248 CENTERING FOR MICRO-PROJECTION [Cn. IX 



the greatest brilliancy is when there is a slight yellowish tinge to the 

 light. It will be pure white if one moves the stage and objective 

 slightly nearer the condenser, but it will not be so brilliant. Guid- 

 ing marks should be made on the apparatus at the best position 

 for the different objectives used (fig. 133, 138). 



377. Table of Can die-Power and Current with Direct Current 

 Arc and Right-Angled Carbons : 



Size of Carbons Amperes Candle-Power 



6 mm. 2 200 



6 mm. 3 400 



6 mm. 4 650 



6 mm. 5 goo 



8 mm. 7.5 1.500 



ii mm. 10 2,200 



1 1 mm. 12.5 2,900 



ii mm. 15 3 ,700 



1 1 mm. 17.5 4>5o 



13 mm. 20 5.400 



13 mm. 25 7.500 



15 mm. 30 9.500 



378. Increase in size of the crater with increase of amperage. 



As the size of the crater and hence its image increases with the 

 increased amperage, the gain for actual micro-projection is not so 

 great as would appear, for the larger crater image will be larger 

 than the lenses of objectives of high power, hence, much light is 

 wasted (fig. 141). 



The heating is also much increased by the higher amperage. 

 It has been found by experience with everything in the best possible 

 condition that 12 amperes is sufficient for most micro-projection. 

 A current above 20 amperes is a pure waste, as well as a source of 

 danger to the specimens and apparatus by overheating. The light 

 given by 10 amperes properly utilized yields far better results than 

 that from 20 amperes only partly utilized. For the candle-power 

 with different amperages see 377. 



