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1138 HANDBOOK OF PROJECTION FOR
you will see a flat copper contact attached to the edge of the rotating table, but insulated therefrom. At B you will see a contact screw. There are two of these for each lamp. They represent the terminals of the lamp filaments. When the base is rotated one-third of a tune, the two you see (A and B) will be brought into electrical contact with the two flat, current carrying "springs" C and C, which automatically completes the circuit of the lamp. The sharp point of contact screw B slips into an indentation in contact bar O when the lamp is in operating position, which acts to locate the lamp in exactly the correct position.
These lamps are so made, and their sockets so placed that when the lamp is in the socket, its filament will, without further attention, be "square" with the slit. In other words, the filament will be exactly at right angles to the optical axis of the slit optical system contained in slit assembly barrel Z, Fig. 413.
We feel bound, in fairness, to say this is a very clever and effective arrangement for having spare exciting lamps always ready for instant replacement of any lamp which may fail.
The exciting lamp itself is a small incandescent globe having a coiled filament of small diameter held or supported in horizontal position. Such lamps and their filaments may be examined in Fig. 394. It is absolutely essential to good work that the exciting lamp filament be and remain in an exactly straight, exactly horizontal line. In this connection, examine and study the matter on pages 1057 to 1066.
WARNING. — Do not neglect to read and understand the matter contained in the above reference. A sagged or warped exciting lamp filament will not only decrease