86 PHILOSOPHICAL TKANSACTIONS. [aNNO I673. 



holds. For to consider only a picture, which is made by an object-glass of 12 

 feet in a dark room, we see, it is too distinct, and too well defined, to be pro- 

 duced by rays that should stray the 50th part* of the aperture. So that, as I 

 believe I have told you heretofore, the difference of the refrangibility does not, 

 it may be, always follow the same proportion in the great and small inclinations 

 of the rays on the surface of the glass. 



Mr. Neivtons Answer to the foregohig Letter, further explaining his Theory of 

 Light and Colours, and particularly that of TVIiiteness ; with his continued Hopes 

 of perfecting Telescopes by Reflections, rather than Refractions. N° 96, p.6087. 



Concerning the business of colours; in my saying that when Mons. N. has 

 shown how white may be produced out of two uncompounded colours, 

 I will tell him, why he can conclude nothing from that; my meaning was, 

 that such a white, were there any such, would have different properties 

 from the white which I had respect to, when I described my theory, that is, from 

 the white of the sun's immediate light, of the ordinary objects of our senses, 

 and of all white phaenomena that have hitherto fallen under my observation. 

 And those different properties would evince it to be of a different constitution; 

 insomuch that such a production of white would be so far from contradicting, 

 that it would rather illustrate and confirm my theory : because by the difference 

 of that from other whites it would appear, that other whites are not compounded 

 of only two- colours like that. And therefore if Mons. N. would prove any 

 thing, it is requisite that he do not only produce out of two primitive colours a 

 white, which to the naked eye shall appear like other whites, but also shall agree 

 with them in all other properties. 



But to let you understand wherein such a white would differ from other whites, 

 and why from thence it would follow that other whites are otherwise compound- 

 ed, I shall lay down this position : that a compounded colour can be resolved 

 into no more simple colours than those of which it is compounded. 



This seems to be self evident, and I have also tried it several ways, and par- 

 ticularly by this which follows. Let a, represent an oblong piece of white paper, 

 about i or i of an inch broad, (fig. 3, pi. 2,) and illuminated in a dark room with 

 a mixture of two colours cast upon it from two prisms, suppose a deep blue and 

 scarlet, which must severally be as uncompounded as they can conveniently be 

 made. Then at a convenient distance, suppose of 6 or 8 yards, view it through 

 a. clear triangular glass or crystal prism, held parallel to the paper, and you shall 



* Compare this with what Mr. Newton says in Numb. 80, page 681, vol. I. 



