METHOD OF EXAMINING CULTURES 23 



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 albumen present, 

 and the bacteria are now fixed down so firmly that they will 

 not be removed 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 un- 

 comfortably 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 pro- 

 cess which sounds 

 more complicated than 

 it really is. The steps 

 are readily learnt, and 

 the whole process FIG. 9. CORNET'S FORCEPS. 



(excluding the time 



spent in staining', which may be long" or short) does not take 

 more than two or three minutes. 



Many 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 with cover-glasses. These forceps are 

 self-retaining, and hold the cover- glass in a horizontal posi- 

 tion, so that stains can be poured upon them whilst the forceps 



