MECHANISM OF REFRACTION IN THE EYE. 



793 



This table illustrates, with great exactness, the gradual diminution in the acuteness 

 of vision as the impressions are made farther and farther from the visual axis. 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 sesthesiometer. 



The fact of the formation of images upon the retina, which are exact only at or imme- 

 diately surrounding the fovea centralis, being settled, it remains to see how these images 

 are rendered perfect, and to study the mechanism of refraction 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 sho\v 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 surfaces of the cornea and the 

 crystalline lens. Careful calculations 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, prac- 

 tically, the refraction is the same as if the cornea and the aqueous humor were one and 

 the same substance. 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. Refraction by the crystalline lens, however, is more complex in its mechan- 

 ism ; depending, first, upon the curvatures of its two surfaces, and, again, upon the differ- 

 ences in the consistence of different portions of its substance. In view of these facts, 

 we may simplify the conditions of refraction in the eye by assuming the following 

 arrangement : 



The cornea presents a convex surface upon which the rays of light are received. At 

 a certain distance behind its anterior border, is the crystalline, a double-convex lens, 

 corrected, sufficiently for all practical purposes, both for spherical and chromatic aberra- 

 tion. This lens is practically suspended in a liquid with an index of refraction equal to 

 that of pure water ; as both the aqueous humor in front and the vitreous humor behind 

 have the same refractive power. Behind the lens, in its axis and exactly in the plane upon 

 which the rays of light are brought to a focus by the action of the cornea and the lens, 

 is the fovea centralis, which is the centre of distinct vision. The anatomical elements of 

 the fovea are capable of receiving visual impressions, which are conveyed to the brain 

 by the optic nerves. All impressions made upon other portions of the retina are com- 

 paratively indistinct ; and the point of entrance of the optic nerve is insensible to light. 

 Inasmuch as the punctum caBcum is situated in either eye upon the nasal side of the retina, 



