METHODS OF OBSERVING BACTERIA. 87 



keep on until it almost touches the glass, then look into 

 the microscope and find the object by slowly and firmly 

 racking up. As soon as the object comes into view 

 leave the rack and pinion and focus with the fine adjust- 

 ment. 



Always select the light from a white cloud if possible ; 

 if there are no white clouds, choose the clearest whitest 

 light possible. Never under any circumstances employ 

 sunlight, which is ruinous to the eyes and useful only 

 for photomicrography. 



In using low-power lenses the Abbe condenser must be 

 moved away from the object and the light modified by 

 the iris-diaphragm. The distance between condenser and 

 object should correspond more or less closely with the 

 distance between objective and object. 



In using high powers the Abbe condenser must be 

 brought near the object and the light modified by the 

 iris-diaphragm. 



If the oil-immersion lens is used, it is perhaps best to 

 employ the plane side of the mirror. When with this 

 lens a section of tissue is examined for details, the light 

 must be modified by the iris-diaphragm, opening and 

 closing it alternately until the best effect of illumina- 

 tion is achieved. If tissue be searched for stained bac- 

 teria, and no cellular detail is required, the diaphragm 

 should be wide open to admit a great flood of light 

 and extinguish everything except the deeply-colored 

 bacteria. 



When unstained bacteria are to be examined with the 

 oil-immersion lens, the diaphragm should be closed so 

 as to leave only a small opening through which the 

 light can pass. 



Bacteria may be examined either stained or unstained. 

 The former condition would always be preferable if the 

 process of coloring the organisms did not injure them. 

 Unfortunately, it is generally the case that the drying, 

 heating, boiling, macerating, and acidulating to which 

 we expose the organisms in the process of staining alter 



