KNOWLEDGE AND OPINION IN EEGAED TO COLOUE-BLTNDNESS. 475 



Comparing the fundamentals determined scientifically with the natural impressions of 

 simple colour, we find that the yellow and blue correspond fairly well. Fraunhofer, in 

 1814, placed the yellow at about w. 1. 570. Donders, after obtaining the impressions of 

 111 eyes, belonging to 76 persons, arrived at a mean, for pure yellow, of w. 1. 582. 

 Chevreul placed it at D ^ E, about 581*5 ; and he placed the blue on F= 486. These 

 agree fairly with the fundamentals of Hering and Helmholtz. But the greens are all 

 discordant ; the natural idea of green is, I believe, about 520 to 530 ; Chevreul puts it 

 as 522 ; while Hering and Konig make their fundamentals a blue-green near 500, and 

 Helmholtz makes his a very yellow-green about 560. Eed also does not agree ; the 

 natural idea of red is at or near the end of the spectrum, Chevreul putting it at the 

 lithium line = 670. But Hering, Konig, and Helmholtz all now choose the extra- 

 spectral purple-red, which I, from my own vision, first suspected to have some peculiar 

 significance. As for violet, in spite of the strong early feeling in its favour, no one ever 

 thinks of putting a fundamental there now. 



The new theory of dichromic vision lately brought out by Helmholtz admits that 

 any arrangement of colours, properly derived from his fundamentals, may consistently form 

 the two dichromic hues ; but it does not help us to understand why, with trichromic 

 fundamentals, the particular selection of yellow and blue should be almost exclusively 

 found. To get this, we are apparently driven to the Leber hypothesis of the amalgama- 

 tion of the red and green elements into one ; but this does not much advance the matter, 

 for no reason has yet been offered for the prevalence of this amalgamation. In 

 fact, notwithstanding the attractiveness of Young's idea of the minimum number of 

 possible elements, the number three does not seem easy to harmonise with the natural 

 binary or quaternary arrangement which makes itself so prominent in dichromic vision. 



There can be no doubt that the tetrachromic hypothesis of the two complementary 

 pairs seems to correspond most nearly with the actual phenomena ; and the peculiar 

 natural prominence of these two pairs which has been found to exist in the normal retina 

 (see " Data," N), is very suggestive in this connexion. It is curious, also, that in the 

 latest investigations, the trichromic authorities have made their fundamentals so much 

 like the hues of the tetrachromic. Surely this looks like a prospect of approximation. 

 It must be recollected that the tetrachromic theory has been bound up with peculiar 

 physiological views, which, though highly ingenious, have not found so wide an assent as 

 the simple explanation of Dichromatism connected with them. Cannot the latter be 

 accepted as a " working hypothesis " till further investigation shall reveal the deeper 

 secrets of the colour-sense ? 



But these are theoretical speculations, which I must not further enlarge upon. The 

 facts are clear enough, and so I must return to my original object; and I cannot avoid 

 drawing from the whole of the evidence the conclusion that the general phenomena of 

 dichromic vision are now, according to the best knowledge we can obtain, no longer sub- 

 ject to any mystery or obscurity other than what attends those of colour- vision generally. 



