Young, George, jr, Baxter, Hubert Eugene, joint author · New York, Macmillan · leaf 176
to the surface as shown in Fig.154. The line of tangency between a surface and its envelopeis called the line of sight. In general the envelope will take theform of a cylinder (see § 145 and Fig. 157) which may be com-bined with planes. Some surfaces, particularly the warped surfaces, can beshown… … fVojecting lines Fig. 154 Projedion ofthe sphere. are invisible in projection. It is well to note that some partsof a surface that are visible on the H projection may be invisibleon the V projection, and vice versa. Also th^re are other partsthat may be visible on both projections, and still others…
Geometry, Descriptive
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