METHOD OF EXAMINING CULTURES 23 



7. Stir the droplet of water which has been deposited on the slide 

 with the tip of the needle, so that the bacteria which it carries 

 are mixed with the water. Now spread out the emulsion thus 

 produced so as to form a patch about J inch in diameter. If it 

 does not spread out uniformly it is a sign that the slide is not clean. 



8. Sterilize your needle. 



9. Allow the film to dry spontaneously. If you have spread it 

 out sufficiently this will take a very short time. 



10. Fix the film by passing the slide slowly through the flame 

 once or twice. This coagulates the albumin present, and the 

 bacteria are now fixed down so firmly that they will not be re- 

 moved by subsequent washing. The exact amount of heat which 

 should be used cannot be stated, as it varies according to the 

 thickness of the slide, etc., and can only be determined by practice. 

 It may be estimated roughly by pressing the finger upon the upper 

 surface of the slide close to the film, but not touching it. The 

 slide should be just uncomfortably warm to the finger, but not hot 

 enough to burn it. 



11. Filter a few drops of the stain on to the surface of the film, 

 and allow it to act for the requisite time. Exact details will be 

 given in each case. 



12. Next wash the slide under the tap, blot it with clean white 

 filter-paper, taking care to avoid rubbing, and warm it gently over 

 the flame until absolutely dry. 



13. Place a drop of balsam on the film, and apply a clean dry 

 cover-glass. 



The preparation is now ready for examination. 



This also is a process 

 which sounds more 

 complicated than it 

 really is. The steps 

 are readily learnt, and 

 the whole process (ex- Fig. 13.— Cornet's Forceps. 



eluding the time spent 



in staining, which may be long or short) does not take more than 

 two or three minutes. 



Most bacteriologists make their films on cover-glasses and not 

 on slides. ■ The steps are just the same, except that the cover- 

 glasses, being much thinner, naturally require much less heating 

 to fix the film ; they are passed rapidly through the flame three 

 times. It is a great advantage to use Cornet's forceps in working 



