VACCINE-THERAPY 



35 



glass slides are used, the circumference being painted with 

 immersion oil, or a wall may be built up with vaseline. It 

 is most important not to place too large a drop on the 

 cover-glass, for by so doing it will reach the well of the 

 slide, and the fluid will run by capillarity, the hanging drop 

 thereby disappearing. Take a drop of emulsion (bac- 

 terial), well diluted with broth or other media, and place 

 it on a cover-glass ; invert the hollow-ground slide over it ; 

 press it down so that the oil round the well adheres to the 

 cover-glass evenly. Now invert the slide, and the hanging 

 drop is complete and ready for examination. Examine with 

 a low-power lens, and move the slide about when the edge 



Fig. 17. — Hanging- Drop Preparation. 



of the hanging drop is seen in the field, opening and 

 shutting the diaphragm until the field becomes faintly 

 illuminated. Now focus a little deeper, and the unstained 

 bacteria will come into view, and note if motile in pairs or 

 clumps, etc. If active they will be darting about in all 

 directions, but if sluggish they can be revived by placing 

 the slide in an incubator for a short time. Needless to 

 add, Brownian movements must not be mistaken for natural 

 bacillary motion. A hanging-drop preparation from cultures 

 should be examined, and the shape, etc., of organisms noted, 



whether 



1. Cocci or bacilli. 



2. Motile or non-motile. 



3. Grouping — clumps, pairs, chains, etc. 



