772 SIGHT. 



light, which thus appear to us distinct, and which we may speak of as 

 " fundamental sensations," are white, black, red, yellow, green, blue. Each 

 of these seems to us to have nothing in common with any of the others, 

 whereas in all other colors we can recognize a mixture of two or more of 

 these. 



This result of common experience suggests the idea that these fundamental 

 sensations are the primary or simple sensations, spoken of above as those out 

 of which all other sensations may be supposed to be compounded. And a 

 theory has been proposed to reconcile the various facts of color vision with 

 the supposition that we possess these six fundamental sensations. This theory, 

 known as that of Hering, is somewhat as follows : The six sensations readily 

 fall into three pairs, the members of each pair having analogous relations 

 to each other. White and black naturally go together, the one being the 

 antagonistic or correlative of the other. There is a similar connection 

 between red and green, the one being the complementary of the other, and 

 between yellow and blue, which are similarly complementary. We saw 

 reason, a short time back (p. 765), for believing that vision originates in 

 the changes taking place in certain visual substances (or a visual substance) 

 in the retina. And the theory of which we are speaking supposes that there 

 exist in the retina, or at least somewhere in the visual apparatus, three dis- 

 tinct visual substances which are continually undergoing a double metab- 

 olism, one constructive, of assimilation or building up, and the other 

 destructive, of dissimilation or breaking down. One of these substances 

 is further of such a nature that when dissimilation is in excess of assimila- 

 tion we have a sensation of white, and when assimilation is in excess a sen- 

 sation of black. With a second substance excess of dissimilation provokes 

 red, of assimilation green ; and with the third substance, yellow and blue 

 respectively. When in the latter two substances dissimilation and assimila- 

 tion are exactly equal, no effect is produced ; but with the first substance 

 this condition produces in us the effect of gray. Further, these substances 

 are of such a kind that while the first or white-black substance is influenced 

 by rays along the whole range of the spectrum, the two other substances are 

 differently influenced by rays of different wave-length. Thus in the part of 

 the spectrum which we call red, the rays promote a rapid dissimilation of 

 the red-green substance with comparatively slight effect in either direction 

 on the yellow-blue substance; hence our sensation of red. In that part of 

 the spectrum which we call yellow the rays effect a marked dissimilation of 

 the yellow-blue substance, but their action on the red-green substance is 

 equal in the direction of both assimilation and dissimilation ; hence our sen- 

 sation of yellow. The green rays, again, promote assimilation of the red- 

 green substance, leaving the assimilation of the yellow-blue substance equal 

 to the dissimilation, and similarly blue rays cause assimilation of the yellow- 

 blue substance, and leave the red-green substance neutral. Finally, at the 

 extreme blue end of the spectrum the rays once more provoke dissimila- 

 tion of the red-green substance. When orange rays fall on the retina, 

 there is an excess of dissimilation of both the red-green and the yellow- 

 blue substance ; when greenish-blue rays are perceived there is an excess 

 of assimilation of both these substances ; and other intermediate tints cor- 

 respond to variable amounts of dissimilation or assimilation of two or more 

 of these substances. 



665. When all the rays together fall on the retina, the red-green and 

 yellow-blue substances remain in equilibrium, but the white-black substance 

 is violently dissimilated ; and we say the light is white. 



Another theory (known .as the Young-Helmholtz theory, because it was 

 introduced by Young and more fully elaborated by Heimholtz) strives to 



