Appearances from the Contemplation of Coloured Objects. 44-3 



insulated from every lateral influence, and contemplated for a 

 long time, seems to become gradually darker. 



During the prolonged contemplation of an insulated ob- 

 ject, there are consequently two contrary forces in presence 

 of each other, namely, the action of direct light, and the op- 

 posite effort of the retina. As long as the first force predo- 

 minates, the eye feels the sensation of the direct colour, al- 

 though more or less faded. But if by any cause whatever, 

 the second force becomes overpowering, then the opposite 

 sensation manifests itself, and the complementary colour is 

 perceived. It is thus, for instance, that the experiment of 

 the red seal is naturally explained : the strong light placed 

 close to the eye paralyses the action of the red rays emanating 

 from the seal, and the effort of reaction of the retina being 

 then overpowering, the green accidental colour shows itself. 

 In a certain point of view, as may be seen, Sir David Brew- 

 ster's theory and mine approach each other ; and indeed, in 

 the first place, they have that in common, that they consider 

 the accidental colour as owing to an impression ot" a peculiar 

 nature, which is spontaneously generated in the oi'gan, and not 

 as the result of a relative insensibility to certain rays. On the 

 other hand, according to Sir David Brewster, the accidental 

 colour unfolds itself on the retina during the contemplation of 

 the direct colour, and combines itself with this latter; and, 

 according to me, the opposite effort of the retina, whence re- 

 sults the negative sensation as soon as that effort ceases to be 

 counteracted, likewise unfolds itself during the contemplation 

 of the direct colour, and combines itself in some respect with 

 this latter, neutralizing it partially. But only, Sir David 

 Brewster maintains that the combination of the two sensa- 

 tions produces whiteness, whereas 1 have shown that, upon 

 an object insulated from every lateral influence, the result is 

 on the contrary blackness. 



At present it must be ascertained how my theory accounts 

 for these lateral influences which tend to give the object a 

 shade of the colour of the ground whereon it rests. 



Let us primarily remark that, at the first sight, this fact 

 seems in opposition with those composing the second section 

 of which I have spoken at the beginning of this article, and 

 constituting, on the retina, the passage to the normal state 

 with regaril to space. Ifj in fact, we bring together the ex- 

 periments made on the phainoniena of simultaneousness by 

 Rumford*, Meusnier f, Prieur:!;, &c., and the results ob- 



* Philosophical Transactions, 1794. 



t Mcusnier's Experiments are related by Monge {Am. dc Chim'w, tome 

 III. 1/89.) :J Annates dc ChimicyiomcVw . 



