292 Mr. Tomlinson on the Theory of [Oct. 



until we arrive at the note, (page 174), when he twice 

 employs the term complementary, and twice the term acci- 

 dental. He then proceeds, (page 175), employing the term 

 accidental only, and speaks of one image as the accidental 

 colour of the other ; and does not loose his hold of the term 

 until he quits the subject of my two papers, and proceeds to 

 speak of the effect of contrast, &;c. (page 177), when he again 

 employs the term " complementary," and only towards the 

 end of his paper does he adopt both terms in such a manner 

 as to shew that he understands their distinction ; but, ap- 

 parently, not having quite satisfied himself as to where he 

 should fix the boundary line, he has, in many instances, 

 avoided the use of either term, and, in other cases, seems 

 to have hesitated which to apply .'^ Now, granting for a 

 moment that M. Plateau would admit (and the admission 

 is not likely to proceed from himf) that the coloured solu- 

 tions on mercury, &c., are accidental, and not complementary 

 colours, his proposition [3.] would be immediately disproved ; 

 for then, as we have already seen, the primitive and the 

 accidental tint if such it be ,can be seen together, the moment 

 the observation is begun to be made, and for a time, how- 

 ever long or short. The difficulty then is, to distinguish 

 the accidental from the complementary tint. It seems to 

 me to rest principally upon the question of equal or unequal 

 intensity ; and, the above extract from Brewster renders the 

 difference very small. I believe, however, that there is a 

 difference between the intensity of the two spectra, when 

 the plain mirror and green coloured disk are so inclined as 

 to form a very acute angle, as recommended by P. C.,J 



* I am thus tiresome, because I feel the distinction between the terms is, I 

 should be sorry to say not seen, but I will say, unattended to by many writers. 

 It is a remarkable fact, that Barlow, in his Treatise on Optics, and Herschel, in 

 his Treatise on Light, both inserted in the Encyc. Metropol. (Mixed Sciences, 

 vol. i. & ii.) have omitted mention of accidental and complementary colours, 

 except in Herschel's admirable exposition of the Laws regulating Polarization, 

 where these colours are referred to only as they incidentally occur in the pheno- 

 mena of polarization. Biot, also, is not altogether free from this sin of omission. 



t But it will be observed that M. Plateau makes free, and, at first sight, indis- 

 criminate use of the two terms, accidental and complementary. 



X I may state that this modification was not neglected by me, for, in addition 

 to other results, I obtained some very amusing ones, to be discussed hereafter, in 

 June last, and which I afterwards had the pleasure of producing to the Editor of 

 this Journal, in company with Dr. Thomas Thomson, when I was in London the 



