OF THE HUMAST SUBJECT. 27 



the cover glass having been prepared exactly as before is inverted 

 so that the hanging drop lies over the centre of the circular hole 

 in the blotting paper (Fig. 3). 



Fig. 3. Section of slide and moist chamber prepared with blotting-paper 

 slightly reduced. 



If the examination be prolonged, it is necessary to moisten 

 the paper from time to time. For the study of such prepara- 

 tions, much of the light must be cut off with the diaphragm, 

 assuming that the Abbe condenser and plane side of the mirror 

 are used; unstained organisms are otherwise scarcely discernible. 



The most suitable area for observation is the edge of the 

 drop where the film of fluid is thin, and where there is what may 

 be called a "still" layer in which an accumulation of the bac- 

 teria takes place, and where their movements (whether active or 

 Brownian) are less lively, and eventually cease. 



The best plan of finding the edge is to place the slide with 

 the hanging drop centrally on the stage of the microscope, 

 transfer a droplet of cedar oil exactly over the hanging drop, 

 and screw down the coarse adjustment until the oil rises up to 

 meet the object glass ; the rest of the focussing is then carried 

 out by means of the fine adjustment which is screwed until 

 some of the bacteria come into focus ; the slide is then moved in 

 any one definite direction until the edge of the drop is reached. 

 Besides the forms, the motility of the bacteria is to be deter- 

 mined by the hanging drop. For the latter purpose, the use of 

 broth cultures is preferable to those made on agar ; nevertheless, 



