6 METHODS OF EXAMINATION 



pipette is then expelled by compressing the rubber tube and twist- 

 ing it. 



One should never blow in the tube as that would be apt to cause 

 saliva to mix with the diluted blood. After a few drops have been 

 expelled a medium sized drop, obtained by simply compressing 

 the rubber tube, is placed on the center of the counting chamber 

 and the cover glass adjusted. In placing the cover glass care must 

 be taken to avoid including air bubbles. If one side of the cover 

 is placed in position and the other gradually lowered there will be 

 no danger of including air bubbles. The cover glass should be 

 placed as quickly as possible. A delay will result in an uneven 

 distribution of the red corpuscles, the surface of the drop before 

 being covered being rounded the corpuscles fall on the ruled disc 

 from unequal thicknesses of fluid. The drop should be large 

 enough to fill or very nearly fill the central disc without running 

 into the trench or between the cover glass and the plate. The 

 proper size of the drop must be learned by experience. With a 

 little practice one can get the proper sized drop at the first trial. 

 If the cover glass fits closely a play of colors (Newton's rings) will 

 be seen between the cover glass and the plate. If they are not 

 seen it is because the cover glass is not in place. The la} r er of 

 blood is consequently more than 0.1 mm. in thickness. Gentle 

 pressure on the corners of the cover glass may facilitate the ap- 

 pearance of the colors, but they must remain after the pressure is 

 removed. The counting chamber and cover glass should be cleane* 1 

 and another drop adjusted if the fluid runs into the trench or be- 

 tween the cover glass and the plate, if the disc is not well filled with 

 fluid, if air bubbles are included, if the interference colors (New- 

 ton's rings) are not seen, or if the blood is found to be unevenly 

 distributed. Time is saved by brushing off dust or lint from both 

 counting chamber and cover glass with a small camel's hair brush 

 before filling the counting chamber. 



Counting. The counting chamber should be kept level after 

 the drop of blood is placed in it. The count is made on a micro- 

 scope with ;i level stage. It is convenient to use a x 4 (2 inch) 

 ocular and a five mm. (^ in.) objective or a special four mm. (| in.) 

 objective with a long working distance, N. A. 66. After the 

 counting chamber is placed on the stage one must wait until the 

 corpuscles have settled on the lines. Count the corpuscles in one 



