It is obvious that we cannot attain any degree of accuracy in focussing, especially with high magnifications, when we must perforce raise or lower the tube by hand. To obviate this difficulty, most microscopes are provided with mechanism known as a coarse adjustment; it consists of milled screws at either end of a metal rod; in the centre of the rod there is a little cog-wheel which engages with a row of notches on the tube. By turning the milled screws slightly in either direction, we can impart a considerable upward or downward movement to the tube carrying the objective and focussing at once becomes a more simple matter. The coarse adjustment is only useful for examining objects with a low magnification; if we use it when objects are being highly magnified we run the risk of screwing our objective down upon our object, to the certain destruction of the latter and the probable injury of the former. To obviate such a catastrophe, most of the better class microscopes are also provided with a fine adjustment. By means of this adjustment, which externally takes the form of a single milled screw, a considerable turn of the screw in either direction only imparts a very slight upward or downward movement to the microscope tube. In the best instruments, movements of as little as one hundredth part of a millimetre may be imparted to the tube by the fine adjustment and, seeing that there are about twenty-five and a half millimetres to the inch, it is obvious that a good fine adjustment is very delicate and, being so, must be treated with care. The fine adjustment is used to supplement coarse adjustment in the final focussing, when using high magnifications. A few words may be devoted to the mirror, for on its intelligent use much depends. Usually we shall find that it is plano-concave, that is to say, flat on one side and hollowed out on the other. The use of the mirror, as we have mentioned already, is to reflect rays of light through the opening of the stage on to the object we desire to examine. Both mirrors will reflect parallel rays of light to a point, just as a double convex lens will so direct them from their course that they meet at a point. The concave mirror gives the more powerful illumination, because it reflects more light rays than a flat mirror of the same diameter. We have mentioned that, to obtain full advantage from the mirror it should be capable of movement to and from the stage. When we desire strong illumination we arrange the mirror so that its reflected rays meet at a point coinciding with our object. Should less intense illumination be required, we slide the mirror nearer to the stage, and of course nearer to our object, so that the reflected rays meet at a point above the object. [Illustration] The two diagrams, given below, show the path of the rays of light, where O is the object, and a trial with our microscope will soon show which position gives the more powerful illumination.