110 ON THE RELATION OF OPTICS TO PAINTING. 



III. COLOUR. 



With these divergences in brightness are connected 

 certain divergences in colour, which, physiologically, 

 are caused by the fact that the scale of sensitiveness 

 is different for different colours. The strength of the 

 sensation produced by light of a particular colour, and 

 for a given intensity of light, depends altogether on 

 the special reaction of that complex of nerves which 

 are set in operation by the action of the light in 

 question. Now all our sensations of colour are ad- 

 mixtures of three simple sensations ; namely, of red, 

 green, and violet, 1 which, by a not improbable suppo- 

 sition of Thomas Young, can be apprehended quite 

 independently of each other by three different systems 

 of nerve-fibres. To this independence of the different 

 sensations of colour corresponds their independence in 

 the gradation of intensity. Kecent measurements 2 

 have shown that the sensitiveness of our eye for feeble 

 shadows is greatest in the blue and least in the 

 red. A difference of -jW to -^-J-g of the intensity 

 can be observed in the blue, and with an untired eye 



1 Helmholtz's Popular Scientific Leuturvs, pp. 232-52. 



2 Dobrowolsky in GTdi'fe's Arcldr fiir Ojththstlntologie, vol. xviii, 

 part i. pp. 24-92. 



