114 ON THE EELATION OF OPTICS TO PAINTING. 



however, enter upon them, for they produce no diver- 

 gence between the picture and reality. 



The second class of phenomena of contrast, and one 

 which, for us, is more important, is met with in 

 \x changes of direction of the glance, and more especially 

 between surfaces in which there are great differences 

 of shade and of colour. As the eye glides over bright 

 and dark, or coloured objects and surfaces, the impres- 

 sion of each colour changes, for it is depicted on por- 

 tions of the retina which directly before were struck 

 by other colours and lights, and were therefore changed 

 in their sensitiveness to an impression. This kind of 

 contrast is therefore essentially dependent on move- 

 ments of the eye, and has been called by Chevreul, 

 'successive Contrast. 9 



We have already seen that the retina is more sen- 

 sitive in the dark to feeble light than it was before. 

 By strong light, on the contrary, it is dulled, and is 

 less sensitive to feeble lights which it had before per- 

 ceived. This latter process is designated as < Fatigue ' 

 of the retina ; an exhaustion of the capability of the 

 retina by its own activity, just as the muscles by their 

 activity become tired. 



I must here remark that the fatigue of the 

 retina by light does not necessarily extend to the 

 whole surface ; but when only a small portion of this 

 membrane is struck by a minute, defined picture it 



also be locally developed in this part only. 



