If I draw a dotted line from E to little a, this represents another visual ray, and o, the point where it passes through the picture, is the perspective of little a. I now draw another line from g to S, and thus form the shaded figure ga·Po, which is the perspective of aAa·g.
Let it be remarked that in the shaded perspective figure the lines a·P and go are both drawn towards S, the point of sight, and that they represent parallel lines Aa· and ag, which are at right angles to the picture plane. This is the most important fact in perspective, and will be more fully explained farther on, when we speak of retreating or so-called vanishing lines.
[RULES]
[ VII]
The Rules and Conditions of Perspective
The conditions of linear perspective are somewhat rigid. In the first place, we are supposed to look at objects with one eye only; that is, the visual rays are drawn from a single point, and not from two. Of this we shall speak later on. Then again, the eye must be placed in a certain position, as at E (Fig. 22), at a given height from the ground, S·E, and at a given distance from the picture, as SE. In the next place, the picture or picture plane itself must be vertical and perpendicular to the ground or horizontal plane, which plane is supposed to be as level as a billiard-table, and to extend from the base line, ef, of the picture to the horizon, that is, to infinity, for it does not partake of the rotundity of the earth.
Fig. 22.
| Fig. 23. Front view of above figure. |