I 4 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



Cause of Color-Blindness. — Color-blindness is a 

 defect of retinal structure. In the case of the color- 

 blind one or more of the color substances are wanting. 

 In the red-green blind, for example, the red color-sub- 

 stance and the green color-substance of Helmholtz are 

 both wanting. Or, according to Hering's better view, the 

 one substance which, by opposite affections, produces 

 these complementaries, is wanting, and that is the rea- 

 son why these two are usually associated. Such persons 

 see yellow and blue perfectly well. According to Mrs. 

 Franklin, color-blindness is an example of atavism — 

 i. e., a reversion to a primitive condition. Total color- 

 blindness, which, though rare, sometimes occurs, is a 

 relapse to the earliest condition. There is only gray 

 substance in the retina. Red-green blindness is a re- 

 lapse to the second stage, in which some of the gray 

 substance has been differentiated into yellow and blue, 

 but the yellow has not been further differentiated; while 

 normal vision is the third or perfect stage, in which the 

 yellow has been further differentiated into red and green. 



What the Color-Blind really See.— By the color- 

 blind pure colors are either seen correctly or not seen at 

 all as colors, but only as shades. The mixed colors they 

 always see incorrectly. Taking the commonest form of 

 color-blindness, the red-green blindness, the following 

 schedule shows what they see and why : 



I. 



See correctly. 



a. White and black and all shades of the same — i. e., grays. 



b. Yellow and all shades of the same — i. e., browns. 

 6. Blue and all shades of the same — i. e., slate blues. 



II. 



■j Do not see at all as Colors, 



a. Reds are seen as shades or grays. 

 „ b. Greens are seen as shades or grays. 



