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CH. V] MAGIC LANTERN WITH PETROLEUM LAMP 121
flame or incandescent mantle) , may be utilized in making the screen image. This is of fundamental importance (fig. 64, 90).
(E) Use of twilight vision. — It is astonishing how dim a picture can be clearly seen after one's twilight vision has become fully established. According to careful investigations the sensitiveness of the eye may be increased from 35 to 2500 times by the adaptation to dim light (§ 281).
The old lanternists used to advise that the exhibition should not begin until the audience had been in the darkened room for half an hour "to get," as they said, "the sunlight out of their eyes." We would say to "get the twilight vision well established."
§ 194. Time required for lighting up. — The gas light and the acetylene light are quickly established, but the petroleum and the alcohol lights require several minutes to get up the best illumination. These two should then burn during the entire time of an exhibition. If the lecturer cannot arrange to have all the slides continuously, but must have them at intervals during the lecture, the operator should make use of an objective shield (fig. 14, 62), and leave the lights on all the time.
§ 195. Rehearsals. — As these lights are more difficult to manage and the results are less satisfactory than with the more powerful radiants, so much the more should the operator rehearse before the lecture and make sure that everything is in as nearly perfect order as human skill can make it.
THE MAGIC LANTERN WITH A PETROLEUM LAMP
§ 196. The petroleum lamps now used as radiants for projection have two, three or four wicks. The wicks are wide (about five cm., two in.) and are placed edgewise to the condenser. If more than two wicks are used the two outer ones are inclined inward (%. 66).
Sometimes instead of being ranked side by side, the different wicks are arranged like the lines forming the letter W, but there is no advantage in this.