REFRACTION IN THE EYE. 49? 



then the diverging rays/c and fd, proceeding from a nearer 

 point, will be harder to bend round, so to speak, and will 

 not meet until a point y, farther behind the system than 

 x is. The more divergent the rays, or what amounts to the 

 same thing, the nearer the point they proceed from, the 

 .farther behind the refracting system will y be. 



y< *. 



FIG. 130. Diagram illustrating the need of "focusing" in an optical instru 

 >ment. 



The refracting media of the eye form a convergent optical 

 system, made up of cornea, aqueous humor, lens, and vitre- 

 ous humor. These four media are reduced to three prac- 

 tically, by the fact that the indices of refraction of the 

 cornea and aqueous humor are the same, so that they act 

 together as one converging lens. The surfaces at which 

 refraction occurs are (1) that between the air and die 

 cornea, (2) that between the aqueous humor and the front 

 of the lens, (3) that between the vitreous humor and the 

 back of the lens. The refractive indices of those media are 

 the air, 1; the aqueous humor, 1.3379; the lens (average), 

 1.4545; the vitreous humor, 1.3379. From the laws of the 

 refraction of light it therefore follows that (Fig. 131) the 

 rays c d will at the corneal surface be refracted towards the 

 :normals N, N, and take the course d e. At the front of 

 lens they will again be refracted towards the normals to 

 that surface and take the course e f; at the back of the lens, 

 passing from a more refracting to a less refracting medium, 

 they will be bent from the normals N" and take the course 

 / g. If the retina be there, these parallel rays will therefore 

 Ibe brought to a focus on it. In the*resting condition of 

 the natural eye this is what happens to parallel rays entering 

 it; and, since distant objects send into the eye rays which 

 .are practically parallel, such obiects are seen distinctly 



