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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    plane mirror, P P, with the eye of the observer at F, and the light L pushed off from the mirror, so that the rays enter the eye as though they came from H, and are per- fectly focused on the retina in the horizontal meridian, ren- dering most distinct the appearance of a vertical band. For illustration, suppose a case [which the student will do well to reproduce for actual study, either in the artificial eye or by lenses placed before the living eye] having com- pound myopic astigmatism, the vertical meridian of the cornea being 2 D. myopic and the horizontal meridian 1 D. myopic. When, with the plane mirror, the observer's eye is one-half metre from the observed eye, it will be at the point of reversal for the vertical meridian, and in a position to see a vertical band of light. But, if the source of light be placed as close to the mirror as possible, the rays from it will be the more accurately focused upon the retina in the vertical meridian and more diffused horizontally, so that the real form of the retinal light area will be rather that of a horizontal line or band. Now, from the observer's position, the retina is most magnified in the vertical direction, and this vertical magni- fication would cause a point of light on the retina to appear as a vertical band in the pupil; but, with the light area really in the form of a horizontal band, the effect of the vertical magnification is largely neutralized, and the ap- pearance in the pupil may be quite indefinite. To bring out the band-like appearance : While keeping the observer's eye and mirror in the same position, the original source of light must be pushed off from the mir- ror one-half metre, the immediate source then retreats cor- respondingly behind the mirror, and approaches the posi- tion of the point of reversal in the horizontal meridian, one metre from the eye. With the light and mirror in this relation to the eye, the rays are perfectly focused upon the retina in the hori-