Appearances from the Contemplation of Coloured Objects. 44'1 



eye is turned from the exciting colour to a white object. These 

 results arc obviously incompatible 'with the theory of accidental 

 colours recently published by M. Plateau." 



This is a very abrupt conchision, and the author will, I 

 trust, permit me to consider how far it is grounded *. 



Before we come to the experiment of the red seal, let us 

 first examine this other fact related, as we have seen, by Sir 

 David Brewster, namely, that during the contemplation of a 

 red object, the colour of this object appears to grow paler, or 

 to mix itself up "with a little "white. Certainly, if this fact was 

 to be admitted in the full extent which the author attributes 

 to it, its explanation in my theory would be difficult. But let 

 us look into the matter more closely. In order to observe an 

 accidental colour, the process frequently employed is to place 

 the coloured object which must produce the phaenomenon, 

 upon a white ground. In this circumstance indeed, it seems 

 that, by a prolonged contemplation, a little white mixes itself 

 with the colour of that object ; and it is, no doubt, in this 

 manner that Sir David Brewster has operated. But, if we 

 wish to notice the effect that a prolonged contemplation may 

 have on the aspect of an object, is it not evident that the latter 

 must be insulated from every extraneous influence, that it must 

 be seen alone, or, in other terms, be placed upon a black 

 ground? Now, in this case, the result is very different: the 

 colour of .the object, instead of growing paler, or blending 

 itself with white, becomes, on the contrary, darker, or blends 

 itself with black. The following facts leave nothing to doubt 

 in that respect. 



" If we look," says Buffonf , " for a length of time at a 

 white spot upon a black ground, we shall see the white spot 

 lose its colour." Now whiteness which loses its colour, what 

 is it but whiteness which becomes dark? 



Instead of the white object observed by Buffon, substitute 

 one of any colour, for instance, a piece of red paper, placing 

 it as before on a black ground ; and, the better to judge the 

 effect, employ an object of comparison. After having looked 

 for a length of time at the red paper, the eye being invariably 

 fixed on the same point, place close to this paper, and with- 

 out altering the position of the eye, another ])iece of the same 

 coloured paper. It is obvious that the image of this latter one 



• I have answered that article of Sir David Brewster, above two years 

 a^o, in my detailed memoir, pages 2!) and 56 ; but I deem it cx[)edicnt now 

 to renew this subject, with the intention of bringing together niy answers 

 to all the objections with which I have become acquainted. I shall besides 

 consider the matter in a new |)oint of view. 



f Mem. of the Acad, of Sciences of Paris, 1743, p. 153. 



