134 UNSTAINED PREPARATIONS 



in the cell is generally quite sufficient for the growth of the organism during 

 this period. 



To improvize a hollow-ground slide. A hollow-ground slide may be improvized 

 by taking a rectangular piece of pasteboard about 3x2 cm. and 1 '5 to 2 mm. thick, 

 and cutting out of its centre a small piece about 15 mm. square. Sterilize the piece 

 of pasteboard in the autoclave at 115 C., take it out with a pair of sterile forceps 

 and lay it on a slide which has been passed through the flame: the cover-glass 

 on which the drop of fluid is placed can be inverted on this to form a hanging drop. 



B. Bcettcher's cell. This cell consists of a glass slide on to which a glass 

 ring (15-20 mm. in diameter and 5 mm. deep) is stuck (fig. 122). The cover- 



FIG. 122. Bcettcher's cell. 



glass carrying the hanging drop is inverted on to the ring. A little drop of 

 water should be put in the bottom of the cell to prevent evaporation of the 

 culture medium. 



C. Ranvier's cell. In the foregoing cells the hanging drop has a spherical 

 lower surface, with the result that the rays of light passing through it are 

 refracted at points which are not equally distant from the lens, and this to 

 some extent interferes with the examination of the preparation. For delicate 

 work it is better to have the two surfaces of the liquid under examination 

 parallel to each other. This can be attained by using Ranvier's cell (fig. 123), 



FIG. 123. Ranvier's cell. 



consists of a rather thick glass slide having a circular groove 15-20 mm. 

 in diameter running round its centre marking off a central elevation which 

 it surrounds on all sides like a moat. The upper surface of this elevated 

 central part is about T Vth mm. below the surface of the slide. The drop of 

 liquid, being placed on the central elevation and covered with a cover-glass, 

 is flattened out between the elevated part and the cover-glass, and forms a 

 layer T ^th mm. deep surrounded on all sides by the air in the groove ; the 

 edges are luted and the subsequent procedure is the same as in the foregoing 

 cases. 



(ii) The cultivation and preservation of hanging drop preparations. 



To grow an organism under these conditions it is necessary to keep it at 

 the temperature best suited to its growth, which in the majority of cases is 

 37 C. This may be done by keeping the slide in the incubator, taking it 

 out when required for microscopical examination ; but it is better to maintain 

 the slide at the temperature required on the stage of the microscope itself, 



