METHODS OF STUDYING BACTERIA 47 



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. 



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 

 bacteria 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 

 stage attachment of the microscope. 



Hanging Block. A small slice of agar containing some 

 of the growth seared to the glass slide with a hot needle. 



Agglutination as observed in Widal's test is best seen in the 

 hanging drop. 



CHAPTER VI 



METHODS OF STUDYING BACTERIA (Continued). 

 SOLUTIONS AND FORMULAS FOR STAINING 



STAINING or coloring bacteria is done in order to make them 

 prominent and to obtain permanent specimens. It is also 

 necessary to bring out the structure of the bacteria, and 

 serves in many instances as a means of diagnosis ; it would be 

 well-nigh impossible to discover them in the tissues without 

 staining. 



Anilin Colors. Of the numerous dyes in the market, 

 nearly all have, at one time or other, been used in staining 



