650 TEXT-BOOK OF PHYSIOLOGY. 



changes in direction before they are finally converged to a focal point. In 

 order to follow mathematically the rays in all their deviations through the 

 media, to determine their focal points and to construct an image, a knowl- 

 edge of the form of the refracting surfaces, the refractive indices of the dif- 

 ferent media, and the distance of the surfaces from one another must be 

 known. 



The following constants are now accepted : The radius of curvature 

 of that portion of each refracting surface used for distinct vision is for the 

 cornea 7.829 mm., for the anterior and posterior surfaces of the lens 10 and 

 6 mm., respectively. The indices of refraction of the different media are 

 as follows: cornea and aqueous humor, 1.3365; lens, 1.4371; vitreous body, 

 1.3365. The distance from the vertex of the cornea to the lens is 3.6 mm.; 

 the thickness of the lens, 3.6 mm.; the distance from the posterior surface 

 of the lens to the retina, 15 mm. As the two surfaces of the cornea are prac- 

 tically parallel, and as the index of refraction of the aqueous humor is the 

 same as that of the cornea, they may be regarded as but one medium. The 

 refracting surfaces may therefore be reduced to the anterior surface of the 

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

 lens. 1 



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

 fraction of 1.00025, into 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 pro- 

 cedures. Perfect vision, however, 



FIG. 308. REFRACTION OF HOMOCENTRIC rprm i r p c tv,,,* tVi P rrmvprapnrp nf 

 RAYS AND THE FORMATION OF AN IMAGE. equires tnat tne convergence 



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 rarer medium the rays are 

 once more converged and to an extent sufficient to focalize them on the 

 retina (Fig. 308). 



While it is thus possible to follow the rays geometrically 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 



1 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 of the aqueous humor, viz,. 

 1.377. But as the increase in the corneal refraction due to the higher index is almost exactly 

 counteracted by a decrease in refraction due to the higher curvature of the posterior corneal 

 surface, the usual assumptions furnish quite accurate results. 



