BLACK AND WHITE FILMS 77 



ing the torn edge of a piece of blotting paper to the 

 opposite side. The reagent may then be observed 

 to flow across the field and come into contact with 

 such of the micro-organisms as lie in its path. 



The non-toxic basic dyes most generally employed 

 for the intra-vitam staining of bacteria are 



Neutral red, 



Quinoleine blue 



Jt ^ , in 0.5 per cent, aqueous solutions. 



Methylene green 



Vesuvin, 



Negative Stain (Burri). By this method of demon- 

 stration the appearances presented by dark ground 

 illumination (by means of a paraboloid condenser) 

 are closely simulated, since minute particles, bacteria, 

 blood or pus cells etc. stand out as brilliantly white or 

 colourless bodies on a dark grey-brown background. 



Reagent required: 



Any one of the liquid waterproof black drawing inks 

 (Chin-chin, Pelican, etc.) . This is prepared for use as 

 follows: 

 Measure out and mix : 



Liquid black ink 25 c.c. 



Tincture of iodine i c.c. 



Allow the mixture to stand 24 hours, centrifugalise 

 thoroughly, pipette off the supernatant liquid to a clean 

 bottle and then add a crystal of thymol or one drop of 

 formalin as a preservative. 



METHOD. 



1. With the sterilised loop deposit one drop of the 

 liquid ink close to one end of a 3 by i slide. 



2. With the sterilised loop deposit a drop of the 

 fluid culture (or of an emulsion from a solid culture) 

 by the side of the drop of ink (Fig. 69, a) ; mix the two 

 drops thoroughly by the aid of the loop. 



3. Sterilise the loop. 



