EXAMINATION OF BACTERIA WITH THE MICROSCOPE. 29 



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

 ferred where the bacteria occur in thick or viscid substances, 

 like pus, mucus and milk, and acts more energetically when 

 made slightly alkaline. 



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

 smear preparation of bacteria having been made, dried and 

 passed through the flame three times in the manner above 

 described, and a watery solution of either fuchsin, gentian- 

 violet or methylene-blue having been prepared, the cover-glass 

 is to be dropped into a dish containing the dye, or the dye may 

 be dropped upon the cover-glass held in the forceps. 



(b) 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 preparation may also be 

 improved by rinsing for a few seconds in \ per cent, acetic acid 

 after staining. If this is done quickly, and the preparation 

 thoroughly washed in water it does not materially affect the 

 stain in the bacteria, and it clears up the background, thus 

 bringing out the bacteria in stronger relief. The acid should 

 be merely poured on and- poured off, otherwise the bacteria will 

 be more or less decolorized. 



Preparations that are mounted at first in water may be made 

 permanent by letting a drop of water fall at the edge of the 

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

 drying and mounting in Canada balsam. Cover-glass prep- 

 arations which have been stained are examined with oil- 

 immersion objective, employing the plane mirror, having the 

 iris diaphragm open and the condenser close to the lower sur- 

 face of the glass slide. The purpose is to obtain the most in- 

 tense illumination possible over a small field. The watery 

 solutions of aniline dyes prepared as above described deterior- 



