160 



ELEMENTARY CHEMICAL MICROSCOPY 



ment as long as the arc is burning. Unless a considerable sum 

 of money is invested in a very high-grade automatic lamp, it 



will be found better to 

 use hand feed arcs. 

 Cheap automatic lamps 

 are rarely satisfactory 

 and it is only when ex- 

 pensive outfits are pur- 

 chased that steady un- 

 interrupted feeding of 

 the carbons takes place, 

 yielding an arc of uni- 

 form brilliancy and non- 

 flickering crater. Hand 

 feed lamps are therefore 



FIG. 93. 



Microscope Lamp; E. Leitz. Arc Type. 

 Automatic. 



to be preferred for ordi- 

 nary work. Satisfactory 

 results can onlv be ob- 



tained from good carbons. These 

 should be moderately soft and of 

 uniform composition. 



In most cases the interposition 

 of a cell filled with water between 

 the arc lamp and preparation is 

 essential in order to prevent 

 damage to optical apparatus and 

 specimens by heat. Filling the 

 cell with a solution of alum or 

 ferrous sulphate is no better than 

 pure water alone. 



Next to the carbon arc, the 

 Nernst lamp is most satisfactory, 

 so far as light intensity and con- 

 venience of mounting are con- 

 cerned. Fig. 94 shows a Nernst glower galvanometer lamp 1 

 which serves admirably for microscopic work, especially for 



1 Made by the Cambridge Scientific Instrument Co., Cambridge, England. 



FIG. 94. Galvanometer Lamp of the 

 Cambridge Scientific Instrument 

 Co. Nernst Type. 



