BLOOD COUNTS _ 13 
monasany. This Thoma ruling, however, is more suitable for counting 
red blood cells. 
The pipettes are two in number and are graduated to deliver differ- 
ent amounts. The pipette for erythrocytes delivers dilution of +45 
or gia; the pipette for leucocytes delivers a dilution of yy or xb. 
The hemocytometer was originally devised for counting the blood 
corpuscles. From this it has been adapted to the enumeration of bac- 
terla in substances containing large numbers. It is best for a person 
not familiar with the technique to apply it to the counting of blood 
corpuscles before attempting to count bacteria. 
Procedure, Red Corpuscles. Collect a drop of blood on a clean 
slide and draw part of it up to the mark on the red corpuscle pipette 
(101). The blood may be secured by cutting the tip of the finger 
(across the lines) or by a syringe from a vein in the forearm. By 
drawing the blood to the 0.5 mark a dilution of 200 is secured; when 
drawn. to the 1.0 mark the dilution is 1 to 100. This first measurement 
of blood is very Important since a slight variation from the mark 
introduces a larger error in the final results. Should the blood be drawn 
up too far in the pipette it may be shaken down carefully to the mark. 
After this the diluting fluid should be drawn into the pipette up to the 
101 mark and the pipette thoroughly shaken. The blood is now suf- 
ficiently diluted for counting the red blood corpuscles. The contents 
of the end of the pipette below the bulb should be discarded since it is 
filled mostly with diluting fluid. A drop of the diluted blood is now 
placed on the disk of the counting chamber. This should be just suf- 
ficient to cover the disk after the cover glass has been put on. The cover 
glass should be carefully applied with a very slight sliding motion. 
This should be placed on the chamber as soon as possible in order to 
secure as even a distribution of corpuscles over the bottom of the 
chamber as possible. The chamber must be left on a level surface for 
about a half heur, after which the corpuscles will have settled to the 
bottom and will be resting on the ruled surface. The number of red 
corpuscles is now determined by putting the chamber under a micro- 
scope fitted with a mechanical stage. 
The small squares cover an area zé> Sq. mam. since they are sy mm. 
on a slide. The depth of the chamber from the bottom of the cover 
glass to the top of the disk is 7s mm., hence the volume of liquid above 
a square is sy Xz5 Xzo equals zooy mm. 4000Xdilution (200) equals 
red corpuscles per cubic millimeter. A sufficient number of squares 
should be counted to give a fairly correct average per square. To 
accomplish this it is best to count about one hundred squares. By 
