IMPRESSION FILMS 85 



8. Mounting. 



1. Wash the film carefully in running water. 



2. Blot off the superfluous water with the filter 

 paper, or dry more completely between two folds of 

 blotting paper. 



3. Complete the drying in the air, or by holding the 

 cover-slip in the fingers at a safe distance above the 

 flame of the Bunsen burner. 



4. Place a drop of xylol balsam on the centre of a 

 clean 3 by i glass slide and invert the cover-slip over 

 the balsam, and lower it carefully to avoid the inclusion 

 of air bubbles. 



NOTE. Xylol is used in preference to chloroform to dissolve 

 Canada balsam, as it does not decolourise the specimen. 



Impression films (Klatschpraeparat) are prepared 

 from isolated colonies of bacteria in order that their 

 characteristic formation may be examined by higher 

 powers than can be brought to bear on the living culti- 

 vation. They are prepared from plate cultivations 

 (vide page 230) in the following manner. 



1. Remove a clean cover-slip from the alcohol pot 

 with sterile forceps and burn off the spirit. 



2. Open the plate and rest one edge of the cover- 

 slip on the surface of the medium a little to one side 

 of the selected colony. Lower it cautiously over the 

 colony until horizontal. Avoid any lateral movement 

 or the inclusion of bubbles of air. 



3. Make gentle vertical pressure on the centre of the 

 cover-slip with the points of the forceps to ensure 

 perfect contact with the colony. 



4. Steady one edge of the cover-slip with the forceps 

 and pass the point of a mounted needle just under 

 the opposite edge and raise the cover-slip carefully; 

 the colony will be adherent to it. When nearly verti- 

 cal, grasp the cover-slip with the forceps and remove 

 it from the plate. Re-cover the plate. 



5. Place the cover-slip, film uppermost, on the rubber 



