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1 6 MAGIC LANTERN WITH DIRECT CURRENT [Cn. I
Rheostat This is for controlling the current. It is inserted in one wire.
Arc Lamp ^ The mechanism for holding and feeding the carbons.
F S Feeding screws for moving the carbons closer together or farther apart. The carbons can be moved separately or both at once.
V A Fine adjustment screw for moving the carbons up or down.
L A Fine adjustment screw for moving the carbons to the right or left. in in Insulation between the carbon holders and the rest of the lamp. This is to prevent the current from leaving the carbons and making a short circuit through the metal part of the lamp.
5 s Set screws for holding the carbons in place.
Lamp-House The metal box enclosing the arc lamp. The feeding screws (F S) and the fine adjustments (V A, L A). should project through the wall of the lamp-house.
Condenser A condenser composed of three lenses with a water-cell in the parallel beam between the plano-convex lenses.
1 The first element of the triple condenser is composed of a meniscus lens next the arc lamp, and a plano-convex lens next the water-cell.
2 The second element of this condenser is a plano-convex lens. The convex surfaces of the plano-convex lenses face each other as in the double condenser (fig. i).
Block i. The block supporting the arc lamp. It is movable back and forth along the track on the base-board. The socket and set screws permit the adjustment of the lamp.
Block 2. The block holding the condenser. It is movable along the track on the base-board. The socket and set screw (S) enable one to adjust the position of the condenser.
Base Board The board on which all the parts of the projection apparatus rest (see fig. 158-159)
The condenser is of two or of three lenses. If of three lenses the first lens, which is nearest the arc lamp, is usually of meniscus form, with the concavity next the lamp. The second lens is a planoconvex, as is also the third (fig. 2). If the condenser is of two lenses both are usually plano-convex with the convex surfaces facing each other and the plane faces looking toward the radiant and toward the lantern slide (fig. i).
The two condensers appear alike in form and relation of the lenses except that in the three-lens type a meniscus has been added.
In the three-lens type the meniscus and first plano-convex together render the diverging light from the lamp parallel, and the third lens or second element renders this parallel beam converging, bringing it to a focus at the center of the projection objective when the condenser and objective are properly proportioned to each other (fig. 1-2).
With the two-lens condenser the usual practice is to bring the condenser closer to the lamp than the focal length of the first lens.