138 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



sight and a small area about it. As we go outward from 

 the fovea in all directions, we find the cones are fewer 

 and larger, until there are none at all on the margins of 

 the retina. So, correspondingly, the perception of color 

 is more and more imperfect as we go from the point of 

 sight to the margins of the field of view, where it is 

 finally lost entirely. 3. There must be some response 

 of the retina characteristic of each color. We may im- 

 agine that different cones are adapted to vibrate respon- 

 sively — co-vibrate — with different colors. Or we may 

 imagine different substances in all the cones which are 

 photochemically affected each by a particular color. 

 This latter seems the more probable view. We shall 

 call such substances color-substances. Thus we have, say, 

 a red color-substance, meaning not that the substance 

 is red, but that it is photochemically affected by a certain 

 rate of vibration and produces the sensation of red. 



Special Theories. — Applying this to the different 

 views as to primary colors, according to Helmholtz, there 

 are in the retinal cones three kinds of color-substance 

 which are responsive to three rates of vibration — viz., red, 

 green, and violet rays, respectively, and these give rise 

 to the corresponding color sensations. If two of them 

 are affected, they produce mixed colors. If all are 

 affected in certain proportions, we have white. Or, to 

 put it another way : pure colors affect only one, mixed 

 colors two or more, white light all in certain propor- 

 tions. According to Hering there are only two color- 

 substances (three, if we include white and black) ; the 

 one by opposite affections produces the complementaries 

 red and green, the other by opposite affections the 

 complementaries yellow and blue; and the essential na- 

 ture of complementariness, especially their mutual de- 

 structiveness, is the necessary result of these opposite 

 affections of the same substance. 



