782 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



and regards all other color sensations as produced by the simultaneous affec- 

 tion of two of these substances in varying proportions. Thus when a ray of 

 blue light falls on the retina it stimulates the violet- and green-perceiving sub- 

 stances, and produces a sensation intermediate between the two, while simul- 

 taneous stimulation of the red- and green-perceiving substances produces the 

 sensations corresponding to yellow and orange ; and when the violet- and red- 

 perceiving substances are affected at the same time, the various shades of 

 purple are produced. Each of these three substances is, however, supposed to 

 be affected to a slight extent by all the rays of the visible spectrum, a suppo- 

 sition which is rendered necessary by the fact that even the pure spectral 

 colors do not appear to be perfectly saturated, as will be explained in connec- 

 tion with the subject of saturation. Furthermore, the disappearance of color 

 when objects are very feebly or very brightly illuminated or when they are 

 seen with the lateral portions of the retina (as described on p. 779) necessitates 

 the additional hypotheses that these three substances are all equally affected by 

 all kinds of rays when the light is of either very small or very great intensity 

 or when it falls on the extreme lateral portions of the retina, and that they 

 manifest their specific irritability for red, green, and violet rays respectively 

 only in light of moderate intensity falling not too far from the fovea centralis 

 of the retina. 



The modifications of the Young- Hemholtz theory introduced by these sub- 

 sidiary hypotheses greatly diminish the simplicity which was its chief claim to 

 acceptance when originally proposed. Moreover, there will always remain a 

 psychological difficulty in supposing that three sensations so different from each 

 other as those of red, green, and violet can by their union produce a fourth 

 sensation absolutely distinct from any of them viz. white. 



The fact that in the Hering theory this difficulty is obviated has contributed 

 greatly to its acceptance by physiologists. In this theory the retina is supposed 

 to contain three substances in which chemical changes may be produced by ether 

 vibrations, but each of these substances is supposed to be affected in two oppo- 

 site ways by rays of light which correspond to complementary color sensa- 

 tions. Thus in one substance viz. the white-black visual substance kata- 

 bolic or destructive changes are supposed to be produced by all the rays of the 

 visible spectrum, the maximum effect being caused by the yellow rays, while 

 anabolic or constructive changes occur when no light at all falls upon the 

 retina. The chemical changes of this substance correspond, therefore, to the 

 sensation of luminosity as distinguished from color. In a second substance red 

 rays are supposed to produce katabolic, and green rays anabolic changes, while 

 a third substance is similarly affected by yellow and blue rays. These two 

 substances are therefore spoken of as red-green and yellow-blue visual sub- 

 stances respectively. 



It has been sometimes urged as an objection to this theory that the effect of 

 a stimulus is usually katabolic and not anabolic. This is true with regard to 

 muscular contraction, from the study of which phenomenon most of our know- 

 ledge of the effect of stimulation has been obtained, but it should be remem- 



