on the vertical plane. At point f, where the horizontal trace intersects the base Bc of the vertical plane, raise perpendicular fP till it cuts a·e· at point P, which is the point required. For it is at the same time on the given line AE and the vertical plane K.
Fig. 20.
This figure is similar to the previous one, except that the extremity A of the given line is raised from the ground, but the same demonstration applies to it.
Fig. 21.
And now let us suppose the vertical plane K to be a sheet of glass, and the given line AE to be the visual ray passing from
the eye to the object A on the other side of the glass. Then if E is the eye of the spectator, its projection on the picture is S, the point of sight.