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

on PERSPECTIVE. Perpendiculars f P, O H, muſt be let fall from
the Points f and O, on the Baſe Line, and the
Line P g drawn; then the Point V, wherein it
cuts the Perpendicular O H, is the Point of Sight
ſought, and the Parts O V, and V H determine
the Height and Diſtance of the Eye.

43. Method V.

35. When the Appearance of a Point is known,

Let A be a Point in the Geometrical Plane,
and a its Repreſentation in the perſpective Plane,
it is requir’d to find the Appearance of the
Point B.


Fig. 11.

44. Operation ,
Without Compaſſes.

Draw a Line from the Point B to the Eye O,
and another from the Point E, wherein the
ſaid Line continued, cuts the Baſe Line, to the
Point A; then draw the Line E a, and where
it cuts B O, is the Point b ſought.

45. Demonstration .

The Point E is its own Repreſentation; and
becauſe the Point a is the Repreſentation of A,
the Line E a is that of E A. Now ſince the
Point B is in the Line E A, the Appearance of
this Point will be likewiſe in E a, as alſo in B O; therefore it is in b the Interſection of the
Lines E a, and B O.



46. Remark .

37. If the Point A be in the Line B O, or
the Line B A be parallel, or a very little inclined
to the Baſe Line, we cannot then uſe this Me-

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