THE SPECIAL SENSES 339 



sclerotic, is the choroid. It consists of pigment cells and 

 blood vessels; its function is to absorb the superfluous 

 light which may pass through the retina, or third and inner- 

 most coat. This light, if reflected to and fro within the 

 ball, would confuse the vision. The continuation of the 

 choroid's layer of pigment in front is the iris, which may 

 be of various colors, and which, besides the pigment, con- 

 tains muscular fibers. Unlike the sclerotic, the choroid is 

 not continuous in front, for the iris is provided with an 

 opening called the//////. The retina is inside the choroid 

 and lines the chamber. It is made by the distribution of 

 the hundred thousand or more fibers of the optic nerve. 

 Just behind the iris is the crystalline lens whose func- 

 tion is to bring the rays of light to a focus on the retina. 

 It is in a transparent capsule from the edges of which mus- 

 cular fibers radiate to the wall of the eyeball. The lens 

 divides the chamber into two cavities. The small front 

 cavity, between it and the cornea, is filled with the watery 

 fluid called aqueous humor. The main cavity is filled with 

 the jelly like vitreous humor. A ray of light in going from 

 the outer world to the retina, passes through four parts 

 of the eye. Name them. This does not count the pupil, 

 which is simply a hole. 



614. Use of the Lens. Take a lens that is rounded 

 outward (convex) on both sides, such as a hand magnifier, 

 or even a strong lens from an old per- 

 son's spectacles. Hold this up on the 

 side of a room opposite to a window 

 and catch the image of the window on FIG. 215 -Refraction of 

 a white card-board held back of the Li s ht - 



lens. This illustrates how the image 

 of an external object is formed by the 



. . , , , f of light rays. 



crystalline lens upon the retina. If 



some one stands up in the window, does he appear in the 



upper or lower part of the image ? If he moves to the 



