the eyes be given some regular work ; in other words, it is r\ot good practice to restrict all use of the eyes. The treat- ment with "tinted glasses," made so much of by the char- latan to " gull " the innocent public, should not be ordered, as the patient grows accustomed to them and they event- ually become an absolute necessity on all occasions. Care- CRAMP OF THE CILIARY MUSCLE. 221 ful attention to the general health is certainly indicated ; tonics, out-door sports, etc., should be prescribed in individual cases. The shade of the trees is to be recom- mended in preference to the seashore and bright reflection from the sand and water. Muscular Asthenopia. This is due to weakness or fatigue of one or more of the extra-ocular muscles, most frequently the interni (cxophoria). Muscular asthenopia of the exophoric kind is the result, as a rule, of a want of power to maintain convergence. The symptoms are in keeping with a cramp followed by a relaxation of converg- ing power. Ocular pains, eyeballs tender to the touch (per- chance the internal recti themselves become sore to the touch or feel sore on movement of the eyes), and in some cases the conjunctiva and subconjunctival tissues overly- ing the muscles become hyperemic during or after the use of the eyes, simulating rheumatism of these structures. In other cases dim vision and diplopia will be occasional mani- festations. Patients with muscular asthenopia occasionally find that they can continue at near work by using one eye, but this does not occur very often. Treatment. This resolves itself into the correction of the ametropia, exercise of the weak muscles, etc. (See chapter on Muscles.) Accommodative Asthenopia. This is by far the most common form of asthenopia, and is due to fatigue of the "-Iciliary muscle ; it is, therefore, to be expected in hyperopic eyes. It is caused in various ways : from overuse of the eyes in too bright or too dim a light, or from using the eyes for too long a time in any kind of a light. The best pair of eyes, if overtaxed, may suffer from accommodative asthenopia, even when wearing the ametropic correction. Or accommodative asthenopia may result from a weakness 222 REFRACTION AND HOW TO REFRACT. of the ciliary muscle as a part of- the general condition of the whole body, and this may come on after or during some long illness, such as typhoid fever. Accommodative asthenopia x^s often present in the early months of presbyopia. Symptoms. The principal symptom is headache fron- tal, frontotemporal, or fronto-occipital ; or this pain or dis- comfort may extend into the neck or between the shoulders. The headache develops during the use of the eyes, and grows worse if the effort is prolonged, and usually ceases after the eyes are rested. See chapter on Hyperopia and Myopia. Treatment. When glasses are necessary, they should be ordered by the static refraction. The general health of the patient should receive careful attention. An out-of- door life will often be necessary, and in certain cases the time for using the eyes at any near-work will have to be very much restricted. Accommodative with Muscular Asthenopia. This variety of asthenopia embraces the two forms just men- tioned, and its description and treatment are included in both. Reflexes Due to Eye-strain. Among the symptoms of the various forms of asthenopia described on the previous pages, the writer has avoided any decided reference to reflex symptoms, preferring to speak of these reflexes in a general way under one heading. Many patients who suffer from headaches, ocular pains, etc., during the use of their eyes, also very frequently suffer from constipation, indigestion, heartburn, nausea, or even vomiting. Other patients may have nervous attacks, a fear of some impending calamity, or they are irritable or despondent ; they may suffer from insomnia, or, if they sleep, it is not a restful sleep. Others may have epileptic attacks, nervous twitch- ings, etc. To just what extent eye-strain is responsible for EXAMINATION OF THE EYES. 223 these and many other reflexes the writer is not prepared to say, though every ophthalmologist has certainly seen some cases of accommodative and muscular asthenopia with gastric symptoms, or nervous symptoms, or epileptic attacks, or irritable tempers, or insomnia, or enuresis, etc., in which these reflex symptoms entirely disappeared after the eye-strain was properly treated. EXAMINATION OF THE EYES. A systematic method should be pursued in the examina- tion of the eyes, and the results recorded in a book or on a card prepared for that purpose. The student should be a careful observer, and also be able to question the patient intelligently for short and definite answers. The following is an excellent method of making records, but there is no arbitrary rule, arid in this respect each surgeon may follow his own desires : Date, Name, Occupation, ...................................... Age, ................... Sex, .................. - Diagnosis, ................... ACCOMMODATION. ASTIGMATISM. MUSCLES. O. D. V ............ p. p. O. S. V., .......... p. p. History, ............................................... S. P. (status prasens, " present condition"). Inspection, .................. Ophthalmometer, O. D .......................... O. S .......................... Ophthalnuacopic examination, O. D .......................... O. S ............. - ...... Manifest refraction. Fields. Color sense. R . 224 REFRACTION AND HOW TO REFRACT. The above record is rilled out as the examination pro- ceeds, but it is not always advisable to follow the exami- nation in the order given ; on the contrary, it is better, after getting the patient's name and address, to ask certain other questions which may appear in keeping with an individual case. 1. Occupation. This is a very important question, as bearing directly upon the amount and character of work done by the eyes ; for example, writing, reading, sewing, music, engraving, weaving, drafting, surveying, painting, typewriting, typesetting, sorting colors, etc. 2. Age. This is of the utmost importance in comparing the range of accommodation (near point) with the emme- tropic condition. Knowing the patient' sage and near point ^x\vill often give a diagnosis of the character of the refraction. 3. The name tells the sex, but the question really is whether the patient is married, single, widow, or widower. /If a young married woman, whether she is nursing a young ' I child. 4. History. Under this heading the questions should bear directly upon the eyes. " In what way do the eyes /cause trouble?" The usual answer to this question is \ tf headache." To get a complete history of the headache, and be able to differentiate it from headache due to other I causes, the succeeding questions seem appropriate : What part of the head aches ? Is it frontal, occipital, temporal, interocular, vertex, or all over the head ? When does the headache come on during or after the use of the eyes ? Does it cease after resting the eyes ? Is the headache worse when using the eyes by artificial light ? Is the headache constant ? Is it periodic ? Is it worse at a certain hour of the day? Is the headache pres- ent when first waking in the morning ? Does the head ache EXAMINATION OF THE EVES. 225 during or after attending a place of public amusement or when shopping ? If a female, is the headache only monthly ? The ophthalmologist must not think because a patient has ^a headache that it is surely and always due to the eyes, and that glasses are going to cure it. It is for the ophthalmolo- gist to find out just what part the eyes take in causing the patient's discomfort, and not always expect to cure with glasses headaches that have no direct relation to the eyes. One of the most common causes of headache which may be mistaken for ocular headache is the " brow ache " due to malaria, but a history of previous malarial attacks, chills and fever, a residence in a malarious district, and the fact that it is periodic in character, should certainly give a clear differential diagnosis. Other patients may not consult the ophthalmologist on account of headache, but for a pain in or back of the eyes, or back part of the head, or between the shoulders, which comes on after any effort of vision. Others may complain of a feeling of sand in the eyes, or a burning in the lids, or a smarting or itching in the lid margins, or excessive lacri- mation, or a feeling of drowsiness as soon as the eyes are used for any length of time, or a feeling as if the eyelids would stick to the eyeballs. The patient's seeing qualities may develop the history of poor distant vision and good near vision, or vice versa ; this should be inquired into very carefully, and it may be well to ask about other members of the family, if they have the same condition. Or a history of the vision gradually fail- ing or of a sudden loss of sight may be obtained, and pres- byopic symptoms should be referred to, if the patient is over forty years of age. If the patient wears glasses, it is well to inquire whether the}' were ordered by an ophthalmologist or if they are 226 REFRACTION AND HOW TO REFRACT. the patient's own selection. In the former instance a record should be made of the character and strength of the lenses, and whether the lenses were ordered with or without /'drops" in the eyes; and if "drops" were used, if the effect lasted for t\vo days or longer (slowly or quickly acting cycloplegic). Ask how long the glasses have been worn, and if the same symptoms are present that existed when the lenses were previously ordered. Having made a note of the patient's history, it is next in order to study the present condition (status prcescns) : 1. Breadth of face, its symmetry or asymmetry; inter- pupillary distance. 2. The eyelids, whether swollen, discolored, or having red margins. 3. The eyelashes (cilia), whether regular, irregular, or absent. If there are chalazia, styes (hordeola), inflammation, moist or dry secretion at the roots of the cilia (blepharitis). 4. Inspect the inner surface of the lids and ocular con- junctiva for inflammation or growths. 5. Inspect the lacrimal apparatus in all its parts. 6. Inspect the cornea for its polish, transparency, and regularity. 7. Depth of the anterior chamber. 8. Iris, its color and mobility. 9. Pupil, its size, shape, and position. 10. Color of reflex from the pupillary area. 1 1. Palpate to measure the intraocular tension. ^ 1 2. Use the cover test at 1 3 inches for any muscular anomaly. Following this record of the history and present condi- ition, the distant vision and near point are taken for each eye, one or more tests for astigmatism are made, the mus- cles are tested for distance (six meters), and the ophthal- EXAMINATION OF THE EYES. 22"J mometric measure of corneal curvature may be recorded. Finally, and most important of all, the ophthalmoscopic examination is made, and the cornea, aqueous, lens cap- sule, lens, vitreous, nerve (shape, size, color, cupping, and vessels), conus, macular region, etc., and periphery of the eye-ground are studied. Lastly, fields and color sense, dynamic or manifest re- fraction. CHAPTER IX. HOW TO REFRACT. General Considerations. Before placing lenses in front of an eye, the surgeon should be acquainted with at least five important facts : 1. The Patient's Age. This tells at once, from the table on page 70 (which the surgeon should commit to memory), what the near point will be if the eyes are emme- tropic or standard. 2. The Near Point. This will usually indicate hyper- opia if beyond, and myopia if closer than, the emmetropic near point for the age. 3. The Distant Vision in Each Eye. If very defective, or if less than and near point closer than the age calls for, myopia is indicated. Good distant vision and near : removed indicate hyperopia. 4. The distant vision, if recorded with question marks, usually indicates astigmatism. 5. The Results of Testing with the Astigmatic Chart. Darkest lines from XII to VI, or I to VII, or XI to V, indicate astigmatism (myopic) with the rule; or darkest lines from IX to III, or VIII to II, or X to IV, indicate astigmatism (hyperopic) with the rule. It is well to remember that about four patients out of js/vt/f^ (five have hyperopia, or one patient in five has myopia, and the minus sphere selected almost invariably requires a cylinder in combination. Remember, also, that astigma- ; tism is usually with the rule and symmetric, and that plus 228 HOW TO REFRACT. 2 29 cylinders are generally selected at axis 90 or within 45 degrees either side of 90, and minus cylinders are gen- erally selected at axis 180 or within 45 degrees either side of 1 80. The Placing of Trial-lenses. i. These should always be placed as close as^ ]3ogg^g^o_the_eYes without interfer- ing with the lashes ; and to accomplish this, the trial-frame should be easy of adjustment. 2. The center of the trial-lens must be opposite- to the center of the pupil. 3. If the distant vision is very defective, -, --, or ~l., a strong lens of 2, 3, or 4 D. will often be re- quired ; whereas, if the vision is ^^ or ~~, a weaker lens would be called for. 4. When a spheric lens placed before an eye improves the vision, it should not be changed for another unless the vision is made better by having its strength increased or dimin- ished by placing in front of it another sphere (plus or minus) of less strength. For instance, if a -f 2 sph. has been placed before the eye and the vision is improved from xY to vHIs ' ^ s + 2 s Pk- snou ld not be changed until a .50 or 0.50 sph. has been held in front of it and the patient states whether he can read more with it or less without it. When a vision of -^j- is approximated, then its accuracy must be determined by placing first a +0.25 and then a 0.25 in front of the correction, so as to learn from the patient which one, if either, of these lenses improves the vision. Or if the correcting lenses selected are weak ones, then o. 12, plus and minus, may be used in place of the 0.25. / 5. Spheric lenses should always be tried before using cylinders, and the vision brought as low as possible with LAA/^-A-" *"*>- '"\ J^U>WA^ WAJUX IM~^^ opia) be necessary to employ a cycloplegic. When the f-vb refraction is estimated without, a cyclonlegic., it is spoken of t-t^jJLt r < r ,' \ lU~ t^?"^ .- as manifest or dynamic (dr., ^jvi/^, power ) refraction. /". When a cycloplegic is used, the refractive estimate is spoken ^ of as static (Gr. frrar-xn-, from, '--n^a:, " to stand at rest "). In \ ^.^-*- ' one instance the ciliary muscle is permitted to act, and in ^ the other it is at rest. A third method is to obtain the / static refraction and then to estimate the strength of the 232 REFRACTION AND HOW TO REFRACT. glasses to be prescribed after the effect of the cycloplegic has passed out of the eyes ; this is spoken of as post- cycloplegic refraction. Eyes for refraction are divided into general classes, according to the age of the patient. In ^,-those under forty-five years of age a cycloplegic is usually employed, but after this age a cycloplegic is often dispensed with. (See Presbyopia.) Fogging Method. This method simulates the static or cycloplegic method, as the ciliary muscle is in great part (if not entirely) placed artificially at rest by having in front of the eye under examination a phis sphere of sufficient strength to more than overcome any ciliary muscle poivcr tJiat the eye might otherwise use ^vhen looking at a distance of six meters. The "fogging" method is so called because dis- tant vision is made obscure or "foggy." The eye under examination with this strong plus sphere in front of it is, to all intents and purposes, for the time being, at least, myopic. This fact should be carefully borne in mind, as the method to pursue in estimating the refractive error is to proceed the same as in any regular case of myopic refrac- tion. Therefore, this method is only of service in estimat- jing the refraction of eyes that have some form of hyperopia 'or simple myopic astigmatism, and is not of service in myopia or compound myopia. How to Proceed in Hyperopia. Having estimated with the ophthalmoscope that the eye is hyperopic about 2.50 D., then place a plus 4 D. sphere before the eye and have the patient look at the card of test letters at a distance of six meters. This has the immediate effect of making distant vision very "foggy," but by waiting a few seconds this foggy vision will clear slightly and the ciliary muscle will relax a part, if not all of its effort to accommodate. Then proceed as if refracting a myopic eye by placing a * ^| > HOW TO REFRACT. 233 0.50 sphere in front of the +4 1). sphere, and gradually increase the strength of the minus sphere until the patient reads ^.|. If the minus sphere is 1.25 D., then the amount of the hyperopia will be -f 2.75 D., which is the difference between the +4 D. and the 1.25 D. How to Proceed in Simple Hyperopic Astigmatism. Having estimated with the ophthalmoscope or retinoscope or ophthalmometer or any of the various ways that the eye has hyperopic astigmatism of about 2.50 D., proceed as in hyperopia by making the eye myopic by the addition of a + 4 D. sphere. Have the patient look first at the card of test letters and then at an astigmatic clock-dial next to the letters. Proceed by adding minus spheres as in the previ- ous case, and as soon as the patient recognizes one series of lines on the clock-dial as much darker than those at right angles to these dark lines, then place minus cylinders in position with their axes at right angles to the darkest lines, and as soon as the lines are uniformly black on the dial then have the patient look at the test letters again and increase the strength of the minus sphere if necessary until the eye can read ^j or ^. Presuming that 2 sphere and 2 cylinder at axis I To degrees were used, then the dif- ference between these and the -j- 4 sphere would give -j- 2 cylinder, axis 90, as the amount of the hyperopic astigma- tism. How to Proceed in Compound Hyperopic Astigma- tism. If the meridian of highest refraction is 4 D., then place a -f 5 D. sphere before the eye, wait a few seconds, then begin by adding minus spheres until a series, of lines on the clock-dial show up very black, then add minus cylinders until all the lines become uniformly black, then turn to the test letters and increase the strength of the minus sphere as necessary until the vision is brought to normal. 234 REFRACTION AND HOW TO REFRACT. With -f- 5 D. sphere before the eye and if I sphere and 3 cylinder at axis 165 were employed, then the com- pound hyperopia would be -f- 1 sphere with -}- 3 cylinder at axis 75. How to Proceed in Mixed Astigmatism. Estimating the refraction approximately as -f- 2 O 3, cylinder; place -(- 5 D. sphere in position as before, then find the meridian I of darkest lines on the clock-dial, add minus cylinder with the axis at right angles to the darkest lines, then when all lines are equally black, diminish the strength of the plus sphere until vision comes to the normal, if it is possible to get it to this point. The result may be -f- 5 sph. O 2.50 sph. O 3- SO cyl. X 1 80, which would equal -{-2.50^ C 3.50 cyl. X 1 80. Advantages of the Fogging Method. It is one of the best approximate ways of estimating the refraction if for any reason a cycloplegic can not be employed, as in cases of glaucoma, nursing n>otners, or of an individual who may 'A i TI r *u A have an idiosyncrasy for a cycloplegic. 1 he fogging method also has the advantage, like the cycloplegic method, of un- covering or bringing out most if not all the latent hyper- opia. Manifest or Dynamic Method. This method is the very reverse of the "fogging" method, in that the refraction is estimated without first making the eye artificially myopic, or placing the ciliary muscle at rest. It is, therefore, a method thatns liable to give all sorts of erroneous results if the subject is hyperopic and less than forty years of o estimate the refraction by the manifest method, the patient is told to look first at the test letters and then at the clock-dial at 6 meters distant ; the astigmatism is corrected and then plus or minus spheres added as necessary to bring HOW TO REFRACT. 235 the vision to the normal. The rule is to employ the strongest plus lenses or the weakest minus lenses which will give normal vision. Advantages of the Manifest Method. This is the method by which the eyes of patients past forty-five years of age are refracted ; a fairly good method in cases of compound myopia in young subjects if drops can not be employee!. Objections to the Manifest Method. It is not a method to be used as a rule in young subjects. If a young subject must be refracted without drops, then the fogging method should be followed. The habit of prescribing glasses from the manifest or "fogging" method without any knowledge of the ophthal- moscopic findings is not a method that merits the attention of the conscientious physician. Such work is very unsatis- factory, often leading to gross errors, ultimate dissatisfaction on the part of the patient, or injury to the eyes. Postcycloplegic Refraction. The ordering of glasses after the static refraction has been recorded and the effect of the cycloplegic has left the eyes. For instance : While the ciliary muscle is paralyzed with atropin the static refraction is found to be -f- I-5O sph. O +2.OO cyl. axis 90 degrees, which gives a vision of -. The atropin is then stopped >dhd the patient told to report in fifteen days, when the ciliary muscle will have regained its original strength and gone back to its old habit of accommodating for distance. The static refraction is placed before the eyes, and the strength of the sphere is gradually reduced until the vision just equals -, as it was when the " drops " were in the eyes. What- ever this correction with the glasses may be, is ordered. Occasionally the strengtlvof the cylinder as well as its axis is also changed 236 REFRACTION AND HOW TO REFRACT. Objections to the Postcycloplegic Method. The pa- tient is annoyed by the long delay to which he ie subjected before getting his glasses. But the principal fault lies in the fact that the eye is not placed in the emmetropic condi- tion ; it is allowed to retain more or less of its accommo- dative power for distance. This, however, can not always be avoided. Static Refraction. By this method the glasses are pre- scribed while the ciliary muscle is under the effect of the cycloplegic. In hyperopia allowance must be made in the strength of the sphere for the distance at which the test is made. At 6 meters 0.25 is deducted from the sphere f\ without any change in the cylinder. The only possible ^objection to this method is in cases of hyperopia, in which, after the effect of the cycloplegic passes away, the ciliary muscle may endeavor to accommodate for distance with the glasses in position, and with the result that the pa- tient can not see clearly except near at hand. To avoid any such contingency the surgeon will have to make a deduction in the strength of the plus sphere to meet such cases. The rule for ordering glasses by the static refrac- tion in hyperopia is to deduct 0.25 from the sphere and / have the glasses worn at once and constantly while the \effect of the " drops " is gradually leaving the eyes. In this way the eyes grow accustomed (slowly) to seeing at a distance without exerting the ciliary muscle ; the eyes are thus placed in an emmetropic condition. If this effect of the cycloplegic passes away before the patient can obtain the glasses, it will be necessary to use the drops for a day or so after the glasses arc received. Unfortunately, how- lever, some hyperopic eyes, in young subjects especial ly, 'with vigorous ciliary muscles, will develop a spasm of the accommodation for distant vision which will make the HOW TO REFRACT. 237 glasses very annoying on account of distant objects look- ing "dim." Such patients should be advised of this fact at the time the glasses are ordered, and if dim distant vision does develop, that it will be transitory, and to per- sist in wearing the glasses. There are two ways of reliev- -ing this " dim " vision if it should occur : 1. To prescribe a weak solution of atropin (^ of a grain to i fluidounce), I drop in each eye once or twice a day, the idea being to slightly relax the accommodation ; this is accomplished, but, unfortunately, the mydriatic effect is a disturbing element which the patient will not submit to long enough, as a rule, to obtain relief. 2. The better way is to make a compromise in the strength of the sphere. An eye which has been in the habit of accommodating 3, 4, 5, or 6 diopters for distance, does not often give up this habit very gracefully, even if assisted by a slowly acting cycloplegic, /so that when the static refraction calls for more than 3 diopters, the surgeon is fre- quently compelled to make a deduction of more than 0.25. ) There is no hard and fast rule as to just hm*.' innck shall be deducted, and very few surgeons agree on this point. Glasses may be ordered as follows, the surgeon being guided in great part by thejpatient's age anoK>ecupation ; also as to whether there is esophoria or exophoria. It will be found that cases of esophoria will accept almost a full correction, whereas cases of exophoria will require a \\-ry liberal deduction in the strength of the glass, the patient being allowed to use his relative hypcropia : Static refraction at 6 meters : -j-l.oo sph. or less deduct o. 12 or 0.25. From -fl-oo sph. up to 3.00 sph. " 0.25 or 0.50 or 0.75. " +3.00 sph. up to 6.00 sph. " 0.50 or 0.75 or i.oo or 1.50. " -f 6- s ph- U P to 8- and above " I. or 1.50 or 2.00 up to 3.00. 238 REFRACTION AND HOW TO REFRACT. It is true that glasses ordered in this way do not leave the eyes in an emmetropic condition, and that, later on, when asthenopic symptoms redevelop, the strength of the glasses will have to be increased. But this method has two advan- tages : first, it gives the patient his glasses without any long delay, and the eyes have an opportunity to become accus- tomed to them while the effect of the " drops " is passing away ; and, second, the patient accepts a much stronger glass in this way than by the postcycloplegic method, which is a decided advantage. The ordering of lenses in low errors for distant vi- sion depends entirely upon the condition of the patient's eyes and symptoms. It is not unusual to find the most distressing asthenopia, headaches, blepharitis, etc., disap- pear as if by magic when corrections are ordered for small defects, especially if there is astigmatism. In other instances slight ametropic errors may not produce any unpleasant symptoms, and such a patient need not wear the correction for distance. The Ordering of Glasses in Myopia. There is no fixed rule for prescribing glasses in myopia. Each case is a law unto itself, and should receive the most careful consid- eration from every point of view. But as the student must have some idea as to how to proceed, the writer would sug- gest the following subdivisions : J. Myopic eyes which can with safety use one pair of glasses for distant and near vision. 2. Myopic eyes which require two pairs of glasses one for distant vision, and another pair for near-work, reading, writing, etc. 3. Myopic eyes which should have the near correction only. CLASS i comprises those cases in which there is an active HOW TO REFRACT. 239 ciliary muscle, and the ophthalmoscope shows but little, if any, change in the eye-ground indicative of stretching (chil- dren, or in beginning myopia). Glasses carefully selected by the static refraction may be ordered in such instances for constant use, but with the distinct understanding that if any discomfort arises at any time they will be subject to change. CLASS 2. Adults who have not previously worn their myopic glasses. In these cases the power of the ciliary muscle is weak, deficient in sphincter fibers, and, if forced into activity, the patient would be very uncomfortable, the eye would stretch, and the myopia increase, the tissues in these eyes yielding more readily than in class I. The glasses selected by the static refraction may be prescribed for distance, but a second pair, I, 2, or 3 diopters weaker, must be ordered for the reading, writing, or working distance, that the accommodative effort may in part be kept in abeyance. Class I, if not carefully watched, may pass into class 2 and class 2 may pass into class 3. CLASS 3. These cases require unusually strong lenses, and it is to these especially that the term " sick," or "stretched" eye particularly belongs. The ophthalmo- scope may show vitreous opacities, areas of retinochoroid- itis, macular choroiditis, a broad myopic conus, and even posterior staphyloma. The eyes are prominent, occupying much of the orbital space. Eyes of such length are limited in their power of comfortable rotation, and hence it is com- mon for one eye to diverge, the patient stating that lie uses only one eye for near vision. The diverging eye is usually more or less amblyopic, due to want of use or pathologic changes, or to both. Such eyes have lost almost or quite all the power of accommodation. These eyes must be placed in such a condition that the desire to converge and 24O REFRACTION AND HOW TO REFRACT. accommodate is at a minimum. The prescribing of glasses for these long eyes must be limited to the one pair for near- work, and yet the patient may, by bringing the glasses closer to the eyes, improve the distant vision for the time being a sort of artificial accommodation. To appreciate what is meant by this statement it is necessary to reconsider the optics of a myopic eye. A myopic eye of 20 D. has a far point of 5 cm., and the minus lens required to make such an eye receive parallel rays of light at a focus upon its retina should be of such strength that the rays passing through it would have a divergence as if they came from this far point (5 cm.). Such a lens would be a 20 D. This means, of course, that the 20 D. would have to be placed with its surface against the surface of the cornea, which is an impossibility. The usual distance for a lens in front of the eye is I or I ^ cm., so that this distance must be subtracted from the distance of the far point. In this instance I cm. from 5 cm. would leave 4 cm., and this would represent 25 D. As just stated, the glasses for this class of patients are limited to the one pair for near-work, and therefore it would be necessary to reduce the strength of these lenses 4 diopters and thus prevent, as far as possi- ble, any accommodative effort. The patient using this 21 D. for near, can, if he wishes, improve his distant vision at any time by pressing the lenses closer to his eyes. The strength of concave lenses increases as they are brought closer to the eyes, and diminishes as they are re- moved from the eyes. Caution. The great danger in any refraction at the trial- case, but especially in myopia, is an overcorrection, and this is very likely to occur if the surgeon is not extraor- dinarily careful in having his lenses placed as close to the eyes as possible while making the test. HOW To K I. TRACT. 24 I Prophylaxis. The prescribing of glasses for myopic eyes is only a part of the general treatment to which these "sick" eyes are entitled. If the treatment stops at this point, then the glasses may be an injury instead of a bless- ing. Myopia once established may pass through the various classes already described, and eventuate in greatly reduced vision or total blindness if certain limitation of their use is not insisted upon. 1. Light. A good, clear, and steady light is always essen- tial ; it should come from the left side, never from in front. 2. Time. The length of time that myopic eyes may be used should be restricted as much as possible, consistent with their condition ; that is to say, they should never be used after they become the least fatigued, and any use of the eyes should be counteracted by life in the open air. 3. Attitudes. The head should have as little inclination as possible in reading, writing, or close work, as so faulty a position invites a congestion of the intraocular tissues. At school or at home the book should be inclined, and its dis- tance from the eyes be regulated by the size of the patient. 4. Print. The use of small print or minute objects must be forbidden. English or Gothic type should be sub- stituted for Greek, German, and other characters. Fine needle-work, embroidery, etc., must be abolished. If nec- essary, music notes must be given up entirely. 5. Health. The health of the patient must be looked after, and all irregularities corrected constipation, etc. These are a few of the major considerations to which the patient's attention must be drawn, the surgeon being limited in his remarks to the exigencies of the individual case. In conclusion it may be well to know how myopia is produced, since it has been stated that the condition is rarely seen in young children. It is well known that astigmatism 242 REFRACTION AND HOW TO REFRACT. (hyperopic) is a congenital defect, and with this in mind it is very easy to appreciate the succeeding steps which lead to the compound myopic condition, showing at the same time the reason why simple myopia is so rare an anomaly. Take a child six years of age who has a compound hyper- opia of say +0.50 sph. O +0.75 cyl. axis 90 degrees ; this child enters upon its course of study without any correcting glasses, and is subjected in its pursuit for knowledge to a faulty school desk and chair, possibly facing a window. The print is defective in many ways. The artificial light for home study in the evening may be of poor quality, and so placed that but few of its rays fall upon the child's book. With these and other hindrances the eyes are strained (stretched). The tissues are very yielding in their growing state, so that at the age of ten years the refraction may show +0.75 cyl. axis 90. The +0.50 sph. (the axial ametropia) has disappeared by an elongation of the optic axis. The vertical meridian is now emmetropic. The same conditions exist for the next three years, during which the number of studies is multiplied and the hours for study are prolonged and the child reaches the age of puberty ; the refraction is now found to be 0.50 sph. O +0.75 cyl. axis 90 degrees, mixed astigmatism. In two years more the refraction is found to be 0.25 sph. O 0.75 cyl. axis 1 80 /. e., compound myopic astigmatism. From this time forward these eyes progressively stretch and are subject to the stretching process unless the progress is stayed with glasses and prophylactic treatment. In the brief detail of this one case the student will fully appreciate another important fact that the vertical meridian of the cornea, as a rule, maintains throughout the shortest radius of curvature. This is abundantly demonstrated by statistics. HOW TO REFRACT. 243 The following summary of refractive errors and direction of meridians of shortest radius of curvature in 2500 pairs of eyes, 1300 in private and 1200 in hospital work, pre- pared by Dr. Risley and the writer, shows the correctness of the above statements : * PRIVATE. HOSPITAL. Monocular astigmatism, 70 $<>% 94 7-8J& Binocular astigmatism, 1151 88.5 828 69.0 Total cases, 1300 1200 Binocular symmetric astigmatism, . . . 694 60.2% 613 74.4% Binocular asymmetric astigmatism, . . . 310 26.8 158 19.8 Heteronymous astigmatism, 123 10.6 40 5.2 Homonymous astigmatism, 24 2.1 17 1.2 Total binocular astigmatism, . . . . 1151 828 Symmetric astigmatism : (a) According to rule (homologous), . 543 78.2^ 559 97.7^0 (6) Against rule (heterologous), . . . 151 21.8 54 2.3 Total symmetric astigmatism, . . . 694 613 Asymmetric astigmatism : (a) According to rule, 223 71.8% 126 79.1$ (t>) Against rule, 87 28.2 32 20.9 Total asymmetric astigmatism, ... 310 158 DIRECTION OF THE MERIDIAN OF SHORTEST RADIUS IN ALL CASES OF SYMMETRIC ASTIGMATISM. Meridian at 90, 57--f $ Meridian inclined 15 or less on each side, 19-74" Meridian inclined from 15 to 30 on each side, 4.0 Meridian inclined from 30 to 45 on each side, I.O Meridian at 180, 12. Meridian inclined 15 or less on each side, 4.0 Meridian inclined from 15 to 30 on each side, 2.O Meridian inclined from 30 to 45 on each side, 0.5 * This report was read in the Section on Ophthalmology at the forty-sixth annual meeting of the American Medical Association, at Baltimore, Md., May 7 to 10, 1895. CHAPTER X. APPLIED REFRACTION. In estimating the refraction of any eye the surgeon will do good work if he will make it a rule never to be satisfied until each eye has a vision of -^j- or more, and if this visual acuity is not attained, to understand the reason why : whether it is his fault or the fault of the eye itself. It is most essential in every instance to have the good-will of the patient. The following cases are detailed so as to demonstrate each form of ametropia in all its phases : CASE I. Simple Hyperopia. This is a common form of ametropia, occurring about 20 times in 100 cases : January 3, 1899. JOHN SMITH. Age, twenty. Single. Stenographer. - D - -VT p - P '= type ' 5 D ' at H cm ' O. S. ^1. p. p. = type o. 5 o D. at 14 cm. ^f\ Add. 22 degrees ; abd.= 6 degrees. History. Frontotemporal headaches almost constantly, but much worse when using eyes at near-work. Had severe headaches when a school -boy. Never liked to study ; preferred out-of-door sports. 5". P. Face symmetric, but narrow. Blepharitis mar- ginalis. Irises blue. Pupils round, 3 mm. in diam. Eyes fix under cover. Oplitlialnwinctcr. Negative. Ophthalmoscope. Both eyes the same. Media clear. 244 APPLIED REFRACTION. 245 Disc small and round, with physiologic cup. All vessels near the disc are seen clearly with +38. Eye-ground flannel -red and accommodation very active. Manifest or dynamic refraction : O. D. +i. 25 S. = ^f. O. S. +i.2 S s.= ^L. R . Atropin and dark glasses for refraction. January 5, 1899. Patient seated at 6 meters from test- card and small point of light. O. D. V.= j^. = O. S. V. ~y. Cobalt-blue glass shows (each eye separately) blue center and red halo. (See Fig. 125.) Retinoscope, with -(-48., developed point of reversal at I meter for each eye. With trial-lenses, O. D. and O. S. each select +3 S., which gives a vision of - y -, and they positively refuse to see v v ' clearly with an addition of +0.25 S. In other words, this + 3 S. is the strongest lens which each eye will accept and maintain clear distant vision. Tlic rule for refraction in hypcropia is to employ the strongest lens which the eye will accept without blurring the distant vision, To prove that the ciliary muscle is at rest, and that the glass selected is correct, add a +4 S. to the distance cor- rection, and the rays of light emerging from the eye must focus at the principal focus of the added +4 S., at 10 inches (25 cm.); and if the patient can read fine print at this distance, the ciliary muscle is at rest and the glass cor- rect. If a +3 S. had been added instead of a +48., then the principal focus would be at 13 inches ; if +5 S., then at 8 inches, etc. The question is, What glasses shall be ordered ? The 246 REFRACTION AND HOW TO REFRACT. writer would give the following prescription, and instruct the patient to stop the drops and wear the glasses constantly : For MR. SMITH. K. O. D. +2.75 sph. O. S. -f 2.75 sph. Sic. For constant use. January 7, 1899. January 8th : Glasses from the optician neutralize ; are centered and accurately adjusted. January 2ist : Add. = 18 degrees. Abd. = 6 degrees. Near point in each eye = 10 cm. No headache or dis- comfort of any kind. Considerations. The static refraction as represented by .00 sph. means that rays of light which pass through this lens and focus at the fovea diverge from six meters' J distance, which heretofore we have considered for purposes Jj& of calculation as parallel ; but when glasses are ordered, ^ allowance must always be made for this small amount of /divergence, and so 0.25 is deducted from the +3 sph., that I/ the eye may have parallel rays focusing on its retina when lit *" looking beyond a distance of six meters. To have been ^ mathematically exact, -f-o. 12 should have been deducted in i * JL lace of +0.25. ^ e P ur P ose ' m a ^ cases of refraction is to place the eye an emmetropic condition, though this is not always advis- able in every instance. The hyperopic eye naturally accom- modates for distance, and the emmetropic eye does not ; then the hyperopic eye is made emmetropic when a spheric lens permits parallel rays to focus upon its fovea without any assistance whatever from the ciliary muscle. Advantages of Atropin in This and Similar Cases. The glasses are ordered while the ciliary muscle is at rest. The accommodation returns gradually. The eye becomes APPLIED REFRACTION. 247 accustomed to seeing at a distance without the assistance of the ciliary muscle. Atropin produces a physiologic rest, which the overacting ciliary muscle, disturbed choroid, and irritated retina require. None of these good results can be expected in a case of this kind from the use of a " quick " cycloplegic like homatropin. SUMMARY. Age of patient, twenty years. Amplitude of accommodation is 10 D. for this age. Near point is 14 cm., which shows only 7 D. Facultative hyperopia (Hf.) equals difference between 10 and 7 D., which is 3 D. Manifest hyperopia (Hm.; equals 1.25 D. Total hyperopia, or static refraction (Ht.), equals 3 D. Latent hyperopia (HI.) equals the difference between the manifest and total, 3 D. and 1.25 D., making 1.75 D. The far point, or conjugate focus, is negative or virtual, and lies back of the retina, where the emergent rays (diverg- ing) from the eye would meet if projected backward ; this point corresponds to the principal focus of the lens which corrects the hyperopia :'. c., in this instance, -j~3 D., and the negative far point is therefore at 1 3 inches. The -(-2.75 makes the eye practically emmetropic ; the near point, after the effect of the atropin passes away, is 10 cm., which is the emmetropic near point for the patient's age. The plus sphere selected represents a shortening of the eye of I mm., as measured on the optic axis. CASE II. Simple Myopia. \Yith the one exception of lemmetropia, it will be found that myopia, plain and simple, r>|whhout astigmatism, is one of the rarest conditions of the eye which the surgeon will meet in careful refractive work. About one and one-half per cent, of all patients, by careful refraction, are found to have simple myopia. Therefore the condition is not common. **-"' V ck REFRACTION AND HOW TO REFRACT. January 3, 1899. Miss RARE. Age, twenty-five years. Single. O. D. V. = L p. p. = type 0.50 D. at 9 cm.; p. r. at 33 cm. O. S. V. = -. p. p. = type 0.50 D. at 9 cm.; p. r. at 33 cm. i\. i- Add. 1 6 degrees. Abd. 6 degrees. Exophoria, 3 degrees at 13 inches. History. Does not suffer much from headache, but eyes ache after any prolonged use at near-work. Never able to see well at a distance. Always stood high in her class at school, though she had to have a front seat to see the fig- ures and writing on the blackboard. Has excellent vision for near-work and does fine embroidery. Has been accused of passing friends on the street without speaking to them. If she drops a pin on the floor, has to get on her knees to find it. Parsnts do not wear glasses. Grandfather had "elegant" sight, had "second sight," and never wore glasses. Patient has postponed getting glasses because parents objected. >S. P. Face symmetric. Interpupillary distance, 65 mm. Irises dark. Pupils large, round, 5 mm. Eyes out under cover. OpJithalmometer. Negative. OphtJialmoscope shows each eye the same. Media clear. Disc large and round. Shallow physiologic cup. Narrow myopic conus at temporal side of disc. Choroidai vessels seen throughout periphery of eye-ground and extending almost to nerve margin. All vessels near nerve-head seen with 3. S. Manifest Refraction. Each eye 3.50 sph. gives vision <"- Cobalt-blue glass gives red center and dark blue halo. (See Fig. 126.) R. Atropin and dark glasses for refraction. APPLIED REFRACTION. 249 January 5, 1899: Patient seated at 6 meters from test- card. O. D. and O. S. vision equals Xl . Retinoscope with 2. S. develops point of reversal at I meter for each eye. 'It will be noticed that the vision in hyperopia with and without drops is decidedly different, whereas in myopia there is little, if any, change. With trial-lenses each eye selects separately 3 sph., which gives a vision of -^-. If a 2.75 sph. is substituted, the vision falls to 5. If a 3.25 is employed, the vision remains ~, but the letters look small, black, and " far away." The rule for refraction in myopia is to employ the weakest lens through which the eye can still maintain clear, distant vision. What Glass to Order. The writer would give the fol- lowing prescription and instruct the patient to stop the drops and wear the glasses constantly : January 7, 1899. For Miss RARE. U. O. I). 3.00 sph. O. S. 3.00 sph. SIG. For constant use. January 9th : Glasses neutralize ; are centered and accu- rately adjusted. Add. 18 degrees. Abd. 10 degrees. Patient is delighted with glasses. January 2 1st: Near point, 12 cm. Considerations. As a rule, concave lenses are ordered without any deductions for the slight amount of diver- gence of the rays of light for the distance (6 meters) at which the estimate is made. To be exact, 0.25 should :be added to the 3.00 sph. in this case ; but the surgeon must avoid the danger of tfrrrcorrecting the myopic eye, and, to be on the safe side, 1(ne glass is usually ordered as the patient selects and the retinoscope confirms j^ These lenses make the eyes, to all intents and purposes, emmetropic. f\ 25O REFRACTION AND HOW TO REFRACT. Advantages of Atropin. The choroid and retina are given a physiologic rest that they could not obtain in any other way. (/The patient will not^sgjfifctiUtflr strong a glass. . as was the case in this very instance w Hwfc"" m aiMgyfcefrft ^ > Myopic eyes usually have large pupils, and do Jiot suffer, therefore, from mydriasis to the same extent as^Tr^K^s eyes. The far point remains unchanged. The power of con- vergence is somewhat relieved by the glasses, \\vhich at the near working distance are of the nature of prisms, bases in. SUMMARY. Age of patient is twenty-fiye years. Ampli- tude of accommodation at this age is 8 ^D. /'Near point, 9 cm. = 11 D., and far point 33 cm. =30. Difference be- tween near point and far point in diopters = 8 D., which is the amplitude of accommodation for the patient's age^/ I Difference between the near point in diopters (i i D.) ( and the amplitude (8 D.) is 3 D., which is the amount of |the distant correction needed. With glasses on, the near point, after the effect^ of the atropin passes away, is 12 cm., which represents the emmetropic near point for the age. This myopia of 3 D. represents an eye i mm. longer than the standard eye, as measured on the optic axis. CASE III. Simple Hyperopic Astigmatism. Not an uncommon condition. About 5.5 percent, of all eyes have this form of refraction. April 3, 1899. Miss ROBINSON. Age, twenty-four years. Single. Dress- maker. VI O. D. V. =-jjj-???. p. p. = type 0.50 at 18 cm. O. S. V.=-j5Jr?? ?. p. p. = type 0.50 at 18 cm. Add., 23 degrees. Abd., 5 degrees. At 6 meters, esophoria 4 degrees ; at 13 inches, I degree of esophoria. Astigmatic clock-dial shows darkest lines from X to IV with O. D. APPLIED REFRACTION. 251 Astigmatic clock-dial shows darkest lines from VIII to II with O. S. History. Headache every day ; seldom entirely free from ocular discomfort. Distress begins in the forehead and extends to the back of the head and into the neck. After a hard day's sewing, has to go to bed and bind the head with a handkerchief. Once a week has a " sick headache," when she has to give up work entirely and take headache powders. Sick headache often ceases after emesis. S. P. Face symmetric. Blepharitis well marked, with many cilia missing. Edges of lids thickened. Irises light blue in color. Pupils apparently oval in vertical meridian. Corneal reflex shows axis inclined from vertical in each eye. OpJitlialmomctcr. O. D., 3.50 ; axis, 75, with the rule ; O. S., 3.50; axis 105 degrees, with the rule. Oplithalnwscopc. O. D., vertically oval nerve axis, 75 degrees. Accommodation very active. Underlying con us down and out. Vessels at 75 best seen with -f- 2.50 ; and at axis 165, without any lens. O. S., same general conditions as in O. D. Vertically oval nerve axis, 105. Vessels at 105 degrees seen with -f- 2. 50, and at axis I 5 without any lens. Manifest Refraction. VI O. D., +2.50 cyl. axis 65 degrees = -yj- ? ? ?. O. S., same cylinder with axis 125 degrees. R. Hyoscyamin and dark glasses for refraction. A/ April ;th : Six meters from test-card and point of light, *^ / t **> O. D. '' ^ <* '< Clock-dial shou^-flie same as at first examination. A *^i i^t &>4 n/ &>*-* *nr***>* ' 252 REFRACTION. AND HOW TO REFRACT. Stenopeic Slit. O. D., axis 75 with +0.25 S., V. = ~ and at axis 165 with +2.508., V. = ~. O. S., axis 105 with +0.25 S., V. = ; and at axis 15 with +2.50 S., V - --^ " vi ' Rctinoscope at I meter shows : O. D., at axis 75 degrees -(-1.25 S., and axis 165 degrees, +4.25 S. O. S., at axis 105 degrees, +1.25 S., and at axis 15 degrees, -f~4- 2 5 S. At Trial-case. O. D. +0.25 sph. O -f 3.00 cyl. axis 75 degrees, V. = _Y_I -f- O. S. +0.25 sph. O +3.00 cyl. axis 105 degrees, V. = . vt . -\-. April 6th : Same result as April 5th. Add., 20 degrees. Abd., 6 degrees. Esophoria, 2 degrees at 6 meters. For Miss ROBINSON. R. O. D. -(-3.00 cyl. axis 75 degrees. O. S. -(-3.00 cyl. axis 105 degrees. Sic. For constant use. > April 7th : Glasses neutralize ; are centered and accu- rately adjusted. April 1 6th : Perfectly comfortable. Free from headache since the first day she used the " drops." Add., 20 de- grees. Abd., 6 degrees. Esophoria at 6 meters, 2 de- grees ; and at 13 inches, o. Near point, 12 cm. Considerations. Apparently, the static refraction in this case would indicate compound hyperopic astigmatism ; but ~*\ when 0.25 is deducted to produce parallel rays, then the prescription becomes one for simple hyperopic astigmatism. General Rule for Prescribing Cylinders. Order the cylinder just as found, without any change in its axis or strength. The vision in each eye at the different visits, before lenses were placed in front of the eyes, was always uncertain, APPLIED REFRACTION. 253 the patient miscalling certain letters, and hence it is that the vision is recorded with as many question marks as there are mistakes in the line of letters /'. e., ?? ?. IX In taking the vision at the first visit, the patient could read part of ^^ if not closely watched. In other words, if she was permitted to tilt her head to one side and nar- row the palpebral fissure by squinting the lids together, and making, as it were, a stenopeic slit out of her eyelids, the vision was improved. But when told to open the eye wide, she could read only part of . This is explained by the fact that when the lids were drawn together, the verti- cal meridian was partly excluded, and then, by accommo- dating, the vision was improved through the horizontal meridian. Astigmatic eyes often take advantage of this condition when the nature of the astigmatism is suitable, but only at the expense of frowning and straining the accommodation. It will also be noticed that the stenopeic slit was not used as a test at the first visit. This is also explained for the same reason that the patient would draw his lids together and therefore annul the virtue of this test. The stenopeic slit is to be used in these cases only when the ciliary muscle is at rest. SUMMARY. When the hyoscyamin has passed out of the eyes and the glasses are in position, the near point be- comes 12 cm., which is quite consistent with the patient's age. < Before using drops, the near point with the eyes wide open was only 18 cm., representing about 5.50 D. ; and this, subtracted from the amplitude for twenty -four years of age, would leave 3 D. for distance uncorrected./ As every 6 D. cylinder represents I mm. of lengthening or shortening of the radius of curvature of the cornea, then this patient, taking a -f- 3 cyl. at axis 7 5 in the right 254 REFRACTION AND HOW TO REFRACT. eye, has the 165 degree meridian l / 2 of a mm. too long as compared with the 75 meridian, which is supposed to have the normal radius of 7.8 mm. The same is true of the meridians of the left eye. CASE IV. Simple Myopic Astigmatism. Not a com- mon condition. About 1.5 per cent, of all eves have this form of refraction. ^^4^- April 10. Miss JENKS. Age, eighteen years. Single. O. D. V. = ~ ? ? ? - P- P- 9 on- VI O. S. V. = ^y-j ? ? ?. p. p. 9 cm. Add., 20 degrees. Abd., 5 degrees. Esophoria at 6 meters = 3 degrees ; and I degree at 13 inches. Pointed Line Test. Each eye selects the series of points from XII to VI as coalescing and appearing as dark lines. Cobalt-blue Glass. O. D. and O. S. each show blue above and below the red. (See Figs. 1 29 and 1 30.) Stenopeic Slit. Axis 90 degrees V. = ~~ ; axis 1 80 V. VI VI* History. Never had good distant vision. Has occa- sional headaches. Comes to find out if glasses will im- prove vision. S. P. Face symmetric. Irises dark in color. Pupils apparently round, 4 mm. in diameter. Eyes out under cover. Ophthalmometer. Each eye 2 D., axis 90. Ophthalmoscope. O. D., media clear. Disc large and round, with underlying conus out. No physiologic cupping. Choroidal circulation everywhere recognized, characteristic a stretching eyeball. Horizontal vessels seen with 2 S. ; vertical vessels seen without any correcting lens. O. S., same general conditions as in O. D. APPLIED REFRACTION. Manifest Refraction. O. D., -2.50 cyl. axis 180 = ^| . O. S., 2.50 cyl. axis 180 = ^ [ . R . Atropin and dark glasses for refraction. April 1 2th : Six meters from test-card and point of light. Retinoscopy at one meter. Vertical meridian +1.25 S. Horizontal meridian i.oo S. Stenopdc slit at axis 1 80 with -f- o. 2 5 == x ' ; at axis 90 = so, >:;. Cobalt-blue glass and pointed line test show same results as at first visit : VI O. D. V. = ~y ? ? ?. O. S. V. = -^V ? ? ? - At Trial -case. O. D., -j-o.25sph. O 2.00 cyl. axis 180 degrees = V1 . VI O. S., -f 0.25 sph. ^ 2.00 cyl. axis iSodegrees = -=77-. April 1 3th: Same results as yesterday. Add. = 20 ; Abd. = 6. Esophoria, 2 degrees. For Miss JENKS. R. O. D., 2.00 cyl. axis 180 O. S., 2.00 cyl. axis 180. April 1 4th: Glasses neutralize. Centered and properly adjusted. April 28th : Comfortable. Enjoys good distant vision. Near point, each eye, 9 cm. Considerations. Apparently, the static refraction would indicate mixed astigmatism, but when -(-0.25 is deducted to produce parallel rays, the prescription resolves itself into one for simple myopic astigmatism. The general rule for ordering cylinders is the same in 256 REFRACTION AND HOW TO REFRACT. myopia as in hyperopia /. e., no change in the strength or in the axis of the cylinder. A cycloplegic is always necessary in such cases, as is shown by the different results obtained by the manifest and stenopeic slit. The vision was always uncertain before lenses were placed before the eyes, as is indicated by the question marks. At the first visit the vision was taken with the eyes -wide open. If allowed to narrow the palpebral fissure by squint- ing the eyelids together and making a stenopeic slit out of them, the patient could read a part of y^g. When the lids were thus drawn together, the myopic vertical meridian was excluded in part and the horizontal meridian was util- ized. cThe stenopeic slit was of some assistance before drops were used, as the accommodation could not be exerted, as in the case of the hyperopeJ SUMMARY. After recovery from the cycloplegic, small type was clear at 9 cm., which was in keeping with the patient's age. The near point before " drops " were used was also 9 cm., but not constant, nor was the type clear. The 2. oo cyl. at axis 180 represents ^ of a mm. of shortening in the vertical radius of curvature as compared with the normal radius of 7.8 mm. in the horizontal. The astigmatism is regular, symmetric, with the rule. CASE V. Compound Hyperopic Astigmatism. The most common form of all refraction. It is a combination of simple hyperopia with simple hyperopic astigmatism. About 44 per cent, of all eyes have this form of refraction. April 1 2th. MR. COMMON. Age, twenty-eight. Married. Bookkeeper. VI O. D. V. =-^-??. p. p. = type 0.75 D. = 22 cm. VI O. S. V. = "x~??- P- P- = type 0.75 D. = 22 cm. Add., 23 degrees. Abd,, 7 degrees. Esophoria, 2 degrees; at 13 inches, o. APPLIED REFRACTION. 257 Astigmatic Clock-dial. O. D. and O. S. each selects darkest series of lines from IX to III. Phicido's disc shows each corneal image as a horizontal oval. Schemer's test shows two lights, separated in all meridians : the vertical have the least separation and the horizontal the most. Ophthalmometer. 2.25 D. with axis 90 in each eye. History. Family physician has tried in vain to stop the headaches, which he said were from biliousness. Headache develops as soon as the patient commences to use his eyes, and gets worse toward noon ; and from that time on, during the rest of the day, he is cross and irritable, and feels dizzy. Unable to read in the evenings as he did a few years ago. Is wearing a pair of " rest " glasses, which he received from an optician ; they were of some benefit for a very short time. 5. P. Lid margins red and excoriated. Many fine scales (looking like dandruff) adhering to the cilia. Irises gray in color. Pupils round, 3 mm. Eyes in, under cover. Ophthalmoscope. O. D. and O. S. each medium clear. Disc small, vertically oval. Shallow physiologic cup. Venous pulsation on disc. Narrow conus to temporal side. Nerve-head prominent and edges somewhat hazy. No path- ologic conditions recognized. Vertical vessels best seen with -f- 3.00 and horizontal with -f-i.oo. U . Duboisin and dark glasses for refraction. April 1 4th : Six meters from test-card and point of light. VI O. D. V. = j*x ? ? ? - O. S. V. = ^???. Retinoscopy develops point of reversal at one meter in each eye; vertical meridian with +2.25 S., and horizontal meridian with +4.00 S. 2$S REFRACTION AND HOW TO REFRACT. Stenopeic slit axis 90 degrees with -}- 1.25 = -^j- ; at axis 1 80 degrees with -f 3-OO - v ,-j-. Cobalt-blue glass, blue center and red all round, more conspicuous on the right and left sides. (See Figs. 131 and 132.) At Trial-case. O. D., +1.25 sph. O -fi-75 cyl. axis 90 = -TT?-. O. S., -)-l.25 sph. O -(-1.75 cyl. axis 90 = -J-. April 1 5th : Same results as April I4th. Add., 22. Abd., 7. Esophoria, I degree. For MR. COMMON : R. O. D., -f i.oo sph. O 4 I -75 cyl. axis 90 degrees. O. S., 4-1.00 sph. O +1.75 cyl. axis 90 " SlG. For constant use. April i /th : Glasses neutralize ; are centered and ad- justed. April 24th : Has been perfectly free from headaches ever since getting glasses. Never realized what a blessing glasses could be. Near point with each eye is now 14 cm. Considerations. The prescription for glasses was the same as the static refraction, with the exception of the re- duction in the strength of the sphere. No change in the cylinder. A cycloplegic as a means of obtaining a prompt, correct, and satisfactory result in such cases can not be dis- pensed with. The decided change in the vision before and with the cycloplegic is quite diagnostic of compound hyperopic as- jtigmatism. When the cylinder and sphere are of any con- siderable strength, the patient can often overcome (faculta- tive hyperopia) the spheric but not the cylindric correction. SUMMARY. After recovery from the cycloplegic the APPLIED REFRACTION. 259 small type becomes clear at about 13 cm., which is the near point consistent with the patient's age. The far point before using drops was really two points, both negative that of the vertical meridian being about I meter, and the horizontal meridian about ^ of a meter back of the retina. The form of the astigmatism is regular, symmetric, and with the rule. CASE VI. Compound Myopic Astigmatism. A com- bination of simple myopia with simple myopic astigmatism. This is the usual form of refraction in myopic eyes. About 8 per cent, of all eyes have compound myopia. The writer's experience is such that he never refracts a case of myopia without searching carefully for a cylinder in combination witli the sphere. April 1 2th. MRS. USUAL. Age, thirty years. Married. Housewife. VI O. D. V., --jj- ? ? ?, type 0.50 = 7 to 14 cm. VI O. S. V., -- ? ? ?, type 0.50 = 7 to 14 cm. Add., 1 6 degrees. Abd., 6 degrees. Exophoria at 6 meters, 2 degrees. History. Suffers from ocular pains, as if knife-points were sticking into the eyes, which come on as soon as near- work is attempted or continued. Says that she constantly sees fine dust particles floating before her vision. Has been wearing glasses from an optician ( 3 sph.). Has all the symptoms of near-sightedness. Family history of father and two sisters wearing glasses for "near sight." S. P. Face symmetric. Eyeballs prominent. Irises dark in color. Pupils small (for a myope) and round, 3 mm. Eyes markedly out under cover. Ophthalmoscope. O. D., many fine floating vitreous opacities. Nerve large and round, with broad underlying conus down and out. Choroidal vessels seen throughout eye -ground. Vessels at about axis 1 20 degrees best seen 26O REFRACTION AND HOW TO REFRACT. with 2, and vessels at axis 30 degrees best seen with 3. O. S., same general conditions as in O. D., except the prin- cipal meridians are about 60 degrees and 1 50 degrees. Indirect method shows a vertically oval nerve, with the conus to the nasal side of the aerial image (as the eye- ground and nerve-head have undergone vertical and lateral inversion). Withdrawing the lens, the nerve grows larger in all meridians, but more so in the vertical. Cobalt-blue glass shows O. D. red center, blue all around, more pronounced on the sides in the 120 meridian. O. S. shows red center, blue all around, more pronounced on the sides in meridian of 60 degrees. (See Figs. 133 and 134.) Stenopeic slit before O. D. at axes 1 20 V. ^^^jFjnT ? ? ? > a t axis 30 V. =" xx ~ ? ? O. S., the same with axes 60 degrees and 150 degrees. Astigmatic Chart. O. D. selects the lines from V to XI as darkest. O. S. selects the lines from VII to I as darkest. Ophthalmometer. O. D., I D., axis 35 degrees. O. S., I D., axis 145 degrees. Manifest. VI O. D., 2.5osph. C 0.75, cyl. axis 35 = - vn ? ? ? ?. O. S., 2.50 sph. O 0.75 cyl. axis 145 = - VI[ ? ? ? ?. 1 . Atropin and dark glasses for rest and refraction. April 1 3th: At six meters from test -card and point of light : O.D.V. =1 -?. .S.V. =] g-?. O. D., 2.00, sph. O i. oo cyl. axis 30 ss -yj- ? ? ?. VI O. S., 2.00 sph. O I. oo cyl. axis 150 = - v j- ? ? ?. Retinoscope confirms this trial-case result. Retinoscope also shows a general cloudiness of the media (vitreous), APPLIKI) REFRACTION. 26 1 which, of course, will account in part for the vision not VI being - v - in each eye with correcting glasses. April 1 4th : Same result as on the ijth. For MRS. USUAL. R. O. D., 2.00 sph. O i.oo cyl. axis 30 degrees. O. S., 2.00 sph. O i.oo cyl. axis 150 " SlG. For distance, as directed. April i 5th: K . Tonics. Rest of eyes. Attention to general health. April i /th : Glasses neutralize ; are centered and ad- justed. April 2pth : Add., 16. Abd., 6. Exophoria, 2. Vision in each eye read slowly. Considerations. The static refraction was ordered just as found, and no deduction whatever was made in the sphere. The rule is to prescribe in the same way as in /simple myopia. But all cases of myopia can not and must not be prescribed for by rule. Each case of myopia is a law unto itself. See description under General Considera- tions, page 238, also pages 227, 228, and 229. SUMMARY. The near point is now 14 cm., which is perfectly consistent with the patient's age. Fourteen centi- meters represents an accommodative power of 7 D., and this was the difference between the near and far points before the drops were used. The astigmatism is regular, symmetric, and with the rule. Vision is not brought up to normal on account of the changes in the vitreous and dis- turbed eye -ground, due, no doubt, to the want of a proper correction the cylinder. The choroid and retina are both in a stretching condition. CASK VII. Mixed Astigmatism. Not an uncommon condition. About 6^ per cent, of all eyes have this form 262 REFRACTION AND HOW TO REFRACT. of refraction. This is a combination of the simple hyper- opic and simple myopic astigmatisms, with their axes oppo- site or at- right angles to each other, as a rule. April 8th. MR. CROOK. Ag, twenty-one years. Single. Clerk. VI O. D. V. = -J(L. p. p. = 14 cm. with type 0.75 D. VI O. S. V. = -j^. p. p. = 14 cm. with type 0.75 D. Add., 20. Abd., to. History of poor sight all his life, but thinks it was better as a boy. Has frequent frontotemporal headaches, which are worse after using eyes at any prolonged near-work. Father has good sight, but his mother and her family have all been near-sighted ; has one aunt that developed cata- racts. Has been to several "stores," but could not get fitted with glasses that would improve his vision. 5. P. Face broad and symmetric. Long interpupillary distance. Irises dark in color. Pupils large, 5 mm. ; round. Eyes out under cover. Ophthalmoscope. O. D., media clear. Disc vertically oval, axis 105. Macular region shows changes. Vessels at 105 best seen with 4-2, and at right angles with 2. O. S., same conditions found, except that the meridians are at 75 degrees and 165 degrees. OphtJialmometer. O. D., 4 D. axis 100. O. S., 4 D. axis 75. Cobalt-blue Glass. O. D. violet center, blue above and below in meridian of 105 and red at the sides in meridian of 15. O. S., violet center, blue above and below in meridian of 75, and red on sides at axis 165. (See Figs. 135 and 136.) Stenopeic Slit. O. D. axis 15 with +2 sph., V.= VI VI IX at axis 105 with 2 sph., V. = . O. S. axis 165 with 4-2 sph., V.=r -y^- ; at axis 75 with 2 sph., V.= . APPLIED REFRACTION. 263 Indirect Method. Each eye shows a lengthening of the vertical meridian as the condensing lens is withdrawn from the eye, and at the same time the horizontal meridian grows narrower. As the lens is advanced toward the eye the vertical meridian grows shorter and the horizontal meridian grows broader. ,/ The astigmatic chart does not show any difference in the /shading of the lines ; they all appear about the same. Retinoscope at 1 meter distance shows myopia in the verti- cal meridian and hyperopia in the horizontal. R . Atropin and dark glasses for refraction. April loth : At six meters from test-card and point of light. O. D. and O. S. V. = ^. LA Cobalt-blue glass shows the same as at first visit. Retinoscope at the distance of one meter shows point of reversal in O. D. at axis 105 degrees with 1.500., and axis 15 with +3 D. O. S., axis 75 with 1.50 D., and axis 165 with +3 D. Stcnopeic Slit. O. D., axis 15 with -\-2 sph., V. = j N x ! ; axis 105 with 2.50. sph., V. = -^. O. S., axis 165 with +2 sph., V. = ^ ; at axis 75 with 2.50 sph., y - .-.XL ' ix ; At Trial-case. O. D, 2.50 cyl. axis 15 degrees O -f-2 cyl. axis 105 degrees, V. = [ l s - O. S., 2.50 cyl. axis 165 O +2 cyl. axis 75, V. = VIISS* Or, VI O. D., 2.50 sph. O 4 4- 50 cyl. axis 105, V. ylfsg. VI O. S., 2.50 sph. O -(-4-5 cyl. axis 75, V. ^ ss> . Or, VI O. D. -f- 2 sph. O 4.50 cyl. axis 15 degrees, V. = yjjgg -f- VI O, S. -j-2 sph. O 4.50 cyl. axis 165, V. = V1ISS -\-. 264 REFRACTION AND HOW TO REFRACT. April nth: Same results as April loth. Add., 2O. Abd., 8. For MR. CROOK. B. O. D. -\-2 sph. O 4.50 cyl. axis 15 degrees. O. S. -f-2 sph. O 4.50 cyl. axis 165 " SlG. For constant use. April 1 2th : Glasses neutralize ; are centered and adjusted. April 26th: Near point 10 cm., which is consistent with age of patient. Considerations. The ophthalmoscope, retinoscope, cobalt-blue glass, indirect method, and stenopeic slit were direct guides to the character of the refractive error. Emphasis is placed upon these different methods, as so many beginners in ophthalmology have a fear or dread of the re- sult in refracting cases of mixed astigmatism. The stenopeic slit shows a difference of 4.50 in the two principal meridians ; bearing this fact in mind, if a 4.50 cylinder at axis 1 5 be placed before the right eye, then all meridians would be made equally hyperopic 2 D. Com- bining -f- 2 sph. with the 4. 50 cylinder at axis 1 5 in the right eye or at axis 165 in the left, the refraction would be corrected. Or, if a +4.50 cylinder at axis 105 be placed before the right eye, then all meridians would be made myopic 2.50 D. Combining a 2.50 sph. with this +4.50 cylin- der at axis 105 in the right eye or at axis 75 in the left eye, the refraction would be corrected. For a further consideration of combination of lenses see page 51. The rule for ordering cylinders is the same in mixed astigmatism as in other cylindric corrections without change. SUMMARY. The character of the astigmatism is regu- APPLIED REFRACTION. 265 lar, symmetric, and with the rule. The near point returns to the normal for the age. ,. Eyes with such errors do not, VI as a rule, obtain a visual acuity of , for the reason that changes have taken place in the eye-ground, especially at the macula. ' CASE VIII. Irregular Astigmatism. April ad. MARY SMILES. Age, ten years. Scholar. VI O. D. V. = xxx slowly. No p. p. obtained. VI O. S. V. = LX~. No p. p. obtained. History of poor sight ever since an attack of measles when two years of age, at which time was kept in a dark room for six weeks. Eyes were never strong afterward ; always very sensitive to light. Child was sent home from school with a note from the teacher : " Mary is near- sighted and should see a doctor." 6". P. Eyelids appear normal. Excessive epiphora. Corneas nebulated, especially O. S., which has a decided leukoma at the pole. Anterior chambers of normal depth. Pupils 3 mm., round. Corneal reflex very irregular. Ophthalmoscope. No view obtained of the eye-ground through the small pupils on account of corneal opacities. Homatropin mydriasis shows O. D. cornea faintly nebu- lated in scattered areas ; rest of media clear. Nerve small and round. Vessels at axis 35 degrees best seen with -f-2 D. O. S., there is a 3 mm. area of opacity at the pole of the cornea ; no clear view of the eye-ground. Indirect method shows a small nerve and refraction hyperopic. R . Atropin and dark glasses for refraction. April 22d : Retinoscope at I meter shows band of light at axis 35, indicating hyperopia. Other meridians very irregular. O. S., nothing definite made out. 23 266 REFRACTION AND HOW TO REFRACT. Placido's disc shows irregular, distorted circles. With Put-hole Disc.Q. D. V. = Y v '-. O. S. V. = ^-. A A 1 , \ With Stcnopeic Slit. Axis 45 degrees before O. D. and with +2.25 S., V. = ~ ? ?. O. S., can not improve vision with any glass. At Trial-case. O. D., +2.00 cyl. axis 145 = ^??. O. S., no glass accepted. April 2$d : At trial-case O. D., +1.75 cyl. axis 35 = ~. For MARY SMILES. R. O. D., 0.25 sph. O +1-75 cyl. axis 35 degrees. O O. S., plane glass. SIG. Constant ^use. Considerations. This case shows the advantage of the stenopeic slit and the use of the pin-hole disc. The near point could not be obtained on account of the poor visual qualities and the child's inability to appreciate what was wanted. CASE IX. Tonic Cramp or Spasm of the Accommo- dation. MRS. L. Age, twenty-four years. VI O. D. V. = xxv ??. p. p. =9 cm. (?) Add., 24 degrees. Abd., 6 degrees. Esophoria, 4 degrees. VI O. S. V. = "xxv ^ *** P- P- 9 cm - (**) ^ v e rt i ca l deviation. History of having had glasses changed on three different occasions during the past year. Drops were used each time, and the three prescriptions were all different. Glasses were always satisfactory for the first w r eek, but after this time she was always able to see better at a distance with- out them. Has pains in her eyes and all over the head whenever she attempts to use the eyes with or without any glasses. Headaches nearly set her " wild" if she tries to AITI.IKI) KK1 KACT1ON. 267 concentrate her vision on a distant or near object. Has not been able to read or write or sew for the past two years. Has been under the care of the gynecologist and neurologist, and they each pronounce her physical condition as normal. The neurologist suggests a diagnosis of " hysteria." Patient sleeps well and has a good appetite, but will suffer from nausea and vomiting if she uses her eyes for any length of time. Patient has been married five years. Has one living child. No miscarriages. Is apparently in the very best of health, and is provoked with her apparent good health as not being consistent with her suffering, and hence she does not receive any sympathy from her family or her friends. 5. P. No external manifestations of any ocular irregu- larity. Manifest Refraction. VI O. D., 0.50 S. O +1.00 cyl. axis 90 degrees = -yj-. O. S., the same as O. D. Ophthalmoscope. O. D. and O. S., media clear. Discs vertically oval, eye -grounds "woolly." Accommodation very active. Shot silk retina. Refraction is compound hyperopic astigmatism. Cobalt-blue glass shows a red center and broad blue halo. (Patient is certainly accommodating.) & . Atropin and dark glasses for refraction. Static Refraction. VI O. D. +1.50 S. = +1.75 cyl. axis 90 degrees = -y-. O. S. +1.50 S. = -j-l.75 cyl. axis 90 degrees = -XL. Patient states that her " pains and headaches all disap- peared after using the drops for the third time." 268 REFRACTION AND HOW TO REFRACT. Refraction repeated on three different occasions, and the following prescription given : For MRS. L. R. O. D., +1.25 S. O +1-75 cyl. axis 90 degrees. O. S., +1.25 S. O +1-75 cyl. axis 90 degrees. Glasses properly centered, and accurately adjusted. r After ten days patient returns with the statement that her pains and aches have recurred as before, and that she can see better at a distance without her glasses. With correction, each eye sees ^~, and with both eyes can see Add., 20 degrees, and abd., 8 degrees. No verti- cal deviation. Has 3 of esophoria at 6 meters. Atropin -$ of a grain to the ounce. SlG. To use one drop in each eye each morning and noon. To wear a pair of dark glasses with her prescription glasses when exposed to any bright light. N^LJxL-attempt ..any near-work. This treatment was continued, off and on, for six months. Patient was always free from ocular pain and headache as long as the atropin was being used, but as soon as the ciliary muscle commenced to contract, then the pains would return with all their former severity. This patient eventually recovered by using her distant correc- tion with a pair of plus 2 spheres as hook -fronts for any near-work. CASE X. Exophoria. Miss. V. B. D. Age, twenty-two years. VI O. D. V. = -yj-. p. p. = 0.50 D., type at n cm. VI O. S. V. = -yy. p. p. = 0.50 D., type at II cm. Add. and abd., 12 degrees. Exophoria = 4. History of seeing double several times a day. Friends and members of her family have told her she was " squint- ' APPL1KD K INFRACTION. 269 ing." Always returns home with a severe occipital head- ache after going shopping or to any place of amusement. Has headache when using her eyes, but it soon passes away after resting the eyes. S. P. Eyes markedly out under cover. Irises react promptly to light, accommodation, and convergence. Fixation test shows the right eye divergent. Ophthalmoscope. O. D. and O. S. No apparent changes, and refraction almost emmctropic ; some small amount of hyperopia and astigmatism. R Atropin and dark glasses for rest and careful refraction. Static refraction, after several repetitions, O. D. and O. S., +0.50 S. Cl 4 0.37 cyl. axis 90 degrees = -~. And this is ordered, less 0.25. With this correction carefully centered, add. =14 degrees and abd. = 1 2 degrees, with 3 of exophoria at 6 meters and 7 degrees of exophoria at 1 3 inches. This patient was given prism exercises for more than two months, and, finally, after the adduction reached 30 degrees and abduction was 10 degrees and 3 degrees of esophoria were obtained, the prism exercises were stopped, and patient told to report promptly if any disconifort arose at any time. To wear . / '^IjJL her glasses constantly, T&I 1 H MR. ALBERT S. Age, twenty-nine years. In general business. VI O. D. V. = LX". p. p. type 0.50 D. = 20 cm. VI O. S. V. = xxx . p. p. type 0.75 D. = 30 cm. Add., 10 degrees. Abd., 6 degrees. Left Hy., 2 degrees. History. Has had three pairs of glasses ordered, with " drops," during the past eighteen months. Has never had any but the very slightest relief from ocular pains and 2/O REFRACTION AND HOW TO REFRACT. frontal headaches, which have been almost constant for the past four years or more. On account of the ocular dis- comfort and headaches, the patient has given up all at- tempts to read for more than fifteen minutes at a time Patient states that if he uses his eyes for more than thu length of time they become bloodshot and very tendci to the touch. General health of patient is excellent ; ha. 1 a good appetite and sleeps well. Docs not use tobacco 01 liquor of any kind. 5. P. Face symmetric. Nose very prominent. Inter pupillary distance, 62 mm. OphtJialmoscope. O. D., nerve-head over capillary. No swollen. Accommodation very active. Eye-ground "fluffy.' Refraction is that of compound hyperopia. O. S., sam< general conditions, but the nerve is vertically oval and th< refraction is compound hyperopic astigmatism. R . Atropin and dark glasses for refraction. Static Refraction. O. D., 4-2.00 S. O 4 I -oo cyl. axis 75 degrees ^.j ~\ ^ O. S., +1.25 S. C +3.00 cyl. axis 105 degrees =-^,| -??? j h >l n>r - K. O. D., -fl-75 S. O +l.oo cyl. axis 75 degrees O ^ A Dase U P- O. S., -f l-ooS. O +3-OOcyl. axis 105 degrees O ^/^ basedowi SlG. For constant use. This patient was rrot made comfortable until he was give five-grain doses of the bromid of potash three times a da for four weeks. Is now able to use his eyes without th least discomfort. , x jf ^ MM. y 1 I / 4-js- CHAPTER XL PRESBYOPIA. APHAKIA. ANISOMETROPIA. SPECTACLES. Presbyopia. The word presbyopia (from the Greek, TT/^'VMT, "old" ; "V', "eye") literally means old sight, and patients at the age of forty-five or more years are univer- sally recognized as presbyopes, and the condition of their eyes as presbyopia There is no exact age limit as to when presbyopia shall begin, the advent of presbyopia being con- trolled by the character of the ametropia and physical con- dition of the eyes themselves. Presbyopia may be described in several different ways, according to the cause /. e., 1. Old sight. 2. The condition of the eyes in which the punctum proxi- mum has receded to such a distance that near vision (close work) is impossible without the aid of convex lenses. 3. The condition of the eye in which the lens fibers have become more or less sclerotic, and, as a consequence, the lens loses some of its inherent quality of becoming more convex during contraction of the ciliary muscle. 4. The condition of the eye in which the power of the ciliary muscle has become weakened. 5. The condition of the eye in which the power of ac- commodation is diminished at the same time that the lens fibers become sclerotic. 6. The condition of the eye in which two different refrac- tions (not necessarily two pairs of glasses) are required, one for distance and one for near vision. 271 272 REFRACTION AND HOW TO REFRACT. 7. The condition of the eye in which one pair of glasses will not answer for distant and also for near vision. 8. Presbyopia may be described as the condition in which nature has instilled a slowly acting but permanent cyclo- plegic (the term cycloplegic is used here in a general sense). Causes of Presbyopia. I. Age. It is a well-estab- lished fact that in childhood the center of the lens begins to harden, becomes sclerotic or sclerosed, to form a nucleus ; and this process continuing, eventuates in complete sclerosis at sixty or seventy-five years. The term sclerosis must not be confounded with opacity. 2. Disease. Ordinarily, presbyopia, as applied to the lens, should be recognized as a physiologic process, as a penalty for growing old, though it is a condition which may be hastened by disease. Any disease, therefore, which will cause the nutrition of the lens to suffer must event- ually interfere with its ability to become more convex during accommodation. The most common ailments that tend to this result are rheumatism, gout, Bright's disease, diabetes, lithiasis, la grippe, etc. Any disease which will weaken the ciliary muscle will produce presbyopic symptoms. Presbyopic Near Points. The near point and power of accommodation in a healthy emmetropic eye, or a healthy eye made emmetropic by the addition of correcting lenses, is as follows for certain ages : POWER OF AGE. H NEAR POINT. ACCOMMODATION. 40 years, . . j^j . . 22cm. 4 5 diopters. 45 ' . . . u'O . 28 ri 3 50 5 ' . . . / (o. . 40 ' 2 50 55 ' ...**. 55 ' I 75 r 2.00 60 ' . . . .t? i loo I 00 65 ' . . . JFp 1 . 133 ' 75 70 ' . . . |*0 ( . 400 o 25 7C ' oo ' oo PRESBYOPIA. 2/3 Ordinarily, the average adult holds a newspaper or book at about 33 cm. (13 inches) from his eyes when reading ; and if he is forty years of age and emmetropic, or is made emmetropic with glasses, he would be using 3 D. of his normal 4.50 of accommodation, which would leave a reserve power of 1.50 D.; and in this condition, other things being equal, he can maintain a reading distance with comfort. In fact, he could, by using all of his 4.50 D. of accommoda- tion, see objects as close as 22 cm., but not for any great length of time, as the ciliary muscle would soon relax. This same patient at forty-five or forty -six years of age will have lost i.oo or 1.50 D. of his accommodation, and now has only about 3 or 3.50 left ; and if he uses all of it at a working distance of 33 cm., the ciliary muscle soon yields. In fact, the ciliary muscle can not be held in such a state of tension without causing all sorts of pains and aches and reflex disturbances ; and the ciliary effort relaxing suddenly, the near vision blurs, and the work or reading or sewing must be put at a greater distance to obtain relief, or else the effort must be abandoned. Symptoms of Presbyopia. The principal symptom is that which indicates a recession of the punctum proximum ; the patient stating that there is an inability to maintain the former reading, writing, or sewing distance, and that all near-work must be held at a greater distance than formerly. Symptoms of accommodative strain may be present if the patient endeavors to force the accommodation to its maximum. Diagnosis. The age of the patient and the history of having to hold reading matter at an uncomfortable distance ; or a history of good distant vision and an inability to retain clear near vision small objects, to be seen, must be held far away or " at arm's length." 274 REFRACTION AND HOW TO REFRACT. ^^ ft Correction of Presbyopia. The presbyopic state repre- sents a class of patients for whom glasses may be pre- scribed by the manifest refraction, although there are exceptional cases in which a quick cycloplegic will be nec- essary when an amount of astigmatism or cylinder axis is uncertain. r a working, reading, writing, or sewing distance of 33 cm. (13 inches), the writer makes it a rule to add to the dis- tance correction at forty- five years of age a + I sphere ; at fifty years of age, a -f- 2 sphere ; at fifty-five years of age, a +2.50 sphere ; and for sixty or more years, a +3 sphere. The following table for emmetropic eyes shows these addi- tions for the different years, and also the near and far points / /A feTJ tt*K/ with these additions as well as the range of accommo- dation or "play " between the near and far points. It will be observed that the range of 78 cm. at forty-five years rapidly diminishes in the succeeding years, until at sixty there is a play of only about 3 inches, and at seventy the range is practically gone. , Iioo cm. 50 40 33 : / 33 / 33 / / 33 'S.-f-f C4*S A+~a** ^ a patient is fifty years of age does not signify at he will be able to read at 33 cm. with a pair of -\-2 spheres, or because he is sixty years of age that he can use his eyes at 33 cm. comfortably with a pair of +3 spheres j on the contrary, this rule that the writer has given applie only to cases of emmetropia. It often happens that pres- byopic patients state that they do not want glasses for dis- I ~ . YEARS. ADD. NEAR POINT. 45 + 1,00 22 cm. So ^^^+2.00 22 " 55 + 2.50 23 " 60 -1-3.00 25 " 65 -f3-oo 27 " fl:LV > 7o +3.00 30 " ,. JW 2. I)/) 75 -h3-oo 33 " IMA* f H rrfa M<^A^rf * lA/fl NT. RANGE i. 78 en ;. ^, 28 17 ' 8 ' 6 3 ' o ' / .0.? /", /~~ PRESBYOPIA. 275 tance ; that they do not need them ; that all they wish is a pair of glasses to use at near-work, reading, ete. When the vision is taken in such cases, it may be found to be or vi VI approximating .- ; but the young ophthalmologist must not be thrown off his guard by this record, as it has already VI been stated that a vision of VI does not by any manner of means prove the existence of emmetropia. Let the sur- geon make it a constant rule in every case of presbyopia to always carefully estimate the amount of the distance ametropia ^/^ first, no matter how ^veak or what its form (sphere or cylinder) ; and to the result thus obtained, add the plus sphere which will be required for the ivorking distance or point at which tJie patient wishes to see clearly. /&t-*~^-yiS '? Illustrative Cases. CASK I. Accepts +0.50 sph. for distance. At forty-five years this case would require -f 1. 50 sph. for reading at a distance of 33 cm.; at fifty years, + 2.50 sph.; at fifty-five years, +3 sph.; and at sixty or more years, +3.50 sph. Only one pair of glasses is neces- CASE II. Accepts -(-2 sph. for distance; at forty-five years these eyes would require +3 sph.; at fifty years they would require -[-4.50 sph.; and at sixty or more years they would require -j~5 s P n - Two pairs of glasses would be indicated in this case. CASE III. Accepts i.oo sph. for distance ; at forty- five years this patient could read without any glasses, as I for distance would be neutralized by the -f I required for reading. At fifty years, however, the patient would require a -f I sphere for near, and at fifty -five a -f- 1.50, and at sixty years a -f-2 sphere. Case II required two correc- tions, one for distance and one for near ; and the same may 'besaid about Case III ; but in this latter instance there was a time at forty-five years when there was no necessity for 276 REFRACTION AND HOW TO REFRACT. glasses for the near-work, as the patient's eyes were in a suitable condition of refraction to read without them. CASE IV. Accepts 3 sph. for distance. At forty-five years would require 2 sph. for reading ; at fifty years would require I sph. for reading ; and at sixty years can read without any glasses. Such a patient says he has gotten his " second sight." CASE V. Accepts -) 0.50 cylinder axis 1 80 for distance and requires the usual additional spheres for the increasing years for his reading distance. CASE VI. Accepts -fi.oo sph. O -j-i-OO C yl. axis 180 for distance and requires the spheric additions as the years increase. Two pairs of glasses should be prescribed. CASE VII. Accepts i cyl. axis 90 for distance, and requires -(- I cyl. axis 180 to read with at forty-five years of age ; at fifty years he requires -f- 1 sph. O -f I cyl. axis 1 80 ; and at sixty years requires +2 sph. O + 1 cyl. axis 1 80. At forty-five years of age this patient is com- monly spoken of as having simple myopic astigmatism for distance (against the rule) and simple hyperopic astigmatism for near (against the rule also) ; two pairs of glasses are in- dicated throughout life. CASE VIII. Accepts i.oo sph. O 1.50 cyl. axis 1 80 for distance, and at forty-five years will need 1.50 cyl. axis 1 80 for reading ; at fifty years will require -(- i.oo sph. O 1.50 cyl., axis 180 degrees; at sixty years, +0.50 sph. O -f- 1.50 cyl. axis Two pairs of glasses At forty-five years this patient has a compound myopic correction for distance and simple myopic astigmatism for near ; at fifty years the correction for near is that of crossed cylinders (mixed astigmatism) ; and at sixty years the near correction is that for compound hyperopic astigmatism. \s *-*.-* / r* 5 QQ degrees. *-fTC -/$"//* ~t~2 rnM$r<~ . *i&u*&i.9T'r* s should be used rTjrniignmiK lif -^ - v< <*V j PRESBYOPIA. 277 CASE IX. Accepts i.oosph. O -f 2 cyl. axis 90 for dis- tance (mixed astigmatism) ; at forty-five years, -\-2 cyl. axis 90 is required for reading ; at fifty years, -)-i.oo sph. O + 2.00 cyl. axis 90. Tu^cTpairs of glasses are required. At forty-five years the distance correction is for mixed astig- matism and the reading correction is for simple hyperopic astigmatism. CASE X. Accepts 2.00 cyl. axis 180 for distance ; at forty-five years requires a mixed astigmatism correction for near; at fifty years, a simple hyperopic correction * and a compound hyperopic correction at sixty years. - In the above illustrative cases the working distance has been calculated at 33 cm., or 13 inches; but as some patients use their eyes at a greater or less distance than this, the additional convex lenses must be calculated accord- ingly. For instance, the weaver at fifty-five years of age who requires -f 2 spheres for distance could not see to weave at 50 cm. ; if -(-2.50 spheres were added to his dis- tance correction, all he needs is -(-3 for his working dis- tance. Or the diamond cutter who wishes glasses to see his work at 8 inches, if he accepted i.oo sph. for distance, lie would require +2 sph. at forty-five years of age. In conclusion, there are three facts in the refraction of presbyopic patients that should receive attention : i. Many accept a weak plus cylinder ( + 0.50 at axis 1 80) against the rule. This is presumptive evidence that the astigmatism is acquired, is lenticular, and is due to the sclerotic changes previously mentioned. The only positive way to prove this fact is by the retinoscope, and by the ab- sence of corneal astigmatism with the ophthalmometer. If the case has been previously refracted by the same sur- geon, his record will also confirm this extremely interesting occurrence. According to able authorities, hyperopic eyes 2/8 REFRACTION AND HOW TO REFRACT. become more hyperopic after the age of seventy years, and emmetropic eyes may become hyperopic, and myopic eyes less myopic, from the same sclerotic or shrinking pro- cess which takes place in all the ocular tissues as a result of senility. The method of correction by glasses, however, is just the same, and that is to correct the distant vision first and then add the near correction. 2. An attack of glaucoma may precipitate presbyopic symptoms, so that when a presbyopic patient asks for fre- quent changes in his corrections, this complication should be borne in mind. 3. The swelling of the lens which occasionally precedes the formation of some forms of cataract should be remem- bered when the patient develops symptoms of myopia /. i\, a reduction in the strength of convex glasses. (_ Aphakia (d, priv. ; erect corneal image, with absence of lenticular images, and by the patient's history. The ametropia of an aphakic eye depends in great part HETEROMETROPIA. 2/9 upon the previous refractive condition of the eye, and also upon the kind of operation that was performed for the re- moval of the lens. It has been calculated that an eye, to be emmetropic after the removal of its lens, would have to be myopic at least twelve diopters. If this is always true, then the correcting lens which is selected by an aphakic eye is a guide to its former ametropia. An eye which selects a weak plus sphere would, therefore, have been myopic before the operation ; and if about a + 12 S., its previous refrac- tion approximated emmetropia ; if a plus sphere stronger than 1 2, then the previous refraction was very likely hyper- opia. An eye which has had its lens removed by absorption/ (needling) is not likely to be astigmatic ; whereas, when the lens has been removed by extraction, astigmatism) , against the rule of one or more diopters almost invariable/. results, and the axis of the correcting cylinder generally coincides with the points of puncture and countcrpuncture in the cornea. If a patient had 2 or 3 D. of myopic astigmatism with the rule, this would be neutralized by the corneal section. Correction of Aphakia. As in presbyopia, two correc-V^. tions are necessary one for distance and one for noaxs^ Astigmatism must always be looked for and carefully cor- rected, especially if the lens has been removed by extrac- tion. CASE I. VI O. D., 48-00 sph. ^ +3-OO cyl. axis 10 degrees. V. = x O. D., 4 n.oo sph. C; 4 3- c >'- ax ' s IO degrees = reading at 33 cm. This patient was presumably myopic before operation. Heterometropia (",""?, "different"; !^r/mv t "a meas- ure" ; &<}', "the eye") literally means that the ametropia of 28O REFRACTION AND HOW TO REFRACT. the two eyes is different in character; examples, O. D. -f- i D. and O. S. I D., or O. D. +3 cyl. axis 90 and O. S. 3 cyl. axis 180, or O. D. 5 D. and O. S. 5 cyl. axis 1 80, etc. Anisometropia (W ?, "unequal"; /j-l-pnv > "a measure"; &<}>, "the eye") literally means that the ametropia of the two eyes is the same in character but of unequal amount. Examples, O. D. +2 D. and O. S. +6 D., or O. D. 0.50 D. and O. S. 5 D., or O. D. +3 cyl. axis 90 and O. S. -{-6 cyl. axis 90, etc. This condition may be slight or one of the most extreme conditions imaginable. ^T'or instance, if both eyes have compound hyperopic astigmatism, they are not considered as heterometropic, even if the sphere and cylinder are of different strength in the two eyes. . Bearing this distinction in mind, the percentages already given for myopia, hyperopia, the different astigma- tisms, etc., have been calculated accordingly, that for j^heterometropia being about thirteen per cent. Causes. Usually the condition is congenital, or it may be acquired. Difficulties. Two difficulties are encountered when or- dering glasses for cases of anisometropia or heterometropia : (i) The lens for one eye may be concave and that for the other may be convex, or both eyes may require a convex or both may require a concave lens, but one very much stronger than the other; under these circumstances, when the eyes are rotated there will be a prismatic result of dif- ferent amount in each eye, and this may mean diplopia, or at least an exertion on the part of the extraocular muscles to prevent diplopia which will cause dizziness, nausea, head- ache, etc. (2) With lenses as just mentioned, the size of the two retinal images will not be exactly the same, and this will mean an interference with clear binocular vision. ANISOMETKOPIA. 28 1 For purposes of study, the writer would divide cases of heterometropia and anisomctropia into four different classes. CLASS I. This class embraces those cases in which the difference in the ametropia between the two eyes is very slight or does not exceed two diopters. In fact, there are very few pairs of eyes that are not slightly anisomctropic ; such eyes usually receive their exact corrections with com- fort, regardless of the condition. CLASS II. Cases that come under this head also accept their exact correction for each eye, but do not attempt binocular single vision, and may never suffer the least in- convenience ; these cases are extremely rare. They do not complain of diplopia, as they have learned to ignore the false image. Cases of alternating squint, one eye myopic and the other eye hyperopic, may be included in this class. CLASS III. This is a class which will accept the exact correction before one eye only, and the eye which has the greatest amount of ametropia will refuse almost any lens except the very weakest. The eye that has the most ame- tropia is often quite amblyopic. CLASS IV. This class includes young children especi- - , ally ; cases of squint. In children the correction as found by the static refraction is usually accepted. The Prescribing of Glasses in Cases of Heterometropia and Anisometropia. Excluding Class I, there is no fixed rule to follow when ordering glasses in decided cases of heterometropia or anisometropia, and, in fact, such eyes are a constant study to the most able ophthalmologist. The younger the patient, however, the more likelihood of a favor- able result from the careful selection of a glass for each eye; but when the patient is an adult, it becomes a very serious question as to what glass to prescribe that will give satisfac- tion. As good results are to be expected in children, they 24 282 REFRACTION AMD HOW TO REFRACT. should receive the most careful retinoscopic refraction. The child comes under observation on account of a squint, and an operation for the deformity is often demanded ; but the opera- tion must be refused until the ametropia has been carefully treated. Glasses having been prescribed, the squinting eye is put to work to develop its seeing qualities, which have been permitted to lie dormant for want of a proper glass. To do this, the "good" eye is shielded or blinded with a hand- kerchief tied over it, or a blinder (see Fig. 1 60) placed over its correcting lens, for an hour or two each day, and in this way an attempt is made to bring the vision in the squinting eye up to that of its fellow. ; Or another way to develop the vision in the squinting ; eye is to use a cycloplegic in the " good " eye, so that the squinting eye must do most of the work. This is rather trying to the little patient, and often means the additional use of dark glasses. As a rule, the "good" eye has the least amount of ametropia, but occasionally the reverse con- dition may exist. In a case like the following, the little girl, five years of age, was brought on account of convergent squint in O. S., which developed or commenced to appear when ten months of age, and the parents attributed it to the habit of sucking her thumb at the time of being weaned. Refraction, with atropin as the cycloplegic, and obtained with the retino- scope, showed O. D., -(-2.00 sph.; O. S., +4.00 sph. O + 1.00 cyl. axis 75 degrees. This child developed the squint on account of the monocular astigmatism and because it could not accom- modate sufficiently with the left eye. To avoid diplopia at the same time that the eyes were converging, the left eye naturally turned inward. With correcting glasses, and practising as above directed, the squint entirely BIFOCALS. 283 VI disappeared, and vision one year later was -_.- in each eye with the correcting glasses. If the glasses are laid aside for any length of time, the squint returns. This child must wear the glasses or have " squint." /To make sure that no injustice is done to an apparently /amblyopic eye in an adult (Class III, p. 270) where ambly- opia exanopsia has existed for many years and nothing has been done to improve its correction, the writer makes it a rule to prescribe the exact correction for each eye, and at the time of ordering the glasses explains to the patient what the purpose and desire is, and that if there is any great amount of discomfort in any way, he must return and have any necessary change made in the glass. These patients should be kept undef observation and the amblyopic eye given some sort of a correction and improved as much as possible ; the purpose being not to allow the eye to degenerate or grow more amblyopic, for if any accident should befall the " good" eye, then the amblyopic eye will often be a friend indeed. Glasses for Presbyopes and Cases of Aphakia. Unless I the distant vision is improved or asthenopic symptoms are > .relieved by glasses, it will be sufficient to prescribe the i near correction only. When a distant and near correc- tion are required, they may be prescribed as two pairs of glasses in separate frames, or two pairs in one frame, known as bifocals. Bifocals, or what is equivalent to bifocals, are made in different ways. i. Franklin* or Split Bifocals (Figs. 175 and 176). This form of bifocals consists of an upper and a lower lens, each with its individual center ; the upper lens is for dis- tance and the lower for near vision. Such lenses must have the frame all' around the edges, so as to hold them in posi- * " History of Spectacles," L. Webster Fox, " Med. and Surg. Reporter," 1890, vol. LXII, 513-519. 284 REFRACTION AND HOW TO REFRACT. tion. Bifocals of this kind are not in common use. The field of distant vision is limited by the unnecessarily large near correction, and where the two lenses come together, A FIG. 175. there is apt to develop chromatic aberration and a decided prismatic effect when the vision is directed through this space. B. O. D., -|-2.oo sph. O. S., -(-a.oosph. SlG. For distance. B- O. D., -f 4.00 sph. O. S., +4.00 sph. SlG. For near. DIRECTIONS TO OPTICIAN. Make into Franklin or split bifocals. . Morck's Patent or "Perfection" Bifocals (Fig. ). These are a modification of the Franklin or split bifocals, and in place of having lenses united in a horizontal line, the near and distant lenses are fitted together with correspond- ing crescent edges. This form of bifocal gives a larger field for the distance correction, and, like the Franklin, is much better for those who work in a damp atmosphere and can not wear the cement bifocal. It is, FIG. 177. I BIFOCALS. 28 5 however, more expensive than the cement form ; but, like the Franklin, it often looks clumsy or heavy on account of the frame. "Perfection" or "Morck" bifocal must be j, . "^^ Mg * . f* signified in writing the prescription. ^j ~ftn_& ' * ' < ^ * f 3. Cement Bifocals-fScTTIgsTi/S and 179*). This is the most common form of bifocal and the least expensive in its original cost, as also when making changes in the near correction. This bifocal is made by cementing a seg- A FIG. 181. FIG. 182. ment of a small periscopic sphere on to the lower part of the distance correction. This periscopic sphere or disc or segment, as it is called, has a prismatic quality (see Fig. 179) suitable to the exigencies of the individual lens to which it is cemented. The segment may be of any shape desired. Those in common use are shown in figures 180, 181, and 182. It is cemented to the distance correction * Described by Dr. Geo. M. Gould, " Med. and Surg. Reporter," Nov. 3, 1888. 286 REFRACTION AND HOW TO REFRACT. with Canada balsam. While this is the usual method of making a cement bifocal, yet it may be made by cementing IB FIG. 183. V FIG. 184. a concave segment to the upper part of the near correction. (Figs. 183 and 184.) This form is not in common use. H. O. D.,-| 2 S. O. S., +2 S. Cement on the lower part of the above O. D. and O. S., -\-2.oo SlG. Make frameless bifocals. Or, li. O. D., + 4 S. O. S., +4 S. Cement on the upper part of O. D. and O. S., 2.00 S. SlG. Make frameless bifocals. 4. Achromatic Bifocals (Figs. 185 and 186*). This FIG. 185. FIG. 1 86. form of bifocal is used principally in cases of aphakia where the plus sphere is quite thick and correspondingly heavy. It * Borsch patent. BIFOCALS. 23 7 is made in one of two ways : (i) By grinding out a portion of the lower part of the distance correction (in crown glass) and cementing into the concavity a biconvex segment of flint glass. This form of bifocal is a combination of the " perfection " and lenticular. (2) In place of grinding the concavity in one lens, as just described, this achromatic bifocal is also made by taking two planoconvex spheres and grinding out a concavity in each, and then inserting a convex sphere of flint glass, as shown in figure 186 ; these three lenses are then cemented together, and when com- pleted, look like the cement bifocal, as shown in figure 182. It is a matter for very careful calculation as to just how strong to make the flint glass segment, so that the result may be just exactly right. The merits of this bifocal are lightness and the absence of chromatic aberration. These lenses are very expensive. 5. Solid or Ground Bifocals (Figs. 187 and 188). Lensts of this character are made in one piece by grinding /* - S: