386 SCIENCE PROGRESS 



dotted curve, which has its maximum at a point between the 

 Tl and Li lines in the spectrum, e.g. in the yellow. Thus red 

 and green combine to form yellow. In the figure the two 

 component curves are drawn the same size. If the thallium 

 curve has all its ordinates increased in the same ratio, the 

 maximum of the resultant curve is moved towards the green, 

 i.e. if the proportion of green in the mixture is increased, the 

 yellow becomes greenish. This is as it should be. 



According to my view the energy curve, i.e. the curve 

 representing the distribution of the energy over the spectrum 

 after it is taken up by the vibrators, is characteristic for the 

 light impression on the brain. We are accustomed to associate 

 three qualities with a light impression, namely luminosity, hue, 

 and saturation. The area of the curve gives the luminosity 

 of the impression, the position of the maximum the hue, and 

 the narrowness of the curve the degree of saturation. 



We have hitherto left the question open as to whether 

 there is only one or whether there are three classes of vibrators 

 in the spectrum. We have found, however, from a study of 

 our model, that one class is quite sufficient to explain the 

 phenomena of colour mixing. The only justification for other 

 two classes would be to satisfy a supposed need in this direction. 

 But as it is non-existent, other two classes are superfluous. 

 The three-colour theory of vision is consequently false if it is 

 taken to mean that there are three independent primary 

 sensations or three independent visual components, one or two 

 of which are wanting in colour blindness. 



In An Introduction to the Study of Colour Vision J. H. 

 Parsons states that the three-components theory becomes less 

 plausible the more concrete the form it takes. Young sug- 

 gested three different types of nerve-fibres, others have sug- 

 gested three substances which undergo chemical change, and 

 others again have suggested electrical change. If the details 

 of any of these hypothetical processes are worked out, and 

 due regard is at the same time paid to the selective action of 

 the eye, I think it will always be found that two of the three 

 components are superfluous. 



But, it will be asked, how does my theory explain the 

 apparent trichromatism of ordinary vision? We may here 

 fall back on the explanation of Dr. Edridge-Green, namely, that 

 the colour-perceiving centre in the brain is not sufficiently 



