THE RETINA 



261 



jects in a dim light and so protected that it will not 

 be injured by a bright light. To adjust the eye to the 

 strength of light, there is stretched in front of the lens 

 a muscular curtain, the iris, which gives to the eye its 

 characteristic color. In the centre of the iris is an 

 opening, the pupil, through which all the light which 

 enters the eye must pass. The iris is capable of con- 

 tracting to make the pupil smaller or of dilating to 

 make it larger. In this way, just enough light is admit- 

 ted to the interior of the eye to give the best vision, 

 but not enough to injure its more sensitive portions. 



Purpose and structure of retina. When the rays of 

 light from objects are focused upon the retina and have 



FIG. 142. Diagram of a section of the retina, showing groups of rods and cones 

 and their nerve connections. 



built upon it, as upon a screen, a reduced and inverted 

 picture, no more has been accomplished than is done by 

 the lens and screen of a camera. It still remains to 

 translate this image on the retina into sensations of 

 sight. The retina consists of several layers of nerve 



