# Full text: Gravesande, Willem Jacob: An essay on perspective

Hitherto I have endeavour’d to give an Idea
of Perſpective in general: But there is yet ano-
ther particular Signification of this Word, which
it is neceſſary ſhould be explain’d, as well as
the other Terms of the Art, which are laid down
in the following Definitions; and which every
one, that intends to underſtand this Treatiſe,
ought to be well acquainted with.

2. The Perſpective, Repreſentation, or Appearance of
an Object, (for theſe Three Words are ſynoni-
mous) * is the Figure which the Rays, by which an
Object is perceiv’d, form in paſſing thro’ the tranſpa-
rent Plane: And the Perſpective of a Point, is the
Interſection of a Ray proceeding from that Point,
and the tranſparent Plane. Which Interſection is
a Point: As the Figure D in the tranſparent
Plane C, is the Perſpective of the Object B; and
the Point e, in the ſame Plane, is the Perſpective
of the Point E, in that Object.

### 8.1.

Def. 1.
Fig. 1.

The Plane parallel to the Horizon, upon which
the Spectator [ſtands, or] is placed, as likewiſe the
Objects that he views, is call’d the Geometrical Plane. As A B C D.

### 8.1.

Def. 2.
Fig. 2.

A Perſpective Plane [or Picture] is that which
is placed between the Spectator and the Object, upon
which the Objects are drawn: As F G R T. This
is commonly perpendicular to the Geometrical
Plane, and conſequently to the Horizon; becauſe
Pictures have generally this Situation: But yet
it may be ſometimes inclin’d, and even parallel
to the Geometrical Plane, according as one
would diſpoſe the Deſign, or Picture that we are
working. And for this Reaſon, in the follow-
ing Chapter, we have laid down General Theo-
rems, and their Corollaries, agreeing to all theſe

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