98 



METHODS OF OBSERVING BACTERIA 



the microscope appears perfectly dark. However, the very central 

 rays of light which strike the cover-glass illuminate the bacteria and 



FIG. 



Dark-field illuminator, or reflecting condenser. 

 FIG. 40 



if I if 



ii i 1 



Optical construction of dark-field illuminator. 



FIG. 41 



Electric arc lamp with hand feed for a current of 4 amperes and illuminating lens to be used 

 with the dark-field illuminator. 



