266 SIE G. STOKES^ BART., ON THE PERCEPTION OF COLOUR. 



extreme red to the extreme violet of the speotrum ; where 

 "then is the difference ? 



The evidence of the difference Hes in the total difference 

 of the result of mixture in the two cases. When two 

 notes of different pitch are sounded together we have the 

 sensation of discord or harmony as the case may be, a 

 sensation altogether different from that of a note of inter- 

 mediate pitch. The two sensations of pitch retain their 

 individuality in the mixture. But when two lights of dif- 

 ferent refrangibility, exhibiting separately different colours, 

 are mixed, we have the sensation of a single colour ; and 

 in many cases, when the places of the two colours in the 

 spectrum are not too far apart, the mixture gives almost 

 exactly the same sensation as an intermediate colour of the 

 spectrum. And the same compound colour may be produced 

 in an infinite number of ways by mixing trios of colours of 

 definite refrangibility. 



The Right Hon. Lord Kelvin. G.C.V.O. — We have ail hstened 

 with great interest to Sir George Stokes' treatment of one of the 

 most difficult subjects in natural philosophy. In using the term 

 " natural philosophy " here, I mean the study that comprehends 

 physics and physiology — and, something beyond both, the mental 

 perceptions and emotions connecting the physical and external 

 with the psychical and nervous processes and with the wonderful 

 sensorium of which we have been hearing Sir George Stokes 

 speak. 



The theory of the perception of colour which he lias so clearly 

 explained (the Young-Helmholtz theory) is, I believe, now 

 universally accepted by scientific men over the world as abso- 

 lutely true in respect of explaining the different qualities of 

 colour ; and as having a possibility of being also mechanically true 

 in respect of this system of nerve fibres by which a hypothetical 

 explanation of known facts is given. I will say nothing on this 

 subject except to express my own intense interest in it, and my 

 desire to know the truth ; but I hope Lord Lister will tell us his 

 view in respect of the triplicity of the nervous sj'stem, connected 

 with the retina of the eye, and of the beautiful experiments cf 

 which the President has told us in respect to the different 



