WIDAL S METHOD. OQ 



lower surface of the cover-glass ; this will not dry up if 

 the seal made by the oil is perfect. 



Place the specimen under the microscope and exa- 

 mine it with the low power, using the fine adjustment 

 and stopping down the diaphragm. Focus until the 

 surface of the cover-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 

 ^ in. 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. 



FIG. 16. Hanging-drop preparation (Crookshank). 



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 micro- 

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

 condenser for a short distance. 



