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 demon- 

 strator 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). 



