ESTIMATION OF THE OPSONIC POWER OF THE BLOOD 131 



come to the bottom and the supernatant fluid is left clear. If the 

 deposit is closely examined the red corpuscles will be seen to be 

 at the bottom, whilst above them there is a thin whitish layer of 

 leucocytes. Then, with a capillary pipette armed with an india- 

 rubber nipple, the whole of the clear fluid is to be pipetted off as 

 close as possible to the leucocyte layer, but without disturbing the 

 latter. Next, this layer and the upper quarter or so of the column 

 of red corpuscles (which also contain leucocytes) are to be 

 pipetted off and put into a small tube, and thoroughly mixed 

 together by repeatedly sucking them into the pipette and then 

 expelling them. The result is an emulsion of living leucocytes 

 mixed with red corpuscles. The presence of the latter is a decided 

 advantage. (It is advantageous, though not absolutely necessary, 

 to rewash the leucocytes in saline solution in order to get rid of 

 the citrate of sodium.) 



5. Two Wright's pipettes. These are drawn out from a piece 

 of ordinary glass tubing about 4 inches long and about as thick 

 as a lead-pencil. This is held at each end, and the central portion 

 is thoroughly softened in a Bunsen or blowpipe flame, the tube 

 being turned round the while. When quite soft the tube is 

 removed from the flame, and the ends then pulled firmly and 

 steadily apart until the softened portion is pulled out into a thin 

 tube (about the thickness of a steel knitting-needle or a little less, 

 and a foot or more long). This will give two pipettes, and to 

 separate them melt the central portion in a small flame, such as 

 that of a wax vesta, and when the glass is softened pull them 

 quickly apart. The whole pipette should be like this — 



Fig. 36. 



The lower figure represents the point, which must have as 

 nearly as possible the shape represented in the figure. The ease 

 and accuracy of the process depends in great measure on this 



being the case. 



The Process.— I. Prepare a pipette by placing an mdiarubber 

 nipple on the thick end. Then with a grease pencil or with pen 

 and ink make a transverse line about i inch from thepomted end. 



