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CLINICAL BACTERIOLOGY AND H/EMATOLOGY 



glass is distinctly seen, and then move the slide about until the 

 edge of the hanging drop runs across the centre of the field. 

 Then turn on the J-inch lens and open and shut the diaphragm 

 until the field is faintly illuminated ; the exact amount of light 

 required can only be learnt by experience. 



Now focus up and down very carefully, using the fine 

 adjustment, until you see a line running across the field and 

 dividing it into a lighter and a slightly darker portion. This 

 is the edge of the drop. Focus a little deeper; you should 

 see numerous small unstained bacilli, and if these are not 

 visible it probably indicates that the illumination is not right. 

 Open and shut the diaphragm, keeping a sharp look-out down 

 the microscope all the time. It may help matters to lower 

 the condenser for a short distance. 



FIG. 19. HANGING-DROP PREPARATION. (CROOKSHANK.) 



Having obtained a clear view of the bacilli, examine them 

 for motility and absence of clumps, and see whether they are 

 present in proper proportion to the amount of fluid. 



If the culture is in good condition, the bacilli should be seen 

 darting about in all directions; but if the movement is but 

 sluggish, the reaction may still be obtained. If the specimen 

 is kept for a short time in a warm place or in the incubator, 

 the movements will usually become more rapid. 



The specimen must be searched thoroughly for clumps of 

 bacilli, and if these are present the emulsion must be filtered 

 through a double thickness of white filter-paper. This 

 examination for -clumps is a most important part of the pro- 

 cess, and must be attended to whether dead or living cultures 

 are in use. 



