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.

    96/122 (page 90)
    The movement in the pupil without any lens will be very slow, and the light areas round and dim. Judging from this appearance the first lens tried may be the concave 5. D. With it, the light in the pupil will appear more brilliant and its movement will be more rapid, but it will still be with the movement of the light on the face. Next, the concave 8. D. will be tried. The movement of light will be found still more rapid, but now against that of the light on the face. With the concave 7. D., it will be found equally rapid, but ivith the light on the face. With the 7.5. D. it will not be distinguishable. Hence, the 7.5. D. lens leaves 1. D. of myopia still uncorrected, and this added to the 7.5 D. corrected by the lens gives 8.5. D., the total myopia present. If the myopia be of low degree, the test without a lens will show either no distinguishable movement of light in the pupil [for myopia of 1. D.], or movement in the pupil against the movement of light on the face [for myopia of less than 1 D.]. In the former case the test is to be repeated with very weak convex and concave lenses [0.25. D. or 0.50. D.]. The convex will give a movement of the light in the pupil ivith the light on the face, and the concave movement against the light on the face. If the movement is found to be against the light on the face to start with, the convex lenses are to be tried, commencing with a 1. D. lens, which will cause the move- ment with the light on the face, and will show, therefore, that the refraction is myopia and not ernmetropia, or low hyperopia. The weaker lenses are then to be tried, and the one which causes 1. D. of myopia thus ascertained. Since this lens is added to the myopia of the eye to cause 1. D. of myopia, it must be subtracted from 1. D. to find the amount of myopia originally in the eye ; the difference between it and 1. D. being the myopia present. Thus, in a case of myopia of 0.50. D. the light will be