MICROSCOPICAL EXAMINATION. 207 



amount of magnification which they yield. Numbers 

 2 and 4 are most useful for bacteriological work. The 

 objective (B) the lens at the distal end of the barrel 

 serves to give the main magnification of the object. For 

 stained bacteria the 1/12 achromatic oil immersion lens 

 is regularly employed; except for photographic purposes 

 the apochromatic lenses are not needed. Even here 

 they are not indispensable. A 1/10 or 1/16 may at 

 times be useful but hardly necessary; a No. 4 ocular and 

 a 1/12 lens give a magnification of about 1000 diame- 

 ters. (Fig. 17.) For unstained bacteria we employ 



FIG. 17. 



Anthrax bacilli and blood-cells. X 1000 diameters. 



either the 1/12 immersion or 1/7 dry lens, according to 

 the purpose for which we study the bacteria; for the 

 examination of colonies where, as a rule, we do not 

 wish to see individual bacteria but only the general 

 appearance of whole groups, we use lenses of much 

 lower magnification. (Fig. 18.) 



The stage (C) the platform upon which the .object 

 rests should be large enough to support the Petri 



