Report of the Committee on Colour-Vision, 289 



the spectrum wliich we call red, the rays promote great dissimi- 

 lative changes of the red-green substance with comparatively 

 slight effect on the yellow-blue substance ; hence our sensation 

 of red. 



The yertical shading represents the red and green, and the horizontal 

 shading the yellow and blue, antagonistic pairs of sensations. The thick 

 line indicates the curve of the white sensation, 



In that part of the spectrum which we call yellow the 

 rays effect great dissimilative changesof the yellow-blue substance, 

 but their action on the red-green substance does not lead to an 

 excess of either dissimilation or assimilation, this substance being 

 neutral to them ; hence our sensation of yellow. The green rays, 

 again, promote assimilation of the red- green substance, leaving 

 the assimilation of the yellow-blue substance equal to its dissimi- 

 lation ; 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 dissimilation of the red-green substance, and by 

 adding red to blue give violet. 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 per- 

 ceived there is an excess of assimilation of both these substances ; 

 and other intermediate hues correspond to varying degrees of 

 dissimilation or assimilation of the several visual substances. 



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

 and yellow-blue substances remain in equilibrium, but the white- 

 black substance undergoes great changes of dissimilation ; and 

 we say the light is white. 



According to this theory what are called red and green blind- 

 ness are identical. The yellow-blue and white-black sensations 

 remain, but the red- green sensation is absent in both. The white 



