Abstract
AN account of the recognised methods of investigating the spherical aberration of the eye is given by Tscherning, “Rapports presentés au Congrès international de Physique réuni à Paris en 1900,” tome iii., pp. 551-557. These methods, for the most part require special experimental appliances, and for some to succeed it is necessary to resort to cocaine or homatropine injections in order, to increase the size of the pupil. The following method/which requires no special apparatus or preparation, appears to have escaped observation, and may therefore be worth describing. Place a piece of white paper, on which a broad black band has been ruled horizontally, just beyond the shortest distance of distinct vision from the eye, and while looking at the upper edge of the black band, cover the pupil progressively from, below by means of a card with its upper edge horizontal, placed as near as possible to the eye. At the moment when the pupil is all but completely covered, the edge of the black band will be seen to suffer a depression, its original position being regained on uncovering the pupil. On raising and lowering the card at a rate of once or twice a second, this displacement is very marked. The best success is obtained in a fairly dim light, when the pupil is expanded; care must be taken to keep the eye carefully focused on the edge of the black band, or an exaggerated displacement, due to relaxation of the accommodation of the eye, may result. The above experiment shows that, when accommodated for near vision, the optical system of the eye is over-corrected for spherical aberration, the rays transmitted near the edge of the pupil being insufficiently deviated. To prove this, let us suppose the edge of the black band to be situated on a continuation of the optic axis of the eye. Then, provided the accommodation of the eye is correct, the rays traversing the middle of the pupil will form an image of the edge of the black band at that point of the retina which is cut by the optic axis. If the rays transmitted through the upper peripheral portion of the pupil are insufficiently deviated, they will cut the retina at a point above the true image, and owing to the mental inversion of retinal images, an image; apparently below the true image will be observed. On covering up the pupil from below, the true image is obscured, and that formed by the rays traversing the upper edge of the pupil is-alone seen.
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EDSER, E. Spherical Aberration of the Eye. Nature 67, 559 (1903). https://doi.org/10.1038/067559a0
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DOI: https://doi.org/10.1038/067559a0
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