114 THE USE OF THE MICROSCOPE 



require a more or less expert worker, and may be tedious at 

 first. The use of an oil-immersion objective of medium 

 aperture is to be advocated in cases where many balsam 

 mounts are to be examined, and especially where a high- 

 aperture, oil-immersion objective is also used. It would 

 also be convenient where stained bacteria are examined 

 without a cover-glass. The water-immersion objective of 

 medium aperture is adapted for use when most of the 

 objects are mounted in aqueous media, and especially when 

 a water-immersion objective of high aperture is also 

 employed. For optimum work all objectives (including 

 oil-immersion objectives) must be used with correct covers. 



Practical Points 



1. It is best to select covers of the right thickness; but in other 

 cases one may correct the 40-times objective for different cover- 

 glass thicknesses by altering the tube length; choosing one of 

 0.65, instead of 0.85, aperture. 



2. We may also omit dry objectives higher than 10, and use 

 only No, 1 covers, between 0.15 and 0.17 millimeter in thickness. 



3. In some cases a high dry objective of 0.85 aperture with 

 correction collar is convenient. 



4. The high dry objective may be replaced by an oil-immersion 

 objective of equal aperture (which requires a much smaller 

 correction for wrong covers). 



5. Or a water-immersion objective with correction collar may 

 sometimes be used with advantage. 



6. The cover-glass itself may be omitted, and special oil- 

 immersion objectives (or specially corrected dry objectives) used 

 throughout. An ordinary 90 oil-immersion objective of 1.3 

 aperture gives nearly as good images without a cover-glass as 

 with one (if the tube is lengthened about Ij/^ centimeters) since 

 the cover deficit is replaced by an equal thickness of immersion 

 oil. 



7. If an extra thick cover-glass is sometimes met with on any 

 object, the remedy is not necessarily to purchase a microscope 

 with a large extension and contraction of tube length. It seems 

 more practical to remove such thick covers, and substitute 

 normal ones. If the objects are mounted on the cover-glass, 



