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 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 If an exact number are not found, the draw-tube at the top end of the body-tube should be withdrawn until an exact number is found to lie within two lines of the lower micrometer.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 × 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 amplification of the drawing after it is made.