SECT, xix.] ACCIDENTAL COLOURS. *185 



that of yellow is indigo ; of green, reddish white ; of blue, 

 orange-red; of violet, yellow; and of white, black; and 

 vice versd. When the direct and accidental colours are of 

 the same intensity, the accidental is then called the com- 

 plementary colour, because any two colours are said to be 

 complementary to one another which produce white when 

 combined. 



From recent experiments by M. Plateau of Brussels, it 

 appears that two complementary colours from direct im- 

 pression, which would produce white when combined, pro- 

 duce black, or extinguish one another by their union, when 

 accidental ; and also that the combination of all the tints of 

 the solar spectrum produces white light if they be from a 

 direct impression on the eye, whereas blackness results from 

 a union of the same tints if they be accidental ; and, in 

 every case where the real colours produce white by their 

 combination, the accidental colours of the same tints pro- 

 duce 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 ac- 

 cidental impression is of an opposite nature to the corre- 

 sponding 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 and then withdrawn from the .excitement, that it endeavours 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, 



