Methods Of Determining The Refraction Of The Eye

Methods Of Determining The Refraction Of The Eye:
OPHTHALMOMETRY; OPHTHALMOSCOPY, SKIASCOPY, OPTOMETRY; THE USE OF MYDRIATICS.
By EDWARD JACKSON, A. M., M. D.,
OF PHILADELPHIA.

Ophthalmometry, more properly called Keratometry, is the measurement of the curvature of the cornea and the astigmatism due to the differences in that curvature in different directions. The ophtalmometer consists essentially of a telescope furnished, in connection with its object lens, with some arrangement for doubling the images formed by it.

In the ophthalmometer of Helmholtz and that of Leroy and Dubois this doubling is effected by covering one half of the object lens by a piece of plate glass inclined in one direction, and the other half with a piece inclined in the opposite direction. The separation of the two images produced by this arrangement is the same at whatever distances the object is placed.

In the ophthalmometer of Java] and Schiotz the doubling is effected by a double prism, and the separation of the two images is only constant at a con¬stant distance. To make sure that the images formed by the instrument shall always have this constant distance cross hairs are placed within the barrel of the telescope. In using the instrument these cross hairs must be in focus when the images are focused; that is, the images must be formed at the plane of the cross hairs. To effect this the eye piece is so adjusted as to accurately focus the cross hairs for the observer's eve, and then the images are, focused by moving the telescope to or from the eye under examination until they become distinct with the cross hairs.

The curvature the cornea is measured by determining how large an obtjec is required to give a reflection from the cornea just equal to the separation of the doubled images Knowing the size of the object, the size of its reflected image, and the distance of the object from the eye, the radius of curvature of the cornea is ascertained by a simple calculation. With the ophthalmometer of Javal to which alone, as of most practical value, we shall refer the distance of the object is always practically the same. It is determined by the distance from which the image of the corneal reflection will be formed A the cross hairs. The size of the corneal reflection is also constant, being the extent to which the doubling prism separates the two images at the constant distance. This being the case, the size of the object and the curvature (or radius of curva of the cornea are inversely proportioned to one another, so that a scale can be calculated upon which a certain size of object will correspond to a 196.

METHOD OF USING THE OPHTHALMOMETER.

certain radius of curvature of the cornea. Such a scale has been calculated and laid off upon the arm of the ophthalmometer. Along with it is placed a scale of diopters of refracting power, corresponding in an average eye to the different lengths of the radius of corneal curvature.

The instrument is shown in Fig. 147. The most striking part of it is the great metal disk which shades the surgeon from the light, and has on its margin figures to indicate the direction in which the arm is turned. Through the center of this disk projects the telescope, and just below it the arm, placed horizontally, is shown, with the two mires upon it, the fixed mire to the right, the movable mire to the left. On the right of the picture is the head rest, with adjustable chin support, and four electric lamps attached to illuminate the mires when good daylight from a space of open sky is not available. The telescope is mounted in a collar which allows it to be freely revolved on its FIG. 147. Javal Schibtz ophthalmometer.

axis, carrying with it the graduated arm and mires, allowing the curvature to be measured in any meridian of the cornea. Unimportant variations as to the disk (which is in some models omitted), form of arm, method of illuminating, etc. are suggested by different writers, but the essential features of the instrument are those above indicated.

Method of Using the Ophthalmometer. To use the ophtbalmometer the instrument should be placed where strong light will fall upon the mires. The patient's face, which should be in comparative shadow, is placed in the head rest, one eve covered with a metal shade and the other directed into the barrel of the telescope. The surgeon, glancing along the telescope, sees that it is turned toward the patient's eve. Then by the large screw passing through one foot of the tripod be adjusts the height of the telescope, and by moving the whole tripod back and forth focusses the corneal images within the instrument. What be sees is the doubled reflection of the disk and mires, one image of each mire (A and B, Fig. 148) being close together. The movable mire is then shifted back or forth along the arm until the edge

198 DETERMINING THE REFRACTION OF THE EYE.

of its central image just touches the edge of the central image of the other mire (1, Fig. 149).

It will be noticed that each mire is crossed by a black line parallel to the arm. If the cornea is astigmatic, these lines oil the adjoining images of the two mires appear continuous only when the arm is turned in the direction of' one of the principal meridians. of astigmatism. In other positions they seem relatively displaced. The telescope is now rotated on its axis until the direction of the arm is found in which the lines on the two mires correspond. The mires are then brought so that their images are quite accurately in contact, and the index oil the movable mire indicates upon the scale on the arm the radius of curvature of the cornea, and corresponding refraction in one of the principal meridians.

The telescope is next rotated until the arm stands at right angles to its former position. If astigmatism be present, it will be found that in this position the mires either separate or overlap. If they overlap, as in Fig. 149,

FIGS. 148,149 Mires or targets ofophthalmometer.

3, the number of steps of overlapping indicates the number of diopters of astigmatism. If in this second position the mires separate, as in 2, Fig. 149, they must again be brought in contact and then rotated back to the former direction, in which they will now overlap and so indicate the amount of astigmatism.

If during the examination the patient looks away from the telescope, so that some portion of the cornea other than the center is presented, the refraction of this other part of the cornea will be indicated, differing, perhaps greatly, from that of the central portion of the cornea. Commonly, the first position in which the mires are brought in contact will be with the arm horizontal. But if it is found that in this position the black lines upon them do not correspond, do riot come opposite one another, the instrument must be rotated either way until these become continuous one with the other. The position of the patient during the examination should be made as comfortable as possible by having the height of the instrument or of the patients chair freely adjustable, and the examination must be completed quickly before the patient has become tired or restless. Ophthalmometry is of special value in cases of aphakia. In other cases the corneal astigmatism which it gives sug ests approximately the meridians and amount of tile total astigmatism.

Objective Methods for the Measurement of Refraction. Rays of light to be focussed on the retina must enter the eye with a certain degree of divergence or convergence for each degree of ametropia. Rays coming

THF. OP UTUALMOSCOPE DIRECT METHOD. 199 ii I from any point of the retina and passing out of the eye travel the same paths in the opposite direction, and leave the eve correspondingly convergent or divergent. The refraction of the eye may ~e determined by ascertaining what divergence or convergence must be given to rays in order that they shall be focussed on the retina. Methods that do this are subjective methods for meas refraction. Or we may take the rays from the retina and ascertain the degree of convergence or divergence which they have on emerging from the eve. Methods of doing this are objective methods for the determination of refraction.

The Ophthalmoscope. I, The Direct Method. The retina of the patient being illuminated by the ophthalmoscope, rays proceeding from it enter the eye of the surgeon and are focussed on his retina. If the surgeon is emmetropic parallel rays will be focussed on his retina, and the lens necessary to focus there the rays coming from the patient's retina is the lens necessary to make those rays parallel i. e. the lens which corrects the patient's ametropia.

To determine which lens does this the surgeon watches the finest visible details of the fundus of the patient's eye. When the focussing is imperfect, these details are blurred; when perfect, they are seen clearly. Suppose a case of hyperopia, illustrated in Fig. 150, in which P represents the eye of the

FiG. 150. Eye of patient and surgeon measuring H.

patient, and 8 the eye of the surgeon. The rays from the patient's retina leave his eye divergent, and are directed to focus back of the surgeon's retina. By trial the convex lens, L, is found, which, rendering the rays parallel (see the dotted lines), causes them to be focussed on the surgeon's retina. This lens, L, which renders parallel the rays coming out of the patient's eve, is the correcting lens, the lens which would make parallel rays from some distant object convergent enough to focus them upon the patient's retina.

fig. 151 Rays in myopia.

In myopia (illustrated in Fig. 151) the rays emerge from the patient's eye convergent. A concave lens, L, is required to render them parallel, so that they can be focussed on the surgeon's retina; and this concave lens is the cor
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200 DETERMIYING THE, REFRACTION OF THE, EYE.

recting lens which, placed in the same position, would render the parallel rays coming from some distant object sufficiently divergent to be focussed on the patient's retina.

If the patient's eye is emmetropic, the rays emerge from it parallel, and require no lens to secure their perfect focussing upon the surgeon's retina.

What has been said of other forms of ametropia holds for regular astigmatism ; only the ametropia differs in different meridians, and its correction in any one meridian affects the distinctness of lines in the fundus running at right angles to that meridian. Thus in an eye where the hyperopia in the horizontal meridian requires a 1 D. convex lens for its correction, and the hyperopia in the vertical meridian requires a 2 D. convex lens for its correc the I D. convex lens renders clear the vessels which run horizontally, and a 2 D. convex lens is required to render clear the vertical vessels; the difference between the two lenses, I D., is the amount of astigmatism present.

In the practical use of the ophthalmoscope to measure refraction the chief difficulty is due to the influence of accommodation in the eye of either the patient or the surgeon. In any case the effect of accommodation is the same as the effect of a convex lens, partly correcting hyperopia and diminishing its apparent amount; or adding to myopia, and to that extent increasing its apparent amount. Accommodation in the surgeon's eye is guarded against by practice. Yet always in young eyes, particularly when tired or irritated, there is a chance of some accommodation being present. In the patient's eye accommodation is reduced to the minimum by making the ophthalmoscopic examination in a thoroughly dark room of sufficient size, with the gaze fixed on blank space to encourage the complete relaxation of the ciliary muscle. Using these precautions, the influence of accommodation is still to be guarded against by choosing, as most nearly correct, the strongest convex or the weakest concave lens with which the details of the fundus are clearly visible.

In determining astigmatism one should first seek the strongest convex or weakest concave lens with which the vessels running in any one direction are still clearly seen. This lens will give the hyperopia or myopia present in the meridian at right angles to those vessels. These vessels run ill One of the principal meridians of astigmatism, the other being at right angles to this. Having determined the direction of the meridians and the lens required by one of them the next point is to find what lens renders clear the Vessels running at right angles to those seen clearly with the first lens. The difference between the two lenses gives the degree of astigmatism.

Another source of error in measuring refraction with the ophthalmoscope lies in the differences in the refraction of the same eye through different parts of the dioptric media. Thus the refraction at the centre is never the same as the refraction at the margin of the widely dilated pupil. In some eyes without a mydriatic the pupil dilated in the dark room shows a very different refraction at its margin from that at its center. The refraction at the center of the pupil is commonly what is of importance, and the error which might occur by measuring refraction through the edge of the pupil must be guarded against.

Again, the refraction of the eye varies at different parts of the retina. In a perfectly spherical eye the refraction at the macula is least hyperopic or most myopic. The refraction of the anterior parts of the retina and choroid may be highly hyperopic, even in eyes quite myopic at the macula. Then, too, the depth of the fundus may vary in other ways, as from posterior staphyloma or cupping or swelling of the optic nerve entrance.

THE 01111TUALMOSCOPE 1ND TRECT JIETJTOD. 201

It is therefore important to select for the measurement of refraction the details of a certain part of the fundus, generally as near the macula as possible. For astigmatism the examiner should take as test lines the vessels runDing from the disk to the macula, with their lateral branches. The large vessels as they pass upward and downward from the optic disk are particularly liable to protrude into the vitreous, and thus give an appearance of astigmatism when none is really present. The pigment layer of the retina and the vessels are usually the parts the refraction of which is measured ; but the attention may be fixed upon any other detail seen within the eye. In glaucoma the refraction of the bottom of the cup may be compared with tile refraction at the margin of the cup, or in optic neuritis the summit of the swelling may have its refraction compared with that of the retina beyond the swelling. By its refraction the surgeon may seek to locate an opacity in the vitreous. The distances in front of the plane of emmetropia indicated by a certain hyperopia, and the distances behind that plane indicated by an equal myopia, are shown in the following table, calculated for the average eye, having an antero posterior axis of 22.824 mm. (see also page 178).

H. Diminution. M. Increase. Diopters. H Diminution. M. Increase.

0.31 0.32 7 2.03 2.52
0.62 0 .66 8 2.28 2.93
0.92 1.01 1 9 2.53 3.35
1.21 1.37 10 2.78 3.80
1.50 1.74 15 3.91 6.28
1.76 2.13 20 4.90 9.31
Indirect Method
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method rays coming from the retina are focussed by the obj~ct lens to form a real inverted image between that lens and the surgeon's eye. When they emerge from the eve parallel, this image is formed at the principal focus of the object lens. When they emerge divergent, as in hyperopia, the image is formed farther from the lens. When they emerge convergent, as in myopia, it is formed close to the lens. By ascertaining the exact distance if the image from the object lens one may determine the refraction of the eye. This has been attempted by placing a screen where the inverted image is most distinct, and measuring its distance from the object lens, but this method is not of practical value.

A modification of this, the Schmidt Rimpler method instead of the screen has a source of light of peculiar form, enabling the surgeon to judge when it is accurately focussed. To use it the object, lens is placed exactly its focal distance from the principal plane of the eye, and by trial the surgeon finds what distance the ophthalmoscopic mirror must be held from the lens to give the most distinct view of the image of the source of light upon the fundus. This is obtained when the focus of the mirror coincides with the focus of the object lens; and a scale attached to the lens gives for each position of this image the amount of hyperopia or myopia to which it corresponds. Fig. 152 represents the course of the rays in this method, the solid lines indicating the rays reflected from the ophthalmoscopic mirror and entering the eye, and the broken lines, the ra s coming from the patient's retina to the eye of the y surgeon.

By the indirect method the nearer to the eye the object lens is held the Smaller is the inverted image in myopia, and the larger it is in hyperopia. The change of size due to the change of distance of the lens in front of the patient's eve varies with the degree of ametropia. Hence the presence and kind of ametropia of high degree can be recognized by varying the dis¬

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202 DE TERMINING THE REFRACTION OF THE EYE.

tance of the lens from the eye. In hyperopia the withdrawal of the lens from the eye makes the image smaller, in myopia it makes it larger. In FIG. 152 Course of rays in Schmidt Rimpler's method.

astigmatism such withdrawal makes the disk relatively larger in the direction of the meridian of greatest refraction, and relatively smaller in the meridian of least refraction. This change in the form of the disk is an evidence of astigmatism, most noticeable in high mixed astigmatism.

Skiascopy; Retinoseopy; The Shadow test. The method of determining refraction with the ophthalmoscope by the position of the inverted image is of little practical value, because of the difficulty of ascertaining the exact position of that image and its nearness to the eye. Skiascopy is essentially a method of determining the distance of the inverted image from the patient's eye with great accuracy. Fig. 153 represents an eye in front of which I

FIG. 153. Eye with convex lens placed before it.

is placed a convex lens ' causing the rays from a point, P, of the retina to be focussed at R ; the lens, L, may be regarded as composed of two lenses, L' and L" LI strong enough to render parallel the rays emerging from the eye, and L" able to take parallel rays and focus them at R. By subtracting the strength of L" from that of L, it is easy to get I', the correcting lens. Suppose L to have a strength of 5 D., and R to be 1 m. (the focal distance of a 1 D. lens) from L. L" will be 1 D., and 5 I = 4 D., the strength of L' required to correct the hyperopia.

The strength of L" to be deducted from that of L is found by determining the distance of R from the lens. In Fig. 154, representing the patient's eye (myopic) focussing the rays from A at C and from B at D, it will be noticed that if the surgeon's eye be placed at N, nearer the patient's eye than C D, the my reaching it from A (tomes through the upper part of the pupil, so that A will appear at a in that direction. But if the surgeon's eye be placed at N, beyond C D, the point A will appear to be located in the lower part of
SKIASCOPY; RETINOSCOPY; THE SHADO W TEST.203

the pupil toward a, the ray from A reaching the surgeon's eye from that direction. In the same way, from N, B will appear in the lower part of the pupil, and from N, in the upper part of the pupil.

FIG. 154 Showing how the rays cross, and so change their relative positions at the plane of reversal, D C.

This reversal in the apparent position of any given points of the retina occurs at the distance of C and D. Closer to the eye the point really above appears above ; the retina is seen in an erect image. Farther from the eve, the point really above appears below, and the point really below appears above ; the retina is seen in the inverted image. The change from the erect to the inverted image occurs at the point for which the patient's eye is focussed, either by its own myopic refraction or a lens placed before it; which point is, therefore, called the point of reversal.

The position of the point of reversal is determined with practical accuracy by observing the apparent direction of movement of light and shade in the pupil. The light is thrown on the eye with a mirror, usually a special form of the ophthalmoscope mirror, which may be either plane or concave. If plane, it should have a small sight hole, 2 or 21 mm. in diameter, with its 2 margin free from reflections. By turning the mirror slightly in different directions the light reflected from it on the patient's face, and also the portion entering his eve and falling on the retina, are made to move correspondingly. The movement of light and shade as it appears in the pupil is now watched. When the apparent movement is in the same direction as the real movement of the light on the retina, the erect image is being watched, and the surgeon's eye must be inside of the point of reversal, as at N (Fig. 154). When the apparent movement in the pupil is the opposite of the real movement of light on the retina, an inverted image is being watched and the surgeon's eye is beyond the point of reversal, as at N". By studying these opposite movements on the two sides of the point of reversal that point is located.

With a certain movement of the mirror there is always the same movement of the light on the face whether the mirror be plane or concave. Thus, when the mirror is made to face upward the light moves upward across the' patient's face. If the mirror is turned down, the light moves down across the patient's face. With the plane mirror the light on the retina always moves in the same direction as the light on the face in the same direction, or with the mirror. With the concave mirror the light on the retina always moves in the direction opposite to that of the light on the face always moves against the mirror (Fig. 155). The reason for this is that with the plane mirror the light enters the eye as though coming from an image (called the immediate source of light) as far behind the mirror as the real or original source is in front; but with the concave mirror the immediate source the point from which the light seems to come to the eye is a. small inverted image of the original source of light, formed in front of the mirror.

Hence, with the plane mirror, if the light in the pupil appears to move with the mirror with the light on the face the surgeon knows that the point

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204 DETERMIXTAW THE REFRACTIOATOF THE EYE.

of reversal is Dot between him and the patient. But when, with the same mirror, the apparent movement of light in the pupil is against the mirrorin the opposite direction to the movement of light on the face he knows that the point of reversal is between him and the patient that be is beyond the point of reversal and looking at the inverted image. On the other hand, when with the concave mirror the light in the pupil appears to move with the mirror with the light on the face the surgeon knows that this is the opposite of the real movement of light on the patient', , retina, and that, therefore, lie is watching an inverted image. But if with the concave mirror the light in the pupil appears to move against the mirror against the light on the face knowing this to be the direction of the real movement of light OD

FiG. 155 Course of rays in skiascopy with concave mirror: A A, one position of mirror giving immediate source of light at 1, and illuminating retina toward a; B Be another position of mirror with imme source of light at I and retina illuminated toward b.

the retina, he knows he is watching an erect image, the point of reversal being somewhere behind him.

Rate of Movement, Form, and Brightness of the Light area. Besides the direction of the movement of light and shadow, the brightness and form and rate of movement of the illuminated area in the pupil are, of practical importance. At the point of reversal a single point of the retina appears to occupy the whole area of the pupil. As the point of reversal is departed from, more and more of the retina is seen in the pupil. Hence, near to the point of reversal a slight movement of the light area on the retina will appear to carry the light entirely across the pupil the light and shadow move in the pupil swiftly. But at a greater distance from the point of reversal they move slower.

The apparent form of the ligbt area in the pupil is also modified by the Dearness of the surgeon to the point of reversal. The actual form of the liglit area on the retina is commonly circular. This circle appears greatly magnified when the surgeon is near the point of reversal, and only a small part of its margin can be seen in the pupil at one time, the boundary between light and shade appearing almost a straight line. While far away from the point of reversal, especially if the surgeon be near the pupil, the whole area of retinal illumination may be seen in the pupil as a complete circle. More important still in determining the apparent form of the light area are regular astigmatism, aberration, and irregular astigmatism, to be presently considered.

The brightness of the light area in the pupil depends on the concentration of the light thrown into the eye and the extent to which the retina is magnified. The immediate source of light being commonly near the mirror, the light is most concentrated on the retina when the mirror is held near the point of reversal. But just at the point of reversal the magnification of the

PRA CTICAL A PPLICA TION OF SKIASCOP Y. 1 205

retina makes the illumination appear feeble, so that the brighest area of light in the pupil is obtained about I or 2 1). from the point of reversal.

Practical Application of Skiascopy. The room should be thoroughly darkened, and the source of light shaded with an opaque chimney having a circular opening opposite the brightest part of the flame.

For the plane mirror the source of light should be so arranged that it can be brought quite close to the mirror and moved with the mirror to or from the patient's eye, and the opening in the shade should be 5 or 10 mm. in diameter. 0

For the concave mirror the flame is to be back of the patient's head, generally as far from the mirror as possible; and if a shade is used, the opening should be 10 to 20 mm. in diameter.

When not otherwise stated, the following description refers to skiascopy with the plane mirror. It may be applied to the concave mirror by reversing the significance of the direction of movement of the light in the pupil :

1. Hyperopia. Without a lens the light moves across the pupil with the light on the face. The convex lens, I (Fig. 153), strong enough to overcome the hyperopia and to give a point of reversal, R, is placed before the eye. The surgeon, then varying his distance from the patient's eye, tries the movement of light and shadow alternately from within _R, where the movement is with, and from beyond R, where the movement is against, the light on the face. The position of the point of reversal is thus determined. Its distance from the patient's eye is then measured or estimated. This is the focal distance of the over correcting effect of the Ions I, which over correcting effect, subtracted from the whole strength of the lens, leaves the strength required to correct the hyperopia.

II Suppose a 5 D. convex lens placed before the eye gives movement with the light on the face at 20 in. (51 cm.), and against the light on the face at 30 in. (76 cm.), the point of reversal taken as midway is at about the focal distance of a 1.5 diopter lens; the over correcting effect of the 5 D. lens equals 5. 1.5 = 3.5 D. the strength of the lens required to correct the hyperopia.

In making the final determination of the refraction, if the freedom of the eye from astigmatism and aberration allows the movement of light and shadow to be easily watched at a greater distance, a weaker lens, giving a point of reversal farther from the e e may be used. But if there be much irregular astigmatism or aberration, the determination can be more correctly made with a point of reversal still closer to the eye.

2. Myopia. From the myopic eye the rays emerge already convergent to meet in a point of reversal that can be determined without the use of any lens, except in myopia of very low degree. Commonly, however, it is too close to the eye for accuracy, and a concave lens partly correcting the myopia should be placed before the eye, and the remaining myopia measured and added to the strength of the concave lens for the total myopia.

For example, in a case of myopia of 10 D., a concave 9 D. lens being placed before the eve, the point of reversal is found at 1 m. This corresponds to myopia of 1 D., which, added to 9D., the strength of the lens, gives 10D., the total myopia. In the case of very low myopia, as only 0.25 D., a convex I D. lens is placed before the eye, and the point of reversal found in this case at 31 in. (78.5 cm.), indicating 1.25 D. of myopia. From this, by subtracting 1 D., the strength of the lens, we get 0.25 D., the myopia originally present.

3. Emmetropia is shown when the convex lens placed before the eye gives a point of reversal just at its focal distance.

206 DETERAMINING THE REFRACTION OF THE EY1

4. Regular Astigmatism. In regular astigmatism the rays coming from the retina emerge from the cornea with different degrees of divergence or convergence in different meridians. For the two principal meridians there are, therefore, always the two separate points of reversal, their distance apart indicating the amount of astigmatism. When in such an eye a point of' reversal is found, it is soon discovered that it is a point of reversal only for the movement in one direction. The surgeon's eye, placed at this point, sees the retina magnified enormously in the direction of' the one inidian, and magnified much less in the other prinericipal meridian. This makes the light area in the pupil appear elongated in the direction of the first meridian, giving it a band like appearance, shown in Fig. 156.

To make this band like appearance most distinct, the immediate source of light should be at the point of reversal for the other meridian. With the plane mirror the surgeon must place his eye at the point of reversal nearest the eye, where lie will get movement undistinguishable in one meridian, and with the light on the face in the other. The original source of light is then to be pushed away from the mirror, its reflection (the immediate source) retreating corresponding] y behind the mirror until it reaches the point of reversal for the other principal meridian. The direction of the band like appearance is to be carefully noted as the direction of the principal meridian of greatest refraction the direction for the axis of a convex cylinder that would correct the astigmatism. The direction of the other principal meridian, the direction for the axis of a concave cylinder to correct the astigmatism, will be at right angles to this.

With the concave mirror the surgeon's eye should be placed at the point of reversal that is the farthest from the eye and the original source of light brought closely to the mirror, causing its conjugate image (the immediate source of light) to go farther from the mirror and closer to the patient's eye, until it reaches the nearer point of reversal, and the band like appearance appears most distinct in the meridian of least refraction. In this position the band cannot be seen to move in the direction of' its length, but at right angles it also moves with the light on the face.

Having determined the direction of the principal meridians of astigmatism, the hyperopia or myopia in each is to be measured separately, just as hyperopia or myopia would be measured in any other case, with the light as near the plane mirror as possible or as far away as convenient from the concave mirror. The difference of refraction between the two meridians is the amount of astigmatism. When it has been determined, a cylindrical lens correcting it should be placed with the proper spherical lens before the eye, and the test applied to ascertain if the correction is really complete.
Aberration. In most eyes the refraction at the edge of' the dilated pupil is more myopic or less hyperopic than at the center. In this form, called positive aberration, the point of reversal for the edge of the pupil is nearer the eye than the point of reversal for the center, and from the latter point movement of light against the light on the face is to be ]noticed in the edge of the pupil. This light in the edge is brighter than the light at the center of the pupil, and great care must be taken to avoid error on this account. When the center of the pupil is the more myopic it is called negative aberration. The circular distribution of aberration largely determines the shape of FiG. 156. Band like appearance in shadow test
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SUBJECTIVE METHODS OF TESTING _REFRACTIOIV~ 207 light and shadow in the pupil, making it more circular when otherwise it would be of different shape, as in regular astigmatism.

When aberration is present the point of reversal for the margin of the. pupil may be close to the surgeon's eye, while the point of' reversal for tile center is far from it. In this case the movement of the light in the center of the pupil will be slow, while in the margin it will be swift. The light area then appears to swing around a fixed center, and assumes an angular shape, shown in Fig. 157. This is the appearance presented in conical cornea where the center of the pupil is more myopic than the margin.

Irregular Astigmatism. The differences of refraction due to the lens changes preceding cataract, or the irregularities of the cornea following phlyetenular keratitis, break up the light and shadow in the FiG. 157. Angular appear into small irregular areas. The surgeon must ance in high aberration.

find which of these areas is most likely to be of use for eye work, and measure the refraction in it. To do this it may be necessary, on account of the smallness of the area, to come quite close to the patient's e ye Here a small source of light and a small sight hole in the mirror are of great importance.

Subjective Methods of Testing Refraction. To determine what lens is required to bring perfectly to a focus the rays entering the eye we may resort to tests based upon a single point of light. Thomson's ametrometer consists of two small gas flames, one fixed and the other movable along a graduated arm, which can be revolved about the first as a center. The distance of the two flames apart when their diffusion areas appear to just touch each other gives the degree of ametropia in the meridian parallel to the graduated arm. Holz uses two small holes in a disk placed in front of a window or lamp flame. To the patient having astigmatism each of the points of light so obtained appears elongated, and by turning the disk so that these elongated images lie in the same line, an index enables the surgeon to read off the direction of the principal meridians of astigmatism.

simple optometer consists essentially of a convex lens which is placed close to the eve, and a graduated arm extending from it on which moves a card bearing test type. In emmetropia the type can be seen distinctly only as far as the focal distance of the lens. In hyperopia it is read to a greater distance and in myopia only to a lesser distance, corresponding to the degree of the ametropia.

Either of the above subjective methods may be found of service where others are not available; but they are not commonly used, and by the subjective method of determining refraction is commonly meant the method with trial lenses and test letters.

The trial case contains a sufficient series of spherical and cylindrical lenses, with trial frames in which they can be placed before the eye, prisms, solid, pin hole, and slit disks, and colored glasses. By combining two or more lenses together a very few convex and concave lenses can be made to answer for any case of ametropia, but where many cases are to be tested convenience and economy of time demand a fairly complete set of lenses. This may include pairs of convex and concave lenses, with 0.12 D. intervals to 1.5 D., 0.25 D. intervals to 4 D., and 0.50 D. intervals to 8 D., for both sphericals and cylindricals. Then for the sphericals, I D. intervals to 20 D., with, perhaps, 25 D. and 30 D. added. The prisms may run by 1 centrad intervals to 10, with the 12, 15, 20, and 30 centrad prisms in addition.

208 DETERMINING THE REFRACTION OF THE EYE.

To use the trial lenses, test letters suited to the distance adopted are to be hung in a strong light, either natural or artificial, the latter being preferable because it can be made more uniform. The test card should always have one or two lilies of letters smaller than those intended to be read at the distance adopted. Thus for 6 m there should be a line of 5 m. letters. Some patients have visual acuteness greater than 6
6'

Use of the Trial Case. The pin hole disk furnishes a ready means of distinguishing between imperfect vision due to ametropia and imperfect vision due to other causes. In the former ease the placing the pin hole opening before the eye lessens the diffusion areas upon the retina and improves vision ; if the imperfection of vision is not due to ametropia, the pin hole disk rather makes it worse.

The slit is used in discovering astigmatism of moderate or high degree and the direction of its principal meridians. In astigmatism the diffusionareas on the retina are wider in the direction of one principal meridian than in the direction of the other. The slit limits them at right angles to its, length, but not in the direction of its length. When, therefore, it is placed before the eye, turned in one direction, it cuts down the diffusion area in its larger dimension, giving the greatest improvement of vision. At right angles to this it limits the diffusion area in the other direction, in which it is already most limited, and gives the least improvement of vision. These directions of the principal meridians of the astigmatism being found, the slit may be turned in the direction of one meridian and spherical lenses, tried until one is found correcting the ametropia in this meridian, and giving the best vision obtainable through the slit. The same is done for the other meridian, and in this way the correcting lenses, both spherical and cylindrical, may be determined. This test has practical value as an approximate and confirmatory test.
In the ordinary use of test lenses each eye is tested separately, the other y being covered by a solid disk or ground glass. When accommodation is absent the aim is to find the lenses which give the best vision. Some idea of the ametropia is given by previous objective tests and the acuteness of vision without a lens. The Concave or convex lens expected to correct it approximately is placed before the eye and the vision with it Doted. Then weak additional convex or concave lenses are held in the hand in front of this, trying first the one, then the other. If the first lens has been Convex, and the additional convex spherical further improves vision, a convex lens correspondingly stronger is substituted. The trial is then repeated, and this is continued until a lens is found which can neither be increased nor diminished in strength without lessening the acuteness of vision.

If file eye is free from astigmatism, this is the lens desired ; but to test such freedom from astigmatism cylindrical lenses should be tried. The cylindrical lens is to be held in front of the spherical lens selected, and its axis turned in different directions, as vertical, horizontal, and oblique to the right and to the left. For this purpose the astigmatic lens, convex in one meridian and equally concave in the meridian perpendicular thereto, is preferable to either the convex or concave cylinder. Such astigmatic lenses should be included in the trial case.
Having ascertained that in some one direction the cylindrical lens improves vision, such a lens is to be placed in the trial frame, either with the spherical lens already there or with one slightly weaker if the cylindrical lens is of the same kind, or a slightly stronger if the cylinder is of the opposite kind. Thus, if the original spherical lens was + 2 D., and + 1 D. is the

M YDRIA TICS.
I
r
cyIinder to be combined with it, the spherical should be changed to + 1.5 A If it is preferred to use a I D. cylinder, the spherical lens should be changed to + 2.5 D. After this the cylinder is to be slightly turned, first to one side and then to the other, the patient being required to indicate when the turning makes his vision worse. This is repeated until it is pretty certain just what direction of the cylinder axis gives the best vision. Then weak convex and concave spherical lenses are to be tried in front of the combined spherical and cylindrical lenses, to see if either will still further improve the vision, and these are followed with the astigmatic lens and a new trial of the direction for the axis. This routine is to be repeated Until any change in any factor of the combination impairs the acuteness of vision.

The combination thus arrived at is the correcting lens of the eye for the distance at which the test is made. If this distance be 4 or 6 m., 0.25 or 0. 17 D. must be subtracted from the convex or added to the concave spherical lens to make it the perfect correction for truly parallel rays from more distant objects. The same process is repeated for the second eye.

When the power of accommodation is present, the aim must be to find the strongest convex or the weakest concave spherical lens that gives the best vision. Cylindrical lenses will be tried as above, preferably before attempting the final determination with the spherical lens. The determination of the spherical lens is best effected by testing both eyes at once and beginning with convex lenses that are too stiong or concave lenses that are too weak to permit of the best vision. Then, if convex, before removing such glasses the next weaker lenses should be placed before the eyes. In this way whatever relaxation of accommodation has been secured tinder the first lenses is preserved. If vision is yet not perfect, a still weaker lens is substituted in the same way, and SO On Until the best vision of which both eyes are capable is obtained.

eyes are then to be tested separately by covering each of them alternately. If it is found that only one eve has attained to its best vision, the lens before the other eve is to be still further weakened until it, too, has obtained its best vision. The lenses thus chosen will be found to correct the total hyperopia in the majority of even young persons.

In myopia the spherical lenses are to be made successively stronger, and when the best vision is obtained the eyes are to be tested separately by alternate covering.

MYDRIATICS.

The drugs atropin, duboisin, hyoseyamin, hyoscin, daturin, and scopolarnin, alkaloids obtained from members of the Solanacea, and homatropin, a derivative of atropin, constitute the true mydriatics. Applied to the eve, they produce dilatation of the pupil and paralysis of the accommodation, which after a time, varying with the drug and the amount of it employed, entirely passes away. In some cases the dilatation of the pupil is of use in the determination of refraction, since it renders easier. the use of the ophthalmoscope, skiascopy, and the test lenses. But the chief value of these drugs in this connection lies in their action as cycloplegics. By paralyzingthe ciliary muscle they eliminate the influence of accommodation.

In healthy eyes a single drop of one of the following solutions is usually sufficient to accomplish this: atropin, 1 : 100, duboisin, hyoscyamin, or scopolamin, I : 250. Of homatropin hydrobromate a single drop of even a saturated solution will not paralyze the accommodation. It must be used by repeated instillations of a 2 to 4 per cent. solution at short intervals. Any of the other drugs will prove effective in weaker solutions if the instillations

14
IIII

210 DETERMINING THE REFRACTION OF TH E.EY E.

are repeated. In practice it is customary to prescribe either atropin, duboisin, or hyoscyamin in solutions of the strength named, to be instilled at the patient's home three times a day for one or more days. The repeated instillations are necessary to guard against their possibly imperfect character.

Homatropin should be instilled by the surgeon or a trained assistant, and the instillations repeated every five or ten minutes until from four to six have been made ; and after its us~ the determination of the refraction should be completed within one or two hours, a's it often begins to lose its control of the ciliary muscle soon after that time.

the choice of the mydriatic homatropin has the advantage of greater brevity of action. The accommodation completely recovers from its effect, usually within forty eight hours, while after atropin two or three weeks are required before it is quite recovered, and after the use of the other drugs named from one to two weeks must elapse. Scopolamin, I : 500, is an efficient mydriatic, used by making two instillations one hour apart. Accommodation will completely return in six days. Even weaker solutions may be efficacious.

In using these drugs certain alarming intoxicating effects must be borne in mind. While in the amount mentioned most people do not experience these, in exceptional cases a single drop of one of the solutions mentioned, except of homatropin, may cause severe symptoms of intoxication. These are dryness and redness of the throat and skin, with delirium and incoordination of movement, especially inability to walk. The patient is not usually much disturbed, but his friends may be greatly alarmed, although from any such dose these symptoms are quite unattended by danger. On their appearance the use of the drug must be suspended, the patient kept quiet, given water freely, and, if decidedly delirious, small doses of an opiate.

Homatropin is much the least likely to produce such symptoms, and duboisin, hyoscyamin, and scopolamin (which may be but different names for the same drug) are the most likely to produce them. In the eyes of a few persons these mydriatics produce marked conjunctival irritation or inflammation, and the homatropin solutions mentioned always produce a temporary hyperemia of the conjunctival and pericorneal vessels during the period of absorption.

Cocaine a drug of an entirely different class, possessing little or no power to paralyze the ciliary muscle, may be useful to dilate the pupil in persons over fifty years of age whose pupils are small and whose power of accommodation is not sufficient to interfere with tests for refraction. A single instillation of a 2 per cent. solution is followed after thirty minutes or an hour by decided enlargement of the pupil, yet with very little inconvenience and no danger.

All drugs which cause dilatation of the pupil, except cocaine are dangerous in eyes presenting the essential changes of glaucoma, since they may produce a glaucomatous outbreak. But if such a revelation of the presence of glaucoma is promptly met by the proper treatment, it can hardly be regarded as unfortunate for the patient. No eye in which this accident can occur is likely long to escape glaucoma, and without the mydriatic the advent of this disease might be so insidious as to escape detection until irreparable damage had been done.

Whether mydriatics should be used in the great mass of refraction cases is a debated question. That with their use the determination of refraction can be more. certainly exact cannot be doubted. The question is as to

I GENERAL PLAN OFEXAM11VATION, 211 whether the increased certainty and accuracy are worth the discomfort and loss of time from ordinary occupations that the mydriatic causes. In deciding this question the desires of the patient and the appreciation of exactness in his work on the part of the surgeon will be the determining factors.

Table of the. Defferent
Name of drug and salt commonly used. Atropin sulphate . . . . Daturin sulphate . . . . Hyoscyamin sulphate . , Duboisin sulphate . . . . Scopolamin hydrochlorate
Hornatropin hydrobromate
Cocain hydrochlorate . .
0
30 60 75 75 75 1 Not comparable.
1: 120 1: 200 1: 240 1: 240 1: 1000 1: 40
1 125
Hydriatic.3.
C. pt's!
U:
12 min. 10 1 10 10 15 15 it
30 "
I hour. 40 min 40 40 1 hour. I It
1 41
0
4 davs. 3 2 2
12 hours 3 "
i 2 "
15 days. 10 11
8 6 2
12 hours.
With cocain the anesthetic effect passes off before. dilatation of the pupil is fairly commenced. The new local anesthetic, eucain, is usually regarded as having no mydriatic effect, but Wagenmann states that, by a strong solution repeatedly applied, some dilatation of the pupil may be produced.

GENERAL PLAN or EXAMINATION.

The acuteness of vision for each eye separately, and the near point of distinct vision, should be first ascertained. If vision be imperfect, the pinhole disk may be tried to see if such imperfection is due to ametropia or to other causes. Then the eye should be examined with the ophthalmoscope by the direct method. This gives a rough approximation of the refraction, especially as regards hyperopia or myopia. After this skiascopy may be used or a mydriatic employed. Then the corneal astigmatism may be measured with the ophthalmometer. When the mydriatic has produced its full effect, the refraction is to be carefully measured by skiascopy, and then to be tested by the trial lenses, commencing with the glass fixed upon by the shadow test. The value of the results obtained by the subjective method depends largely on the patient not being wearied by prolonged testing. After the correcting lenses have been thus ascertained, the eye should be allowed to recover from the mydriatic and the trial with lenses repeated. Such a routine, carefully followed by one of fair skill, cannot fail to give accurate and reliable results.

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