144 BACTERIOLOGICAL DIAGNOSIS. 
on the instrument (fig. 23, a). This is ruled in squares, 
and the squares are of known size. The amount of 
blood lying upon each square is thus known, and the 
number of corpuscles which lie upon it being counted 
under the microscope, all the data for the calculation 
are obtained. 
In blood examinations it is absolutely necessary that 
all points in the technique should receive the most 
careful attention, or the result will be worse than 
useless. For this reason we shall describe each step in 
the process at some length, and advise the practitioner 
to make several estimations before placing any reliance 
whatever on his results. 
Requisites.—1. The hemocytometer. 
2. A needle suitable for obtaining a small quantity of 
blood. A Hagedorn’s needle is the very best that can 
be used, and an ordinary triangular surgical needle will 
answer very well. 
3. Diluting fluid. There are a good many formule 
for this, and some are rather complicated. Isotonic 
saline solution (common salt ‘5 per cent.) will answer 
perfectly ; it is advisable to add to it a small quantity 
of some stain, methyl violet being the best, although 
gentian violet will do very well. This colours the leuco- 
cytes, so that they are readily distinguished from the 
red corpuscles.* 
4. A microscope having an 4 inch lens which will 
focus through the thick cover-glass supplied with the 
hemocytometer. Ifthe examination is not to be made 
by the bedside a strong india-rubber band a little 
shorter than the pipette should be carried. 
* The following formula is better:—Distilled water, 160 c.c.; 
glycerine, 30 c.c.; sodium sulphate, 8 grams; sodium chloride, 1 
gram; methyl violet, a trace. 
