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 
Fia. 17. 
Anthrax bacilli and blood-cells. > 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 baeteria 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 
