92 PHILOSOPHICAL TRANSACTIONS. [aNNO 1673. 



to show, that de facto they are original and immutable quahties of the rays 

 which exhibit them ; and to leave it to others to explicate by mechanical hypo- 

 theses the nature and difference of those quahties : which I take to be no diffi- 

 cult matter. But I would not be understood, as if their difference consisted in 

 the different refrangibility of those rays; for that different refrangibility 

 conduces to their production no otherwise, than by separating the rays whose 

 qualities they are. Whence it is, that the same rays exhibit the same colours 

 when separated by any other means ; as by their different refiexibility, a quality 

 not yet discoursed of. 



In the next particular, where N. would show, that it is not necessary to mix 

 air colours for the production of white ; the mixture of yellow, green, and blue, 

 without red and violet, which he propounds for that end, will not produce 

 white, but green; and the brightest part of the yellow will afford no other colour 

 but yellow, if the experiment be made in a room well darkened, as it ought; 

 because the coloured light is much weakened by the reflexion, and so apt to be 

 diluted by the mixing of any other scattering light. But yet there is an expe- 

 riment or two mentioned in my letter in the Transactions, Numb. 88, by which 

 I have produced white out of two colours alone, and that variously, as out of 

 orange and a full blue, and out of red and pale blue, and out of yellow and 

 violet, as also out of other pairs of intermediate colours. The most convenient 

 experiment for performing this, was that of casting the colours of one prism 

 upon those of another, after a due manner. But what N. can deduce from 

 hence I see not. For the two colours were compounded of all others, and so 

 the resulting white, to speak properly, was compounded of them all, and only 

 decompounded of those two. For instance, the orange was compounded of 

 red, orange, yellow, and some green ; and the blue of violet, full blue, light 

 blue, and some green, with all their intermediate degrees; and consequently the 

 orange and blue together made an aggregate of all colours to constitute the 

 white. Thus if one mix red, orange, and yellow powders, to make an orange; 

 and green, blue, and violet colours, to make a blue; and lastly, the two mix- 

 tures, to make a grey; that grey, though decompounded of no more than 

 two mixtures, is yet compounded of all the six powders, as truly as if the pow- 

 ders had been all mixed at once. 



This is so plain, that I conceive there can be no further scruple ; especially to 

 those who know how to examine whether a colour be simple or compounded, 

 and of what colours it is compounded; which having explained in another place, 

 I need not now repeat. If therefore N. would conclude any thing, he must 

 show, how white maybe produced out of two uncompounded colours; which 

 when he has done, I will further tell him why he can conclude nothing from 



