THE SPECIAL SENSES. 209 



presents itself in whatever direction we look. This spot 

 passes away as the retina resumes its activity. 



72. If a bright color be gazed at intently, and the eyes 

 then be turned to a white surface, a spot will appear ; but 

 its color will be the complement of that of the object. Fix 

 the eye upon a red wafer upon a white ground, and on re- 

 moving the wafer a greenish spot of the same shape takes 

 its place. This result happens because a certain portion of 

 the retina has exhausted its power to perceive the red ray, 

 and perceives only its complementary ray, which is green. 

 The color thus substituted by the exhausted retina is called 

 a physiological or accidental color. In some persons the 

 retina is incapable of distinguishing different colors, when 

 they are said to be affected with " color-blindness." Thus, 

 red and green may appear alike, and then a cherry-tree, 

 full of ripe fruit, will seem'bf the same color in every part. 

 Railroad accidents have occurred because the engineer of 

 the train, who was color-blind, has mistaken the color of a 

 signal. 



73. The Crystalline Lens. Across the front of the 

 eye, just behind the iris, is situated the Crystalline lens, 

 enclosed within its own capsule. It is supported in its 

 place partly by a delicate circular ligament, and partly by 

 the pressure of adjacent structures. It is. colorless and per- 

 fectly transparent, and has a firm but elastic texture. In 

 shape it is doubly convex, and may be rudely compared to 

 a small lemon-drop. The front face of the lens is flatter 

 than the other, and is in contact with the iris^near its 

 pupillary margin, as is represented in the diagram on page 

 214. It is only one-fourth of an inch thick. 



74. When this little body becomes opaque, and no longer 

 affords free passage to the rays of light, as often happens 



72. How further shown? How is the result accounted for? " Color-blind- 

 uesB ?" 



73. The location of the crystalline lens? How supported? Its color and 

 texture? Shape? Size? 



Y4. Cataract? Aqueous humor ? Vitreous humor ? 



