HANGING-DROP PREPARATIONS. 



eyelet of sterile bouillon is placed on the cover-glass in the 

 same position, and a very small quantity of the culture 

 (picked up with a platinum needle) is rubbed up in the 

 bouillon. The cover is then carefully lowered over the cell 

 on the slide, the drop not being allowed to touch the wall 

 or the edge of the cell. The edge of the cover-glass is 

 covered with vaseline, and the preparation is then complete 

 and may be placed under the microscope. If necessary, it 

 may be first incubated and then examined on a warm stage. 

 (2) The sterile cover-glass is placed on a sterile plate (an 

 ordinary glass plate used for plate cultures is convenient). 

 The drop is then placed on its upper surface, the details 

 being the same as in the last case. The edge of the cell 

 in the slide is then painted with vaseline, and the slide, 

 held with the hollow surface downwards, is lowered on to 

 the cover-glass, to the rim of which it of course adheres. 

 The slide with the cover attached is then turned right side 

 up, and the preparation is complete. 



In the case of B the drop of fluid is placed on the centre 

 of the table x. The drop must be thick enough to come in 

 contact with the cover-glass when the latter is lowered on 

 the slide, and not large enough to run over into the sur- 

 rounding trench y. The cover-glass is then lowered on to 

 the drop, and vaseline is painted along the margin of the 

 cover - glass. The 

 method of micro- 

 scopic examination is 

 described on page 



83- 



The Counting of 

 Colonies. An ap- 

 proximate estimate of 

 the number of bac- 

 teria present in a 

 given amount of a 

 fluid (say, water) can 

 be arrived at by counting the number of colonies which 

 develop when that amount is added to a tube of suitable 



FIG. 26. Apparatus for counting colonies. 



