The Artist's Repository, Or, Encyclopedia of the Fine Arts (Band 2): Perspective, Architecture (London, 1808)

lect. ii.] on perspective. 55

the objects reprefented in the picture may be molt
conveniently feen, we proceed to prepare the picture
for practice: by which, I mean nothing more than
infcribing upon it thofe imaginary lines, to whofe
properties we have already paid fome attention. Firft,
rule the horizontal line, then, having determined its
center, erect there the vertical line; thus we have
two lines, whereon all relative planes (i. e. horizontal
or vertical) will vaniih. Moreover, as all horizontal
planes will vanifh on the horizontal line, we have the
proper vanishing point for them in the center; be-
caufe the vanifhing point to any plane (in whatever
direction) is that point at which a line drawn from the
eye, parallel to that particular plane, ftrik.es the pic-
ture.

- Obferve further, that, thefe two planes are of ne-
ceffity perpendicular to each other; I fay the hori-
zontal plane is perpendicular to the vertical plane,
and the vertical plane to that: for as to the fituation
of this or of any plane, with refpedt to the natural
horizon, let that now be forgot.

We have already obferved, that planes are in fact of
a fimilar conftru6tion, whatever be their pofition; and
therefore the vertical plane is perfectly correfpondent
in its conftru£tion, and its properties to the horizontal,
and differs only by fituation, as being erect upon it.
The center beam, or ray, from the eye, is parallel
likewife to the vertical plane, and gives the central
point of this plane for its vanifhing point : i. e. where
the horizontal and the vertical planes interfect each

h 2 other.