474 DE WILLIAM POLE ON THE PRESENT STATE OF 



probably be soon settled by observation, now that the depressing influence of erroneous 

 theory is removed. 



9. Although the dichromic patient sees only two hues, he receives a great variety of 

 colour-impressions therefrom, arising from differences in their saturation or luminosity, or 

 both ; and these, under the special sensitiveness derived from constant experience, largely 

 counteract his defect in regard to colour, in judging of the different appearances of objects 

 around him. 



10. Although it is no part of the object of this paper to discuss colour- vision 

 generally, or the theories proposed to explain it, yet I may briefly mention some 

 aspects which the phenomena, as now understood, would seem to bear towards points 

 connected with these theories. 



The most salient fact in dichromic vision is its remarkably simple and symmetrical 

 character, consisting of one pair of complementary colours, with gradations of nuance 

 perfectly symmetrically disposed. This is shown to some extent in the picture of the 

 spectrum, but it becomes more evident in the circular diagram designed by Newton, 

 and explained in Helmholtz's work, page 325. Donders perfected this in a 

 beautiful picture which he showed at Cambridge in 1881. He completed the 

 naturally re-entering circle, and so arranged the colours (giving about 100 inter- 

 mediate varieties of hue) that the complementaries should be diametrically opposite each 

 other. The sketch in the Plate, fig. 4, may give a faint idea of the arrangement, without 

 pretending to accuracy, which even Donders himself found it very difficult to attain. 

 Fig. 5 is similarly constructed, but adapted to the dichromic spectrum. In this the pair 

 of visible colours are shown with their maxima diametrically opposite ; and from these 

 two points they become modified, in one direction by darkening only, in the other by 

 darkening combined with dilution, till, at the top and bottom, the colours meet, and 

 become lost in neutral points. And these two neutral points, also diametrically opposite 

 each other, correspond to the other pair of complementary colours, the purple-red and 

 blue-green, which, though most prominent colours to the normal-eyed, are invisible as 

 colours to the colour-blind. It will be at once seen what a remarkable symmetry this 

 structure presents, and how greatly the regular arrangement of the dichromic series of 

 colour impressions differs from the irregular structure of the normal series in the adjacent 

 figure. 



And, as having some connexion with this greater simplicity of the dichromic arrange- 

 ment, it would seem that Donders's idea of the evolution of colour- vision, is worthy of 

 more attention than it has yet received. 



In the interior of fig. 4 are shown the approximate hues of the " fundamental sensa- 

 tions," as determined by the three authorities who have devoted most attention to 

 them — Hering, Konig, and Helmholtz. It will be seen that Hering's are symmetrically 

 placed on the diameters crossing at right angles ; Konig's coincided with three of these ; 

 but Helmholtz found fault with Konig's green, and removed it nearer the yellow, 

 keeping the other two as they were. 



