436 Sir D. Brewster's Observations on Prof. Plateau's 



truth will not be compromised by a further analysis of those 

 pha?nomena of our visual sensations respecting which these 

 difFerences of opinidii have arisen. 



Although I have "devoted much time to the study of the 

 phaenomena of accidental colours, yet my observations have 

 been made at distant intervals, and recorded from time to 

 time without any design of establishing a particular theory. 

 At one time I believed in the theory of the diminution of 

 sensibility, till a series of specific observations led me to a 

 new theory, which, as I shall afterwards show, differs very little, 

 if it differ at all, from that of M. Plateau in its leading aspect. 



To this general theory M. Plateau has added two distinct 

 propositions respecting the combination of accidental colours, 

 and their lauo of succession ', and it is on these points that I 

 have the misfortune of differing with him in opinion. 



On the first of these points M. Plateau maintains, that while 

 *' the combination of real colours produces iiohite, the com- 

 bination of accidental colours produces the contrary to "uohite, 

 or black." That is, " whereas two real complementary colours 

 produce together niohite, two accidental complementary colours 

 produce together black*." 



With respect to this proposition, I have stated in the Edin- 

 burgh Review, that we cannot, with any propriety of lan- 

 guage, consider ojie acci'dental colour as added to, or comlmied 

 with another ; that the proposition itself is a verbal illusion ; 

 and that the physical fact which it expresses has been long 

 known to philosophers, and is indeed the necessary result of 

 our previous knowledge on the subject. 



After endeavouring to controvert this opinion, M. Plateau 

 does not conclude that it is erroneous ; he concludes only, and 

 he puts his conclusion in italics, that he has not merely ex- 

 pressed what has been long known. The ground upon which 

 he rests this conclusion is, that I did not seem to be aware 

 that his proposition included the new fact, that accidental co- 

 lours were seen in total obscurity. Now I admit the force of 

 this exception, and I acknowledge that the visibility of ac- 

 cidental colours in the dark was not known to Buffbn, Darwin, 

 or Rumford ; but this fact has no connexion with the pro- 

 position under discussion, any further than that it overturns 

 the theory of sensibility, and weakens my argument in so far 

 as it was supjioscd. to depend upon that theory. 



Now though I abandoned as incorrect, and have long ago 

 abandoned, the theory of u diminution of sensibility as furnish- 

 ing the true exiplanation of the phaenomena of accidental co- 

 lours, I still maintain that a diminution of sensibility is a ne- 

 * Ann. de Cliim., Aug. 1833, p. 388, and this Journal, vol. xiv, p. 334. 



