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

The Interſection V H of this Plane, and the Per-
ſpective Plane, is the Vertical Line.

### 8.1.

Def. 11.

And S H I, the Interſection of it, and the Geo-
metrical Plane, is the Station Line.

### 8.1.

Def. 12.

Points of Diſtance, are two Points in the Hori-
zontal Line, each way diſtant from the Point of Sight
by the length of the principal Ray; as MN.

### 8.1.

Def. 13.

The Geometrical Line, is a Line, that paſſes
through the Station Point, and is parallel to the baſe
Line, as A B.

### 8.1.

Def. 14.

The Seat of an Object, is the Concurrence of Per-
pendiculars let fall from every of its Points upon the
Geometrical Plane, and the ſaid Plane.

### 8.1.

Def. 15.

The Direction of a Line inclined to the Geometri-
cal Plane, is the Interſection of the ſaid Plane, and
another Plane perpendicular thereto, paſſing through
the ſaid inclined Line.

Def. 16.

## 9.CHAP. II.The Theory of Perſpective. Lemma .

3. THE Perſpective, or Appearance of a
Right Line, as A B, which being con-
tinued, does not paſsthrough the Eye O, is like-
wiſe a right Line: For the Rays, by which the
Line A B is perceived, form a Plane cutting
the perſpective Plane; and the common Section
of theſe two Planes is a right Line, as a b.

Fig. 3.

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