Gravesande, Willem Jacob 's - An essay on perspective

Gravesande, Willem Jacob 's, An essay on perspective

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235on PERSPECTIVE.

The Interſection V H of this Plane, and the Per-
11Def. 11. ſpective Plane, is the Vertical Line.

And S H I, the Interſection of it, and the Geo-
22Def. 12. metrical Plane, is the Station Line.

Points of Diſtance, are two Points in the Hori-
33Def. 13. zontal Line, each way diſtant from the Point of Sight
by the length of the principal Ray; as MN.

The Geometrical Line, is a Line, that paſſes
44Def. 14. through the Station Point, and is parallel to the baſe
Line, as A B.

The Seat of an Object, is the Concurrence of Per-
55Def. 15. pendiculars let fall from every of its Points upon the
Geometrical Plane, and the ſaid Plane.

The Direction of a Line inclined to the Geometri-
66Def. 16. cal Plane, is the Interſection of the ſaid Plane, and
another Plane perpendicular thereto, paſſing through
the ſaid inclined Line.

CHAP. II.

The Theory of Perſpective.

3. THE Perſpective, or Appearance of a
Right Line, as A B, which being con-
tinued, does not paſsthrough the Eye O, is like-
77Fig. 3. wiſe a right Line: For the Rays, by which the
Line A B is perceived, form a Plane cutting
the perſpective Plane; and the common Section
of theſe two Planes is a right Line, as a b.

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