We all allow the Air to be much grosser and more dense in some Places than in others; and that in proportion, as it is higher from the Earth, it is more subtil, pure, and transparent: for this Reason high Objects seen at a good Distance, do not show their under Parts, so clearly as the upper; the Visual Rays by means of which we view the former, travelling through a long track of thick foggy Air; whereas the Rays, by which we see the latter, tho' on the side of the Eye, they begin in a gross Air; yet do they terminate in a much purer, and refined Air on the side of the Object: So that in Proportion, as they remove farther from the Eye, they become still finer; passing continually out of a pure Air, into a Purer. In painting Landskips, therefore, a Painter must observe to make his Mountain-tops clearer than the bottoms; and even his Hillocks rising over one another, must appear conformable to this Rule: the farther they are removed, the clearer alwas must their Tops be seen; and the higher they are raised, the more visible and distinct must their Forms, and different Colours discover themselves.