Elements of descriptive geometry, with applications to isometric projection and othe forms of one-plane projection; a text-book for colleges and ingineering schools by O. E. Randall.

SURFACES WITH PLANE FACES

91

Fig. 96 represents a square pyramid with base parallel to H and
with axis perpendicular to H.
Fig. 97 represents a pentagonal pyramid with base parallel to
H and with axis perpendicular to H.
Fig. 98 represents the frustum of a square pyramid with bases
parallel to H.
GAL G, D
FI(G. 97 FIG. 98
230. Shade Lines. Objects in nature, as a rule, are exposed to
some source of light so that some portions of their surface are in
the light and other portions are in the dark.
The sun is usually taken as the source of light, and on account
of its great distance from the earth we may safely consider such
solar rays as fall upon terrestrial objects of finite dimensions as
parallel.
The source of light is conventionally assumed in such a position that the rays shall be parallel to that diagonal of a cube (the
cube resting on H in the first quadrant with one face coincident
with V) which slopes downward to the right toward V.
Under these conditions the horizontal and vertical projections
of a ray of light will each make an angle of 45 degrees with G-L.
When the position of the object is definitely known it will be
easy to determine which portions of the surface are in the light
and which portions are in the dark.