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

181. II. When the Perſpective Plane is inclined.
Prob . III.

115. To find the Repreſentation of Figures which
are in the Geometrical Plane.

The Operation laid down for the perſpective Plane being perpendicular, may be uſed here,
becauſe the inclined perſpective Plane may be
changed into a perpendicular or upright one.



182. Prob . IV.

116. To find the Appearances of any Number of
Lines of the ſame Length, which are perpendicular
to the Geometrical Plane.

Raiſe a Perpendicular R C, in ſome Point on
the Baſe Line, in which aſſume R L equal in
Length to the given Lines; and draw the Line
L P, through the Point L, ſo that the Angle
L P R, be equal to the Angle of Inclination of
the perſpective Plane; then having made R S
equal to P L, and S C equal to P R, draw the
Lines S Q and C D, parallel to the Baſe Line. This being done, raiſe up the Figures of the
Geometrical Plane, ſo that the Point R coincides
with c, and the Line R H, with C D; then
proceed as is directed for the perpendicular perſpective Plane, in uſing S Q for the Baſe


Fig. 62.
112, 113.

This is demonſtrated in n. 114.

183. Remarks .

The Point C muſt be aſſumed below the Point
S, when the perſpective Plane is inclined towards


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