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 µ. 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, Abbé pattern.] (a) By means of the Camera Lucida. 1. Attach a camera lucida (of the Wollaston, Beale, or Abbé 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 10µ. 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 obtained. To measure an object by this method simply project the image on to the scale corresponding to the particular optical combination in use at the moment. Read off the number of divisions it occupies and express them as _micra_. In place of preserving a scale for each optical combination, the object to be measured and the micrometer scale may be projected and sketched, in turn, on the same piece of paper, taking particular care that the centre of the eyepiece is 25 cm. from the paper on which the divisions are drawn. [Illustration: FIG. 56.--Eyepiece micrometer, ordinary.] [Illustration: FIG. 57.--Eyepiece micrometer, net.] (b) By means of the Eyepiece Micrometer. The ~eyepiece micrometer~ is a circular glass disc having engraved on it a scale divided to tenths of a millimetre (0.1 mm.) (Fig. 56), or the entire surface ruled in 0.1 mm. squares (the net micrometer) (Fig. 57). It can be fitted inside the mount of any ocular just above the aperture of the diaphragm and must be adjusted exactly in the focus of the eye lens.