HERMANN VON HELMHOLTZ 



these three primaries. The quality of the compound 

 colour so produced depends on the proportion of the 

 intensities of the components, and its brightness 

 depends on the sum of these intensities. There is no 

 proof that these effects depend entirely on changes 

 occurring in the retina ; the probability is, as was 

 indeed suggested by Young himself, that they are 

 connected with phenomena occurring in the brain. 



Sir David Brewster l developed a new theory of 

 colour sensation, in which the three primitive colours 

 were red, yellow and blue, 2 and it was assumed that 

 they corresponded to three kinds of objective light. 

 Each of these varieties gave throughout the spectrum 

 rays of all degrees of refrangibility, but the red pre- 

 dominated at the lower end, blue at the upper end, 

 while yellow ruled the middle. Coloured media 

 absorbed in different proportions rays of the same re- 

 frangibility, but of different colours. This theory was 

 combated by Airy, Draper and Melloni, and it has 

 now been entirely abandoned. It was founded mainly 

 on the colours apparently assumed by light in passing 

 through various transparent and colourless media ; 

 phenomena, however, that can be explained by disper- 

 sion or diffusion even in clear prisms and in the media 

 of the eye. Brewster's investigations undoubtedly 

 led to renewed research, and it was at this point 



1 Trans, of Royal Soc. of Edinburgh, ix., p. 433 ; xii., p. 123. Pogg. 

 Annalen, xxiii., p. 435. 



2 Leonardo da Vinci, Trattato della pittura, Paris, 1651, named four 

 simple colours, yellow, green, blue and red. 



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