When clouds come between the sun and the eye all the upper edges of
their round forms are light, and towards the middle they are dark,
and this happens because towards the top these edges have the sun
above them while you are below them; and the same thing happens with
the position of the branches of trees; and again the clouds, like
the trees, being somewhat transparent, are lighted up in part, and
at the edges they show thinner.

But, when the eye is between the cloud and the sun, the cloud has
the contrary effect to the former, for the edges of its mass are
dark and it is light towards the middle; and this happens because
you see the same side as faces the sun, and because the edges have
some transparency and reveal to the eye that portion which is hidden
beyond them, and which, as it does not catch the sunlight like that
portion turned towards it, is necessarily somewhat darker. Again, it
may be that you see the details of these rounded masses from the
lower side, while the sun shines on the upper side and as they are
not so situated as to reflect the light of the sun, as in the first
instance they remain dark.

The black clouds which are often seen higher up than those which are
illuminated by the sun are shaded by other clouds, lying between
them and the sun.

Again, the rounded forms of the clouds that face the sun, show their
edges dark because they lie against the light background; and to see
that this is true, you may look at the top of any cloud that is
wholly light because it lies against the blue of the atmosphere,
which is darker than the cloud.

[Footnote: A drawing in red chalk from the Windsor collection (see
Pl. XXIX), representing a landscape with storm-clouds, may serve to
illustrate this section as well as the following one.]