THE BLOOD. 49 



tity, but if only a small drop is obtainable, such, for example, 

 as is got by pricking the finger, a mixer is better. This con- 

 sists of a capillary tube terminating in a bulb, the capacity of 

 the bulb between the marks 1 and 101 being exactly 100 times 

 that of the tube from its point to the mark 1. A small glass 

 ball is inclosed in the bulb, and serves, by its movements, to 

 facilitate the mixing. The capillary tube is allowed to fill with 

 blood as far as the mark 1 ; sulphate of soda solution is then 

 sucked up as far as the mark 101- As it passes in, it of course 

 pushes the blood before it into the bulb, and the two are there 

 thoroughly mixed by gentle agitation. 



The next thing is to count the corpuscles in a known quan- 

 tity of the mixture. The most convenient plan is that of 

 Hay em and Nachet. A slide is used, having a glass ring mm. 

 in depth, cemented on to its upper surface. A drop of the 

 mixture, but 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 % mm. square, this will give the number which 

 were contained in mm. cube of the mixture, and multiplying 

 this by the number of times the blood was diluted, the result 

 will be the number of corpuscles in | mm. cube of blood. 

 Schaefer thinks that it is more convenient to have the quad- 

 ratic markings upon the micrometer glass of the eye-piece than 

 upon the slide, which is a practical point. The quadratic 

 markings are shown in Fig. 22. To measure any square, it is 

 only necessary to take the stage micrometer, ruled in milli- 

 metres and decimals, and adjusting the draw tube, make the 

 side of one square correspond exactly to an interval of | mm. 

 on the stage micrometer. It will then be convenient to mark 

 the tube at this point, and then, in all subsequent work, if the 

 tube be kept at this line and a slide is used of the thickness of 

 the micrometer and the same lens and eye-piece, the side of a 

 square will always be mm. This method is the one in general 

 use. 



Another less frequently employed is that of Malassez, which 

 is also described by Schaefer as follows : A little of the mixture 

 of blood and sulphate of soda is transferred to a very fine flat- 

 4 



