PROFESSOR AT KONIGSBERG 95 



which could be produced from any two simple colours. The 

 relative intensity of the mixed colours could be altered by 

 shifting the prism from its vertical position to one more or 

 less oblique, and care was taken to examine the points of 

 which the colour was to be determined through a small 

 diaphragm (since it is impossible to judge the colour of the 

 field so long as it is surrounded by equally saturated colours) ; 

 thus all the combinations could be investigated at all degrees 

 of their relative intensity by means of a telescope, the cross- 

 wires of which were set parallel with the colour-bands of 

 the spectra. Helmholtz then discovered the surprising fact 

 that contrary to the views hitherto entertained there are 

 only two among the colours of the spectrum, yellow and 

 indigo-blue, which together yield pure white, that is, are 

 complementary to each other, whereas their combination 

 had always been supposed till then to produce green. The 

 whole width of the spectrum is divided into three sections 

 by the rays which produce white: the colours of the first 

 and second sections combine to tones of yellow, with transi- 

 tions to red, flesh-colour, white, and green ; colours of the 

 second and third to blue, with transitions to green, white, 

 and violet; colours of the first and third to purple-red, with 

 transitions to flesh-colour, rose, and violet. Now since the 

 mixture of yellow and blue does not produce green, but 

 at most a faint greenish-white, Helmholtz had to conclude 

 that the hypothesis of the composition of all colours from 

 red, yellow, and blue was erroneous. But he further investi- 

 gated the composition of pigment colours, and was thereby 

 enabled to explain the obtained results. If a yellow and a 

 blue powder are mixed, the blue particles which lie upon 

 the surface will yield blue, and the yellow particles on the 

 surface yellow light, which combine to form white or greenish- 

 white. From within the mixture, only such light will return as 

 can penetrate the blue as well as the yellow particles, and 

 since blue substances can only let green, blue, and violet 

 light through, and yellow substances only red, yellow, and 

 green light, green light alone is able to return from within the 

 pigment ; this combines with the whitish light reflected from the 

 surface, so that green predominates. Thus Helmholtz estab- 

 lished the composition of pigmentary colours on purely physical 



