so that the
drop of fluid is in the optical axis of the instrument, and secure it in
that position by means of the spring clips.

3. Use the 1/6-inch objective, rack down the body tube until the front
lens of the objective is almost in contact with the cover-slip--that is,
well within its focal distance. This is best done whilst bending down
the head to one side of the microscope, so that the eyes are on a level
with the stage.

4. Apply the eye to the ocular and adjust the plane mirror to the
position which secures the best illumination.

5. Rack the condenser down slightly and cut down the aperture of the
iris diaphragm so that the light, although even, is dim.

6. Rack up the body tube by means of the coarse adjustment until the
bacteria come into view; then focus exactly by means of the fine
adjustment.

Some difficulty is often experienced at first in finding the hanging
drop, and if the first attempt is unsuccessful, the student must not on
any account, whilst still applying his eye to the ocular, rack the body
tube down (for by so doing there is every likelihood of the front lens
of the objective being forced through the cover-glass, and not only
spoiling the specimen, but also contaminating the objective); but, on
the contrary, withdraw his eye, rack the tube up, and commence again
from step 2.

~Dark Ground Illumination.~--

1. Set up the microscope stand in the vertical position and insert the
highest eyepiece available.

2. Remove the nosepiece from the microscope tube and fit the 2/3 inch
objective in place.

3. Remove the substage condenser and replace it by the dark ground
condenser.

4. Fit up the source of illumination some 30-50 cm. distant from the
microscope. (This should be the Liliput Arc Lamp (Leitz), Nernst Lamp or
incandescent gas lamp; if either of the two latter are employed, a
bull's eye condenser to produce parallel rays must be interposed between
light and microscope); and adjust illuminant and microscope so that the
substage plane mirror is completely filled with light.

5. Focus the two concentric rings engraved upon the upper surface of the
condenser and centre them accurately by means of the centring screws.

6. Prepare a "fresh" specimen (see pages 74-76) of the material it is
desired to observe, using selected, new, 3 by 1 glass slips of less than
1 mm. thickness, and No. 1 cover-glasses (0.17 mm. thick), which should
be cleaned with a piece of soft washleather and n