ON THE RELATION OF OPTICS TO PAINTING. 117 



If this same part of the retina is afterwards illuminated 

 with white light, the sensation of green is enfeebled, 

 while that of red and violet is vivid and predominant ; 

 their sum gives the sensation of purple, which mixed 

 with the unchanged white ground forms rose-red. 



In the ordinary way of looking at light and coloured 

 objects, we are not accustomed to fix continuously one 

 and the same point ; for following with the gaze the 

 play of our attentiveness, we are always turning it to 

 new parts of the object as they happen to interest us. 

 This way of looking, in which the eye is continually 

 moving, and therefore the retinal image is also shift- 

 ing about on the retina, has moreover the advantage 

 of avoiding disturbances of sight, which powerful and 

 continuous after-images would bring with them. Yet 

 here also, after-images are not wanting ; only they are 

 shadowy in their contours, and of very short duration. 



If a red surface be laid upon a grey ground, and if 

 we look from the red over the edge towards the grey, 

 the edges of the grey will seem as if struck by such an 

 after-image of red, and will seem to be of a faint 

 bluish green. But as the after-image rapidly disappears, 

 it is mostly only those parts of the grey, which are nearest 

 the red, which show the change in a marked degree. 



This also is a phenomenon which is produced more 

 strongly by bright light and brilliant saturated colours 

 than by fainter light and duller colours. The artist, 



