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CH. I] MAGIC LANTERN WITH DIRECT CURRENT
FIG. 4. MAGIC LANTERN WITH INCANDESCENT LAMP IN THE CIRCUIT AFTER THE RHEOSTAT (Compare fig. 2).
W W Supply wires.
F Fuses in the supply wires (see fig. 3).
R R Rheostat for controlling the current.
A Ammeter for indicating the amount of current.
p p The two binding posts of the knife switch. The two wires of the incandescent lamp are connected at these points.
b s The incandescent bulb and the key switch of the lamp socket. From the connections of the supply wires to the incandescent lamp it will shine whenever the socket key is closed whether the knife switch to the arc lamp is opened or closed. When the arc lamp is burning the incandescent lamp will be very dim and when the arc lamp is out it will shine with full brilliance.
51 The table, knife switch,
L The source of light.
The + 's, — 's and arrows indicate the polarity and course of the electric current.
Condenser A two-lens condenser with water-cell (W).
L S Lantern slide.
Axis The principal optic axis of the condenser and of the objective.
Objective The objective for projecting an image of the lantern slide upon a screen.
Screen Image. The image projected on the screen by the objective.
This gives a somewhat diverging beam between the two lenses. The second lens brings this diverging beam to a focus beyond its own principal focus.
This condenser is sometimes placed so that the crater of the arc lamp is at the principal focus of the first lens and the center of the projection objective at the focus of the second lens, as in fig. 2.
Whatever the form of the condenser, the lenses must be so mounted that there is freedom for expansion; and they must be so arranged that the proper lens is next the radiant (see fig. 2,3, 36 B).