398 HERMANN VON HELMHOLTZ 



ground colour, which must therefore be a highly saturated 

 carmine-red ; spectral violet is a pale red modification of the 

 third ground colour, which may therefore be compared with 

 ultramarine in its tone, while the second fundamental colour 

 corresponds more or less with the green of vegetation. These 

 results are contradictory to Helmholtz's own earlier opinion, that 

 dichromatic subjects are simply wanting in one of the funda- 

 mental sensations of the trichromatic eye. But Helmholtz 

 now gave up this view (as he had already informed Lord 

 Rayleigh), and adopted the position that coloured lights which 

 appear equal to the normal trichromatic eye must do so to the 

 dichromatic eye also. If Newton's Law of Mixture is appli- 

 cable to the colours of the dichromatic system, it follows 

 that (granting every plane, the rectangular co-ordinates of 

 which represent the values of the prime colours of the tri- 

 chromatic system, to be available as a colour table) all the 

 isochromatic planes in a dichromatic colour system must pass 

 through a line of intersection. It further follows that in the 

 colour table constructed after Newton all isochromatic lines 

 of a dichromatic system intersect at a point beyond, or at the 

 limit of, the trichromatic colour triangle. He concludes the 

 investigation with a comparison of sensitiveness to differences 

 of brightness, and to differences of colour. 



After the heavy afflictions that had befallen Helmholtz and 

 his family, the second half of the year 1891 brought a flood of 

 ovations from the scientific, and indeed from the whole learned 

 world, such as have seldom fallen to the lot of any scholar. 



After taking up the Presidency of the Test Commission at 

 the International Technical Exhibition in Frankfurt-a-M., and 

 again devoting the summer to optical problems, he went with 

 his family to Campiglio in the middle of August, to avoid the 

 excitement and exertion incident on his seventieth birthday, and 

 only returned to Berlin when the anniversary was over. 



Both at Campiglio and subsequently at Feldafing, he received 

 innumerable congratulations from the whole world. On 

 September 21 he writes to Ludwig : 



1 Best thanks for your kind in my opinion far too kind- 

 appreciation of my labours. When two friends are working 

 in somewhat different directions, it is natural that one should 

 occasionally be able to help the other, and I am glad if I have 



