Prof. J. D. Forbes on the Classification of Colours. 167 



coincide with the centre of the circle, the primary colours 

 must be in proportion to the sines of those angles, which are 

 •9785, -9031 and •4'593; the first being blue, the second red, 

 and the third yellow, which give the proportions above stated. 

 And there can be little doubt that this rule is sufficiently cor- 

 rect, though we restrict the colours of the spectrum to three 

 only; for the centre of gravity of the blue (for example) may 

 be regarded as the centre of gravity likewise of the blue con- 

 tained in the green, and that in the indigo and violet ; and so 

 of the other colours. 



Let then the proportions 0'386 = R, 0'196 = Y, 0-41 8 = B 

 represent the constituents of white light in the spectrum, their 

 sum being =1. But by what has been said of Laml)ert's expe- 

 riments, it appears that red, yellow, and blue pigments reflect 

 but ^rd, y'^^lhs, and ^th of the rays ot those respective colours 

 which fell upon them. Therefore to \vA\e reflected Wghioiihe 

 same composition with the white of our tricoloured spectrum, 

 we must have the surfaces of the colours larger in proportion 

 as their reflecting power is less. Hence the spaces in our 

 coloured wheel must be 



Consequently, of all the red rays which fall upon our tri- 



l r 

 coloured surface only the fraction - •- are reflected (for of 



^ S n ^ 



those which fall on the yellow and blue spaces, none are re- 

 flected, and but one-third of those falling on the red), that is, 



R 



— or '0843 



71 



Y 



Of the whole yellow rays are reflected — or "0429 



Of the whole blue rays are reflected . — or •0914 

 •' n 



In short, the reflected light is white light) whose intensity is 

 attenuated by reflexion m or 4*57 times ; whereas had it been 

 incident on white paper, it would have still had y^oths of its first 

 brightness, or been attenuated only 2*50 times. This abun- 

 dantly explains why the result is a grai/ colour, not a bright 



