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

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

S^r

Concrning {sic} y^e Experim^t of y^e glas & papers I should add these two to y^e former directions. One that y^e glas be rubd w^th a full handfull of stuff w^ch may cover & rub all y^e glas at once: for thus its' electric vertue will be more easily & vigorously excited then if rub'd w^th a little only doubled up but once or twice. This rubbing w^th y^e stuff I suppose rarefies & diffuses y^e electric effluvia from y^e glas into y^e air, & y^e knocking or rubbing w^th y^e finger ends put{illeg}|s| y^e diffused {illeg} effluvia into irregular motions: The other thing I would note is that y^e papers may \perhaps/ be too little as well as too great. Too small ones will be apter to stick to y^e glas or table. If y^e Experim^t be tryed w^th a glas three or four inches broad set about $\frac{1}{6}$ of an inch from y^e table, & y^e papers of a thin sort of paper cut into triangular pieces, y^e sides of those trianges|le|s may not unfitly be about y^e 20^th or 25^t part of an inch, more or less. It may be best tryed w^th bits of several sizes put in at once, & if there be put in a piece or two of y^e wing of a fly, those, I find, will move more easily though scarce so variously. These & y^e 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 w^ch may make a difference.

I am obliged to you S^r for your candor in {illeg}|a|cquainting me w^th M^r 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 c{illeg}|a|ndor; though y^e b{illeg} {illeg} \I think the present buisines/ of no great moment \as/ to me, not imagining y^t y^e R. Society are to be imposed on in a thing so plain, or that M^r Hook himself will persist in {illeg} \mistake/ when he hears y^e difference stated. The only thing I said he could pretend taken from his Hypothesis was y^e disposition of æther to vibrate, & yet whilst he grasps at all he is likely to fa{illeg}|ll| sh{illeg}|o|rt {illeg} of this too. That light consists of æthereal vibrations \are light/ is his; but y^t æther may {illeg} vibrate (w^ch 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 & y^e principles I go upon. I desire M^r Hook to shew me therefore, I say not y^e only y^e summ of y^e Hypothesis I wrote, w^ch 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 y^e application of vibrations to y^e solution of y^e Phænomena of thin plates: & yet all y^e use I make of vibrations is to strengthen or weaken y^e reflecting power of y^e æthereal superficies, w^ch is so far from being in his Micrographia, y^t y^e la{illeg}|s|t Spring when I told him of y^e reflecting power of y^e æthereal superficies, he took it for a new notion, having till then supposed that light to be reflected \{illeg}/ by y^e parts of gross{illeg} bodies onely, such as are air, {tinder}, glas {illeg}. To y^e things that he has from Des Cartes, pray add this, that y^e 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 y^e reduction of all colours to two you may if need be explain further for me thus <48v> That as Des Cartes puts every globulus to be urged forward on one side by y^e illuminated Medium & impeded on y^e other by y^{illeg}|e| {illeg} dark one, so M^r Hook puts every vibration to be promoted at one end & retarded at y^e other by those Mediums, & thence both alike derive two modifications of light on y^e two sides of y^e refracted beam for y^e production of all colours.

^[1] By M^r Gascoin's letter one might suspect y^t M^r Linus tri{illeg}|y|ed y^e exp^t some other way then I did, & therefore I shall expect till his friends have tried it according to my late directions: in w^ch trial it may possibly conduce to be a further guidance to 'em to acquaint 'em that y^e Prism casts from it several papers images. One is that oblong one of colours w^ch I {illeg} mean & this is made by two refractions only. Another there is, made by two refractions & an intervening reflexion, & this is round & colourless if y^e angles of y^e Prism be exactly equal, but if y^e angles at y^e reflecting base be not equal it will be coloured, & that so much y^e more by how much unequaller y^e angles are, but yet not much unround unles y^e 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 y^e second for y^e first. But they are distinguishable not only by y^e length & lively colours of y^e first, but by its' different motion too: for whilst y^e Prism is turned continually the same way about its' axis, y^e second & third move swiftly, & go a{illeg}|l|ways \on/ y^e same way till they dissappear, but y^e first moves slow & grows {illeg} continually slower till it be stationary & then turns back again & goes back faster & faster till it vanish in y^e place where it began to appear. If before they \without/ dark{illeg}|n|ing their Room they |"| hold y^e prism at their window in y^e sun's open light in such a posture y^t |"| its' axis be perpendicular to y^e sun beams & then turn it about its' axi{illeg}|s|, they |"| cannot miss of se{illeg}|i|ng y^e first image: w^ch having found, let them proceed to darken |"| their room they may double up a paper once or twice, & make a round hole |"| in y^e middle of it about $\frac{1}{2}$ or $\frac{3}{4}$ of an inch broad, & hold y^e paper immedi|"| ately before y^e Prism being st{illeg}ayed & held steddy in that posture w^ch makes y^e |"| image stationary that y^e sun may shine on y^e Prism through that hole; & y^e |"| Prism being stayed & held steddy in that posture w^ch makes y^e image stationary, if |"| y^e image then fall directly on an opposite wall, or on a sheet of paper placed |"| at y^e wall suppose 15 or 20 foot from y^e prism or further {illeg}|o|ff, |"| they will \see/ y^t image in such an oblong figure as I have described, w^th y^e red at |"| one end, {illeg} {illeg}|th|e violet at y^e other & a blewish green in y^e middle; & if they ob|"| scure their Room as much as thy|e|y can by drawing curtains or otherwise, it |"| will make y^e colours so much y^e 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 M^r Linus's friends have tryed it thus, they may proceed to repeat it in a dark room w^th a less hole mad{illeg}|e| in their window shut. And then I shal desire that they will send you a full & clear description how they tryed it, expressing y^e length breadth & angles of y^e Prism, its position to y^e incident rays & to y^e window shut, y^e bignes of y^e hole in y^e shut through w^ch y^e sun shined on y^e Prism, what side of y^e Prism y^e sun shine{d} on & at what side y^e light came out of it again, y^e distance of y^e Prism from y^e opposite paper or wall on w^ch y^e refracted light was cast perpendicularly, & y^e length breadth & figure of y^e space there illuminated by that light, & y^e situation of each colour w^thin that figure: & if they please to illustrate their description w^th a scheme or two it {illeg}|w|ill make y^e buisines plain{illeg}|e|r. By this means if there be any difference in o^r way of experimenting, I shall be the better enabled to discern it & give 'em notice of it where y^e failure is & how to rectify it. I should be glad too if they would favour me w^th a description of y^e Experiment as it has been hitherto tryed by M^r Linus, y^t I <49r> may have an opportunity to consider what there is in that w^ch makes against me. And because M^r Gascoin seems to suspect th{illeg}|a|t my directions sent M^r Linus may differ from w^t I have printed, I desire also y^t 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 M^r Linus might possibly rely on old Exp^ts, his quoting S^r 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 \M^r Gascoin's/ insinuation as if I influenced y^e Press in what concerns M^r Linus & me. You know S^r I never spake nor hinted a syllable to you conc{illeg}|e|rning printing or not printing any thing of M^r Linus, nor so much as knew of y^e printing his first letter till I saw it was out in y^e Transactions. When you sent it to me I out of a great desire to avoid controversies (w^ch as you know I had enterteined long before) wrote back to you that I had no mind to meddle w^th it: but as I was \ready to/ sealing that letter, I added a Post-script to this purpose: That seing M^r Linus was designing somthing \about light/ for y^e Pres, to prevent pu{illeg}|b|lishing his mistake you might if you thought fit signify to him (but not from me) y^t y^e Exp^t was tryed otherwise then he suggested & y^t in such & such respects w^ch I {illeg} there named. And y^e substance of this Postscript was that you published at y^e end of his first letter on w^ch M^r Gascoin here animadverts, but was so far from being designd for y^e Press by me, y^t y^e first sight of it \together/ w^th his letter in y^e Transactions made me say to one, that I wisht they had {illeg}|b|een supprest for I doubted y^e printing them would make M^r Linus unquiet & so in y^e end create me trouble. As for his 2^d letter w^ch you shewed me at Londō I returnd it again to you so soon as I had read it & never saw it since, persisting in my desire to avoid y^e controversy. And at my returning it you moved me for an answer w^th this argum^t y^t if I wavd it M^r Linus was like to make y^e more stir: to w^ch I replied y^t y^e buisines being about matter of fact was not proper to be decided by writing but by trying it before competent witnesses. Whereupon at yo^r motion I told you what was requisite, & by your procurem^t preparations were accordingly made for it's tryall at y^e next Assembly of y^e R. Society as I M^r Hook I understood by M^r Hook: {illeg}|B|ut y^e day provd cloudy & before another Assembly I returnd to Ca{illeg}|m|bridge, & from y^t time never enquired after nor regarded y^e {illeg} \matter/ further till you sent me M^r Linus's {illeg}|t|hird letter. This is y^e history of M^r Linu{illeg}|s| buisines so far as I know't: w^ch I have set down y^t his friends may see he has not been dealt w^th 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 y^e controversy, & to {illeg}|me| y^t I had no mind to be ingag'd: a liberty every body has a right to & may gladly make use of, someti{illeg}|me|s at least, & especialy if he want leisure or meet w^th prejudice or groundles {illeg} \insinuations/. But I hope to find none of this in M^r Gascoin. The hansome genius of his present Letter makes me hope it for y^e future. In y^e mean time I desire w^th him y^t you would publish M^r Linus's letters as soon as you can conveniently to pre{illeg}|v|ent further misapprehensions.

S^r I am

Yo^r obliged & humble Servant

Is. Newton.

< insertion from the bottom of the page >

Pray S^r let not my papers
go out of yo^r hands till you hear
from me about registring them.

In printing my former letter to M^r Linus you
may leave out w^t I mention of M^r Hill & M^r 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 M^r Hook, for I suppose there is nothing but misapprehension in w^t has lately happend.

< text from f 49r resumes >

<49v>

|Newtons Letter
Rec. jan. 12. $\frac{75}{76}$.
Answ. jan. 15. 75/76.|