74 CLINICAL BACTERIOLOGY AND HiEMATOLOGY 



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. 



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. It is hardly 

 necessary to say that when dead cultures are used there will be 

 no movements of translation, though the bacilli may show 

 oscillatory (Brownian) movements. 



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 examina- 

 tion for clumps is a most important part of the process, and must 

 be attended to whether dead or living cultures are in use. 



Next see that the emulsion is neither too thin nor too thick. 

 No definite rules can be given, but if there are very few bacilli 

 in the field a further supply of growth must be added to the stock 

 of emulsion, and a further specimen examined. If the bacilli are 

 thickly crowded together, the emulsion must be diluted with a 

 little water and re-examined. 



When you are satisfied that the emulsion is right, slide off the 

 cover-glass and drop it into some antiseptic lotion ; of course, this 

 is unnecessary if dead cultures are used. 



2. Making the Dilution. — You are now about to dilute a drop of 

 the serum from the patient with a known multiple (in this case 

 thirty times) of its bulk of the emulsion which you have just 

 prepared. To do so you will take advantage of the fact that the 

 platinum loop, if dipped into a fluid and pressed against a surface, 

 so that every part of the loop touches that surface, will deposit a 

 drop of fluid of definite size. You are about to mix one loopful 

 of the serum with twenty-nine loopfuls of the emulsion just 

 prepared and examined. 



