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CH. I] MAGIC LANTERN WITH DIRECT CURRENT 19
9. The darkness of the projection room (see §608). 10. The proper adjustment of the eyes of the spectators to either daylight or twilight vision (§ 281).
USE OF A MAGIC LANTERN FOR EXHIBITIONS AND FOR DEMONSTRATIONS
SUGGESTIONS TO THE LECTURER OR DEMONSTRATOR!
§ 21. Order of the lantern slides. — The lecturer or demonstrator should have his slides in the exact order in which they are to be shown. They should not only be in the exact order of exhibi tion, but they should all be in the same relative position so that the operator can insert them correctly without the trouble of looking at them individually.
§ 22. Duplication of lantern slides. — It frequently happens that the same slide, for example, of a map or some other general subject, should be shown at two or more stages of a lecture. There is always difficulty in doing this unless the operator is carefully instructed, and the slide is marked to be repeated, and a slip of paper inserted in the pile of slides at the proper level. With a small audience, and for an informal talk the difficulty is, perhaps, not great; but for a large audience and anything like a formal presentation, the repetition of the same slide almost always causes confusion and delay.
To avoid this confusion, one can have duplicate lantern slides. Then the slides can be put exactly in order, and no confusion is possible.
If a person has ever exhibited lantern slides for a friend, and one or more of the slides had to be shown two or three times, he can understand the troubles of the operator when the same slide must be shown more than once, and will agree that it is better to have the slide duplicated.
§ 23. Marking or "spotting" lantern slides. — In order that lantern slides may be inserted in the carrier by the operator correctly, and without hesitation or worry, the slides must be marked or "spotted" in some conspicuous way (fig. 7, 8, 13).