334 M. Plateau's Defence ofJiis Theori/ of the Visual 



His first attack is directed against one of the proofs by 

 which I intend to establish an opposition of nature between 

 accidental and dii'ect impressions. My proposition was thus 

 expressed * : — 



" In the cases where the combination of real colours pro- 

 duces lohite^ the combination of accidental colours produces 

 the contrary to "white, or black. Whereas, for instance, two 

 real complementary colours produce together isohite, two ac- 

 cidental complementary colours produce together black. 



" This fact may be ascertained by an experiment : place 

 on a black ground a rectangle of paper, the two halves of 

 which are painted with two complementary colours, for in- 

 stance red and green, as indicated by the fig. l.t? the middle 

 of each coloured portions being marked with a black point. 

 Then, if you alternately direct your sight from one of these 

 points to the other, for a sufficient length of time, the result 

 will be an image, in the bottom of the eye, formed by the su- 

 perposition of the accidental green produced by the red half, 

 and of the accidental red produced by the green half; or, in 

 other terms, by the superposition of two complementary ac- 

 cidental colours. Now, if you suddenly and completely cover 

 your eyes with a handkerchief, this image will appear perfectly 

 black, having on the right hand a red image, and on the lett 

 a green image (supposing that the green and red halves of 

 the rectangle be placed as in the figure). The production 

 of these two lateral images sufficiently explains itself" 



I shall here subjoin some particulars for persons who may 

 wish to repeat the experiment. The rectangle I employed 

 was ^0 centimetres long by 10 high, and the ground on which 

 I placed it was a large black shawl spread on the floor, in a 

 light room. I stood with my back to the windows, but so as 

 not to throw a shadow on the object. I then directed my 

 sight alternately on the two black points, looking at each of 

 them for nearly a second, and I continued thus about the 

 space of a minute, after which I covered my eyes, as I before 

 said. 



These are now the objections. The anonymous author be- 

 gins by saying that this property of accidental colours advanced 

 by me, must appear, at first sight, an important one, but that 

 a slight examination of the subject will show, " that this pro- 

 position is a verbal illusion, and that the physical fact which 

 it so erroneously expresses, has been long known to philoso- 

 phers." 



* Ann. de Chimie et de Physique, August 1833, p. 388. 

 •j- In the figure which accompanies the article, (in the Ann. de Chim.) the 

 green halt' of the rectangle is on the right, and the red half on the left. 



