THE SENSATION OF SIGHT. 243 



ners are the three colours I have mentioned, 1 and the sides 

 of the triangle show the transitions from red through yellow 

 into green, from green through bluish-green and ultra- 

 marine to violet, and from violet through purple to scarlet. 



Newton used the diagram of the colours of the spectrum 

 (in a somewhat different form from that just given) 

 only as a convenient way of representing the facts to 

 the eye ; but recently Maxwell has succeeded in de- 

 monstrating the strict and even quantitative accuracy 

 of the principles involved in the construction of this 

 diagram. His method is to produce combinations of 

 colours on swiftly rotating discs, painted of various tints 

 in sectors. When such a disc is turned rapidly round, 

 so that the eye can no longer follow the separate hues, 

 they melt into a uniform combination-colour, and the 

 quantity of light which belongs to each can be directly 

 measured by the breadth of the sector of the circle it 

 occupies. Now the combination-colours which are pro- 

 duced in this manner are exactly those which would result 

 if the same qualities of coloured light illuminated the 

 same surface continuously, as can be experimentally 

 proved. Thus have the relations of size and number 

 been introduced into the apparently inaccessible region 

 of colours, and their differences in quality have been 

 reduced to relations of quantity. 



All differences between colours may be reduced to three, 

 which may be described as difference of tone, difference of 

 fulness, or, as it is technically called, * saturation,' and 

 difference of brightness. The differences of tone are those 

 which exist between the several colours of the spectrum, 

 and to which we give the names red, yellow, blue, violet, 

 purple. Thus, with regard to tone, colours form a series 



1 The author has restored violet as a primitive colour in accordance with 

 the experiments of J. J. Miiller, having in the first edition adopted the 

 opinion of Maxwell that it is blue. 



