OF SHADOWS.

Where a shadow ends in the light, note carefully where it is paler
or deeper and where it is more or less indistinct towards the light;
and, above all, in [painting] youthful figures I remind you not to
make the shadow end like a stone, because flesh has a certain
transparency, as may be seen by looking at a hand held between the
eye and the sun, which shines through it ruddy and bright. Place the
most highly coloured part between the light and shadow. And to see
what shadow tint is needed on the flesh, cast a shadow on it with
your finger, and according as you wish to see it lighter or darker
hold your finger nearer to or farther from your picture, and copy
that [shadow].

On the lighting of the background (562-565).