208 BACTERIOLOGY. 
plates if culture work is to be done. The iris dia- 
phragm (D), which is now regularly used in bacterio- 
logical work, opens and closes like the iris of the eye, 
and so controls the amount of light. Its opening is 
diminished or increased by moving a small arm, which 
is underneath the stage, in one or another direction. 
The reflector placed beneath the stage serves to direct 
the light to the object to be examined. It has two sur- 
faces—one concave and one convex. The coarse ad- 
Fie. 18. 
Colonies of diphtheria bacilli. >< 200 diameters. 
justment (F) is the rack-and-pinion arrangement by 
which the barrel of the microscope can be quickly 
raised or lowered. It is used to bring the bacteria 
roughly into focus. The fine adjustment (G) serves to 
raise and lower the barrel very slowly and evenly, and 
is used for the exact study of the bacteria when high- 
power lenses are used. or the microscopical study of 
bacteria it is essential that we magnify the bacteria as 
much as possible and still have their definition clear and 
