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A MANUAL OF BACTERIOLOGY 



observer then, looking down the microscope, very cautiously 

 and gradually racks down again with the coarse adjust- 

 ment until the object comes into view, and finishes the 

 focussing with the fine adjustment. The fine adjustment 

 should only be used after the object has been brought into 

 view by means of the coarse adjustment. After the 

 examination has been concluded for the day, the lens 



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y 



FIG. 24. Diagram to illustrate the refraction of light. 



should be carefully wiped with a soft rag, or preferably 

 with a piece of soft Japanese paper, to remove the water or 

 oil. If the oil should happen to dry on the lens, it may be 

 removed by wiping with a soft rag or Japanese paper moist- 

 ened with xylol, quickly drying with another rag or paper. 

 Instead of cedar-oil, a liquid paraffin has also been used. 



The T ^ in. (2 mm.) oil-immersion lens is the one usually 

 selected. It combines sufficient magnification for most 

 purposes with adequate working distance for convenience 

 in using. If expense is not an object, the Zeiss J in. 

 (3 mm.) apochromatic oil-immersion lens is a very fine one 

 for general use. By means of the compensating oculars 



