DIOPTRICS OF THE NORMAL EYE 29 



Dioptrics of the Normal Eye — As light enters the eyeball it first 

 encounters the tissue of the cornea, then in succession the aqueous 

 humor, the lens, the vitreous humor and the transparent retina on whose 

 posterior, outer surface the sensory rod and cone cells lie. These trans- 

 parent structures and substances, exclusive of the retina, are known col- 

 lectively as the dioptric media. When a light ray comes through the air 

 into the cornea at one side of the latter's center, it is bent sharply toward 

 the antero-posterior axis of the eyeball. Upon leaving the cornea and 

 entering the aqueous humor, the ray is bent again but only very slightly 

 since the corneal tissue and the aqueous have nearly the same optical 

 density. The refractive index of the cornea is 1.376, and that of the 

 aqueous is 1.336, which is about the same as that of water. 



Now upon entering the lens, the ray is bent further, again toward the 

 axis of the eye. The index of refraction of the lens can be taken as 1.42. 

 Actually, the values for the lens are 1.406 at the center, 1.386 at the 

 surface, but because of its zoned structure the lens behaves as would a 

 homogenous body whose index was actually higher than that of any part 

 of the lens. This figure, 1.42, for the effective index of the lens, does 

 not exceed the index of the aqueous (1.336) by as much as the latter 

 value exceeds the index of air (l.OO). This, together with the fact that 

 the anterior surface of the lens is not as sharply curved as the cornea, 

 is responsible for the fact — often overlooked — that the cornea does most 

 of the job of placing the image on the retina. In the optically normal 

 eye the lens acts like the fine adjustment of a microscope — it adjusts the 

 position of the image only in a minor way. Some highly myopic persons, 

 in fact, see clearly without spectacles after the lens has been removed 

 because of cataract — with the lens in the eye, they have too much focus- 

 ing power, the focal length of the cornea alone being equal to the 

 length of their abnormally elongated eyeballs. 



Upon travelling through the posterior surface of the lens into the 

 vitreous humor, our light ray for the first time passes from a medium 

 of higher density into one of lower density — the vitreous having the 

 same index as the aqueous. If it were passing through a convex surface, 

 it would be bent away from the axis of the eye; but since it is here 

 travelling through a concave surface it is still further converged toward 

 the axis. In fact, since both surfaces of the lens are in contact with media 

 whose refractive indices are the same, and the posterior surface of the 

 lens is more sharply curved than the anterior, the posterior face is the 

 more important of the two in the static refraction of the eye. 



