station point, Fig. 3. This is the eye of the observer.
The picture plane is perpendicular to the ground plane.
Example. Place a large plane of glass perpendicular to the ground. Place a cube on the other side a little way back. Keep the eye steady and trace on the glass the outline of the cube. If this is done accurately, you will have a picture of the cube in perspective. Close one eye while doing this.
All parallel lines which run directly away from the observer are called converging lines. Converging lines which are perpendicular to the picture plane vanish in the point of sight. (Fig. 1) and (Fig. 5 3 ft.)
Converging lines which are not perpendicular to the picture plane but run obliquely away from the observer, vanish in a point on the horizon, but not in the point of sight. (Fig. 5 (2 ft.))
The prime vertical (PV) is a vertical line drawn perpendicular to the horizon. It passes through the point of sight and through the station point. (Fig. 3.)
Study Fig. 3. H is the horizon, PS is the point of sight, S is the station point. Place the station point at a distance of 1 1/2 the diagonal of the picture plane. PV is the prime vertical, abed is the picture plane.
D and D' are distance points, or as far as the observer can see on each side of the point of sight. From PS to D equals from PS to S. D2 is one-half this distance; its use will be explained later.
Study Fig. 5. Find H and PS.
When wishing to represent objects by measurement, a measuring scale can be used in the foreground. This measuring
scale should be the actual measurement in feet and inches. As objects recede, they appear smaller, just how much smaller can be determined by using this scale.
In Fig. 5 let us call the spaces in the foreground feet. If a six-foot post were placed close to the (glass) picture plane (on the other side), it would appear six feet, or life size; if placed farther back it would look smaller. See the posts.
To place a six-foot post a distance back, count off three feet on the scale, connect the ends with PS. Any horizontal line (parallel to the picture plane) between these lines will equal three feet. The farther back the shorter three feet will appear.