50. To find the Repreſentation of a Point, elevated

above the Geometrical Planc.

Let G S be the Geometrical Line, and S the

Station Point: Make S F, in the Geometrical

Line, equal to the Height of the Eye; and let

A be the Seat of the given Line.

Fig. 18.

Aſſume F C in the Geometrical Line, equal to

the Height of the Eye, above the Geometrical

Plane: Then draw Lines from the Point A to

the Points S and C, and on the Point B, the In-

terſection of the Line AS and the Baſe Line,

raiſe the Perpendicular BI to the Baſe Line,

equal to E B, plus FC; and the Point I will be

the Perſpective ſought.

51. Let us ſuppoſe a Plane to paſs thro’ the

given Point, and the Eye perpendicular to the

Geometrical Plane; then it is manifeſt, that the

Interſection of theſe two Planes is the Line

A B S, and the Interſection of the ſaid ſuppos’d

Plane and the perſpective Plane, is B I. Now,

let X be this ſuppos’d Plane; a, b, s, the Point

mark’d with the ſame Letters in the precedent

Figure, bi the Interſection of this Plane and

the perſpective Plane; O the Eye, and D the

propos’d Point: We are to prove, that if O D

be drawn, the Line B I of the precedent Figure

will be equal to b i in this Figure.

Fig. 19.