'310 



SCIENCE. 



[N. S. Vol. VIII. No. 193. 



Young-Helmholtz color-theory has, in fact, 

 always been the explanation of the white- 

 sensation, and one of the principal advan- 

 tages of the theory proposed by Hering 

 in 1874 was the separation of the white- 

 ■eensation, which he identified with the sen- 

 sation of brightness, from special connec- 

 tion with the color-sensations, or rather his 

 including it, upon the same level, in the list 

 of his three sets of opposing colors, black- 

 white, red-green, yellow-blue. 



A photo-chemical substance is supposed 

 to exist in the eye — where it is not decided 

 — which possesses the remarkable property 

 that if acted upon by light of one wave- 

 length its molecules are dissociated or dis- 

 similated ; if acted upon by light of another 

 wave-length they are built up or assimi- 

 lated again. This substance exists in three 

 forms : One is, let us say, dissimilated by 

 red light, assimilated by green ; one is sim- 

 ilarly acted upon by yellow and blue, and 

 one by white and black. There are thus 

 six color-processes, arranged in three pairs. 

 They are antagonistic in character, so that 

 if red and green, or yellow and blue light 

 act on the retina at the same time and with 

 equal strength they neutralize each other 

 and the sensation of color is completely de- 

 stroyed. It is difficult to form a clear con- 

 ception of these processes. Photo-chemical 

 actions of somewhat antagonistic character 

 are well known, but the analogies between 

 them and the visual processes are hardly 

 close enough to be of great assistance. 



The hypothesis, however difficult to con- 

 ceive of in itself, is veiy definite, and ex- 

 plains as well as the Young-Helmholtz 

 theory the results of ordinary color- vision, 

 and in respect to subjective phenomena, as 

 contrast, or after-images, is much more 

 satisfactor}', in formal statements at least. 

 But it fails equally, though for different 

 reasons, to explain satisfactorily the more 

 lately discovered or less evident phenomena 

 of vision. 



Complementary colors are regarded, on 

 this theory, as mutually destructive, the 

 one representing an assimilative, the other 

 a dissimilative process. The white or gray 

 which results from their combination is due 

 to the action on the white-black visual sub- 

 stance, which was unperceived in the sep- 

 arate colors, being masked by their greater 

 brilliancy, but becomes effective when they 

 are neutralized by mixture. If two equal 

 grays are formed, one let us say from red 

 and green, the other from yellow and blue, 

 they must according to the theory contain 

 equal amounts of white, and the colors in 

 each are completely neutralized. They 

 should, therefore, remain equal at all degrees 

 of brightness. But Ebbinghaus,* by mixing 

 spectral colors, and Mrs. Franklin,! with 

 color-disks, have shown that this is not 

 true. If they are made equal when bright, 

 and the intensity gradually diminished, the 

 red- green mixture greatly exceeds in bright- 

 ness. If they are equated when of feeble 

 intensity, and then made brighter, the yel- 

 low-blue mixture surpasses. Konig| has 

 lately examined this with much elabora- 

 tion, confirming these results, and showing 

 also that the brightness of a gray obtained 

 thus by mixture is always equal to the sum. 

 of the brightness of the separate colors, 

 whether by bright or faint light. 



This single experiment, so amply con- 

 firmed, appears completely destructive to 

 Hering's fundamental hypothesis, at least 

 in its original and simple form. 



It may be remarked in passing that photo- 

 metric methods which have been proposed, 

 of comparing lights of different colors by 

 reducing their brightness until the color- 

 differences vanish, are worthless. 



Other phenomena of similar character 

 exist which are equally difficult of explana- 



* Zeitsohrift fur Psychol, und Physiol. Sinnesor- 

 gane, V., p. 145, 1893. 



t Mind, N. S., II., p. 487, 1893. 



X Ber. d. Preuss. Akad., p. 945, 1896. 



