Perspective, then, is the art of drawing or represent- ing any object on a plane, or flat surface, as the object appears to the eye of the spectator.
The simplest way to represent an object in perspec- tive is to stand before a window, and, holding the head still, to draw on a pane of glass the outline of any ob- ject seen through it. But a more convenient apparatus is generally used.
This, and some of the more important terms used in Perspective Drawing, the pupil must endeavour to un- derstand.
1. The Perspective Plane is an upright square of glass, usually framed like a picture, with a base, so that it can stand up alone. This is placed between the eye of the spectator and the object to be drawn, and as the drawing is sometimes made directly upon it, it is sometimes called the Picture or the Plane of the Pic- ture.
2. Visual Rays are lines drawn from every part of the object, through the Perspective Plane to a point in the spectator's eye, which point, it has been said, is called the Point of Sight.
3. The Horizontal Line is a line drawn di- rectly across the Perspective plane at the height of the spectator's eye, and this may be of any height, although it is customary to draw the line about one third from the base or bottom of the Perspective Plane.
4. The Centre of the Picture is that point on the Perspective Plane, where a line, drawn from the spec- tator's eye to the Perspective Plane, would strike the Horizontal Line. This point is generally placed as near the centre of the Perspective Plane or Picture, as possible, and hence its name.
5. The Prime Vertical Line is a perpendicular line raised on the base of the picture, and passing through that point which is called the Centre of the Picture.
