HERMANN VON HELMHOLTZ 



The true explanation of why yellow and blue 

 pigments yield green may thus be shortly stated, and 

 almost in the words of Helmholtz : l When light 

 falls on a powder composed of transparent particles, 

 only a small portion is reflected from the surface ; the 

 rest penetrates the particles, and is only returned by 

 the surfaces of separation of particles situated more 

 deeply. Thus a single plate of clear glass reflects 

 o^th of the light which strikes its surface, two plates 

 will reflect ^th, and many plates will reflect nearly 

 the whole of it. If the glass reflects only ^ 7 th of 

 the incident light, the rest must be reflected by the 

 deeper layer. In the same way the surface of coloured 

 powders furnishes only a small part of the light which 

 emerges from them ; the deeper parts give back a 

 greater proportion. Light reflected from the surface 

 is always white ; that alone which is returned by the 

 deeper layers is coloured by absorption, and the tint 

 will be deeper as the light penetrates more deeply. 

 Consequently coloured powders are more deeply 

 coloured if the grains are of considerable size than if 

 they are very minute. Reflection depends on the 

 number of surfaces, and not on the thickness of the 

 particles. Consequently, if the particles are large, the 

 light must go through a greater thickness to reach the 

 same number of reflecting surfaces than if the particles 

 are small, and thus, if the particles in a thick powder 

 are large, the rays absorbable by the substance will be 



1 Optiyue Physiologique. Paris, 1867, p. 363. 

 120 



