and the ordinary student may, by means of very simple and inexpensive
apparatus, obtain fairly satisfactory results in the measurement of
fibres.

There is a choice of one of three methods, viz.:--

1. By having the mechanical stage so arranged that the slightest
displacement either to the left or right can be measured, and having
the eye-piece so marked (generally a hair stretched across it) that
when an object is to be measured, one side of it is made to coincide
with this central line and the stage rack is worked left or right
until the opposite side of the object is brought coincident with the
central line again; the amount of displacement can then be readily
obtained on referring to the graduated stage.

2. By having a stage micrometer and camera lucida.

3. By having two micrometers, a stage micrometer and eye-piece
micrometer.

This latter method is certainly the least expensive, and for all
practical purposes can be safely recommended.

A stage micrometer consists of a slip of glass 3" x 1" on which are
marked divisions of an inch, usually 1/100ths and 1/1000ths. As a rule
these markings are protected by means of a small cover-glass.

Eye-piece micrometers vary much in form, size and value, but the one
which is here described is of the simplest type. It consists of two
circular pieces of glass carefully cemented together. On one of the
inner surfaces are marked usually the 1/100ths divisions of an inch. In
some 1/200ths are marked. If the top lense of the eye-piece be
unscrewed, a diaphragm will be found on which the eye-piece micrometer
will easily rest. Screw on the top lense again and, generally, the
eye-piece will be ready for use. If the micrometer is not properly in
focus after a few trials, it may easily be made right. In order, then,
to measure the diameter of a single fibre of Sea Islands cotton, fit in
the quarter inch objective and place the stage micrometer in position
on the stage. First, focus the fine lines which are plainly to be seen,
and remember the lines which are farthest apart are 1/100th of an inch;
the others 1/1000th of an inch.

As a rule, these lines run from N. to S. of the field; in other words,
from top to bottom across the circles of light. Now look at the
divisions in the eye-piece micrometer, which are 1/100th of an inch
apart.

It will be found often that an exact number of these divisions fill up