144 BACTEBIOT^OGY. 



Examination of the hanging-d/rop. — Place the slide, with 

 the cover-glass uppermost, on the stag'e of the microscope 

 and find the edge of the drop with a low power — No. 3, or 

 frd inch objective. If too much light is present, constrict 

 the diaphragm. The edge of the drop should be seen as a 

 sharp line passing through the field of the microscope. 

 Holding the slide between the thumb and forefinger of the 

 left hand, slowly move it so that the edge of the drop con- 

 stantly remains in the field. In this way the entire edge, or 

 circumference of the drop, should be examined, chiefly for the 

 purpose of practice in moving a slide under the microscope. 

 Owing to the minute size of bacteria, they cannot be seen 

 under this magnification. At most, fine granules can be 

 detected. To observe the individual cells, therefore, recourse 

 must be had to a higher power — No. 7, or ^ inch objective, 

 or to the T^sth inch homogeneous, oil immersion objective. 



Examination with the No. 7 objective. — Having found the 

 edge of the drop with the No. 3 objective, replace this with 

 No. 7, by rotating the nose-piece. Then lower the tube of 

 the microscope by the coarse adjustment, till the objective 

 almost touches the cover-glass. The field of the micro- 

 scope is now, usually, very dark; hence open the diaphragm 

 a trifle to admit enough light, so as to see distinctly. . With 

 the flne adjustment now raise the microscope tube, till the 

 edge of the drop is brought out distinctly. By focussing 

 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 micro-organisms present, especially 

 with reference to form, structure and motility. 



In working with a high power, while focussing, it is 

 desirable to constantly hold the slide between the thumb 

 and foreflnger of the left hand, imparting to it a slight mo- 

 tion. 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. The 



