Letter from Newton to Henry Oldenburg, dated 28 November 1676

SrSir

~~SrSir~~ I esteem my self much obliged to Mr Lucas for his case in making yethe ExꝑtumExperimentum Crucis: but his reasoning I still forbear to meddle with becaus I will not run into any other dispute till I see a full end of what relates to Mr Linus. For if this running from one thing to another be allowed we shall never come to a conclusion in any thing. Mr Lucas would represent yethe question about yethe length of yethe Image improper if not preposterous to spend time on till yethe different refrangibility of rays differently coloured be agreeed on. That is he would wholly decline it. For by his asserting demonstrations against me it's plain he thinks yethe different refrangibility will never be agreed on. But however he represents it others may thinks it more preposterous to run from one thing to another before we come to a conclusion & esteem yethe question as materiall now as when twas Started & thought worthy of being put upon Mr Linus's account. The examining my Theory is but a new attempted digression. 'Tis yethe truth of my experiments which is yethe business in hand. On this my Theory depends, & which is of more consequence, yethe credit of my being wary, accurate and faithfull in yethe reports I have made or shall make of experiments in any subject, seeing ytthat a trip in any one will bring all yethe rest into suspicion. And besides yethe full truth of what I have said in my first letter of colours about yethe impossibility of promoting much further Catadioptrical Telescopes depends on yethe length of yethe Image, and is of great moment, be yethe Theory true or or false Wherefore though I was willing to let pass any way of concluding Mr Linus's proper objection; presuming ytthat he would have concluded it himself as much to his own honour as to my satisfaction had he lived to see yethe exꝑtexpert duely tryed: yet I cannot admit to have yethe remainder of yethe business wholly jostled out by a new Subject. Let what relates to Mr Linus be made an end of & then Mr Lucas may begin upon his own scoare as soon as he pleases.

And becaus I hope his next letter will grant me what remains of yethe contended exꝑtexpert, ytthat there be no further delays, I desire him to send me word which of his experiments he would have me begin with. For though whilst he urged his experiments but as exceptions which he hoped would not be unwelcome, I esteemed esteemed them not so proper to be considered in competition wthwith a Demonstration, yet now he adds weight to their number presenting them for Demonstrations, & one Demonstration deservesesdeserves to be considered as well as another: I intend to take into consideration one or two of them which he shall recommend for yethe best: & when there appears to be no weight in those let others judge what there may be in yethe numbers of yethe rest. But I shall consider but one at a time, ytthat neither of us may have room to run up and down in till we are agreed. You know how weary I have profest myself of this subject even before Mr Linus's first letter came to me; & as much might be gathered by my deferring to meddle with ytthat business for a year together till I was urged to it, & also by some expressions in my last letter. And therefore I hope Mr Lucas will not press me beyond one, or two of his objections though afterwards I may say something ~~more~~ of ~~that~~ yethe rest.

But becaus we stick at yethe Question about yethe length of yethe Image, to prevent unnecessary demurrs I cannot but say something more of ytthat. Mr Lucas tells you when convenience of weather, and Prismes shall serve he shall be glad to see how farr yethe length of yethe Spectrūum made by Prismes of different angle. sett down in my last letter will agree with observations at Liege: but yet ytthat he thinks it improper to spend time on ytthat precise length of yethe Spectrūum by severall Prisms before yethe different refrangibility of rays be agreed on. That is he still turns off yethe Quæstion in hand to a new one, as if yethe business now was onely to examin my last lLetter. The length made by several angles I sent not to create new unnecessary disputes but onely to accommodate my oObservations to yethe Prisms of men abroad to try and judge between us, and therefore though Mr Lucas may try them also for his private satisfaction, yet I think it as improper as he does to spend time on preciseness of all those The Question in hand is this. TWhether yethe Image in yethe eExperiment set down set down in my first letter about colours printed in yethe Transactions Num 80 could be five times longer than broad as I have there exprest it, or but three or at most three and a half as Mr Lucas has [39] has represented it. To this I desire a direct answer: which I hope will be so free as (wthoutwithout putting me to any further wayies of justifying myself) may take off all suspicion of my misrepresenting matter of fact. If Mr Lucas hath not yet procured a Prism with an angle about yethe bigness of mine there used, & with sides not at all concave, but plain or onely a very little convex, let him onely upon yethe receipt of this signify without any insinuation of suspicion ytthat he sees no reason to oppose or question me further upon yethe experiments they have hitherto made and proceeded on, & I shall acquiesce and begin wthwith his oObjections; & this I think he need not stick at seeing he is now to seek for a due Prism to try yethe ExptExperiment wthwith, & in his former letter where he started yethe Question he set down yethe angle of his Prism less than ytthat in my experiment he wrote against: not attending to this pPrinciple ytthat yethe greater angle makes yethe greater length of yethe Image. Yet I cannot but say ytthat I had rather have my exptexperiment confirm'd by his assent than yethe suspicion only taken of. But yet I leave it to him to conclude yethe buisiness any way, so it be but concluded before any thing els be urged upon me. So I am

YorYour humble Servant

~~Is. Newton~~

Is. Newton.

In yethe last clause of Mr Lucas's Letter where he says ytthat I calculated yethe length of yethe Spectrum by divers Prisms: the word calculate misrepresentes what I sent him; yethe force of wchwhich lay in their being not calculated but measured. I desire Mr Lucas would be very exact in measuring yethe angle of yethe Prism he tries yethe ExꝑtExperiment with: for I must confess I cannot but suspect ytthat yethe angle he Set down before (I imagin by guess) for 60 degrees, was nearer to 50. I could wish we knew yethe just measure of that angle. If it be as I conjecture it would end yethe Question. The way to examin whether an angle be justly measured is to measure all yethe 3three angles of yethe Prism by yethe same method & see if their summ be 180 degrees.

Cambridge. Nov. 28. 1676.

YorsYours about Planting I received, b& am obliged to you & yoryour Friends for making orour way to Mr Austin. He whom I should communicate yoryour letter to is at prsentpresent out of town, so ytthat at prsentpresent I can say nothing to it in particular.