290 APPENDIX. 



quantity of the mixture. The most convenient plan is 

 that of Hay em and Nachet. A slide is used having a 

 glass ring, 1 millimetre in depth, cemented on to its upper 

 surface. A drop of the mixture, not enough to fill the 

 cell so formed, is placed in the middle of the ring, and a 

 perfectly flat cover-glass is so laid on that the drop touches 

 and adheres to it without reaching the sides of the cell. 

 The slide is placed on the microscope, and as soon as the 

 corpuscles have settled down to the bottom of the drop 

 the number in a definite area is counted. If the area 

 chosen is J of a millimetre square, this will give the num- 

 ber which were contained in millimetre cube of the 

 mixture, and multiplying this by the number of times the 

 blood was diluted, the result will be the number of cor- 

 puscles in J millimetre cube of blood. 



Areas of J millimetre square might be marked with a 

 diamond (like the lines of a stage micrometer) upon the 

 centre of the slide, and subdivided into smaller squares 

 to facilitate the counting. But it is found more conve- 

 nient to measure them off by using an ocular micrometer 

 similar to the one shown in Fig. 33, but with a large 

 square subdivided into smaller squares substituted for 

 the scale on the micrometer glass, m. To obtain the de- 

 sired value for the large square, a stage micrometer, ruled 

 in parts of a millimetre, is placed under the microscope, 

 and by adjusting the draw-tube, the sides of the square 

 on the ocular are made exactly to subtend the interval of 

 \ millimetre on the stage micrometer. A mark is then 

 made on the tube to indicate the extent to which it is 

 drawn out, and whenever an examination is made the 

 draw-tube is always adjusted to this mark, the same ob- 

 jective (one of moderate power) being of course used in 

 every case. 



By another method that of Malassez a little of the 

 mixture of blood and sulphate of soda is transferred to a 

 very fine flattened capillary tube (Fig. 36 A) the capacity 

 of a given length of which is ascertained previously and 

 marked on the slide to which the tube is fixed. Thus in 

 the capillary tube shown in the figure, a length of 400 

 micro millimetres 1 represents the TF V.s part of a cubic 



1 A micro-millimetre (/*) is the one-thousandth part of a milli- 

 metre. 



