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.

_Wright's Eikonometer_ is really a modification of the eyepiece
micrometer for rapidly measuring microscopical objects by direct
inspection, having previously determined the magnifying power of the
particular optical combination employed. It is a small piece of
apparatus resembling an eyepiece, with a sliding eye lens, which can be
accurately focussed on a micrometer scale fixed within the instrument.
When placed over the microscope ocular the divisions of this scale
measure the actual size of the virtual image in millimetres.

In order to use this instrument for direct measurement, it is first
necessary to determine the magnifying power of each combination of
ocular, tube length and objective.

Place a stage micrometer divided into hundredths of a millimetre on the
microscope stage and focus accurately.

Rest the eikonometer on the eyepiece. Observation through the
eikonometer shows its micrometer scale superposed on the image of the
stage micrometer.

Rotate the eikonometer until the lines on the two scales are parallel,
and make the various adjustments to ensure that two lines on the
eikonometer scale coincide with two lines on the stage micrometer.

For the sake of illustration it may be assumed that five of the
divisions on the stage micrometer accurately fill one of the divisions
of the eikonometer scale; this indicates a magnifying power of 500 as
the constant for that particular optical combination, and a record
should be made of the fact.

The magnification constants of the various other optical combinations
should be similarly made and recorded.

To measure any object subsequently it should be first focussed carefully
in the ordinary way.

The eikonometer should then be applied to the eyepiece and the size of
the object read off