The major defects in faulty lantern slides are usually: (1) the scale of lettering on the original copy is too small; and (2) the lines, letters, and figures are overcrowded on the original copy. These defects result in the projected image being illegible on the screen. As mechanical or photographic defects can be eliminated by reliable lantern-slide makers they are not discussed here.

A standardized procedure is presented for use in drafting maps, charts, tables, et cetera, for lantern-slide presentation. This procedure consists of a simple formula and a series of charts to illustrate the minimum letter and figure heights that should be used on the original lantern-slide material. The formula is based on the following three factors that control the legibility of lantern slides: (1) maximum size of lecture room, or maximum audience distance, (2) size or width of original lantern-slide copy, and (3) size or width of screen on which lantern slide is to be projected.

Illustrations of geological maps, cross sections, and electric logs have been prepared following the recommended formula.

The dimensions set forth here for original lantern-slide copy represent a figure and letter size appreciably larger than the minimum size employed for routine drafting work. This discussion of dimensions also supports the general conclusion that most original copy must be redrafted for lantern-slide reproduction in order to provide larger figure and letter sizes, if good results are to be secured.