LIGHT 237 



nating colors in the iris. The outer edge of the iris is 

 fastened to the sclerotic coat by the ciliary ligament and 

 to the choroid coat by folds known as ciliary processes. 



The third layer of the eye or retina is a delicate trans- 

 parent membrane of connective tissue containing an 

 expansion of the optic nerve. It lines the interior of the 

 eyeball with the exception of the front, where it stops with 

 the ciliary processes and at the entrance of the optic 

 nerve. The retina is the only part of the eye that is 

 sensitive to light, and it is the part of the eye in which 

 visual impulses originate. 



In addition to the transparent parts of the eye al- 

 ready mentioned there are three other important media. 

 The main part of the interior of the eyeball is filled with 

 a colorless, transparent, jelly-like substance called the 

 vitreous humor. It aids in preserving the form of the 

 eyeball. In front of the vitreous humor and just behind 

 the iris is the crystalline lens. It is a biconvex, transpar- 

 ent body which focuses the rays of light as they enter the 

 eye, so that a clear image of the reflecting object is pro- 

 duced. Between the crystalline lens and the cornea is 

 a small space which is filled with a clear, watery liquid 

 called the aqueous humor. 



The place where the optic nerve enters the eye is called 

 the " blind spot," since the eye at that point is not sen- 

 sitive to light. 



How the Eye Does its Work. The human eye re- 

 sembles a camera in many essential respects. It con- 

 tains a focusing lens, the crystalline lens, the iris, which 

 acts as a shutter, and the retina, which takes the place of 

 the camera film or sensitive plate (Figure 213). The eye, 

 however, is vastly superior to the camera. The focusing 

 organs of the eye possess the power of accommodating 



