STAINING OF 6aCTEEIA. 145 



student should acquire, as soon as possible, the habit of 

 keeping both eyes open, when using the microscope. 



Examination with the -hth inch homogeneous oil immersion 

 objective. — Having studied the bacteria in the hanging-drop 

 with the No. 7 objective, replace the No. 3 objective, and 

 again find the edge of the drop. Now raise the tube of the 

 microscope, by means of the coarse adjustment, and bring 

 the iVth inch objective into position. Place a drop of cedar 

 oil on the center of the cover-glass, and lower the tube till 

 the objective touches the oil. As the field is now very 

 dark, open the diaphragm slightly. Focus the edge of the 

 drop with the fine adjustment, holding the slide between 

 the fingers of the left hand. Carefully study, in the man- 

 ner already indicated, the bacteria which are present at 

 the edge, and in the different parts of the drop. 



Laboratory wm-h. — The student will make hang-ing-drops of the 

 several chromogenic bacteria, which have been grown on potatoes, 

 according to the directions given in the following chapter. Too 

 much time cannot be devoted to this method of examination, inas- 

 much as the practice thus obtained is indispensable to the easy and 

 successful manipulation of the microscope in the subsequent work. 

 A delicate touch, and a quick perception of the smallest detail, can 

 only be acquired by repeated effort. 



When the examination of a specimen of living bacteria 

 is completed, the cover-glass should be carefully removed, 

 and placed in boiling water for 10 or 15 minutes, or in mer- 

 curic chloride solution (1 — 1000) for some hours. If, in re- 

 moving the cover-glass, the suspension touches the slide, 

 the latter should likewise be at once disinfected. 



Staining of Bacteria. 



One of the most important conditions when staining 

 bacteria is to have perfectly clean cover-glasses. The 



10 



