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

The Reaſon of this is evident, from the Demon-
ſtration of the upright Cone: For, it is manifeſt, that
X is the Seat of the Center of the Circle, which the
Cone continued forms in the Horizontal Plane; and
conſequently, the Point s, in regard to the Circle BED,
is in the ſame Situation as the Eye is, in reſpect of
the Interſection of the continued Cone, and the Hori-
zontal Plane.

Note, moreover, that a Cone can ſcarcely ever
be thrown into Perſpective, by the common Me-
thod, ſo exact as by this.

## 71. Problem VII. 55. To find the Perſpective of a Line, perpendicular to the Geometrical Plane.

It is requir’d to find the Appearance of a Line
equal to B C, and perpendicular to the Geome-
trical Plane, in the Point A.

Fig. 22.

## 72. Operation .

Aſſume E D, any where in the Baſe Line,
equal to B C; and from the Points D and E,
draw D F and E F to ſome Point F, taken at
pleaſure in the Horizontal Line. Then having
found a, the Repreſentation of the Point A; draw a H parallel to the Baſe Line, and aI per-
pendicular thereto: And if aI be made equal to
G H, the ſaid a I will be the Perſpective
ſought.

22.

## 73. Demonstration .

56. The Appearance of the ſaid Line, is per- pendicular to the Baſe Line, and equal to the Perſpective of the Line A L, drawn from the

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