...you will judge the things seen to be so minute that not only the limbs, but the whole, will seem to you almost impossible to portray. As if the eye were o and the hole of a quarter of a braccio, equal to your painted panel, were a b, distant from the eye half a braccio; then through that space you will see all the things that could be seen within the length of a horizon of a hundred miles, in such confused diminution that, not to speak of portraying any part of them that has a shape, you will scarcely be able to place so small a dab of the brush that it will not be larger than every great building set at ten miles' distance.

[798] Why mountains at long distance show darker at the peak than at the base.
The air, which acquires degrees of thickness at every degree of its lowness and of its distance, is the cause that the peaks of mountains which rise higher show more of their natural darkness, because they are less hindered by the thickness of the air at the peak than at their base, or in nearness than in remoteness.
It is proved: o p, d s, c r, a k are degrees of the air that always grow thinner the higher they rise; a f, f h, h k are the other transverse degrees where the air acquires...