This rule is important, as we shall see when we come to the consideration of the perspective vanishing scale. Its use may be illustrated by this sketch, where the houses, walls, &c., are parallel to the base of the picture. When that is the case, then objects exactly facing us, such as windows, doors, rows of boards, or of bricks or palings, &c., are drawn with their horizontal lines parallel to the base; hence it is called parallel perspective.
All lines situated in a plane that is parallel to the picture plane diminish in proportion as they become more distant, but do not undergo any perspective deformation; and remain in the same relation and proportion each to each as the original lines. This is called the front view.
All horizontals which are at right angles to the picture plane are drawn to the point of sight.
Thus the lines _AB_ and _CD_ (Fig. 28) are horizontal or parallel to the ground plane, and are also at right angles to the picture plane _K_. It will be seen that the perspective lines _Ba·_, _Dc·_, must, according to the laws of projection, be drawn to the point of sight.
This is the most important rule in perspective (see Fig. 7 at beginning of Definitions).
An arrangement such as there indicated is the best means of illustrating this rule. But instead of tracing the outline of the square or cube on the glass, as there shown, I have a hole drilled through at the point _S_ (Fig. 29), which I select for the point of sight, and through which I pass two loose strings _A_ and _B_, fixing their ends at _S_.
As _SD_ represents the distance the spectator is from the glass or picture, I make string _SA_ equal in length to _SD_. Now if the pupil takes this string in one hand and holds it at right angles to the glass, that is, exactly in front of _S_, and then places one eye at the end _A_ (of course with the string extended), he will be at the proper distance from the picture. Let him then take the other string, _SB_, in the other hand, and apply it to point _b´_ where the square touches the glass, and he will find that it exactly tallies with the side _b´f_ of the square _a·b´fe_. If he applies the same string to _a·_, the other corner of the square, his string will exactly tally or cover the side _a·e_, and he will thus have ocular demonstration of this important rule.