CH. X] DRAWING WITH HOUSE CURRENT 349 



502. Concave mirror without a substage condenser. For 



objectives of 20, 25, 30, 35, 40 and 50 mm. focus use the concave 

 mirror without a substage condenser. 



503. Immersion objective. -For immersion objectives used 

 in drawing do not forget to use the proper immersion liquid 

 between the cover-glass and the objective ; cedar oil for the oil 

 immersions, and distilled water for the water immersions. 



FIG. 195. SHIELD AT THE END OF THE ARC LAMP CONDENSER TUBE TO 

 AID IN CENTERING THE LIGHT. 



This disc is of blackened sheet iron, asbestos or cardboard and is 125 to 150 

 mm. in diameter. It is placed at the end of the lamp condenser tube. If the 

 light is centered, then that reflected back from the substage condenser and 

 microscope mirror will enter the lamp condenser (C). If the light is not cen- 

 tered there will be a round spot of light somewhere outside the lamp condenser. 

 In that case the mirror must be turned slightly until the reflected light enters 

 the lamp condenser. 



If the plane mirror is used the spot of light will be nearly circular; with the 

 concave mirror it will be crescentic. 



C The lamp condenser. 



I Spot of light outside the condenser showing that the light is off the center. 



