256 SIE G. STOKES, BAET., ON THE PEKCEPTION OF COLOUR. 



as the tympanum of the ear. It is not apparently essentia] 

 to the perception of sound that the tympanum should be 

 thrown into vibration, for if we press a watch against our 

 skull and stop our ears, we hear the ticking. In that case 

 the skull is no doubt thrown into vibration ; and whether 

 it be the tympanum or the skull which acts on the nerves of 

 hearing, they are apparently in that manner excited, and 

 they carry a certain stimulus, which passes along them into 

 the brain, which somehow or other becomes to us the means 

 of our sensation of sound. 



Now we must not go too far here in our analogy of light 

 Avith sound. We have no reason to believe — but quite the 

 contrary — that when light falls on the body or any portion 

 of the body (when it falls, for example, on the eye) a portion 

 of the structure of our bodies is thrown into vibrations 

 which synchronise with the vibrations in light which fall 

 upon us, I do not say that there is nothing in our bodily 

 organism which receives vibration — what I said was that we 

 have no reason to suppose that any part of the structure of 

 our body is thrown into vibrations in the manner of the 

 tympanum of the ear when sound is heard. What is thrown 

 into vibration, what is disturbed, as we have reason to 

 believe, when light falls upon the eyes is, in the first instance, 

 some of the ultimate molecules of which, we have reason 

 to believe, matter consists. These differ from a structui-e as 

 the individual bricks in a heap differ from a house. 



Now it may be (I do not say it is, because we know very 

 little about it) that it is bound up Avitli this difference, that 

 we have in the case of sound one phenomenon which has 

 absolutely no coiniterpart in light. If two musical notes are 

 sounded together, if there is a simple ratio between the times 

 of vibration we experience a pleasing sensation which we 

 call harmony ; if, for example, the frequencies are as two to 

 three we have a perfect fifth, and so forth. Now we have 

 nothing in light answering to the sensation of harnioii}'' in 

 sound. People talk it may be of colours harmonising with 

 one another, but that is merely a metaphorical form of 

 expression. In sound harmony is a pleasin g sensation, and 

 when an assortment of colour pleases us, we sometimes say 

 that the colours are harmonious, using the word in a 

 pm'ely metaphorical sense. But there is actually no 

 phenomenon known in light answering to the sensation of 

 harmony in sound. How then do different lights affect us 

 when they are put together? I am not at present speculating 



