THE SPECIAL SENSES 285 



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 sub- 

 stituted 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 of the same 

 color in every part. Kailroad accidents have occurred because 

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

 color of a signal. (Read Note 9.) 



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 deli- 

 cate circular ligament, and partly by the pressure of adjacent 

 structures. It is colorless and perfectly 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 natter than the other, and is in contact with 

 the iris near its pupillary margin, as is represented in the dia- 

 gram of page 290. It is only one-fourth of an inch thick. 



9. Color-blindness. " Daltonism, or color-blindness, receives its 

 name from the eminent English chemist who described this infirmity 

 as it existed in his own case. It arises from an unnatural condition 

 of the organs of vision which prevents the discrimination of certain 

 colors. Some persons will mistake red for green ; so that ripe cherries on 

 a tree appear the same as the leaves; others recognize only black and 

 white. Persons thus affected are sometimes incapable of discriminating 

 musical tones. The healthy eye ordinarily fails to discriminate between 

 certain colors, blue and green especially, when viewed by artificial 

 light. But even this may in a measure be overcome by training, so 

 that an expert dealer in silk obtains a knowledge of the shades of blue, 

 green, and violet, which is proof against the confusing influence of gas- 

 light and tinted curtains. The eyes of persons who have much to do with 

 colors are more liable to become overstrained than those dealing chiefly 

 with rays of white light." Flint's Physiology (in part). 



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

 Shape ? Size ? 



