122 ON THE RELATION OF OPTICS TO PAINTING. 



have given, or would give, which is to be imitated, so 

 as to produce as distinct and vivid a conception as pos- 

 sible of those objects. As the painter must change 

 the scale of light and colour in which he executes his 

 picture, he only alters something which is subject to 

 manifold change according to the lighting, and the 

 degree of fatigue of the eye. He retains the more 

 essential, that is, the gradations of brightness and tint. 

 Here present themselves a series of phenomena which 

 are occasioned by the manner in which the eye replies 

 to an external irritation ; and since they depend upon 

 the intensity of this irritation they are not directly 

 produced by the varied luminous intensity and colours 

 of the picture. These objective phenomena, which 

 occur on looking at the object, would be wanting if the 

 painter did not represent them objectively on his can- 

 vas. The fact that they are represented is particu- 

 larly significant for the kind of problem which is to be 

 solved by a pictorial representation. 



Now, in all translations, the individuality of the 

 translator plays a part. In artistic productions many 

 important points are left to the choice of the artist, 

 which he can decide according to his individual taste, 

 or according to the requirements of his subject. 

 Within certain limits he can freely select the absolute 

 brightness of his colours, as well as the strength of the 

 shadows. Like Rembrandt, he may exaggerate them 



