d.

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.

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 for normal vision.
Special eyepieces are also obtainable having a sledging adjustment to
the eye lens for focussing the micrometer.

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 mu, by means of the following formula:

x = 10 y.

Where x = the number of included divisions of the
eyepiece micrometer.

y = the number of included divisions of the
stage micrometer.

5. Note the optical combination employed in