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 If an exact number are not found, the draw-tube at the top end of the body-tube should be withdrawn until an exact number is found to lie within two lines of the lower micrometer.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