thicker than the
two-thousandth of an inch, under the microscope it is convenient to
use a film of glycerine stained with some kind of dye, in order to
render the thread more sharply visible. The thread is mounted beneath
a cover slip, and a drop of the stained glycerine allowed to run in.
Such a treatment gives the image of the thread a sharply defined edge
3 and the contrast between the whiteness of the thread and the colour
of the background allows measurements to be made with great ease.

On the whole the easiest way of measuring the diameter of a thick
thread is to use a measuring microscope, i.e. one in which the lens
system can be displaced along a plane bed by means of a finely cut
micrometer screw. The instruments made by the Cambridge Scientific
Instrument Company do fairly well. Direct measurements up to 0.0001
inch are easily made by means of a microscope provided with a Zeiss
"A" objective, and rather smaller differences of thickness can be made
out by it. For thin threads the method next to be described is more
fitting, because higher powers can be more conveniently used.

In this method an ordinary microscope is employed together with a
scale micrometer, and either an eyepiece micrometer, or a camera and
subsidiary scale. The eyepiece micrometer is the more convenient. If
a camera be employed, i.e. such an one as is supplied by Zeiss, it is
astonishing how the accuracy of observation may be increased by
attending carefully to the illumination of both the subsidiary scale
and of the thread. The two images should be as far as possible of
equal brightness, and for this purpose it will be found requisite to
employ small screens.

The detail of making a measurement by means of the micrometer eyepiece
is very simple. The thread is arranged on the stage so as to point
towards the observer, and the apparent diameter is read off on the
eyepiece scale. In order to calibrate the latter it is only necessary
to replace the thread by the stage micrometer, and to observe the
number of stage micrometer divisions occupying the space in the
eyepiece micrometer formerly occupied by the thread. It is essential
that both thread and stage micrometer should occupy the same position
in the field, for errors due to unequal distortion may otherwise
become of importance. For this reason it is best to utilise the
centre of the field only.

The same remark applies to measurements by means of the camera, where
the i