444- M. Plateau's Defence of his Theory of the Visual 



tained by M. Chevreul*, we shall come to this conclusion; 

 that, whenever an object of small dimensions is placed upon 

 a large coloured surface, the small object must take a tint 

 more or less intense complementary to the colour of this sur- 

 face t, and not one identical to that colour. 



But there is an essential difference between the circum- 

 stances in which these two phicnomena, apparently irrecon- 

 cilable, are produced ; for the one is the result of an instaii- 

 taneous contemplation, and the other of a 2}rolonged contem- 

 plation. In fact, by repeating the experiments which relate 

 to the appearance on the small object of a tint complementary 

 to that of the surrounding surface, it is easy to verify that this 

 phsenomenon shows itself in an instant with all its intensity ; 

 and this particularity had already been experienced by M. 

 Chevreul (see the memoir quoted, page ^l.). Now, what must 

 happen, if the contemplation be prolonged? Since the com- 

 plementary tint which covers the object results from the la- 

 teral action exercised on the retina by the impression of the 

 surrounding colour, it is obvious that, if this latter impression 

 becomes fainter, die complementary tint must undergo the same 

 change. But the first effect being the necessary result of the 

 length of the contemplation, the second is not a less inevitable 

 consequence of it. Thus a prolonged contemplation will 

 begin by weakening the impression of the complementary 

 tint. And indeed I have verified this fact, (see my detailed 

 memoir, page 33,) which can also be easily ascertained by 

 others. But there is another cause which contributes to pro- 

 duce this degradation. To make it understood, let us take 

 an example. Suppose the coloured surface to be red, and 

 the small object black or gray. Then the prolonged con- 

 templation will develop, on the part of the retina exposed to 

 red ravs, a contrary action ; that is to say, an action tending 

 to produce the corresponding negative sensation, or the acci- 

 dental green ; but the part of the organ which receives the 

 image of the small object, and which is consequently alien 

 to that negative action of the retina, must, according to the 

 principles of my theory, exercise an effort in an opposite 

 sense ; that is to say, tending to give the corresponding posi- 

 tive sensation, namely a sensation of positive red. Now this 



• Memoires de rinstitut, torn. xi. 1832. 



t It is only to be remarked, (as I have pointed out in the article of 

 Annates de Chim. et de Physique which has been before alluded to, agreeably 

 to the objections of the anonymous author) that in some particular circum- 

 stances the complementary colour does not extend itself on the whole 

 surface of the small object, and the middle of this becomes then slightly 

 tinted with the colour of the ground whereon the object is placed. 



