6i8 



LABORATORY EXERCISES 



As soon as the cover-glass has been put into position the chamber is laid under 

 the microscope, and if a haematimeter is being used as a counting chamber the "net 

 eyepiece" is required. It is not advisable to use a greater magnification than is 

 necessary. After waiting a short time, the counting is proceeded with when all the 

 cells in the preparation have sunk to the bottom. The "net eyepiece" consists of 

 a large square divided into sixteen or twenty-five smaller squares, the latter being 

 used as aids in counting. The cells inside the large square are counted; it does 

 not matter how the cells lying on the side lines of the square are counted, if the 

 same rule is always followed. Many squares in each haematimeter may be counted 

 by di^placihg the haematimeter. It is to be recommended always to count a certain 



d e c b 



[8. — Details of Thoma's haematimeter. A, Surface view of thick glass slide 

 with chamber and ruled center; B, cover glass; C, sectional view. 



number of squares, e.g. ten — two in the middle and eight along the edge of the drop. 

 As soon as these ten countings are performed, the haematimeter is well cleaned and 

 dried, the second test-tube well shaken and then a drop taken from it and counted in 

 the same manner. This alternation is repeated until a constant average is obtained. 



When it is not necessar)' to determine the number of cells in a given volume, 

 the same unit of volume is always employed, viz., that of a column of liquid of which 

 the base is the large square of the "net eyepiece" for the particular magnification 

 employed, the height being the thickness of the perforated cover-glass. 



For example, 3 cc. of beerwort with yeast cells and i c.c. of sulphuric acid give 

 the following results. 



