on the eikonometer scale as millimetres, and the
actual size calculated by dividing the observed size by the
magnification constant for the particular optical combination employed
in the observation.

(c) By means of the filar micrometer.

[Illustration: FIG. 58.--Ramsden's Filar micrometer.]

[Illustration: FIG. 59.--Ramsden's micrometer field, a, fixed wire;
b, reference wire (fixed); c, travelling wire.]

The ~Filar~ or cobweb Micrometer (Ramsden's micrometer) eyepiece (Fig. 58)
consists of an ocular having a fine "fixed" wire stretching horizontally
across the field (Fig. 59), 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 drum head is divided into one hundred parts, which
successively pass a fixed index as the head is turned. In the lower part
of the field is 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 drum head.
(If when the drum head is at zero the two wires do not exactly coincide
they must be adjusted by loosening the drum screw and resetting the
drum.)

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
transversed 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 mu.

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