<46r>

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

Sr[1]

Upon yor letter I took another glas 4 inches broad & 14 of an inch wide /thick\, of such glas as Telescopes are made of & placed it a 16 part of an inch from ye Tap|b|le. It was set in such a piece of wood as ye object glasses of Telescopes use to be set in. And ye experiment succeed|ed| well. After ye rubbing was still & all was {illeg}|st|ill ye motion of ye 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 wth two other glasses yt belong to a Telescope & it suceeded wth both, & I make no question but any glas will do yt 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 ye failure was in ye manner of rubbing: for I have observd yt ye rubbing variously or wth various things alters ye case. At one time I rub'd ye afforesaid great glass wth a napkin twice as much as I used to do wth my gown & nothing would stir, & yet presently rubbing it wth something els ye motions soon began. After ye glass has been \much/ rubd too, ye motions are not so lasting, & ye next day I found ye motions fainter & difficulter to excite then ye first. If ye 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'd (perhaps one of ye old ones may do well enough after it has lain still a while,) & let this be rub'd not wth linnen nor soft nappy woo{illeg}|l|len but wth stuff whose threds may rake ye surface of ye glass, suppose Tamerine or ye like doubled up in ye hand, & this wth a brisk motion as may \be/ till a hundred or 150 may be counted, ye glass lying all ye while over ye papers, then if nothing stirr rub ye glas wth yor finger ends half a score times to & fro or knock it as of wth yor finger ends as oft upon ye glas; for this rubbing or knocking wth your fingers, after ye former rub{illeg}|b|ing, conduces most to excite ye papers. If nothing stir yet, rub ag{illeg}|a|in wth ye cloth {illeg}|ti|ll {illeg} 60 or 80 may be counted & then rub or knock again wth your fingers, & re{illeg}t|pe|at thi{illeg}|s| till ye electric virtue of ye glass be so far excited as to take up ye papers, & then a \very/ little rub{illeg}|b|ing {illeg} or knocking now & then will revive ye motions. In doing all this let ye rubbing be always done as nimbly as may be, & if ye motion be circulars {sic} like yt 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 ye suspicion of ye papers bein moved by ye air I am secure from that. Yet in ye other of drawing leaf gold to above a foot distance, I could not make it wch I never went about to try my self till ye last week, I suspect ye Air might raise ye gold & then a small attraction might determin it toward ye glas. For I could not make it succeed.

As for Mr Hook's insinuation yt ye summ of ye Hypothesis I sent you had been delivered by him in his Micrography, I need not be much concerned <46v> at the liberty he takes in yt kind. Yet because you think it \may/ do well {illeg} if I state ye difference I take to be between them, I shal do it as briefly as I can, & yt \ye rather that/ I may avoyd ye savour of having done any thing un{illeg}justifiable or unhan{illeg}|s|ome 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 ye action of this Medium; That this Medium is less implicated in ye parts of solid bodies & so moves more freely \in them/ & transmits light more readily through them & that after such a manner as to accelerate ye 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 ye colours of a Prism are made by ye action \means/ of ye quiescent Medium accelerating {illeg} some motion of ye rays on one hand side where red appears & retarding it where on ye other side where blew appears, & that there are but \these/ two original colours or colour-making modifications of light, wch by their various degrees, or, as Mr Hook call's it, dilutings, produce all intermediate ones. This rejected, ye remainder i|o|f his Hypothesis is yt he has changed Des-Cartes pressing or progressive motion of ye Medium to a vibrating one, ye rotation of ye globuli {illeg} to ye obliquation of pulses, & ye accelerating yt \their/ rotation of ye globuli on ye one hand & retarding it on ye other by ye quiescent Medium to produce colours, to ye like action of ye Medium on ye two ends of his pulses for ye same end. T|A|nd having thus far modified his by ye Cartesian Hypothesis he has added extended it further to {illeg} explicate ye phænomena of thin plates & added another explication of ye colours of natural bodies fluid & solid.

This I think is \in short/ ye summ of his Hypothesis & in all this I have nothing common wth him but ye supposition yt æther is a vibrating Medium \susceptible of vibrations/ of wch supposition I make a very different use: he supposing it light it self & I using it wch I suppose it is not. This is as great a difference as is between him & Des Cartes. But besides \this/ ye manner of refraction & reflexion & ye nature & production of colours in all cases (wch takes up ye body of my discours) I explain very differently from him; & even in ye colours of thin transparent substances I explain every thing after a way so differing from him, yt ye experiments I grownd my discours on destroy all he has said about them, & ye two main experiments wthout wch ye manner of ye production of those colours is not to be found out, he was were not only unknown to him when he wrote his Micrograph{illeg}|y|, but even last spring, as I understood in mentioning them to him. This therefore is ye 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 ye various bignes of pulses (wthout wch his Hypothesis will do nothing) his will borrow as much from my answer to his Ojections {sic} as yt I sent you does from his Micrograph{illeg}|y|.

But it may be he means yt I have made use of {illeg}|h|is Observations & of some I did; as yt of ye inflexion of rays, for wch I quoted him; that of opacicity {sic} arising from ye intt|e|rstices of ye parts of bodies, wch I insist not on; & that of plated bodies exhibiting colours, a Phænomenon for ye notice of wch I t{illeg}|h|ank him: But he left me to find out & make such experiments about it as might inform me of /ye\ <47r> manner of ye production of those colours, to ground an Hypothesis on; he having given no further insight into it then this yt ye colour depended on some certain thicknes of ye 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 ye pains to find out. And this I hope may vindicate me from {ye expectation} what Mr Hook has been pleasd to charge me wth. Sr I doubt I have already troubled you wth too long a letter, & so break of abruptly

Yor humble Servant

Is. Newton.

Decemb. 21.
1675.

<47v>

For Henry Oldenburg Esq
at his house about ye middle of ye
old Pal-mall in

Westminster
London.

|Rec. Dec. 23. 75.
Answ. Dec. 29 \30/. 75.|

[1] Dec. 21. 1675.

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