<48r>

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

Sr

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

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

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

Sr I am

Yor obliged & humble Servant

Is. Newton.

< insertion from the bottom >

Pray Sr let not my papers
go out of yor hands till you hear

In printing my former letter to Mr Linus you
may leave out wt 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 wt has lately happend.

< text from f 49r resumes >
<49v>

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

[1] Ph. Tr. N. 121. p. 503.