Fig. 69.
When the cube is placed below the horizon (as in [Fig. 59]), we see the top of it; when on the horizon, as in the above (Fig. 69), if the side facing us is removed we see both top and bottom of it, or if a room, we see floor and ceiling, but otherwise we should see but one side (that facing us), or at most two sides. When the cube is above the horizon we see underneath it.
We shall find this simple cube of great use to us in architectural subjects, such as towers, houses, roofs, interiors of rooms, &c.
In this little picture by de Hoogh we have the application of the perspective of the cube and other foregoing problems.
[ XXII]
The Square at an Angle of 45°
When the square is at an angle of 45° to the base line, then its sides are drawn respectively to the points of distance, DD, and one of its diagonals which is at right angles to the base is drawn to the point of sight S, and the other ab, is parallel to that base or ground line.