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



white*. The proportions of the surfaces of bright colours 

 whose mixture produces white (r, i/, b in the preceding nota- 

 tion) is 5, 3 and 8, as given by Field f. 



It is to Mayer, the mathematician, that we owe a complete 

 and perfect diagram of mixed colours, staiting from red, yel- 

 low and blue, as constituents. Let the extreme corners of a 

 triangle be painted of these colours, and let the periphery of 

 the triangle be composed of graduating colours between each 

 pair of these respectively ; then the centres of the sides of the 

 triangle will be occupied by perfect orange, perfect green and 

 perfect purple, each of which will pass in each direction towards 

 the predominating primary colour. The periphery of Mayer's 

 triangle includes, therefore, all the colours of the spectrum, or 

 primary colours mixed two and two. But combinations of 

 three colours may be represented by selecting points in the 

 interior of the triangle which shall be the centre of gravity of 

 the constituent colours. Thus if the three colours, red, yel- 

 low and blue, be mixed in equal proportions, the resulting 

 colour, which will be neutral gray, will be found at the centre 



of gravity of the 

 triangleatW. But 

 this would also re- 

 sult from the mix- 

 ture of one portion 

 of red and one of 

 blue united at P to 

 form twoofpurple, 

 which then being 

 compounded with 

 one of yellow, Y, 

 will give the centre 

 of gravity at one- 

 third of the di- 

 stance from P to- 

 wards Y. The 



Fig. 1. 



* Goethe, in his Theory of Colours, seems to think that he has over- 

 turned Newton's experimental demonstrations by calling the opinion that 

 " all the colours mixed together produce white," " an absurdity which 

 people have credulously been accustomed to repeat for a century in oppo- 

 sition to theevidenceof their senses." (Eastlake's translation, p. 225.) 'Ihe 

 truth is, that " gray " is not an affection of Light at ail, but of Surface 

 merely. All Light combining the coloured elements in due proportion is 

 essentially white, though more or less intense ; but no Surface can be said 

 to be 'perfectly/ white rather than graj/, except by comparison with another. 

 A surface of white paper illuminated by common daylight is gray relatively 

 to a similar one placed in full sunshine. 



t Field's Chromatography, p. 247. 



