264 SIR a. STOKES^ bart., on the perception of colour. 



fails to reveal any difference in these different nerves ; but 

 Avliy should we expect that it should be capable of revealing 

 anything different ? For aught we know to the contrary, the 

 nerve threads of these three classes may be just like one 

 another, and the difference in their function may arise from 

 the difference in the mode of stimulation at the distal endof the 

 nerve apparatus, and from some difference in the way in which 

 they affect the sensorium at the other end. In relation to 

 Goldschneider's experiments, I am told that one of the marked 

 heat points and one of the cold points were selected, and 

 self- vivisection in a small way was performed by punching 

 out little bits of skin, so as to catch the ends of the two 

 Jfinds of nerves. On examining them under the microscope, 

 no particular difference could be made out. Hence, if we 

 cannot make out any difference in the distal ends of the 

 nerve fibres of the retina, Ave cannot say that there is there- 

 fore no difference. 



As I said the subject that I have ventured to bring before 

 you is not only out my line but it is rather speculative. 

 Still, I think it leads us to some interesting contemplations, 

 and one thing I think we cannot fail to be strongly impressed 

 with — viz., the astonishing complexity of this marvellous 

 organ, the eye, and the wonderful proof which (to my own 

 mind at least) it gives of design in its construction. 



Addition made while going through the Press. 



The question naturally presents itself, if there are three 

 primary sensations of colour, brought about by the stimula- 

 tion of three sets of nerves respectively, how is it that the 

 sensation of unity of direction is preserved ? If, when the 

 image of a star falls in focus on the retina there are three 

 classes of nerve fibres excited, leading, it may be, to 

 different places in the brain, how is it that we see but one 

 white star, instead of three stars showing respectively the 

 three primary colours ? 



The only answer, as I conceive, that it is possible to make 

 to such a question is to show that the phenomenon is in 

 perfect analogy with what we know by experience in the 

 case of other nerves of sensation. Suppose, for example, 

 that a toe or a finger or an elbow is gently pressed, or else 

 that a small Avarm body is held against it The quality of 

 the sensation, be it that of pressure or of Avarmth, is alike 



