_Simple microscope._--For examining the larger kinds of objects, and for dissection, a simple microscope is very useful. This consists of a stand, a stage, and an arm supporting a simple lens or combination of lenses, but without the body of the compound microscope (as the ordinary microscope is distinctively called). For most purposes, common plano-convex or doubly convex lenses are sufficient to form the object-glasses of a simple microscope. With the best microscopes, an “erector,” or tube containing a pair of lenses, fitted within the body, renders the compound microscope capable of answering most of the purposes of a simple microscope. _Polariscope._--An expensive but interesting and useful addition to a microscope is a polarizing apparatus, or polariscope. This consists of a Nicol’s prism, or a plate of tourmaline, placed beneath the stage, and another in the body of the microscope or above the eye-piece; both in brass fittings. The former is called the polarizer, and the latter the analyzer. _Rotating disk._--Another most useful piece of apparatus, for moving opake objects whilst under the microscope, in all directions, is Smith and Beck’s “rotating disk.” _Slides._--The slides upon which objects, especially those to be viewed as transparent objects, are to be placed, should be made of crown or plate glass. They are usually 3 inches long, and 1 inch wide; but I prefer them 2½ inches long, and 1 inch wide, simply because they take up less room in a cabinet, and because they do not project beyond the stage on either side. They should not be more than 1/20th of an inch thick, and as colourless and clear as possible. The edges should be ground or filed, to prevent their scratching the stage. _Covers._--The covers are square pieces of very thin glass, less in breadth than the slides, so as not to reach their margins; and of various thicknesses, the thicker and stronger being used to cover large objects for examination under the lower powers, and the thinner serving to cover very delicate objects requiring the higher powers. _Side condenser._--For illuminating opake objects, a large plano-convex or doubly convex “bull’s-eye” lens, or side condenser, is used; this is fixed to an arm, which slides on a stand, so as to be capable of being raised or lowered to a suitable height. This is placed between the source of light and the stage, and at such a distance from the latter that the light may be brought to a focus upon the object. Sometimes a “Lieberkuhn” or concave silver reflector is used for this purpose. These are the most important pieces of apparatus required in examining microscopic bodies. But the beginner will do well, if he have the achromatic condenser and the polarizing apparatus, to lay these aside until he has had considerable practice in examining objects simply with the mirror and the lower powers.