262 OUR BODIES AND HOW WE LIVE 



cornea uniformly convex and allows the curtain with the 

 hole in it the iris to float and move at freedom. The 

 back chamber contains a jelly like fluid called the vitreous, or 

 glassy, humor. 



361. The Course of the Rays of Light in the Eye. Let 

 us now master a few points about the mechanism of vision. 

 Let us trace the course of the rays of light going from any 

 luminous body a lighted candle, for example through 

 the different parts of the eye. Imagine the candle to be 

 placed about ten inches in front of the eye. 



Some of the rays fall on the outer coat, or the white, 

 of the eye, and, being turned back or reflected, take no 

 part in vision. The more central rays fall upon the cornea. 

 Some of these are reflected, giving to the surface of the eye 

 its beautiful, glistening appearance. 



Now, if the rays of light passed directly to the retina, 

 they would pass in parallel lines and produce the impres- 

 sion of light, but everything would be dim and confused. 

 Therefore it is necessary that the rays coming from any 

 object should be brought together (converged) by being 

 bent (refracted). That is, they must be refracted and 

 brought to a focus. 



362. The Work done by the Crystalline Lens. The rays 

 of light are refracted and brought to a focus to a certain 

 extent by the cornea and the fluids, or humors, of the 

 eye, but mainly by the crystalline lens. It is thus the duty 

 of the crystalline lens to bring the rays of light nearer 

 together as they pass through it, and to bring them to a 

 focus on the retina. 



It is a familiar fact in the use of optical instruments that 

 they must be differently adjusted for objects at different 

 distances. The boy changes the focus of his spyglass by 



