78 



PRINCIPLES OF BACTERIOLOGY 



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 surfaces one concave and one 

 convex. The coarse adjustment F is the rack-and-pinion arrange- 

 ment by which the barrel of the microscope can be quickly raised or 

 lowered. It is used to bring the bacteria roughly into focus. If the 



FIG. 53 



FIG. 54 



-Eye-lens 



'Body Tube 



of the Ocular 



Draw Tube Diaphragm 

 with Society Screw 



--Society Screw 



.--Mount 



Back-lens 

 Middle-lens 

 Front-lens 

 Working Distance 



ofthe 

 Objective 



Microscope. 



Slide-" 

 Object' 



Internal structure of the microscope. 

 (After Spencer Lens Co.) 



bearings become loose tighten the little screws at the back of the pinion 

 box. Keep the teeth clean. If the bearings need oiling use an acid-free 

 lubricant, such as paraffin oil. 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. It is neces- 

 sarily of limited range and delicate in its mechanism. If, when look- 



