2l6 PHYSIOLOGICAL CHEMISTRY. 



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 erythrocytes and the leucocytes from the same 

 blood sample. To insure accuracy the number of leucocytes within 



FIG. 73- 



ZAPPERT'S MODIFIED RULING OF THOMA-ZEISS COUNTING CHAMBER. (Da Costa.} 



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 3,600 of the small squares and, if dupli- 

 cate examinations were made, the total number of small squares 

 examined would aggregate 7,200. The calculation would be as 

 follows : 



Number of leucocytes in _ Number of leucocytes per 



7,200 squares cubic millimeter. 



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 con- 

 sisting 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 



1 Ehrlich's mechanical eye-piece with iris diaphragm is also very satisfactory 

 for this purpose. 



