216 COLOUR VISION 



gauging values belonging to any other group of curves. In any given 

 spectrum, therefore, the R, G, V values must be some linear function 

 of the three empirically observed gauging values (R, G, V). The 

 three gauging curves therefore represent the relative values of the three 

 sensations for each light throughout the given spectrum. 



The simplest concrete conception of the bases of the three sensation 

 elements or ingredients is that there are three components which are 

 counterparts of the physical stimuli. We shall at present use the term 

 component in the broadest sense : it may represent a chemical, an 

 electrical or some other process acting upon different substances or 

 nerve-fibres and giving rise to nervous activity, but the general state- 

 ment of the theory necessitates no such concrete conception. 



In this, its most generalised form, the Young-Helmholtz theory 

 explains satisfactorily the sensations resulting from colour-mixtures in 

 the normal visual system, since it is indeed founded upon them. 



It also explains satisfactorily the gross divergences from the normal 

 system. Thus dichromatic vision is due to absence of one of the theo- 

 retical components ; in the protanopes this is the R component, in the 

 deuteranopes the G component, and in the tritanopes the V component. 

 Indeed, the determination of the absent component follows mathe- 

 matically from the correlation of the facts of colour-mixtures in dichro- 

 mats. In their colour-diagram all the colours which appear to them to 

 match must lie upon a straight line, since the line joining any two points 

 representing homogeneous colours contains all the points representing 

 the colours which can be mixed from those homogeneous colours. 

 Similarly the mixtures of any of these colours with any other colours 

 lies on a series of straight lines, v. Helmholtz has shown that all these 

 lines either meet in a point or are parallel 1 . The point of intersection 

 corresponds to a colour which has no stimulus value for the dichromatic 

 eye. It is generally called the Null-point (Fehlpunkt] of the system. 



Conversely, the normal colour diagram can be constructed from the 

 combined protanopic and deuteranopic observations (Fig. 57). It 

 cannot be expected to coincide precisely with that of a single individual 

 on account of differences in macular pigmentation, variations in the 

 spectra used, and so on, but it shows a remarkable similarity 2 . 



Anomalous trichromatic vision may be regarded in various ways 

 on this theory. The simplest explanation is that it is a reduction system 



1 v. Helmholtz, 3rd ed. p. 123 ; Greenwood, Physiology of the Special Senses, p. 153. 



2 Greenwood, loc. cit. p. 150 ; v. Kries, in Nagel's Handb. d. Physiol. d. Menschen, m. 

 p, 161. 



