72 MAGIC LANTERN WITH ALTERNATING CURRENT [Ch. II
1 The first element of the condenser consisting in a meniscus lens next the
arc lamp and a plano-convex lens.
2 Plano-convex lens toward the lantern slide. The lenses of this condenser
should be arranged as here shown.
Objective The projection objective.
c The optic center where the rays from the condenser should cross when the
objective is in focus.
Base Board The board bearing the track and the blocks for supporting the
different parts.
Block i, Block 2, Block j The blocks supporting the arc lamp, condenser
and objective.
Rods The rods or tubes on the base-board and serving as a track for the
blocks to move upon.
§ 108. Arc lamps for alternating current.—These are almost
invariably of the hand-feed type. Lamps are made to hold the
carbons: (i) at right angles (fig. 1-3); (2) inclined backward 30
degrees (fig. 23, 39); (3) converging in the form of a V (fig. 23
D); or (4) even in a vertical position (fig. 22). Each form is
best adapted to some special purpose.
With carbons of the same size and composition both carbons
bum away at the same rate, and therefore must be fed forward at
the same rate. If the carbons are of different size or material, then
the mechanism must be adjusted to move the two at a rate which
shall hold the ends at the same level.
§ 109. Fine adjustments for the lamp.—As indicated for the
direct current arc lamp (§ 10), there should be some means of
moving one or both carbons separately to compensate for any
unequal burning. There must also be some means of raising and
lowering the lamp and moving it sidewise so that any slight varia¬
tions of the source of light from the axis may be corrected
(§ 10, fig. 3).
§ 110. Lamp-House.—There should be a well ventilated metal
lamp-house of good size and with large doors, so that all the
apparatus within can be easily got at. There should also be a good
sized window (say 5 cm., 2 in. square) glazed with smoky mica
or a combination of green and red glass or some smoked glass of
sufficient depth of tint for the protection of the eyes. This window