Septembek 9, 1898.] 



SCIENCE. 



309 



character of the three fundamental sensa- 

 tions. Albert, in 1882, from a study of the 

 color changes in faint light, pointed out the 

 direction and character of the corresponding 

 sensation- changes. * 



A method by which such changes might 

 be brought about was not difficult to sug- 

 gest. The idea of Newton and Young, of 

 nerve-fibres vibrating in unison with the 

 light-waves, had grown increasingly im- 

 probable as, on the one hand, the rate of 

 vibration of light became known, and on 

 the other the maximum rapidity of vibra- 

 tion transmissible by a nerve-fibre, which is 

 a quantity of an entirely different order. 

 Hence most modern theorists have turned 

 to photo -chemical action, and Helmholtz 

 among the rest, although with character- 

 istic caution, he advocates no particular 

 form of light efiect, resting the claims 

 of his theory, in the second edition of 

 his great book as in the first, on the 

 physical possibilities of representing all 

 colors by means of three. But if we suppose 

 red-sensitive, green-sensitive and violet- 

 sensitive molecules, there is no difficulty, 

 when one thinks of modern photographic 

 processes, in supposing that the green -sen- 

 fiitive substance, for example, may be so 

 chemically changed as to coincide in char- 

 acter with the red-sensitive. Each would 

 send to the brain the impression of its own 

 characteristic color, while each would be 

 affected in exactly the same manner by any 

 given light. In other words, the color 

 curve corresponding to the sensation of the 

 fundamental green in the Young-Helmholtz 

 theory would be more or less perfectly su- 

 perposed upon that of the fundamental red. 



As a result, there would be no more sen- 

 sation of green or red, but only of their 

 compound, yellow. Now this is just what 

 a color-blind person sees, as Hippel and 

 Holmgren showed in their long-neglected 

 papers already referred to. 



* Albert, Wied. Ann., 16, p. 129, 1882. 



So peripheral color-blindness may be ex- 

 plained by a gradual approximation of the 

 three color-substances, and a gradual super- 

 position of the corresponding sensation- 

 curves, until in the outer zone all three 

 coincide. 



But when we attempt to apply this sug- 

 gestion to all the changes noted before, in 

 ordinary color-blindness, peripheral color- 

 blindness and twilight color-blindness, as 

 von Kries happily calls it, the shifting of 

 the curves becomes so great and so various 

 that we realize that we are dealing no 

 longer with a scientific theory, but with 

 fanciful and arbitrary arrangement. As a 

 first approximation, the Young-Helmholtz 

 hypothesis is valuable, leading to simple 

 and definite connections between great 

 numbers of facts. As a detailed explana- 

 tion of existing phenomena it is unsatis- 

 factory, and growing more so daily. 



One thing is becoming steadily evident, 

 that the sensation of brightness, perhaps 

 also the sensation of white, must be ac- 

 counted for in some other way than as a 

 summation-effect of separate color-sensa- 

 tions. Another class of phenomena points 

 yet more directly to this view. I refer to 

 the discovery of Eood that the effect of 

 sudden variations in brightness, the 'flicker' 

 sensation, is dependent on brightness alone, 

 and not upon color. It is difficult for one 

 who has seen how easily and definitely a 

 red and a gray, or a red and a blue, can be 

 compared in brightness by this method, to 

 believe that brightness is not an indepen- 

 dent sensation. The words of Helmholtz 

 are worth quoting here. " As to myself," 

 he says, " I have always the feeling that in 

 photometric compai'ison of different colors 

 it is a question, not of the comparison of 

 one sort of magnitude, but of the combined 

 effects of two, brightness and color, of which 

 I cannot form a simple summation and of 

 which I can give no scientific definition." 



The strongest point of attack upon the 



