METHODS OP EXAMINATION. 29 



is very slight, it does not require much turning to break the 

 cover-glass and ruin the specimen. Having found the bacte- 

 rium, we see whether it be bacillus, micrococcus, or spirillum ; 

 discover if it be motile, or not. That is about all we can ascer- 

 tain by this method. 



FIG. 10. 



Hanging Drop in Concave Glass Slide. 



Hanging Drop. When the looped platinum needle is dipped 

 into a liquid, a very finely-formed globule will hang to it ; this 

 can be brought into a little cupped glass slide (an ordinary 

 microscopic glass slide with a circular depression in the centre) 

 in the following manner : The drop is first brought upon a 

 cover-glass ; the edges of the concavity on the glass slide are 

 smeared with vaseline, and the slide inverted over the drop ; 

 the cover-glass sticks to the smeared slide, which, when turned 

 over, holds the drop in the depression covered by the cover- 

 glass, thus forming an air-tight cell ; here the drop cannot 

 evaporate. 



Search for the bacteria with a weak lens ; having found them, 

 place a drop of cedar-oil upon the cover-glass, and bring the oil 

 immersion into place (here is where a nose-piece comes in very 

 usefully), careful not to press against the cell, for the cover- 

 glasses are very fragile in this position. 



Search the edges of the drop rather than the middle ; it will 

 usually be very thick in the centre and the bacteria not so easily 

 distinguished. 



