FORMATION OF THE IMAGE UPON THE RETINA 847 



practically parallel, while the refractive power of the tears and aqueous 

 humor are nearly equal. It is strongly deviated, however, at the 

 anterior and posterior surfaces of the lens, because the refractive 

 indices of the aqueous and vitreous humors are less than that of the 

 lens. In general, the refractive power of this entire system may be 

 calculated without difficulty, provided the following factors are open 

 to analysis: 



(a) The indices of refraction of the different media. 



(b) The radii of the different curved surfaces. 



(c) The distances between them. 



Regarding the first factor, the following values have been obtained r 1 



Air 1.0 



Cornea 1.3771 



Aqueous humor 1 . 3374 



( Capsule 1 .3599 ] 



Lensi Ext. layer 1.3880 \ 1.4371 



(Body 1.4107J 



Vitreous humor 1 .3360 



It will be seen that the indices of the aqueous and vitreous humors 



are practically the same a-nd correspond 



to that of water. Furthermore, it will 



be observed that the total refractive 



power of the lens (1.4371) is greater 



than that of its different layers, as well 



as of that of its body. This apparent 



discrepancy is explained by the fact that 



its central substance, when isolated, 



possesses a greater curvature than its 



entire mass and, therefore, gives rise to a FIG 447 _ DlAGRAM T0 ILLTJS . 



Stronger refraction in relation to its TRATE THE REDUCED OR SCHEMATIC 



index. EYE - 



In Order to simplify matters Listing 2 R > imaginary refracting surface; 

 , . . , ,, !. < < ./V, nodal point of this system. 



has combined these different refractive 



media into a single one possessing a general refractive index of 1.33. 

 If united in this manner, the entire eye may be regarded as being 

 composed of a homogeneous substance presenting to the air a single 

 convex surface with a refractive index of 1.33, and a radius of cur- 

 vature of 5.017 mm. (Fig. 447). The principal point of the re- 

 fracting surface of this reduced or schematic eye lies 2.1 mm. behind 

 the anterior surface of the cornea, and its nodal point (N) or optical 

 center 0.04 mm. in front of the posterior surface of the lens, i.e., 7.3 

 mm. behind the anterior surface of the cornea. The principal focus 

 of this imaginary refracting surface lies 22.2 mm. behind the anterior 

 surface of the cornea of the actual eye. The optical power of this 



1 Matthiessen, Pfliiger's Archiv, xxxvi, 1885. 



2 Wagner's Handworterbuch der Physiol., 1853, iv, 451. 



