SPECIAL PHASES 299vertical edges of the cube, one of which is at the rear of the cube
and so invisible, but its position must be known nevertheless.
Assuming that our eyes are level with the bottom of the cube,
the point of intersection of the nearest edge vertical and the
horizon is used as a starting point. The length of the edge is
measured upward from the horizon. From the upper end of this
line, other lines are drawn to the vanishing points. These form
the two front-upper edges of the cube. The side edges are the
projected verticals between these oblique edges and the horizon.
This is the simplest aspect of the cube of two sides; just one hori-
zontal base, three verticals and two oblique upper edges. With
the eye above or below the cube, the upper or lower face would
be seen, with edges running to the vanishing points.
Such is the very superficial description of the perspective draw-
ing. It is to be hoped it is sufficient to identify the steps in rela-
tion to a more detailed treatise.
Stereo Drawing.-This brings us to the important phase of
producing a drawing which will have three dimensions when
properly viewed.
The plan is put into perspective exactly as if the conventional
perspective drawing were to be made. Then the station point is
moved to one side a sufficient distance to permit a second plan to
Fig. 20-2. Target figures as used for testing and inclusion in
certain stereo-optical instruments.