HERMANN VON HELMHOLTZ 



the colours in the two spectra, and the mixed rays, 

 passing through a lens, were directed on the same 

 spot of the observer's retina. This method differs 

 from that of using a rotating disc, sectors of which 

 could be coloured at pleasure, a method used by 

 Thomas Young, but worked out with great exacti- 

 tude by Clerk Maxwell, in the form of his well-known 

 colour top. The arrangement of the rotating disc is 

 such that a little area of retina is struck in rapid 

 succession with reflected rays of different wave- 

 lengths, say now the long waves of red, then the 

 short of violet, with the result that the sensation is 

 that of purple. 



By mixing the pure colours of the spectrum, Helm- 

 holtz showed that red and violet gave purple ; red 

 and blue, rose ; red and green, dull yellow ; red and 

 yellow, orange ; yellow and violet, rose ; yellow and 

 and blue, white ; yellow and green, yellow-green ; 

 green and violet, pale blue ; green and blue, blue- 

 green ; and blue and violet, indigo ; but he was un- 

 able by any combination to produce red, green and 

 violet. Further, he formulated several important 

 principles with regard to sensations of colour. Thus 

 the quality of every luminous sensation depends on 

 three variables luminous intensity, tone, and degree 

 of saturation. A sensation of colour produced by a 

 certain quantity, *, of coloured rays mixed in any way 

 whatever may be always reproduced by a certain 

 amount, #, of white light with a certain quantity, />, 

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