Sun and Moon have both been seen above the Horizon, whil'st the Moon has been in an Eclipse. I shall not here instance in the great refractions, that the tops of high mountains, seen at a distance, have been found to have; all which seem to argue the Horizontal refraction, much greater then it is hitherto generally believ'd.
I have further taken notice, that not onely the Sun, Moon and Starrs, and high tops of mountains have suffer'd these kinds of refraction, but Trees, and several bright Objects on the ground: I have often taken notice of the twinkling of the reflections of the Sun from a Glass-window at a good distance, and of a Candle in the night, but that is not so conspicuous, and in observing the setting Sun, I have often taken notice of the tremulation of the Trees and Bushes, as well as of the edges of the Sun. Divers of these Phænomena have been taken notice of by several, who have given several reasons of them, but I have not yet met with any altogether satisfactory, though some of their conjectures have been partly true, but partly also false. Setting my self therfore upon the inquiry of these Phænomena, I first endeavour'd to be very diligent in taking notice of the several particulars and circumstances observable in them; and next, in making divers particular Experiments, that might cleer some doubts, and serve to determine, confirm, and illustrate the true and adæquate cause of each; and upon the whole, I find much reason to think, that the true cause of all these Phænomena is from the inflection, or multiplicate refraction of those Rays of light within the body of the Atmosphere, and that it does not proceed from a refraction caus'd by any terminating superficies of the Air above, nor from any such exactly defin'd superficies within the body of the Atmosphere.
This Conclusion is grounded upon these two Propositions:
First, that a medium, whose parts are unequally dense, and mov'd by various motions and transpositions as to one another, will produce all these visible effects upon the Rays of light, without any other coefficient cause.
Secondly, that there is in the Air or Atmosphere such a variety in the constituent parts of it, both as to their density and rarity, and as to their divers mutations and positions one to another.
By Density and Rarity, I understand a property of a transparent body, that does either more or less refract a Ray of light (coming obliquely upon its superficies out of a third medium) toward its perpendicular: As I call Glass a more dense body then Water, and Water a more rare body then Glass, because of the refractions (more or less deflecting towards the perpendicular) that are made in them, of a Ray of light out of the Air that has the same inclination upon either of their superficies.
So as to the business of Refraction, spirit of Wine is a more dense body then Water, it having been found by an accurate Instrument that measures the angles of Refractions to Minutes that for the same refracted angle of 30°.00'. in both those Mediums, the angle of incidence in Water was but 41°.35'. but the angle of the incidence in the trial with spirit of Wine was 42°.45'. But as to gravity, Water is a more dense body then
