166 ACCIDENTAL COLOURS. SECT. XIX. 



light if they be from a direct impression on the eye, whereas 

 blackness results from a union of the same tints if they be acci- 

 dental ; and in every case where the real colours produce white 

 by their combination, the accidental colours of the same tints 

 produce black. When the image of an object is impressed on 

 the retina only for a few moments, the picture left is exactly of 

 the same colour with the object, but in an extremely short time 

 the picture is succeeded by the accidental image. M. Plateau 

 attributes this .phenomenon to a reaction of the retina after being 

 excited by direct vision, so that the accidental impression is of 

 an opposite nature to the corresponding direct impression. He 

 conceives that when the eye is excited by being fixed for a time 

 on a coloured object, and then withdrawn from the excite- 

 ment, it endeavours to return to its state of repose ; but in so 

 doing, that it passes this point, and spontaneously assumes an 

 opposite condition, like a spring which, bent in one direction, 

 in returning to its state of rest bends as much the contrary way. 

 The accidental image thus results from a particular modification 

 of the organ of sight, in virtue of which it "spontaneously gives 

 us a new sensation after it has been excited by direct vision. 

 If the prevailing impression be a very strong white .light, its 

 accidental image is not black, but a variety of colours in succes- 

 sion. 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. 



