BRITISH ASSOCIATION TOE THE ADVANCEMENT OF SCIENCE. 61 



sometimes to yellow or blue, but never to green. Prof. J. D. Forbes, of Edin- 

 burgh, made similar experiments in 1849, with the same result. Prof. Helm- 

 holtz, 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 a particle 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, yel- 

 low, and orange, and the yellow stops the blue and violet. I have made experi- 

 ments on the mixture of blue and yellow light— hy rapid rotation, by combined 

 reflection and transmission, by viewing them out of a 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 spec- 

 trum. 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 yel- 

 low 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 

 *r eated 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 g Y 3 means a mixture of three parts blue and five parts yellow. 



Given Colour. Standard Colours. Co-efficient, 



V. U. E. 



B 9 100 2 36 7 45 



B 7 Y, 100 = 1 18 17 37 



B 6 Y g 100 = 4 11 34 49 



B 6 Y 3 100 = 9 5 40 64 



B 4 Y 4 100 = 15 1 40 56 



B s Y 5 100 = 22 -2 44 64 



B 8 Y 6 100 = 35 -10 51 76 



B, Y 7 100 = 64-19 64 109 



Y 8 100 = 180 -27 124 277 



— The columns Y., XL, E. give the proportions of the stardard colours which nre 

 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 admix- 

 ture of yellow diminishes the brightness of the blue. The negative values of TJ. 

 indicate that a mixture of V., U., and E. cannot be made equivalent to the given 

 eolour. The experiments from which these results were taken had the negative 



