Howard, Frank: The sketcher's manual: or, the whole art of picture making reduced to the simplest principles by which amateurs may instruct themselves without the aid of a master (London, 1841)

Section III.

BELIEF.

Any object behind another, or of which the view
is partially concealed or intercepted by another,
becomes a part of the back-gixmnd to that other;
or, the other is said to be relieved against, or
opposed to it.

Dark objects should be relieved against light
ones, and light against dark ones (Plate XXI.
fig. 1).

But in neither case should the object be equally
relieved on all sides; or it will appear to be cut
out and stuck on the surface of the picture. Some
portion of the outline should tell more strongly
than the rest; other portions should not be
relieved at all, but merge into the back-ground.
To effect this, shadows may be thrown upon some