| Fig. 29. |
| Fig. 30. |
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.
In this little picture (Fig. 30) in parallel perspective it will be seen that the lines which retreat from us at right angles to the picture plane are directed to the point of sight S.
[Rule 6]
All horizontals which are at 45°, or half a right angle to the picture plane, are drawn to the point of distance.
We have already seen that the diagonal of the perspective square, if produced to meet the horizon on the picture, will mark on that horizon the distance that the spectator is from the point of sight (see [definition], p. 16). This point of distance becomes then the measuring point for all horizontals at right angles to the picture plane.