276 



MR J. CLERK MAXWELL ON COLOUR, 



The colours used for Mr Purdie's papers were- 



Vermilion, 



V 



Ultramarine, 



U 



Carmine, 



c 



Prussian Blue, 



PB 



Red Lead, 



RL 



Verditer Blue, 



VB 



Orange Orpiment, 



00 







Orange Chrome, 



OC 







Chrome Yellow, 



CY 







Gamboge, 



Gam 







Pale Chrome, . 



PC 







Emerald Green, . EG 



Brunswick Green, . BG 

 Mixture of Ultramarine 



and Chrome, . UC 



Ivory Black, . . Bk 



Snow White, . . SW 



White Paper (Pirie, Aberdeen). 



The colours in the first column are reds, oranges, and yellows ; those in the 

 second, blues; and those in the third, greens. Vermilion, ultramarine, and 

 emerald green, seem the best colours to adopt in referring the rest to a uniform 

 standard. They are therefore put at the head of the list, as types of three con- 

 venient divisions of colour, red, blue, and green. 



It may be asked, why some variety of yellow was not chosen in place of 

 green, which is commonly placed among the secondary colours, while yellow 

 ranks as a primary ? The reason for this deviation from the received system is, 

 that the colours on the discs do not represent primary colours at all, but are 

 simply specimens of different kinds of paint, and the choice of these was deter- 

 mined solely by the power of forming the requisite variety of combinations. Now, 

 if red, blue, and yellow, had been adopted, there would have been a difficulty in 

 forming green by any compound of blue and yellow, while the yellow formed by 

 vermilion and emerald green is tolerably distinct. This will be more clearly per- 

 ceived after the experiments have been discussed, by referring to the diagram. 



As an example of the method of experimenting, let us endeavour to form a 

 neutral gray by the combination of vermilion, ultramarine, and emerald green. 

 The most perfect results are obtained by two persons acting in concert, when the 

 operator arranges the colours and spins the top, leaving the eye of the observer 

 free from the distracting effect of the bright colours of the papers when at rest. 



After placing discs of these three colours on the circular plate of the top, and 

 smaller discs of white and black above them, the operator must spin the top, and 

 demand the opinion of the observer respecting the relation of the outer ring to 

 the inner circle. He will be told that the outer circle is too red, too blue, or too 

 green, as the case may be, and that the inner one is too light or too dark, as com- 

 pared with the outer. The arrangement must then be changed, so as to render 

 the resultant tint of the outer and inner circles more nearly alike. Sometimes 

 the observer will see the inner circle tinted with the complementary colour of the 

 outer one. In this case the operator must interpret the observation with respect 

 to the outer circle, as the inner circle contains only black and white. 



By a little experience the operator will learn how to put his questions, and 





