ring.]

4. Fill the fluid to be filtered into the cylinder and screw on the nut
carrying the pressure gauge. (This nut should be immersed in boiling
water for a few minutes previous to screwing on, in order to sterilise
it.)

5. Connect the horizontal arm of the entry tube with a cylinder of
compressed oxygen (or carbon dioxide, Fig. 38, b), by means of
pressure tubing.

6. Connect the lateral arm of the filter flask with the exhaust pump
(Fig. 38, c) and start the latter working.

7. Open the tap of the gas cylinder; then open the tap on the entry tube
of the filter cylinder and raise the pressure in its interior until the
desired point is recorded on the manometer. Maintain this pressure,
usually one or one and a half atmospheres, until filtration is
completed, by regulating the tap on the entry tube.

Some forms of filter candle are made with the open end contracted into a
delivery nozzle, which is glazed. In this case the apparatus is fitted
up in a slightly different manner; the fluid to be filtered is contained
in an open cylinder into which the candle is plunged, while its delivery
nozzle is connected with the filter flask by means of a piece of
flexible pressure tubing (previously sterilised by boiling), as in
figure 39.

IV. THE MICROSCOPE.

The essentials of a microscope for bacteriological work may be briefly
summed up as follows:

[Illustration: FIG. 40.--Microscope stand.]

The instrument, of the monocular type, must be of good workmanship and
well finished, rigid, firm, and free from vibration, not only when
upright, but also when inclined to an angle or in the horizontal
position. The various joints and movements must work smoothly and
precisely, equally free from the defects of "loss of time" and
"slipping." All screws, etc., should conform to the Royal Microscopical
Society's standard. It must also be provided with good lenses and a
sufficiently large stage. The details of its component parts, to which
attention must be specially directed, are as follows:

[Illustration: FIG. 41.--Foot, three types.]

~1. The Base or Foot~ (Fig. 40, a).--Two elementary forms--the tripod
(Fig. 41, a) and the vertical column set into a plate known as the
"horse-shoe" (Fig. 41, b)--serve as the patterns for countless
modifications in shape and size of this portion of the stand. The chief
desiderata--stability and ease of manipulation--are attained in the
first by means of the "spread" of the three fe