CH. X] DRAWING WITH HOUSE CURRENT 343 



If direct current is used, the lamp will burn for about six 

 minutes and the carbons should be fed together every three to five 

 minutes. (See fig. 205). 



CONDENSER, STAGE AND MICROSCOPE FOR DRAWING WITH THE 

 HOUSE LIGHTING SYSTEM 



492. Drawing outfit. If one has a drawing outfit consisting 

 of the projection apparatus shown in figure 182, all that is necessary 

 to do is to place the arc lamp with its small carbons in the lamp- 

 house and arrange it exactly as for projection. 



The procedure is precisely as described above for the ordinary 

 arc lamp on the usual special lantern lighting system (Ch. IX). 



493. Small Current Outfit. This consists of an arc lamp 

 using small carbons (6 to 8 mm. in diameter) and a rheostat or an 

 inductor (fig. 197) not allowing over 5 to 6 amperes of current to 

 flow. Instead of the usual large condenser (fig. 1 2 1) , a small, single, 

 convex lens is used. This is of 70 to 100 mm. (3 to 4 in.) focus, 

 and 37 to 50 mm. (i^ to 2 in.) in diameter, and is placed in a tube 

 extending straight out from the upper carbon. Usually, also, the 

 lens is in a sliding tube, so that it may be varied in distance from 

 the source of light. If it is at its focal distance from the light, the 

 beam will be approximately parallel (fig. 189); if farther from the 

 light, the beam will be converging. 



494. Method of using the lamp with a special condenser. 



There are three methods of using this arc lamp and special con- 

 denser : 



(1) The lamp can be put in line with the drawing microscope 

 and a converging beam thrown directly on the specimen as for the 

 large apparatus (fig. 132), the mirror and sometimes the substage 

 condenser having been removed or turned aside. 



(2) The mirror is removed from the microscope, but the sub- 

 stage condenser is left in position, and a parallel beam of light 

 thrown directly into the substage condenser along the optic axis 



(fig. 20lA). 



