VERTICAL ILLUMINATORS, METALLURGICAL MICROSCOPES 87 



than the size of the illuminator threads it may be moved to the 

 right or left for alignment, and clamped fast by turning the screw 

 collar of I. L is a 6-volt i6-candle-power tungsten lamp coated 

 with "frosting compound" in which fine graphite has been sus- 

 pended. This provides an absolutely opaque covering for the 

 lamp and prevents annoying side lights in working. A tiny 

 clear area is made in the coating at O by means of a little alcohol 



FIG. 40. Vertical Illuminator with Simple Device for a Tungsten 

 Lamp Radiant. 



on a bit of rag or cotton. The lamp L is attached to the end of 

 S in a slot cut for the purpose and is secured in place by wires 

 soldered to the terminals of the lamp. To obtain rigidity these 

 wires pass downward through bits of glass tubing i as insulators. 

 S is so bent as to bring the glowing lamp filament in line with the 

 center of the illuminator diaphragms. As shown in the illus- 

 tration the whole device, including the coating of the lamp, can 

 be made in any workshop in about an hour. A better arrange- 

 ment, when shop facilities permit, is to fasten an attaching 

 socket with bayonet catch to S. The lamp is connected with 

 the usual no- volt lighting circuit with the interposition of a 

 suitable rheostat or lamp bank, allowing the passage of 2 to 3 

 amperes (five i6-candle carbon filament lamps or two 32 and one 



