[From the Report of the British Association, 1856.] 



XIII. On the Tfieory of Compound Colours with reference to Mixtures of Blue 



and Yellow Light. 



WHEN we mix together blue and yellow paint, we obtain green paint. This 

 fact is well known to all who have handled colours ; and it is universally 

 admitted that blue and yellow make green. Red, yellow, and blue, being the 

 primary colours among painters, green is regarded as a secondary colour, arising 

 from the mixture of blue and yellow. Newton, however, found that the green 

 of the spectrum was not the same thing as the mixture of two colours of the 

 spectrum, for such a mixture could be separated by the prism, while the green 

 of the spectrum resisted further decomposition. But still it was believed that 

 yellow and blue would make a green, though not that of the spectrum. As 

 far as I am aware, the first experiment on the subject is that of M. Plateau, 

 who, before 1819, made a disc with alternate sectors of prussian blue and gam- 

 boge, and observed that, when spinning, the resultant tint was not green, but 

 a neutral gray, inclining sometimes to yellow or blue, but never to green. Prof. 

 J. D. Forbes of Edinburgh made similar experiments in 1849, with the same 

 result. Prof. Helmholtz of Konigsberg, to whom we owe the most complete 

 investigation on visible colour, has given the true explanation of this phenomenon. 

 The result of mixing two coloured powders is not by any means the same as 

 mixing the beams of light which flow from each separately. In the latter case 

 we receive all the light which comes either from the one powder or the other. 

 In the former, much of the light coming from one powder falls on particles of 

 the other, and we receive only that portion which has escaped absorption by one 

 or other. Thus the light coming from a mixture of blue and yellow powder, 

 consists partly of light coming directly from blue particles or yellow particles, 

 and partly of light acted on by both blue and yellow particles. This latter light 

 is green, since the blue stops the red, yellow, and orange, and the yellow stops 



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