"Idl On the Colours that enter into the [Sept. 



means of comparison, and will enable us to appreciate the 

 apparent alteration experienced by the colour of the first. 

 Now, this change is rendered very sensible ; the paper 

 which has produced a prolonged impression appears darker 

 than the other. We are also certain, by this means, that 

 the alteration in the colour of the object is progressive : 

 that is to say, at first feeble, and, increasing with the length 

 of time that it is contemplated. In the second experiment. 

 Plateau conceives that the conclusion drawn is not at all 

 evident : viz. that the two colours exist at the same time, 

 since it is obvious, from the wax appearing black, that the 

 presence of the flame prevents the red rays from being 

 perceived. 



The accidental colours developed in experiments like the 

 last, (3.) Plateau considers as distinguished by particular 

 properties, different from those pertaining to colours which 

 succeed the action of coloured light. For, the intensity of 

 the latter is proportionally greater as the action of the 

 coloured light is prolonged. The former, on the contrary, 

 possess all their intensity in a very short space of time, and, 

 if we continue to look at them, the colour becomes feebler. 

 Thus, if we hold between a window and the eye a piece 

 of semi-transparent red paper, upon which a strip of white 

 card is applied, the small strip will appear of a green shade; 

 and, if we continue to look at it, the colour will become less 

 intense. Hence, he concludes, we should beware of con- 

 founding the two kinds of accidental colours, and that the 

 existence of the one set does not necessarily lead us to infer 

 the co-existence of the other set. 



9. Having proved, as he conceives, that accidental colours 

 do not proceed from a moral cause, but originate from a true 

 affection of the retina ; and, since they can be developed 

 without any participation of external light. Plateau con- 

 cludes, 1. That the accidental image results from a peculiar 

 modification of the organ, by which a new sensation is spon- 

 taneously produced : 2. He infers, from experiments, that 

 *' the accidental image is always preceded by the continued 

 action of the primary image;" 3. " The mixture of two 

 real complementary colours produces white, and that of 

 two corresponding accidental colours forms the opposite of 

 white or black." 



