If the slope of a hill comes between the eye and the horizon,
sloping towards the eye, while the eye is opposite the middle of the
height of this slope, then that hill will increase in darkness
throughout its length. This is proved by the 7th of this which says
that a tree looks darkest when it is seen from below; the
proposition is verified, since this hill will, on its upper half
show all its trees as much from the side which is lighted by the
light of the sky, as from that which is in shade from the darkness
of the earth; whence it must result that these trees are of a medium
darkness. And from this [middle] spot towards the base of the hill,
these trees will be lighter by degrees by the converse of the 7th
and by the said 7th: For trees so placed, the nearer they are to the
summit of the hill the darker they necessarily become. But this
darkness is not in proportion to the distance, by the 8th of this
which says: That object shows darkest which is [seen] in the
clearest atmosphere; and by the 10th: That shows darkest which
stands out against a lighter background.
[Footnote: The quotation in this passage again cannot be verified.]
463.