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makes it a very difficult matter to obtain the focus of a good objective. And it not unfrequently gives an apparent motion to the object on the stage, which may be seen gliding backwards and forwards to the extent of an inch or more over the field. Such a microscope, however cheap it may be, should be studiously avoided, as it will inevitably prove a source of constant irritation to its possessor. In some of the cheapest instruments, the rack-work and pinion movement is dispensed with altogether. The focussing of the lenses is accomplished by sliding the body up and down an outer tube which embraces and supports it. A microscope of this character may be used with low powers, but it can never be made to work with the same degree of accuracy as those which possess the higher mechanical movements. There is one other mode of adjustment which I think it right to mention here. It is known as the chain movement. A watch chain is in this case substituted for the rack-work. The chain is wound round a small drum, connected with the usual milled heads. The movement thus obtained is so beautifully smooth and so little likely to get uneven by wear, besides obviating the necessity of a fine adjustment, that I cannot understand why it is not more universally adopted. The only objection I have heard urged against it, is that the chain, being made up of so many small links and rivets, is, like a fortress, no stronger than its weakest point: it is liable to snap asunder. The old chain may, however, be easily replaced by a new one, at the coast of a few shillings. Besides the movements already alluded to, and known as the coarse adjustment, there is in all the better class of microscopes a much more delicate mechanical contrivance termed the fine adjustment, which, as its name implies, gives greater facilities for securing to the greatest nicety the focus of a one-eighth or one-twelfth objective. In the former arrangement the entire body or tube is moved up or down by the rack-work; in the latter, it is the object-glass alone that is affected. This advantage is secured by attaching the object-glass to a short piece of tubing that fits accurately the interior of the lower part of the body, and at the same time slides freely in it. The tube is kept down in its place by a spiral spring, which presses upon it from behind. Motion is communicated to the tube by a graduated milled-head screw, acting on the end of a lever. The screw itself contains 150 threads to an inch. One revolution, therefore, of its thread raises or depresses the tube, carrying the object-glass the 150th of an inch. The tenth part of a revolution gives us the 1,500th, while the half of a division realizes a movement not exceeding the 3,000th of an inch. This extreme nicety of adjustment may, to the beginner, seem unnecessary, but to the practised operator it is found to be not a bit too sensitive for his minute investigations. It also proves a safeguard for the object-glass, when, through inadvertence, it is brought in contact with the glass that covers an object. The spring tube yields to the pressure, carrying with it the objective, which is thus sometimes saved from destruction.

We now come to the stage: this, in its simplest form, consists of a flat brass plate, with a circular hole in its centre for the transmission of light from the mirror beneath it. It should be broad and roomy, and firmly attached to the bar that supports it; not too thick or deep for oblique illumination, nor so thin as to be springy under a slight pressure of the manipulator's hand. Such a stage is furnished with a sliding ledge, against which objects rest when the instrument is used in an inclined position; but there is no mechanical arrangement for giving motion to an object; this must be effected by the fingers of the observer. It is not, however, every microscopist who possesses a touch delicate enough for such a purpose. Although stage movements necessarily render a microscope more expensive, the advantage more than counterbalances the extra cost. A moveable stage consists of two or three flat plates, sliding one over the other in rectangular directions, by means of screw and rack-work. Such movements should be as perfect as those required for the body of the instrument. The value of a stage with such mechanical appliance is best appreciated when high powers are employed.

By turning the milled heads on either side of the stage, the stage-plate carrying the object slowly traverses the field either in the direction of its length or breadth, and the whole of an object too large to be seen in the field at one view is thus made to pass slowly before the eye. If both heads are turned at the same time, the object then takes a diagonal direction. In the best microscopes there is also a circular motion produced by rack and pinion, which enables the stage-plate to revolve three-fourths of a circle, or, in some cases, to make a whole revolution. Such an arrangement is advantageous, provided it be not obtained by sacrificing other conveniences of more importance. The ordinary traversing motion of a stage should not be less than three-quarters of an inch in each direction; if extended to an inch, or even more, so much the better for the operator, who will be saved the trouble of shifting his specimen. There is yet another contrivance, known as White's Lever-Stage, in which motion is communicated to the stage by a lever. By applying a slight force to its handle or free end, the stage-plates glide quickly one upon another, thereby presenting considerable facilities for following the rapid evolutions of the numerous agile animals found in stagnant water. We had almost forgotten another useful and simple arrangement—the magnetic stage, which consists of a single plate of brass, having two narrow tongues projecting slightly above its surface, and running longitudinally at equal distances on each side of the circular opening. The tongues being in