ot with the emery
paper, as scratches on the glass produce haziness in the preparation.

7. Deposit a large drop of immersion oil (or pure water) on the upper
surface of the condenser and rack it down a few millimetres.

8. Adjust the fresh preparation on the microscope stage and fasten it in
position with the stage clips.

9. Rack up the condenser until the immersion fluid makes contact with
the under surface of the slide; avoid the formation of air bubbles.

10. Adjust the substage mirror so that the light is reflected upward. A
bright spot will be seen on the fresh preparation near the centre of the
field.

11. Replace the 2/3-inch objective by the 1/12-inch oil immersion lens
which has been fitted with the special stop to reduce its N. A.; place a
drop of immersion oil upon the centre of the cover-glasses of the fresh
preparation and lower the microscope tube until the front lens of the
objective has entered the oil drop.

12. Focus the bright spot referred to in step 10. If it no longer
occupies the centre of the field, alter the angle of the substage mirror
until it does.

13. Now focus the lens accurately on the film, cautiously vary the
height of the dark ground condenser until the best position is found.
The intensely illuminated bacteria will stand out in vivid contrast to
the dark background.

[Illustration: FIG. 70.--Immersion oil bottle.]

~Microscopical Examination of the Stained Specimen.~--(The body tube of
the microscope may be vertical or inclined to an angle.)

1. Secure the slide on the stage of the microscope by means of the
spring clips.

2. Place a drop of cedarwood oil on the centre of the cover-slip.

The immersion oil is pure cedarwood oil, and is kept in a
small bottle of stout glass (Fig. 70), the cavity of which
is shaped like an inverted cone, and is provided with a
safety funnel (so that the oil does not escape if the bottle
is accidentally overturned) and a dust cap of boxwood fitted
with a wooden rod with which the drop of oil is applied to
the cover-glass or lens.

3. Use the 1/12-inch oil immersion lens of the microscope. Rack down the
body tube till the front lens of the objective is in contact with the
oil and nearly touching the cover-slip.

4. Rack up the condenser until it is in contact with the under surface
of the slide.

5. Apply the eye to the ocular and arrange the plane mirror so as to
obtain the greatest possible amount