72 MAGIC LANTERN WITH ALTERNATING CURRENT [Cn. II 



/ 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 

 burn 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 sidewisc 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 



