is also essential. Details of those in
general use will be found in the following pages.

[Illustration: FIG. 54--Diamond Object marker.]

_Object Marker_ (Fig. 54).--This is an exceedingly useful piece of
apparatus. Made in the form of an objective, the lenses are replaced by
a diamond point, set slightly out of the centre, which can be rotated by
means of a milled plate. Screwed on to the nosepiece in place of the
objective, rotation of the diamond point will rule a small circle on the
object slide to permanently record the position of an interesting
portion of the specimen. The diamond is mounted on a spring which
regulates the pressure, and the size of the circle can be adjusted by
means of a lateral screw.

METHODS OF MICROMETRY.

The unit of length as applied to the measurement of microscopical
objects is the one-thousandth part of a millimetre (0.001 mm.),
denominated a _micron_ (sometimes, and erroneously, referred to as a
micro-millimetre), and indicated in writing by the Greek letter mu. Of
the many methods in use for the measurement of bacteria, three only will
be here described, viz.:

(a) By means of the Camera Lucida.

(b) By means of the ocular or Eyepiece Micrometer.

(c) By means of the Filar Micrometer (Ramsden's micrometer eyepiece).

For each of these methods a ~stage micrometer~ is necessary. This is a 3
by 1 inch glass slip having engraved on it a scale divided to hundredths
of a millimetre (0.01 mm.), every tenth line being made longer than the
intervening ones, to facilitate counting; and from these engraved lines
the measurement in every case is evaluated. A cover-glass is cemented
over the scale to protect it from injury.

[Illustration: FIG. 55.--Camera lucida, Abbe pattern.]

(a) By means of the Camera Lucida.

1. Attach a camera lucida (of the Wollaston, Beale, or Abbe pattern)
(Fig. 55) to the eyepiece of the microscope.

2. Adjust the micrometer on the stage of the microscope and accurately
focus the divisions.

3. Project the scale of the stage micrometer on to a piece of paper and
with pen or pencil sketch in the magnified image, each division of which
corresponds to 10 mu. Mark on the paper the optical combination (ocular
objective and tube length) employed to produce this particular
magnification.

4. Repeat this procedure for each of the possible combinations of
oculars and objectives fitted to the microscope supplied, and carefully
preserve the scales thus obtaine