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

An ESSAY different Situations of the Perſpective Plane, [or
Picture] which ought to be well obſerv’d.

### 8.1.

Def. 3.

The Interſection of the Perſpective Plane and the
Geometrical Plane, is call’d the Baſe-Line: As
F G.

### 8.1.

Def. 4.

The different Situation of the Eye, alters the
Repreſentation of Objects in the perſpective Plane; for the Rays proceeding from the Object, and
concurring in ſome other Point, will likewiſe fall
upon the Perſpective Plane in different Places. And
for determining this Situation of the Eye, in re-
ſpect to the Perſpective Plane, we ſuppoſe,

A Plane parallel to the Horizon, paſſing thro’ the
Eye, and every way extending it ſelf; and this is
call’d the Horizontal Plane: As OMVNL.

### 8.1.

Def. 5.

The Interſection of this Plane and the Perſpective
Plane, is the Horizontal Line. As M V N.

### 8.1.

Def. 6.

The Perpendicular drawn from the Eye to the Ho-
rizontal Line, is the principal Ray. As O V.

### 8.1.

Def. 7.

The Point V, wherein the ſaid Perpendicular meets
the Horizontal Line, is the Point of Sight, or prin-
cipal Point.

### 8.1.

Def. 8.

Note, There is a Perpendicular, let fall from
the Eye upon the Geometrical Plane, meaſuring
the Height of the Eye.

The Point S, wherein the ſaid Perpendicular meets
the Geometrical Plane, is the Station-Point.

### 8.1.

Def. 9.

The Plane paſſing thro’ the aforeſaid Perpendicular,
and the principal Ray, is call’d the Vertical Plane. As SOLI.

Def. 10.

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