HANGING DROP PREPARATIONS 



135 



by making use of some form of warm stage for the purpose, VignaFs for 

 example (fig. 124) or Malassez's or Ranvier's. These really are small incu- 



FIG. 124. Vignal's warm stage. 



bators, allowing of the examination of the culture through a circular aperture 

 cut in the apparatus. 



Pfeiffer's warm stage is simpler than those already mentioned and serves 

 the same purpose. It consists of a rectangular glass box (fig. 125A), the upper 

 surface of which is hollowed out to form a cell, in which the culture is placed. 

 The box is filled with water and is connected by means of two lateral tubulures 

 to a thermostat. The temperature is indicated by a thermometer placed as 

 shown in the figure. 



FIG. 125A. Pfeiffer's warm 



FIG. 125B. Pfeiffer's stage in section. 



The apparatus is placed on the stage of the microscope like an ordinary 

 slide. 



By another method the lower part of the microscope is enclosed in a box 

 a small incubator which entirely surrounds the stand ; the box has a window 

 for lighting purposes and lateral openings to allow of the preparation being 

 moved (Zeiss, Plehn). The apparatus is fitted with a regulator and is 

 heated by a gas burner. The temperature must not exceed 45 C., to avoid 

 injury to the microscope. 



SECTION III. THE EXAMINATION OF STAINED PREPARATIONS. 



Staining methods allow a more detailed study of the morphology of micro- 

 organisms than is possible with unstained preparations, and furnish important 

 data for the diagnosis of species. For different species of bacteria do not 

 react in the same way to stains : some are readily stained and cannot be 

 decolourized with alcohol, others which stain with equal readiness lose the 



