THE METHODS OF MOUNTING MICROBES 89 



section, which ought to be so thin as to be almost 

 invisible.' 



After staining, etc., the sections are mounted in 

 various media on glass slides (3 in. X 1 in.), and 

 covered with thin cover-glasses. 1 ' When high- 

 power lenses are to be used it facilitates the work 

 very much to know the exact thickness of the cover- 

 glass under which the specimen is mounted, and 

 with very high powers, or those with wide angles of 

 aperture, the cover-glass must be at least 0'004 in. 

 to enable the lens to work through it.' All Zeiss' 

 objectives in fixed mounts are corrected for a cover- 

 glass of medium thick- 

 ness (between 0'15 

 and 0'2 mm., or 0'006 

 and 0-008 in.). In the 

 higher series from CC 

 upwards the thickness 

 of the cover-glass con- 

 sistent With the mOSt Flo> . 27 , z EIS8 - COVER-GLASS TESTER. 



perfect correction is 



indicated on the side of the mount by small figures 

 (mm.). As a rule, it is sufficient for ordinary work 

 to use cover-glasses of an estimated medium thick- 

 ness. 



Oil-immersion objectives are within wide limits 

 independent of the thickness of the cover-glass. 

 But considerable variations in the thickness of the 

 cover-glass may be compensated for by slightly 

 lengthening the body-tube for thinner cover-glasses ; 

 and by slightly shortening the body-tube of the 



1 The round ones are better than those that are square. 



