THE OPTICAL CONSTANTS OF THE EYE 



509 



ing surfaces; (3) the distances of the different refracting surfaces from one 

 another. These measurements are called the optical constants of the eye. 



The following table after Helmholtz contains a summary of values found 

 by different authors for the refractive indices of the various media in the 

 human eye : 



Cornea 1.330-1.357 



Aqueous humor 4 1 335-! 355 



Vitreous body 1 .336-1 .357 



Crystalline lens, outer layer \ 1 338-1 .474 



" median 1.352-1.478 



core 1 . 39Q_i . 431 



The lens, as appears from the table, has a different refractive index in its 

 different layers, the value increasing from without inward. As a consequence, 

 the focal distances of the component lenses become smaller in the same order 

 and the total refracting power greater than it would be, if the whole lens had 

 the refractive index of its core (Young, 

 Listing). 



Hence it is a mistake to try to re- 

 place the crystalline lens by a homoge- 

 nous lens of the same form and with an 

 average refractive index. Such a lens 

 must have a higher total refractive in- a 

 dex than that of its densest part. 



In calculating the course of the light 

 rays in the eye, we shall follow Helm- 

 holtz in supposing the crystalline lens to 

 be replaced by a homogenous lens with 

 a refractive index of 1.4371. 



The problem to be solved is rendered 

 much easier by this simplification and 



we can now treat the optical system of the eye as if it were composed of 

 two relatively simple systems. The first consists of (1) air, (2) cornea, 

 (3) aqueous humor; the second of (1) aqueous humor, (2) crystalline lens 

 and (3) vitreous body. 



The system of the cornea can be simplified still further for three reasons : 

 it is very thin, its surfaces are almost concentric and its refractive index is 

 only a little greater than that of the aqueous humor. Since the refractive 

 index of the layer of tears on the outside differs but slightly from that of 

 the aqueous humor inside, we may think of the cornea as a watch-glass-shaped 



FIG. 206. 



is but slightly less than in a vacuum, the refractive index of a medium is ordinarily given 

 as the retardation which the light suffers in passing from air into that medium. The re- 

 fractive index may be found by measuring the angle of incidence and the angle of refrac- 

 tion e. g., the angles o and formed by the light ray/ g in Fig. 206. The refractive 

 index of the medium below the line a b, supposing the medium above that line to be 



air, is given by the formula n = S ? ne g . 



sine ft 



