222 Transactions of the Society. 



|-in. diameter iipon ordinary 3x1 glass slides with vaseline or 

 Canada balsam. 



Square |-in. cover-glasses, No. 1 thickness, clean and stored in 

 absolute alcohol. 



Platinum loop (very small), or spatula. 



Filter paper. 



Two cork borers (or preferably " hollow punches ") |-in. and 

 ^-in. diameter respectively. 



Normal saline solution (sterile). 



Tube of liquefied nutrient agar, cooled to 42° C, in water bath. 



Method. 



The organism to be studied is grown upon a solid medium for 

 a short period, say six to eight hours, and the resulting growth 

 emulsified in sterile broth or normal saline solution. When work- 

 ing with delicate parasitic bacteria, such as streptococci, it is 

 essential to use saline at or near incubator temperature for this 

 purpose. 



Narrow rings of filter paper (C) are now stamped out with the 

 help of the hollow punches and one or two placed in the hanging 

 drop cell and moistened with saline. The rim of the cell is 

 prepared with vaseline in the usual manner (A and B). 



A perfectly clean coverslip is flamed, and as soon as it is cool 

 a minute drop (micro-drop), which should not be of greater 

 diameter than about 0*5 mm. to 0*75 mm., of the emulsion of the 

 bacterium is placed in its centre by means of a very small loop of 

 platinum wire. A suitable gauge of wire is • 1 mm. The slip is 

 immediately placed in position over the moist chamber. 



This procedure should be carried out with considerable speed 

 to prevent the drop from drying up prematurely. When once the 

 slip has been applied to the moist chamber, the drop neither 

 shrinks nor grows, because its vapour tension is accurately balanced 

 by that of the saline on the paper ring. If water is used to moisten 

 the paper instead of saline, the drop grows in size and quickly 

 assumes unsuitable proportions. 



The drop is then carefully searched with a -^-in. objective, and 

 any doubtful particles noted for more detailed scrutiny with an 

 ■jVin- oil immersion. 



A series of such drops can be prepared and examined rapidly, 

 and the dilution of the original emulsion adjusted, until a drop 

 containing a solitary organism is found. 



The cover-slip of this particular preparation is then raised from 

 the cell wdth forceps, and a large drop of liquefied nutrient agar, or 

 blood broth, or other suitable medium, placed close to the micro- 

 drop and the slip tilted until the two coalesce. 



