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

97. Operation .

In ſome ſeparate Place, draw the Lines C E
and C P, making an Angle with each other
equal to the Angle of Inclination of the given
Line; and in one of theſe Lines, aſſume C E
equal to the given Line, and let fall the Perpen-
dicular E P, from the Point E upon the other
Line. Then aſſume A B, in the Direction of
the propos’d Line, equal to C P; and after ha-
ving found a, the Perſpective of A, and the
Point T , the Perſpective of a Point elevated above B, the Height of P E; join the Points a
and T by a right Line; and the ſought Appear-
ance will be had.



98. Demonstration .

If from the Extremity of the inclin’d Line, a
Perpendicular be let fall upon the Geometrical
Plane, the ſaid Perpendicular will meet this
Plane in the Point B, and will be equal to P E; as is evident by the Conſtruction of the Figure
C P E. But the Point T is the Repreſentation
of the Extremity of this Perpendicular; and
therefore it is alſo the Extremity of the inclin’d
Line. Which was to be demonſtrated.

99. Remarks .

There are ſome Caſes of this Propoſition, that
may be ſhorten’d. As, 1. When there are ſeve-
ral Lines of this Kind parallel between them-
ſelves, and whoſe accidental Point can be
found : And, 2. When an inclin’d Line is pa- rallel to the perſpective Plane. The Manner of


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