1 6 MAGIC LANTERN WITH DIRECT CURRENT [CH. 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. 



.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 line 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. 



/ 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 con- 

 vex surfaces of the plano-convex lenses face each other as in the double con- 

 denser (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 

 cm 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 plano- 

 convex, as is also the third (fig. 2). If the condenser is of two 

 lenses both are usually plano-convex with the convex surfaces fac- 

 ing 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). 



AVith the two-lens condenser the usual practice is to bring the 

 condenser closer to the lam]) than the focal length of the first lens. 



