528 M. H. Helmholtz on the Theory of Compound Colours. 



made to balance each other that pure wliite is procured. Thus 

 we obtain white from red, gi'een, and violet, which may be com- 

 bined to three pairs of complementary colours ; namely, 



Simple red and compound dull blue-green. 



Simple green and compound purple-red. 



Simple violet and compound dull yellow. 

 It is a striking fact, that, while the complementary colours of 

 simple red and violet are only distinguished from certain tones 

 of the spectrum by their less saturated appearance, the former 

 nevertheless give, with simple red and violet, white, the latter 

 not. 



Newton's few observations on the combinations of every two 

 prismatic colours coincide with my results. He finds that the 

 primitive colours can be obtained by the combination of the 

 neighbouring ones at both sides of the former; for example, 

 orange can be formed from red and yellow ; yellow, from orange 

 and green-yellow ; green, from green-yellow and sea-green, and 

 also, but not so good, from yellow and blue [cijaneum) ; blue, 

 from sea-green and indigo. He has also formed from red and 

 violet, purple-red. White he could only obtain from the three 

 colours, red, violet, and green ; and in order to render the ex- 

 periment successful, he even recommended the application of 

 spectra whose colours were not completely separated. In this 

 case more than three single colours are mixed together. 



It will, on the contrary, be observed, that my i-esults on the 

 action of prismatic colours diifer materially from those obtained 

 by the mixing of colouring substances. In particular, that yellow 

 and blue do not furnish green, but at most a weak greenish 

 white, contradicts in the most decided manner the experience of 

 all painters during the last thousand years. The reason of the con- 

 tradiction will, however, be rendered quite plain by reflecting a 

 little upon the manner in which colouring substances act upon 

 light. The substances used in painting, as all coloured bodies 

 of regular structure which we possess in large ])ieces, for example, 

 ciystallized cinnabar, crystallized chromate of lead, cobalt-glass, 

 from which smalt colours are made, are transparent, or at least 

 translucent. When light falls upon them, a portion of the 

 latter will be reflected from the exterior surface as white light ; 

 another ])ortion enters the substance, and by the unequal ab- 

 sorption of the component simple rays becomes coloured, is 

 reflected at the posterior limiting surface of the body, and I'cturns 

 to the eye of the observer, which, by means of this particular por- 

 tion of light which has entered the body and been reflected within 

 it, sees the latter coloured. When, however, we grind a colour- 

 ing substance to powder, the observer sees not only that portion 

 of the incident light which is reflected at the forward and poste- 



