186* ACCIDENTAL COLOURS. [SECT. xix. 



but a variety of colours in succession. According to 

 M. Plateau, the retina offers a resistance to the action of 

 light, which increases with the duration of this action; 

 whence, after looking intently at an object for a long time, it 

 appears to decrease in brilliancy. The imagination has a 

 powerful influence on our optical impressions, and has been 

 known to revive the images of highly luminous objects 

 months, and even years, afterwards. 



