CONTRIBUTIONS TO SCIENCE. 473 



should not look at the coloured papers, nor be told the propor- 

 tions of the colours during the experiments. When these ad- 

 justments have been properly made, the resultant tints of the 

 outer and inner circles ought to be perfectly indistinguishable 

 when the top has a sufficient velocity of rotation. The number 

 of divisions occupied by the different colours must then be read 

 off on the edge of the plate, and registered in the form of an 

 equation. Thus in the preceding experiment we have vermilion, 

 ultramarine, and emerald green outside, and black and white 

 inside. The numbers, as given by an experiment on the 6th 

 March 1855, in daylight without sun, are 



37V + -27U + -36EG = -28SW + '72Bk. 



In a similar way matches were obtained in which the 

 resulting tint was a decided colour and not a neutral gray. 

 Tims on the 5th of March 1855 a match was obtained as 

 follows 



39PC + -21U + -40Bk = -59V + -41EG, 

 where PC represents pale chrome. The resulting tint in 

 this case is an impure yellow. 



Mixtures which appear to make perfect matches by 

 one kind of light are far from matching one another when 

 viewed by a different light. Thus, C representing carmine, 

 the following match was obtained by daylight, viz. 



44C + -22U + -34EG = -iTSW+'SSBk ; 

 while by gaslight the match was 



which shows that the yellowing effect of the gaslight tells 

 more on the white than on the combination of colours." 



Maxwell, following Young, represented the results of his 

 experiments graphically by means of a triangle, at the 

 angular points of which he placed the three colours which 

 he believed to most nearly correspond with the primary 

 sensations. (See Plate I.) The colours he selected for 

 these positions were vermilion, ultramarine, and emerald 

 green, of such strength that, when mixed in equal propor- 

 tions, they produced a neutral gray which therefore appeared 

 at the centre of the triangle. In the paper published in the 

 Edinburgh Transactions the reason alleged for selecting green 

 in preference to yellow is the fact that it was found possible 



