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 
to be preferred for ordi¬ 
nary work. Satisfactory 
results can only be ob- 
Fig. 93. 
Microscope Lamp; E. Leitz. Arc Type. 
Automatic. 
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- 
Fig. 94. Galvanometer Lamp of the 
Cambridge Scientific Instrument 
Co. Nernst Type. 
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. 
