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

on PERSPECTIVE. fore the Point X is the Repreſentation of the
ſaid Extremity.


13, 14.
Figure 1. Page 36
Plate 22
Fig. 50
Figure 2. Fig. 51
I H T a X
Figure 3. Fig. 52
C D X I H G a F E L b T
Figure 4. Fig. 53
H I F T x d X L B C

147. Corollary .

94. It is manifeſt from hence, that when the
Perſpective of a Line, perpendicular to the
Geometrical Plane is once found, it is eaſy af-
terwards to find the Repreſentations of any Per-
pendiculars of the ſame Length as that.

148. Method II.

The Perſpective Plane being conſider’d as the Geo-
metrical Plane.

95. Let T (as in Fig. 51.) be the accidental
Point of perpendicular Lines to the Geometri-
cal Plane; H I the Arc of a Circle, whoſe Cen-
ter is T, and ſemidiameter the Eye’s Diſtance
from the perſpective Plane: Alſo let a be the
Point where the Perpendicular, whoſe Appear-
ance is ſought, meets the perſpective Plane, and
B C the Length of this Perpendicular.


Fig. 53.

149. Operation ,

About the Point a, as a Center, and with the
Semidiameter B C, deſcribe the Circle L F, and
draw the Line I L, or H F, touching each of the
Circles H I, and F L; and then a X or a x, is
the Appearance ſought, viz. a X, when the Per-
pendicular is above that Surface of the per-
ſpective Plane next to the Eye, and a x, when
the Perpendicular is on the oppoſite Side.

150. Demonstration .

Draw the Radii a F, a L, T I, T H, to the
Points of Contact F, L, I, and H. Then be-

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