...growing brighter. This arises by the third proposition of the ninth book, which says: The thickness of the air seen from below looking up is much clearer and more brilliant than that seen from above looking down; and this comes about because the air seen from above downward is somewhat penetrated by the dark images of the earth that lies beneath it, and so it appears to the eye darker than that which is seen from below upward, which is penetrated by the rays of the sun that come to the eye with great brightness. Therefore the same happens with the mountains and landscapes here proposed, whose images, passing through the aforesaid layers of air, will appear either dark or bright according to the darkness or brightness of the air.

[793] Of the air that shows the bases of mountains brighter than their summits.
The summits of mountains will always appear darker than their bases. This happens because such mountain summits penetrate into thinner air than their bases do, by the second proposition of the first book, which says that a region of air will be the more transparent and thin the more remote it is from the water and the earth. It follows, then, that such mountain summits which reach into that thin air show more of their natural darkness than those parts which penetrate the low air, which, as has been proved, is much thicker.

[794] Why distant mountains show more...