170 Prof. J. D. Forbes 07i the Classification of Colours. 



colours, but the third was used to confirm them. He thus 

 found the combining proportions by weight of carmine, gam- 

 boge and Berlin blue, to be 1, 10 and 3*. 



We are now to consider how far this triangle carries us 

 towards a complete scale of colours. It is manifest that the 

 ««/f»5iV?/ of the colours depends upon the reflective power of 

 the pigments used, and that this essentially varies for the dif- 

 ferent primary colours. In no sense, tiien, can we be said to 

 have red, yellow and blue, of equal brightness at the corners 

 of our triangle; for even if we assume as merely convenient 

 definition, that by equal brightness of different colours we 

 mean the proportions in which, when combined, white light 

 results, we have already seen that the yellow pigment, being 

 far most reflective, will be brightest, then the red, and after all 

 the blue. But in fact we have no scale at all for comparative 

 brightness of heterogeneous colours. We must take the Tp\g- 

 rvxQnls purest in quality, and most lucid or reflective as regards 

 the quantity of light which they scatter, and consider these 

 as the primary colours. The mixed colours also vary in their 

 lucidity, according to the prevalence of a more or less lucid 

 component; the yellow hues will be most lucid, the blue least. 



When the triangle is exposed to a brighter light, the pro- 

 portions of the colours remain unchanged, and the whole will 

 be more lucid. 



It is probable, however, that the decomposing action of all 

 pigments upon light is limited; and that a coloured surface 

 may be so drowned in white light, that much of the light is 

 returned undecomposed, and the colour is thus diluted. 



If less light tall on the triangle, a different kind of dilution 

 occurs, only piu'e coloured light will be reflected, but so little 

 of it as to affect the eye but slightly, or not at all with the 

 sense of colour. 



In the latter case all colours pass into indistinguishable 

 blackness, in the former case into indistinguishable whiteness. 



If we mix black and white pigments with coloured pigments, 

 we may have both these variations exhibited at once under a 

 common external illumination. 



If we have a series of triangles thus constructed, they will 

 embrace under one common illuminating influence (as ordinary 

 daylight) all possible varieties of hue and shade under that 

 illumination. Every conceivable natural or artificial object, 

 such as a piece of stuff", a feather, or a flower, ought to be 

 capable of being matched with one or other of the spaces in 

 these triangles. This is all that we propose to accomplish. 

 If we choose the most lucid known bodies for our primary 

 * Farbenpyramide, § 63. 



