The main use of the microscope in bacteriology is in the study of _stained preparations_ of the organisms. Staining makes bacteria opaque and hence more easily seen than the transparent unstained forms. Some methods of staining also show morphological structures which are either imperfectly recognized in the unstained cell, spores, or are not visible at all--capsules, metachromatic granules, flagella. Finally certain bacteria are colored by special methods of staining which do not affect others, so that under proper conditions these bacteria may be recognized by staining methods alone--tubercle bacilli in the organs of animals. The phenomena of staining are essentially chemical, though sometimes the chemical union is a very weak one, even resembling an absorption of the dye rather than true chemical union--most watery stains. In other cases the chemical compounds formed are decidedly stable and are not decomposed even by strong mineral acids--staining of tubercle bacilli and other “acid-fast” organisms. In still other cases the principal action is a precipitation on the surface of the object stained--methods for staining flagella. In many methods of staining in addition to the dyes used other substances are added to the solution which assist in fixing the dye in or on the organism stained. Such substances are called _mordants_. The principal mordants used are alkalies, anilin, carbolic acid, iodine, metallic salts, tannic acid. While it is true that some bacteria may be stained by that standard histological nuclear dye, hematoxylin, it is of little value for this purpose. Practically all bacteriological stains are solutions of the _anilin dyes_. These dyes, as is well known, are of nearly every conceivable color and shade but relatively very few are used in bacteriological work. The beginning student will rarely use solutions of other than the three dyes _fuchsin_ (red), _methylene blue_ and _gentian violet_ for staining bacteria, with occasionally Bismarck brown, or eosin, or safranin as tissue contrast stains. The bacteriological dyes are kept “in stock” as saturated solutions in 95 per cent. alcohol which are _never used as stains_, but merely for convenience in making the various staining solutions. The approximate percentages of the three common dyes in such solutions are indicated in the following table adapted from Woods _Chemical and Microscopical Diagnosis_, Third Edition, 1917, Appendix: Fuchsin 3.0% Gentian Violet 4.8% Methylene Blue 2.0% The stains made from these dyes which are in most common use are the following: 1. Aqueous (watery) gentian violet solution. Saturated alcoholic solution of gentian violet 1 part Distilled water 20 parts Mix well and filter. 2. Anilin gentian violet. Saturated alcoholic solution of gentian violet 1 part Anilin water (see below) 10 parts Mix well and filter. 3. Anilin Fuchsin.