Fig. 7.
The line HD is the Horizontal-line or Horizon, the chief line in perspective, as upon it are placed the principal points to which our perspective lines are drawn. First, the Point of Sight and next D, the Point of Distance. The chief vanishing points and measuring points are also placed on this line.
Another important line is AB, the Base or Ground line, as it is on this that we measure the width of any object to be represented, such as ef, the base of the square efgh, on which the cube C is raised. E is the position of the eye of the spectator, being drawn in perspective, and is called the Station-point.
Note that the perspective of the board, and the line SE, is not
the same as that of the cube in the picture AKB, and also that so much of the board which is behind the picture plane partially represents the Perspective-plane, supposed to be perfectly level and to extend from the base line to the horizon. Of this we shall speak further on. In nature it is not really level, but partakes in extended views of the rotundity of the earth, though in small areas such as ponds the roundness is infinitesimal.
Fig. 8.
Fig. 8. This is a side view of the previous figure, the picture plane K being represented edgeways, and the line SE its full length. It also shows the position of the eye in front of the point of sight S. The horizontal-line HD and the base or ground-line AB are represented as receding from us, and in that case are called vanishing lines, a not quite satisfactory term.
It is to be noted that the cube C is placed close to the transparent picture plane, indeed touches it, and that the square fj faces the spectator E, and although here drawn in perspective it appears to him as in the other figure. Also, it is at the same time a perspective and a geometrical figure, and can therefore be measured with the compasses. Or in other words, we can