50 SPECIAL SENSES. 



talline lens is divided into two compartments, called .he an 

 terior and posterior chambers. The fluid which fi is tnese 

 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. Tnis 

 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 again to a point 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 o- lo ss, according 

 to the difference of this density, and the obliquity at which 

 the ray strikes the surface. This may be illustrated by the 

 following figure, (Fig. 14.) 



Fig. 14. 



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

 continues without deviation, until it reaches the bottom ot 

 the eye at c. But the rays a m and a n, which strike the eye 

 obliquely, change their direction, and instead of proceeding 

 onward to m g and n d, take the direction m i and n f A 

 still further refraction, though less considerable, is occasioned 

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

 humor, so that the two rays, m i and nf, will at last meet in 

 ?jt point. This point is called the focus, (c,) and in distinc 

 vision \s always precisely at the retina, E F. 



78. From this arrangement, the image found upon the 



