WIDAL’S METHOD. 89 
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 
4 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. 
PM a eis es areata 
Fic. 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. 
