96 HERMANN VON HELMHOLTZ 



principles. If the theory of three elementary colours is to 

 go (Young proposed red, green, and violet), then Young's 

 theory of the three fundamental colours as the three funda- 

 mental qualities of sensation must also be given up. For 

 if e. g. the sensation of yellow is only aroused by the yellow 

 rays of the spectrum because they simultaneously excite the 

 sensations of red and green, which in combination give yellow, 

 then it would follow that the same sensation must be excited 

 by the simultaneous action of the red and green rays, which, 

 however, never produce so bright and vivid a yellow as that 

 due to the yellow rays. 



Helmholtz was led to these investigations in following up 

 the phenomena described by Brewster, which were in apparent 

 contradiction with Newton's theory, and involved a more 

 searching analysis of coloured light than had been made by 

 Newton, Goethe, or Brewster. Brewster, like Goethe, had 

 stated (and built up his entire theory of colour on that state- 

 ment) that it was not the differing refrangibility of the rays 

 that determined the colours of the prismatic image, but that 

 there were three different kinds of light, red, yellow, and 

 blue, exhibiting every degree of refrangibility, and so arranged 

 that the red light contains a preponderance of rays of less 

 refrangibility, the yellow more rays of mean refrangibility, and 

 the blue more of greater refrangibility; hence the first pre- 

 dominates at the less refrangible end of the spectrum, the 

 second in the middle, the third at the most refrangible end. 

 The remaining colours of the spectrum would be produced 

 by the mixture of the three primitive colours. The object of 

 Helmholtz's inaugural dissertation was to prove this view 

 untenable, but he now went farther. Brewster had recog- 

 nized that if different coloured rays of equal refrangibility 

 exist, the compound light formed by them must behave as 

 simple light in prismatic analysis, but declared that such 

 rays might be separated by taking advantage of their difference 

 of absorption in coloured media ; thus there would be rays of all 

 three descriptions in all portions of the spectrum, and conse- 

 quently the white light due to their union : this is in direct 

 contradiction with Newton's theory, according to which homo- 

 geneous light passing through coloured media may indeed be 

 weakened or extinguished, but can never exhibit changes of 



