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1921-22.] New Method of investigating Colour Blindness. 
Young, Helmholtz, and Maxwell made the assumption that to each of 
the three primary colours there corresponded a primary sensation. The 
fact underlying Newton’s diagram, namely, that there are three primary 
colours, is beyond all dispute : it is accepted by all theorists, and is the 
basis of various systems of colour photography. The additional assumption, 
that there are three primary colour sensations corresponding to these 
primary colours, is the fundamental hypothesis of the Young-Helmholtz 
theory, and has been a subject of debate for years. 
Let us suppose that fig. 2 represents the colour-patch system of a normal 
individual, that the Young-Helmholtz theory is true, and that his red 
sensation is suddenly completely destroyed. Then his colour-patch system 
should be given by fig. 3. No matter how much red is added to a colour, he 
cannot see it : consequently, he should be able to move in a straight line to 
GREEN GREEN GREEN 
Fig. 2. Fig. 3. Fig. 4. 
the red corner from any point on the green-blue side without crossing 
a boundary. 
In Hering’s theory, instead of primary colours there are primary pro- 
cesses, the red-green process and the blue-yellow process. The one acts 
parallel to the red-green side (fig. 1), and the other along the blue-yellow 
median. Suppose the red-green process completely destroyed, then the 
colour-patch system should be given by fig. 4. 
Edridge-Green’s non-elemental theory does not require that for colour 
blindness the number of patches should grow less in any particular manner. 
It merely requires that it should grow less. It is not generally known that 
Hering’s theory and the non-elemental theory explain the facts of colour- 
mixing quite as well as the Young-Helmholtz theory ; this has been shown 
in the case of Hering’s theory by Helmholtz,* and in the case of the non- 
elemental theory by myself.f 
It will be seen from the foregoing that the colour triangle, although 
* Fhysiologische Optik, 2nd edition, p. 377. 
t Phil. Mag., 38, p. 402, 1919. 
