Text-book of ophthalmology . Eye is Focused for the Source of Light. observer only in case the latter was in identically the same spot as the source of solution of this problem is another of the discoveries that we owe to the geniusof Helmholtz. The conditions are different when the eye is not focused for the source of lightbefore it. Suppose, for example, that the eye is hypermetropic (Fig. 16). Then therays springing from the illuminated portion of the retina, I, leave the eye as a divergentbeam, so that only a part of the rays are returned to the source of light, L, while anotherp
Text-book of ophthalmology . Eye is Focused for the Source of Light. observer only in case the latter was in identically the same spot as the source of solution of this problem is another of the discoveries that we owe to the geniusof Helmholtz. The conditions are different when the eye is not focused for the source of lightbefore it. Suppose, for example, that the eye is hypermetropic (Fig. 16). Then therays springing from the illuminated portion of the retina, I, leave the eye as a divergentbeam, so that only a part of the rays are returned to the source of light, L, while anotherpart passes to the side of the latter and can be seen by an observer stationed near comes the striking luminosity of the pupil in so-called amaurotic cats eye (see§ 535), in which a markedly hypermetropic condition of the refraction is produced bythe pushing forward of the retina. In like manner, luminosity is frequently apparentin eyes which are deprived of their lens by the operation for cataract and are therefore. .*H=a Fig. 16.—Explanation of Luminosity of the Eye. The source of light, L, throws the conical beam of rays, k, into the eye. The further course ofthese rays is not shown in the figure, but only that of the rays reflected from the retina at I. strongly hypermetropic. The enlargement of the pupil, which is at the same timepresent, and which is due to the iridectomy, renders it still easier to observe the lumi-nosity. The luminosity of the eyes of many beasts, especially the carnivora, is alsoin part to be ascribed to the hypermetropic character of their refraction, although herethe presence of a strongly reflecting layer (the so-called tapetum) in the chorioid ofthese eyes contributes to this result. The luminosity of the pupils of albinos eyes is to be explained in a different such eyes the light passes not only through the pupil, but also through the unpig-mented iris, and even through the sclera. Accordingly, in these eyes, not simply alimited
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