HELMHOLTZ IN KONIGSBERG 



taken up to a greater extent, and the coloured light 

 coming back from the powder will be deeper and 

 more saturated than if the particles were small. Sup- 

 pose, now, that yellow chrome is mixed with indigo 

 blue. The light reflected from the particles of chrome 

 will be orange, yellow and green, the blue and violet 

 being absorbed, and that from the indigo blue will be 

 green, blue and violet, the orange and yellow being 

 absorbed. If, now, the light that has passed through 

 a particle of chrome traverses a particle of indigo blue, 

 all the colours will be absorbed except the green. 

 Consequently the green alone will reach the eye. 



On the other hand, if the pure spectral colours, 

 yellow and blue, pass into the eye so as to affect the 

 same spot on the retina, the result is a sensation of 

 white, because, in this case, both the yellow and the 

 blue wave-lengths fall on the terminal organ. This 

 ingenious explanation has many important applica- 

 tions, not only as regards the colours .reflected from 

 mixtures of powders, but also the mode of action of 

 all coloured surfaces both in the inorganic world and 

 on the surfaces of plants and animals. It shows how 

 the texture of the surface modifies the result. 



Helmholtz then devised an ingenious method by 

 which two spectra could be simultaneously examined, 

 through a slit shaped like the letter V, in such a way 

 that a portion of one spectrum was superposed on the 

 other. In this way all possible mixtures of two 

 simple colours could be made with the intensities of 

 121 



