15675on PERSPECTIVE.
CHAP. VI.
Of throwing Figures into Perſpective, when
the Perſpective Plane is conſider’d as being
parallel to the Geometrical Plane.
the Perſpective Plane is conſider’d as being
parallel to the Geometrical Plane.
Prob. I.
92.
To find the Perſpective of a Figure, which
is in the Geometrical Plane.
is in the Geometrical Plane.
When the perſpective Plane is parallel to the
Horizon, we commonly conſider it, as being
it ſelf the Geometrical Plane, in which Caſe,
the Problem is fully reſolved: But when it
happens, that another Geometrical Plane is
ſuppoſed, either above or below the perſpective
Plane, upon which the Figures upon the Geo-
metrical Plane are requir’d to be drawn; we
muſt draw geometrically thereon, Figures ſimi-
lar to thoſe in the Geometrical Plane; ſo that
the Lines on the perſpective Plane, be to their
correſpondent ones on the Geometrical Plane,
as the Eye’s Diſtance from the perſpective Plane,
is to its Diſtance from the Geometrical Plane.
Horizon, we commonly conſider it, as being
it ſelf the Geometrical Plane, in which Caſe,
the Problem is fully reſolved: But when it
happens, that another Geometrical Plane is
ſuppoſed, either above or below the perſpective
Plane, upon which the Figures upon the Geo-
metrical Plane are requir’d to be drawn; we
muſt draw geometrically thereon, Figures ſimi-
lar to thoſe in the Geometrical Plane; ſo that
the Lines on the perſpective Plane, be to their
correſpondent ones on the Geometrical Plane,
as the Eye’s Diſtance from the perſpective Plane,
is to its Diſtance from the Geometrical Plane.
The Demonſtration of this is evident from
n. 8, and 9.
n. 8, and 9.
Prob. II.
93.
To find the Perſpective of a Line, perpen-
dicular to the Geometrical Plane.
dicular to the Geometrical Plane.
Draw the Line O S, in which aſſume O R e-
11Fig. 50. qual to the Eye’s Diſtance from the
11Fig. 50. qual to the Eye’s Diſtance from the
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