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JSTOR helps people discover, use, and build upon a wide range of content through a powerful research and teaching platform, and preserves this content for future generations. JSTOR is part of ITHAKA, a not-for-profit organization that also includes Ithaka S+R and Portico. For more information about JSTOR, please contact support@jstor.org. STOR Philosophical Transactions Please note: Due to an error in the print volume, the page numbering in this article may contain either page numbering skips, or page numbering repetitions, or both. However, the article content is presented in its entirety and in correct reading order. Please click on "Next Page" (at the top of the screen) to begin viewing the article. ( 5«o ) Mr. Ifaac Newton'* Confiderathns on the former Reply \ together with further Directions \ how to mike the Experiments controver- ted aright: Writtento the Publijherfrcm Cambridge,Novemb. 1 3; 1675. SIR, "« x 7 Hen you fljew'd me Mr. Line's fecond Letter, I remember V V I told you, that I thought aa anfwer in writing would be infignificant, becaufe the difpute was not about any Ratiocination, but my veracity in relating an Experiment, which he denies will fucceed as it is defcribed in my printed Letters : For this is to be decided not by difcourfe, but new trya! of the Experiment. What it is that impofes upon Mr. Line I cannot imagin; but I fufpefthe has not tryed the Experiment fince he acquainted himfelf with my Theory, but depends upon his old notions taken up before be had any hint given to obferve the figure of the coloured Image. I fliall defire him therefore, before he returns any anfwer, to try it once more for his fatisfa&ion, and that according to this manner. Let him take any Prifme, and hold it fo that its Axis may be perpendicular to the Sun's rays, and in this pofture let it be placed as clofe as may be to the hole through which the Sun fiiines into a dark room, which hole may be about the bignefsof a Peafe. Then let him turn the Prifm flowly about its Axis, and heflhall fee the colours move upon the oppofite wall firft towards that place to which the Sun's direft light would pafs, if the Prifm were taken away, and then back again. When they are in the middle of thefe two contrary motions, that is,when they areneareft that place to which the Sun's direft ray tends, there let him (top 3 for then are the rays equally refrafted on both fides the Prifm. In this pofture of the Prifm let him obferve the figure of the colours, and he fhall find it not round as he contends,butoblong,and fo much the more oblong as the Angle of the Prifm, comprehended by the refrafting plains, is bigger, and the wall, on which the colours are caft,roore diftant from the Prifm 5 the colours red, yellow, green, blev,pur- ple, fucceeding in order not from one fide of the figure to the o- ther, as in Mr. Line's con;efture,but from one end to the other; and the length of the Figure being not parallel but tranverfe to the Axis of the Prifm, After this manner I ufed to try the Experi- T 1 1 2 menc C 502 ) merit: Fori have try'd it often ; fometimes toobferve thecircum- fiances of i t, fometimes in order to further Experiments,and fome- times to fhow it to others, and in all my tryals the fuccefs was the fame. But whereas Mr, Line thinks,! tryed it in a cloudy day,and placed the Prifm at a great diftance from the hole of the window; the Experiment will not fucceed well if the day be not clear , and the Prifm placed clofe to the hole, or fo near at leaft, that all the Sun's light that comes from the hole may pafs through the Frifm alfo, fo as to appear in a round form if intercepted by a paper im- mediately after it has pad the Prifm. When Mr. Line has tryed this, I could wifh,he would proceed a little further to try that which I call'd the Exferimentum Cruris, fc€\r\g(\flmfazmembzYWt)hzder\\e$th*tas well as the other. For when he has tryed them (which by his denying them, I know he has not done yet as they fhould be tryed)I prefume he will reft fatisfied. Three or four days after you gave me a fight of Mr.JUw'sfecond Letter, I remember I thereupon fhow'd the firft of thefe two Expe- riments to thatGentleman whom you found with me,whenyougave me that vifit,and whilft 1 was (hewing it to him, ^.27. (a member of the R.Society) came in and I fhewed it to him alio. And you may re- member,that R, H. two or three years agoe in a Letter read before theR.SW/cfyand tranfmitted tome,gave teftimony not only to the Experiments, questioned by Mr. Line,bm to all thofe fet down in my firft Letter about Colours, as having tryed them himfelf ; and* wien you read Mr. Line's Letter at a meeting of thefaid Society and was pleafed to do me the favour to propound the Experi- ment to be tryed in their prefence , R.H. fpake of it to them as a thing not to be queftioned. But if ic have not yet been tryed be- fore them, and any of them, upon Mr. Line 's confidence,doubt of it, I promife when I fhall have the happinefs to be at any more of their AfTemblies, upon the leaft hint, to fhew 'em the trya! of it - and 1 hope, I fhall not be troublefome, becaufe it may be tryed (though not fo perfectly) even without darkning a room, or the expence ofany more time than half a quarter of an hour ; although, if Mr. Line perfift in his denyal of it , I could wifh it might be tryed fooner there , than I fhall have an opportunity to be among them. C 5^3 ) AnExtrad of mother Letter of e^fr. Newton, written totheTu- bltfher the ioth of January 167-J, relating to the fame- Area- ment, BY Mr. Gafeoins Letter* one might fufpeft, that Mr. £*>>/& tryed the Experiment fome other way than I did j and therefore I fhallexpeft, till his friends have tryed itaccording to my late Directions. In , , which tryal it may poffibly be a further guidance wnuen^Vhe p" co them.to acquaint them, that the Prifm cafts biffher, vtcmb.\*, from it feveral Images: One is.that Obion? one of l6?s * /j om :f £ '^» Colours which J mean ; and rhis is made by two ha^ng been aScko- Refractions only. Another there is, made by two larofMr.z/*«,now Refractions and an intervening Reflexion ; and t"if a^'coScd this is Round md Colour lefs, if the Angles of the thefe words , to Prifm beexa&ly equal ; but if the Angles at the wbi( : h M . r ^ w ^ r> n «• i /- L I • mi i > i r tO Whom It WSSCOm- Reflecting bafe be nor equal , it will be colour d, rou nicated , Teems and that fo much the more, by how much unequal- here to have refpea; Jer the Angles are, but yet not much unround, un- ^i^TJPgli lefs the angles be very unequal. A third Jniage andegain, and c Mr, Newton theydifappear; but the firft moves flow, and 'Z7L \I'fid"£n grows continually flower till it be ftationary, and »« ««*>«»•« ^ */>« a . then turns back again, and goes back fafter and ft- %&%%£$ Iter, till it vanilhin the place where it began to tht dntrpty ef f u- appear. ' ''*»'*« £">» «»-**« /i»?e c/W yici ?/»•- cumftance , &e thtceuft rfikt difftrtnet Ittvrixt tktm , A2V, Newtor.'* Experiment wilt hardly /land. If without darkning their Room they hold the Prifm at their window in the Sun's open Light, in fuch a pofture that it's axis be per pen C 5^4 ) perpendicular to the S^n-beams.and then turn it about its axu,they cannot mifs of fee i.ig the firji Image; which having found,they may double up a paper once or twice, and make a round hole in the middle of it about \ or^of an inch broad, and hold the paper im- mediately before the Pnfm, that the Sun may fhine on the Prifm through that hole ; and the Prifm being ftay'd, and held fteddy io that poftpre which makes the Image Stationary ; if the Image then fail directly on anoppofite wall, or on a flieet of paper placed at the wall, fuppofe 1 5 or 20 foot from the Prifm, or further or7;chey will fee the Image in fuch an ObUng figure as I have defcribed,with the Red at one end, the Violet at theoiher, and a Bleveifh green in the middle : And if they obfeure their Room, as much as they can, by drawing curtains or otherwife, it will make the Colours the more confpicuous. This direction I have fet down, that no body, into whofe hands aPrifm (hall happen,may find difficulty or trouble in trying it.Buc whenMr.L/»»/sfriendshavetryeditthus, they may proceed ta repeat itin a dark Room with a lefs hole made in theirwindow (hut. And then Khali defire,that they will fend you a full and clear de« fcription,How theytryedit, exprefling the length, breadch and angles of the Prifm; its pofition to the Incident rays and to the window (hut; thebignefsoftheholein the window (hut through which theSunfhined on the Prifm; what fide of the Prifm the Sun fhin'd on ; and at what fide the light came out of it again ; the di- ftance of the Prifm from the oppofite paper or wall on which the Refratfed light was caft perpendicularly ; and the iength,breadch, and figure of the fpace there illuminated by rhat light,and the fci- tuaticnof each colour within that figure. And, it they pleafe to illuflrate their defcription with a Scheme or two, it will make the bufinefs plainer. By this means, if there be any difference in our way of experimenting, I (hall be the better enabled to difcern ir , and give them not ice,where the failure is, and how to refiifie ir. I ihould be g'ad too, if they would favour me with a defcription of the Experiments it hath been hitherto tryed by Mr. Linus, that I may have an opportunity to confider, what there is in that which makes againft me. So far Mr. Newton; which was thought fit to make publick with che reft,that fo the Curious every where, who have a mind to try tV Experiment, may find the fuller diredions for their tryal.