STAINING AND EXAMINATION OF MICRO-ORGANISMS 59 



whilst applying the vaseline, so as to avoid any con- 

 tamination through air-organisms falling on to the sur- 

 face. A cover-glass is held with forceps, and sterilised 

 by means of the flame, and when cool a drop of bouillon 

 is placed on the centre . of its under-surface, either by 

 means of a platinum loop or a sterilised pipette. The 

 merest trace of material containing the desired organism 

 is then conveyed on the point of a platinum-needle 

 and introduced into the broth. The cover-glass is now 

 placed, with the drop of infected bouillon downwards, 

 on to the excavated slide, so that the drop hangs down 

 into the middle of the depression in the slide. The 

 vaseline forms a seal from the outer air, which may be 

 further increased by another touch with vaseline all 

 round the edges of the cover-glass. Of course no 

 vaseline must be allowed to get on to the surface of 

 the cover-glass, where the oil will be subsequently 

 placed prior to examination with the immersion lens. 



FIG. 11. SUSPENDED DROP-CULTURES. 



FIG. A. A, glass slide ; B, cover-glass ; c, glass ring forming the wall of the chamber ; 



P, drop of nutritive liquid in which the micro-organisms grow (Klein). 

 FIG. B. Side view of above. 



Drop-cultures thus prepared will last for a long time, 

 and may be studied from hour to hour or day to day. 



Instead of an excavated glass slide, a small glass 

 ring may be cemented on to a plane slide, so as to form 

 a little cell in which the drop-culture hangs, as shown 

 in the figures. 



