528 COLOUR SENSATIONS [BOOK in. 



when dissimilation is in excess of assimilation, \ve have a sensation 

 of white, and when assimilation is in excess a sensation 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 

 assimilation are exactly equal, 110 effect is produced ; but with the 

 first substance, this condition produces in us the effect of grey. 

 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 di- 

 rection 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 sensation 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 assimila- 

 tion of the yellow-blue substance, and leave the red-green sub- 

 stance neutral. Finally at the extreme blue end of the spectrum, 

 the rays once more provoke dissimilation of the red-green sub- 

 stance. When orange rays fall on the retina, there is a:i excess of 

 dissimilation of both the red-green and the yellow-blue substance ; 

 when greenish-blue rays are perceived there is an excess of assimi- 

 lation of both these substances ; and other intermediate tints 

 correspond to variable amounts of dissimilation or assimilation of 

 two or more of these substances. 



When all the rays together fall on the retina^the red-green and 

 yellow-blue substance 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, be- 

 cause it was introduced by Young and more fully elaborated by 

 Helmholtz) strives to reduce the matter to still further simplicity. 

 Starting from the fact mentioned a short time since, that all colour 

 sensations, including the sensation of white, may be obtained by 

 the appropriate mixture of three standard sensations, this theory 

 teaches that our visual apparatus is so constituted as, when excited, 

 to give rise to three primary sensations, and that these primary 

 sensations are called forth in different degrees by different rays of 

 light, so that each ray gives rise to a different mixture of the three. 

 Several sets of three such primary sensations might be chosen, 

 which would satisfy the conditions of giving rise, by appropriate 

 mixture, to all sensations of colour including white ; but for reasons, 

 into which we cannot enter fully here, the sensations which may 

 thus be taken as primary sensations appear to correspond to our 



