CHAPTER IV.
METHODS OF OBSERVING BACTERIA.

WHOEVER would study bacteria must be equipped with
a good microscope. The instruments generally provided
for the use of medical students in college laboratories, as
well as those seldom-employed u show microscopes n seen
in physicians' offices, are ill adapted for the purpose.
The essential features' of a bacteriological instrument
are lenses giving a clear magnification extending as
high as one thousand diameters, and a good condenser
for intensifying the lights thrown upon the objects. It
naturally follows that the best work requires the best
lenses. The cheapest good microscope which is at pres-
ent offered to the public is the BB. Continental stand,
made by Bausch and Lomb. This stand is provided with
everything necessary, is fitted with very creditable objec-
tives, including an excellent Ty' oil-immersion lens, and
seems capable of doing very good work. I do not
recommend this as the best instrument obtainable, but
as one that is both good and cheap. For those who desire
the very best the rather costly outfits made by Carl Zeiss
of Jena are unexcelled.

For those who may begin the use of the Abbe con-
denser and oil-immersion lenses without the advantage
of personal instruction a few hints will not be out of
place:

Always employ good slides without bubbles, and thin
cover-glasses; No. i are best.

Place a drop of oil of cedar upon the cover-glass of
the specimen to be examined; rack the body of the instru-
ment down until the oil-immersion lens touches the oil;