Extracts of Two Letters, the One of Mr. Newton, Concerning the Number of Colors, and theNecessity of Mixing Them All for the Production of White, &c; the Other, of a Philosopherat Paris, by Way of Answer to the FormerAuthor(s): Isaac NewtonSource: Philosophical Transactions (1665-1678), Vol. 8 (1673), pp. 6108-6112Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/101373 .

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( 6io8 )

An t_xtraJil of Mr, Iraac Newton's Letter, written to the Pub/i.o Jhier from Cambrildge April 3. i1673. concernuPig the Number of Colors, and the Necefjityv of mixing them all for the pro- dultioni of White - as a!Po touchiocg the caufe why a Piilure caft by Gl4je.s into a dark!zed room appearsfo diJiind notwithftand. ing its Irregutar refraThon : (Wfhich Letter, being an Imme- diat antfwer to that from Paris,prntaed N½.6,p,6o86. ofihefe Tra 6ts, Ihould <0f, if it had not been mif-laid, have immedi ately folowed tbe fame.)

IT feems to me,that N. takes an im-proper way of examinling thle inature of ColorswhiXit he proceeds upon compouniding

thofe that are already compouinded ; as he doth in tht former part of his Letter, Perhaps he would foonvr fatisfie himfeif by refolvilng Light into Colors, as far as may be doieby Art,, and then, by examining the propertiles of thofe~ cobtArs apart, and af'terwards by tryn ch ff&~4s of rc-conjoim too mocre or 3ll ofthofc5 and lafily3, by feparat'ing them ag,~kn to

examine, what changes that rcoconjunation h-ad wrought i'n them. This,I conffiTs, willl prove a ted'ious and diffic..upt ta,k to do it as it ought to be done;- but I could not be fati'fi-d, t'lilIh4d gone thruough it Howcver,l only propoundityand leave evvry mani to his own method.

As to the Contenits of his Letter,! conceive,my former An. fwer to thegm~ere about the Nummber of Colors is fufficienr,which was to this ectf<Ct T'hat all Colors cannot praatically be dern- ved out of the Yel'low and B/emr, and confkquently that thofe H.ypothefes are ground lcfs whiich implt' they may. If you ask, What colors cantiet be derived out of yellow at-d blew? Ian. fwe.r.none ofallitchofe whijch I defin'd t'o bt OriginaJI; and if he can fbiew by experiment,how they mayJ! willl acknowledge my felt in an error. Nor is it eafier to f'rame an Hjjah4efis by

fhmi ng o tlIy two-7 Original colors rather tha anlindfiuit variet y uilefs'it beecafier to fuppofe, that there are but two

flg~re',izs d dc-grees of velocity or force of the iL.hereal corp,uicles or pulf'es,rdAthtr than indefitu variety; whi..;ch cer- tainly wouild be a harfh fruppofit"ion. No min wonder-s at the indetialit varilety of Waves of the Sca,or of fands on thu fimcre;

bur.,

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( 61c9 ) but, were they all but two fizes, it would be a very puzling phaenomenon. And I hould think it asunaccountable, if the feveral parts or corpufcles, of which a Ihining body confifs, which rmuf be fuppos'd of various figures, fizes and motions, fiould imprcfs but two forts of motion on the adjacent A/Ethe- real medium, or any other way beget but two forts of Rays. But to examine,how Colors may be explain'd hypotheically is bcfides my purpofe. I never intended to Ibew, wherein con- fifts the Nature and Difference of colors,but only to fhew, that defaao they are Original and Immutable qualities of the Rays which exhibit them ; and to leave it to others to explicate by Mechanical HypotheleC the Nature and Difference of thofe qualities: which I take to be no difficult matter. But I would not be underflood, as if their Difference confifted in the Dif- ferent Refrangibiiity ofthofe rays ; for,that different Refran- gibility conduces to their produ&ion no otherwife, than by ftparating the Rays whofe qualities they are. Whence it is that the fame Rays exhibit the fame Colors when feparated by any other means as by their different cflcxibiliLy, a quality not yet difcourfed of.

In the next particular, where N. would fhew, that it is not neceffary to mix all Colors for the produCtion of White; the mixture of Te/low, Green and Blew, without Red and Violet, which hepropounds for that end, will not produce white,but Green 5 and the brighteft part of the Yellow will afford no o- ther colour but Yellow, if the Experiment be made in a room well darkn'd,as it ought i becaufe the Colour'd light is much weaken'd by the Reflexion, and fo apt to be diluted by the mixing of any ,t-her fcattering light. But yet there is an Ex- periment or tv : mention'd in my Letter in the TraiJations Nnb.,8, by which I have prcduced White out of two colors alone,and tha- varioufly; as out of Orange and a full Blew, and out of Red anm pale Blew) and out of rellow and Violet, as alfo out f other ?nirs ofIntermediat colors. The molt conveni- ent Experimt. : for performing this,was that ofcafting the co- lors of one PF.ile up< n thofe ofanother, after a due manner. But w-hat N. C;I deduce; from hence, I fee not. For the two cclors were , mapounded of all others, and Co the refulting White, (to .peak properly,) was compounded of them all,

q qqqqq 2 and

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( 6jto ) and only de-compounded of thofe two. For i*ftjance, the orange was compounded of Red, Oranige, Yellow and fome G;reen - and the Blerv,of Vio1et.,full1 Blew,light Bllew,and fume Green,with all thei'r Intermnediat degrees anid conifiLquently the Orange and Blew together made an Aggregate of all co. lors to contlitute the White. Thus,if one mix red,orange and yellow Powders to make an Orange- i arid green,blew anid vi-' olet colors to) ma'ke a Blew;5 and laffly, the two mixtures,, to

miake a Grey ; that Grty,though de- compounded of no more than two Mixtures, is yet compoundcd of all.the fix Powders, as truly as if the powders had been all mixt at once.

This is fo plain, that J coniceive there can be no further fcru pie; efpecially to them who.know how to cxamine,w he- ther a colour be -fimple or -compounded,and of what colors it is compounded; which having xplaitied inaoher place, I need not now repeat. If therefo~re-N would conclude any thing) he miuft~ (hew, how White may be produced out of two Va. c,otpounded colors A which when hehbath done., I i4lfltl fin hr tell him,why hit can conclulde. nothing fro-m that.But I bel'ieve, there cannot bte found arn Experiment of that kind;I bec,,,ufe., as I remember,1 once tryed,by gradual fucceflion,the mixture of all Pairs of Una-compounded colors; and, tht.ugh fo.mC of them were paler, and nearer to Wh-ite, than others, yet none could be truly call'dWhite, But it beingfCome years fin-e this tryal was moodcs I remember not well the circumfiances, and the-refcGre recommend it to others to be tryed again.

In the laft place, had Ithought, the Difiin&tnefisof the Pi-. ctrwih (fo ijiance) a-Twelf foot ObJect'glafs cafth 1into

a darket-i'd room,to be fo contrr-ry to me as N. is pleaf7'd to af-. firm,l mbould have waved my Thtifwy ini that point bef6re I propounoded it. For) tha-.t I had th,-~ught on that diffi.-ulty ,

5eeumb ~ YOU may e4iiy guicis by an expreffi'il1, fume- *SeNum. S~.wherc in my firfi Lctcer r( to h'is purpo-fe;That ~~ I

~wond--i-d,how 'TeL fopes couild be brought to fo great perfr&ion byA lKcfr icti url wh' ch wvere- to Irregular. But2ro take away rhe dlffiCu~i~yj muit acqaimnt y- lfirll,Thar, though I put the greateft Lateral error of the ray3s fromn oine anothtr to be aboUlf 1- of the: G'af:l~ ditmercr; yet "herTgrea- ter error from the Po'ints on whi'ch. th-y ought to

'_ ll, wil.) be

but

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but'~ of that dameter: And then,that-the rays, whofe error is fo greatr,are but very few in comnparilfon to thofe, which are refraated more Juftlv 5 for, the rays which fall upon the mid- die-parts of the Glafs, are refractecd with fufficilent exa&nefs, as a)lfo are tho{eth3t fall ne-ar the perimeter and have a mean de- gree of Rcfrangibility ; So that there- remain only the rays, which fall near the perimeter anid are mofi or leaji refrangible to caufet any fetofible c rnfution in the Pi'lure. And thefe are yet fCo mucii farther wcakeCn'd by the greater fpace, through which the-y are fi6vttr'd, that the Light which falls on-the due poi;)t, is infiitcly mi)re deliCe than that which falls ',on any other Pown rounid abr%ur it. Which though it miy feem a Pitar&x ye iscrinly demonftrable. Yea, although the Light, whichl pdffs thirough the middle parts of the Glafs, were wholly intercepted, yet would the rema'in'ing light con- vene it.finitdly more denfe at the duie points,than at other pla- ces. Ar-d by this excefs of Dezdfity,the Light, wh'ich falls in or invifibly near the jufk po'int ,may'l conceive, firike the (en.. forium f'o vigoroufly,that the imnprefs of the weak light, which errs round about ir,fhbll,in comparifon, not be fitrooag enough to be aoimad'verted.or to caufe anly more fenfible confufibn in the Pi&ure than is found by Experience.

ThisjI conceive,is enough to fhew,Why the Pi&ure appears fo d'ifiioQknotwithftandiDg the Irregul4r refrat§tion. But, if thi's fatisfie nor,N. mnay try,if he plcafe,how dif?kin& the Pi&lure will appear,when all the Leovs is cover'd excepting a little hole next its edge on one fide, only : And if in this c&fchIm- plcafe to meafure thei brcadth of the colors thus made at the edge of the Suns p'iaure, he will perhaps find it to approach neater to my proportion than he expc&s.

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( 6112 )

An Alvtfwer (to tbe former- Letter, written to the PubljJler junie Io.16 7 3 bY the 'ame Pari/ian Phi<eorhfr. that, was lais4Jj2,'id to

have written the Letter alread.y extant in N0, 96,p.6c86.

~Ouchirvg thlc Solurions,givcn by M Newton tu t'he J crupA!es by mo~ propo)s'd abreut fhibTtieory ok'Co-

lorsyhere were Mattcr tc aifwcr the-m, and to form iiew diffi- culties ; but fcv.ir-g thit.- h?naimtair~shis -epiiion with fo much conc,,rn5I liU niot to difpute. But wh;at nmeaios it,I pray, that he Lith ; Though Ifrouldft3ew him,t hat the UAkite couldbeprodus- ced of fnly twoVni-compoundcd colors ,yet I could conclude not hihg

from that. And ycc ht: hath z,ffirm'd 'in P. 3c8 3 of the Trauf- at/ions, that to cumpofe the White, all primitive colors arc nece ffitry.

As to the mnanner,where by he reconciles the effea of Con- vex glaffes for fo well afk'mbling the rays, with what he efla4 blifhes concerning the different refrangibility, I- am fatisfied with it - but then he is alfo to acknowledge,that this aberrati- on of the rays is not fo difadvantag'ious to Optio glaffes as he feems to have been willing to make us believe, when he pro- pofed Concave fpeculums as the only hopes of pt rfeLtiig Tele- fcopes. H'is invention certainly was very good 5 but,,as fi~r as I could perceive by experience, the defta of the Matter ren- ders it as impoffiblc to execute, as the difficulty of' th'- Form obftru&s the ufe of the Hyperbole ofCM,Des-Cartesr: So that, 'in my opinDion,we mnufi ftick to our Spheric Glaffes,whom we are already fo much obliged to,and that are yet capable of great- er perfeafion,as well by increafing the length of Tektfcopcs,aB by corrcaing the nature of Glafs i't felf so far He.

7io thisr Letter is to be referr'd that, which is already extant in N.96.P.6087. Os bein an Afwer thereto,~

A Letter

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