378 



PHYSIOLOGY 



CHAP. 



simultaneous stimulation of the green and violet elements. In 

 fact, on looking at spectral red when the green and violet elements 

 have been fatigued by prolonged stimulation by the comple- 

 mentary colours (blue-green) the sensation of red appears more 

 saturated than before. 



Mixed colours are less saturated than their components because 

 they result from the unequal stimulation of the three receptor 

 elements. White is the result of the approximately equal stimula- 

 tion of the three elements ; grey is only white, feebly illuminated ; 

 black is white with the least possible degree of illumination. So 

 that between black, grey, and white there is no qualitative but 

 only a quantitative difference. The transformation of a coloured 

 sensation into white owing to increased intensity of light is 

 explained on the assumption that in this case the* excitation of 

 the three separate elements is at its maximum. 



Inasmuch as the stimulation of the receptor apparatus of the 



Or 



Or 



FIG. 181. Excitability-curve of the three fundamental components of colour-vision. (Helmholtz.) 

 1 , for red ; 2, for green ; 3, for violet rays. Abscissa = colours of the spectrum in their 

 relative positions. 



retina by light most probably consists in chemical changes, v. 



Helmholtz, in the second edition of his Physiological Optics 



. (1889), modified his theory by substituting for Young's three 



. nerve-fibres three kinds of photochemical substances, with which 



each receptor element in the retina that is capable of giving rise 



to coloured sensation must be provided. In the cerebral cortex 



three different kinds of perceptor cells must correspond to the 



three substances sensitive to the red, green, and violet rays. 



As a working hypothesis Helmholtz' theory has done good 

 service in countless experiments on vision : it is also simple and 

 helps to explain many phenomena. It does not, however, interpret 

 all the facts, and many objections have been raised against it. 



Why, as Pick remarked, should violet be taken as the third 

 fundamental colour ? It is obvious that there is a less qualitative 

 difference between red and violet than between red and green 

 and between green and violet. The three primary colours must 

 be chosen so as to make the difference between them, respectively, 



