Culture Techniques 149 



of depression slide so as to seal the drop into a 

 chamber which will reduce evaporation from the 

 drop, and the culture is then incubated. This 

 technique permits many modifications which have 

 been described in more or less detail in other man- 

 uals of microbiological methods. 



Slides. A variety of slides have been developed 

 for use with hanging drops (Fig. 42). The sim- 

 plest of these are the hollow ground slides used 

 by the protozoologists. The curvature of these 

 depressions provides only a very poor optical 

 medium through which to illuminate cultures for 

 observation by transmitted light. Slides with a 

 deep hot-pressed well with "flat" bottom instead 

 of the ground depression are somewhat better in 

 this respect. Low ' ' Van Tiegham ' ' rings of glass, 

 or the brass rings used by Dr. and Mrs. Warren 

 Lewis (Parker, 1938, 26), cemented to ordinary 

 flat slides, provide still better optical properties 

 but are somewhat clumsy to handle. The pierced 

 slide described in Chapter IV (Fig. 27), in spite 

 of its fragility, offers the best, most easily handled, 

 most widely useful type of slide for hanging-drop 

 cultures. While the optical properties of this 

 slide are excellent, the curvature of the surface of 

 the drop may cause some trouble. This can be 

 obviated by floating a 5 X 5 mm. sq. piece of No. 

 cover on the surface of the drop over the culture. 



