272 COLOUR VISION 



the atoms oi each group are synchronous with, and probably sub- 

 multiples of certain vibration periods of the ether, i.e., the vibration 

 periods of the three fundamental colour-tones. Hence, when light of 

 a fundamental colour-tone, e.g., green, falls on the retina it will have the 

 effect of tearing off from a large number of molecules those atom-groups 

 whose periodicity is such as to render them particularly exposed to its 

 shocks, and hence that special substance will be set free which is the 

 exciter of the sensation of green. 



When the wave-length of the light which falls onthe retina is any- 

 where between the wave-lengths of two fundamental colour-tones — for 

 example, blue and green — then a certain number of molecules lose their 

 blue constituents and a certain number their green constituents, and 

 the resulting sensation is a mixture of green and blue. This fact explains 

 why we are unable to distinguish between a single intermediate wave- 

 length and the appropriate mixture of two out- lying wave-lengths. 



There will be certain mixtures of objective light which will set free 

 all three kinds of nerve-exciting substances in equal amounts. These 

 three substances, however, are the chemical constituents of the exciter 

 of the grey sensation. Hence, when they are present in the right amount 

 they recombine to form that substance and the sensation produced 

 is exactly the same as that caused by the decomposition of the grey 

 molecules. 



There are five instances in which we are incapable of receiving any 

 sensation but that of grey. In all, the grey molecule is alone decomposed. 

 Thus in the peripheral totally colour-blind zone of the retina the diffe- 

 rentiation of the grey molecule into colour-molecules has not taken 

 place. Similarly, total colour blindness is an example of atavism, the 

 grey molecules remaining undifferentiated. When such small areas of 

 the retina are stimulated, or areas are stimulated with light of such weak 

 intensity, that no colour-sensation is aroused it is to be concluded that 

 the colour-molecules are not decomposed in sufficient quantity. When 

 the stimulus is so intense that the colour-sensation is blotted out, it is 

 to be supposed that the colour-molecules have become exhausted sooner 

 than the grey, or that a strong energy of ether vibrations affects all 

 the colour constituents equally without reference to their periodicity. 

 In all these cases the important feature is the capacity of the substance 

 which excites the sensation of grey for independent existence. 



Mrs Ladd-Franklin holds that the theory explains not only the facts 

 of colour-mixture, but also after-images and simultaneous contrast. 

 Thus, in her own words : " When a red light, say, has fallen for some 



