It is easy to perceive that the atmosphere which lies closest to the
level ground is denser than the rest, and that where it is higher
up, it is rarer and more transparent. The lower portions of large
and lofty objects which are at a distance are not much seen, because
you see them along a line which passes through a denser and thicker
section of the atmosphere. The summits of such heights are seen
along a line which, though it starts from your eye in a dense
atmosphere, still, as it ends at the top of those lofty objects,
ceases in a much rarer atmosphere than exists at their base; for
this reason the farther this line extends from your eye, from point
to point the atmosphere becomes more and more rare. Hence, O
Painter! when you represent mountains, see that from hill to hill
the bases are paler than the summits, and in proportion as they
recede beyond each other make the bases paler than the summits;
while, the higher they are the more you must show of their true form
and colour.
On the colour of the atmosphere (300-307).
300.