VISUAL SENSATIONS 



645 



pigment absorbs the blue rays which the blue allows to pass. The 

 only rays left over are the green, and the mixture of pigment has a 

 green colour. 



THEORIES OF COLOUR VISION 



These facts show that in all probability the primitive colour- 

 sensations are few in number, and that the various sensations excited 

 by the different parts of the spectrum are not simple, but are com- 

 pounded of mixtures of these primary sensations. The two theories 

 which have obtained most vogue, and which enable us to account for 

 a certain number of the phenomena associated with colour-vision, are 

 those known as the Young-Helmholtz theory and the Hering theory. 



(a) THE YOUNG-HELMHOLTZ THEORY. This theory, which 

 Was first put forward by Young and elaborated by Helmholtz, 



FIG. 294. Curves showing sensitiveness of the three varieties of nerve fibres 

 to different parts of the spectrum. 



1, red fibres ; 2, green fibres ; 3, violet fibres. 



assumes that there are three primary colour-sensations red, green, 

 and violet which are represented by three separate sets of elements 

 in the retina, or in each cone, or by separate substances, each of 

 which is affected by one of these colours. 



Thus the rays with a longer wave-length excite chiefly the red 

 fibres or elements ; those of medium wave-length the green percipient 

 element ; those of short wave-length the violet percipient element 

 of the retina. The excitability of these three elements by the rays 

 of different parts of the spectrum are represented in the figure 

 (Fig. 294). At the extreme red and violet end of the spectrum 

 alterations of the wave-length cause no alteration in colour, showing 

 that these rays only excite either the red or the violet fibres. All 

 other parts of the spectrum excite the three sets of fibres simultaneously 

 but to varying degrees. Thus the greater part of the red, e.g. about 

 ' C,' excites the red percipient element strongly, the other two 

 only weakly ; the result is a sensation of red. The yellow rays at 

 ' D ' excite almost equally the red and the green percipient elements, 

 but the violet only slightly. Yellow is therefore a mixed sensation. 



