Skiascopy and its practical application to the study of refraction / by Edward Jackson, A.M.,M. D.

  • Jackson, Edward, 1856-1942.
Date:
1896
Catalogue details

Licence: Public Domain Mark

Credit: Skiascopy and its practical application to the study of refraction / by Edward Jackson, A.M.,M. D. Source: Wellcome Collection.

Provider: This material has been provided by University of Bristol Library. The original may be consulted at University of Bristol Library.

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    for the less myopic meridian [JI, figure 9] is reached, the movement in its direction ceases, but it is impossible, at this point (with the plane mirror), to bring out so distinct a band as was seen in the direction of the other meridian. Withdrawing still farther, the light in the direction of the less myopic meridian begins to move against the light on the face, at first very rapidly as compared with the movement in the more myopic meridian. But, as the ob- server withdraws farther from this second point of reversal, the difference in rate of movement in the two meridians becomes less noticeable. With the concave mirror, the same series of appearan- ces are presented, except that the directions of movement are reversed—the erect image seen from within the point of reversal giving movement of the light in the pupil against the movement of the light on the face, and against the mirror; and the inverted image seen from beyond the point of reversal giving movement of the light in the pupil with the mirror and with the light on the face. (See page 26.) With the concave mirror the meridian in which it is possible to bring out the band-like appearance of the light most dis- tinctly is the meridian of less myopia. With such a mirror it will also be necessary to bring about the series of changes in the movement of the light area, which has been referred to, by changes of the lens placed before the eye, and not by changes in the observer's distance from the eye studied. Direction and Movement of the Bands in Astigma- tism.—The reason for the constant conformity of the di- rection of these bands of light to the principal meridians of refraction is obvious from their dependence on the magnifi- cation of the retina. That conformity sharply separates them from the somewhat similar appearance seen near the point of reversal in eyes free from astigmatism (page 31).