98 JAMES CLEItK MAXWKIJ, 



other cjlour; the words "proper mixture," however, 

 need, as will appear shortly, some development. 



\Ve may with advantage compare the problem 

 with one in acoustics. 



When a compound musical note consisting of 

 a pure tone and its overtones is sounded, the 

 trained ear can distinguish the various overtones 

 and analyse the sound into its simple components. 

 The same sensation cannot be excited in two different 

 ways. The eye has no such corresponding power. 

 A given yellow may be a pure spectral yellow, corre- 

 sponding to a pure tone in music, or it may bo a 

 mixture of a number of other pure tones; in either 

 case it can be matched by a proper combination of 

 three standard colours this Maxwell proved. It 

 may be, as Young supposed, that if the three standard 

 colours be projicrly selected they correspond exactly 

 to three primary sensations of the brain. Maxwell's 

 experiments do not afford any light on this point, 

 which still remains more than doubtful 



When Maxwell be^an his work the theory of 

 colours was exciting considerable interest. Sir David 

 Brewster had recently developed a new theory of 

 colour sensation which had formed the basis of some 

 discussions, and in 1852 von Ilelmholtx. published 

 his first paper on the subject. According to Brewster, 

 the three primitive colours were red, yellow and blue, 

 and he supposed that they corresponded to three 

 different kinds of objective light. Ilelmholtx pointed 

 out that experiments up to that date had been con- 

 ducted by mixing pigments, with the exception of those 

 in which the rotating disc was used, and that it is 



