40 MANUAL OF BACTERIOLOGY. 



just transparent in a layer about 12 mm. in thickness, after 

 filtering. 



Fuchsin and gentian-violet operate rapidly and intensely. 

 Methylene-blue works more slowly and feebly; it is to be 

 preferred where the bacteria occur in thick or viscid sub- 

 stances, like pus, mucus, and milk. 



Method of Staining Cover-glass Preparations. (a) A 

 smear preparation of bacteria having been made in the 

 manner above described, and a watery solution of either 

 fuchsin, gentian-violet or methylene-blue having been pre- 

 pared, the cover-glass is to be dropped into a dish contain- 

 ing the dye, or the dye may be dropped upon the cover- 

 glass held in the forceps. 



(fr) Allow the stain to act for about thirty seconds. 



(c) Wash in water. 



(d) Examine with the microscope in water directly or 

 after drying and mounting in Canada balsam. 



The rapidity and intensity of staining may be increased 

 by warming the solution slightly. The bacteria will usually 

 appear more distinct if, directly after pouring off the stain, 

 the preparation is rinsed for a few seconds in i per cent, 

 solution of acetic acid, and then thoroughly washed in water. 

 The acetic acid solution serves to remove in a measure any 

 color which has been imparted to the background, and which 

 is undesirable. 



Preparations that are mounted at first in water may be 

 made permanent by moistening the edge of the cover-glass 

 so that it may easily be removed from the slide, then dry- 

 ing and mounting in Canada balsam. Cover-glass prepara- 

 tions which have been stained are examined with the oil- 

 immersion objective, employing the plane mirror, having 

 the iris diaphragm open and the condenser close to the 

 lower surface of the glass slide. The purpose is to obtain 

 the most intense illumination possible over a small field. 



