3. To show how to exactly measure the lengths and diameters of fibres by means of micrometers. First, as to the instrument: a good substantial stand is desirable, one that will not readily vibrate. The microscope shown in Fig. 6 is a cheap and commendable form, and good work can be done by this instrument, which is made by Ross, London. The stand carries the body-tube, and at the lower end is placed the objective, so called, because the image of the object (which rests upon the stage as shown) under examination is first focussed by it and conveyed along the body-tube. The top end of the said tube contains the eye-piece, so named because by its aid the eye is allowed to receive the image duly focussed and enlarged. As a rule, beginners work with one objective only, generally a one inch. Image: FIG. 6.--Microscope in position for drawing objects. A much higher power than this is necessary if the fibre in question is to be seen at its best, and for the purpose of this chapter a quarter inch objective will be used. Underneath the stage, which is pierced by a circular aperture, is a diaphragm. This regulates the quantity of light which is to be transmitted by means of the silvered reflector shown in the illustration. As a rule, two reflectors are fixed in the same holder; one a concave mirror, the other a plane one. The former brings the rays of light to a point or focus while the latter simply passes the beam of light along just as it received it, viz., as a parallel beam of light. In examining fibres the concave mirror will be of most use. An ordinary lamp is usually good enough for the light required, the one figured being very suitable and having a tube-like arrangement of wick. Behind the body-tube are two forms of adjustment, coarse and fine. The latter is worked by means of the milled screw, conical in shape, which is found immediately behind the coarse adjustment. The operator is supposed to have had some slight experience in the manipulation of the microscope. The slide is now placed upon the stage. Fine Sea Islands cotton is mounted in Canada Balsam and protected by a small circular cover glass. Now rack down the body-tube by means of the coarse adjustment until within 1/16 of an inch of the cover-glass of the slide. Now see that the light from the lamp is fully on the cotton strands. Rack up or down, as the case may be, with the fine adjustment, and a wonderful sight meets the eye, for the cotton viewed through the microscope is altogether unlike what we should expect it to be. Running completely across the field are a number of strands, varying in thickness, form and natural twist. What is meant by natural twist is very clearly shown in Fig. 7.