THE SENSE OF SIGHT. 245 



accomplished by the crystalline lens, a body denser than the cornea 

 and possessing a higher refractive power. The manner in which a 

 biconvex lens focuses both parallel and divergent rays is shown in 

 figures 32 and 33. 



The function of the crystalline lens, therefore, is to focus the rays 

 of light with the formation of an image on the retina. 



The retinal image corresponds in all respects to the object from 

 which the light proceeds. The existence of this image can be demon- 

 strated by removing from the eye of a recently killed animal a cir- 

 cular portion of* the sclerotic and choroid posteriorly, and then 

 placing at the proper distance in front of the cornea a lighted candle ; 

 an inverted image of the candle will be seen upon the retina. The 

 size of the retinal image depends upon the visual angle, which in 

 turn depends upon the size of the object and its distance from the 

 eye. At a distance of 15.2596 meters the image of an object one meter 

 high would be one millimeter, or a thousand times smaller than 

 the object. 



Accommodation. By accommodation is understood the power 

 which the eye possesses of adjusting itself to vision at different 

 distances. In a normal or emmetropic eye parallel rays of light 

 are brought to a focus on the retina ; but divergent rays that is, 

 rays coming from a near luminous point will be brought to a focus 

 behind the retina, provided the refractive media remains the same ; 

 as a result, vision would be indistinct, from the formation of 

 diffusion circles. It is impossible to see distinctly, therefore, a 

 near and a distant object at the same time. We must alternately 

 direct the vision from one to the other. A normal eye does not 

 require adjusting for parallel rays ; but for divergent rays a change 

 in the eye is necessitated ; this is termed accommodation. In the 

 accommodation for near vision the lens becomes more convex, par- 

 ticularly on its anterior surface. The increase in convexity aug- 

 ments its refractive power ; the greater the degree of divergence of 

 the rays previous to entering the eye, the greater the increase of con- 

 vexity of the lens and convergence of the rays after passing through 

 it. By this alteration in the shape of the lens we are enabled to 

 focus light rays coming from, and to see distinctly, near as well as 

 distant objects. 



Function of the Ciliary Muscle. Though it is admitted that the 

 change in the convexity of the lens is caused by the contraction of 



