Gravesande, Willem Jacob 's - An essay on perspective

Note, When V I and T a cut each other ve-
ry obliquely, recourſe muſt be had to the Obſer-
vation at the End of the aforegoing Method, or
the following Way may be uſed.

Method III.

86. Let T be the accidental Point of the Lines
11Fig. 47. perpendicular to the Geometrical Plane, thro’
which Point draw a parallel to the Baſe Line,
in which aſſume T R equal to O T of Fig. 44.

Operation.

Aſſume D N ſomewhere in the Baſe Line, e-
qual to the propoſed Line, and draw the Lines
D F and N F to the Point F, taken at Pleaſure
in the Horizontal Line; then through the Point
a, the Appearance of A, draw the parallel a H,
to the Baſe Line, in which aſſume a Q equal
to G H. Then if the Lines T a, and R Q be
drawn, and continued, till they cut each other
in the Point X, a X will be the Appearance
ſought.

Demonstration.

The Part a Q of the Line a H, is the 2256. pearance of a Line proceeding from A in the
Geometrical Plane, and which is equal to the
propoſed Line, and parallel to the Baſe Line;
and conſequently , the Line R Q paſſes 3320. the Perſpective of the Extremity of the propo-
ſed Line: And therefore X the Interſection of
R Q and T a, is the Appearance of the ſaid
Extremity.

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