Letter from Newton to Henry Oldenburg, dated 19 February 1676
\{to} Oldenburg/
Feb. 19. 1676.
Sr
I received yor letter conteining yt of Mr Lucas & this afternoon another from yor self by Mr White wth ye last Transactions, for wch I thank you. [[1]The speculation of ye figures of confining liquors. I am apt to think wth Mr Boyll may by degrees administer very considerable thoughts to such as are contriving mechanical systems systems of Phy: but ye last of ye Phænomena, where an oyle lighter yn spt of wine poured on ye Alcaly deliquated made a very specular superficies in their confine, being more in ye way of these|in|gs I have been som{e}times considering; I shal rather prsent you wth {an}{illeg}|wt| thoughts occur to me about yt, presuming you will excuse my randome guesses] because it is by yor desire yt I write them.
[I imagin then that as spt of wine, though lighter yn water, is yet a more strongly refracting liquor; so ye refractive density of this lighter oyle may be still greater, so as much to exceed yt of ye Alcalizate liquor. And if so then ye propounded Phonomenon may keep analogy wth that of other reflecting superficies. For ye reflexive power of any superficies (so far as my observation reaches) is proportional to ye difference of ye refractive densities of {the} {illeg}|bo|rdering mediums. And (wch I take to be ye prsent case,) ye same superficies reflex|c|ts \much/ more strongly towards on yt side towards ye denser (I mean ye more strongly refractive) medium then towards ye rarer on ye opposite side. B{ut}|es||id|to|es| when that side towards ye denser Mediu{illeg}|m| is viewed, obliquely enough, it always reflects all ye light to ye eye as much as quicksilver does: & ye greater ye difference of ye \refractive/ density {illeg}|o|f ye mediums is, ye less obliquity is requi{illeg}|s|ite to produce this phænomenon. This total reflexion holds to|in| all positions of ye eye. not less oblique then to a certain limit: & if ye eye be held less oblique then \at/ yt limit, ye reflexion grows very sensibly fainter on a se{illeg}|u|ddem {sic}, & is faintest of all wn ye superficies is viewed perpendicularly. The mentioned limit, a skil\ful/ Optician, by knowing ye refractive power of ye liquors would es|a|sily assigne.
If ye expt were therefore to be tryed again it might not be amis to observe whither if ye common superficies of ye liquors be viewed by degrees less & less obliquely, ye reflexion does not \begin to/ grow faint at a certain position of ye eye & from thence forward grow insensibly fainter & fainter till ye eye be perpendicularly over ye superficies. 2 Whether ye lighter oyle be not more strongly refractive yn ye oyle of Tartar. 3 Whether ye obliquity at wch ye reflec|x|tion grows begins to grow faint, be not such as a Mathematician would compute it by having ye different densities \refractive powers/ of ye two liquors given, that is, ye rathios of their sines of incidence & refraction. 4: Whether if ye light \oyle/ be powred on ye \some/ more strongly refracting liquor then ye alcalizate one (such as butter of Antimony per deliquium (as I may call ye solution of it \made/ by exposing it to ye air) may be) the common superficies be not less stronl|g|ly reflexive then wn ye 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 s|a|nswer to Mr Lucas, in ye mean time I rest
Yor humble servant
Is. Newton.
I would not have you write to me till you hear yt I am returned. Excis|u|se this hasty scri{illeg}|b|ble.
Feb 19 1676
[1] N. 131. p. 785. Exp. 13.
