BLOOD ANALYSIS 



307 



solution the leucocytes may be counted in the same specimen of blood in which 

 the erythrocytes are counted. When this is done it is customary to use a slide 

 provided with Zappert's modified ruling (Fig. 98). This method is rather more 

 accurate than the older one of counting the leucocytes in a separate specimen 

 of blood. Furthermore, it is obviously preferable to count both the eryth- 

 rocytes and the leucocytes from the same blood sample. To insure accuracy 

 the number of leucocytes within the whole ruled region should be determined in 

 duplicate blood samples. This includes the examination of an area eighteen 

 times as great as the old style Thoma-Zeiss central ruling. This region then 

 would correspond to 3600 of the small squares and, if duplicate examinations were 

 made, the total bumber of small squares examined would aggregate 7200. 



The calculation would be as follows: 



Number of leucocytes in 7200 

 squares 



X 200 X 4000 4- 7 200 : 



Number of leucocytes per cubic 

 millimeter. 



I 

 FIG. 98. ZAPPERT'S MODIFIED RULING OF THOMA-ZEISS COUNTING CHAMBER. (Da Costa.) 



If a Zappert slide is not available, a good plan to follow is to place a 

 diaphragm in the tube of the ocular of the microscope consisting of a 

 circle of black cardboard or metal 1 having a square hole in the center of 

 such a size as to allow of the examination of exactly 100 squares or one- 

 fourth of a square millimeter at one time. With this arrangement any 

 portion of the specimen may be examined and counted whether within 

 or without the ruled area. In counting by means of this device it is, of 

 course, helpful if the microscope is provided with a mechanical stage, 

 but even without this arrangement, if the observer is careful to see that 

 the leucocytes at the extreme boundary of one field move to the opposite 

 boundary when the position of the slide is changed, the device may be 

 very satisfactorily employed. The leucocytes should be counted in 36 



1 Ehrlich's mechanical eyepiece with iris diaphragm is also very satisfactory for this 

 purpose. 



