Why mountain summits look darker than their bases from afar
Aerial perspective: air thins with height, so higher peaks appear less tinted than distance alone would predict
A leaf of the Treatise on Painting on aerial perspective applied to mountains. The argument holds that a peak looks darker at its summit than at its base because the air is thicker (denser) below than above, and that mountain e reads darker than mountain g since less air lies between the eye and e. A following lettered chapter explains why distant summits appear darker than their bases, arguing that air grows thinner and colder the higher and more remote it is, so the brightness of peaks does not track their distance in simple proportion. A faint sketch of receding ridges illustrates the case, and the sheet also bears the running head and a catchword not included in the transcription.
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A summit reads darker than its base because lower air is denser
The top of mountain e is darker than its base because the air is thicker below than above. Mountain e is darker than mountain g since less air lies between d and e than between d and g; and summit g, rising higher, likewise darkens toward its top.
Brightness of peaks does not track distance in simple proportion
The thickness of the air varies with height, growing thinner and colder the more remote it is from earth and water. Mountain p shows clearer than mountain o, and q clearer than p, because more air lies between the eye a and the nearer peak; yet this clearness does not keep the same proportion as the distances, because a higher peak such as g penetrates into thinner air.
Faint sketch of receding mountain ridges
Amid the lower lines of text a light pen sketch shows the profiles of successive hills or mountains dropping toward the horizon, drawn to accompany the comparison of near and far peaks in the argument.