Defence of his Theory of Accidental Colours. 437 



cessary accompaniment of accidental colours; that is, that the 

 portion of the retina which affords the accidental colour is 

 more or less insensible to every other colour but that which 

 it affords. To deny the influence of diminished sensibility as a 

 came, is a very different thing from denying it as an effect, 

 and it is by confounding these two propositions that M. 

 Plateau seems to me to have erred. 



Now if we admit, what I think is capable of the most ri- 

 gorous proof, that there is a diminished sensibility of the part 

 of the retina affected, it follows as a matter of course that the 

 sum of those diminished sensibilities for all the rays of white 

 light must be an insensibilitj/ to them all, that is hlacJcncss. 



But why should we examine M. Plfileau's proposition, by 



means o^i theories, or oi opiiiions, that are in any way disputed 



or ambiguous? Is it not a demonstrated fact, admitted by 



every philosopher, that the accidental colour of "white is hlack'i 



that is, the sum of the actions of all the component colours of 



white light, when their action has ceased, or the combination 



of all their accidental colours, is blackness. If we take two 



complementary colours,.namely the red^iW^X green tints forming 



the ordinary and extraordinary pencils in the polarized ring, 



which by overlapping form 'white light, then it is manifest that 



the accidental colour of the overlapping part is black, and 



hence the sum of the action of the red and sreen acting se- 

 es o 



parately must also be black. 



I have no hesitation, therefore, in averring that the view 

 which I have taken in the Edinburgh Review and elsewhere 

 of Prof. Plateau's proposition is in every respect correct. 



In replying to the observations of Professor Plateau in de- 

 fence of his theory of accidental colours, I feel a difficulty of a 

 very peculiar kind. He admits that his theory is more fre- 

 quently opposed by the residts of experiment^ than supported by 

 them ; and as this is the very opinion which I have maintained, 

 it would seem almost unnecessary to continue the discussion. 

 In case, however, I may have misapprehended that part of his 

 reply in which this admission is made, it will be necessary to 

 enter more minutely into the subject. 



But before I do this, I must acknowledge a mistake which 

 1 have committed, and which he has pointed out, in ascri- 

 bing to him the assertion, that the regular alternation of the 

 PRIMITIVE and ACCIDENTAL colow, is the effect most frequently 

 observed, whereas he has maintained the contrary proposition, 

 that the effect 'which he most frequently observed 'was that of the 

 DISAPPEARANCE and REAPPEARANCE of the negative or acci- 

 dental impression alone. In expressing my regret for this 

 mistake, into which I was led by believing Professor Plateau's 

 • In this Journal, vol. xiv. p. 340. 



