Schweigger uses the indirect method in comparison with the direct method to determine the presence of astigmatism. It has already been explained that the disc appears elongated in the direction of the meridian of greatest refraction when seen in the upright image, so it is only necessary to state that the reverse obtains in the inverted image.
Skiascopy (Retinoscopy, or the Shadow Test).- This is a method of determining the refraction by observing the direction in which the light appears to move across the pupil when the mirror is rotated in various directions. The test should be made in a dark room at a distance of one metre from the patient. It is convenient to place a tape on the wall extending from the position of the examined eye to a distance beyond that of the observer. This is graduated in centimetres, and at appropriate intervals the corresponding number of dioptres marked. Either a plane or concave mirror can be employed, but the preference is with the former. The light is covered with an opaque asbestos shade having an aperture 1 cm. in diameter. If the plane mirror is used the light should be near the surgeon, but if the concave mirror is used, behind the patient. The arrangement is shown in Fig. 25.
Skiascopy has been elaborated especially by Jackson, whose description is here followed and whose work is the ablest published.
By rotating the mirror the area of light it throws on the face is made to move in the direction the mirror is rotated. Those rays which enter the pupil from a small light area on the retina, which also moves when the mirror is rotated. This area moves with the light on the face when the plane mirror is used and in the opposite direction if the concave mirror is employed.
For the plane mirror this movement is shown in Fig. 26.
When the mirror occupies the position A A the rays from L which enter the eye are reflected as if they came from L, and after passing through the eye are condensed at a, on the lower part of the retina. At the same time the light falls on the lower part of
Skiascopy by Dr. Edward Jackson. the face. If now the mirror is rotated to B B the light which enters the eye is reflected as if it came from l, and is condensed toward b, on the upper part of the retina. Simultaneously the light which falls on the face moves upward also. The same movement of the light with the light on the face occurs in hyperopia and myopia as well as in emmetropia.
The movement of the light area on the retina caused by a concave mirror is shown in Fig. 27.
When the mirror occupies the position A A the rays which enter the eye come from the focus of the mirror at l and are condensed towards a, on the upper part of the retina, while the light falls on the lower part of the face. When the mirror is rotated to B B the rays which enter the eye come from l and are condensed towards b, on the lower part of the retina, while the light on the face moves upward. The same is true in hyperopia and myopia.
The real movement of the light on the retina, as it would appear from behind, is always with the light on the face with the plane mirror and in the opposite direction with the concave mirror.
In our examination, however, we do not see it in that way, but we watch the apparent movement as seen through the pupil. When the plane mirror is used the apparent movement in the pupil and the real movement on the retina are the same when the observer sees an erect image, and in the opposite direction when he sees an inverted image.
The reverse of this obtains with the concave mirror.
The rays of light coming from a myopic eye are convergent and cross each other at its far point, and proceed divergently. The point B (Fig. 28), at which they cross and which corresponds to the far point, is known as the point of reversal. As has been explained under myopia, the distance of the far point from the eye represents the focal length of the glass required to correct it, and, therefore, if the point of reversal is known the amount of myopia is also known.
Retinoscopy is an accurate method of determining the point of reversal.
In the following description it is assumed that the plane mirror is used, though it will apply equally to the concave mirror if we reverse the movement in the pupil and change the lenses. If the mirror is held closer to the eye than its point of reversal, as at A (Fig.28 ), an erect image is seen, and the light in the pupil will seem to move with the light on the face. Beyond the point of reversal, as at C, an inverted image is seen, and the light will appear to move in an opposite direction to the light on the face.
At the point of reversal it is impossible to see which way the light moves, and the illumination is much more feeble. At a distance of one or two metres from the point of reversal the illumination is very bright, but as the distance increases it becomes more and more feeble. Without altering the rapidity of the movement of the mirror, the apparent movement of the light is more rapid as we approach the point of reversal.
While the test depends mainly on the direction of the movement of the light in the pupil, the degree of illumination and the rapidity of movement aid in arriving at a diagnosis.
Myopia is estimated by finding the nearest point that an inverted image is seen (C, Fig. 28 ) and the most distant point at which an upright image is seen, as at A. Midway between the two is the point of reversal, B, whose distance from the eye should be noted on the tape for that purpose, and the number of dioptres corresponding is the measure of the myopia. Thus if C is at 55 cm. and A at 45 cm., which corresponds to 2. D. giving a myopia of that amount. When the myopia is of a high degree, the point of reversal lies very close to the eye, and in this situation a slight error in marking the distance may mean an error of a dioptre or more in estimating the myopia; whereas if a similar error is made when the point of reversal is situated at a metre or more from the eye it is unimportant. Therefore, in high degrees of myopia, in order to check results, it is well to correct all but about one dioptre by placing a suitable concave lens in a frame before the eye and measure the remainder, which is to be added to the lens in the frame.
If the myopia is less than one dioptre, the point of reversal lies so far away from the eye that when near it one cannot see which way the light moves. In this case put a weak convex glass in the frame to increase the myopia, then determine the point of reversal, and deduct the convex glass from the amount of myopia found.
Hyperopia gives an upright image no matter how far we recede from the eye, because the rays leave it divergently. In order to obtain a point of reversal, it is necessary to convert it into an artificial myopia by putting a convex glass in the frame and then finding the point of reversal as in myopia. The amount of myopia is to be deducted from the convex lens, the hyperopia being represented by the remainder.
Thus, if a convex 5. D lens is placed in the frame, and the point of reversal is found at 1 metre (1. D), the hyperopia will be 4. D. That is 4. D out of the five were necessary to render the divergent rays parallel; and the other dioptre to bring them to a focus at 1 metre.
Emmetropia acts the same as hyperopia, but when a convex lens is added the myopia produced equals the strength of the lens, proving that the rays were parallel in the first place.
In astigmatism the refraction of the principal meridians is obtained in the same way as in myopia or hyperopia. In order to determine the refraction of a certain meridian, it is necessary to rotate the mirror about an axis at right angles to it, which causes the light to traverse the length of the meridian. The direction of the meridian is revealed by the areas of light assuming a band-like shape as its point of reversal is approached. This is most marked in the higher degrees of astigmatism. Near the point of reversal, where the band-like appearance is most distinct, it is easy to cause the apparent movement from side to side, but more difficult in the direction of the length of the band. The latter, however, is to be watched, as it determines the point of reversal.
If the astigmatism is of low degree, this band-like appearance may not be perceptible; but when we have determined the point of reversal of one meridian it will be found that there is still distinct movement, either upright or inverted, in the direction at right angles to this. Supposing the case to be one of myopic astigmatism, either natural or artificial, and the surgeon starts at a point nearer the eye than either point of reversal and gradually withdraws from it, the following phenomena occur: At the start the movement will be with the light on the face in all directions. Withdrawing to the nearest point of reversal there will be no movement in the meridian whose reversal-point it is, but direct movement at right angles to it. Between the two points of reversal the former meridian gives an inverted movement and the latter direct. At the farthest reversal-point the direct movement for the meridian ceases and the other remains indirect. Beyond both points of reversal the movement is against the mirror in all directions. The degree of the myopia of both principal meridians, either natural or artificial, having been determined, the astigmatism present is represented by the difference between them. As a final test the cylinder correcting the astigmatism should be put in the frame together with the concave or convex lens which will remove the point of reversal to about 1 metre, and the movements watched again.
Professor of Ophthalmology in the College of the New York Ophthalmic Hospital; Surgeon to the New York Ophthalmic Hospital. Visiting Oculist to the Laura Franklin Free Hospital for Children; Ex-President American Homoeopathic Ophthalmological, Otological and Laryngological Society. First Vice-President American Institute of Homoeopathy : President Homoeopathic Medical Society of the State of New York ; Editor Homoeopathic Eye. Ear and Throat Journal : Associate Editor. Department of Ophthalmology, North American Journal of Homoeopathy, etc.