## 63. Prob . V. 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.

## 64. Operation .

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.

## 65. Demonstration .

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.