26 SPECIAL SENSES. 



talline is divided into two compartments, called the anterior 

 and posterior chambers (i). The fluid which fills these 

 chambers is a clear watery liquid, called the aqueous 

 humor. The portion of the globe behind the lens, which is 

 much the largest, is filled by a gelatinous liquid, perfectly 

 transparent, like that of the chambers, but somewhat more 

 dense. This is called the vitreous humor (h). 



77. The object of this apparatus is to receive the rays of 

 light, which diverge from all points of bodies placed before 

 it, and to bring them to a point again upon the retina. 

 It is a well-known fact, that when a ray of light passes 

 obliquely from one medium to another of different density, 

 it will be refracted or turned out of its course more or 

 less, according to the difference of this density, and the ob- 

 liquity at which the ray strikes the surface. This may 

 be illustrated by the following figure. (Fig. 14). 



A/ E 



Fig. 14. 



The ray a c, which strikes the cornea A B perpendicularly, 

 continues without deviation, until it reaches the bottom of 

 the eye at c. But the rays am and an, which strike the eye 

 obliquely, change their direction, and instead of proceeding 

 onward to mg and nd, take the direction mi and nf. A 

 still further refraction, though less considerable, is occa- 

 sioned by passing through the crystalline lens C D, and the 

 vitreous humor, so that the tw r o rays m i and nf, will at last 

 meet in a point. This point is called the focus (c), and in 

 distinct vision, is always precisely at the retina, E F. 



78. From this arrangement, the image found upon the 



