METHODS OF EXAMINATION 2 9 



Hanging Drop (Fig. 10). 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 center) 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 vaselin, 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. Both slide and cover-glass should 

 first be sterilized by heat. 



Fig. 10. A "concave slide" with "hanging drop" (McFarland). 



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 

 useful) , 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; the bac- 

 teria will usually be very thick in the center and not so easily 

 distinguished. 



Spores, automatic movements, fission, and cultivation in 

 general can be studied for several days. This moist chamber 

 can be placed in a brood-oven or on the ordinary warming 

 stages of the microscope. 



Agglutination as observed in WidaPs test is best seen in the 

 hanging drop. 



