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 spirit of Wine, for the proportion of the same Water, to the same very well rectify’d spirit of Wine was, as 21. to 19.

So as to Refraction, Water is more Dense then Ice; for I have found by a most certain Experiment, which I exhibited before divers illustrious Persons of the Royal Society, that the Refraction of Water was greater then that of Ice, though some considerable Authors have affirm’d the contrary, and though the Ice be a very hard, and the Water a very fluid body.

That the former of the two preceding Propositions is true, may be manifested by several Experiments; As first, if you take any two liquors differing from one another in density, but yet such as will readily mix: as Salt Water, or Brine, & Fresh; almost any kind of Salt dissolv’d in Water, and filtrated, so that it be cleer, spirit of Wine and Water; nay, spirit of Wine, and spirit of Wine, one more highly rectify’d then the other, and very many other liquors; if (I say) you take any two of these liquors, and mixing them in a Glass Viol, against one side of which you have fix’d or glued a small round piece of Paper, and shaking them well together (so that the parts of them may be somewhat disturb’d and move up and down) you endeavour to see that round piece of Paper through the body of the liquors, you shall plainly perceive the Figure to wave, and to be indented much after the same manner as the limb of the Sun through a Telescope seems to be, save onely that the mutations here, are much quicker. And if, in steed of this bigger Circle, you take a very small spot, and fasten and view it as the former, you will find it to appear much like the twinkling of the Starrs, though much quicker: which two Phænomena, (for I shall take notice of no more at present, though I could instance in multitudes of others) must necessarily be caus’d by an inflection of the Rays within the terminating superficies of the compounded medium, since the surfaces of the transparent body through which the Rays pass to the eye, are not at all altered or chang’d.

This inflection (if I may so call it) I imagine to be nothing else, but a multiplicate refraction, caused by the unequal density of the constituent parts of the medium, whereby the motion, action or progress of the Ray of light is hindred from proceeding in a streight line, and inflected or deflected by a curve. Now, that it is a curve line is manifest by this [Schem. 37.]
Fig. 1. Experiment: I took a Box, such as ADGE, in the first Figure of the 37. Scheme, whose sides ABCD, and EFGH, were made of two smooth flat plates of Glass, then filling it half full with a very strong solution of Salt, I filled the other half with very fair fresh water, then exposing the opacous side, DHGC, to the Sun, I observ’d both the refraction and inflection of the Sun beams, ID & KH, and marking as exactly as I could, the points, P, N, O, M, by which the Ray, KH, passed through the compounded medium, I found them to be in a curve line; for the parts of the medium being continually more dense the neerer they were to the bottom, the Ray pf was continually more and more deflected downwards from the streight line.

This Inflection may be mechanically explained, either by Monsieur Des Cartes principles by conceiving the Globuls of the third Element to find less and less resistance against that side of them which is downwards, or by a way, which I have further explicated in the Inquisition about Colours, to be from an obliquation of the pulse of light, whence the under part is continually promoted, and consequently refracted towards the perpendicular, which cuts the Orbs at right angles. What the particular Figure of the Curve line, describ’d by this way of light, is, I shall not now stand to examine, especially since there may be so many sorts of it as there may be varieties of the Positions of the intermediate degrees of density and rarity between the bottom and the top of the inflecting Medium.