Vision (Classic Reprint)

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Excerpt from Vision To the values of the optical constants of the eye as above given may be added the following: Distance from the anterior surface of the cornea to the anterior surface of the lens, millimeters, distance from the posterior sur face Of the lens to the retina, 15. Millimeters, thickness of lens, millimeters. The methods usually employed for determining these constants are the fol lowing: The indices of refraction of the aqueous and vitreous humor are determined by filling the space between a glass lens and a glass plate with the fresh humor. The aqueous or vitreous humor thus forms a convex or concave lens, from the form and focal distance of which the index can be calculated. Another method consists in placing a thin layer of the medium between the hypothenuse surfaces of two right-angled prisms and determining the angle at which total internal re¿ection takes place. In the case of the crystalline lens the index is found by determining its focal distance as for an ordinary lens, and solving the equation which expresses the value of the index in terms of radius of curvature and focal distance, thickness, and focal length. The refractive index Of the lens increases from the surface toward the centre, a peculiarity which tends to correct the disturbances due to spherical aberration, as well as to increase the refractive power of the lens as a whole. The curvature of the refracting surfaces of the eye is determined by an instrument known as an ophthalmometer, which measures the size of the re¿ected image of a known object in the various curved surfaces. The radius of curvature Of the surface is determined by the following formula. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully, any imperfections that remain are intentionally left to preserve the state of such historical works.