Data on Colour-Blindness. 105 



chromic strength. In class 5 this will be the same thing as 

 the degree of saturation ; in class 6 it will be the same as the 



degree of luminosity ; but in class 7 it will be = — 7 



te * y + ic + b' 



Now, having defined quantitatively the different varieties 

 of dichromic colour-impressions, we ought to be able to repre- 

 sent them in graphic form ; and this is another point in which 

 my original paper required supplementing. Professor Clerk 

 Maxwell, who was kind enough to take much interest in my 

 case, pointed this out to me, referring me to the explanation 

 he had given on the subject in the Phil. Trans, for 1860. 

 The system he there laid down, founded on Newton's law 

 for colour mixtures, has since become well known to earnest 

 students. For normal vision an equilateral triangle is drawn, 

 and on its three angles are supposed to be located the three 

 fundamental colour-sensations — red, green, and violet, by 

 combinations of which, according to Young's theory, all 

 hues are formed. Then every possible hue will be repre- 

 sented by a point within the triangle, which can be determined 

 by Newton's rale. 



The application of this, however, to dichromic vision has 

 never yet received proper attention. Maxwell, in his fig. 11, 

 gave an example, the object of which was to compare di- 

 chromic with trichromic vision, and for which, therefore, he 

 used the normal triangle. But, to show simply the pheno- 

 mena of dichromic cases, or to compare them with each other, 

 this becomes unnecessarily complicated, and makes a clumsy 

 figure. We know nothing of red, green, or violet (none of 

 them existing in our system), and have no inducement to base 

 our triangle upon them ; and, moreover, we have none of the 

 varieties of hue, to depict which the normal triangle is formed. 



For these reasons, availing myself of a suggestion made to 

 me by Lord Rayleigh, I have adopted a different arrangement 

 of the diagram, which, while founded on the same principles, 

 shall be specially adapted to the facts of our vision. I retain 

 the equilateral triangle, but I place on its three angles the 

 three chief sensations received by dichromic eyes; i.e. the warm 

 colour, the cold colour, and the sensation of their " equilibrium,'" 

 white. I then find an additional point for their "negation," 

 black, and thus I get the four starting-points of the colour- 

 impressions which it is the object of the diagram to portray. 



The construction will be readily understood from fig. 1, where 

 Y is the warm colour, yellow ; U is the cold colour, ultra- 

 marine blue ; and W is white ; the point for black is deter- 

 mined for each special patient by equation I. For my vision 



