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CH. I] MAGIC LANTERN WITH DIRECT CURRENT 23
§ 28. See if the lantern is centered. — Make sure that the different elements of the lantern are centered along one longitudinal axis (fig. 1,2). Then and then only will a perfect screen image be produced. If the apparatus was installed correctly in the beginning the only part liable to be out of line is the crater of the positive carbon. In burning the carbons frequently so wear away that the crater is at one side of the axis. Slight decentering of the crater can be easily corrected by using the fine adjustment designed for the purpose (§ 10, fig. 3, see also Troubles § 79).
§ 29. Slide-carrier. — Be sure that the slide-carrier works properly and easily. The "push-through" form (fig. 6), is very convenient, for while one slide is on exhibition the one previously shown can be removed and another put in place, and it can be instantly put in front of the condenser when the lecturer signals.
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FIG. 6. "PUSH-THROUGH" OR DOUBLE SLIDE-CARRIER.
1 The frame which remains in one position in front of the condenser and serves as a container and guide for the "push-through" part.
2 The movable slide-holder or "push-through" part of the carrier. It moves easily to the right and to the left. It contains two slides in the proper position for an erect image on a vertical opaque screen (see fig. 7, 8).
3 3 Notches in the movable part to enable one to grasp the slide easily.
4 Elevator serving to lift the slide wrhen at either end. In some forms the elevator lifts the entire slide from the middle instead of tilting one end.
5 6 Inclined planes at each end. These raise the elevator when the carrier is moved to either end of the base.
5 i Lantern slide in the carrier in front of the condenser.
S 2 Lantern slide in the carrier at the left end. It is of the first magic lantern (1665) and is in position to be removed or to be pushed to the right for exhibition.
§ 30. To start the arc light. — Turn on the current by closing the switch (fig. 1-4). If the lamp is of the automatic type the