mr. isaac newtons answer to some considerations upon his doctrine of light and colors; which...

Mr. Isaac Newtons Answer to Some Considerations upon His Doctrine of Light and Colors;Which Doctrine Was Printed in Numb. 80. of These TractsAuthor(s): Isaac NewtonSource: Philosophical Transactions (1665-1678), Vol. 7 (1672), pp. 5084-5103Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/100964 .

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Philosophical Transactions

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( 5084 ) AMr. Ifaac Newtons Anf,er tofomne Coniderations upon bis Dot

ftrine of Light and Colors ; wibicb Doerine was printed i Numb. So. ofthefe 7?raFs.

ClP, I have already told you, that at the perufal of the con. lilderations, you fent met on my Litter concerning Refra;-

tions alid Colors, I found nothing, that, as I conceived,niight not without difficulty be anfwer'd, And though I finld the Coafidtrer fomewhat more concern'd for an [iypotbefli than I expeated ; yet I doubt not0 but we have ole commoni defign 5 I mean, a fincere endeaavour after knowledge, without valuing uncertain fpeculations for their fubtleries, or defpifilng cer- tainties for their plainniefs : And on confidence of this it is, that I make this rctturn to his difcourfc.*

Thefihj? thing ltat offers ic felf w aWicts D,o;be here pta, tlo,h.f i; Icfs agreeable to me, and I begin

becaufein the boly of tbhi, nfwr with it becaufe it is fo. The confl- are tobe met ittbechiefparticu- derer is pleafed to reprehend me larf, wbet,in the Anfwcrer w

fl,a cognerS'd, for laying afide the thoughts of im-

,. Of th Pr4if,, pdr, of o. Pprovin'g Optiques by IQ,fraEtionf. . 0 thOf PrYa piJ f O - t?tiUc , If he had obliged me by a private

Letter on this occafion, I would have acquainted him with my fucceffes on the Tryals I have made of that kind, which I fiall now fay have been lefs than I fometimes expe6ted, and perhaps than he at prefent hopes for. But fince he is pleafed to take it for granted, that I have let this fubject pafs without due examination, I ihall refer him

to my former Letter, * by which

Pr;ntsd i

Numb. . o, oeft that conjecture will appear to be un-grounded. For, what I faid

there, was in refpe& of Telefcopes of the ordinary conftru&i. on, fignifyitng, that their improvement is not to be expected from the well-fg.iuring of Glaffes, as Opticians have imagin'd ; but I defpaired not of their improvement by other conftru- tions 5 which made me cautious to infert nothing that might

intimate the contrary. For, although fucceffive refrations that are all made the fame way, do neceffarily more and more augment the errors of the firft refradion ; yet it feem'd not impoflible for contrary refraations fo to corred each others inequalities, as to make their difference regular - and, if that

could



C So85 ) could be conveniently effe~ted, there would be no further dif&. ficulty. Now to this end I examin'd, what may be done not only by Gle/fes alone, but more efpecially by a Complication of divers fucceflive Mediumr, as by two or more GlaMts or Cry- itais with Water or fome other fluid between them; all which togeither mnay perform the office of one Glaft, efpecially of the Objeatglafs, on whofe cou.rualion the perfeaion of the in. frirment chiefly depends4 But what the refults in Theory or by Tryals have been, I may poilibly finid a more properocca- fion to declare.

To the Affertion, that Kays are lefs true reflected to a point by a Concave, thanr efracted by a Convex, I cannot affent ; nor do r underifaid, that the foc;vs of the latter is Icfs a line than that ofthe Former. The truth of the contrary you will rather perceive by this following Table, computed for fuich a Reflec4 tinpg Concave, and Refractingo convex, on fuppofirion that they have equal Apertures, and colle6t parallel rays at an equal di- fiance from their vertex ; which diftance beinig divided into i 5ooo parts, the Diameter of the Concave Sphere will be 6cooo ofthofe parts, and of the Convex, I oooo f fupofinig the Sinesr of Incidence anid Refra6tion to be, in round num- bers, as 2 to 3, And this Table fhews, how much the exterior rays, at feveral Apertures, fil fhiort of their principal fAcnM.

The paRts oftthe Axis iitereepted The Diametcr between he vcrtex bind the rays. The Error by c/the_Apertare. Reflet7ed. Rr*fr'ed. -Reflexion, Refrtaio

2.000 149911 14865 13

4-000 14966 14449 33 $51 6ooo 14924 13699 76 1301

oOO 14865$ 12475 13$ 25. .

10000 I4787 9+72 _ 213 528

By this you may perceive, that the Errors of the Refracting Convex are fo far from being kfs, that they are iuore tlian ifix teeni times greater than the like errors of the eflctin,gconcq, efpecially in great Apertures j and that withiout refped to the Heterogeneous coifticution of light. So that, however the contrary fuppofition might make the Author of thefe Anixnade verfions rejea 1efiections as uftlefs for the promuLuing of Op0

Kkkkk2 tiquc-s



( 086 ) tiques; yet I muft for this as well as other coufideratio ns pre fcr them in theTheory before Refractions.

Whether the Parabola be more difficult to defcribe tlhan the Hyperbola or Ellipfrs, may be a e,ere: But I lee no abfolute neceflity of endeavouring after any of their defcriptions, For, if Metals can be ground truly Sphericfl, they will bear as great Apertures, as I believe men will be well able to communicate an exact ptlifli to. And for Dioptrique Telefiopes, I told you, that the difficulty colifitted ot in the Figureof the glafs, but in the Diflbrmity of Refradtions : Which if it did not, I could tell you a better and more eafie remedy than the ufe of tle Conic Seaions.

Tllus mlch concerning the Praetique art. of i part of Opriqucs. 1 fliall ow take a view

cf the Confiderations oni mvThcories. And thofe confift in afcribing an Hyiothe/i to me,which is not mine; i; Afferting an ItypothfJi, which, as to the principal parts, is uot agaitft me 5 in Granting the greateft part of my difcouirfe ifexplicated by tlht Hiypthefisr alnd in Deinying fome things, the truth of which would have appear'd by an experimental examination,

Of thefe Particulars I flall difcourfe in ,k, of H, bpoi, s m order. And firft of the Hypothdfi, . hich

is afcribed to me in thefe words: But' rant hi f rjf/ uppofition, that light is a body, and that as many colours or deg,rees as there ma be, /o many bodies there may be; al nvbicb comr poundld ltogether nv uldrtmafe "hite, &c. This, it feems, is taken fcr my I]ypothbffi. 'Tis true, tlat from my Theory I argue the Cuoporaeiy ofLight; but I do it without any abolute pofitive- nefs, as the word perihaps intimates; and make it at iaoft but a very plaufible con/equence of thle Dorilne, and not a funda. mental SuppoftiorJ, nor fo much as any part of it ; which was wholly comprehended in the precedent PropofitionsD And I lomewhat wonder, how the Objector could imagine, that,when I had afferted the Theory witl) the greateft rigour, 1 fhould be fo forgetful as afterwards to aftert the fiundamental fippo- fition it felfwith no more thain a perhaps. Had 1 inltended any iIch Hypotbefi, 1 fliould fomcwhere have cxplain'd it. But I Iknew, that the Propertiesr which I declar'd of Light, were in

fome:



( o587 ) fome meafure capable of being explicated not only by that, but by many other Mechanical Hypothefe., And therefore I chofe to decline them all, and to fpeak of Light in geveral terms, confidering itabftratly, as fomehing or other propa, gated every way in itreight lines from luminous bodies, with. out determining, what that Thing is; whether a confufed Mixture of difform qualities, or Modes of bodies, or of,BoO4

-bies themfclves or of any Virtues, Powers, or Beings what. foever. And for the fame reafon I chofe to fpeak of Colours according to the information of our Senfes, as if they were Qualities ofLight itbout, us. Whereas by that HypotbtJis I muft have confidered them rather as Modes of Senfation, ex, cited in the mind by various motions, figures, or fizes of the corpufcles of Light, making various Mechanical imprefions oNthe Organ of Senfe as 1 exprefid it in that place, where I Jpake of the Corporeity of Light.

But fuppofing I had propounded that Hypothtfi, I underftand:not, why the Objetor Ihould fo much endeavour to op. pofe it.. For certainly it has a much greater affinity with his own Hypothefis) than he feems to be aware of; the Vibrations of the Ether being as ufeful and neceffary in thif, as in hisF'or; affuming the Rays of Light to be fiali bodies, emitted evety way from Shining fubftances, thofe, when they iirmiin e on any Refra6ting or Reficting fuperficies,. muft as tieccflTrily excite Vibrations in the ether, as Stones (do in water te.i:i thrown into it. And fuppofingthefe Vibrations to be ofi it veral depths or thickneffes, accordingly as they are exc'ted by the faid corpufcular rays of various fizes and velocitieaoif w:at ufe they will be for explicating the manner of Rtflkctionl atdi Refraction, the production of Heat by the Sun-beatiis, the , , miffion of Light from burning putrifying, or other fibitarce whofe parts are vehemently agitated, the dh.nomenea of }Ait tranfparent Plates and Bubles, and of all Natural bcdi&e, tJe. Manner of Vifion, and the Difference of Colors, as alfo thei' Hkarmony and Difcord; I fihall leave to their colnfid rationi. who may think it worth their endeavor to apdly this HIotintfit,r to the f0lution ofphlenomen,.



C 3568#) 1-iki the icond44ee, I cold you,that the Objector'smTypothejr4

4. 1:e ujel O'sHviotefi,ad 2s-to the furndam ental part of it, is' ~ai'd moi fre ~d jnuin nt aginft m e. That-fundomental

.fiittiu t aud all otber 4-fcb - Suppofition is ; That the parts of nical HI potbefes is comformable to myl bodies, when) briskly agiae,d x

Do~~~rine. ~~cite Vibratio-ns inl the 44 tber, which are

propaga ted ev.ery way from tbofe bodies i'n [Irei,-ht lines, and caufea &enlpation of Light b beating and dajhing againfi t1he bottom of'the Eye,/reh~ afier th)e manner t'hat Vibratin inthe A4ir caufe a Senifation of'Soutid-hybe,at-ing againfithe organ.r-of Hearing. N6ow the. moft freecand natural Application ot this HyPotbefi- to the Solutioun of pbcrnomen~a I take to be this : That the agitated parts of bo'dies, accordinig to their feveral fizes, figures, and mo: cions, do excite Vibratioins in the ather of various depths or b:igneffes,w,hich,being pro-mifcuoufly propagated throughi that Medium to our Eyes,effe6t in us a Seinf'ation of Light of a White colour;- but if by any means thofe of uniequal big neffes be leel parated, from one another, the largyeft begett a Senf ationi of a Redcolourl, the leaft or iborteftl, of a deep Vliolet, and the in. termediat ones, of intermnediar colors; much after the mnan. tier that, bodies,. according to their feveral fizes, lhapes, and motions, excite vibrations in the Air of ~various bignieffes, which, accordin)g to thofe bi-gne'ffes, make feveral Tones in Sound: 'thiat the largeft Vibrat'ions are beft able to o-ver- come the refiftaince of a Refra6ting fuperficies, and fo break through it with leaft Refra&ion; whence the Vibrationis of feveral bigneffes, that is, the Rays of feveral Colors., which are blended together in Light, muft be parted from one ano. ther by Kefra6t ion., and fo caufe the Penomena of Prfime: and other refra6thng fubftances,: And that it depends on the th,icK'f nefs of a thiin tranfparent Plate or Buble, whether a Vibration fhall be refled5ed at its furtther fuperficies,or tranfmitted; fo that, according to the number of vibrations, interceding the two fuperficies,they may, be reflealed or tranfmitted for many fucceffive tlhickn-effes. And fice the Vibrations which make Blewv and;Violet, are fuppofed fhorter -than thofe which make Red and YTellow, they-muft be reflected at a lefs thickuefs of the Plate:- Which is fufficient to expli-cate all the ordinary p-henea Mena of thofe Plates or Bubhis, and alfo of all natural bodi'es,

whofe



( 3s99 ) whofe parts are [like fo many fragments of fuch Plates,

Thefe feem to be the mft plain, genuine-and neceffary conditions ofthis Hiypothiji: And they agree fo jutfy with my Theory, that if tb AnAmaidverr think it to apply them, he need not, on that acoQun, .apprehend a divorce from it, But yet how he willdefend itfroa other diflicuIties, I know not. For, to me, the Fuadalital Suppofition it lelf feems impof. fible namely,That the WaVec or Vibrations of any Fluid,can, like the Rays of Light, be propagated in Streight lines,without a continual andvery extravagant fpreading and bending every way into the quiefcent. Medium, where they are terminated by it. I miftake, if there be not both Experiment and De- monftration to the contrary. And as to the other two or three Hypothefes, which he mentions, I had rather believe them fub. jeCt to the like difficulties,than fufpetdtheAnimadverfvr fhould Ieled the worft for his own.

What I have faid ofthis, may be eafily applied to all other Mechanical Hypothbes, in which Light is fuppofed to be caufed by any Preffion or Motion whatfoever, excited in the ether by, the agitated parts of Luminous bodies.For,it feems impoffible, that anyof thofe Motions or Preffions can be propagated in streigbt lines without the like fpreading every way into the- flladow'd Medium, on which they border. But yet) if any man can thinkit poffible, he muft at leafallow,that thofe Mos tions or Endeavors to motion, caufed in the etker by the feveral parts ofany Lucid body that differ in fize figure, and agi. tation, muft neceffarily be unequal: Which is enough to de- nominate Light an Aggregat ot diffrmrays, according to any ofthofe Hypothees.- And if thofe-Originai inequalities may fuffice to difference the Rays in Colour and Refrangibility, I fee no reafon, why they, that adhere to any ofthofe hypothe/et, flould feek for other Caufes of thete Effeits, unlefs (to ufe the Objectors argument) they will multiply entities without ne- ccffity.

The thirdthing to be confidered is, the Condition of tLe Animadver/or's Conceflions, which is that I would explicate my The s. Of th Animadverfor's Cocef;.

1S^ mat , .OUiQ ̂ P11030 .]

1 ^st, and rbW imiton to bs Vo.

ries by his Hypotbefi: And if I could ,. f

comoly with him in that point there



(o 5090)I

tere wu'ld be little--or no differenc-e between Us' orh grants, th'at without any-refpect to a different incidence ofT rays there aroa difterent Refra&tions;i but he would have it ex. plica ted, not'by the diffierent RLefrangibility of feveral Rays, but by the' Splitting and Rarefying of wthereal pulfes. He grants my tbird, fiObri and fixtb I-ropofitions ; the fenfe of which is, That Un:compounded Colors-are unchanigeable,and that Compound.ed ones are changeabl onily by refolving them. into ~the colors, of which, they are compounded ; and thiat all the Changes, whiich can be' wrougyht in Colours, are effe6ted only by varioufly mixing or parting them : But he grants them

oncndition that I will expliat Colorsby the two ieso .fjlit pulfe, and fo make buit two Ipecieir of them, accounting all other Colors in the world to be but vari-ous degrees and dij latings of-,thofe two. And he further grants, that Wb5iteieffe is produced by the Conventilon of all Co"Ffo's; but then I muft al: low -it to be not only by Mixture 6f-tb&fe Colors, but by a far-0 ther Unitinlgof the parts of' the Ray -fuppofed to be former-ly frplit.

I'f I would proceed to examine thefe his Explicati'on.s, I think it would-be -no difficult matter to flhew, that th-ey are not only in/ufflcient, but in fome refpects to me (at teaRl) un'-intelli.. gible. For, though it be earietocon-ceive, how Motionlmay be- dilated :and fpread, or'how parallel motions may become diverging; yet I underftand not, by what artifice any Linear mnotion can by a refracting fuperficies be infinitely dilated and rarefied, fo as to become stuperficial: Or, if that be fuppofed,, yet I underftand as little, why it fhiould be fplirt at fo fmallI ani angle only, and not rather fpread and difperfed through the wholean1gle of Refraction. And further, though I cani eafily imagine, how Unlike motions may crofs onie anothier;5 yet I canno-t well,conceive how'te (bould coalefce inato one uniform motion), and then part again, and recover their former ULI- likenefs ; notwithftanding that I conjecture the ways,by which the,xAnimadv~erfor may endeavour to explai'n it. So that the Di. rect, uniform and undifturbed Pulfes fhould be fplit and difturbed by Recfraction ; and yet the Oblique and difturbed Pulfes perfift wi1thout fplitti'ng or f-urther difturbance by following Rkefractionsj, is (to me) as unintelligible. And there is

as



( 509I ) as great a difficulty In the Nuimber of Colous,s -as, you will fee hereafter.

But whatever be the advantages6*ti . e

or difadvanrages of this Hypaotbejir, xpat yD~ rm. by any t

orp

I hiope I may be excuied from ta. thefis. king it up, fince I do not think it needful to explicate my Doctrinie by any dypothI)J7 at all, For if Ligbt be confider4d abftra6tedly with out refp)eLt to any Hy.~ patkeflsr, I can au eafily conceive, that the feveral parts ot'a fbi- ning body may em'it rays ofdiffering -co'lours and other quali% ties, of all which Light i-s conftiutued, as that the feveral. parts of a falfe or uneven iftring, or of uneavenly agitated water in a Brook or Catara&k, or the feveral Pi'pes of an Organ infpi-- red all at once, or all the variety of Sounding bodies in the world together., fhould produce founds of feveral Tones, and propagate them through the Air confufedly inrermixt. And, if there were any natural bodies that could refteR founds of one tone., and ftifle or tranfmit thofe ofanother; thlen, as the Ecbo of a confufed Aggregat of.all Tonies would be that particular Tone., which the Echoing body is difpofed to refle&; fo,fince (even by the Amnmadver/or"i conceffions) there are bodies apt to reflect rays of one colour, and iftifle or tranfnit thofe of another; I can as cafily concei've, that thofe bodies, when illum;- nated ~y a mixture of all colours, mull appear of that colouir only which they refle&%.

But when the Objector wouild infinuate a difficulty in thefz thinigs, by alluding to Sounds in the ftri-ng of aM Mufical i'nftru. mneat before percufiion,or in the Air of an Organ Bellowes be, fore its arrival at the Pipes i I mull confefs, [un2oderit and itr as littlea, as if one had fpoken of Light ina pi'ece of Wood before -it be-. fet on fire, or in the oyl of a Lamp before **C afcend up the match to feed the flame.

You fee therefore, hoW mnuch A.Tedffiute Fb nravr

it is befides the bufiuiefs in hanid, fo)r s di c;, w fe ox,JTh a tl d fro -i H y- to dii pute about HypoAnefe;. For pothefes, and cqnfider'd rn;oz sen.,

whi'ch reafon I iballt now in the ~~Y liaft place,prcceed to abftra&t the difficulties in the Animadverfor's difco-urfe, at:id,without havinag regard-to any Hypotheftis confider them in general tcrmv. And they may be .reduced to thefe ;OLmres: 1..t liii%



( 5092 ) i. Whether the unoequal Refra&tions, made without ref" peC

to au..y inequality of inicidence, be caufedi by the dilThrent Re- fratngibility of feveral RLays; or by the flitting, breakiDg or difflpiaEring the fam e Ray inito divergingi parts?

;z. Whlether there be more than two forts of Colours? kV hether Whitenefs be a mixture, of all Colours?

3. -That the R&.y is note Fpli't . cr The F:ft freel ~ c o .therai ~ diated.may fiwd already d etermin'd- by

an Experiment i-n my former Let- ter; the ddLigti of whi'ch was to fliew,- That the length of the culour'd. Image proceeded not 'from aniy unevennefs in the 'Glafs, or any other contingent Irregularity in the Refra&ions, Amongft other Irregularities I ktiow nor, what is more obviz ous to fufpe6t., than, a fortu-itous diltating and, fpread'Ing of Light after fome fuch manner, as Des-Caries hath defcribed in his £,,hereal Refraations for explicating the Tayle of a Comet3 or as the Animadver for now fappofes to be effe6led by the S-plitti'ng and RLarifying-ofhi's AEthcreal jpulfics. And to p1 e: i'cnt the fufpicion of any fuck Irregularities, I told you, that I refra&ted-the Light contrary ways with two Prifmes fuccef- fivdy, tdeftroy thereby the RegPular effeds of theflrft Prifmne by the feceond, and to difcovet the Irregular effedh by augment. ingL them with iterated refra&ions. Now, amon gtoher Irs regularities'-)if theflrfl Pri'fme had fpread anid diffipated every ray into an indefinit number of diverg'ing- parts, the fecond fhould in like manner have fpread aiad diffipated every one of thof,e parts into a further indefinite number, whereby the I. mnage would have been Rill more dilated, contrary to the e- venc, And thi's ought to have hapned, becaufe thofe Linear diverging parts depend not on one another for the manner of their Refraaion, but are every one.of them as truly and com-t pleatly Rays as i he. whole was before its lncidence5 as. may ap. pear by intterceptiang them feverally.

The reafonoablenefs of this proceedi'ng wi'll perhaps better appear by acqu-ainting you with this further circumftance, Limetimes placed the fecond Prifme i-n a pofition Tranfverfe to the-flrft, on derign to try, if it would mak-e -the long Image be. come fourfquare by refra&tions crofling-thofe that had drawtn the round Image into a long -one. For, ifa'mongit othcr Ir. r-egLularities the Kcfradien of theffrfl Prifme, did. by Splitting

dilate



(5093) dilate a Linear ray i'nto a Superfici'al., the Crois. refraalions of- that ftcond Prifmae ought by further fplittigt di-late and dra-w that Superficial ray inlto a Pyramidal folid. But, upon tryai,

Ifound it otherwife; the Image beinig as regularly Ob- long as before, and inclini'd to buth the Prifmes at an aeigled 45. degrees.

I tryed alfo all other Pofitions of the fecond P~rifrnie -by tur- niDg the Ends about its middle part; and in nDo cafe cotuld of-, ferve any fuch Irregularity. The Image was ever alike 'incb -

ned to'.both Prifinesj, its Breadth anfweri'ng to the Suns Diao. meter, and i'ts length bei'ng greater or lefs accordingly as tfhe~. R-efraCtions -more or Iefs agreed, or contradi6ted one another,

And by thefe Obfervati'ons, fince the Breadth of the Image was not augmented by theCrofs ref'raation ofthe fecondPrifrne, that refractionl mufl have beenDperform'd7w:thout any fp'littitig or dilating of the ray ; and therefore at lea ft the Light. in ci- dent on th;a~t Prifme muft -be granted an Aggregat of Rays un- eqalaly refrangible in'My fenfe. And fince the image was e-

qulyinclin'd to both Prifmes, and confequently the Refra.. tiotns alik-e i'nboth, it argues,thar they were perfQrm'd accor-

4mg to fome Con f/ant Lawp without any irregularity. To det'ermine thefecondQuj ktbeeiemeetwt.oOir

,re,the Animadverfor referrs to an -Experiment made withn two Wedgel~ boerecited in the M icrog raplyothIneou Mr. Hook Obhcrv. zo 0-Pag. 73. th-e defign of whiich was to produce all Colours out ofavmixture of two. But there i, i conceive, a double defed in this inftacce,, For, it appears niOt, that by this Experiment all colours can be produced out of two;i and, if they could, yet the Inferenice would niot fol- lowe

That all Colours cannot by that Experimenit be,,O p-roduced out of two, will appear by confidering, that the Tlinaure of Alo0es, Which afforded one of thofe Colours, was riot all over ofone uniform colour, but appear'd yellorv near the edg onf t

the Box and red at other pl aces w'here it wastikr afItrdi9g all vari'ety o-fcolours from apale yelltom to a deep", red4 urscar1eli, accordinngto the various th-ickuefs of the liquor, 116An bj -th --

L I l1" 2



folution, of CaPPer, which affiorde-d the- other Colou-r, was o'f ,various B Jew-s and Tndigo'sr. So that iuftead of two, 'c'olours., here is a great variety made ufe of for the pro d w$tion of all o- thers. Thus, for i'njlance, to Droduce all forts of Greens ,the fe- iieral degrees of Yellow and Pale Blew, muff be mixed 5 but to Compou. nd Purpl e:, the Scarlet andf deep Blewp are to be the In-b

Now, If the Animadverfor contend, that all the Reds arnd Tel. lows of the on-e Liquor) or Blew: and InidigoI of the othier, are 90nly various degrees anid dilutinigs of the fame Colour, and not divers colours, that is a Begginlg of the Qtieftion : Anid I ilhould as Coon grant, that the two Thi'rds or sixths in Mufick are but feveral degrees. of the fame found, and not divr founids, Ce'rtainly it is much be,2tter to believe our'Senfes, in-

frIng us, tht_dadyellow are divers colours, and to make it a Fhilofophical fj~re, Why the fame Liquor doth,, according to its- various thicknefs, appear of thole divers colo'urs, than to fuppofe them to.be the. fam'e colour becaufe exhibited by the farwe liquor ? For, if that were a fufficient reafon, th'en Lien' and Yellowv muft alfo be the fame colour, fince they' are both exhibited by the fame T31'in&9ure of NePliritic4Wood. Biut that they are divers ccA ours, you will more ftilly u.nderftand by the reafon, which, inmy judgme)t, is this: %-The Tinature of A4loesr is qiualified to tranfmit mofi eafily the rays indue'd with red, =mo difficultly the rays indued with vi'olet, and with interv,edia

atdgrees oflacility the rays ifidued with intermeditcorsS that where the liquor is very thini, it may fuflice to 'intercept m1off of the violet, anid yet tranfimit moftrof the othe'r colours ; .al1 which together mutt compound a middle Colour, that is, a 'fainrlt yellow. And where it is fo much thicker as alfo to, intercept moft- of the Blew and Green, the remaining Green, Yellow, .And ii-ed, t muftf comrpound an Orenge An hr tetik ncefs is fo great, that fcarce any rays cran paTs through it be. tides thofe indued with I~fd) ,.nuft appear of that Colour,and that Co much the deepe-r and 'obf curer, by how the liquor is thiicker. And the fame may be underftood of the- variouis de-' grees of Blev, exhibited by the- Solutio~n of Copper, by reafoti of its dif oofitioni to intercept RkO moft eafily, anld tr-tnfmilt a deep ~IC w Ox Id'o-Colour molt freelv%



C' 509.5 ) ~1ut, fuppofing that 4ll Colours mnight,accordinrg, to- this e. r

pe.rimetit,be produced out of two by mixture i yet it follows not tha thf w r h nyOginaIlcolours, and that for

a double reafon. Firf/, becaufe thofe two are not themfelve5 Original colours, but compounided ofothers; there beinlg no li'quor nor any other body in nature,whcfe colour in Day-lighlt is wholly un-cormpounded. And then, becaufie, though thofe two were Orig'inal, and all' others might be compounided of them, yet it folilows not, that clhey cannot be otherw-ife prcfd.us ce.d. For I faid, that they had a double Origin, the farme Co- lours to fenfe being in fome cafes compounAded and in1 others un compounded ; and fufficiently declar 'd imythinrd and f'ourth Propofitions,) and in the Concltnfion, by what Properties the one- might be known and diftingudh'it from the other. But, becaufe I fufpe6t by fome Circunaftances, that the Di/1in&1ion. might not be rightly apprehended, I fhall once more declare it, and further e-xplain it by Examples.

That Colour is. Primary or original, which cannot by a;ny Art be, changed, and whofe Rays are no(t a.ike refrangyible: And that compounded, which is changeable into other -colours, and whofe Rays are not afik~e refrang`lblIe. Fo r zf1a nc e, t o kniow, whether the colour of any Green ubje6t be, conmpoutii ded or not, view" it through a Prime, an)d if it appear confefed and the edges tinged With Blewr, Ye/low, or any variety of o- ther colours, then is that Green compounide-d of fiuch C.olcu.rs as at its edges emerge cut of it:. But if it a.ppear d0,inc'1, atnd well defin'd, and entirely Gree-n to the very -edges. wi;thout any other culours emerging, it is of ainOriginal and ut -c,o -n pounded Green. In like maniner, ifa refra6ted beam of I WY> being caft on a white wall, exhibi't a Greeni colouar, to knowvv whether thait be compounided, refraat the be_am- witLh au m'r- terpofed Prifme and it you f'ind. any Difforni:'ty ina othe refirtA~ ations, and -the Green be transform'd into Blew,) YeLTioi,or an,,y

variety,of other colours, you may conclude, 'that it was compounded of thofe which emerge: Bu-t if thle Refra&ions be uniform, and the Green perfift without any chan1ge of colouir, then is it Original and -un -compounided. Auzdthe reafon whf-y

Icall it fo, is, bepcaufe a Green iindued with fuch propertie5 catlm irot be produced boy any mixing of other. cololbirs,



;( 5097 ) Novr,if two GCreen,Objea&s.may to the naked eye appear -of

the famne colours and yet one of them through a prifine teem c,on(ufed and -variegated with other colouirs at the edges, -aud the. other ~dijlinR? and entirely Green ; or, if there imay be two Beams of-Light, -which falling on a white wall do -to the ua- ked eye exhibit -the fame Green colour., anid yet one of them., when tranfmitted through a Prifine, be uniformly and regw

lary rfraledandretinitjs-colour unchanged, and t'he other be irregularly refraated ar id7t divaricateinoamliuef other colours - I fuppofc., thefe two greens will in both cafes be granted of a different Origin and conftitution.Anifb mnixing colours, a green cannot be ccompounded -with the pro.- perties of the Vnckangal Gre,Itik I may cal that an vn-compouinded colour,,efpecially lince its rays are alike refraa. gible,and-uniform in -all refpects.

The fame rule is to be obferv'd ',n examining, whether TQd, Orenge, Y'e/low Bkerv or any other colour be compounded or not. And, by the way, fince all White obje&ts through the Prifme appear confus'd and terminated with colours, Whitenef: muft., according to this diftination, be ever compounded, and that the moft of all colours,. becaufe it is the moft confusk(I and changed by Refra&tioDss

From hence I may take occafion to communi*cate a way for the&impro-vement ofMicro (copes by Kefraation. The way is, by illuminating the Obje6t in a darkned room with Light of any convenient colour not too much compounded: for by that means the Microficope will with diftin2niefs bear a deeper Charge and larger Aperture, efpecially if its conftru&tion be fuch, as I may hereafter defcribe;- for, the advantage in Or- dinary Microfcopes will not be fo fienfible.

10.7'Jat"ienes s a of There remains now the tJird ,t!whitenrfs i~at;t fQuaxre to'be -confider'd, which is,

Whether Whitenefir be an Uniform Colour, or a difTimilar Mixture of all colours ? The Experiment which I broughi to decide it, the Animadverfor thinks may be otherwife ex-plain'd, and fo concludes niothing. But he might e,afily have' fatisfied himfelf by tryinDg, what would 'be t'he re-c fult of a Mixture of all colours. And that very Experiment might have, fatisfied him,' if -he had pleafed t-o eZamine it'by

hc 4



( to96)

the various circtumftances. One Circuznftance I'there decla-- red?~ of which I fee no' notice taken; and it is~ That if any. co.- lour at the Leni be iintercepted, the Whitenef:r wilLbe changed into the other colours:w If all the colours but red be -intercepted, that Red alone in the concourfe.or.crofling of the Rays will not conftitu:11e Whitenefs, but cotin"l,ues as much Red as- before ; and fo of the other colours. So- that the buftnefs is not only to fh2ew, how rays, which before the concourfe exl-;. bit colours, do. in the concourfe exhib-it TFYite but to fh cw, How in the fame place, where the feveral forts of rays apart exhibit feveral colours,a Confufiou of all together tnake White For iuftance, if red alone be firft tranfmirted~ to the paper at the place of concourfe, and Ithen. the.- other colours be let fall on that Red, the Zeftion will be, WVhether tlh:-y c9nvert it ias

-White, by mixing withi it only, as Blew fallinig on Yello)w light is fuppos 'd to compound Green i or, Whethier there be, 1irme further change wrought in the colours by their mnutual aating On one another, untill, like contrary Peripateti-c euali* ties, they become afflhmla ted. And-he that fhall explicate this lfaa Cafe mechanical,y, nmuft conquer a double impoffibil.'ty, He muftfirfl ffiew, that many unlike motions in a Fluid cani by clafhing fo aft on one another, and change each other, as to become one Uniform motion ; and tben, that an Uniform mo. tion can ofit felf, without any new unequal impreffiorns, de- part into a gyreat variety ofmotions regularly un-equlal. And after this he muft further tell me, Why all Obje6h1 appear not of the fame colour, that is, why their colours in the Air- where the rays that coinvey themi every way are confufdly mixt, do not aflimilate-one another and- become Uliiform before they arri've at the Spe&ators eye?

But i'f there be yet any doubting,'ti-; better to put the EventXu on further Circumnftances of the Experiment, than to acquiefce in the poffibiliry of any Hipod$etical Explication. As, for in.i ftance, by trying,)Whar will be the appariti-on of thnefe coiouri in a very quick Confecution of cne another. And thisma b- cafily pertorm'.d by the rapid gyrati'on of'a Wheel with fi:a:-,y spoaks or cogg inispriee,whofe Jnterfticer n thick,; inefles maybe equa andoffuh l rgnfs, ?hat, if thfe Whee-l beinterpofed between the Prifm= and-the v.vh.it ozr!



CS098 of the colours, one halfof the Colours may be intercepted by a fpoake or cogg, and the other half pafs through an iurer, ft;ce. The- Wireel being in th'is poftare, you may firft turn it flowly about, to fee all -the, colouirs fall fu'ccefiively oni the fame place of the paper, held at their aforefaid concourfe j and if you then accelerate its -gyration, until the Confecution of thofe colcurs be-.. fo quick, that you cannot diftinguifhi them feverally, the relfultingr colour willbe- a.,White,nefs per. fe6tly like that, which an gan-refracted be,am o'f Light exhibits,, when in like manner fucceflively interrupted by thel jpoaks or coggs of that circulating? Wheel. And that this Whi'tenef: is produced by a fucceffive Intermixture of the Colours, with4 c~ut their bz-ing affimilated , or reduc'd to any Unifor. mity, is' certainly beyond all doubt, unlefs things that exift not at the fame time may notwithftanding a&t on onae air nother.

T here are yet other Circamftances, by which the Truth might have been decided; as.by viewing the White concourfe of the Colours through another Prifme placid clofe to the eye, by whofe Refraction that whitenefs may appear again tranf. form'd into Colours : And then, to examine their Origin, if an Affiftant intercept any of the colours at the Lens before their airrival at the Whitenefs, the fame colours will vanifh from a- inongft thofe, iato which that Whitenefs is converted by the fecond Prifme. Now, if the rays which difappear be the fame with thofe that are intercepted, then it muff be acknowled: ged, that the fecond Prifme. makes no new colours in2any'rays,, which were not in them before their concourfe at the. paper. Which is a plain indicat'ion, that the rays of feveral colours re- main difticLt from one another in'the WhiteneLs, and that fromn their previousr difpofit'ions are deriv'd the Colours of'the fecond Pri(he. And, by the way, what is faid of their Colors may be applied to their Refrangibility.

The aforefaid Wheel may be alfo herec made Ufe Of 2 nd, if its gyration be neilther ~too quick nor two flo,w, the fuc- cefs ion of the colours may be difccrn'd through the Prifme, whilat to the naked eye of a Byftander they exhibit white,

ThLere is fometh'ing ftiiAi remainaing to be faid of this Experi'. ment



C o099 ) mert. But this, I conceive, is enough to enforce it, and fo to dew cide the controverfy. H. ow- ever, I Ilhall now proceed to IIie w fom other ways of producing whirexefrs by mixts7ef, lince I pCrtwvV~jl c lily ielf', that this Affertion above the refi app-ars 5Para4'1ox:i(%d, .d is with moft difficulty admitted. And becaufe the /fic n(h;;Ir ICfjr4 an inftance of it in Bodies of divers colours, I Ihall hegin wi( that. But in order thereto it muft be con(ider'd, that fucli colourld 4 coJik:s refleft but fome part of the Light incident oni thrn ; as is CViJC!'t by the i 3 PropoFtion ; And therefore the Light refleficd fi-oniaru ta ~1-

gregat of them will be much weakned by the lots of nvmtiy r'ayc WVihence a perfeft and intenfe Whitenefs is not to be exn(tIed, but rather a Colour between thofe of Light and Shldow, or fuich a Gray or Dirty colour as may be made by mixing Wlhite an413Blck together.

And that fuch a Colour will refult, may be collafted from the colour of Dufi found in every corner of an houfe, which hath been obferv'd to confitt of many colour'd particles. There may be alfo produced the like Dirty colour by mixing feveral Pdiiters co/olorf together. And the fame niay be effeaed by Painting a Top (tYich as Boys play with) of divers colours. For, when it is made to circulatc by whipping it, it will appear of fuch a dirty colour*

Now, the Compounding of thefe colours is proper to my pur- pofe, becaufe they differ not from Whitenefs in the Species of co.. lour, but only in degree of Luminoufnefs: which (did not the An- imadverfor concede it) I might thus evince. A beam of the Suns Light being tranfrnitted into a darkned room, if you illuminate a Tlieet of White Paper by that Light, refelted from a body of a-, zy colour, the paper will always appear of the colour of that body, by whofe refle&ed light it is illuminated. If it be 'a red bo. dy, the paper will be reds if a green body, it will be green; and fo of the other colours. The reafon is, that the fibers or threds, of which the paper confifts, are all tranflparent and fpecular ; and fucli fubftances are known to reflef colours without changing them. To know therefore, to what Species of colour a Grey belongs, place a-

ny Gray body(fuppofe a Mixture of Pdinters coloirsr,)in the f'aid Lighit, and the paper, being illuminated by its reflexion, Iflhall appear White. And the f ame thiing will happen, if it be illuminated by reflexion from a blackfubftance,

Thefe therefore are all of one Species ; but yet they fecm diftin:' guifht not only by dcegrees of Luminouthefs, but alfo by fome octher Inequalities, whereby they become more harfh or pleafant. And the ditiinfIion feems to be, that Greys and perhaps Blackjs are miade by an uneven defe~tof Light, conifutin(g as it were of mnly little veins or fItrcams, whichi ditffr either in Luminoulhefs or in thc Unequal di.

M rn m in m f1ikribution



f 5-1Oao) ftribution of divcrfly colour'd rays ; fuch,a ought to be caus'd by I(cflexilon from a M'ixture of white and black, or of d'iverily co. lour'd corpufcies. But when fuch imperfe&Ily mixt L'ight is by afe. con'd Reflcyion from the paper more evenly and uniformly I'blended, it becomes more pleafant, and exhibits a-faint or fliadow'd Whitte- nefs. An htfihlttle irregularilties as thefe may caufe thefe dif. fereaces, is not improbable,, 'if we c-onfider, how much variety t1hay be caufed 'in souinds of thec fame tone by 'irrcgulair and uneven jar-

rings. And befides, thefe differences are fo little, th'at I have fome'1 times doubted, whiethier they be any at all, whien I have con(ilder'd that a Black and White Body being plac'd together, the one in a

itrong light, and thle other in a very faint light, fo proporti. on'd that they might appear equally luminous i it has been dif. ficult to diftinguifh tthem, when view'd at diatancc, unlefs when the

Black feem'd more blewifhi ; and the WAti'te body in a lgtfl fainter, hath, in comparifon of the Black body, 'it felf appear'd Black. ofCmonI bteex w'c

Thiis leads me to anothier wayo onoIAn rienf, hc

is, That, if four or five Bodie ottemr mnn oor;o L~aer ained all over, in feveral parts of 'it, with thofe feveral

colIours in a due proportion, be placed in the faid- Beam of L'ight;- the Lighit, refle&ted from thofe Colours to another Wh'ite paper, held at a convenient diftance, fhiall make that paper appear White; Ifit be held too near the Colours, 'its parts will feemn of thoft colours

th.at are neareft them ; but by removing it further, that all 'its parts

may be equally illuminated by-all the colours, they will be more and

more diluted, until they become, perfte&ly White. And, you may

further obferve, thiat 'if any of the colours be intercepted, the Paper

wIll no longer appear Whi te., but of the other colours which are not

interepted Now,that this Wbiteqefs is a Mixture of thN feverally

colour'd rays, falling confufedly on the papier, I fece no reafon to

doubt of ; becaufe, if the Light became- Uniform and Sim'ilar before

kfitel confufed4y on the paper, it muft much more be Uniform, wvhen

at a greater dii{ance it falls on the Spe&tators eye, and fo the rays;,

which come from feveral colours, would in no qualities differ from

one anothier, but all of them. exLhibit the fame. colour to the Spcf&aa

tar, contrary to what he feCs.

Not much unlike this inflance 'it is,That,if a polifht pilece of Metal

b,e fo placed, that thel colours appear in it as in a Looking. glafs, and hen he etalbe ade rough,, that by a confus'd refleciv

,IoI thofe apparent colours may be bleddtgehr he hl

dIfap ,adb their mumxturtC cauafe the Metal1 to look

~Whitc, a



( 101) But further to enforce this EXperimont - if, inffead of the paper"I

any White Froth, confiffing ofml ubies, be illuminated by re4 fiexion fro-m the aforefaid Colours, it fhall to the naked eye feem White, and yet thirough a good Microfcope the feveral Colours will appe'ar diftin& on the bubles, as if fecn by reflexion from fo many fpberical %furfacess With my naked eye, bei'ngvery near,lIhave alfo~ difcern'd the feveral colours on eachi buble i and yet at a greater difrance, whiere I could not diflinguifli thecm apart, Elhe Froth lbath appear-'d ent-irely Whiite. And at the fame diflance, whien I looied intently, I hiave feen the colours diflindfly on each buble ; and yet, by firain'ing my eyes as 'if I would look at fomethiing far off beyondt them, thereby to render the Vifion confus'd, the F"rothl has appear'd without any other colour than Whitenef's. And what is here faii

of motsnay eafilly be underftood of the Paper or Metal in the fore going Experirments. For, their parts are fpecular bodies, like thcfe B3ubles -.And perhaps with an excel lent Microfcope the Colours may be alfo feen intertvixedly reflec&ed from thiem.

In proportioning the feverallty Colour'd bodies to produce thefe efVeds,there may be fome nicenef's i and it will be more convenient, to make ufe of the colours of thie Prifrne, caft on a Wall, by whofe reflexion the Paper, Metral, Frothi, anid o-ther Whiite fubilances may. be illuminated. And I ufually made my Tryals thtis way, becaufe I could better exclude any fcattcring Lighit from mii'xi'ng withi the colours to d'ilate them. k- To this way of Compounding Whiiteners mlay be referrdj that o. tlier, by Mixing light after it biathi been traj'eded throughi tranfpa. rently colour'd fubtlances. For i4iftance, if no Light be admitted in.. to a roomi but only thiroughi Colour'd glafs, wh1ofe feveral parts are of feveral,colours in a pretty equal proportilon; all White thiings in thec room thiall appear White, 'if they be not lheld too near thec Cl Ia fs. Anld yet this lighit, with whiichlhyaeiluiae,cn not poflibly be uniforml, becaufe, if the IRays, whichi at their entrance are of divers colours, do in their progref's thirough the room fuffer any alteration to be reduced to an Uniformity; the Glaf's would not in the rcmoteil p-arts of the room appear of the very fame colour, whiich it doth when the Spectators eye is very- ne-ar -it :Nor wvould the rays, when tranfmitted i'nto anothier dark roomi thirough a little hole in an oppol'ite door or partition-wall, projeif on a Paper the .Spe.Cicf or reprefentationl of the glafs in its prop-er colours,

And, by the by, this feems a very fit and cogent Inftance of fome othier parts of miy Theory, and particularly of the 1 3 P?ropojitwio. For, in this room allI natural B3odies whatever appear in thieir PrOper colouirs. And allI thie Pb~evarne.4 of colouirs in nature, made eithecr by Refra&tion or withiout It, arc hiere the f'ame as in the Open Air. -Now, the Lighit inthis room being fuich a Difl'irli1ar mixtu.-c,Ali

~mimm m-2



lihave defcrib'd in my 71'eory the Caufres Of all there PI.I4eOr(j4,0j,, muuil. he thec fame thiat I have thiere afiroignd. An'd I fee no raont fuf(~pedt, thiat thie famec Pbx%omcna mlouild hiave other caufes -in the O

pcn Air. The fLuccefs of thi's Experimenit miay be eaifily con jefur'd by the ap.

pear-ances, of thiings in a Chiurch or Chlappel, whiofe windores are of colour'd glafs, i or in thle Open Ar, wheni it is illutlrated with Clouds of various colouirs.

The.re are ye-t otlher way;, by. whiichi I have produced JPh;teneff. a Zs

bv caftng feveral Colours fromi two or miore Prifmies upon thie fame place , /'v Refradiing a B(a ii of Lighit withi two or three Prifmeis fuc- C.cfi riely, to make the divcrging colour-s convxerge again ; by Rlefle- ding. oecolour to anothier; and by looking throucn ga Prifme onan ()bjcf.t of miany colours ; arid, (which is equivalent to the above men. tion'd way of mix ing colours by concave W4gc(ks fiil'd with colour'd liquors,) I hiave obferv'd thie tliadows of a Painited Glafi-wvindow to become white, where tliofe of many colours hiave at a gireat d iftance inter.fered. But yet, for further fatisfadilon, the Anim4dvcrfror may try,, if he pleafe, thle cifeds of four or five of fuch Wcldges filled wvithl l iquors of as m'any feveral colours.

Befides allI thefe,o the Colours of Watcr44'b/les and othecr thiin pellu;, cid fibitanices afford feveral, inflances of Wlhitenefs produced by thecir mixture ., withi one of which I fliall conclude this particular. Let fome Water, in whiichi a conivenient quantity of Soap or wahi- ball i!;

ditflvd,be agitate itFrh,and, after that froth has ftood a whiile withiouit f'urthier agitation, tillI you fee the bubbles, of which it confif{s,begin to break-, there will appear a great varie ty Of colours, allI over the top of eveiy bubble, if you view them near at hand ; but; if you view thiem at fo great a diftancc that you cannot diflinguith the colours one from another.,the Froth will appear perfedtly White.

Thlus muchi concerning the defi'gn I i. 71it the EXPCrimciitum crucis, and fubilance of the Anim-adv'rfor*.

~~fiscb. C~~(onfideratiorns. There are yet fome particulars to be taken notice of, be.

f'ore T conclude; as the denyal of the Experimentwm Crmcis. Oil tbit ) chofe to lay the whiole flrefs of my difeour-fe ; whichi thierefore was thle prinicipal thiing to have been objeeded againfi. But I cannot be convinced ot its infufficiency by a bare denyal without allignings a RIeaf'on for it. I am apt to believe, it has been mifuniderftood ; for othcrwife it would have prevented thec dfcouirfes about Rarifying aiid Splittinig of rays j becaufe thec defign of it i-s, to flicw, that I1 ays of divers colours;, confider'd a part, do at PqIIncidences fuffer Vn eq; iat Refraffions, Withoutibeing fplit, rarificd, or anly ways di" 1,Ated.



In'the Conflderitidns of my firft and 1.Sm atclnrcm.eiet fecond Propofitions, the Ani'madverfor- a. Somr pat;raticuas eommr-d' hath-rendred my Doct,rine of Vn-efmaI utI~rc~drK A&fr4ngribility very imperfect -and maim- ed by explicating it wholly by the Splitting of rays; whereas I chiefly intended it in thofe Refr'acti'ons'thiat are perflorm'd without that fuppos'd Irreguilarity ; fuch as the Experimentumn Cruczu m'ight have in- form,d him of. And, in general I find, that, whiilal he hathi endeavour'd to explicate my Propolitions Hypotbeticglly, the more material fuggeftilons, by whiIch I def'ign-d to recommend them, have efcap'd his confideration ; fuch as are, Thec Unchangeablenefs- of the degree of Refrangibility p*uliar to any fort of rays; thie ftind Analogy between th-e degrees of Rtefrangibi'lity and Colours ;thec Diflindifion between compounded and un-compounded colours thec U-nchangta'4 blenefs of un- comp-ounded colours~ and thec Affertion, that if any one of the Prifmat'ique colours be wholly intercepted, that colour carinot be ,new produced out of the remaining Light by any further V '.frac tionor Retlexion whatfoever. And of what firetngth anid cf-icacy thefe Particulars are for enforcing 'thec Tbeery, I def'ire therefore may be now confider'd..

An Accom-pt of two BookF*

I1. Ottonis de Gsiericke EXPERIMENT A NOVA4 Af/IG,DR,BVR( r. CA,kdeACVO SPATIO, &c- AmftelodarniA. i67z, in fol.

AFter. that the famous Author of this Book hathi made a Naraw of the c'hief Ii'yothefes and Opinions of botli Antient and f( ece Ir.

Aftronomers concerning the Syfteme of the world, and reptrefen?"ed the, great difficulties in the Ptolem,iiqiie, and Tycboxilme, and repeatcd the Anfwers to the Objedtions againfi the Copermic4n hei at larg%. gives us his own Thoug-hts of the Frame and Contlitut ion of tbhey- World; By which worl4dhe underifands- in this Treatife the CotnpIe~r of the Planets, difpofed-and order'd much after the Coperxiocan wavx' the Sun being feated,in the midft, having his Spot-s about hirn1D and moving and influencing all the reft of thec Planets- according to their feveral diftances from him i Sacwrn making the utmoft of all thec Planets, and -the End of thi-s hiis World bcing there, whecre the dif-~ fuCi-ve power and vertue -of the Sun,- thie King and Governor of them.~ all, term'inates ; which bounds he con-je&ures to extend themifelve.,~ beyond Satarn, to thofe Fixt Stars that are of the. nearer raink to S¾- turns Orbe..

Concerning the Bodies-lodg%ed in ~thefePrjlanctsl, he thinks 'itcor" fonant to thie Power and Wifdom of thec G'reat Creator, that tlheta. fMtuld be fuch a -variety of them, as t-Qk fltok each, of thc (aid 'PIAnet-S

In'the Conflderitidns of my firft and 1.Sm atclnrcm.eiet fecond Propofitions, the Ani'madverfor- a. Somr pat;raticuas eommr-d' hath-rendred my Doct,rine of Vn-efmaI utI~rc~drK A&fr4ngribility very imperfect -and maim- ed by explicating it wholly by the Splitting of rays; whereas I chiefly intended it in thofe Refr'acti'ons'thiat are perflorm'd without that fuppos'd Irreguilarity ; fuch as the Experimentumn Cruczu m'ight have in- form,d him of. And, in general I find, that, whiilal he hathi endeavour'd to explicate my Propolitions Hypotbeticglly, the more material fuggeftilons, by whiIch I def'ign-d to recommend them, have efcap'd his confideration ; fuch as are, Thec Unchangeablenefs- of the degree of Refrangibility p*uliar to any fort of rays; thie ftind Analogy between th-e degrees of Rtefrangibi'lity and Colours ;thec Diflindifion between compounded and un-compounded colours thec U-nchangta'4 blenefs of un- comp-ounded colours~ and thec Affertion, that if any one of the Prifmat'ique colours be wholly intercepted, that colour carinot be ,new produced out of the remaining Light by any further V '.frac tionor Retlexion whatfoever. And of what firetngth anid cf-icacy thefe Particulars are for enforcing 'thec Tbeery, I def'ire therefore may be now confider'd..

An Accom-pt of two BookF*

I1. Ottonis de Gsiericke EXPERIMENT A NOVA4 Af/IG,DR,BVR( r. CA,kdeACVO SPATIO, &c- AmftelodarniA. i67z, in fol.

AFter. that the famous Author of this Book hathi made a Naraw of the c'hief Ii'yothefes and Opinions of botli Antient and f( ece Ir.

Aftronomers concerning the Syfteme of the world, and reptrefen?"ed the, great difficulties in the Ptolem,iiqiie, and Tycboxilme, and repeatcd the Anfwers to the Objedtions againfi the Copermic4n hei at larg%. gives us his own Thoug-hts of the Frame and Contlitut ion of tbhey- World; By which worl4dhe underifands- in this Treatife the CotnpIe~r of the Planets, difpofed-and order'd much after the Coperxiocan wavx' the Sun being feated,in the midft, having his Spot-s about hirn1D and moving and influencing all the reft of thec Planets- according to their feveral diftances from him i Sacwrn making the utmoft of all thec Planets, and -the End of thi-s hiis World bcing there, whecre the dif-~ fuCi-ve power and vertue -of the Sun,- thie King and Governor of them.~ all, term'inates ; which bounds he con-je&ures to extend themifelve.,~ beyond Satarn, to thofe Fixt Stars that are of the. nearer raink to S¾- turns Orbe..

Concerning the Bodies-lodg%ed in ~thefePrjlanctsl, he thinks 'itcor" fonant to thie Power and Wifdom of thec G'reat Creator, that tlheta. fMtuld be fuch a -variety of them, as t-Qk fltok each, of thc (aid 'PIAnet-S



mr. isaac newtons answer to some considerations upon his doctrine of light and colors; which...

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