Gravesande, Willem Jacob 's - An essay on perspective

Therefore, I lay my Perſpective Plane upon
the Geometrical Plane; ſo that it be between
the Horizontal Plane, and the Figures requir’d
to be thrown into Perſpective.

Problem I.

22. To find the Appearance of a Point, which is in
the Geometrical Plane.

Let Z be the Geometrical Plane, I E the Baſe
11Fig. 7. Line, D V the Horizontal Line, V the Point
of Sight, D one of the Points of Diſtance, and
A the given Point.

Operation.

From the Point A, let fall the Perpendicular
A B upon the Baſe Line; and from the Point of
Concurrence B, draw the Line B V to the Point
of Sight; then aſſume B E in the Baſe Line
equal to B A, and from the Point E draw the
Line E D to the Point of Diſtance D: And the
Point (a), the Interſection of B V and E D, is
the Repreſentation ſought.

Demonstration.

23. The Appearance of the Line A B, is a Part of the Line B V. Now, if we conceive a Line
2216. iſſuing from the Eye towards the Point D, and
another from the Point A towards the Point E;
theſe two Lines will be parallel, becauſe they
are in parallel Planes, aud each make half a
right Angle with the Perſpective Plane;

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