Robson, William: Grammigraphia; or the Grammar of Drawing. (London, 1799)

72

Imagination has thus, without altering appearance,
made nature a picture only, on the Real Perspective
Plane, which is parallel with the Plane of the observer
(or, with the eyes of the observer, as he may be supposed
to look, directly forward], and perpendicular to the
ground : just as a scene is viewed in a mirror ; only here
it is not reversed.

In drawing from nature this idea (for it is no more)
has great advantages; and should be perfectly impressed
on the mind. It gives truth to the design, and facility
in remarking what appears on the Real Perspective
Plane. Banishing from the thoughts any conception of
natural situation, state, use, or magnitude ; it directs the
observation only to the Proportion and Position formed
on this magical picture.

The perfect knowledge of Appearance, in this way,
will point out the Natural Situation, by means of the
rule, according to which, thus governing this Situation,
the Appearance is formed. The use of this Plane, in
Perspective (as of the others already observed), is to
ascertain the Positions parallel with it.

In The Process of a Drawing from Nature, these two,
the Artificial and Real Perspective Planes, though
of such different sizes, when formed by scale, must
perfectly correspond in every proportion and division