Prof. J. D. Forbes oti the Classification of Colours. 169 



same result will evidently flow from mixing two parts of orange 

 with one of blue, or two of green with one of red. 



It also clearly follows from this construction, that a point in 

 the triangle may always be found which shall represent any 

 2)ossible proportional mixture of the three colours, because the 

 centre of gravity of the three elements, however unequal, must 

 of necessity be found within the triangle. 



Also, a complex colour of three elements may be regarded 

 as composed of primary colours and their binary compounds 

 in an infinite variety of ways. Thus, the colour called citrine 

 by some authors, and which is described as a compound of 

 equal parts of orange and green, has its place in the triangle 

 at a, which shows that it is intermediate between pure yellow 

 and neutral gray in the proportion of 1 of the first to 3 of the 

 second ; or it is a mixture of pure yellow and pure purple in 

 equal proportions. 



The annexed diagram shows the principle of Mayer's mix- 

 ture of colours, the subscribed figures denoting the relative 

 portions of each colour in any compound, the sum of the units 

 making up 8 in every case. The same principle of numerical 

 ratios may be extended to any degree of nicety ; but it is soon 

 found that the power of the eye in distinguishing hues is over- 

 passed. 



Fig. 2. 



.^4 u lyA^'sKy^r^Uy^rM y^ h 



1.% ^5 \y^^^fi^\y^^'A\y?r^Ky^^\K\y^ h 



\y-i ^ 6 lj/2^-5^i \y'i^'Myi^Myz ^ Myi^\h\yi h I 

 \y\ ^7 \y \>A \yxrA\y\rA\yx^s h\yi^'A\yirA\y ^ ^7 1 



I r^ I ^7 ^1 I ^6 h \rsh\ f'4 f>4 I ^3 h I ^'2 ^6 kl ^7 I ^8 | 



The unit of mass for any primary colour or pigment is the 

 proportion which, mixed with the other two primaries, forms 

 a perfectly neutral gray. This must be found by experiment, 

 and resembles the atomic weight or equivalent of the simple 

 bodies of chemistry. Lambert found it by uniting carmine 

 and gamboge until a perfect orange was formed, which (judging 

 by the eye) inclined neither to red nor yellow ; so with yellow 

 and blue forming green, and with blue and red forming purple. 

 The quantities being weighed in each case, two such expe- 

 riments were sufficient to determine the relative powers of the 



