BLOOD COUNTS 15 
means of the mechanical stage the counting chamber should be moved 
carefully from square to square. It is useful to always count the cells 
when they lie on the lines, which are above and to the left. By fol- 
lowing some definite system the chance of counting the same corpuscle 
twice is eliminated. 
White Corpuscles. The blood should be drawn into the pipette 
up to the 0.5 mark after which the special diluting fluid for white 
corpuscles should be drawn up to the mark. This will give a dilution 
of yy. <A special diluting fluid which will destroy the red and leave 
the white corpuscles, must be used. The preparation of the disk of 
the counting chamber is identical with that for counting red corpuscles. 
A systematic procedure should be followed in going over the slide in 
order not to count the same square twice. In a well-prepared mount, 
the number of cells per square should not vary very much from the 
average for 100 ) squares. 
Fic. 14.—Diluting Pipettes for Heemocytometer. 
A counting chamber of more recent construction has been made 
for the Arthur H. Thomas Co., Philadelphia, known as the Thoma- 
Levy hemacytometer. This has certain advantages over the types 
of the older construction and is described as follows: 
“Tn the Levy construction a rectangular depression is cut into the slide itself 
extending across the entire width. In the middle of this depression is perma- 
nently fixed a rectangular strip of glass also extending entirely across the slide, 
and on this are the rulings. When the cover glass is placed in position on the 
slide itself the solution over the ruled areas is of the required depth. The Levy 
method of construction avoids the cemented cell and the attendant danger of 
its loosening by the drying out of the balsam cement, and the possibility of the 
loosening of the ruled counting surface is also greatly reduced by this construc- 
tion. The parallel form of cell greatly facilitates cleaning as compared with the 
circular type and the method of ruling used in the manufacture of these chambers 
provides a line with absolutely cleancut edges and of distinctly increased visi- 
bility when the chamber is filled with solution for the count. This increase 
in visibility of the ruling greatly lessens the eye fatigue experienced in making 
repeated counts.” 
The manufacturers ¢laim for this counting chamber the following 
advantages: 
Increased visibility of the rulings when chamber is filled with solution. 
