556 



SCIENCE 



[Vol. LV, No. 1430 



tention should be called at the same time to the 

 curious fact that though you may easily have 

 the physical conditions (the proper light-ray 

 mixtures) for the two other possible dual color- 

 blends (the red-greens and the yellow-blues), 

 these are sensations that never occur — their 

 places are taken, respectively, by yellow and 

 by white. 



I should like to mention that the color theory 

 which I have proposed (the development color 

 theory) is the only one in existence which holds 

 together (the function of a theory), and makes 

 reasonable, the three fundamental color-sensa- 

 tion facts (and the other, subsidiary, facts as 

 well). These are: 



A. The Helmholtz fact : the basis of color- 

 vision is a three-receptor (chemical) process, 

 — the "red," "green" and "blue" light-rays are 

 sufficient (on mixing) to reproduce the whole 

 gamut of the color sensations. 



B. Nevertheless (the Hering fact) yellow 

 and white are also unitary sensations and not 

 any sort of sensational color blends, although 

 they may be produced by physical light-ray 

 mixtures. Hering thus corrects what I have 

 called the psychical color-blindness (to yellow 

 and to white) of the Helmholtz school, but at 

 the cost of concealing from his followers all the 

 facts which are mapped out in the Helmholtz 

 triangle, or what the metallographers call when 

 they make a diagram of their ternary alloys — 

 a less frightening word perhaps — the "tri- 

 axial diagram." The color-triangle, in other 

 words, is nothing more than the representation 

 of mixed color-constitution in terms of trilinear 

 coordinates; what could be more natural when 

 the variables which are both sufficient and in- 

 dispensable are three in number"? 



C. Of equal importance is the fact of the 

 order of phylogenetie development of the light- 

 sensations (achromatic and chromatic). It is, 

 in its three successive stages, as has -been per- 

 fectly well made out, this : 



(1) A white-sense only, achromatic vision 

 (furnished by the more primitive retinal ele- 

 ments, the rods), which occurs (o) in the lower 

 animals, such as lived, for instance, in car- 

 boniferous times (when colored flowers and 



colored birds did not yet exist) ^, (b) in those 

 defective individuals who have achromatic 

 vision only, and (c) in the far periphery of 

 our own retina. 



(2) Dichromatic vision — the spectrum is 

 yellow at one end, blue at the other (but in 

 place of what should be the yellow-blues ap- 

 pears white). This is the vision (a) of the bees 

 (v. Friseh), (b) of the partially color-blind, 

 and (c) of our own mid-periphery. 



(3) Complete, tetraehromatic, color- vision, 

 — the red and green sensations have been added ; 

 but where we have the physical conditions for 

 seeing the red-greens, or the red-green-blues, 

 yellow and ivhite, respectively, take their places. 



The theory of Helmholtz is (as Professor 

 Cattell has well said) both pre-psychologieal 

 and pre-evolutionary. That of Hering (be- 

 sides being otherwise impossible) is pre-evo- 

 lutionary : there is no question that red and 

 green (which revert to yellow) are developed 

 out of yellow. But worse than this — each of 

 these theories is utterly contradictory to the 

 facts which the other theory is expressly built 

 up upon. This circumstance has not hitherto 

 been sufficiently noticed; this Mr. Troland 

 says ("The Enigma of Color Vision," Amer- 

 ican Optical Society, p. 8) : ''The Young- 

 Helmholtz theory is preferred by physicists 

 because it lays emphasis primarily upon the 

 stimuli to vision, while the Hering theory re- 

 ceives more attention at the hands of the 

 ' psychologists because its fundamental concep- 

 tions are derived from introspective analysis." 

 This is true, but it is very far from being an 

 adequate account of the situation. (1) The 

 Young-Helmholtz school not only assumes but 

 proves (not, as is often said, by means of the 

 Konig-Dieteriei spectral distribution curves by 

 themselves, but by the complete coincidence of 

 these curves with those, respectively, of the 



1 We have no means of knowing whether our 

 own background sensation, the non-light sensa- 

 tion, that of blackness (which exists for the pur- 

 pose of filling up OUT visual field), came in with 

 the first, non-specifie, light-sensations, or only 

 later. There is some ground for thinking it arose 

 later. I discuss this question in my coming arti- 

 cle on ' ' The Sensation of Blackness. ' ' 



