884 Prof. Challis's Theory of the Composition of Colours 



and if /L6 be considerable, the whiteness may overcome the sensa- 

 tion of colour. Also it is evident, that, to produce the interme- 

 diate colour, the quantities of the two colours must be mixed in 

 certain proportions. The eflfect of an excess of one of the colours 

 will be to alter the tint ; but as the change of tint must be con- 

 tinuous, the only change that can occur is an approach of the 

 compound colour to that of the rays which are in excess. Speak- 

 ing theoretically, the intermediate colour is most perfectly pro- 

 duced when the number of rays of each kind is the same, and 

 when m=m' for each pair of rays. 



(5) The following passage occurs in Sir John HerschePs 

 Treatise on Light (art. 516): — "Blue and yellow combined 

 produce green. The green thus arising is vivid and rich ; and, 

 when proper proportions of the elementary colours are used, no 

 way to be distinguished from the prismatic green. Nothing can 

 be more striking, and even surprising, than the effect of mixing 

 together a blue and yellow powder, or of covering a paper with 

 blue and yellow lines drawn close together and alternating with 

 each other. The elementary tints totally disappear, and cannot 

 even be recalled by the imagination. One of the most marked 

 facts in favour of the existence of three primary colours, and of 

 the possibility of an analysis of white light distinct from that of 

 the prism, is to see the prismatic green thus completely imitated 

 by a mixture of adjacent rays totally distinct from it both in 

 refrangibility and colour." According to this statement, a mix- 

 ture of blue and yellow powders has the same effect as a mixture 

 of blue and yellow light ; for in the second mode of making the 

 experiment, it is clear that the eye receives a mixture of blue and 

 yellow rays. I have made this experiment in another manner, 

 by placing one upon the other two pieces of blue and yellow rib- 

 bon, and holding them up to the light. The resulting green is 

 very decided, and is plainly due to a mixture of lights. M. 

 Helmholtz has established by his experiments, that the mixing 

 of coloured substances produces effects different from those of 

 mixing prismatic colours, and in particular that prismatic blue 

 and yellow do not produce green, or only a greenish-white. The 

 existence of green, in however small a degree, is a phsenomenon 

 which the theory ought to account for, and which in fact it does 

 account for in the manner explained in art. (4). Why the green 

 is so much less conspicuous than the white will be shortly con- 

 sidered. At present it is important to remark, that the experi- 

 ment above adduced proves that blue and yellow light may pro- 

 duce a vivid green. Consequently it must be concluded that a 

 difference of some kind distinct from tint exists between the blue 

 and yellow of that experiment, and prismatic blue and yellow. 

 Now the theory I am proposing involves a mathematical condition 



