MECHANISM OF EEFRACTION IN THE EYE. 89 



This table illustrates, with great exactness, the gradual dimi- 

 nution in the acuteness of vision as the impressions are made 

 farther and farther from the visual axis. 1 The experiments 

 were made upon the same principle as that of observations 

 upon the tactile sensibility of different portions of the skin 

 by testing the power of distinguishing the two points of the 

 8esthesiometer. a 



The fact of the formation of images upon the retina, 

 which are exact only at or immediately surrounding the f ovea 

 centralis, being settled, it remains to see how these images 

 are rendered perfect, and to study the mechanism of refrac- 

 tion by the transparent media of the eye. 



Mechanism of Refraction in the Eye. 



A visible object sends rays from every point of its surface 

 to the cornea. If the object be near, the rays from each and 

 every point are divergent as they strike the eye. Rays from 

 distant objects are practically parallel. It is evident that the 

 refraction for diverging rays must be greater than for parallel 

 rays, as a necessity of distinct vision ; in other words, the 

 eye must be accommodated for vision at different distances. 

 Leaving, however, the mechanism of accommodation for 

 future consideration, we shall endeavor to show how the rays 

 of light as they penetrate the eye are refracted and brought 

 to a focus at the retina. 



The important agents in refraction in the eye are the sur- 

 faces of the cornea and the crystalline lens. Careful calcula- 

 tions have shown that the index of refraction of the aqueous 

 humor is sensibly the same as that of the substance of the 

 cornea, so that, practically, the refraction is the same as if the 

 cornea and the aqueous humor were one and the same sub- 

 stance. The index of refraction of the vitreous humor is 

 practically the same as that of the aqueous humor, both being 

 about equal to the index of refraction of pure water. Re- 



1 VOLKMANN, Sehen, in WAGNER, Handworterbuch der Physiologic, Braun- 

 schweig, 1846, Bd. ill, Erste Abtheilung, S. 334. 8 See page 19. 



