276 MR J. CLERK MAXWELL ON COLOUR, 
The colours used for Mr Purpre’s papers were— 
Vermilion, é . Vv Ultramarine, . : U Emerald Green, é EG 
Carmine, : : C Prussian Blue, : PB Brunswick Green, . BG 
Red Lead, , : RL Verditer Blue, 3 VB Misture of Ultramarine 
Orange Orpiment, . OO and Chrome, 4 UC 
Orange Chrome, ; OC 
Chrome Yellow, ‘ cY 
Gamboge, 2 F Gam 
Pale Chrome, . é PC 
Ivory Black, . 5 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 
