METHODS OF OBSERVING BACTERIA. 87

keep on until it almost touches the glass, then look into
the microscope and find the object by slowly and firmly
racking up. As soon as the object comes into view
leave the rack and pinion and focus with the fine adjust-
ment.

Always select the light from a white cloud if possible ;
if there are no white clouds, choose the clearest whitest
light possible. Never under any circumstances employ
sunlight, which is ruinous to the eyes and useful only
for photomicrography.

In using low-power lenses the Abbe condenser must be
moved away from the object and the light modified by
the iris-diaphragm. The distance between condenser and
object should correspond more or less closely with the
distance between objective and object.

In using high powers the Abbe condenser must be
brought near the object and the light modified by the
iris-diaphragm.

If the oil-immersion lens is used, it is perhaps best to
employ the plane side of the mirror. When with this
lens a section of tissue is examined for details, the light
must be modified by the iris-diaphragm, opening and
closing it alternately until the best effect of illumina-
tion is achieved. If tissue be searched for stained bac-
teria, and no cellular detail is required, the diaphragm
should be wide open to admit a great flood of light
and extinguish everything except the deeply-colored
bacteria.

When unstained bacteria are to be examined with the
oil-immersion lens, the diaphragm should be closed so
as to leave only a small opening through which the
light can pass.

Bacteria may be examined either stained or unstained.
The former condition would always be preferable if the
process of coloring the organisms did not injure them.
Unfortunately, it is generally the case that the drying,
heating, boiling, macerating, and acidulating to which
we expose the organisms in the process of staining alter