30 ESSENTIALS OF BACTERIOLOGY. 



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 

 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 ; the bac- 

 teria will usually be very thick in the centre 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 Widal's test is best seen in the 

 hanging drop. 



CHAPTER IV. 



STAINING OF BACTERIA. 



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 ; and lastly, 

 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 bacteria. 

 But now only a very few find general use, and with methyline 

 blue and fuchsin nearly every object can be accomplished. 



Basic and Acid Dyes. Ehrlich was the first to divide the 

 anilin dyes into two groups, the basic colors to which belong 

 Gentian violet, or pyoktanin, Basic fuchsin. 



Methyl violet, or dahlia, Bismarck-brown, 



Methylin blue (not methyl blue), Thionin, 



SafFranin. 

 And the acid colors to which eosin and acid-fuchsin belong. 



