Here again the use of absolutely clean coverslips is essential, and the blood must be taken immediately it escapes from the puncture. A little blood is taken on a cover-glass which is held horizontally. Another cover-glass is lowered on to this and by its weight and by capillary attraction, the drop of blood quickly becomes transformed into a thin film. The two covers are separated as soon as the film is formed by rapidly sliding them off one another. This manœuvre requires a little practice and dexterity. The movement of the slips must be in an exactly parallel direction otherwise the coating left will be uneven, just as when two pieces of bread and butter are pulled apart. Even with practice it is difficult to get more than one good film, the lower being usually best. There are four ways of fixing the film. 1. Exposure to *osmic acid vapour*. The film while still moist is held over the mouth of a bottle containing at least one per cent. solution of osmic acid. In a minute or two the fixation will be complete, and the film becomes of a dirty brown colour. It is then left exposed to the air to get rid of all traces of osmic acid, and may afterwards be stained as described below. 2. Treatment with *saturated aqueous solution of corrosive sublimate* (Muir’s method). The cover-glass on which the film has been spread, is floated before the latter has time to dry, film downwards on a saturated solution of corrosive sublimate in a watch glass for half an hour. The cover-glass is placed in distilled water and then in alcohol to remove excess of corrosive sublimate, and then stained. A little care is required when washing the film to prevent it sliding bodily off the cover-glass. 3. By drying and passing rapidly through the flame of a Bunsen burner, exactly as in preparing specimens of sputum, &c. (p. 111). This method is handy for ordinary clinical purposes. 4. By keeping the coverslips at a temperature of about 200° F. (Ehrlich’s method). Ehrlich uses for this purpose a strip of copper about two inches wide and a foot long which is supported on a retort stand in a horizontal position. One end is heated by a Bunsen’s burner beneath. The point in the copper strip at which the temperature is at boiling point is readily ascertained by dropping a little water on. The point at which a drop of water assumes the spherical state indicates a temperature there of 212° F. The coverslips are placed an inch or two further than this point, and kept there at a temperature of about 200° F. for some hours. STAINING METHODS. Fresh blood may be stained by mixing with Ferrier’s fuchsine solution:-- Fuchsine 1 grm. Distilled water 150 c.c. Dissolve and add Alcohol (80 per cent.) 50 c.c. Neutral glycerine 200 c.c.