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.

92

DESCRIPTIVE GEOMETRY

Those lines on a surface which separate light portions from
dark portions are called shade lines, and in the drawing, for purposes of clearness, are represented a little heavier than the ordinary line.
In Fig. 89, remembering the direction taken by rays of light,
it is evident that the upper, the left-hand, and the front faces of
the cube are in the light and that all the others are in the dark.
Therefore the shade lines in this case are B-D, D-E, E-L, L-F,
F-G, and G-B, and are therefore represented by heavy lines.
In Fig. 90 the shade line B-G is invisible and is therefore
represented by a heavy broken line.
When two lines of the surface, one a shadp line and the other
an ordinary line, are projected upon the same line, it is customary
to give preference to that line which is visible. For example, in
Fig. 89 the lines A-E and F-L have a common horizontal projection a,-e,, but the line A-E is the visible line, and inasmuch
as it is not a shade line the projection a,-e, is made light.
In Fig. 90, B-D and G-K have a common horizontal projection,
but since B-D is the visible line and is a shade line, b,-d, is made
a heavy line.
Following the directions now given, it will be easy to determine
the shade lines in the remaining diagrams.