Letter from Newton to Henry Oldenburg, dated 10 January 1675/6

jan. 10. 167 $\frac{5}{6}$ .

SrSir ConcrningConcerning yethe ExperimtExperiment of yethe glas & papers I should add these two to yethe former directions. One that yethe glas be rubd wthwith a full handfull of stuff wchwhich may cover & rub all yethe glas at once: for thus its' electric vertue will be more easily & vigorously excited then if rub'd wthwith a little only doubled up but once or twice. This rubbing wthwith yethe stuff I suppose rarefies & diffuses yethe electric effluvia from yethe glas into yethe air, & yethe knocking or rubbing wthwith yethe finger ends puts yethe diffused effluvia into irregular motions: The other thing I would note is that yethe papers may perhaps be too little as well as too great. Too small ones will be apter to stick to yethe glas or table. If yethe ExperimtExperiment be tryed wthwith a glas three or four inches broad set about

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of an inch from yethe table, & yethe papers of a thin sort of paper cut into triangular pieces, yethe sides of those triangesles may not unfitly be about yethe 20th or 25t part of an inch, more or less. It may be best tryed wthwith bits of several sizes put in at once, & if there be put in a piece or two of yethe wing of a fly, those, I find, will move more easily though scarce so variously. These & yethe former directions observed, I cannot imagin how you should miss, though I cannot promis al things will appear justly to you as they did to me, there being unaccountable circumstances wchwhich may make a difference. I am obliged to you SrSir for your candor in acquainting me wthwith Mr Hook's insinuations. It's but a reasonable piece of justice I should have an opportunity to vindicate my self from what may be undeservedly cast on me: & therefore since you have been pleased to be my representative there, & I have no means of knowing what's done but by you, I hope you will continue that equitable candor; though yethe b I think the present buisines of no great moment as to me, not imagining ytthat yethe R.Royal Society are to be imposed on in a thing so plain, or that Mr Hook himself will persist in mistake when he hears yethe difference stated. The only thing I said he could pretend taken from his Hypothesis was yethe disposition of æther to vibrate, & yet whilst he grasps at all he is likely to fall short of this too. That light consists of æthereal vibrations are light is his; but ytthat æther may vibrate (wchwhich is all I suppose) is to be had from a higher fountain: for that æther is a finer degree of air & air a vibrating Medium are old notions & yethe principles I go upon. I desire Mr Hook to shew me therefore, I say not yethe only yethe summ of yethe Hypothesis I wrote, wchwhich is his insinuation, but any part of it taken out of his Micrographia: but then I expect too that he instance in what's his own. It's most likely he'l pretend I had from him yethe application of vibrations to yethe solution of yethe Phænomena of thin plates: & yet all yethe use I make of vibrations is to strengthen or weaken yethe reflecting power of yethe æthereal superficies, wchwhich is so far from being in his Micrographia, ytthat yethe last Spring when I told him of yethe reflecting power of yethe æthereal superficies, he took it for a new notion, having till then supposed that light to be reflected by yethe parts of gross bodies onely, such as are air, tinder, glas . To yethe things that he has from Des Cartes, pray add this, that yethe parts of solid bodies have a vibrating motion least he should say I had from him what I say about heat. And his having from Des Cartes yethe reduction of all colours to two you may if need be explain further for me thus That as Des Cartes puts every globulus to be urged forward on one side by yethe illuminated Medium & impeded on yethe other by yethe dark one, so Mr Hook puts every vibration to be promoted at one end & retarded at yethe other by those Mediums, & thence both alike derive two modifications of light on yethe two sides of yethe refracted beam for yethe production of all colours. Ph.Philosophical Tr.Transactions N. 121. p. 503. By Mr Gascoin's letter one might suspect ytthat Mr Linus triyed yethe exptexperiment some other way then I did, & therefore I shall expect till his friends have tried it according to my late directions: in wchwhich trial it may possibly conduce to be a further guidance to 'em to acquaint 'em that yethe Prism casts from it several papers images. One is that oblong one of colours wchwhich I mean & this is made by two refractions only. Another there is, made by two refractions & an intervening reflexion, & this is round & colourless if yethe angles of yethe Prism be exactly equal, but if yethe angles at yethe reflecting base be not equal it will be coloured, & that so much yethe more by how much unequaller yethe angles are, but yet not much unround unles yethe angles be very unequal. A third image there is made by one single reflexion & this is always round & colourles. The only danger is in mistaking yethe second for yethe first. But they are distinguishable not only by yethe length & lively colours of yethe first, but by its' different motion too: for whilst yethe Prism is turned continually the same way about its' axis, yethe second & third move swiftly, & go always on yethe same way till they dissappear, but yethe first moves slow & grows continually slower till it be stationary & then turns back again & goes back faster & faster till it vanish in yethe place where it began to appear. If before they without darkning their Room they " hold yethe prism at their window in yethe sun's open light in such a posture ytthat " its' axis be perpendicular to yethe sun beams & then turn it about its' axis, they " cannot miss of seing yethe first image: wchwhich having found, let them proceed to darken " their room they may double up a paper once or twice, & make a round hole " in yethe middle of it about

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of an inch broad, & hold yethe paper immedi" ately before yethe Prism being stayed & held steddy in that posture wchwhich makes yethe " image stationary that yethe sun may shine on yethe Prism through that hole; & yethe " Prism being stayed & held steddy in that posture wchwhich makes yethe image stationary, if " yethe image then fall directly on an opposite wall, or on a sheet of paper placed " at yethe wall suppose 15 or 20 foot from yethe prism or further off, " they will see ytthat image in such an oblong figure as I have described, wthwith yethe red at " one end, the violet at yethe other & a blewish green in yethe middle; & if they ob" scure their Room as much as thyey can by drawing curtains or otherwise, it " will make yethe colours so much yethe more conspicuous. This direction I have set down that no body into whose hands a Prism shall fall happen may find difficulty or trouble in trying it. But when Mr Linus's friends have tryed it thus, they may proceed to repeat it in a dark room wthwith a less hole made in their window shut. And then I shal desire that they will send you a full & clear description how they tryed it, expressing yethe length breadth & angles of yethe Prism, its position to yethe incident rays & to yethe window shut, yethe bignes of yethe hole in yethe shut through wchwhich yethe sun shined on yethe Prism, what side of yethe Prism yethe sun shined on & at what side yethe light came out of it again, yethe distance of yethe Prism from yethe opposite paper or wall on wchwhich yethe refracted light was cast perpendicularly, & yethe length breadth & figure of yethe space there illuminated by that light, & yethe situation of each colour wthwithin that figure: & if they please to illustrate their description wthwith a scheme or two it will make yethe buisines plainer. By this means if there be any difference in orour way of experimenting, I shall be the better enabled to discern it & give 'em notice of it where yethe failure is & how to rectify it. I should be glad too if they would favour me wthwith a description of yethe Experiment as it has been hitherto tryed by Mr Linus, ytthat I 49. may have an opportunity to consider what there is in that wchwhich makes against me. And because Mr Gascoin seems to suspect that my directions sent Mr Linus may differ from wtwhat I have printed, I desire also ytthat he would signify wherein he thinks they may differ so as to need reconciling. Fuller they are, but not different, nor any other then I have followed these above these seven years. As for my suspicion that Mr Linus might possibly rely on old ExptsExperiments, his quoting SrSir Kenelm Digby for a By-stander might have made any other stranger to his way suspect it as well as I me suspect it: but I wonder most at his Mr Gascoin's insinuation as if I influenced yethe Press in what concerns Mr Linus & me. You know SrSir I never spake nor hinted a syllable to you concerning printing or not printing any thing of Mr Linus, nor so much as knew of yethe printing his first letter till I saw it was out in yethe Transactions. When you sent it to me I out of a great desire to avoid controversies (wchwhich as you know I had enterteined long before) wrote back to you that I had no mind to meddle wthwith it: but as I was ready to sealing that letter, I added a Post-script to this purpose: That seing Mr Linus was designing somthing about light for yethe Pres, to prevent publishing his mistake you might if you thought fit signify to him (but not from me) ytthat yethe ExptExperiment was tryed otherwise then he suggested & ytthat in such & such respects wchwhich I there named. And yethe substance of this Postscript was that you published at yethe end of his first letter on wchwhich Mr Gascoin here animadverts, but was so far from being designd for yethe Press by me, ytthat yethe first sight of it together wthwith his letter in yethe Transactions made me say to one, that I wisht they had been supprest for I doubted yethe printing them would make Mr Linus unquiet & so in yethe end create me trouble. As for his 2d letter wchwhich you shewed me at Londōon I returnd it again to you so soon as I had read it & never saw it since, persisting in my desire to avoid yethe controversy. And at my returning it you moved me for an answer wthwith this argumtargument ytthat if I wavd it Mr Linus was like to make yethe more stir: to wchwhich I replied ytthat yethe buisines being about matter of fact was not proper to be decided by writing but by trying it before competent witnesses. Whereupon at yoryour motion I told you what was requisite, & by your procuremtprocurement preparations were accordingly made for it's tryall at yethe next Assembly of yethe R.Royal Society as I Mr Hook I understood by Mr Hook: But yethe day provd cloudy & before another Assembly I returnd to Cambridge, & from ytthat time never enquired after nor regarded yethe matter further till you sent me Mr Linus's third letter. This is yethe history of Mr Linus buisines so far as I know't: wchwhich I have set down ytthat his friends may see he has not been dealt wthwith obliquely as they seem to apprehend. All I think that they can object to you is that you were at a stand becaus you could not ingage me in yethe controversy, & to me ytthat I had no mind to be ingag'd: a liberty every body has a right to & may gladly make use of, sometimes at least, & especialy if he want leisure or meet wthwith prejudice or groundles insinuations. But I hope to find none of this in Mr Gascoin. The hansome genius of his present Letter makes me hope it for yethe future. In yethe mean time I desire wthwith him ytthat you would publish Mr Linus's letters as soon as you can conveniently to prevent further misapprehensions. SrSir I am YorYour obliged & humble Servant Is. Newton. Pray SrSir let not my papers go out of yoryour hands till you hear from me about registring them. In printing my former letter to Mr Linus you may leave out wtwhat I mention of Mr Hill & Mr Hook, or at least put letters for their names: for I beleive they had rather not be mentiond. If you have opportunity pray present my service to Mr Hook, for I suppose there is nothing but misapprehension in wtwhat has lately happend.

Newtons Letter Rec. jan. 12.

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. Answ. jan. 15. 75/76.