178 
ELEMENTARY CHEMICAL MICROSCOPY 
able by micrometer screws attached to graduated drums D^, D^. 
The pitch of the micrometer screws is identical in each instru¬ 
ment. One complete revolution of a drum is an aliquot part 
of one division on the scale of the stage S^. The object to be 
studied is placed upon the section S ^ of the stage and clamped in 
place, the stage S having been first moved by hand by the knob 
K to the most convenient place for beginning the measurements. 
