68 
ELEMENTARY CHEMICAL MICROSCOPY 
is adjustable up and down by a friction collar. To the upper 
end of each microscope tube is attached a large chamber C, 
containing reflecting erecting 
prisms. Above the cham¬ 
bers are the oculars E, E^, 
provided with sliding dia¬ 
phragms D^, Dl The prism 
chambers are so constructed 
as to rotate through a small 
arc in the directions of the 
arrows, thus bringing the 
eyepieces nearer together or 
farther apart for adjustment 
of the proper pupillary dis¬ 
tance of the observer. The 
upper half of each eyepiece 
can also be rotated so that 
when the diaphragms 
are inserted to cut off half the 
field in each ocular, they may 
be turned until the diam¬ 
eters of each half field are 
parallel or coincident. After 
turning through the proper 
arc the thumb screws T\ T^ 
are tightened to prevent the 
adjustment from changing. 
By proper manipulation of 
the sliding diaphragms, the 
observer looking into the 
Fig. 28. TheLeitz Comparison Microscope. 
instrument with an eye above each ocular sees half the field 
from one preparation and half from the other in close juxtaposi¬ 
tion. A very rapid yet critical comparison of one preparation 
with another is thus easily accomplished. Or D^, may be so 
placed as to cut out the field of either tube, or if both are pushed 
in as far as they will go the fields will be superimposed, and the 
symmetry of two objects may be compared. 
