8. _Microtome._—Although the preceding plans may be sufficient to answer all his requirements, if the student wishes to obtain only one or two sections of small dimensions, of a given object, if he requires a number of such sections he will find these methods fail him, for even though by practice he may have attained to considerable aptitude in the use of the knife, it will still unquestionably happen that the vast majority of his sections will be more or less imperfect. If, therefore, it be desired to procure a number of perfect sections, of equable thickness and large area, it is absolutely necessary to resort to the use of some form or other of microtome, or section-cutter. This instrument, in its simplest form, merely consists of a stout brass tube closed at one end, and being by the other fixed at right angles into a smooth plate of metal. A plug or disk of brass, accurately fitting the interior of the tube, is acted upon by a fine threaded screw piercing the base of the tube, and by means of which the plug, and any object it may support, can be elevated at pleasure. The object by this means being made gradually to rise out of the tube, sections are cut from it by simply gliding a sharp knife along the smooth cutting-plate, and hence across the specimen. Any intelligent worker in brass would make an instrument of this kind at a very small cost, and although perhaps it might lack the finish of an instrument bought at the optician’s, it would, if accurately made, do its work as well as the most complicated and expensive. If, however, the student resolves to purchase a microtome, there are a variety of forms in the market from which he may choose. A few hints may perhaps be of service in enabling him to make a judicious selection. At the outset we may say that unless the student intends to devote himself solely to the production of sections of wood, etc., he ought not to procure one of those forms of microtome known as wood section-cutters, in which the object to be cut is held in position in the tube by means of a binding screw which pierces its side.[4] Although these machines are all that can be desired for cutting hard bodies, they are not so suitable for soft ones. The chief points to be attended to in selecting a microtome are, (1) that the cutting-plate of the instrument be made of glass, or in default of this, of very hard metal of the most perfect smoothness;[5] (2) that the diameter of the tube be neither too large nor too small—it ought not to be less than 5/8-in., or greater than 1 inch;[6] (3) that the screw, which should be _fine_ and well cut, be provided with a graduated head; (4) that there be some kind of index by which fractional portions of a revolution of the screw may be measured; and (5) that the plug fit the tube of the microtome so accurately that when melted paraffine, gum, or other imbedding agent be poured into it, it may not find its way between the plug and side of the tube (§ 18). It