SrSir

When you shew'ded me Mr Line's second Letter, I remember I told you, ytthat I thought an answer in writing would be insignificant because yethe dispute was not about any ratiocination, but my veracity in relating an experiment, wchwhich he denies will succeed as it is described in my printed letters: For this is to be decided not by discourse, but new tryall of yethe experiment. What it is ytthat imposes upon Mr Line I cannot imagin, but I suspect he has not tryed yethe exptexperiment since he acquainted himself wthwith my Theory, but depends upon his old notions taken up before he had any hint given to observe yethe figure of yethe 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 yethe suns rays, & in this posture let it be placed as close as may be to yethe hole through wchwhich yethe sun shines into a dark room, wchwhich hole may be about yethe bigness of a pease. Then let him turn yethe Prism slowly about its axis & he shall see yethe colours move upon yethe opposite wall first towards that place to wchwhich yethe sun's direct light would pass if yethe Prism were taken away & then back again. When they are in yethe middle of these two contrary motions, that is when they are nearest that place to wchwhich yethe sun's direct light would p ray tends, there let him stop, for then are yethe rays equally refracted on both sides yethe Prism. In this posture of yethe Prism let him observe yethe figure of yethe colours & he shal find it not round as he contends, but oblong & so much yethe more oblong as yethe angle of yethe Prism, comprehended by yethe refracting plains, is bigger & yethe wall on wchwhich yethe age is colours are cast, more distant from yethe Prism: After this the colours red, yellow, green, blew, purple succeeding in order not from one side of yethe figure to yethe other as in Mr Line's conjecture, but from one end to yethe other. After & yethe length of yethe figure being not parallel but transvers to yethe axis of yethe Prism. After this manner I used to try yethe Tttt 4 502ExptExperiment; For I have try'ded it often; sometimes to observe yethe circumstances of it, sometimes in order to further exptsexperiments & sometimes to show it to others, & in all my tryals yethe success was yethe same. But whereas Mr Line thinks I tryed it in a cloudy day & placed yethe Prism at a gtgreat distance from yethe hole of yethe window: yethe experiment will not succeed well if yethe day be not cleare & yethe Prism placed close to yethe hole, or so neare at least that all yethe sun's light that comes from yethe hole may pass through yethe Prism also, also, so as to appear in a round form if intercepted by a paper immediately after it has past yethe Prism.

When Mr Line has tryed this I could wish he would proceed a little further to try ytthat wchwhich I calld yethe Experimentum Crucis, seing (if I misremember not) he denies that as well as yethe other. For when he has tryed ymthem (wchwhich 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 yethe first of these two experiments to ytthat Gentleman whom you found wthwith me when you gave me that visit: & whilst I was showing it to him, Mr Hill . . . . A. H. (a member of yethe R. Society,) whom you are well acquainted wthwith, came in & I showed it to him also. And you may remember, ytthat Mr Hook R. H. R. H. D. two or three years agoe in a letter read before yethe R. Society, & transmitted to me, gave testimony not only to yethe Experiments questioned by Mr Line, but to all those set down in my first Letter about cColours, as having tryed them himself; & when you read Mr Line's letter at a meeting of yethe sdsaid Society, & was pleasd to do me yethe favour to ppropound yethe eExperimtExperiment 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 Mr Line's confidence, doubt of it, I promise when I shall have yethe happines to be at any more of their Assemblies, upon yethe least hint, to show 'emthem yethe tryal of it; & I hope, I shall not be troublesome, because it may be tryed (though not so perfectly) even wthwithout darkning a room, or yethe expence of any more time then half a qterquarter 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 letter. 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 yoryour Assemblies, but find it yet against yethe grain to write put pen to paper any more on ytthat subject. But however I have one discourse by me of ytthat subject written when I sent my first letters to you about colours & of wchwhich I then gave you notice. This you may command wnwhen you think it will be convenient if yethe custome of reading weekly discourses still continue. In yethe meane while I am SrSir

YorYour humble ServtServant

Is. Newton.