592 THE NERVOUS SYSTEM. 



suiting in the sensation of luminosity only ; that they are more 

 easily stimulated than the cones, and are particularly responsive 

 to waves of short wave-lengths ; and that they adapt themselves 

 to light of varying intensity. 



Hering Theory of Color. This theory supposes the existence 

 of three substances in the retina, and of six primary color sensa- 

 tions, arranged in pairs, white and black forming one pair, red 

 and green another, and yellow and blue the third. These corre- 

 spond, it will be noticed, to complementary color sensations. The 

 three substances are the white-black, the red-green, and the yellow- 

 blue. These substances are supposed to be susceptible of being 

 affected in two opposite ways : In one a constructive or anabolic 

 change is produced, and in the other a disintegrate ve or katabolic 

 change. If, for example, all the spectral colors fall upon the 

 white-black substance, katabolic changes occur in this substance 

 producing the sensation of luminosity ; while if no light enters the 

 eye, anabolic changes occur, with the effect of producing blackness. 

 The red rays falling upon the retina produce katabolic changes 

 in the red-green substance, producing the sensation of red, while 

 the green produces anabolic changes, and the resulting sensation is 

 that of green. Blue rays cause anabolic changes in the yellow- 

 blue substance, and yellow rays cause katabolic changes in the same 

 substance. These changes in the retinal substances produce the 

 sensations of color when transmitted through the fibers of the 

 optic nerve to the brain. 



Franklin Theory of Color-sensation. This theory supposes that 

 the eye, in the early periods of development, possesses only the 

 white-black or gray visual substance, and is therefore sensitive to 

 luminosity only, and not to color. Later this substance becomes 

 modified into the blue and yellow substance, and then into the red 

 and green. For a further account of this theory the reader is 

 referred to the American Text-book of Physiology, vol. ii., p. 337. 



Birch Modification of the Young-Helmholtz Theory. This ex- 

 perimenter has exposed the eye to sunlight in the focus of a burn- 

 ing-glass behind transparent screens of different colors, with the 

 result of producing a temporary color-blindness. If a red screen 

 is used, the eye is red-blind i. e., cannot distinguish the color, so 

 that if scarlet is looked at, it appears black, while yellow appears 

 green and purple appears violet. If a violet-colored screen is used, 

 violet appears black ; purple appears crimson ; and green, a bright 

 green. These effects are due to fatigue of the retina, so that the 

 color to which the retina is exposed for a time ceases to stimulate, 

 and that color ceases to be recognized while the other colors con- 

 tinue to stimulate. Birch found that after exposure to yellow 

 the eye was blind not only to yellow, but to red and green also, 

 which primary colors in the Young-Helmholtz theory produce 

 yellow. He concludes that there are not only the three primary 



