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, C 1 



containing reflecting erecting 

 prisms. Above the cham- 

 bers are the oculars E, E 1 , 

 provided with sliding dia- 

 phragms D 1 , D 2 . 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 D 1 , D 2 

 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 1 , T 2 

 J. are tightened to prevent the 

 adjustment from changing. 

 By proper manipulation of 

 the sliding diaphragms, the 

 observer looking into the 

 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 1 , D 2 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. 



FIG. 28. The Leitz Comparison Microscope. 



