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Cambridge
Novemb. 13th 1675.

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[1]Mr Isaac Newtons Reflexions \Considerations/ /Letter\ /Answer\ on ye Re former Reply \to Mr Line's Letter/; together wth further directions, how to make the Experiments controverted aright written to ye Publisher \Mr Oldenburg/ from Cambridge Novemb. 13. 1675.

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Entd LB. 7. 275.
Pr. Trans. 121.

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Sr

When you shew'd me Mr Line's second Letter, I remember I told you, yt I thought an answer in writing would be insignificant because ye dispute was not about any ratiocination, but my veracity in relating an experiment, wch he de{illeg}|n|ies will succeed as it is described in my printed letters: For this is to be decided not by discourse, but new tryall of ye experiment. What it is yt imposes upon Mr Line I cannot imagin, but I suspect he has not tryed ye expt since he acquainted himself wth my Theory, but depends upon his old notions taken up before he had any hint given to observe ye figure of ye coloured image. I shall desire him therefore, before he returns any answer, to try it once more for his satisfaction & that according to this manner.

Let him take any Prism & hold it so that its axis may be perpendicular to ye suns rays, & in this posture let it be placed as close as may be to ye hole through wch ye sun shines into a dark room, wch hole may be about ye bigness of a pease. Then let him turn ye Prism slowly about its axis & he shall see ye colours move upon ye opposite wall first towards that place to wch ye sun's direct light would pass if ye Prism were taken away & then back again. When t{illeg}|he|y {illeg}|a|re in ye middle of these two contrary motions, that is when they are nearest that place to wch ye sun's direct light would p{illeg} ray tends, there let him stop, for then are ye rays equally refracted on both sides ye {illeg}|P|rism. In this posture of ye Prism let him observe ye figure of ye colours & he shal find it not round as he contends, but oblong & so much ye more oblong{illeg} as ye angl{illeg}|e| of ye Prism, comprehended by ye refracting plains, is bigger & ye wall on wch ye {illeg}age is \colours are/ cast, more distant from ye Prism: After this the colours red, yellow, green, blew{,} purple succeeding in order no{illeg}|t| from one side of ye figure to ye other as in Mr Line's conjecture, but from one end to ye other. After & ye length of ye figure being not parall{illeg}|e|l but transvers to ye axis of ye Prism. After this manner I used to try ye [2]Expt; For I have try'd it often; sometimes to observe ye circumstances of it, sometimes in order to further expts & sometimes to show it to others, & in all my tryals ye success was ye sam{illeg}|e|. But whereas Mr Line thinks I tryed it in a cloudy d{illeg}|a|y & placed ye Prism at a gt distance from ye hole of ye window: ye experiment will not succeed well if ye day be not cleare & ye Prism placed close to ye hole, or so neare \at least/ that all ye sun's light that comes from {ye h}ole may pass through ye Prism <48v> also, so as to appear in a round form if intercepted by a paper immediately after it has past ye Prism.

When Mr Line has tryed this I could wish he {illeg}|w|ould proceed a little further to try yt wch I calld ye Experimentum Crucis, seing (if I misremember not) he {illeg} denies that as well as ye other. For when he has tryed ym (wch by his denying them I know he has not done yet as they should be tryed) I presume he will rest satisfied.

Three or four days after you gave me a sight of Mr Line's second Letter, I remember I thereupon showd ye first of these two experiments to yt Gentleman whom you found wth me when you gave me that visit: & whilst I was showing it to him, Mr Hill \{illeg}. {illeg}. {illeg}. {illeg}. A. H./ (a member of ye R. Society,) whom you are well acquainted wth, came in & I showed it to him also. And you may remember, yt Mr Hook \R. H. R. H. {illeg} D./ two or {illeg}|t|hree years agoe in a letter read before ye R. Society, & transmitted to me, gave testimony not only to ye Exper{illeg}|i|ments questioned by Mr Line, but to all those set down in my first Letter about c|C|olours, as having tryed them himself; & when you read Mr Line's letter at a meeting of ye sd Society, & was pleasd to do me ye favour to ppound ye e|E|xperimt to be tryed in their presence, Mr Hook \R. H./ spake of it to them as a thing not to be questiond. But if it have not yet been tryed before them, & any of them, upon {illeg}|M|r Line's confidence, doubt of it, I promise when I shall have ye happines to be at any more of their Assemblies, upon ye least hint, to show 'em ye tryal of it; & I hope, I shall not be troublesome, because it may be tryed (though {illeg}|n|ot so perfectly) even wthout darkning a room, or ye expence of any more time then half a qter of an hower; although, if Mr Line persist in his denyal of it, I could wish it might be tryed sooner there, than I shall have an opportunity to enjoy be among them.

I have returnd you Mr Line's le{illeg}|tt|er. It came to my hands but this week the Gentleman by whom you sent it having not yet been at Cambridge but transmitting it to me from Oxford.

I had some thoughts of writing a further discours about colours to be read at one of yor Assemblies, but find it yet against ye grain to write put pen to paper any more on yt subject. But however I have one discourse by me of yt subject written when I sent my first letters to you about colours & of wch I then gave you notice. This you may comma{nd} wn you think it will be convenient if y{illeg}|e| custome of {illeg}|r|eading weekly discourses still continue. In ye meane while I am Sr

Yor humble Serv{illeg}t

Is. Newton.

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An Extract

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[1] Ttt 3 501

[2] Tttt 4 502