Answer to Huygens' critique of Newton's new theory of light and colours, dated 10 June 1673 Isaac Newton c. 2,573 words The Newton Project Falmer 2014 Newton Project, University of Sussex

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 10 June 1673, in English, c. 2,714 words, 4 pp. 4 pp.

Published in Philosophical Transactions of the Royal Society, No. 96 (21 July 1673), pp. 6087-6092.

An Extract of a Letter lately written by an ingenious person from Paris [Christiaan Huygens] [Philosophical Transactions 96 (21 July 1673)]Mr. Newtons Answer to the foregoing Letter [of Christiaan Huygens] [Philosophical Transactions 96 (21 July 1673)]

in English

 EL/N1/47, Royal Society Library, London, UK UKLondonRoyal Society Library EL/N1/47 10 June 1673 England English Latin French Holograph Henry Oldenburg Unknown Hand Unknown Hand (2) Unknown Cataloguer (1) Unknown Cataloguer (2) ScienceOpticsCorrespondence Transcription by Daniele Cassisa Code audited by Michael Hawkins
47Newton 847

p in d. 28 jun. 73.

Entred LB. 6. 152. Ent (Pr. Trans: 96.)

Mr Newton's Letter upon the his receipt of MonsrMonsieur Hugen'ssHugens's Book de Motu Pendulorum, together wthwith his some considerations upon it; as also an Answer to yethe same M. Hugens's Letter of June 10. 1673; in wchwhich Answer yethe he further explains his New Theory of Light and Colors, and particularly ytthat of Whiteness, etc.

SrSir

I received yoryour letters wthwith M. Hugens kind prsentpresent, wchwhich I have viewed wthwith great satisfaction, finding it full of very subtile & usefull speculations very worthy of yethe Author. I am glad ytthat we are to expect another discours of yethe vis centrifuga, which speculation may prove of good use in naturall Philosophy & Astronomy as well as mechanicks. Thus for instance if the reason why the same side of yethe Moon is ever towards yethe earth be yethe greater conatus of yethe other side to recede from it; it will follow (upon supposition of yethe Earths motion about yethe Sun) that yethe greatest distance of yethe sun from yethe earth is to yethe greatest distance of yethe Moon from yethe earth, not greater then 100000 to 55912 10000 to 56 & therefore the parallax of yethe Sun not less then 5610000 of yethe Parallax of yethe Moon: Because were the sun's distance less in proportion to ytthat of yethe Moon, she would have a greater conatus from yethe sun then from yethe earth. I thought also sometime that yethe moons libration might bdepend upon ytthat her conatus from yethe Sun & Earth compared together, till I apprehended a better cause.

In yethe Demonstration of yethe 8th Proposition, dDe descensu gravium, there seems to be an illegitimate supposition, namely ytthat yethe flexures at B & C do not hinder yethe motion of yethe descending body. For in reality they will hinder it, so ytthat a body wchwhich descends from A shall not acquire so great velocity when arrived to D as one wchwhich descends from E. If this supposition be made becaus a body descending by a curve line meets with no such opposition, & this Proposition is laid down in order to yethe contemplation of motion in curve lines: then it should have been shown that though rectilinear flexures do hinder, yet yethe infinitely little flexures which are in curves, though infinite in number, do not at all hinder the motion. The rectifying curve lines by that way wchwhich M. Hugens calls Evolution, I have been sometimes considering also, & have met wthwith a way of resolving it wchwhich seems more ready & free from yethe trouble of calculatioon then that of M. Hugens. If he please I will send it him. The Problem also is capable of being improved by being propounded thus more generally. "Curvas invenire quotascunqꝫue quarum longitudines cum propositæ alicujus Curvæ longitudine, vel cum area ejus ad datam lineam applicata, comparari possunt." No 6086Pr. 4.Mr Newtons Answer to yethe foregoing letter further explaining his Theory of Light and Colors, and particularly ytthat of Whitenes; together wthwith his continued hopes of perfecting Telescopes by Reflections rather than Refractions. Concerning the bussiness of colours; in my saying that when M. Hugens MonsrMonsieur M. Hugens hath shown how White may be produced out of two uncompounded colours, I will tell him why he can conclude nothing from that; my meaning was, that such a White, (were there any such,) would have different properties from the White, wchwhich I had respect to, when I described my Theory, that is, from yethe white of yethe Sun's immediate light, of yethe ordinary objects of orour senses, & of all white Phænomena that have hitherto faln under my observation. And those different properties would evince it to be of a different constitution: Insomuch that such a production of white would be so far from contradicting, that it would rather illustrate & confirm my Theory; because by yethe difference of that from other whites it would appear that other whites are not compounded of onely two colours like that. And therefore if M. Hugens MonsrMonsieur N. would prove any thing, it is requisite that he do not onely produce out of white two primitive N 6 8. 6.p. 6.n. 6.colours a white wchwhich to yethe naked eye shall appear like other whites, but also shall agree wthwith them in all other properties. But to let you understand wherein such a white would differ from other whites & why from thence it would follow that other whites are otherwise compounded, I shall lay down this position That a compounded colour can be resolved into no more simple colours then those of wchwhich it is compounded. This seems to be self evident, & I have also tryed it severall ways, & particularly by this wchwhich follows: Let α represent an poblong piece of white paper about 12 or 14 of an inch broad & illuminated in a dark room with a mixture of two colours as little compounded cast upon it from two Prisms, suppose a deep blew & scarlet, wchwhich are must severally be as uncompounded as they can conveniently be made. Then at a convenient distance, suppose of six or eight yards, view it through a clear triangular glass or crystall Prism held parallel to yethe paper & you shall see the two colours parted from one another in the fashion of two images of yethe paper as they are represented at β & γ where suppose β the scarlet & γ yethe blew, without green or any other colour between them. Now from the afforesaid Position I deduce these two conclusions: 1 That if there were found out a way to compound white of two simple colours onely, that white would be again resolvable into no more then two. 2dlySecondly That if other whites (as that of the suns light &c) be resolvable into more then two simple colours (as I find by experiment that they are) then they must be compounded of more then two. To make this plainer, suppose that A represents a white body illuminated by a direct beam of the sun transmitted through a small hole into a dark room, & α such another body illuminated by a mixture of two simple colours wchwhich if possible N 67may make it also appear of a white colour exactly like A. Then at a convenient distance view these two whites through a Prism & A will be changed into a series of all colours, rRed, yYellow, gGreen, Blew, Purple with all their intermediate degrees succeeding in order from B to C. But α, according to the afforesaid experiment, will onely yeild those two colours of wchwhich 'twasit was compounded, & those not conterminate like yethe colours at BC but separate from one another, as at β & γ, by means of the different refrangibility of yethe rays to wchwhich they belong. And thus by comparing these two whites, they would appear to be of a different constitution & A to consist of more colours then α. So that what M. Hugens MonsrMonsieur N. contends for, would rather advance my Theory by the access of a new kind of white then conclude against it. But I see no hopes of compounding such a white. As for M. Hugens MonsrMonsieur N. his expression that I maintain my doctrine wthwith some concern I confess it was a little ungratefull to me to meet wthwith objections wchwhich had been answered before, without having the least reason given me why those answers were insufficient. The answers wchwhich I speak of are in yethe Transactions from pag 5093 to pag 5102. And particularly in pag 5095, to show that there are other simple colours besides blew & yellow, I instance in a simple or homogeneal green such as cannot be made by mixing blew & yellow or any other colours. And there also I show why, supposing that all colours might be produced out of two, yet it would not follow that those two are the onely originall colours. The reasons iIn desire you would compare with what hath 13. been now said of white. And so the necessity of all colours to produce white might have appeared by yethe experiment pag 5097 where I say that if any colour at yethe Lens be intercepted yethe whiteness (wchwhich is compounded of them all) will be changed into (yethe result of) the other colours. However, since there seems to have happened some misunderstanding between us, I shall indeavour to explain my self a little further in these things according to the following method. Definitions. 1. I call that Light homogeneal, similar or uniform whose rays are equally refrangible. 2. And that heterogeneal whose rays are unequally refrangible. Note. There are but three affections of light in wchwhich I have observed its rays to differ. viz: Refrangibility, Reflexibility & Colour, & those rays wchwhich agree in refrangibility agree also in the other two & therefore may well be defined homogeneal: especially since men usually call those things homogeneal wchwhich are so in all qualities that come under their knowledg, though in other qualities that their knowledg extends not to there may possibly be some heterogeneity. 3. Those colours I call simple, or homogeneal, which are exhibited by homogeneal light. 4. And those compound or heterogeneal wchwhich are exhibited by heterogeneal light. 5. Different colours I call not, onely the more eminent species, red, yellow, green, blew, purple, but all other gradations the minutest gradations: much after yethe same manner that not onely the more eminent degrees in musick but all yethe least gradations are esteemed different sounds. Propositions. 1. The Sun's light consists of rays differing by indefinite degrees of refrangibility. 2. Rays wchwhich differ in refrangibility, when parted from one another do proportionally differ in the colours wchwhich they exhibit. These two Propositions are matter of fact. 3. There are as many simple or homogeneal colours as degrees of refrangibility. For to every degree of refrangibility belongs a different colour by Prop:Proposition 2. And that colour is simple by Def:Definition 1, & 3. 4. Whiteness such as is in all respects like that of the Sun's immediate light & of all yethe usuall objects of orour senses cannot be compounded of two simple colours alone. For such a composition must be made by rays that have onely two degrees of refrangibility by Def.Definition 1, & 3.; & therefore it cannot be like that of the suns light by Prop.Proposition 1; Nor for yethe same reason like that of ordinary white objects. Oooooo. 60915. That Whiteness in all respects like that of the Sun's immediate light may cannot be compounded of simple colours, there are requisite without an indefinite variety of them. For to such a composition there are requisite rays indued wthwith all yethe indefinite degrees of refrangibility by Prop.Proposition 1. And those infer as many simple colours, by Def.Definition 1 & 3. & Prop.Proposition 2 & 3. To make these a little plainer, I have added also the Propositions that follow. follow. 76. The rays of light do not act on one another in passing through the same Medium. This appeares by several passages in yethe Transactions pag. 5097, 5098, 5100, & 5101, & is capable of further proof. 7. The rays of light suffer not any change of their qualities from refraction. 8. Nor afterwards from the adjacent quiet Medium. These two Propositions are manifest de facto in homogeneal light, whose colour & refrangibility is not at all changeable either by refraction or by the contermination of a quiet Medium. And as for heterogeneal light, it is but an aggregate of severall sorts of homogeneal light no one sort of wchwhich suffers any more alteration then if it were alone becaus the rays act not on one another by Prop.Proposition 6. And therefore the aggregate can suffer none. These two Propositions also might be further proved apart by Experiments, too long to be here described. 9. There can no homogeneal colours be educed out of light by refraction wchwhich were not commixt in it before: Because by PropProposition 7, & 8, Refraction changeth not yethe qualities of yethe rays, but onely separates those wchwhich have divers qualities, by means of their different Refrangibility. 10. The sun's light is an aggregate of an indefinite variety of homogeneal colours; by PropProposition 1, 3, & 9. And hence it is, that I call homogeneal colours also primitive or original. And thus much concerning colours. M. Hugens MonsrMonsieur N. has thought fit to insinuate ytthat yethe aberration of rays (by their different refrangibility) is not so considerable a disadvantage in glasses as I seemed to make be willing to make men beleive when I propounded concave mirrors as yethe onely hopes of perfecting Telescopes. But if he please to take his pen & compute the errors of a Glass & Speculum that N 6 P 2 6092collect rays at equall distances, he will find how much he is mistaken, & that I have not been extravagant, as he imagins, in preferring reflexions. And as for what he adds says of yethe difficulty of yethe praxis I know it is very difficult, & by those ways wchwhich he attempted it I beleive it next to an impossibility unpracticable. But there is a way insinuated in yethe Transactions pag. 3080 whereby by wchwhich it is not improbable but that as much may be done in large Telescopes, as I have thereby done in short ones, but yet not without more then ordinary diligence & curiosity. Pray wthwith these Notes return my thanks to M. Hugens for his book. By a former letter of yorsyours I was a little dubious whether M. Slusius might not apprehend, by wtwhat you wrote to him concerning me, ytthat I pretended to his Method of drawing tangents; untill I understood by M. Collins ytthat you signifyed to him ytthat you thought it here of a later date. For it seems to me that he was acquainted wthwith it some yeares before he printed his Mesolabum & consequently before I understood it. But if it had been otherwise yet since he first imparted it to yethe worl his friends & yethe world, it ought deservedly to be accounted his. As for yethe Methods they seem to be are yethe same, though I beleive derived from different principles. AndBut I know not whether his Principles afford it so generall as mine wchwhich extends to Equations affected wthwith surd terms, wthwithout reducing them to another form. But if you please let this pass. The incongruities you speak of, I pass by. But I must, as formerly, signify to you ytthat I intend to be no further sollicitous about matters of Philosophy. And therefore I hope you will not take it ill if you find me ever refusingrefraining doing any thing more in ytthat kind, or rather ytthat you will favour me in my determination by preventing so far as you can conveniently any objections or other philosophicall letters that may concern me. For your profer about yethe my Quarterly payments I thank you. But I would not have you trouble yoryour self to get them excused if you have not done it already. And now being tired with this long letter, I must in hast write my self YorYour humble Servant I. Newton Cambridg. juneJune 23. 73. H Relation