230 THE SPECIAL SENSES 



violet are the fundamental colors, and all others can be made 

 by combinations of these three. Working on this basis, Young 

 and Helmholtz assumed three chemical substances in the retina 

 capable of responding to the three fundamental colors. Hering 

 assumed three substances corresponding respectively to white or 

 black, red or green, and yellow or blue light. In this theory 

 the white, red, and yellow rays are katabolic in their effects 

 on their individual recipient substances, while black (absence 

 of light), green, and blue are anabolic, thus having an antago- 

 nistic effect. Mrs. Franklin assumes in her theory that in early 

 life the eye possesses no color perception, but merely the power 

 of perceiving luminosity, i. e., distinguishing between white and 

 black. The substance responding to luminosity is called gray- 

 perceiving. As the development progresses, some of the gray 

 is differentiated into a blue- and a yellow-perceiving substance. 

 The yellow-perceiving substance is still further differentiated 

 in the course of development into a red- and a green-perceiving 

 substance; thus: 



Gray. 



Blue. Yellow. 



| 

 Green. Red. 



Many objections have been raised against each of these theories, 

 but Mrs. Franklin's is so far the best, and explains more readily 

 the causes of color blindness. According to the theories of 

 Helmholtz and Hering, color blindness is due to an absence of 

 of one or more of the fundamental color-perceiving substances. 

 Mrs. Franklin's theory assumes a lack of full development or 

 complete absence of development of gray substance. If the 

 development should cease after the blue- and yellow-perceiving 

 substances have been formed, the individuals would be capable 

 of distinguishing the blues and yellows, but could not recognize 

 reds and greens. Clinically, such cases often are met with. 

 Males are far more likely to be color-blind than females (16 to 

 1). Only 1 woman in 400 is color blind. The reason for this 

 is partly at least that the development of the gray-perceiving 

 substance is favored by practice and color education. 



