134 ELEMENTARY CHEMICAL MICROSCOPY 



into the vertical illuminator without getting seriously out of 

 alignment 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 expensive 



FIG. 79. Microscope Lamp; E. Leitz. Arc Type. Automatic. 



outfits are purchased that steady uninterrupted feeding of the 

 carbons takes place, yielding an arc of uniform brilliancy and 

 non-flickering crater. Hand feed lamps are therefore to be pre- 

 ferred for ordinary work. Satisfactory results can only be 

 obtained 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 convenience of mounting are con- 

 cerned. Fig. 80 shows a Nernst glower galvanometer lamp 1 

 which serves admirably for microscopic work, especially for ob- 

 taining oblique illumination in the study of opaque objects and 



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



