In order to make the zinc plate from which Fig. 233 was actually printed, a red pencil was used for the bottom portion of the illustration referring to Ben Day work. Red photographs as black, and a zinc plate was obtained which shows the marking such as would be used with a blue pencil when Ben Day work is ordered from an engraver. Ben Day work has been used on a great many illustrations in this book, and it is believed that the reader will have no difficulty in distinguishing the cuts with Ben Day work from those cuts for which hand shading was used.

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Fig- 233. Copy from Which a Zmc Plate Is to Be Made with Ben Day Mechanical

Shading

This is the copy from which the plate for Fig. 1 was made. The areas to receive the Ben Day work are designated by arrows and numbers naming the particular kind of shading desired. Sample books give a wide choice of shadings. The markings on the face of the copy regarding the Ben Day work are made with blue pencil, since blue does not photograph dark enough to affect the line engraving

GENERAL METHODS

There are a great many problems in graphic work which puzzle the person getting up a chart if there are three different variables to deal with. The problem, as ordinarily found, involves two different independent variables, and a dependent variable depending upon each of the two independent variables. Isometric drawings like Fig. 235, or solid models such as are seen in Fig. 236 and Fig. 237, can be used, but they require a great deal of labor and care to make and are accordingly not often seen.