Now, because that heat and cold are equally diffus’d every way; and that the further it is spread, the weaker it grows; hence it will follow, that the most part of the under Region of the Air will be made up of several kinds of lentes, some whereof will have the properties of Convex, others of Concave glasses, which, that I may [Schem. 37.]
Fig. 8. the more intelligibly make out, we will suppose in the eighth Figure of the 37. Scheme, that A represents an ascending vapour, which, by reason of its being somewhat Heterogeneous to the ambient Air, is thereby thrust into a kind of Globular form, not any where terminated, but gradually finished, that is, it is most rarify’d in the middle about A. somewhat more condens’d about BB, more then that about CC; yet further, about DD, almost of the same density with the ambient Air about EE;, and lastly, inclosed with the more dense Air FF, so that from A, to FF, there is a continual increase of density. The reason of which will be manifest, if we consider the rising vapour to be much warmer then the ambient heavie Air; for by the coldness of the ambient Air, the shell EE will be more refrigerated then DD, and that then CC, which will be yet more then BB, and that more then A; so that from F to A, there is a continual increase of heat, and consequently of rarity; from whence it will necessarily follow, that the Rays of light will be inflected or refracted in it, in the same manner as they would be in a Concave-glase; for the Rays GKI, GKI will be inflected by GKH, GKH, which will easily follow from what I before explained concerning the inflection of the Atmosphere.

On the other side, a descending vapour, or any part of the air included by an ascending vapour, will exhibit the same effects with a Convex lens; for, if we suppose, in the former Figure, the quite contrary constitution to that last describ’d; that is, the ambient Air FF being hotter then any part of that matter within any circle, therefore the coldest part must necessarily be A, as being farthest remov’d from the heat, all the intermediate spaces will be gradually discriminated by the continuall mixture of heat and cold, so that it will be hotter at EE, then DD, in DD then CC, in CC then BB, and in BB then A. From which, a like refraction and condensation will follow, and consequently a lesser or greater refraction, so that every included part will refract more then the including, by which means the Rays, GKI, GKI, coming from a Starr, or some remote Object, are so inflected, that they will again concurr and meet, in the point M. By the interposition therefore of this desending vapour the visible body of the Star, or other Object, is very much augmented, as by the former it was diminished.

From the quick consecutions of these two, one after another, between the Object and your eye, caused by their motion upwards or downwards, proceeding from their levity or gravity, or to the right or left, proceeding from the wind, a Starr may appear, now bigger, now less, then really it would otherwise without them; and this is that property of a Starr, which is commonly call’d twinkling, or scintillation.

The reason why a Star will now appear of one colour, now of another, which for the most part happens when ’tis neer the Horizon, may very easily be deduc’d from its appearing now in the middle of the vapour, other whiles neer the edge; for if you look against the body of a Starr with a Telescope that has a pretty deep Convex Eye-glass, and so order it, that the Star may appear sometimes in one place, and sometimes in another of it; you may perceive this or that particular colour to be predominant in the apparent Figure of the Starr, according as it is more or less remote from the middle of the Lens. This I had here further explain’d, but that it does more properly belong to another place.

I shall therefore onely add some few Quæries, which the consideration of these particulars hinted, and so finish this Section.

And the first I shall propound is, Whether there may not be made an artificial transparent body of an exact Globular Figure that shall so inflect or refract all the Rays, that, coming from one point, fall upon any Hemisphere of it; that every one of them may meet on the opposite side, and cross one another exactly in a point; and that it may do the like also with all the Rays that, coming from a lateral point, fall upon any other Hemisphere; for if so, there were to be hoped a perfection of Dioptricks, and a transmigration into heaven, even whil’st we remain here upon earth in the flesh, and a descending or penetrating into the center and innermost recesses of the earth, and all earthly bodies; nay, it would open not onely a cranney, but a large window (as I may so speak) into the Shop of Nature, whereby we might be enabled to see both the tools and operators, and the very manner of the operation it self of Nature; this, could it be effected, would as farr surpass all other kind of perspectives as the vast extent of Heaven does the small point of the Earth, which distance it would immediately remove, and unite them, as ’twere, into one, at least, that there should appear no more distance between them then the length of the Tube, into the ends of which these Glasses should be inserted: Now, whether this may not be effected with parcels of Glass of several densities, I have sometimes proceeded so farr as to doubt (though in truth, as to the general, I have wholly despair’d of it) for I have often observ’d in Optical Glasses a very great variety of the parts, which are commonly called Veins; nay, some of them round enough (for they are for the most part, drawn out into strings) to constitute a kind of lens.

This I should further proceed to hope, had any one been so inquisitive as to have found out the way of making any transparent body, either more dense or more rare, for then it might be possible to compose a Globule that should be more dense in the middle of it, then in any other part, and to compose the whole bulk, so as that there should be a continual gradual transition from one degree of density to another; such as should be found requisite for the desired inflection of the transmigrating Rays; but of this enough at present, because I may say more of it when I set down my own Trials concerning the melioration of Dioptricks, where I shall enumerate with how many several substances I have made both Microscopes, and Telescopes, and by what and how many, ways: Let such as have leisure and opportunity farther consider it.

The next Quæry shall be, whether by the same collection of a more dense body then the other, or at least, of the denser part of the other, there might not be imagin’d a reason of the apparition of some new fix’d Stars, as those in the Swan, Cassiope’s Charr, Serpentarius, Piscis, Cetus, &c.

Thirdly, Whether it be possible to define the height of the Atmosphere from this inflection of the Rays, or from the Quicksilver Experiment of the rarifaction or extension of the Air.

Fourthly, Whether the disparity between the upper and under Air be not sometimes so great, as to make a reflecting superficies; I have had several Observations which seem to have proceeded from some such cause, but it would be too long to relate and examine them. An Experiment, also somewhat analogous to this, I have made with Salt-water and Fresh, which two liquors, in most Positions, seem’d the same, and not to be separated by any determinate superficies, which separating surface yet in some other Positions did plainly appear.