CHAPTER III 

 MICROSCOPY 



Bacteriology is an outgrowth of microscopy. For many years the study of 

 bacteria was confined to microscopic observations. At present other methods 

 described in the following chapters, are also employed but microscopic studies 

 are still a large and indispensable portion of most investigations. 



For ordinary bacteriological work any of the microscopes of standard make 

 do equally well, whether of American or European manufacture. If but one 

 eyepiece or ocular is obtained with the microscope, a 4 or 6 will be found most 

 satisfactory. At least three objectives are required, % inch, % mcn an d M.2 

 inch. The % inch should have a working distance that will permit blood-cell 

 counting and the ^f 2 mcn objective is an oil immersion lens. Although a 

 mechanical stage is not essential, it is a great convenience and often a time-saving 

 adjunct. 



Microscopes are constructed to rest upon and be supported by the base. 

 When it is necessary to move or carry one from place to place the safest way is 

 to grasp the instrument below the stage; never lift it by the barrel, coarse ad- 

 justment or fine adjustment. Hold it upright, do not tilt it; failure to observe 

 these precautions in handling, often results in injury to the instrument. 



Dust, acid fumes and continuous exposure to direct sunlight are injurious; 

 therefore, when not in use a microscope is protected by a suitable covering. 



Proper illumination is very important. Direct sunlight is not desirable, 

 north light, especially from a white cloud, is the best natural light. If gas light 

 is used it should come from a Welsbach burner. Incandescent electric lights 

 should have frosted globes and be 100 candlepower or more. 



When unstained preparations are to be examined, as in the observation of 

 hanging-drops, agglutination tests and red blood-cell counting, oblique illumina- 

 tion will generally be most satisfactory; when examining stained bacteria and 

 tissue, central light is best. 



ILLUSTRATION OF MICROSCOPE INDICATING THE VARIOUS PARTS 



To bring the rays of light upon the object to be examined, the diaphragm 

 must be open. Beginners frequently overlook this. The mirror is manipu- 

 lated until the best possible illumination is obtained, using the plane side of the 

 mirror usually. Then by focusing the condenser, one determines at what 

 distance from the object it gives best results. Start with the condenser flush 

 with the stage. 



When using a low-power objective, the % or %, one may look in the ocular 

 and focus downward until the field is in focus. This, however, is a dangerous 

 procedure when using a high-power objective. 



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