202 Newton's Letters, Hypothesis and Experiments 



free-stone floor of a large hall ; so as I believe the immediate 

 stroke of five hundred drum-sticks could not have done, un- 

 less perhaps quickly succeeding one another at equal intervals 

 of time. iEtherial vibrations are therefore the best means by 

 which such a subtle agent as light can shake the gross parti- 

 cles of solid bodies to heat them. And so supposing that light 

 impinging on a refracting or reflecting aetherial superficies 

 puts it into a vibrating motion, that physical superficies being 

 by the perpetual appulse of rays always kept in a vibrating 

 motion, and the aether therein continually expanded and com- 

 pressed by turns ; if a ray of light impinge upon it while it is 

 much compressed, I suppose it is then too dense and stiff to 

 let the ray pass through, and so reflects it; but the rays that 

 impinge on it at other times, when it is either expanded by 

 the interval of two vibrations, or not too much compressed and 

 condensed, go through, and are refracted. 



These may be the causes of refractions and reflexions in all 

 cases, but for understanding how they come to be so regular, 

 it's further to be considered, that as in a heap of sand, although 

 the surface be rugged, yet if water be poured on it to fill its 

 pores, the water, so soon as its pores are filled, will evenly 

 overspread the surface, and so much the more evenly as the 

 sand is finer ; so, although the surface of all bodies, even the 

 most polished, be rugged, as I conceive, yet when that rug- 

 gedness is not too gross and coarse, the refracting aetherial 

 superficies may evenly overspread it. In polishing glass or 

 metal, it is not to be imagined that sand, putty, or other fret- 

 ting powders should wear the surface so regularly as to make 

 the front of every particle exactly plain, and all those planes 

 look the same way, as they ought to do in well-polished bodies, 

 were reflexion performed by their parts; but, that those fret- 

 ting powders should wear the bodies first to a coarse rugged- 

 ness, such as is sensible, and then to a finer and finer rugged- 

 ness, till it be so fine that the aetherial superficies evenly over- 

 spreads it, and so makes the body put on the appearance of a 

 polish, is a very natural and intelligible supposition. So in 

 fluids it is not well to be conceived that the surfaces of their 

 parts should be all plain, and the planes of the superficial 

 parts always kept looking all the same way, notwithstanding 

 that they are in perpetual motion, and yet without these two 

 suppositions, the superficies of fluids could not be so regularly 

 reflexive as they are, were the reflexion done by the parts 

 themselves, and not by an aetherial superficies evenly over- 

 spreading the fluid. 



Further, concerning the regular motion of light, it might 

 be suspected whether the various vibrations of the fluid 



