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 G K I, G K I will be inflected by G K H, G K H, 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 F F 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 E E, then DD, in D D then C C, in C C then B B, and in B B 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, G K I, G K I, 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,
