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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. (3075 ) NtmkBo, PHILOSOPHICAL TRANSACTIONS. FebrHary 19. 16!;. x_» The C ONTENTS. A Letter of Mr.Ifaac ^tv^toxi^lStathcmatick^ProfejJhr in the Vniver/i. ty of Cambridge '^ contaimnghis NetvTbeory about Light and Co- lors : Where Light is declared to be not Similar or Homogeneal , but con^fiingof difformraysj fome of vehuh are more refrangible than 0- tbers : ^W Colors are affirm d to be not ^ualtfications of Light, de^ rivd from 1{efraBions of natural Bodies^ {as 'tis generally bekeved 5 ) But Original and Connate properties ^ rvhich in divers rays are divers : Where Jeveral Obfervations and Experiments are alkdged to prove the [aid Theory, An Accrimpt of fome Books ; I. A Defcription of the EJST-INDJAN COASTS, MALABAR^, CO\OUA'NDEL, r£rL<9N,«^f.m Dutch, ^3/ Phil. Baldseus, II. Antonii le Grarsd INSTITVTIO PHI LOS OP H I ^jkcnndhm principia Renati Des-Cartes; tiovd methodo adornata ^ explicate. III. An Efjay to the Advancement of MVSICK^--) by Thomas Salmon M.A, Advertifement about ThxonSmyvDXmt Anlndcx for the Tracts of the Tear 1671. A Letter of Mr, Ifaac Newton, Profeffor of the Mathematicks in the Vniverjity of Cambridge ; containing his New Theory about Light and Colors ; fent by the Author to the PubVfoer from Cambridge^ Fcbr.6. 1 6— '-i in order to be communicated to the R, Socie ty» S I R, TO perform my late proraife to yots, I (halUvitnout further ceremony acquaint you, tliat in the beginning ot the Year i666(atwhichtirae lapplyed my felfto the grinding of Optick glaffes of other figures than Spherical,') I procured me a Triangu- lar glafs-Prifme, to try therewith the celebrated Phmi?msna of G g g g Colours, ( %o-}e ) Cohnru And in order thereto having darkened my chamber.and made a rm:dl hole in my window-ihuts , to let in a convenient quantity of the Suns light, I placed my Prifme at his entraGCe,that ic HTgh: bs thereby rctrsdted to the oppolue walL It was at firft a very pteafir.gdivcrtifcmenr, to view the vivid and intenfe co- lours produced thereby j but aiter a while applying my felf to con- fiJer ihcm more circumrpe€tly, I became furprifed to fee them ia an6v5/ow^form^ whch. according to the received laws of Refra- ftion, I expe 44 deg.56'. And the vertical Angle of the Prifrae, 63 deg. 12'. Alfo the RefracTtions on both fides the Prifme, that is, of the In- cident, and Emergent R.ays, were as near, as I could make them, equal,and confeqasntly about 54 deg. 4'. And the Rays fell per- pendicularly upon tlie wall. Now fubduding the diameter of the hole from the length and breadth of the Image, there remains 13 Inches the length, and 2|- the breadth, comprehended by thofe Rays, which paffed through the center of the faid hole, and cons (equently the angle of the hole, which that breadth fubtended, wasabout^i', anfwerable to the Suns Diameter; but the angle, which its length fubtended, was more then five fuch diameters, namely 2 deg. 49'* Having made thefeobfervations, I firft computed from them the refradive power of that glafs, and found it meafured by the rfl^«5 of the lines, 20 to 31. And then, h'^thztratio^ I computed theRefradionsof two Rays flowing from oppofite parts of the Sun's difcut^ fo as to differ 51' in their obliquity of Incidence, and found, that the emergent Rays ihould have comprehended an angle of about 31', as they did, before they were incident* But becaufe this computation was founded on the Hypothefis ofthe proportionality of the /«e/ of Incidence, and Refradion, which though by my own Experience I could not imagine to be fo erroneous, as to make that Angle but 31', which in reality wjs 2 deg. 49' 5 yetmycuriofityCaufedme again to take my Prifme. And having placed it at ray window, as before, I obferved.diat by turning it a little about its axis to and f o, fo as to vary its obh- quity to the light, more then an angle of 4 or 5 degreej,the Co- lours were not thereby fenfibly tranflated from their place on the wall, andconfequendyby that variation of Incidence, the quan- tity of Refradion was not fenfibly varied. By this Experiment therefore, as well as by the former cotnputation , it was' evident, thatthedifference of the Incidence of Pvays, flowing from divers Gggga parts ( 3'^78 ; parts of the Sua, could not make them after decuflation diverge at a lenfibly greater angle, than that at which they before conver- ged ; which being, at mo ft, bur2bout3i or 32 minutes, there Itill remained fome other caufe to be found out, from whence it could be 2 degr. 49'. Then I began to fufpedl', whether the Rays, after their tr ajedi* on through the Prifme, did not move in curve lines, and accord- ing to vheir more or lefs curvi[y tend to divers parts of the wall. And itincreafed my fufpition, whenlremembred thati had often feen a Tennis ball, ftruck with an oblique Racket, defcribe fuch a carve line. For, a circular as well as a progreffive motion being communicated to it by that ftroak, its parts on that fide, where the motions confpire, muft prefs and beat the contiguous Air more violently than on the other, and there excite a relu<2:ancy and readion of the Air proportionably greater. And for the fame reafon, if the Rays sf light fhould poffibly be globular bodies, and by their oblique pafiage out of one medium into another ac- quire a circulating motion5they ought to feel the greater refiftance ftom the ambient iEther, on that fide, where the motions con» fpire, and thence be continually bowed to the other. But note v.'ithftanding this plaufible ground of fufpitioD , when I came to examine it, I could obferve no fuch curvity in them. And be- fides (which was enough for my purpofe) I obferved, that the difference 'twixt the length ojf the Image, and diameter of the hole, through which the light was tranfmitted,was proportionable to their diftance. The gradual removal of thefe fufpitions.at length led me to the Experimentum CruciSf which was this .• I took two boards, and pla- ced one of them clofe behind the Prifme at the window , fo that the light might pafs through a fraall hole, made in it for the pur- pofe, and fall on the other board, which f placed at about 12 feet diftance, having firft made a fmall hole in it alfo, forfomeof that Incident light to pafs through. Then I placed another yrifme be« hind this fecond board, fothat the light, trajecfited through both the boards, might pafs through that alfo, and be again xcUidicd before it arrived at the wall. This done, I took the firft Prifme in mv hand , and turned it to and fro flowly abrjut lis Axis, io much as'to make -he fevcral parts of tht Image,C£ft un the fecond board, fucccffively pafs through the hols in it, that I might oblerve to what places on the' wall the fecond Prifme would refradt them. And ( 3079 ) And I faw by the variation of thofe places, that the light, tending to that end of the Image, towards which the rcfradion of the firlt Prifme was made, did in the fecond Prifme fuffer a Refradion confiderably greater then the light tending to the other end. And fo the true caufeof the length of that Image was dctedied to bi no other, then that Li^^t confifts of J^ys differently refrangible^ whichjWidioui: any refpeiSt to a difference in their ipxidence,were, according to their degrees of refrangibility, traafmitted towards divers parts of the wail. When I underftood this, I left off my aforefaid Glafs works; for I fav;, that the perfection of Telcfcopi's was hirherto limited, not (o much for want of glaffes truly figured according to the pre- fcriptions of Optick Authors, (which all men have hitherto ima- gined,) asbecaufe that Light it fclf is a Heterogeneous mixture of differently refrangible 'Rap. So that.were a glafs fo cxadly figured, as to colle(5t any one fort of rays into one point, it could not col' led thofe alfo into the fame point, which having the fame Inci- dence upon the fame Medium are apt to fufier a diff ;rent refracti- on. Nay, I wondered, thatfeeing the difference of refrangibili- ty was fo great, as I found it, Telef copes fhould arrive to that per* fedion they arc now at, Forjmeafuring the refraftions in one of my Prifmes, I found, that fuppofing the common fine of Inci- dence upononeof its planes was 44 parts, the/«5of refradtion of the utmofl Rays on the red end of the Colours, made out of the glafs into the Air, would be 68 parts, and the/«e of refraction of the utmofl rays on the other end, 69 parts : So that the difference is about a 2\th or 25^/; part of the whole refradion. And c )nfe. quently, theobjeft-glafsof any Telefcope cannot collcdt all the rays, which Corac from one point of an objc Ct/o as to make them convene at vis Jqchs in kfs room then in a circular fpacc , whofs diameter is the 5o^,!'partof the Diameter of its Aperture ; which is an irregularity, fome hundreds of times greater, then a circu- larly figured Lens^ of fofmall a fedion as the Objedtglafl'es of long Telefcopes are, would caufe by chcuaSmefs of its figure,were Light uniform. This made me take l{efieBicns into confideration, and finditig them regular, fo that ths AngleofReficdion of all forts of Hays was equal to their Angle of Incidence ; I underftood, that by their mediation Optick initruments might be brought to any degree ot perfection imaginable, provided a RefieUing fubftaace could bs found. ( 3o8o ) found, which would polifh as finely as Glafs, and reflcB as much light, as glafs tran/mit/y and the art of communicating to it a Pa- rabo{ic\ figure be alfo attained. Eut there feemed very great dif- ficulties, and i have almoft thought them infuperable, when I fur- ther confidcred, that every irregularity in a reflcifl'.ng fuperficics makes the rays ftray 5 or 6 times more out of their due couifc, than the like irregularities in a refra<3:ing one : So that a much greater curiofity would be here requifite, than in figuring gl.iflcs torRefracStion. Amidft thefe thoughts I was forced fvomCambridge by the Inter, veoing Plague, audit was more then two years,bef ore I proceed* ed further. But then having thought on a tender way of polifli- ing, proper for merall, whereby, as I imagined, the figure alfo would be corrected to the laft ; I began to try, what might be ef- f edicd in this kind, and by degrees fo far perfe<9:ed an Inftrument (inthe effentialpartsof itlikethat I fent 10 London,^ by w^hich I could difcern Jupiters 4 Concomitants, and {hewed them divers times to two others of my acquaintance. I could alfo difcern the Moon.like phafe of Fenu/, but not very diflindly, nor without feme nicenefs in difpofing the Inftrument. From that time I was interrupted till this laft Autumn, when I made the other* And as that was feofibly better then the firft (efpecially for Day-Objeasj)fo I doubt not, but they will be flill brought to a much greater pcrfedion by their endeavours, who, as you inform me, are taking care about it at London. I havefometimes thought to make a Micro fcope, which in like manner ftiould have, inftead of an Objeera!Iy be« Heved,) but Or;^?W and connate propsrtiei^which in divers Rays are divers. Some Rays are difpofed to exhibit a red colour and no other J fomea yellow and no other, fome a green and no other, and fo of the reft. Nor are there only Rays proper and partcu- Iar to the more eminent colours, but even to all their intermediate gradations. 2. To the fame degree of Refrangibility ever belongs the fame colour, and to the fame colour ever belongs tha fame degree of Refrangibility. The lea^ Infrangible Rays are all difpofed to ex- hibit a7((?rt? colour, and contrarily thofe Rays, which are difpofed to exhibit a Zfe(5^ colour, are all the leaft refrangible: So the mo[t refrangible Rays are all difpofed to exhibit a deep F^ioletColow^^nd contrarily thofe which are apt to exhibit fach a violet colour^ are all the moft Refrangible. And fo to all the intermediate colours in a continued feries belong intermediate degrees of refrangbili- ty. And this Analogy 'twixt colours, and refrangibility, is very precife and ftrid: 5 the Rays always eicher exat^ly agreeing in DDth, or proportionally difagreeing in both. :\. The fpecies of colour, and degree of Refrangibiliry proper to any particular fort of Rays, is not mutable by Refra(5t;onj iior by Fvcfledion from natural bodies, nor by any other caud^ tisiC I could yet obferve. When any one fort of Rays hath bzz n well prrt'-d ( 3082 ) parted from thofe of other kinds, it hath afterwards obftinately retained its colour, notwithftanding my utmoft endeavours to change it. I have refradted it with Prifmesj and refledled it with Bodies, which in Daydight were of other colours 5 I have inter- cepted it with the coloured film of Air interceding two compreG led plates ofglafs j tranfmitted it through coloured Mediums,and through Mediums irradiated with other forts of Rays, and di- vcrfly terminated itj and yet could never produce any new co- lour out of it. It would by contrading or dilating become more brisk, or faint, and by the lofs of many Rays^ in fome cafes vary obfcure and dark j but I could never fee it changed ift fpscie^ Yet feeming tranfmutations of Colours may be made , where there is any mixture of divers forts of Rays. For in fuch mixtures, the component colours appear not, but, by their mutual allaying each other, conftitute a midling colour. And therefore, if by refraftion, or any other of the aforefaid eaufes, the difform Rays, latent in fuch a mixture, be feparated, there fhall emerge colours different from the colour of the corapofition. Which colours are not New generated,buc only made Apparent by being parted^ for if they be again intirely mix'C and blended together, they will again compofe that colour, which they did before reparation. And for the fame rcafon, Tranfmutations made by the conveningof divers colours are not real ; for when the difform R.ays are again fevered, they will exhibit the very fame colours, which ihey did before they entered the coropofition ; as you fee, Blew and Yellsw powders, when finely mixed, appear to the naked eye Crecn^ and yet the Colours of the Component corpufcles are not thereby really tranfmutedj but only blended. For, when viewed with a good Micrcfcope,they ftill appear Blew and Tellcve interfper/edly, 5. There are therefore two forts of Colours* The one original andfimple, the other compounded of thcfe. The Original or pri; miry colour? are, T{ed, Telkw, Green, Blcvo^ and ?i. Viekt-.purple, together v,-i:h Orange, Indico, and an indefinite variety of Inter- mediate griidations. 6. The fame colours in Specie mihthdtVnmzrj ones may be aUo produced by compolition : For, a mixture of Tellow and Bhiv m kes Green ^ of 'Red and Telhw makes Orange ; oi Orange and Tel= lo:-v,frjqre.eninzhcs,ydld)v. And iu general, if any two Colours be mixed; v.'hich va the ferics of thofe, gens«rated by the Prifmejare no!: ( 3085 ) nottoo far diftaot one from another, they by their mutual allo^ compound that colour, which in the faid feries appearetli in the mid-way between them* But thofe, which are fituated at too great a diftancCj do not fo. Orange ^wA Indico produce not the intermediate Green, nor Scarlet and Green the intermediate yellow. 7. Butthemoftfurprifing.aud wonderful compofition was that of Whiienefs^ There is no one fort of Rays which alone can ex- hibit this. 'Tis ever compounded,and to its corapofirion are re- quifite all the aforefaid prrraary Colours, mixed in a due propor* tion. I have often with Admiration beheld, that all the Colours of the Prifme being made to converge, and thereby to be again mixed as they were in the light before it was Incident upon the Pnfme, reproduced light, intirely and perfedly white, and not at all fenfibly differing from a (iireB Light of the Sun , unlefs when the glaffes, I ufed,were not fufficiently clear ^ for then they would a little incline it to iheir colour. 8. Hence therefore it comes to p!L^s,ihit}Fkitenefj is the ufual co- lour of Light 5 for, Light is a confufed aggregate of Rays indued with all forts of Colors,as they are promifcuoufly darted from the various parts of luminous bodies* And of fuch a confufed aggre- gatC5as I faid,is generated Whitenefs, if there be a due proporti- on of the Ingredients 5 but if any one predominatejshi Light muft incline to that colour ^ as it happens in the Blew flame of Brira- ftone; theyellowflameof a Candle ; and the various colours of the Fixed ftars. 9. Thefe things confidered, thtmannery how colours are pro- duced by the Prifme, is evident. For, of the Rays,conftituting the incident light, fince thofe which differ in Colour proportio- nally differ in Refrangibility, they by their unequall rcfradions muftbe fevered and difperfed into an oblong form in an orderly fuccefCou froratlie leaft refracted Scarlet to the moft refraded Violet. And for the fame reafon it is, that objeds, when looked upon through a Prifme^appear coloured. For,the difform Rays, by their unequal Refradtions, are made to diverge towards fe- veral parts of the Retim^ and there exprefs the Icnages of things coloured, as in the former cafe they did the Suns hnao-e upon a wall. And by this inequality of refra and fuch arc the drops on the exteriour part of the Primary, "and interiour part of the Secondary Bow. 1 1. The odd Phseuomena of an infufion o^ Lignum Nephriticumy Leaf gold f Fragments of coloured glafs^ and fome other traofparently coloured bodies, appearing in one poficion of one colour,and of another in another, are on chcfe grounds no longer riddles* For, thofe are fubffanccs apt to refled: one fort of light and tranfmit another , as may be feen in a dark room, by illuminating them with firailar or uncompounded light. For, then they appear of that colour only, with wliich they arc illuminated, but yet in one pofition more vivid and luminous than in another, accordingly as they are difpofed more or lefs to refle<5l or tranfmit the incident colour* 12. From hence alfo is manifeft the reafon of an uncxpcdled Experiment, which Mr. /:^o^ fomewhere in his Micrography te- lates to have made wi:h two wedg-like tranfparent veiTelsjfiird the one with a red, the other with a blew liquor : namely,that though they were feverally tranfparent enough, yet both together became opake ; For,if one tranfmitted only red,and the other only blew, no rays could pafs through both. 15. I might add more inftances of this nature, but I fliall con- clude with this general one, that the Colours of all natural Bodies have no other origin than this, that they are varioufly qualified to reSe<9: one fort of light in greater plenty then another. And this 1 havd experimented in a dark Room by illuminating thofe bodies with uncompounded light of divers colours. For by that means any body may be made to appear of any colour. They have there no appropriate colour , but ever appear of the co- lour of the light cafl upon them, bnt yet with this difference, that they are moft brisk and vivid in the light of their own day- light-colour. Minium appeareth there of any colour indifferently. With which 'tis illuftrated, but yet moft luminous in red, and fo ( 3©85 ) Bi/f appearcth indifferently of any colour with which 'tis tlluftra- ted, but yecmoftlumiuousiablew. And therefore Mm/aw re- fledeth Rays of any colour, butmoft copioufly thofe indued with red; and confequently when illuftrated with day-light, that is, with all forts oi Rays promifcuoufly blended, thofe qualified with red (lull abound moft in the refledied light, and by their preva- lence caufe it to appear of that colour. And for the fame reafoa Bife, refie(5ting blew moft copioufly, ihall appear blev/ by the ex= cefsofthole Rays in its^-efieded lights and the like of other bo» dies. And that this is the intire and adequate caufe of their co« lours, is manifeft, beca-iie they have no power to change or alter the colours of any fort of Rays incident apart, but put on all co- lours indifferently^ with which they are inlightned. Thefe things being fo, it can be no longer difputed, whethf.r there be colours in the dark, nor whether they be the qualities of the objetfis we fee, no nor perhaps, whether Light be a Body* For, fince Colours are the qualities of Light, having its Rays for their intire and immediate fabjeft, how can we think thofe Rays qualities alfo, unlef.i one quality may be the fubjed of and fuftaia another ; which in efTed is to call it Subjianee. We fliould nat knowBodies for fubftancesjwere it not for their fenfiible qualities, and the Principal of thofe being now found due tofomething elfe, we have as good reafon to believe that to be a Subftance alfo. BefideSj whoever thought any quality to be a heterogeneous ag- gregate, fuch as Light is difcovered to be. But, to determine more abfolutely, what Light is, after what manner refradiedjand by what raodesor anions it produceth in our minds the Phan- tafms of Colours, is not fo cafic. And I fhall not mingle cou- je^lures with certainties. Reviewing what I have written, I fee the difcourfe it felfwill lead to divers Experiments fuflScient for its examination : And therefore I (hall not trouble you further, than to defcribe one of thofe,which I have already infinuared. la a darkened Room make a hole in the Chut of a window, whofe diameter may conveniently be about a third part of an inch, to admit a convenient quantity of the Suns light : And there place a clear and colourlefs Prifme, to refraft the entring light towards the further part of the RoomjWhich,as Ifaid,will thereby be diffufed into an oblong colonred Image. Then place a Lens of H h h h 2 about C 3086 ) about three foot radias (fuppofe a broad Objeft-glafs of a three foot Telelcope,) at the diftanceof about four or five foot from chcnce, through which ali thofe colouri may at oace be traDfmit- ted, andmade by its Refraction to convene at a further diftance of about ten or twelve feet, if at that diftance you intercept this light with a fheet of white paper, you will fee the colours convert- ed into whitenefs again by being mingled* But it is requifice,thac thtPrifme and Lent be placed fteddy, and that the paper, on which the colours are caft, be moved to and fro i for, by fueh motion, youwilinot only find, at what diftance the whitenefs is iiioft perfeft,but alfo fee.how the colours gradual'y convene, and vanilli into whitenefs, and afterv/ards having eroded one another iuthat place where they compound Whiceneis, are again diffipa- ted, and fevered, and m an inverted order retain the fame co- lours, which they had before they entered the compoficion. You may alfo fee, that, if any of the Colours at the Lens be intercept- ed, the Whitenefs will be changed into the other colours. And therefore, that the compofition of whitenefs be perfed;,caremuft be taken, that none of the colours fall beildes the Lens. In the annexed defign of this Experiment, ABC exprcfTeth the Prifra fet endwifc to fight, clofe by the hole F of the window E G. Its vertical Angle A C B may conveniently be about ^o d-egrees : M N defigneth the Lens. Its breadth 2 J or 3 inches. S F one of the flreighc lines, in which difform Rays may be con- ceived to flow fucceffively from the San. F P,and F B. two of thofe Rays unequally refraded, which the Lens makes to converge towards Q, and after decuffation to diverge again. And H I ihz Bapcr, at divers diftances, on which the colours are projeded: wh'ch in Q,cooft:!tute whilene/s, but are Bsdzud TcUow'va K.t, acd f, ■^vABl.ti^ra^'.Furfk m P, p, and ^ If ( 3087 ) If you proceed further to try the impoffibility of changing any uDCompounded CGloiir (v='hich 1 have aiferted m the third and thirteenth Propofitions,) 'tis rcquifite that the Room be made ve« ry darkj leaft any fcattering hght^mixing with the colourjdifturb and aiiayit, and render it compound, contrary to thedefignof the Experiment. 'Tis alfo requifite, that there be a perfedter re- paration of the Colours, thanjafter the manner above defcribed, can be made by the Refradition of one fingle Prifme, and how to make fuch further feparationsjwill i'carce be difficult to them,that confider thedifcovered laws of Refradiions. But if tryallhall be made with colours not throughly Icparated, there mutt be al- lowed changes proportionable to the mixture. Thus if com- pound Yellow light fall upon Blew Bife, the Rife will not appear perfectly yellow, but rather green, becaufe there are in the yel- low mixture many rays indued with green, and Green being lefs remote from the ufual blew colour of Bife than yellow, is the more copioufly refledled by ir» In like manner, if any one of the Prifmatick colours, fuppofe Red, be intercepted, ondefignto try the afferrcd impoffibility of reproducing that Colour out of tru; ochcrs wl;ich are preter- mitted ; 'cis neceflaryjcitherthruihc c-jlours be very well parted before the red be intercepted, or that together with the red the neighbouring colours, into which any red is fecretly difperfed, (that is, theyellowj and perlians green too) be intercepted, or elfe, tliat allowance be made for the emerging of (o much red out of the yellow green, as may pofTibly have been dtffufed, and fcatteringly blended in thofe colours. And if thefe things be ob- lerved, the new Produiftion of Red, or any intercepted colour will be found impoffible. This,! conceive, is enough for an Introduction to Experiments of this kind ; whichif any. of the /(^.^or/f^y (hall be fo curious as to profecute^I iliould be very glad to be informed with whatfuccefs: That.if any thing ieem to be defe<^ive,or to thwart this relation, ff may have an opportunity of giving further direction about it, or of acknowledging my errorSv if I have committed any. Sofar this Learned and very Ingenious Letter i which having httTiby xhzl lilujinoHs Company^ before whom it was read, with much applaule cotrimitted to the confideration of fome of their Fellows, well verfed in this argument, the Reader may poffibly in an other Tract be inforiricd of fome report gjv.en in upoa this Dif* courlc, J%.