SIR G. STOKES, BAKT., ON THE PERCEPTION OF COLOUR. 255 



as we really know relating to the perception of colour, 

 something' has been clone experimentally, partly in the way 

 of establishing certain laws as to the direct perception of 

 colour, partly in the wa}^ of investigating other perceptions 

 which may guide us by analogy towards forming some 

 conception of the requirements Avhich are demanded in our 

 bodily organisation in order to the perception of colour. 

 The subject lies on the border-land between physics and 

 physiology, or indeed more properly belongs to the latter, 

 though physical considerations relating to the nature of 

 light largely come in. I have no claim whatsoever to be 

 a physiologist, and in consequence felt some hesitation in 

 venturing to bring before this meeting the subject I have 

 named. Still, I have read something even on the physio- 

 logical side bearing on the question, and so far as my 

 knowledge extends, it seems to me that there is one theory 

 v^hich appears to have by far the greatest probabilities in 

 its favour. That is the one known as the Young-Helmholtz 

 theory. 



It is now universally admitted that hght consists of 

 vibrations, somewhat in the manner in which sound is 

 produced by vibrations in the air; and in the early days of 

 the study of the theory of undulations in light, very great 

 assistance was obtained from what was known of the 

 analogous phenomenon of sound. But Avhile there is a good 

 deal in the theories of light and of sound that is connnon to 

 the two, still there are some wide differences ; and Ave must 

 not lean too strongly on the analogy of light to sound in 

 our attempt to explain the phenomena of light, and more 

 especially in our endeavour to explain, so far as any explana- 

 tion can be given at all, the manner in which the dbjective 

 state of things (vibrations) is perceived by our senses. We 

 must not lean too strongly on the analogy of sound lest we 

 should be led into error. One great point of difference is 

 the nature of the vibrating medium in the two cases. In 

 the one case there are vibrations of the air — a substance 

 which we can weigh and examine chemically and so forth — 

 and in the other case we have vibrations of the mysterious 

 medium, which we believe to exist between us and the 

 remotest fixed star, to which we have given the name of 

 luminifei-ous ether. Now when sound acts upon our bodies 

 there are certain portions of the structure of our bodies 

 which are thrown into vibration, and which vibrate sympa- 

 thetically with the vibrations of the air, such, for example, 



