1. Attach a camera lucida (of the Wollaston, Beale, or Abbé pattern) 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 μ. 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.

Fig. 43.-Eyepiece micrometer, Fig. 44.-Eyepiece micrometer, net. ordinary.

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.

(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. 43), or the entire surface ruled in o.1

mm. squares (the net micrometer) (Fig. 44). 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 eyeglass.

Some makers mount the glass disc together with a circular cover-glass in such a way that when placed in position in any Huyghenian eyepiece of their own manufacture, the scale is exactly in focus.

The value of one division of the micrometer scale. must first be ascertained for each optical combination by the aid of the stage micrometer, thus:

1. Insert the eyepiece micrometer inside the ocular and adjust the stage micrometer on the stage of the microscope.

2. Focus the scale of the stage micrometer accurately; the lines will appear to be immediately below those of the eyepiece micrometer. Make the lines on the two micrometers parallel by rotating the ocular.

3. Make two of the lines on the ocular micrometer coincide with those bounding one division of the stage micrometer; this is effected by increasing or diminishing the tube length; and note the number of included divisions.

4. Calculate the value of each division of the eyepiece micrometer in terms of μ, by means of the following formula:

x

=

IO y.

Where x = the number of included divisions of the

eyepiece micrometer.

ν the number of included divisions of the

=

stage micrometer.

5. Note the optical combination employed in this experiment and record it with the calculated micrometer value.

Repeat this process for each of the other combinations. Carefully record the results.

To measure an object by this method read off the

number of divisions of the eyepiece micrometer it occupies and express the result in micra by a reference to the standard value for the particular optical combination employed.

Zeiss prepares a compensating eyepiece micrometer for use with his apochromatic objectives, the divisions of which are so computed that (with a tube length of 160 mm.) the value of each is equivalent to as many micra as there are millimetres in the focal length of the objective employed.

(c) By means of the filar micrometer.

The filar or cobweb micrometer (Ramsden's microm

eter eyepiece (Fig. 45) consists of an ocular having a fine "fixed" wire stretching horizontally across the field (Fig. 46), a vertical reference wire-fixed-adjusted at right angles to the first; and a fine wire, parallel to the reference wire, which can be moved across the field by the action of a micrometer screw; the trap head is divided into one hundred parts, which successively pass a fixed index as the head is turned. In the field is also fixed a comb with the intervals between its teeth corresponding to one complete revolution of this screw-head.

As in the previous method, the value of each division of the micrometer scale (i. e., the comb) must first be determined for each optical combination. This is effected as follows:

1. Place the filar micrometer and the stage micrometer in their respective positions.

2. Rotate the screw of the filar micrometer until the movable wire coincides with the fixed one, and the index marks zero on the screw-head.

3. Focus the scale of each micrometer accurately, and make the lines on them parallel.

4. Rotate the head of the micrometer screw until the movable line has traversed one division of the stage micrometer. Note the number of complete revolutions (by means of the recording comb) and the fractions of a revolution (by means of scale on the head of the micrometer screw), which are required to measure the 0.01 mm.

5. Make several such estimations and average the results.

6. Note the optical combination employed in this experiment and record it carefully, together with the micrometer value in terms of μ.

7. Repeat this process for each of the different optical combinations and record the results.

To measure an object by this method, simply note the number of revolutions and fractions of a revolution of the screw-head required to traverse such object from edge to edge, and express the result as micra by reference to the recorded values for that particular optical combination.

IV. MICROSCOPICAL EXAMINATION OF BACTERIA AND OTHER MICRO-FUNGI.

APPARATUS AND REAGENTS USED IN ORDINARY
MICROSCOPICAL EXAMINATION.

EACH student is provided with a set consisting of the following pieces of apparatus and reagents.

1. India-rubber "change-mat" upon which coverglasses may be rested during the process of staining.

2. Squares of blotting paper about 10 cm., for drying cover-slips and slides.

(The filter paper known as "German lined"-a highly absorbent, closely woven paper, having an even surface and no loose "fluff" to adhere to the specimens -is the most useful for this purpose.)

3. Glass jar filled with 2 per cent. lysol solution for the reception of infected cover-glasses and useless slides.

4. Bunsen burner provided with by-pass.

5. Porcelain trough holding five or six hanging-drop slides (Fig. 47).

A hanging-drop slide is prepared by cementing a circular cell of tin, 13 to 15 mm. diameter, and 1 to 2 mm. in height, to the centre of a 3 by 1 slip by means of Canada balsam. It is often extremely convenient. to have two of these cells cemented close together on one slide.

Another form of hanging-drop slide is made in which a circular or oval cell is ground out of the centre of a 3 by 1 slip. These are more expensive, less convenient. to work with, and are more easily contaminated by drops of material under examination, and should be carefully avoided.