244 ON THE THEORY OF COMPOUND COLOURS. 



the blue and violet. I have made experiments on the mixture of blue and 

 yellow light by rapid rotation, by combined reflexion and transmission, by view- 

 ing them out of focus, in stripes, at a great distance, by throwing the colours 

 of the spectrum on a screen, and by receiving them into the eye directly ; and 

 I have arranged a portable apparatus by which any one may see the result of 

 this or any other mixture of the colours of the spectrum. In all these cases 

 blue and yellow do not make green. I have also made experiments on the 

 mixture of coloured powders. Those which I used principally were " mineral 

 blue" (from copper) and "chrome-yellow." Other blue and yellow pigments gave 

 curious results, but it was more difficult to make the mixtures, and the greens 

 were less uniform in tint. The mixtures of these colours were made by weight, 

 and were painted on discs of paper, which were afterwards treated in the manner 

 described in my paper " On Colour as perceived by the Eye," in the Transactions 

 of the Royal Society of Edinburgh, Vol. xxi. Part 2. The visible effect of the 

 colour is estimated in terms of the standard-coloured papers : vermilion (V), 

 ultramarine (U), and emerald-green (E). The accuracy of the results, and their 

 significance, can be best understood by referring to the paper before mentioned. 

 I shall denote mineral blue by B, and chrome-yellow by Y ; and B, Y, means 

 a mixture of three parts blue and five parts yellow. 



Coefficient 

 of brightness. 



45 



37 



49 



54 



56 



64 



76 



109 



277 



The columns V, U, E give the proportions of the standard colours which 

 are equivalent to 100 of the given colour; and the sum of V, U, E gives a co- 

 efficient, which gives a general idea of the brightness. It will be seen that the 

 first admixture of yellow diminishes the brightness of the blue. The negative 

 values of U indicate that a mixture of V, U, and E cannot be made equivalent 

 to the given colour. The experiments from which these results were taken had 



