KNOWLEDGE AND OPINION IN REGARD TO COLOUR-BLINDNESS. 471 



The most important feature in the second edition is an entirely new determination of 

 the three fundamental colour-sensations, by a method never adopted before, dependent on 

 complicated relations of colour and luminosity. He takes advantage of the elaborate 

 series of experiments on vision made shortly before by Konig and Dieterici ; and by high 

 mathematical processes of calculation, he determines not only the hue, but also the 

 intensity of these sensations, which, according to the principles he follows, he believes to 

 be fundamental. In hue they are — 



1. A Eed, rather more purple than the red end of the spectrum. 



2. A Yellow-Green, about w. 1. 560, something like the green of vegetation. 



3. A Blue, about 482, like ultramarine. 



The red and the blue are stated to correspond with those chosen by Hering, the former 

 also being the colour that I in 1856 had pointed out as the variety of red invisible to me. 



The strength of the fundamental colours is stated to be very much greater than any 

 in the spectrum, the latter being not only much mixed, but very largely diluted with 

 white. It has often been suspected, and indeed confidently asserted by Bering, that 

 the spectral colours had peculiarities of this kind. 



The fundamental colours thus determined by Helmholtz differ materially from those 

 previously found by Konig and Dieterici, who had used the same observations as their 

 foundation. They had arrived at pretty nearly the same hues of red and blue, but their 

 third fundamental, green, was nothing like that of Helmholtz ; and the intensities were 

 very different in all. Helmholtz takes some pains to explain and comment on this 

 discrepancy, and in doing so he discloses most unequivocally his change of view in regard 

 to the " older" explanation of dichromatism. He finds that the discrepant results of his 

 predecessors are due entirely to their attempt to apply the old assumption, and to make 

 their fundamentals conform thereto. And, though he gives a good excuse for their 

 attempt, he leaves no doubt that it has vitiated their labours, leading, indeed, to conse- 

 quences which are irrational and inapplicable. This statement completes and emphasizes 

 the abandonment, by Helmholtz, of the older explanation of dichromatism, which he in 

 so many other places now speaks of as antiquated and superseded. 



Students of colour- vision must be heartily grateful to Professor Von Helmholtz for 

 this decision. When the original explanation of the defect was first proposed, knowledge 

 was scanty, and the ingenious hypothesis, originating with Young and advocated by such 

 men as Helmholtz and Clerk Maxwell, was freely received ; but, as observations 

 became multiplied, the accumulation of adverse facts gradually changed the opinion, 

 and the explanation may be said now to have been long generally discredited on the 

 Continent. The continued adherence to it, by a small minority, has undoubtedly been an 

 evil, as it has often blocked up the way to reasonable and useful inquiry, and has tended 

 to bring into disrepute the admirable theory it was intended to support. But its present 

 renunciation by Helmholtz cannot, one would hope, fail to give it its death-blow. 

 Requiescat in pace ! 



Helmholtz, wishing to do his work thoroughly, goes on to show that a new rationale 



