one of the 1/100th divisions of the stage micrometer markings. If an
exact number are not found, the draw-tube at the top end of the
body-tube should be withdrawn until an exact number is found to lie
within two lines of the lower micrometer.

Suppose twenty-two of the spaces on the eye-piece micrometer just cover
one of the divisions (1/100th of an inch) on the stage micrometer. Then
it is clear that each division of the former represents 1/100 x 1/22 of
an inch, or 1/2200th of an inch. For every fresh objective used, a fresh
estimation of eye-piece and stage micrometer ratio is necessary. Having
now got in the eye-piece micrometer a unit of measurement, it becomes a
comparatively easy matter to measure the fibre.

Remove the stage-micrometer and put a slide of Sea Islands cotton in its
place. Focus the fibre and observe the number of divisions or parts of a
division covered by any particular fibre, and its measurement is at once
known. Thus if a single filament covers two of the divisions then it is
2/2200th of an inch in diameter, or 1/1100th of an inch. Exactly the
same method is adopted if it is desired to measure the diameters of
sections of the same fibres.

The making of the drawing of a fibre, either transverse or horizontal
section, is not at all a difficult matter.

All that is needed is what is known as a camera lucida. This consists of
a brass fixing for the eye-piece end of the body-tube and a small
reflecting prism. This prism receives the image of the objective, and
reflects it in this case at right angles downward on to a sheet of
paper, which is placed beneath for the purpose of tracing the said
image.

Focus the object, first having the microscope in a horizontal position.
This will not be a difficult matter. Now remove the cap which fits on
the eye-piece, and fix on the camera lucida as shown in the illustration
(see Fig. 6). Adjust this until the image of the fibre is seen. Usually
one or two smoke-coloured glasses are fixed below the prism, and these
are now brought into position so as to allow the image of the fibre to
pass through them. Place a sheet of drawing paper directly under the
camera lucida, sitting as shown in the illustration. After a few trials
it will not be a difficult matter to follow the outline of the image by
means of a black lead on the paper as is shown in the figure. In this
way many useful working drawings can be made, and a little careful
calculation will give the ampl