and Coloured Shadows, 439 



colour disappears while the hand is before the eyes, and 

 re-appears when it has passed beyond them, and this may 

 be repeated several times ; but if we suffer the hand, or 

 any other object which intercepts the light, to remain for 

 some time before the eyes, the accidental colours will re- 

 appear after a short interval, though much lessened in 

 brilliancy and distinctness. 



Now, with such evidence before us, that in every other 

 instance the accidental image is dependent upon the light 

 which has access to the eyes, and that in many cases it is 

 modified by the character of this light ; and this evidence 

 being brought up to the very point at which the difficulty 

 occurs, it would, in my opinion, be contrary to the rules 

 of sound philosophy to introduce any other cause to 

 account for it ; particularly in our present state of un- 

 certainty with regard to the ultimate destination of the 

 light which disappears when it is intercepted by opaque 

 bodies. 



M. Plateau's theory appears to be founded entirely upon 

 this single fact. [2] The hypothesis by which he endeavours 

 to explain it, that the retina spontaneously assumes an 

 opposite state after the cessation of direct impressions, [3] 

 has no other support ; and the numerous experiments in 

 which the accidental image is modified in colour by the 

 colour of the surface upon which it is formed are so 

 decidedly opposed to it, that it appears to me to be quite 

 untenable. 



He states, " that the accidental colours of impressions 

 destroy direct corresponding impressions ;" [5] and in the 

 experiment which is brought forward to support it, con- 

 cludes that the direct impression of a small piece of red 

 paper, upon a black ground, is destroyed when. the eye is 

 directed to a larger piece of paper of the same colour, by 

 the accidental impressions, green. According to our theory, 

 no accidental green image can be formed upon a surface 

 where the only colour present is red : but the part of the 

 eye impressed with the small piece of paper of the same 

 colour, being in a great measure insensible to red light, 

 the image of the paper will appear somewhat black, when 

 seen in contrast with the surrounding paper of the same 



