Perspective — We know now that lines receding from the eye appear to meet at the horizon, the lines above the point of sight going down to the horizon, those below rising to it. This appearance is in line with the rule of perspective which says: A level line below the eye when not parallel to it, must be drawn upward from its nearest point, .and, inversely, similar lines above the eye must be drawn downward.
To illustrate the matter practically, one has only to step into the middle of the street. Look along the lines of the curbstone, the lower lines of the houses, the windows and the lines of the roofs. Then, by holding a pencil parallel to one's eyes, but about a foot away from them, it will be found that the lines which are above the eye run down to some point on a line level with the eye and that the lines below run up, but meet or tend to meet at the same point.
For further purposes of illustration, let us return to our beach by the sea. Let us suppose that there are three little huts there, a few feet apart and nearly on a line with each other.
Stand in front of one of them and you will see nothing of its sides. The sides of the others will be visible, but with the lines of the boards converging to a vanishing point, which is also the point of sight, in the center of the middle hut. Now walk away to the left of the huts and you may observe that all the lines of the sides that faced you before recede to a vanishing point within the picture. On the other hand, the sides that before receded from your line of. vision now are almost facing you. (Fig. 26.)
Let us suppose that the tide has come and gone and taken the little huts a few feet out to sea. The plane diagram (Fig. 27) shows where they were carried. Looking at them now they appear as in Fig. 28. They are no longer parallel, but they are on a common level; therefore, they have the same horizon line; the line of vision is the same for each, but each has its individual vanishing point. The two huts at the right and the left each have different vanishing points.
The drawings of the three huts clearly explain that, although each one has its own vanishing point, C having the point of sight as its only vanishing point, nevertheless, lines described from each pair of points to D, the point of station, form a right angle. The lines referred to are drawn in varying styles in order to show the angles more plainly. The hut on the left with single lines, the hut in the middle with dotted lines, and the hut on the right with double lines.