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14 MAGIC LANTERN WITH DIRECT CURRENT [Cn. I
§ 13. Lamp-house. — This is a metal box in which the arc lamp is enclosed. It should be of good size, and be well ventilated by means of openings at the bottom, and a flue at the top. There should be one or more large doors, so that the lamp can be reached for changing the carbons and making any necessary adjustments. Opposite the crater at the end of the positive carbon there should be a window about 2 to 3 cm. (2 in.) square so that the ends of the carbons can be observed when the lamp is burning without opening the door. This window should be provided with a combination of red and green, or red and blue glass, or with smoky mica or with deeply tinted glass so that the eyes will not be injured when looking at the crater (fig. 133, 147).
§ 14. Incandescent lamp. — If experiments are to be made it is desirable to have an incandescent lamp with wire guard to use in connection with the lantern. It should have a flexible cord of sufficient length so that it can be carried to any desired position. This lamp must be connected with the supply wires before the rheostat is inserted; then it will burn brightly while the arc lamp is going. By consulting fig. 2, it will be seen that the two wires for this lamp are connected one with each of the supply wires. That is the incandescent lamp is not connected with one wire like the rheostat and the ammeter but with both wires.
§ 15. Electric flash-light. — An electric flash-light is a great convenience about a lantern; and is almost a necessity when an incandescent light (fig. 1,2) is absent. It should lock, so that it will burn continuously ; then carbons may be changed by its light and other corrections made. It is an absolutely safe light also.
§ 16. Incandescent lamp to burn when the arc lamp is turned off. — To avoid the great darkness in the room when the arc lamp is turned out, it is advantageous to have an incandescent lamp connected with the line, as indicated in fig. 4.
§ 17. Condenser.— This collects the light from the arc lamp and directs it through the objective. In passing from the condenser to the objective it passes though the lantern slide or other object whose image is to be projected (fig. i, 24),