FILM PREPARATIONS. 89 



of this being accomplished is that, when the drop of fluid con- 

 taining the bacteria is placed upon the glass, it can be uniformly 

 spread with the platinum needle all over the surface without 

 showing any tendency to retract into droplets. 



The best method to pursue is to keep the cover-slips in a 

 vessel containing 95 per cent alcohol. When required for use a 

 slip is taken and polished with a piece of old linen or cotton cloth, 

 which must be perfectly clean ; then placing the cover-slip in 

 the forceps, pass it through the flame 

 six or eight times, not too slowly so 

 as to risk destroying it, and it will be 

 found that the lowermost surface will FlG - 49- Comet's forceps for hoid- 



. . . ing cover-glasses. 



be perfectly free from any greasy 



substance. Another method is that recommended by Van 

 Ermengem. The cover-glasses are placed for some time in a 

 mixture of concentrated sulphuric acid 6 parts, potassium bichro- 

 mate 6 parts, water 100 parts, then washed thoroughly in water 

 and stored in absolute alcohol. For use, a cover-glass is either 

 dried by wiping with a clean duster or is simply allowed to dry. 

 This method will amply repay the trouble, and really saves time 

 in the end. A clean cover having been obtained, the film pre- 

 paration can now be made. If a fluid is to be examined, a 

 loopful may be placed on the cover-glass, and either spread out 



FlG. 50. Stewart's cover-glass forceps. 



over the surface with the needle, or another clean cover may be 

 placed on the top of the first, the drop thus spread out between 

 them and the two then drawn apart. When a culture on a solid 

 medium is to be examined a loopful of distilled water is placed 

 on the cover-glass and a minute particle of growth rubbed up in 

 it and spread over the glass. The great mistake made by be- 

 ginners is to take too much of the growth. The point of the 

 straight needle should just touch the surface of the culture, and 

 when this is rubbed up in the droplet of water and the film 

 dried, there should be an opaque cloud just visible on the cover- 

 glass. When the film has been spread it must next be dried by 



