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

For the Horizontal Plane, is parallel to the
Geometrical Plane.

## 21. Corollary III.

16. The Repreſentations of all Lines parallel to
the ſtation Line, concur in the Point of Sight.

This follows, becauſe the principal Ray is
parallel to the ſaid Lines.

## 22. Corollary IV.

17. Two or more equal Lines being perpendicular,
or equally inclined the ſame Way, to the ſame Line pa-
rallel to the Station Line, have their Repreſentations
concurring in the principal Point.

Becauſe all theſe Lines are parallel and equal,
the Line paſſing through their Vertices, is pa-
rallel to that paſſing through their Baſes, and
this being parallel to the Station Line, it fol-
lows, that the Appearances of the ſaid equal and parallel Lines concur in the principal
Point.

16.

## 23. Theorem V.

18. The Appearance of an indefinite Line does not
alter, when the Eye moves in a Line parallel to a
propoſed Line.

The Repreſentation of this Line, is the In-
terſection of the perſpective Plane, and a Plane
paſſing through the Eye and the ſaid Line. Now
the Eye remains in the ſame Plane, when it
moves in a Line parallel to the propoſed Line; and conſequently the Appearance of this laſt
Line, will not be changed by that Motion.

Note, This Demonſtration doth not extend
to any particular Part of the given Line, but on-
ly to the Line in general.

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