1. Spreading the Film.--Thin, even films are essential to accurate and pleasant work. They more than compensate for the time spent in learning to make them. There are certain requisites for success with any method: (_a_) The slides and covers must be perfectly clean; thorough washing with soap and water and rubbing with alcohol will usually suffice; (_b_) the drop of blood must not be too large; (_c_) the work must be done quickly, before coagulation begins. The blood is obtained from the finger-tip or the lobe of the ear, as for a blood count; only a very small drop is required. * * * * * Ehrlich's Two-cover-glass Method.--This method is very widely used, but considerable practice is required to get good results. Touch a cover-glass to the top of a small drop of blood, and place it, blood side down, upon another cover-glass. If the drop be not too large, and the covers be perfectly clean, the blood will spread out in a very thin layer. Just as it stops spreading, before it begins to coagulate, pull the covers quickly but firmly apart on a line parallel to their plane (Fig. 74). It is best to handle the covers with forceps, since the moisture of the fingers may produce artifacts. Two-slide Method.--Place a small drop of blood upon a {169} clean slide and push it along with the edge of a second slide held at an angle of 45 degrees to the surface of the first (Fig. 75). [Illustration: FIG. 74.--Spreading the film: two-cover-glass method.] [Illustration: FIG. 75.--Spreading the film: two-slide method.] Cigarette-paper Method.--This gives better results in the hands of the inexperienced than any of the methods in general use, and may be used with either slides or covers. A very thin paper, such as the "Zig-zag" brand, is best. Ordinary cigarette paper and thin tissue-paper will answer, but do not give nearly so good results. Cut the paper into strips about ¾ inch wide, _across the ribs_. Pick up one of the strips by the gummed edge, and touch its opposite end to the drop of blood. Quickly place the end which has the blood against a slide or a large cover-glass held {170} in a forceps. The blood will spread along the edge of the paper. Now draw the paper evenly across the slide or cover. A thin film of blood will be left behind (Fig. 76). [Illustration: FIG. 76.--Spreading the film. Cigarette-paper method applied to cover-glasses.] * * * * * The films may be allowed to dry in the air, or may be dried by gently heating high above a flame (where one can comfortably hold the hand). Such films will keep for years, but for some stains they must not be more than a few weeks old. They must be kept away from flies--a fly can work havoc with a film in a few minutes.