A Champaign sometimes appears larger, and at other times smaller than ordinary; this is owing to the Air, interposed between the Eye, and the Horizon, which at some times is grosser, and at other times more subtile than usual.

Among several Horizons equally distant from the Eye, that seen through the grossest Air, will appear the most remote; and on the contrary, that will seem the nearest, which is seen through an Air, the most subtile.

Objects of unequal Bulks, and seen at equal distances, will appear equally big, when the several Airs through which they are seen, bear the same proportions with regard to grossness, which the unequal Bodies bear to each other with regard to bigness: with this restriction, that the grossest Air be found between the Eye, and the smallest Body; and the rest, in the same order. Now, this may be proved, by the Perspective of Colours; by means of which, a Mountain, which wou'd be found very small, shou'd you come to measure it, is never the less made to appear larger than a Hillock, which is seen nearer the Eye, and whose Dimensions are considerably larger; just as a little Finger held near the Eye, is found to cover a large Mountain, when further removed.