ACCESSORY EYE STRUCTURES 



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The latter is the only way that will be considered here. 

 The principle is familiar, since it is made use of in or- 

 dinary photographic cameras. It consists in focusing 

 beams of light by means of a lens. In all the higher 

 animals the front part of the eye acts as a lens through 

 which light reflected from the object is focused in such 

 a way that the image of the object is formed upon the 



Fig. 38. — Diagram of vertebrate eye. (a) cornea; (6) crystalline 

 lens; these together make up the focussing apparatus; the lens 

 serves also in accommodation; (c) the retina or sensitive surface upon 

 which images are formed; (d) the beginning of the optic nerve; 

 (e) the iris. 



sensitive surface known as the retina, which is at the 

 back of the eye in line with the lens. 



Accessory Eye Structures. — In the eyes of the 

 higher animals there are one or two additional parts which 

 call for brief description. First is the arrangement for 

 changing the focus, or for accommodation as it is called. 

 This is necessary in order that objects at different dis- 

 tances from the eye may be seen with equal clearness. 

 Everyone knows that an adjustment takes place in his 

 own eyes when he turns from looking at distant objects 



