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” (7). 
If this happens, or if there are any bubbles under the 
cover-glass, you must begin again. Ifthe 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 (4 in. or 4 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. 
