SIK G. STOKESj BART,, ON THE PERCEPTION OF COLOUR. 261 



fibres which, if excited separately, would give rise to the 

 three supposed primary colour sensations respectively'-, but 

 which usually are excited together. Microscopic examination 

 shows that the same cone or rod is provided, not with a 

 single nerve fibre, but with a whole set of nerve fibres. 

 Therefore we cannot say a jn-iori but that it may really be 

 true that there are different nerve fibres appropriated to the 

 different supposed colour sensations ; and we have to explain, 

 if we can, two things in order to account for what we 

 observe. We have to explain for one thing— if we can 

 explain it — how it is that the nerve fibres of these three sets 

 respectively are affected in different proportions by the same 

 incident light, according to the nature of that light; so 

 that if light be taken from the red end of the spectrum, those 

 fibres which give rise to the sensation of red (supposed 

 provisionally to be one of the primaries) are the most 

 affected, though the others may be aftected to a less 

 degree, and so in other cases. AVe must seek in the distal 

 ends of the nerves, or in some apparatus connected with 

 them, for something enabling differentiation of stimula- 

 tion to take place. I do not mean to say that that has 

 been explained yet. It is not, however, incomprehensible 

 that it is a thing which may hereafter be explained. 

 At present w^e can only form certain conjectures regard- 

 ing it. 



Then there is another thing concerned with tliese three 

 primary sensations of colour, viz., that these three classes 

 of nerves being affected, some sort of stimulus appears to 

 be propagated along the nerves to the sensorium, and there 

 gives rise to a sensation differing from one to another of 

 the three classes. How sensation is there produced is a 

 question belonging to that mj^sterious region in which, so 

 to speak, mhid and matter come together, and I do not 

 suppose we shall ever be able to explain how it is that this 

 stimulation of the nerves (if they are stimulated) produces 

 in us these three sensations. But at the other end — the 

 distal end — there may possibly, as I have said, be some 

 chance of our doing something. 



Different conjectures might be formed as to how these 

 three sets of nerves might at the distal end be affected 

 in a proportion differing according to the nature of the light. 

 As regards the mode of stimulation, I may mention that 

 the most probable theory seems to be that it is due to the 

 result of a photo-chemical change, possibly it may be in 



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