Mr Newton to Mr Oldenburg containing a more particular direction for trying yethe Experiment of glas rubbed to cause various motions, mention'dmentioned in his former letter of Dec. 14. 1675.

SrSirDec. 21. 1675.

Upon yoryour letter I took another glas 4 inches broad & $\frac{1}{4}$ of an inch wide thick, of such glas as Telescopes are made of & placed it a $\frac{1}{6}$ part of an inch from yethe Tapble. It was set in such a piece of wood as yethe object glasses of Telescopes use to be set in. And yethe experiment succeeded well. After yethe rubbing was still & all was still yethe motion of yethe papers would continue sometimes while I counted a hundred, every paper leaping up about 20 times more or les & down as often. I tryed it also wthwith two other glasses ytthat belong to a Telescope & it suceeded wthwith both, & I make no question but any glas will do ytthat be excited to electric virtue as I think any may. If you have a mind to any of these glasses you may have them but I suppose if you cannot make it do in other glas you will faile in any I can send you. I am apt to suspect yethe failure was in yethe manner of rubbing: for I have observd ytthat yethe rubbing variously or wthwith various things alters yethe case. At one time I rub'drubbed yethe afforesaid great glass wthwith a napkin twice as much as I used to do wthwith my gown & nothing would stir, & yet presently rubbing it wthwith something els yethe motions soon began. After yethe glass has been much rubd too, yethe motions are not so lasting, & yethe next day I found yethe motions fainter & difficulter to excite then yethe first. If yethe Society have a mind to attempt it any more, I can give no other better advice then this: To take a new glass not yet rub'drubbed (perhaps one of yethe old ones may do well enough after it has lain still a while,) & let this be rub'drubbed not wthwith linnen nor soft nappy woollen but wthwith stuff whose threds may rake yethe surface of yethe glass, suppose Tamerine or yethe like doubled up in yethe hand, & this wthwith a brisk motion as may be till a hundred or 150 may be counted, yethe glass lying all yethe while over yethe papers, then if nothing stirr rub yethe glas wthwith yoryour finger ends half a score times to & fro or knock it as of wthwith yoryour finger ends as oft upon yethe glas; for this rubbing or knocking wthwith your fingers, after yethe former rubbing, conduces most to excite yethe papers. If nothing stir yet, rub again wthwith yethe cloth till 60 or 80 may be counted & then rub or knock again wthwith your fingers, & retpeat this till yethe electric virtue of yethe glass be so far excited as to take up yethe papers, & then a very little rubbing or knocking now & then will revive yethe motions. In doing all this let yethe rubbing be always done as nimbly as may be, & if yethe motion be circularscircular like ytthat of glas grinders it may do better. But if you cannot make it yet succeed, it must be let alone till I have some opportunity of trying it before you. As for yethe suspicion of yethe papers bein moved by yethe air I am secure from that. Yet in yethe other of drawing leaf gold to above a foot distance, I could not make it wchwhich I never went about to try my self till yethe last week, I suspect yethe Air might raise yethe gold & then a small attraction might determin it toward yethe glas. For I could not make it succeed.

As for Mr Hook's insinuation ytthat yethe summ of yethe Hypothesis I sent you had been delivered by him in his Micrography, I need not be much concerned at the liberty he takes in ytthat kind. Yet because you think it may do well if I state yethe difference I take to be between them, I shal do it as briefly as I can, & ytthat yethe rather that I may avoyd yethe savour of having done any thing unjustifiable or unhansome towards Mr Hooke. But for this end I must first (to see what is his) cast out what Mr Hooke he has borrowed from Des Cartes: or others viz: That there is an æthereal Medium. That light is yethe action of this Medium; That this Medium is less implicated in yethe parts of solid bodies & so moves more freely in them & transmits light more readily through them & that after such a manner as to accelerate yethe rays in a certain proportion. That refraction arises from this acceleration & has sines proportional. That light is at first uniform. That its colours are some disturbance or new modification of its rays by refraction or reflexion, That yethe colours of a Prism are made by yethe action means of yethe quiescent Medium accelerating some motion of yethe rays on one hand side where red appears & retarding it where on yethe other side where blew appears, & that there are but these two original colours or colour-making modifications of light, wchwhich by their various degrees, or, as Mr Hook call's it, dilutings, produce all intermediate ones. This rejected, yethe remainder iof his Hypothesis is ytthat he has changed Des-Cartes pressing or progressive motion of yethe Medium to a vibrating one, yethe rotation of yethe globuli to yethe obliquation of pulses, & yethe accelerating ytthat their rotation of yethe globuli on yethe one hand & retarding it on yethe other by yethe quiescent Medium to produce colours, to yethe like action of yethe Medium on yethe two ends of his pulses for yethe same end. TAnd having thus far modified his by yethe Cartesian Hypothesis he has added extended it further to explicate yethe phænomena of thin plates & added another explication of yethe colours of natural bodies fluid & solid. This I think is in short yethe summ of his Hypothesis & in all this I have nothing common wthwith him but yethe supposition ytthat æther is a vibrating Medium susceptible of vibrations of wchwhich supposition I make a very different use: he supposing it light it self & I using it wchwhich I suppose it is not. This is as great a difference as is between him & Des Cartes. But besides this yethe manner of refraction & reflexion & yethe nature & production of colours in all cases (wchwhich takes up yethe body of my discours) I explain very differently from him; & even in yethe colours of thin transparent substances I explain every thing after a way so differing from him, ytthat yethe experiments I grownd my discours on destroy all he has said about them, & yethe two main experiments wthwithout wchwhich yethe manner of yethe production of those colours is not to be found out, he was were not only unknown to him when he wrote his Micrography, but even last spring, as I understood in mentioning them to him. This therefore is yethe sum of what's common to us, that there are æther may vibrate, & so if he think fit to use that notion of colours arising from yethe various bignes of pulses (wthwithout wchwhich his Hypothesis will do nothing) his will borrow as much from my answer to his OjectionsObjections as ytthat I sent you does from his Micrography. But it may be he means ytthat I have made use of his Observations & of some I did; as ytthat of yethe inflexion of rays, for wchwhich I quoted him; that of opacicityopacity arising from yethe intterstices of yethe parts of bodies, wchwhich I insist not on; & that of plated bodies exhibiting colours, a Phænomenon for yethe notice of wchwhich I thank him: But he left me to find out & make such experiments about it as might inform me of yethe 47 manner of yethe production of those colours, to ground an Hypothesis on; he having given no further insight into it then this ytthat yethe colour depended on some certain thicknes of yethe plate: though what that thicknes was at every colour he confesses he ha in his Micrographia he had attempted in vain to learn & therefore seing I was left to measure it my self I suppose he will allow me to make use of what I tooke yethe pains to find out. And this I hope may vindicate me from yethe expectation what Mr Hook has been pleasd to charge me wthwith. SrSir I doubt I have already troubled you wthwith too long a letter, & so break of abruptly YorYour humble Servant Is. Newton. Decemb. 21. 1675.