September 9, 189S.] 



SCIENCE. 



307 



in motion less or more forcibly, by undula- 

 tions differing less or more from a perfect 

 unison ; * * * and each sensitive filament of 

 the nerve may consist of three portions, one 

 for each principal colour." Young, as is 

 well known, afterward substituted green for 

 yellow, in his triad of principal colors. 



It is to be observed that the ' particles ' 

 spoken of in the above quotation, as vibra- 

 ting in unison with the undulations of light, 

 are not to be considered as molecules, in 

 the modern sense. He speaks of them as 

 particles, then as sensitive filaments, and in 

 a later paper on the same subject as ' sym- 

 pathetic fibres.' A supposition, such as has 

 been urged, that Young is here speaking of 

 molecular constitution, and anticipating the 

 photo-chemical theories of our own day, is 

 an anachronism. Young is thinking of the 

 wave-theory of light, and of sympathetic 

 vibrations, or, as we should say, of reso- 

 nance. One who meant chemical decompo- 

 sition would not speak of the shattered 

 molecules as sympathetic fibres. Young, in 

 fact, is not consciously proposing a new 

 theory, much less one so startlingly differ- 

 ent. He is simplifying Newton's, making 

 for that purpose four hypotheses, three of 

 which, as he remarks himself, ' are liter- 

 ally parts of the more complicated New- 

 tonian system.' Young's theory seems to 

 have attracted little attention, until brought 

 once more to public notice by Helmholtz, 

 in 1860. 



Helmholtz suggests one objection to the 

 theory, and a modification to obviate the 

 objection, but remarks that the essence of 

 the theory does not lie in this or that spe- 

 cial assumption, but in the fact that the 

 color-sensations are conceived as com- 

 pounded out of three entirely independent 

 properties in the nerve-substance. 



Therefore, he says, if only for the sake 

 of clearness of representation, he uses 

 Young's original and simple form of state- 

 ment, ascribing the different color-sensa- 



tions to three-kinds of nerve endings in the 

 retina, sensitive respectively to red, green 

 and violet light, the sensation of white to 

 the equal excitation of all three sets of 

 nerve-fibres, and color-blindness to the ab- 

 sence of one or more of the sets of such 

 fibres. He puts into Young's words, how- 

 ever, an entirely different meaning, by clas- 

 sifying Young's hypothesis as ' only a spe- 

 cial application of the law of specific sense- 

 activities. ' This statement, a most natural 

 one to a pupil of Johann Miiller, changes 

 the whole character of the theory, and 

 makes it really a new one. 



The hypothesis in this simple form met 

 very well the conditions of normal vision, 

 and the cases of color-blindness thus far 

 studied. Dif&culties soon arose. Color- 

 blindness, especially red-blind persons, per- 

 sisted in calling the principal colors seen in 

 the spectrum, yellow and blue, instead of 

 green and blue, as they should have done 

 if the red sensation were simply absent. It 

 was long supposed that this was merely a 

 question of naming the colors, and that 

 those actually seen were green and blue. 

 So lately as 1892 the report of the British 

 Association committee on color-vision con- 

 tains colored spectrum-plates, in which the 

 spectrum as seen by a red-blind person is 

 shown as composed of green and blue, 

 while that seen by a green-blind person is 

 formed of red and blue. Abney prefixes 

 the same plate to his Tyndall lectures on 

 color-vision, published in 1894. But in 

 1881 Hippel* and Holmgrenf had examined 

 the vision of a person, one of whose eyes 

 was normal, the other typically color-blind. 

 All the long-waved end of the spectrum 

 was described as yellow, the sensation be- 

 ing tested by comparison with the normal 

 eye. Other cases of color-blindness, arising 

 from disease, corroborate this testimony. 



*Aroh. f. Ophthal., XXVI. (2), p. 176, 1880; 

 XXVII. (3), p. 47, 1881. 

 tSeeProo. Roy. Soc, No. 209. 



