ADDITIONAL TOPICS IN HUMAN BIOLOGY 163 



is white in color, except in front, where it becomes the transparent 



cornea. 



Inside of the outer coat is a second layer which is seen beneath 



the cornea as a colored ring known as the iris. In the center of the 



iris is a circular opening, the pupil, which is black hi appearance. 



Through the pupil enter the rays of light into the interior of the 



eyeball. If one comes suddenly from a dark room into the light, it 



is possible to see this opening 



quickly decrease in size. The inner 



lining of the eyeball is extremely 



thin and black in color ; it is known 



as the retina, and connected with 



it are the many nerve fibers that 



carry messages to the brain. 



Behind the iris is a beautiful 



transparent object, the crystalline 

 lens, both surfaces of which are 

 convex. The space within the eye- 

 ball in front of this lens is occupied 

 by a liquid, and behind the lens is 

 a jellylike substance. 



FIG. 52. Section of the eye. 

 246. The eye as a camera. 

 Any one who is at all familiar with 

 a camera knows that by means of 

 a lens, gr a combination of lenses, 

 the scene to be photographed is 

 made to appear upside down on the ground glass plate at the back 

 of the camera. If the image is not clear, it is brought into focus 

 by moving the lens nearer to, or farther from, the object. 



In the eye, too, we have an arrangement similar to that of a 

 camera, since the convex surfaces of the cornea and crystalline lens 

 (Fig. 53) focus the rays of light so that an image is formed on the 

 sensitive retina at the back of the eye. Since, however, the lenses 

 within the eye cannot be moved backwards and forwards, as in a 

 camera, focusing or accommodation of the eye must be accomplished 



C = Cornea. 

 I = Iris. 



L = Crystalline lens. 

 ON = Optic nerve. 



R = Retina. 

 V. H. = Jellylike substance. 



