If similar polygons be inscribed in similar positions in the circles
_kn_ and _lm_ (Coroll. Prob. XII.), and the corresponding angles of
the polygons joined by right lines, the resulting figure will be a
portion of a polygonal pyramid. (The dotted lines in Fig. 38.,
connecting the extremities of two diameters and one diagonal in the
respective circles, occupy the position of the three nearest angles of
a regular octagonal pyramid, having its angles set on the diagonals
and diameters of the square _ad_, inclosing its base.)
If the cone or polygonal pyramid is not truncated, its apex will be
the center of the upper square, as in Fig. 26.