THE SPECIAL SENSES 



401 



it and the cornea is a space filled with a transparent 

 watery fluid, the aqueous humor. The entire cavity of 

 the eyeball back of the lens is filled with a jelly-like 

 mass, the vitreous humor. 



. Sclerotic 



Choroid 



Cornea 



Aqueous 

 humor 



Fig. 181. Section- of the eye. 



Light. The nerve endings in the retina when stimu- 

 lated by light start impulses to the brain which produce 

 sensations of sight. The source of this light varies. 

 It may be the sun, an electric light, or it may be light 

 reflected by objects. 



We can not give a definite answer to the question, 

 "What is light?" We do know how to produce it, 

 and we do know many things about its behavior. It 

 is supposed that vibrations, or light waves, are given 

 off in all directions by luminous bodies. These waves 

 travel in straight lines ; a single line of light is called a 



