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

on PERSPECTIVE. one of the Directions drawn upon the Geome- trical Plane; but it is ſufficient here, that the
Angle the ſaid Directions make with the Baſe
Line, be only known: And ſo, as the Problem
expreſſes it, the Geometrical Plane may be en-
tirely laid aſide.


Figure 1. page 64.
Plate 23.
Fig. 54
O M P Q t A X x Q R N
Figure 2. Fig. 55
G F b T L a
Figure 3. Fig. 56
I F a X b E T C P
68, 88.

105. When the Perſpective Plane is parallel,
the Sun’s Rays will have no accidental Point; for their Repreſentations are then parallel; in
which Caſe, one of the Parallels muſt be drawn
through the Point a, inſtead of the Line D a. Moreover, when the Perſpective Plane is perpen-
dicular, or inclin’d, and the Sun’s Rays are pa-
rallel thereto; a Line muſt be drawn through the
Point a, parallel to the Baſe Line; as likewiſe
another Line through the Point I, parallel to the
Sun’s Rays; cutting the firſt Line in the Point

169. Problem III.

106. To find the Perſpective of the Shadow of an
elevated Point, when there is ſome Body hindring its
falling upon the Geometrical Plane.

The Perſpective of the Section of the Body
made by a Plane paſſing through the given Point
perpendicular to the Geometrical Plane, and pa-
rallel to the Sun’s Rays, muſt be found: And then
the Interſection of the ſaid Perſpective, and a
Line drawn from the Appearance of the given
Point to the Repreſentation of its Shadow, is the
Repreſentation ſought.

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