330 M. Plateau's Defence of his Theory of the Visual 



stance tres facile a chaTbonnei\ mentioned by Thenard, is pro- 

 bably aldehyd, which from its behaviour towards alkalis 

 might apparently merit that character. 



LI I. Ansixier to the Objections published against a general 

 Theory of the Visual Appearances which arise from the Con- 

 templation of Coloured Objects. By J. Plateau, Professor 

 at the University of Ghent. 



To the Editors of the Philosophical Magazine and Jourfial. 

 Gentlemen, Ghent, April 15, 1837.t 



I HAVE the honour to transmit you herewith an article 

 which I should thank you to insert in your Journal. 

 Although the main object of this paper is not to make known 

 new facts, you are well aware that theoretical discussions are 

 not void of interest with regard to science. Besides, when a 

 theory is attacked, it is naturally to the organs of scientific 

 publicity that the office belongs to furnish the author of that 

 theory with the means of defending his ideas. As it was in 

 England that the principal objections against my statements 

 appeared, and as a part of those objections were published 

 in your Journal ; lastly, as my article contains a mere scien- 

 tific defence, I make no doubt. Gentlemen, that you will 

 willingly devote your pages to it, and do me the favour to 

 insert it in your next Number. 



I remain. Gentlemen, yours, &c., 



Jh. Plateau, 



Professor at the University of Ghent. 



In publishing my theory on the visual appearances which 

 succeed the contemplation of coloured objects, and on those 

 which manifest themselves during that contemplation :j:, I con- 

 ceived that if this work deserved any attention, it was princi- 

 pally on account of the generality and simplicity of the point 

 of view under which I have united four different classes of 

 phaenomena whose affinity to each other has not been acknow- 

 ledo-ed ; namely, the duration of impressions on the retina, the 



t This communication, as I subsequently learnt from M. Plateau, was 

 forwarded from Ghent at the date which it bears, and having, after being 

 loniT oiven up for lost, at length come to hand through some unknown 

 channel, it is now published without further delay. — R. T. 



X See An. de Chim. et de ir'hysique of Messrs. Gay-Lussac and Arago, 

 August 1833, page 386, and April 1835, page 337. Supplement to the 

 Treatise on Light of Sir J. Herschel. by M. Quetelet, page 515. Memoirs 

 of the Academy of Brussels, torn. viii. 



