70 ALTERNATING AND DIRECT CURRENT LANTERNS [Cn. II 



But in installing a magic lantern for either current, it must 

 never be forgotten that the arc lamp must not be connected with the 

 main line without some form of rheostat or regulating device in the 

 circuit (fig. 3, 40, and 744). 



FIG. 39. MAGIC LANTERN WITH INCLINED CARBONS. 



U C, L C The upper and the lower carbon. Only the carbons of the arc 

 lamp are shown. 



A C Alternating current supply wires. 



F Fuses at the outlet box (see fig. 40). 



L Incandescent lamp for use in working around the magic lantern. 



S Double-pole, knife switch. 



R Rheostat in one wire. 



A Ammeter for indicating the amount of current. 



Condenser A two-lens condenser. The light is shown as a parallel beam 

 between the lenses. It is usually diverging (see fig. i). 



L S Lantern slide next the condenser. 



Axis Axis The principal optic axis of the condenser and the projection 

 objective. 



Objective The projection objective for forming the screen image. 



c Center of the projection objective. The objective and condenser should 

 be so related that the light from the condenser crosses at the center when the 

 image is in focus on the screen. 



Screen Image The image of the lantern slide on the screen. 



107. Double-pole table switch. This is especially necessary 

 when using an alternating current, because with it the current can 

 be turned completely off the lamp whenever desired. Any changes 

 in the carbons or in the lamp mechanism can then be made with 

 safety, as the lamp is completely cut off from the electric supply, 

 which would not be the case if a single-pole switch were used. 

 The shock from an alternating current supply of no volts is much 



