Among Continental makers, excellent microscopes for histological work are turned out by many makers. Zeiss’s lenses stand deservedly high in reputation, as no faulty Zeiss lens ever leaves the works, and their optical properties are nearly perfect. For this guarantee, however, the purchaser has to pay somewhat higher prices, but the money is well invested. Zeiss’s agency is at 29 Margaret Street, Regent Street, W. Reichert, of Vienna, sells microscopes and lenses which are modelled on the lines of those of Zeiss, and though cheaper are often equal to them in excellence, but the quality is not quite uniform. His instruments can be obtained through any optician, but his agent in this country is Mr. A. Frazer, Teviot Place, Edinburgh. Before buying a microscope the student should obtain an illustrated price list from any of the firms mentioned above, and, having selected an instrument, he should test it very carefully, or better get some experienced friend to test it for him, before deciding to purchase it. Delicate test objects such as diatoms, scales of butterfly’s wing, or a stained specimen of micro-organisms should be employed. The coarse and fine adjustments should be tried. They should work freely and smoothly and without any delay. The definition of the lens must be tested with the fine objects mentioned. The field should be quite flat, _i.e._, every part should be in focus at the same time, and the definition should be perfectly sharp and accurate, and the test objects without double contour. The field should be totally free from prismatic colours. If there is a halo of colour around the objects it indicates a defect in the optical properties of the objective, and another should be selected. A microscope must always be treated with the greatest care. Jars and falls tend to slightly loosen and shift the lenses, and to permanently impair its optical properties. Dust must be most carefully excluded. This is best effected by keeping the instrument under a glass bell jar when not in use. The lenses should be wiped as little as possible, and when it is necessary, very soft chamois leather should be employed. The microscope must be kept in a dry room, or the brass work will soon tarnish and the steel parts will tend to rust. CHAPTER II. HARDENING PROCESSES. For the satisfactory examination of tissues it is necessary that they should be “hardened” in certain fluids. The object of this is to give the specimens greater consistence, so that thin sections may be more readily obtained and more safely manipulated, and also to “fix” the tissue element as far as possible in the same relative position as in the living body. The hardening process also acts on the protoplasm of the cells, and prevents their swelling up when placed in water, and in the various staining fluids.