VOL. VIII.] PHILOSOPHICAL TRANSACTIONS. Ql 



also a Discovery of the Pcrviousness of Glass to Ponderable parts of Flame, 

 with some reflexions on it by way of Corollary. 



This long title sufficiently shows the nature and contents of these Tracts, 

 which are in the hands of the public. 



On the Number of Colours, and the Necessity of mixing them all for the production 



of White ; also why a Picture cast by Glasses into a darkened Room appears so 



distinct i notwithstanding its irregular Refractions. In a Letter of April 3, 1673, 



from Mr. Newton. Being an Answer to the Remarks in the last Number. N° 97, 



p. 61O8. 



It seems to me, that N. takes an improper way of examining the nature of 

 colours, whilst he proceeds upon compounding those that are already com- 

 pounded, as he does in the former part of his letter. Perhaps he would sooner 

 satisfy himself by resolving light into colours, as far as may be done by art, and 

 then by examining the properties of those colours apart, and afterwards by try- 

 ing the effects of re-conjoining two or more, or all of those^ and lastly, by sepa- 

 rating them again, to examine what changes that re-conjunction had wrought 

 in them. This I confess will prove a tedious and diffidilt task, to do it as it 

 ought to be done ; but I could not be satisfied till I had gone through it. How- 

 ever, I only propound it, and leave every man tahis own method. 



As to the contents of his letter, I conceive my former answer to the quaere 

 about the number of colours is sufficient, which was to this effect; that all 

 colours cannot practically be derived out of the yellow and blue, and conse- 

 quently that those hypotheses are groundless which imply they may. If you 

 ask, what colours cannot be derived out of yellow and blue ? I answer, none of 

 all those which I defined to be original ; and if he can show by experiment, 

 how they may, I will acknowledge myself in an error. Nor is it easier to frame 

 an hypothesis by assuming only two original colours, rather than an indefinite 

 variety; unless it be easier to suppose, that there are but two figures, sizes and 

 degrees of velocity, or force, of the asthereal corpuscles or pulses, rather than 

 indefinite variety ; which certainly would be a harsh supposition. No man won- 

 ders at the indefinite variety of waves of the sea, or of sands on the shore ; but 

 w^ere they all but two sizes, it would be a very puzzling phenomenon. And I 

 should think it as unaccountable, if the several parts or corpuscles, of which a 

 shining body consists, which must be supposed of various figures, sizes and 

 motions, should impress but two sorts of motion on the adjacent aethereal 

 medium, or any other way beget but two sorts of rays. But to examine how 

 colours may be explained hypothetically, is besides my purpose. I never in- 

 tended to show wherein consists the nature and difference of colours, but only 



N 2 



