282 M. Plateau on the Phenomena of [Oct. 



the portion of the retina which has, for a certain time, 

 received the impression of a certain colour, has become less 

 sensible to the rays of that colour, and that its sensibility 

 becomes relatively greater to the rays which form the com- 

 plementary tint. 



[2.] *' It is necessary for me to cite here, in favour of my 

 opinion, only one single fact, stated long ago, but totally 

 forgotten, that accidental colours may be seen in perfect 

 darkness, when, consequently there exist no luminous rays 

 which can produce the sensation of the complementary tint. 

 [3.] " Accidental colours result from an opposite state 

 which the retina spontaneously assumes after the cessation 

 of direct impressions. 



[4.] " Accidental colours may be seen independently of 

 the immediate action of light : that is, that they result from 

 a spontaneous modification of the retina, and that such is 

 the case the following are proofs : 1st, This opposition is 

 evident, as in the case of Mack, which follows the contem- 

 plation of a white object ; and secondly, 



[5.] " That the accidental colours of impressions destroy 

 direct corresponding impressions. If we observe for a suf- 

 ficient time, a small piece of red paper on a black ground, 

 and afterwards direct the eyes to a large piece of red paper, 

 the space occupied by the image of the small piece of red 

 paper will appear somewhat Mack, (noiratre), without any 

 mixture of red: Thus, the direct impression, rec?, is here 

 destroyed by the accidental impression, green, 



[6.] " In cases where the combination of real colours 

 produces white, the combination of accidental colours pro- 

 duces the opposite of white, namely. Mack. For example, 

 while two real complementary colours produce, together, 

 white, two accidental complementary colours produce, to- 

 gether. Mack. This may be proved by experiment. By 

 placing, on a black ground, two square pieces of coloured 

 paper, the colours being complementary to each other, such 

 as a red and a green, the centre of each piece being marked 

 with a black dot. Then, if the eyes be directed from one 

 of these dots to the other, and so on alternately, for a suffi- 

 cient time, the result upon the retina (le fond de I'oeil) will 

 be the formation of an image, by the superposition of the 

 accidental green, produced by the red square; and of the 



