30 The Perception of Colour. 



colour impressions there must be a number of distinct 

 nerve terminations, each sensitive to the impression produced 

 by a single colour. An analysis of the components of white 

 light led him to fix on three as the least possible number 

 of these nerve terminations capable of being acted on by 

 red, green, and violet respectively. By the combination of 

 these three colours, or of two of them in varying proportions, 

 either white light or any intermediate colour can be pro- 

 duced. White light is the combined sensation resulting 

 from the equal stimulation of all three nervous elements ; 

 and so with varying degrees of stimulation of one or more, 

 the particular colour perception results, yellow, for instance, 

 being the colour perceived when the terminations for red 

 and green are about equally stimulated, and the one for 

 violet little or not at all. This hypothetical explanation of 

 the phenomena has been almost universally accepted as a 

 satisfactory one, since with the help of the minimum of 

 secondary hypotheses it could be applied so as to account for 

 certain peculiarities and abnormalities of the colour-sense. 

 The theory as a whole of course rests on the doctrine of the 

 specific energy of different nerves and nerve terminations ; 

 the doctrine, namely, that each nerve responds only to one 

 particular stimulus, the optic nerve to light, the auditory 

 nerve to sound, and so on. On the Young-Helmholz 

 theory it is assumed that, in addition to the specific energy 

 of the optic nerve, as a whole, there are fibres or fibre- 

 terminations endowed with specific energies adapting 

 them for receiving different colour impressions. It might 

 be questioned in how far such an extension of the doctrine 

 is allowable, unless we are prepared to accept a similar 

 differentiation of the elements of the other nerves of 

 special sense. It would perhaps be applying the reductio 

 ad ahsurdum test to such an extension of the doctrine, to 

 what might be called secondary specific energies, to assume 

 that there must be in the olfactory nerve, or its surface 

 endings, a special element susceptible only to the stimulus of 

 one odorous substance, one each for every possible smell 

 between otto of roses and assafoetida. I do not know that 

 it is allowable to make that extension of the doctrine in the 

 case of the optic nerve, merely because we can indicate a 

 possible minimum number of elements in it so endowed, 

 while in the case of the other special senses there is no 

 approach to such a limitation. I make this criticism with 

 all humility, knowing that it is in opposition to the opinion 



