76 DARK-GROUND ILLUMINATION WITH HIGH POWERS [Cn. II 



light is next in brilliancy to sunlight. This also has its drawbacks, 

 for on account of the burning away of the carbons the light shifts and 

 requires frequent adjustment of the carbons or of the mirror or both. 

 The newly invented "Pointolite" overcomes the shifting, but is 

 available only where direct current can be had. The most satis- 

 factory light up to the present is produced by the " headlight " 



FIG. sob. SMALL ARC LAMP AND CONNECTIONS. 

 (From Optic Projection.) 



A Small arc lamp base with right-angle carbons (HC, VC) with insulation 

 (In. In). 



The arc and the condenser (C) are in position to give a parallel beam. 



Ch Chimney over the arc, T, tube holding the condenser; sh, metal shield 

 at the end of the condenser tube. 



Wi Wire cable to the lamp socket (So) with its key switch (K). 



Sp Separable plug. 



W 2, W 3 Wire to the upper or horizontal carbon (H C). 



W 4 Wire to the rheostat (R) and to the lower or vertical carbon (VC). 



C Tips of carbons for alternating current. 



D Tips of carbons for direct current, the positive pole always being on the 

 horizontal carbon (+), and the negative pole on the lower or vertical carbon ( ). 



E Shield at the end of the condenser tube; the face of the condenser is shown 

 at (C). 



