M. H. Helmholtz on the Theory of Compound Colours. 529 



rior surfaces of the uppermost layer of powder-particles^ but also 

 that reflected from the second^ thirds fourth, &c. Of light per- 

 pendicularlj'- incident, a single glass plate reflects only ~j, two 

 such plates only -^^, very many plates almost the whole. We 

 can conclude from this, that of the light which falls upon the 

 fine white powdered glass, the smallest portion only is reflected 

 from the uppermost particles, a much greater portion being 

 reflected by those beneath. This must also be the case with 

 coloured powders, at least with those simple rays whose colour 

 they bear, and which are permitted to pass without absorption ; 

 the greater portion of light of this kind comes from the deeper 

 layers, having traversed in their passage a number of powdered 

 particles. 



Let us consider what will be the case when we mix together 

 powders of difi"erent colours, for example, yellow and blue. The 

 blue particles which lie upon the surface will give blue light, the 

 yellow which lie upon the surface will give yellow light ; both toge- 

 ther will combine to form white or greenish white. It is quite 

 otherwise, however, with the light which returns from greater 

 depths. This must pass through yellow and blue particles, and 

 hence from a distance below the surface such light only will 

 return as can penetrate both the yellow particles and the blue 

 ones. Blue substances generally permit green, blue, and violet 

 light to pass through them in sensible quantity ; yellow, on the 

 contrary, permits red, yellow, and green to jiass. Green, there- 

 fore, is the only light which will pass through both, and hence 

 from the deeper layers of the mixed powder only green light can 

 return. Now, as the quantity of light reflected from the super- 

 ficial portions of the powder is, according to what has been 

 already said, generally much smaller thau that which returns 

 from the deeper layers, the consequence is, that the green of the 

 latter is by far the most predominant, and thus determines the 

 colour of the mixture. 



When therefore to a blue powder we add a yellow one, the 

 colour of the mixture is less altered by the addition of the yellow 

 rays to the blue, than by the circumstance that of the latter rays 

 the violet and blue ])ortions are lost, and the green alone remains. 

 For this reason also mixtures of two colouring substances of 

 nearly equal intensities are in general darker than their consti- 

 tuents, especially when the latter possess such colours as stand 

 far apart in the prismatic series, and hence contain but few rays 

 of a common nature. Thus cinnabar and ultramarine, instead 

 of the rose colour, which corresponds to the composition of their 

 rays, give a black-gray which approximates somewhat to violet. 



Tlic theory tjf ])igmentary colours here presented is simply de- 

 rived from the generally recognized laws of physics ; it explains 



Phil. May. S. k No.*28. Suppl. Vol. 4. 2 M 



