Sr I esteem my self much obliged to Mr Lucas for his case in making ye Exꝑtum 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 ye question about ye length of ye Image improper if not preposterous to spend time on till ye 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 ye 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 ye question as materiall now as when twas Started & thought worthy of being put upon Mr Linus's account. The examin\in/g my Theory is but a new attempted digression. 'Tis ye truth of my experiments which is ye business in hand. On this my Theory depends, & which is of more consequence, ye credit of my being wary, accurate and faithfull in ye reports I have made or shall make of experiments in any subject, seeing yt a trip in any one will bring all ye rest into suspicion. {illeg}|A|nd besides ye full truth of what I have said in my first letter of colours about ye impossibility of promoting much further Catadioptrical Telescopes depends on ye leng\t/h of ye Image, and is of great moment, be ye Theory true {or} \or/ fal{se} Wherefore though I was willing to let pass any way of concluding Mr Linus's proper objection; presuming yt he would have concluded it himself as much to his own honour as to my satisfaction had he lived to see ye exꝑt duely tryed: yet I cannot admit to have ye remainder of ye 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 scoar{e} as soon as he pleases.

And becaus I hope his next letter will grant me what remains of ye contended exꝑt, yt 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 <38v> esteemed them not so proper to be considered in competition wth a Demonstration, yet now he adds weight to their number presenting them for Demonstrations, & one Demonstration deserv\e/s|e||s| {sic} to be considered as well as another: I intend to take into consideration one or two of them which he shall recommend for ye best: & when there appears to be no weight in those let others judge what there may be in ye numbers of ye rest. But I shall consider but one \at a/ {illeg}time, yt 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 yt 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 ye rest.

But becaus we stick at ye Question about ye length of ye Image, to prevent unnecessary demurrs I cannot but say something more of {yt}. Mr Lucas tells you when convenience of weather, and Prismes shall serve he shall be glad to see how farr ye length of ye Spectrū made by Prismes of different angle. sett down in my last letter will agree with observations at Liege: but yet yt he thinks it improper to spend time on yt precise length of ye Spectrū by severall Prisms before ye different refrangibility of rays be agreed on. That is he still tu\r/n{illeg}|s| off ye Quæstion in hand to a new one, as if ye business now was onely to examin{illeg} my last l|L|etter. The length made by several angles I sent not to create new unnecessary disputes but one|l|y to accommodate my o|O|bservations to ye Prisms of men abroad to try and judge between us, and therefore thoug\h/ 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 {illeg}      The Question in hand is this. T|W|hether ye Image \{illeg}/ in ye e|E|xperiment set down set down in my first letter about colours printed in ye 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 <39r> has represented it. To this I desire a direct answer: which I hope will be so free as (wthout putting me to any further way|ie|s 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 ye bigness of mine there used, & with sides not at all concave, but plain or onely a very little convex, let him onely upon ye receipt of this signify without any insinuation of suspicion yt he sees no reason to oppose or question me further upon ye experiments they have hitherto made and proceeded on, & I shall acquiesce and begin wth his o|O|bjections; & this I think he need not stick at seeing he is now to seek for a due Prism to try ye Expt wth, & in his former letter where he started ye Question he set down ye angle of his Prism less than yt in my experiment he wrote against: not attending to this p|P|rinciple yt ye greater angle makes ye greater leng\t/h of ye Image. Yet I cannot but say yt I had rather have my expt confirm'd by his assent {illeg} than ye suspicion only taken of. But yet I leave it to him to conclude ye buisiness any way, so it be but concluded before any thing els be urged upon me. So I am

Yor humble Servant

Is. Newton

Is. Newton.

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

Cambridge. Nov. 28. 1676.

Yors about Planting I r{illeg}|e|ceived, b{illeg}|&| am obliged to you & yor Friends for making or way to Mr Austin. He whom I should communicate yor letter to is at prsent out of town, so yt at prsent I can say nothing to it in particular.


For Henry Oldenburg Es
at his house about ye middle of ye old Pal-mail in


Of this I sent a copy to Mr Lucas {illeg}|D|e{illeg}|c|emb. 4. 76.


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