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- 



FIG. 18. 



Colonies of diphtheria bacilli. X 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. For the microscopical study of 

 bacteria it is essential that we magnify the bacteria as 

 much as possible and still have their definition clear and 



