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

An ESSAY ctive Plane; and then (from the Station Point,
thro’ the Extremities of the Perſpective Plane)
he muſt draw right Lines; which will limit the
Space wherein the Figures muſt be placed; ſince
the Rays of Figures, without thoſe Lines coming
towards the Eye, will not paſs thro’ the Perſpe-
ctive Plane.

21. The Figures being thus drawn on the Geo-
metrical Plane, the next Thing is to find their Ap-
pearance upon the Perſpective Plane. Now,
theſe Figures are made up of either ſtraight
Lines, or crooked ones. To find the Repreſen-
tation of a ſtraight Line, its Extremes need on-
ly be ſought: And to have the Appearance of a
crooked Line, ſeveral Points thereof need only
be found. Since all this is equally applicable
to Figures, as well in the Geometrical Plane,
as thoſe above it; it follows, that the whole Bu-
ſineſs of Perſpective conſiſts in only finding the
Repreſentation of a Point.

And to find this Repreſentation in the follow-
ing Problems, we only uſe certain Lines drawn
in the Geometrical and Horizontal Planes; which, by their Interſection with the Baſe and
Horizontal Lines, ſhew the manner of drawing
new Lines upon the Perſpective Plane, which
determine the propos’d Appearances. Now, it
is plain, that in finding the ſaid Interſections, it
is not neceſſary to place the Perſpective Plane
perpendicular to the Geometrical and Horizontal
Planes; which would render the Work extream-
ly laborious: Whence the Perſpective and Hori-
zontal Planes may be conſider’d as lying upon
the Geometrical Plane, and ſo coinciding there-
with.

The Perſpective Plane may lye upon the Geo-
metrical Plane two ways; viz. Either upon the
Face reſpecting the Objects, or upon that next

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