METHODS FOR THE MICROSCOPIC STUDY OF BACTERIA 177 



about one-quarter inch to one-third 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 inoculated surface 

 of the block in close contact with it, carefully avoiding air-bubbles. 

 Remove the slide from the lower surface of the block and invert 



FIG. 11. Warm stage, electrically heated, for the cultivation of bacteria. 



the cover-slip so that the agar block is uppermost. With a platinum 

 .loop, run a drop or two of melted agar along each side of the agar 

 block, to fill the angles between the sides of the block and the cover- 

 slip. This seal hardens at once, preventing slipping of the block. 

 Place the preparation in the incubator again for five or ten minutes, 

 to dry the agar-agar seal. Invert this preparation over a moist 

 chamber and seal the cover-slip in place with white wax or paraffin. 

 Vaselin softens too readily at 37 C., allowing shifting of the cover- 

 slip. The preparation may then be examined at leisure. 1 



1 A light, detachable, electrically heated warm-stage incubator, manufactured by 

 the Chicago Surgical and Electrical Company according to specifications furnished 

 by the writer is very satisfactory for this purpose. Bacteria may be maintained con- 

 stantly at any desired temperature between that of the room and 45 C. for several 

 days, and observed continuously without difficulty. If the warm-stage incubator is 

 attached to a graduated mechanical stage, many individual bacteria may be observed 

 in the same preparation by recording their respective positions as indicated on the 

 graduated rectilinear stage verniers. 

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