Prof. J. D. Forbes on the Classification of Colours. \ 6.5 



ofnr.tural bodies (which refers them to the colours of thin 

 plates), the reasoning of the above paragraph will hardly be 

 questioned. There will therefore be always this essential 

 difference between compounding rays of the spectrum and 

 compounding pigments ; that in the former case, by throwing 

 light of two or more colours upon a white screen, each of 

 these colours being reflected with equal vividness, the bright- 

 ness of the screen will be the sum of the brightnesses due to 

 the several rays (and if a sufficient number of rays be com- 

 bined, the result will be a dazzling white) ; but, on the other 

 hand, by combining pigments we do not add together lights, 

 but merely construct a ground or screen capable of scattering 

 a greater number of the constituents of a beam of white light 

 which falls upon it. Thus there will be an inevitable quantity 

 of darkness or absorbent faculty in the constitution of every 

 artificial colour, whatever be its predominant reflecting hue; 

 and the mixture of pigments will not tend to increase the 

 brightness, as the mixture of lights would do, but only to mix 

 with the fundamental darkness of the surface a portion of light 

 which shall be of a mixed instead of a simple hue. 



Let us suppose for a moment a simple case. Let us admit 

 that a paper thickly coated with ultramarine can reflect none 

 but blue rays, and that a paper coated with chrome-yellow 

 reflects only yellow rays. But further than this, a share of 

 the blue and of the yellow light falling on each is absorbed; 

 suppose one-half to be thus lost. If a compound pigment of 

 blue and yellow be formed and exposed to the same white ray 

 as before, we cannot expect that it should have more brilliancy 

 than either one or other of the primitive colours, whilst it is 

 evident that the union of rays of yellow and blue light upon a 

 white screen would have a twofold splendour*. For we must 

 admit the reflecting particles in each of the separate pigments 

 to be so densely spread that a ray of light can fall nowhere, 

 on the ultramarine for instance, but it finds a particle of 

 colour ready to decompose and reflect it, and the same of the 

 pure yellow pigment; in a mixture, therefore, of the two, the 

 surface may be regarded as equally divided amongst an infi- 

 nite number of the blue and the yellow reflecting points, so 

 that the reflected light is half yellow, half blue, but altogether 

 is no more than the amount which either pigment covering 

 the whole surface would have reflected. We must not there- 

 fore suppose that by mixing pigments we render the surface 

 on the whole more reflective, that is to say, more luminous, 

 than before. Experience confirms this anticipation. On the 



• What is meant here by speaking of rays of different colours having 

 "equal" or " two-fold" brilliancy will be explained by and by. 



