We have already noticed the most essential use of the iris—viz. its power, under the influence of light upon the retina, of modifying the size of the pupil so as to regulate the amount of light entering the eye. But this is not its only use, one of its ofilees being to prevent the passage of rays through the circumferential part of the lens, and thus to obviate the indistinctness of NINA would arise from spherical aberra tion (the unequal refraction of the rays passing thr•ngh the centre aml near the margin of the lea ), in the same manner as the diaphragms em by the optician. But there are addition HR. Iwo other means by which this spherical alwrr•t ion is prevented, which so well illustrate the r•arv4 tons weehanisin of the eye that we can not ontit to notice them. They are described by Wharton a-, follows: (1 ) "The surfaces of the dioptrie parts of the eye are not spherical, but those of the cornea and posterior surface of the lens are hyperbolical, and that of the anterior surface of the lens tical—configurations found by theory titted to prevent spherical aberration. This discovery was made at a time hen it was not known hut that the dioptric parts of -the eye had spherical sur faces.
(2) "The density of the lens diminishing (as we have already shown) from the centre to its periphery, the circumferential rays are less refracted than they would have been by a homo geneous lens with similar surfaces. This ele gantly simple contrivance has been hitherto in imitable by human art." Chromatic aberration, which is caused by the unequal refrangibility of the primitive rays of which white light is composed. when trans mitted through an ordinary lens, whereby colored fringes are produced, is practically corrected in the eye, although it is doubtful whether it is entirely absent. The provision, however. on which the achromatism depends has not been deter mined with certainty, probably because we do not yet know the relative refractive and dispersive powers of the cornea and humors of the eye. Sir David Brewster denies that the chromatic aber ration receives any correction in the eye, and maintains that it is imperceptible only in conse quence of its being extremely slight. See also