Mat 26, 1922] 



SCIENCE 



555 



social organization we can never know, nor can 

 we guess what acceleration the future may 

 bring to it if more of the test minds are set 

 free within the state for work of this highest 

 kind, what riches may be added to intellectual 

 life, or what fuller service may be given to the 

 practical affairs of man and to the merciful 

 work of medicine. 



Walter rLETCHER 



TETRACHROMATIC VISION AND 



THE DEVELOPMENT THEORY 



OF COLOR 



It would seem to be time for the poor child- 

 ren in the kindergarten to be taught that the 

 number of different "colors" in the spectrum 

 (and in the whole world of natural objects 

 as well) is not seven, nor six, but simply four 

 — red, yellow, green and blue. We have been 

 told lately by Dr. Jennings, in the American 

 Journal of Physiological Optics, that the num- 

 ber is seven, and by U. S. Public Health Bul- 

 letin No. 92 (prepared by direction of the 

 Surgeon General) that the number is six. The 

 Milton Bradley Company, which furnishes 

 countless delightful kindergarten objects for 

 the children, follows the customary delusion 

 that there are six. But every psychologist 

 knows by this time, thanks to the life-long 

 labors of Hering, that the number of different 

 chromatic sensations (chromata) furnished by 

 the spectrum, and by all of nature too, is 

 four. No physicist, however, is as yet aware 

 that there are more than three; I am in the 

 habit of saying that the physicists are all 

 psychically blind to both yellow and white, all 

 save one. Professor Robert Wood, who in his 

 Physical Optics explicity recognizes the ex- 

 istence of a "subjective" yellow. In course of 

 time, no doubt, even the physicists will recog- 

 nize the fact that all the color sensations are 

 "subjective" — that there are no reds, greens, 

 etc., in the extra-corporeal world, but that there 

 are simply the erythrogenic, xanthogenie, 

 ehlorogenic and cyanogenic light rays — and 

 that any ray-combination that looks white (as, 

 for instance, a mixture of "yellow" and "blue" 

 light) is a leueogenic combination and due to a 

 "leuco-base." 



The reason that led Newton to find seven 

 "colors in the spectrum was an aesthetic one — 

 the spectrum is, counted in wave-lengths, about 

 an octave long; in the music octave we recognize 

 seven notes, so why not assign seven tones also 

 to the color octave? In this way what was 

 common knowledge in regard to the number 

 of colors in the world from the time of Leon- 

 ardo da Vinci became vitiated for a hundred 

 and fifty years by an error which it is still 

 hard to recover from. Hering, in opposition 

 to Helmholtz, recognized that there are four 

 chromatic sensations, but he too was led astray 

 by a logico-aesthetic consideration; he thought 

 it would be nice if, since red and green are, 

 like blue and yellow, a "disappearing" color 

 pair, they were also a white-constitutive color 

 pair. So he said we will assume that they are 

 a white-constitutive color pair, and to make 

 the situation still more pleasing we will assume 

 that black and white too are at least a disap- 

 pearing color pair. But I have shown that 

 when you take the exact red and green (or, in 

 fact, anything near them) you get, on mixing, 

 not white but yellow. My contention on this 

 point has been accepted by Westphal, by v. 

 Kries and others; the colors which are comple- 

 mentary, or white-constitutive, are, as Titchener, 

 with a degree of honesty which is unusual in 

 the followers of Hering, admits, not red and 

 green, but crimson and verdigris, — in other 

 words, white is here, as elsewhere, made out of 

 red, green and blue. 



Normal, mid-retinal, vision is tetrachromatic. 

 It is to be hoped that we may sometime per- 

 suade the Milton Bradley people (whose red, 

 green, yellow and blue papers are, as I have 

 shown, very near to the exact, unitary, Eed, 

 Green, Yellow and Blue — I write these color- 

 names with capitals when the colors are exact), 

 and the United States government as well, that 

 the different colors in the spectrum are four 

 in number, and that if one adds to one's papers 

 two of the dual color-blends, red-blue and red- 

 yellow (the so-called purple and orange), one 

 should add also the remaining dual color-blends, 

 green-blue and green-yellow. (The fact that 

 these last two color-blends have no misleading 

 unitary names is so much to the good). At- 



