OTHEE THEORIES 273 



time upon the retina a large number of molecules have lost their red 

 constituents — they have become partly mutilated molecules. But in 

 this condition they are extremely unstable ; they gradually go to pieces 

 completely, and the setting free of their remaining constituents, the blue 

 and the green producing parts of the molecules, causes a sensation 

 of blue-green. The red sensation, therefore, in the case of careful 

 fixation, becomes paler and paler ; if the objective illumination is 

 weakened, it may even be overpowered by the blue-green sensation ; 

 and if the eyes are closed, the blue-green sensation alone remains after 

 a few seconds and continues until the injured molecules have all become 

 completely destroyed. Since, as is well-known, the circulation of the 

 retina is extremely rapid, the half mutilated molecules are in large 

 numbers dragged across the border of the original image and there their 

 complete destruction causes the phenomenon of simultaneous contrast." 



The facts of scotopic vision are explained by the theory on the view 

 that the grey substance is present in the rods, which subserve vision 

 at low intensities, whilst the differentiated material is present in the 

 cones, which subserve colour-vision and vision at higher intensities. 

 A novel suggestion is that the grey substance in the rods is responsible 

 for the change in the luminosity curve at higher intensities, its decom- 

 position undergoing change as the intensity increases. 



The theory is held to offer a satisfactory explanation of the relative 

 saturation of colours in the spectrum. The number of molecules 

 decomposed by a given light depends on the closeness of the coincidence 

 of the vibration periods. For wave-lengths half-way between those of 

 two fundamental colour-tones this coincidence will be very slight, and 

 hence the number of molecules decomposed will be small, so that the 

 resulting colour-tone is very little saturated. Green is less saturated 

 than red and blue because the grey substance is most decomposed by 

 lights of this region of the spectrum. 



It also explains the relative sensitiveness of the eye to change of 

 colour per change of wave-length, which is greater in the yellow and 

 blue-green than anywhere else (Part I, Section II, Chap. ii). Where 

 the number of colour-molecules decomposed by a light of given wave- 

 length is relatively small a given amount of change of wave-length 

 is necessarily more effective in changing the quality of the sensation. 



Mrs Ladd-Franklin severely criticises both the Young-Helmholtz 

 and Hering theories, yet it must be admitted that her theory is a 

 legitimate offspring of the three-components theory. It perhaps owes 

 more than she would readily admit to Bonders' theory. 



p. c. V. 18 



