OF PAINTING.

The conditions of shadow and light [as seen] by the eye are 3. Of
these the first is when the eye and the light are on the same side
of the object seen; the 2nd is when the eye is in front of the
object and the light is behind it. The 3rd is when the eye is in
front of the object and the light is on one side, in such a way as
that a line drawn from the object to the eye and one from the object
to the light should form a right angle where they meet.