8o 



CLINICAL BACTERIOLOGY AND HiEMATOLOGY 



growth and mix it with some water in a watch-glass. In either 

 case stir it round with the platinum needle for a few minutes, so 

 that the bacilli are evenly distributed throughout the water and 

 form an emulsion. 



Next take the hollow-ground slide and paint a ring of immersion 

 oil round the well (Fig. 22, b). If you are using a built-up cell, 

 paint the top of the card with the oil. In either case vaseline 

 may be used. 



Place a drop of the emulsion on a clean dry cover-glass, and 

 invert the hollow-ground slide over it ; press it down so that the 

 oil round the well adheres to the cover-glass ; now invert the 

 slide, and you will have a hanging-drop specimen. The bacilli will 

 be contained in the droplet of water (Fig. 22, a) which hangs 



7 7 



.'5 



Fig. 22. — Hanging-drop Preparation. (Crookshank.) 



from the lower surface of the cover-glass : this will not dry it up 

 if the seal made by the oil is perfect. 



Place the specimen under the microscope and examine 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 ^-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 



