mage of the thread is projected against the reflected image of
the subsidiary scale laid alongside the microscope. In this case the
value of the subsidiary scale divisions must be obtained from the
divisions of the stage micrometer, coinciding as nearly as possible
with the position occupied by the thread. Before commencing a
measurement the screens are moved about till both images appear
equally bright.

Threads up to about one twenty-thousandth of an inch in diameter may
be sufficiently well measured by means of a Zeiss "4 centimetre
apochromatic object-glass" and an eyepiece "No. 6" with sixteen
centimetre tube length. [Footnote: The objective certainly had "4 cm."
marked on it, but the focal length appeared to be about I.5 mm. only.]

Sec. 86. Drawing Threads by the Catapult.

The bow-and-arrow method fails when threads of a greater diameter than
about 0.0015 inch are required--at least if any reasonable uniformity
be demanded, and no radical change in the bow and arrow be carried
out.

Thus in the writer's laboratory a thread of about this diameter,
within 1/10000 of an inch-13 inches long and free from air
bubbles--was required. A fortnight's work by a most skilful operator
only resulted in the production of two lengths satisfying the
conditions.

The greatest loss of time occurs in the examination of the thread by
means of the microscope.

Threads for galvanometer suspensions are conveniently from 0.0001 to
0.0004 inch in diameter, and are much more easily made and got uniform
than thicker threads, to the production of which the catapult method
applies.

A reference to the diagram will make the construction of the
instrument quite clear. The moving end of the quartz is attached to a
small boxwood slider working on a tubular girder or between wires.
The quartz is secured in position by clamps shown at A and B, and
motion is imparted to the slider by a stretched piece of catapult
elastic (C). An easy means of regulating the pull of the elastic is
to hold it back by a loop of string whose length can be varied by
twisting it round a pin.

Fig. 69. [Footnote: For greater clearness of drawing, the tube
carrying the slider is shown somewhat higher above the base than is
convenient in practice; and the slide itself is shown too thin in the
direction of the hole through it.]

Since it is not permissible to allow the slider to rebound at the end
of its journey, some such arrangement of breaks as is s