186 Newton's Letters, Hypothesis and Experiments 



medium, certainly deserves more notice than it has obtained. 

 It has scarcely, I think, been quoted, except by Dr. Young; 

 and its existence is but little known, even among the best-in- 

 formed scientific men, notwithstanding its publication (with 

 some verbal inaccuracies) by Dr. Birch in his History of the 

 Royal Society. Under these circumstances, I have no doubt 

 that a reprint of it would be very acceptable as a curious 

 matter of history, and especially as tending to throw light on 

 the character of Newton's mind. 



The letter to Boyle is more accessible; but the circumstance 

 of its containing the earliest attempt, as I have pointed out in 

 my letter to Lord Brougham, to explain the nature of gase- 

 ous substances, has failed to attract the notice which it merits 

 at the hands of the historians of chemistry. 



I remain, dear Sir, 



Yours faithfully, 

 Bolton Percy, August. William Vernon Harcourt. 



Mr. Isaac Newton's Letter, Hypothesis, Observations and 

 Experiments touching his Theory of Light and Colours ; in 

 confirmation and illustration of his former discourse on the 

 same subject*. 



Sir, 



HAVE sent you the papers I mentioned by John Stiles. 

 ■*• Upon reviewing them, I find something so obscure as 

 might have deserved a further explanation by schemes, and 

 some other things I guess will not be new to you, though al- 

 most all was new to me when I wrote them. But as tbey are, 

 I hope you will accept of them, though not worth the ample 

 thanks you sent. I remember in some discourse with Mr. 

 Hook, I happened to say that I thought light was reflected, 

 not by the parts of glass, water, air, or other sensible bodies, 

 but by the same confine or superficies of the aetherial me- 

 diums which refracts it, the rays finding some difficulty to get 

 through it, in passing out of the denser into the rarer medium, 

 and a greater difficulty in passing out of the rarer into the 

 denser; and so, being either refracted or reflected by that 

 superficies as the circumstances they happened to be in at 

 their incidence, make them able, or unable, to get through it. 

 And for confirmation of this, I said further, that I thought 

 the reflexion of light at its tending out of glass into air, would 

 not be diminished or weakened by drawing away the air in 

 an air-pump, as it ought to be, if they were the parts of air 



* Read before the Royal Society, December 9, 1675, and some following 

 days of their meeting. 



