STOP Early Journal Content on JSTOR, Free to Anyone in the World This article is one of nearly 500,000 scholarly works digitized and made freely available to everyone in the world by JSTOR. Known as the Early Journal Content, this set of works include research articles, news, letters, and other writings published in more than 200 of the oldest leading academic journals. The works date from the mid-seventeenth to the early twentieth centuries. We encourage people to read and share the Early Journal Content openly and to tell others that this resource exists. People may post this content online or redistribute in any way for non-commercial purposes. Read more about Early Journal Content at http://about.jstor.org/participate-jstor/individuals/early- journal-content . JSTOR is a digital library of academic journals, books, and primary source objects. 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. (556) jind Mr. Boyle di [cover 'd andpublijhed the ufe of the Ba rofcope , to remark the weight of thekiuv.XpYzrz and the changes of that weioht in hisjirji'freatife of i'tieumaticals an. r6 ! '^ that the Mercury af- cended in England to 30 inches, and above \ , Lxper. 1 7'A ; that it changed within five weeks to full two inches, hxp 1 % lh ;and the changes, in reference to heat., coU. t wind, weather , and other alterations of the air, or whatever fecrct circumflances, have' Urn remarked at London, Oxford , far in the Weft and other parts of England , thefe 1 c years andvpw&rds. the We el Barometer perfected,Num. 1 3. Another Sta- tical Sarofcope accuratly devifed, N. 14- And many other Inflru- ments contrived and Materials prepared, which may make a deeper andclofer difcovery of the nature of a Place, than hath forme 1 ly been obfirv'd. And befides what is done as proper for this Ifland, there was beounby Dr. Ger. Boate, and fubltfbt an. x 52. Ireland's Natu- ral Hiftory, which if the Author had lived tojimfh ., had been much fuller i, and 'tis hoped that others will foon give it a larger meafurr of perfection. So that, though we are in this addrefs fomewhal tardy , yet, I think ,we are not much behind our neighbour:. And fame have alfo made confide ruble researches which extend far beyond thefe our Cli* mats. But I mufl forbear. If I am prolix here, it may be confidered, that this is a pregnant part of our main bufinefs in fhyjiology ; and it mxy be interpreted for a good proof of no monopolizing envy of us, in that we are willing to provoke our Neighbors to excel us in things that are truly good and excellent. J particular Anfwcr of Mr. Ifaak Newton *0 Mr. Linus his Letter, printed in Numb. 1 2 r. p.4^9. about an Experiment relating to the New DoSlrine of Light and Colours: This Anfwcr fent from Cam- bridge in a Letter to the Fublifher Febr. 29. i 6 7?- . Sir . TQ Y reading Mr. Linms Letter when you fihew'd it to me at Lcn- j don,\ retained only a general reniembrance,that Mr. Linmdt- _J don,\ retained only a gent vfd what I afnrmed.and f > could lately fay nothing in particular to it; but having the opportunity to read it again in Numb. 1 2 r. of tYitfranfaciions, I perceive he would perfuade you, that the in- formation you gave him about the Experiment is as inconfiftent with my printed Letters as with experience; and therefore, left any who have not read thofe Letters fhould take my filence in this point for an acknowledgment , I thought it not amifsj to fend you flmething in anfwer to this alfo. He c 557; He tells you thztJVhereasyou afiurehim^irfijhutthe Experiment was made in tie at days 5 fecondlyjhat the Frifm was placed clofe to the bole Jo that the light had no room to diverge\and thirdly, that the Image was not parallel but tranfverfetotheaxU of the Prifm: If the fed f- fertionsbe compared with my Relation of the Experiment in the Phil. Tranjaclion N> Ho. p. 3076. it will evidently appear ;, they cannot be admitted at being directly contrary to what is there delivered. H.s rea Pons are thefe: Firft, that I faid, the ends of the long Image feemed femicircular, •which, faics he, never happens in any of the three cafes above faid. But this is not to fet me at odds with my fe!f, b'.it with the experiment, for it is there defenbed co happen in than all; and I frill fay, it dcth happen in them. Let others try the Experiment, and judge. Further he fa its, that the Prifm is placed at a diflance from the hole in the Scheme of the Experiment in N. 84. p 409 i . Bur, what ir it were fo there ? For, that is the Scheme of a demonftration, not of the experiment , and would have ferved for the detnonftratiun, had the diitance been put twenty timesgreater than it is. Jn the Schemes of the Experiment A r .8o./>. 3086, and N- Hi. p. 5or6. it is repre- fented clofe, and clofe enough in the Scheme, N. 83. p.4.06 1 : But Mc. Linus thought fit to wink at thefe, and pitch upon the Scheme of a Demon ft rat ion , and fuch a Scheme too as hath no hole at all represented in it. For, the Scheme f Numb. 84. p. 49 1 is this ; t See' Fig. in which the rays are not fo far diftant from one a- notherat GL, buc that the hole, had i cxpreft it, might have been put there , and yet have compre- hended them. But if we fliould put the ho:e at x, their decuffation } yet wi' \ it no: be any thing to his purp')fe i the diftance x G or x L being but abouc half the breadth of a fide of the Prifm (-AC) which I conceive is n >r the twentieth part of the diftance requifke in Ills conjecture, hirdiy. A. * 3-He fays,that more might be [aid out of my relation tofbewjhat the Image was not tranfuers, for if it had been tranjvers, I could not have been [urpriz,ed(as Ifaid I vt>a*)to fee the length thereof Jo much exceed the breadth Jt bewga thing fo obvious & eafie to be explicated by the or- dinary rules ofRefraclion.But on the contrary, it may rather befaid, that if the Image had been parallel , I could not have beenfurpri- zed to fee the length thereof foruuch exceed the breadth , it being athingfocxtrcamly obvious as not co need any explication. For who that had but common fenfe 3 and faw the whole Prifm or a good part of it illuminated, could not expect the ligh fhould have the fame long figure upon the wall that it had when it came out of the Prifm ? Mr. Linus therefore, while he would ftrergthen his argu- ment by reprefenting me well skilled inOpticks, does but over- throw it. But whereas he fayes, Icouldnot ha-ve beenfurprized at.the length, had the Image been parallel, it being a thing Jo obvious and eajy to be explicated by the ordinary rules of refraction: Let any Man take the Experiment intireas I have there delivered it,that is, with this condition, that the refractions on both fides the Prifm were equal, and try if he can reconcile it with the ordinary rules of refraction. On the contrary, he may find the impoffibility of fucha reconciliation, demonftrated in my Anfwer to P. Par dies JSf. 84,^.409 r. In thelaff p!ace,he objects, that my faying in N. 80. ^.3 077,^4/ the incident refractions mere in the Experiment equal to the emer- gent, pre ves again, that the long Image was parallel. And yet that very faying is a fufficient argument, that I meant the contrary, be. caufeit becomes wholly impertintnt,if apply'd to a paraliejimage; but in the o her cafe is a very necefTary circumftance. What is ad- ded therefore of P. Pardies,imght have been fpared , efipeciaily fince that Learned Perfun understood my difco'.irfe to be meant of a tranfvers Image, and acquiesced in my Anfwers. This in anfwer to Mr. Linut\ Letter: And now to takeaway the like fufpicions from his Friends, if my declara'ion of my meaning fat is fie nor, I fhall notefome further pafTages in my Let- ters, whereby they may fee, how I was to be understood from the beginnmg, as totheaforefaid three circumftances. For the Day ; I exprefs every where that the Experiment was tried in the Sun's light, and in JSf. 80.^.3077, that the breadth of the 1 mage by meafure anfwered to the Sun's diameter : But becaufe n is pretended., I was impofed upon, I would ask, what the Ex- p-riment as it is advanced to that which I called the Experiment- t'im C ssO turn Cruris, can have to do with a cloudy day t For, if the Exfe- rimentum Cruets (which is that which I depend on) can have no- thing to do with a cloudy day,then is it tono purpofe to talk of a cloudy day in thefirft Experi men t,which does but lead on to that. But if this fatisfie not, let the Tranfatfions N. 83./*. 4060, be'con- fulted : For. there I tell you, how by applying a hens to the Prifin, the ftreight edges of the oblong Image became diftinfter than they would have been without the hens : A circumfiance which cannot happen in Mr. hinuis cafe of a bright Cloud. Forthe Portion of the Prifms I tell you J\T.B<2>.j>.^o-j6, that it was placed */ the Sun's entrance into the Chamber,and in/>3o8$.I bad to make a hole in the fhur,and there place the Prifm,and in the next page I fay again, that the Prifm ABC is to be fet clofe by the hole F of the window EG'; and accordingly reprefent it clofe in the Figure. Alfoin/>^. 3077 I tell you, thatthediftanceof the Image from the hole or prifm was 22 foot ; which is as much as to fay, that the Prifm (fuppofe that fide of it next the hole) was as farfromthe Image as the hole it felfwas, and confequently that the Prifm and Hole were contiguous, Alfo in/>.3c>78,where inftcad of the Window fliuc 1 made life of a hole in a loofe board , 1 tell youexprefly.that I placed the board clofe behind the Prifin. All thefepaflages are in my very firft Letter about Colours ; and who therefore would imagine, that any one that had read that Lerter fhouldfomuch asfufpeft,that I placed the Prifin, I fay notatfo greatadiftanceasMr.L/»«* fuppofes,but at any diftance worth confidering ? Laftly, for the Pojttkn of the Image , it is reprefented tranfvers to the axis of the Prifm in the figures iOo.p.3086. N.S^.p.^o6i r and M$.p,$ox6. And in iv r .88./».so93, where 1 made life of two crofs Prifins, I tell you exprefly , that the Image was crofis to both of them at an angle of 4$ degrees. The calca ations alfo N. 80. p. 3077. are not to be underftood without fuppofing the Image crofs. Nor are my notions about different Refrangibilityotherwife in- telligible : For in Mr. Linus's fuppofition , the rays that go to the two ends of the Image,are equally refra&ed.So for colours,rhe red, according to my defcription, falls at one end of the (mage, and the blew at the other ; which cannot happen but in a tranfvers Image. The fame pofition is alfo demonftrable from what I faid in AT* 80. f, 3076, about turning the long Image into a round one, by the Bddd contrary (5«°) contrary refra&ionof a fecond Priftn, further explained in Km, 83./. 406 1 . For this is not to be done in Mr. Limn furmifc of a parallel Image,and therefore bad Mr. Limn confidered it, he could never have run into that furmifc. This I fuppofe is enough tomanifeft the three particulars ; any one of which being evidenced,is fufficient to take away the fcruple. And therefore Mr. L inus Friends need not fear but that the further directions I fent them lately for trying the Experiment are the fame with thofe i have follow'd from the beginning 5 nor trouble them- felves about any thing but to try the Experiment right. But yer, becaufe Mr. Gafeei* has been pleafed to infinuate his fufpicion that I do differfrom himfelf in thofe directions, I fliall not fcruple here to reduce them into particulars, and fhtw where each particular is to be found. r.Then,he is to get aPrifin with an angleabout 60 or 6$ degrees, N. So, p. 3077, and p. 3086. If the angle be about 63 degrees, as that was which I madeufe of N. 80. f.3077, he will find all things fucceed exactly as I defcribed them there. But if it be bigger or kf>, as 30, 40, 50, or 70 degrees, the Refraction will be accord- ing! y bigger or lefs, and confequencly the Image longer or ihorter. If his Prifm be pretty nearly equilateral (foch asl fuppofe arenfu- ally fold in other places as well as in EngUad) he may make ufe of the biggeft angle. But he mnft be fure to place the Prifm fo, that the Refraction be made by the two planes which comprehend this angle. I could almoft fufpect, by considering fome circumftances in Mr. Linuat Letter, that his error was in this point, he expecting the Image fhould become as long by a little refraction as by a great one ; which yet being too grofs an error to be fufpefied of any O- ptician, I fay nothing of it, but only hint this to Mr. Gafeciv, that he may examine all things. 2. Having fuch a Prifm, he muit place it fo, that its Axis be per- pendicular to the rays N. 84,^. 409 \ % lin. 1 8, 19. A little error in this point makes no fenfible variation of the effect. 3. The Prifin muft be fo placed, that the Refraftions on both fides be equal N. 80, f. 3077 ; which how it was to be readily done by turning it about its Axis, and flaying it when you fee the Image reft between too contrary motions, as I explained in my late De- fer! ptions,fo I hinted before M 80.^.3077,^. 34,35, 36. If there fhould be a little error in this point alfo, it can do no hurt. 4. The C s*i ) 4» The Diameter of the hole I put ~ of an inch N, So, /. 307 7 , and placed the Prifm dole to it, even fo clofe as to becontiguous* Jtf. 80,/. 3077, Un. 4, 5* But yet there needs nocuriofity in thefts circumirances. The hole may be of any other bignefs,and the Prifm at a diftance from the hole, , provided things be fo ordered, that the light appear of a round form, if intercepted perpendicularly at its coming out of the Prifm. Nor needs there any curiofity in the day. The clearer it is the better; butifitbea little cloudy, that cannot much prejudice the Experiment, fo the Sun do but fliine diftinfily through the cloudy Thefe things being thus ordered, if the refraded light fall per- pendicularly on a wall or paper at 2©footormore from the Prifm, it will appear in an cblong form, crofs to the axis of the Prifm, r^i at one end,and violet at the others the length five times the breadth (more or lef> according to the quantity of the refra&ion,) the fides, ftreight lines, parallel to one another, and the ends confufed, but yet feeming femi-circular. I hope tbcrtfore,Mr. Linu\ Friends will not entertain them- felvesany further about incongruous furmifes, but try the Experi- ment as Mr. Gajcoin has promifed. And then, fince Mr. Gafco'm tel 1 s you, That the Experiment being of it [elf extr aor dinary and fur pri- zing , and beftdes ufbering in new Principles into Of>ticks,quite con- trary to the common and received , it will be hard to ferfivade it as a truth, till it be made fo viftble to all as it were afhame to deny it: if he efteem it fo extraordinary, he may have the priviledg of making it fovifibleroall, that it will be afhame to deny it. For, I dare fay, after his teftimony no body elfe will fcruple it. And I make no queftion but he wi 1 hit of it, it being fo plain and eafy, that lam very much at a lofs to imagine what way Mr. Linus took tomifs. Dat. Cambridge Feb. 29. 167/. An Extr all of a Latin Lttter of Signer Caflini, containing both his Con- Jiderations upon Afr.Yhm!\