6o8 



TEXT-BOOK OF PHYSIOLOGY. 



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 i 



cornea, the anterior surface of the lens, and the posterior surface of 

 the lens.* 



Parallel rays of light entering the eye pass from air, with an index 

 of refraction of 1.00025, m ^ the cornea, with an index of refraction 

 of 1.3365. In passing from the rarer into the denser medium they 

 undergo refraction in accordance with the laws of optics and are 

 rendered somewhat convergent. The extent of this first refraction 

 and convergence is sufficiently great to bring parallel rays, if con- 

 tinued, to a focus about 10 mm. behind the retina. This would be 

 the condition in aphakia whether the lens is congenitally absent or 

 has been removed by surgical procedures. Perfect vision, however, 

 requires that the convergence of the light must be great enough to 

 bring the focal point, the image, on the retina. This is accomplished 

 by the introduction of an additional refracting body, the lens. On 

 entering the lens the rays are for the same reason i. e., the passage 

 from a rarer into a denser medium again refracted and converged, 



and if continued would 



^^ - come to a focus about 6.5 



mm. behind the retina. On 

 passing from the lens into 

 the vitreous i. e., from,, a 

 denser into a rfliffi* medium 

 ire once more 



FIG. 283. REFRACTION OF HOMOCENTRIC RAYS 

 AND THE FORMATION OF AN IMAGE. 



converged and to an extent 

 sufficient to focalize them 

 on the retina (Fig. 283). 



While it is thus possible 

 to geometrically follow the 



rays through these media by means of the above-mentioned factors, 

 the procedure is attended with many difficulties. Moreover, as the 

 relations all change when rays enter the eye from objects situated 

 progressively nearer the eye, a separate calculation is necessitated for 

 each distance for the determination of the size of the image. 



A method by which these difficulties are much reduced was sug- 

 gested by Gauss and developed by Listing. It was demonstrated by 

 Gauss that in every complicated system of refracting media separated 

 by centered spheric surfaces there may be assumed certain ideal 

 or cardinal points, to which the system may be reduced, and which, 

 if their relative position and properties be known, permit of the de- 



* Strictly speaking, the posterior surface of the cornea is not parallel to the 

 anterior surface, and the index of refraction of the cornea is a trifle greater than that 

 : the aqueous humor, viz., 1.377. But as the increase in the corneal refraction due 

 l u a i.- mdeX is almost ex actly counteracted by a decrease in refraction, due to 

 the higher curvature of the posterior corneal surface, the usual assumptions furnish 

 quite accurate results. 



