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

An ESSAY and drawing a Line from the Point I to the
other Point of Diſtance; which, by its Inter-
ſection with E D, will give the Appearance of
the Point A.

## 31. Method II.

26. γ is the Horizontal Plane, X the Perſpe-
ctive Plane, Z the Geometrical Plane, O the Eye,
D C the Horizontal Line, B E the Baſe Line, and
A the given Point.

Fig. 8.

## 32. Operation .

Draw a Line from the Point A, to the Eye O, cut-
ting the Baſe Line in the Point B, and the Horizon-
tal Line in the Point C: Then aſſume B E in the
Baſe Line, equal to B A; and C D in the Hori-
zontal Line, equal to C O; and join the Points
E and D, by a Line cutting the Line A O in
the Point a; which will be the Appearance
ſought.

## 33. Demonstration .

27. The Triangle O D C in the Horizontal
Plane, is ſimilar to the Triangle A B E in the
Geometrical Plane; and conſequently A B is pa-
rallel to O C, and A E to O D. But the Appear-
ance of A muſt be in the Lines B C, and E D; and therefore it will be in a, their Interſection.

[?] 13.

## 34. Remarks .

28. If the Place wherein the Eye ought to be in
the Horizontal Plane be not known, but the Point
of Sight is; then, to find the Place of the Eye,
a Perpendicular muſt be rais’d from the Point of

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