196 MICROSCOPIC AND CULTURAL STUDY OF BACTERIA 
1. Examination of Living Bacteria.— A. Hanging Drop.— The motil- 
ity, shape and size of bacteria may be studied in a "hanging-drop" 
preparation. A drop of fluid from a bacterial culture in liquid media 
is transferred to the center of a thin cover-glass. If the growth is 
upon solid media a drop of physiological salt solution^ is placed upon 
the center of the cover-glass as before, and a very small amount 
of the culture is removed with a platinum needle and emulsified in 
it. Next, the rim of the concavity in a "hollow-ground slide" is 
ringed with vaseline and the cover-glass is inverted over it in such a 
manner that the drop is suspended in the hollow, but touches neither 
the sides nor the bottom. The vaseline seals the preparation, causing 
it to adhere to the slide, and also prevents evaporation. The prepa- 
ration is now ready for microscopic examination. The i or | inch 
objective should be used, with the diaphragm partly closed to reduce 
the intensity of illumination. It is desirable to focus first upon the 
edge of the drop; the edge is sharply defined and readily located. 
Bacteria are usually more numerous at the edge than in the center of 
the drop. 
L • — " I 
Fig. 10.— Hollow-ground slide for hanging drop. 
B. Hanging Block. — It is desirable occasionally to follow the devel- 
opment of bacteria through several generations, to study the germi- 
nation of spores, or to examine special structures within the bodies 
of individual organisms. The hanging-drop method is unsuited for 
this purpose, which presupposes immobilization of the organism. 
Hill- has invented an ingenious modification of the hanging-drop 
method, the hanging block, which fulfils this requirement. His 
directions for preparing it are : 
"Pour melted nutrient agar into a Petri dish to the depth of about 
I or J inch. Cool this agar and cut from it a block about j inch to ^ 
inch square and of the thickness of the agar layer in the dish. This 
block has a smooth upper and under surface. Place it, under side 
down, on a slide and protect it from dust. Prepare an emulsion, in 
sterile water, of the organism to be examined if it has been grown on a 
solid medium, or use a broth culture; spread the emulsion or broth upon 
the upper surface of the block as if making an ordinary cover-slip 
preparation. Place the slide and block in a 37° C. incubator for five 
to ten minutes to dry slightly. Then lay a clean sterile cover-slip on 
the inocidated surface of the block in close contact with it, carefully 
a\'oiding air-bubbles. liemove the slide from the lower surface of the 
block and in\'ert the cover-slip so that the agar block is uppermost. 
With a platinum loop, run 1 or 2 drops of melted agar along each side of 
1 Physiological salt solution is prepared by dissolving 8.5 grams NaCl in distilled 
water, 1000 ec. 
•= Jour. Med. Res.. 1902. 7, 202. 
