The bacilli are readily detected by means of the staining method devised by Koch. Various modifications have been presented from time to time, of which that of Ehrlich[72] has proved the most satisfactory. The essential features are to obtain a dry, thin layer of a selected portion of the suspected sputum, which is then to be deeply stained with fuchsin or methyl-violet; the excess of color is to be removed with nitric acid, and the preparation is then ready for examination with the microscope. A power of four or five hundred diameters is sufficient for the recognition, and the object should be illuminated with a flood of light through a large diaphragm or an achromatic condenser. The bacillus retains the color notwithstanding its exposure to the acid, and the violet colors are more strongly presented if the preparation is tinted yellow after the action of the acid. If the bacilli are stained red with fuchsin, the background should be made blue. It is important that the reagents should be freshly prepared and filtered, that other bacteria may not obscure the picture, and that all the apparatus employed should be thoroughly clean. [Footnote 72: _Allg. med. Centr. Zeitung_, 1882, xxxvii. 458.] A fragment of thick, opaque sputum is to be taken in forceps, placed on a cover-glass, and spread into a thin layer by means of a second cover-glass. The prepared slide is then to be passed slowly through an alcoholic flame, or that of a Bunsen burner, till the layer of sputum is dried. A saturated alcoholic solution of methyl-violet or fuchsin is made and filtered, and added, drop by drop, to a filtered, saturated solution of aniline oil shaken in water. The color is to be added with stirring till an opalescent film forms on the surface of the mixture. The slide containing the dried sputum is to be placed in or on this staining fluid, and allowed to remain for half an hour or less, the application of warmth hastening the process, when it is removed, and the specimen is decolorized in a solution of one part of nitric acid and two parts of water. The preparation is then washed in water, and may be examined directly in water, glycerin, or, after dehydration in alcohol, in oil of cloves. The tinted bacilli are made more prominent by a secondary staining, for a minute or two, of the red (fuchsin) preparation {103} in a concentrated solution of methyl-blue, the violet preparation being secondarily stained in a like solution of aniline-brown. If the preparation is to be permanently preserved, it should be dehydrated in strong alcohol after washing with water, and it may then be treated with oil of cloves and mounted in Canada balsam.