cations, The American Museum of Natural History, Central Park West at 79th Street, New York City. MICROSCOPE SLIDES Of the million or more known species of arthro- pods, a great many are too small to be appreciated by the naked human eye. Of the hypothetical ten million yet to be described, the majority are probably minute. Adequate study of such creatures is impossible with- out the aid of some sort of magnifier, and if you are an enthusiastic student, you will want the best that you can find. The best is not necessarily the strongest. With increasing magnification, the size of the field and the depth of focus decrease, so that less of the specimen is visible at a time. For most insects and spiders, a four- or six-power hand-lens is more satisfactory than one with a magnification of twenty diameters. Take a few specimens with you to the opticians's, and try a variety of lenses critically before making your choice. For laboratory work, a low-power binocular "dissecting" microscope is better than any hand-lens; but since it costs over a hundred dollars, few amateurs can hope to possess their own. Most high-school and college biology laboratories do have such instruments. Perhaps you can arrange to use one of these, when the class is not in session. Have someone who thoroughly understands it demonstrate its operation. You will be amply repaid for any trouble you may take in learning to use the microscope properly. Specimens which are so small that they must be examined through lenses are likely to be correspond- ingly fragile. They should be handled as seldom as possible. Hard-bodied ones may be mounted dry, on cardboard "points," as described in Leaflet Four. Soft-bodied ones may be preserved in liquid, as dis- cussed above. Either may be mounted upon slides. The standard microscope slide is a slip of clear glass, one inch wide by three inches long. The specimen, suspended in a transparent medium, is sandwiched in between this slide and a smaller and much thinner cover-glass. So mounted, it is relatively safe from damage, easy to handle, and convenient to store. There is, however, this serious disadvantage; the position of the specimen is fixed, and one, or at most, two sides only are exposed to view. For this reason, it is desirable, when practical, to mount several identical animals in different positions upon the same slide, so that what is concealed in one may be displayed in another. The distortion apparent in some slide- mounted specimens is largely the result of faulty preparation and can be avoided. Slide making for the medical sciences is a fine art about which many scholarly volumes have been written. Entomological slide making is, fortunately, easier. To an histologist or a bacteriologist, the smallest whole insect or spider is a gross object which can be treated adequately by processes simple to the point of crudeness. This does not mean that you can afford to be careless. It does mean that, working with a minimum of equipment and with such materials as are readily available and relatively harmless, you can expect to produce a satisfactory product most of the time. The method of slide preparation here described, though not quite the simplest possible, is yet frankly a beginner's method. It requires no expensive tools, and no chemicals which are extravagant, explosive, deadly in small quantities, or proscribed by law. Nevertheless, it is a workable method of mounting most kinds of small insects and spiders, and a sound foundation upon which to build more elaborate procedures. Many of the necessary materials you have already; a large drugstore may be able to provide the rest. If not, you can order them from a scientific supply house. Here is a list of the things that you will need: 1. "Completely denatured" alcohol of graduated concentrations, including full strength, as described above under "Liquid Preservation." 2. Xylol (xylene). Although a liquid, this is sold by the pound. One pound is about a pint and a half, and should last a long time. 3. Carbol-ylene. Four ounces will be plenty. This is a saturated solution of phenol (carbolic acid) in xylene. Phenol is very corrosive, and can "burn" you badly. If you spill any of this mixture on your skin, wash it off at once with strong alcohol. 4. Canada balsam, the transparent medium in which the specimens are to be suspended. A resin soluble in xylene, balsam is usually sold as a viscous liquid, packed in a bottle or a tube. It is almost colorless when new, but turns yellow with age. 5. "Duco" or other clear plastic cement. 6. Containers for the specimens during processing. The number and kind will depend upon the scale and frequency of your operations. If you wish to make a few slides only, at long intervals, you may leave each lot of specimens in its own labeled vial throughout the process, changing the fluids with a medicine dropper. If, on the other hand, you expect to be working steadily for a season, you may prefer to keep each liquid in a marked and covered dish, and to transfer the speci- mens from one to another. In this case you will need: 7. Some gelatine capsules, such as druggists use for powdered medicines. You can buy an assortment of sizes at any pharmacy. When each end is punctured several times with a strong needle, such a capsule becomes a miniature strainer in which one or several specimens can travel safely through a whole series of baths. To prevent confusion, mark both the capsule and the data label of the insect which it contains with