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

170. Of the Shadows of a ſmall Light.
Prob . IV.

107. To find the Perſpective of the Shadow of a
Point, whoſe Seat, and Height, above the Geometri-
cal Plane is known.

Let Z be the Geometrical Plane, A the Seat
of the given Point; and C that of the Light: Draw the indefinite Line C A B; and about C,
as a Center, with a Semidiameter equal to the
Height of the Light above the Geometrical
Plane, deſcribe the Arc F: Again, about the
Point A, with a Semidiameter equal to the
Height of the given Point, deſcribe the Arc E. This being done, draw the Line E F touching
the ſaid two Arcs, and cutting the Line C A in
B. Then if the Perſpective of B be found, the
Perſpective of the Shadow of the Point will be
had.

170.1.

Fig. 59.

171. Problem V.

108. To find the Perſpective of the Shadow of an
elevated Point, the Repreſentation of which, as alſo
of its Seat being known, without uſing the Geometri-
cal Plane.

The Operation of Prob. 3. for Solar Sha- dows muſt be uſed here, with this Difference; that inſtead of the accidental Point of the Sun’s
Rays, the Perſpective of the ſmall Sight muſt
be uſed; and in the Room of the accidental
Points of the Directions of the ſaid Rays, the
Perſpective of the Seat of the Light muſt be
aſſumed.

171.1.

104.

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