APPARATUS FOR HOLDING SPECIMENS 61 



will therefore be seen that the amount of liquid resting upon 

 each square has a cubic capacity of l/4,000th of a cubic mm. The 

 hquid which has been placed in the counting chamber is allowed 

 to settle, and the corpuscles will therefore be in contact with the 

 bottom of the cell. It will be found that it is a simple matter 

 to count the corpuscles contained in each square. The usual 

 method is to count, say, 100 squares, and it must be noted that 

 m dealing with those actually on the lines, only those on two sides 

 of the square should be counted, and this rule should be applied 

 throughout. 



The number of corpuscles in 1 cubic mm. "of undiluted blood 

 is then obtained by multiplying together the rate of dilution, the 

 number of corpuscles counted, the volume of each square (l/4,000th 

 of a cubic mm.), and dividing by the number of squares counted. 

 The above is the general method of counting the red corpuscles ; 

 but in the case of the white corpuscles, as there are a very much 

 smaller number of these, the method generally employed is to 

 count the total number of the whole ri3ed area of the counting 

 chamber, which is 1 sq. mm. 



^^ There are other forms of counting chambers, such as the 

 Biirker, Fiichs -Rosenthal, Breuer, and Zapperts ; the method of 

 employment in all these is the same, but the ruHng and also the 

 counting are different in each case. 



The use of a mechanical stage greatly assists the counting. 



A simpler form of haemacytometer can be used which depends 

 for its action on Mr. Rheinberg's beautiful process of making 

 graticules and glass scales. A glass 

 plate is photographed with squares 

 in the pattern of a chess-board, so 

 that alternate squares are tinted, al- 

 though they are transparent. This 

 plate is dropped into an eyepiece be- Fig. 64. 



tween the lenses, and by means of a 



stage micrometer the drawtube can be varied until a definite 

 number of squares are equal to '1 of a millimetre in the mi- 

 crometer. The chess-board glass plates are supplied with squares 

 either 1/4, 1/2, 1, or 2 mm. in size. They are made to cover the 

 whole field of view, or as a small block of squares in the centre 

 of the field. The latter are to be preferred for blood counts. 



The only other requirement is a 3 X 1-inch sHp with a metal 

 ring cemented to it which is '1 mm. thick, into which the blood 

 is placed covered with an ordinary cover glass. Suppose a 1/6-inch 

 object glass is being used, a 1-mm. chess-board plate dropped into 

 the eyepiece can be made by drawing out the drawtube to the 

 required position according to the eyepiece and object glass 

 employed, of such an apparent size that nine squares, three each 

 way, correspond to '1 mm., and the count of nine squares will 

 give the number in a cubic tenth of a millimetre. If a 1/2-nmi. 



