824 A MANUAL OF PHYSIOLOGY 



and blue, although so long considered the primary colours, 

 from data yielded by the mixture of pigments, will not do ; 

 for no possible combination of them will produce either a 

 pure green or white light. 



The Young - Helmholtz Theory. The theory which best 

 explains the facts, and has been most widely accepted, 

 is that of Young, generally called, on account of its adop- 

 tion and extension by Helmholtz, the Young-Helmholtz theory. 

 It assumes that in the retina, or in the retino - cerebral 

 apparatus, there are three kinds of elements * (i) * red 

 fibres,' which are chiefly excited by light of comparatively long 

 wave-length (red), to a less extent by light of medium wave- 

 length (green), and to a still less extent by the shortest 

 visible waves (violet) ; (2) ' green fibres,' mainly excited by 

 medium, but also to a certain extent by long and short 

 waves ; (3) ' violet fibres,' chiefly affected by the short 

 vibrations, less by the medium, and still less by the long 

 waves. The curves in Fig. 316 illustrate these relations. 

 It must be carefully remembered that here the word ' fibre ' 

 is merely a convenient term to avoid some such cumbrous 

 phrase as * physiological unit.' There is no ground for 

 believing that an anatomical distinction of three ' fibre ' 

 groups can be made in retina, optic nerve, or brain. 



This assumption explains the phenomena of colour- 

 mixture to which we have referred above. When all the 

 rays of the spectrum act upon the retina together, the three 

 groups of fibres are about equally excited, and this equal 

 excitation may be supposed to be the condition of the sensa- 

 tion of white light. When the green of the spectrum alone 

 falls on the retina, the green fibres are strongly excited, the 

 other two groups only slightly ; this is the relation between 

 the amount of excitation in the three groups which is 

 associated with a sensation of spectral green. When two 

 complementary colours, such as red and bluish-green, fall 

 together on the same portion of the retina, the three fibre 

 groups are excited in the relative proportions associated 

 with the sensation of white light, 



* Or, it may be, three kinds of visual substance capable of being acted 

 on by light. 



