Chap. CCCXIII.—Of the inferior Extremities of distant Objects.
The inferior or lower extremities of distant objects are not so apparent as the upper extremities. This is observable in mountains and hills, the tops of which detach from the sides of other mountains behind. We see the tops of these more determined and distinctly than their bases; because the upper extremities are darker, being less encompassed by thick air, which always remains in the lower regions, and makes them appear dim and confused. It is the same with trees, buildings, and other objects high up. From this effect it often happens that a high tower, seen at a great distance, will appear broad at top, and narrow at bottom; because the thin air towards the top does not prevent the angles on the sides and other different parts of the tower from being seen, as the thick air does at bottom. This is demonstrated by the seventh proposition[1], which says, that the thick air interposed between the eye and the sun, is lighter below than above, and where the air is whiteish, it confuses the dark objects more than if such air were blueish or thinner, as it is higher up. The battlements of a fortress have the spaces between equal to the breadth of the battlement, and yet the space will appear wider; at a great distance the battlements will appear very much diminished, and being removed still farther, will disappear entirely, and the fort shew only the straight wall, as if there were no battlements.
- ↑ See chapters cccvii. cccxxii.