1889.] ' MICROSCOPICAL JOURNAL. 221 



BiondVs Method. — I have already described this method in vol. ix, 

 p. 112, of this Journal. 



Cover-glass Preparations. — The blood should be spread on a cover- 

 glass in as thin a layer as possible. To do this, place a clean cover- 

 glass on a piece of filter-paper on the table, then put a small drop of 

 blood on the cover near one edge. Then bring the edge of a slide in 

 contact with the di'op of blood, and with slight pressure draw the 

 slide quickly across the cover-glass. By this means the blood is 

 spread out in such a thin layer that it dries before the cells change 

 their form. Preparations made in this manner, as soon as they are 

 thoroughly dry, may be mounted in balsam by inverting the cover- 

 glass on a drop of the same placed in the centre of a slide, and allow- 

 ing the cover to settle by its own weight. 



If the above preparations are to be submitted to the action of any 

 aqueous reagent, they must be fixed or the film will be washed off by 

 the subsequent manipulations. The fixing coagulates the albumen and 

 makes the film very adherent. The fixing is done by submitting the 

 cover-glass preparation to the action of osmic acid. It is immersed in 

 a i% solution, and allowed to remain for 5 to 10 minutes, or it may be 

 inverted over the mouth of a bottle containing a solution of osmic acid. 

 The preparations are then washed well in water and dried in the air. 

 In place of osmic acid, dilute solutions of chromic acid, mercuric chlo- 

 ride, or alcohol may be used. 



Staining. — Cover-glass preparations that have been fixed may be 

 stained with any of the usual staining reagents used in dilute solutions. 

 A few drops of the stain are placed on the prepared side of the cover 

 and allowed to act for about one quarter of an hour. The stain is then 

 washed off with water, the cover-glass allowed to dry in the air, and 

 then it is mounted in balsam. 



For the white cells and nucleated red cells, double staining gives 

 beautiful pictures. The cover-glass preparation is first stained with a 

 nuclei stain and then with a contrast stain. 



Blood-cell Counting. —To collect the blood for counting, a punc- 

 ture is made in the end of the finger and the required amount of blood 

 sucked up into the mixer (Fig, 2). The mixer is so constructed that 

 the capillary tube is exactly one one-hundredth the capacity of the bulb. 

 Before counting the blood is always diluted in a known proportion. 

 The dilutions used are generally 1 to 100 or 1 to 200. For diluting the 

 blood a f-% solution of sodium chloride is used by Thoma ; Malassez uses 

 a mixture of 1 part of a solution of gum arabic, of the sp. gr. 1,020, 

 and 2 parts of a solution of sodium sulphate and sodium chloride, in 

 equal parts, and having a sp. gr. of 1,020. The method of dilution 

 is as follows : The blood is sucked up into the mixer to a fixed mark. 

 The point of the mixer is then wiped, and it is then filled to the mark 

 (101) with the dilutant. The end of the mixer is then closed with the 

 finger and the mixer carefully shaken, the glass bead in the bulb aiding 

 in distributing the cells. The fluid in the capillaiy tube is allowed to 

 run out and only that portion of the mixture contained in the bulb is 

 used for the counting. 



If the mixer has been filled to the mark 1 with blood and then after- 

 wards the dilutant added until the contents reach the mark 101, the 

 bulb will then contain a mixture of 1 part blood to 99 parts of the dilu- 



