ON THE EELATIOX OF OPTICS TO PAINTING. 109 



low lights, are not without some influence in the region 

 of the middle lights. We have to observe more closely 

 in order to perceive this influence. It is found, in fact, 

 that when the very finest differences of shade are pro- 

 duced on a rotating disk, they are only visible by a 

 light which about corresponds to the illumination of a 

 white paper on a bright day, which is lighted by the 

 light of the sky, but is not directly struck by the 

 sun. With such a light, shades of y^- or y^- of 

 the total intensity can be recognised. The light in 

 which pictures are looked at is, on the contrary, much 

 feebler ; and if we are to retain the same distinctness 

 of the finest shadows and of the modelling of the 

 contours which it produces, the gradations of shade 

 in the picture must be somewhat stronger than cor- 

 responds to the exact luminous intensities. The 

 darkest objects of the picture thereby become un- 

 naturally dark, which is however not detrimental to 

 the object of the artist if the attention of the observer 

 is to be directed to the brighter parts. The great 

 artistic effectiveness of this manner shows us that the 

 chief emphasis is to be laid on imitating difference of 

 brightness and not on absolute brightness ; and that the 

 greatest differences in this latter respect can be borne 

 without perceptible incongruity, if only their grada- 

 tions are imitated with expression. 



