ON THE KELATION OF OPTICS TO PAINTING. 121 



The phenomena of irradiation also occur with 

 moderate degrees of brightness. A dark aperture 

 in a sheet of paper illuminated by the sun, or a small 

 dark object on a coloured glass plate which is held 

 against the clear sky, appear as if the colour of the 

 adjacent surface were diffused over them. 



Hence the phenomena of irradiation are very similar 

 to those which produce the opacity of the air. The 

 only essential difference lies in this, that the opacity 

 by luminous air is stronger before distant objects which 

 have a greater mass of air in front of them than before 

 near ones ; while irradiation in the eye sheds its halo 

 uniformly over near and over distant objects. 



Irradiation also belongs to the subjective pheno- 

 mena of the eye which the artist represents objectively, 

 because painted lights and painted sunlight are not 

 bright enough to produce a distinct irradiation in the 

 eye of the observer. 



The representation which the painter has to give 

 of the lights and colours of his object I have described 

 as a translation, and I have urged that, as a general 

 rule, it cannot give a copy true in all its details. The 

 altered scale of brightness which the artist must 

 apply in many cases is opposed to this. It is not the 

 colours of the objects, but the impression which they 



a diffusion of the excitation in the substance of the nerves, as this 

 appears to me too hypothetical. Moreover, we are here concerned 

 with the phenomena and not with their cause. 



