TRANSACTIONS OF THE SECTIONS. 13 



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 

 phsenomenon. 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 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 viewing 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 : — ver- 

 milion (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 B3 Yj 

 means a mixture of three parts blue and five parts yellow. 



Given Colour. Standard Coloiurs. Coefficient 



V. U. E. of brightness. 



Bs , 100 = 2 36 7 45 



By Yi , 100 = 1 18 17 37 



Bg Y., , 100 = 4 11 34 49 



B5 Y3 , 100 = 9 5 40 54 



B4 Y4 , 100 = 15 1 40 56 



B3 Yg ,100 = 22-2 44 64 



Bj Y5 , 100 = 35 -10 51 76 



Bi Y7 , 100 = 64 -19 64 109 



Yg , 100 = 180 -27 124 277 



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 the negative 

 values trEinsferred to the other side of the equation. They were all made by means 

 of the colour-top, and were verified by repetition at different times. It may be 

 necessary to remark, in conclusion, with reference to the mode of registering visible 

 colours in terms of three arbitrary standard colours, that it proceeds upon that theory 

 of three primary elements in the sensation of colour, which treats the investigation of 

 the laws of visible colour as a branch of human physiology, incapable of being 

 deduced from the laws of light itself, as set forth in physical optics. It takes advan- 

 tage of the methods of optics to study vision itself ; and its appeal is not to physical 

 principles, but to our consciousness of our own sensations. 



