AND MODERN PHYSICS. 99 



necessary to make them on the rays of the spectrum 

 itself, lie then describes a method of mixing the 

 light from two spectra so as to obtain the combination 

 of every two of the simple prismatic rays in all 

 degrees of relative strength. 



From these experiments results, which at the time 

 were unexpected, but some of which must have been 

 known to Young, were obtained. Among them it 

 was shown that a mixture of red and green made 

 yellow, while one of green and violet produced blue. 



In a later paper (Philosophical Magazine, 1854) 

 Helmholtz described a method for ascertaining the 

 various pairs of complementary colours colours, that 

 is, which when mixed will give white which had 

 been shown by Grassman to exist if Newton's theory 

 were true. He also gave a provisional diagram of 

 the curve formed by the spectrum, which ought to 

 take the place of the circle in Newton's diagram ; 

 for this, however, his experiments did not give the 

 complete data. 



Such was the state of the question when Maxwell 

 began. His first colour-box was made in ^Si-- 

 Others were designed in 1855 and 1856, and the final 

 paper appeared in 18CO. Hut before that time he 

 had established important results by means of his 

 rotatory discs and colour top. In his own description 

 of this he says : u The coloured paper is cut into the 

 ton a of disc, each with a hole in the centre and 

 divided along a radius so as to admit of several of 

 them being placed on the same axis, so that part of 

 each is exposed. By slipping one disc over another 

 wo can expose any given portion of each colour. 

 G 2 



