174 Prof. J. D. Forbes on the Classification of Colours. 



** Their distinctions arise from a double occurrence of yel- 

 low in the first, of red in the second, and of blue in the third*." 

 In like manner, by the combination of the tertiary colours (or 

 primary hues, as Mr. Hay calls them), he produced a system 

 of quaternaries (or secondary hues), and so on until the small 

 predominance of any one or two primary colours in the com- 

 pound reduces the whole to a neutral gray. Now a simple 

 inspection of Mayer's triangle, p. 168, shows that the confusion 

 of the colours, by drawing a series of perpetually inscribed 

 triangles, increases with great rapidity, and that consequently 

 the gradations of shades will not be such as affect the eye most 

 sensibly, but will be deficient in the brighter and redundant 

 in the grayer colours. 



The law of composition of the secondary and tertiary colours 

 is however worthy of notice, and may be represented in the 

 following diagram (employing the notation of page 169). 



Hence citrine^ for instance, is a mixture of four parts of yellow, 

 with two of blue and two of red, which is equivalent to two 

 parts of yellow and six of neutral gray (since gray ^ybr) ; 

 citrine is therefore yellow verging into gray, one-fourth of the 

 mixture being yellow and three-fourths gray ; its place in 

 Mayer's triangle, fig. 1, will therefore be at Qi dividing the line 

 YW in the ratio of 3 to 1 ; and so it does by construction, 

 taking the centre of gravity of the orange and green, O, G. 



Let us take an instance of a still more indefinite colour, a 

 colour of the fourth order, which Mr. Hay calls a secondary 

 hue, and to which he gives the distinguishing names of orange 

 hue, green hue^ and purple hue; we find these to be thus 

 composed : 



Hy?,K yA'>\-> h^^y^^ 



or thus: 

 2 orange + 6 gray, 2 green + 6 gray, 2 purple + 6 gray. 

 * Hay, p. 18. 



