148 BACTERIOLOGICAL DIAGNOSIS. 



Cover the slide in this way : Place your finger at the 

 side on the glass square on the slide, and apply the 

 cover-glass, letting it rest against your finger ; lower it 

 gently in place with a needle or other suitable object. 

 When it is in place press it gently with the needle at 

 each corner in succession, and look at it obliquely so 

 as to see the light reflected from the surface. If the 

 slide and cover-glass are in sufficiently close contact 

 you will see Newton's rings (looking like the eye of a 

 peacock's feather) round the point at which you are 

 applying pressure. If you do not see this the inference 

 is that there is some dust between the slide and cover- 

 glass ; you must clean both and begin again. 



If you have taken the right amount of fluid the drop 

 should extend exactly to the edge of the central glass 

 disc, but should not run over into the "moat" (r). 

 If this happens, or if there are any bubbles under the 

 cover-glass, you must begin again. If the drop does not 

 quite extend to the edge of the central disc no great 

 harm is done. 



4. Focussing the specimen. This is somewhat difficult 

 for beginners, and merits a short description. Place 

 the slide under the microscope, taking care to get it 

 accurately centred, and examine it with the low power. 

 You will find that the central disc is ruled into squares 

 like a chess-board (c). Get these squares into the 

 centre of the field. 



Do not forget you are dealing with an unstained 

 object ; use a flat mirror and a small diaphragm. The 

 examination is often easier if artificial light is used. 



Now turn on the high power (J in. or i in.) and 

 screw it downwards until it almost touches the cover- 

 glass ; look down the microscope and focus gently 

 upwards, using the fine adjustment and keeping a 

 careful lookout for the rulings. 



