190 THE PERCEPTION OP LIGHT AND COLOR. 



Nevertheless, a rapid review of the principal hypotheses which have 

 been advanced by scientists may not be entirely without interest. 



The first hypothesis that presents itself to the mind is the assump- 

 tion that for each wave length of light there corresponds a distinct 

 kind of excitation of the optic nerve, and consequently a distinct sen- 

 sation, but this idea, which at first sight appears so simple, is open 

 to a very grave objection, which has given birth to the trichromatic 

 theory which the name of the great Helmholtz has endowed with a 

 remarkable authority, notwithstanding that he is not the author of the 

 theory; neither does he claim authorship, for he has himself declared 

 that it was borrowed from Young. It seems, moreover, that it was 

 developed several years before the publication of his treatise on physio- 

 logical optics, with all the accompaniment of general conceptions that 

 give to it its philosophic scope. 



We now turn to the consideration of a treatise that appeared in 

 1855, entitled, "Electro-dynamisme vital," under the nom de plume of 

 Philips, under which the real author, M. Durand (de Gros), then pro- 

 scribed, hid his identity. The basis of the trichromatic theory is found 

 in the doctrine of the specific energy of the nerves, according to which 

 each nerve filament can only act in a single manner, and consequently 

 can only provoke sensations differentiated by their intensity. Apply- 

 ing this theory to the whole nervous system, Durand summarizes it as 

 follows : 



"1. The nature of each animal or vegetal function depends essen- 

 tially on the characteristic activity of its corresponding nerve fiber or 

 fibers. 



"2. The characteristic activity of every nerve fiber is its invariable 

 attribute." ' 



Applying this theory further on he expresses himself thus : 



" It would be carrying these principles too far to assume the existence 

 of a distinct faculty and of a distinct individual nerve fiber for each of 

 the varieties of sensation that can be experienced. It is conceivable, 

 indeed, that the same agent might excite the same sensitive faculty 

 with an unequal intensity * * *. Thus, although still remaining 

 invariable in its nature, a sensitive faculty might vary in the degree of 

 excitation to which it is susceptible, and, moreover, it is very easy to 

 comprehend that two or more sensitive faculties excited simultaneously 

 might give rise to a compound sensation, which it would consequently 

 be improper to attribute to a new elementary faculty or to a particular 

 nerve fiber." 2 



In a special consideration of the sense of sight Durand remarks 

 that, although the advocates of the theory of special nerve fibers are 

 correct in not admitting that the nerve fibers affected in color sen- 

 sations are localized in distinct portions of the retina (under these con- 

 ditions objects would change color if displaced), there would be 

 no objection to a different subdivision of the fibers. He states that 



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