VOL. X.] PHILOSOPHICAL TRANSACTIONS. 203 



it to him also. And you may remember that R. H. two or three years ago in a 

 letter read before the Royal Society, and transmitted to me, gave testimony, 

 not only to the experiments questioned by Mr. Line, but to all those set down 

 in my first letter about colours, as having tried them himself; and when you 

 read Mr, Line's letter at a meeting of the said Society, and was pleased to do 

 me the favour to propound the experiment to be tried in their presence, R. H. 

 spake of it to them as a thing not to be questioned. But if it have not yet been 

 tried before them, and any of them upon Mr. Line's confidence doubt of it, I 

 promise when I shall have the happiness to be at any more of their assemblies, 

 upon the least hint, to show them the trial of it; and I hope I shall not be 

 troublesome, because it may be tried, though not so perfectly, even without 

 darkening a room, or the expence of any more than half a quarter of an hour ; 

 although if Mr. Line persist in his denial of it, I could wish it might be tried 

 sooner there, than I shall have an opportunity to be among them. 



An Extract of another Letter of Mr. Newton, the lOth of January ]67i, relating 

 to the same Argument. N" 121, p. 503. 



By Mr. Gascoin's letter* one might suspect, that Mr. Linus tried the 



experiment some other way than I did; and therefore I shall expect till his 

 friends hav'e tried it according to my late directions. In which trial it may 

 possibly be a further guidance to them, to acquaint them, that the prism casts 

 from it several images : one is, that oblong one of colours which I mean; and 

 this is made by two refractions only. Another there is, made by two refractions 

 and an intervening reflection ; and this is round and colourless, if the angles of 

 the prism be exactly equal ; but if the angles at the reflecting base be not equal, 

 it will be coloured, and that so much the more, by how much unequaller the 

 angles are, but yet not much unround, unless the angles be very unequal. A 

 third image there is, made by one single reflection, and this is always round 

 and colourless. The only danger is in mistaking the second for the first. But 

 thev are distinguishable, not only by the length and lively colours of the first, 



* This letter was written to the editor, December 15, l67o, from Leige, where M. Gascoine, 

 havino- been a scholar of Mr. Linus, now deceased, resides. In it are contained these words, to 

 which Mr. Newton, to whom it was communicated, seems here to have respect, viz. Mr. Linus 

 tried the experiment again and again, and called divers on purpose to see it, nor ever made ditficulty 

 to show it to any one, who either by chance came to his chamber as he was doing it, or showed the 

 least desire to see the same. So. tliat for point of experience, Mr. Newton cannot be more confi- 

 dent on his side than we are here on the other j who are fully persuaded, that, unless the diversity of 

 placing the prism, or tlie largeness of the hole, or some other such circumstance, be the cause of the 

 difference betwixt them, Mr. Newton's experiment will hardly stand. — Orig. 



