to Oldenburg

Feb. 19. 1676.

SrSir

I received yoryour letter conteining ytthat of Mr Lucas & this afternoon another from yoryour self by Mr White wthwith yethe last Transactions, for wchwhich I thank you. [N. 131. p. 785. Exp. 13.The speculation of yethe figures of confining liquors. I am apt to think wthwith Mr Boyll may by degrees administer very considerable thoughts to such as are contriving mechanical systems systems of Phy: but yethe last of yethe Phænomena, where an oyle lighter ynthan sptspirit of wine poured on yethe Alcaly deliquated made a very specular superficies in their confine, being more in yethe way of theseings I have been sometimes considering; I shal rather prsentpresent you wthwith anwtwhat thoughts occur to me about ytthat, presuming you will excuse my randome guesses] because it is by yoryour desire ytthat I write them.

[I imagin then that as sptspirit of wine, though lighter ynthan water, is yet a more strongly refracting liquor; so yethe refractive density of this lighter oyle may be still greater, so as much to exceed ytthat of yethe Alcalizate liquor. And if so then yethe propounded Phonomenon may keep analogy wthwith that of other reflecting superficies. For yethe reflexive power of any superficies (so far as my observation reaches) is proportional to yethe difference of yethe refractive densities of the bordering mediums. And (wchwhich I take to be yethe prsentpresent case,) yethe same superficies reflexcts much more strongly towards on ytthat side towards yethe denser (I mean yethe more strongly refractive) medium then towards yethe rarer on yethe opposite side. Butesidtoes when that side towards yethe denser Medium is viewed, obliquely enough, it always reflects all yethe light to yethe eye as much as quicksilver does: & yethe greater yethe difference of yethe refractive density of yethe mediums is, yethe less obliquity is requisite to produce this phænomenon. This total reflexion holds toin all positions of yethe eye. not less oblique then to a certain limit: & if yethe eye be held less oblique then at ytthat limit, yethe reflexion grows very sensibly fainter on a seuddemsudden, & is faintest of all wnwhen yethe superficies is viewed perpendicularly. The mentioned limit, a skilful Optician, by knowing yethe refractive power of yethe liquors would esasily assigne.

If yethe exptexperiment were therefore to be tryed again it might not be amis to observe whither if yethe common superficies of yethe liquors be viewed by degrees less & less obliquely, yethe reflexion does not begin to grow faint at a certain position of yethe eye & from thence forward grow insensibly fainter & fainter till yethe eye be perpendicularly over yethe superficies. 2 Whether yethe lighter oyle be not more strongly refractive ynthan yethe oyle of Tartar. 3 Whether yethe obliquity at wchwhich yethe reflecxtion grows begins to grow faint, be not such as a Mathematician would compute it by having yethe different densities refractive powers of yethe two liquors given, that is, yethe rathios of their sines of incidence & refraction. 4: Whether if yethe light oyle be powred on yethe some more strongly refracting liquor then yethe alcalizate one (such as butter of Antimony per deliquium (as I may call yethe solution of it made by exposing it to yethe air) may be) the common superficies be not less stronlgly reflexive then wnwhen yethe Alcalizate liquor or common water of is used.]

The Phosophorus seems to me a very extraordinary thing, but I cannot yet deduce any thing from it. I am going out of town for a few days, & when I return I shall think of an sanswer to Mr Lucas, in yethe mean time I rest

YorYour humble servant

Is. Newton.

I would not have you write to me till you hear ytthat I am returned. Excisuse this hasty scribble.

Feb 19 1676

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

Westminster

2 London

Letters about Reflecting Telescopes, & Refraction.