14 



the edge carefully the bacteria will be readily detected. 

 Now move the slide as mentioned above, so as to examine 

 the entire edge of the drop, and also the center. Study the 

 characteristics of the microorganisms present. 



In working with high powers, while focussing, it is 

 desirable to constantly hold the slide between the thumb 

 and forefinger of the left hand, imparting to it a slight 

 motion. If this motion is arrested it is due to pressure of 

 the objective, which has been lowered too far, and unless 

 the pressure is promptly relieved, damage may result. 



Examination with A- homogeneous oil immersion 

 objective. Having studied the bacteria in the hanging 

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

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

 the microscope, bring the iV 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. 

 Examine carefully the bacteria present, their motion, 

 structure, etc., and also different parts of the drop in the 

 manner already indicated. 



LABORATORY WORK. Make hanging drops of the four 

 kinds of bacteria growing on the potatoes, and examine as 

 above. Too much time cannot be devoted to the work at 

 this point, as the practice thus obtained is indispensable 

 to the easy and successful manipulation of the microscope. 



