THOMA-ZEISS COUNTING CHAMBER 491 



specimen. If milk must be brought from a distance to the laboratory 

 it is advantageous to add a few drops of formalin to 100 c.c. of the 

 milk to prevent changes in reaction. The milk should be well shaken, 

 then 10 c.c. are filled into a centrifuge tube. The latter is now im- 

 mersed in a water bath at 65 to 70 C. for ten minutes, or at 80 to 

 85 C. for one minute. The tube, while still warm, is at once centri- 

 fuged for ten minutes. 



2. Draw off 5 or 6 c.c. of the fat and watery fluid, leaving the 

 sediment undisturbed. Add enough warm distilled water to make up 

 to 10 c.c. Shake well and centrifuge again. Repeat this procedure 

 several times and a clear sediment free from fat is obtained. Finally, 

 draw off all fluid except 1 c.c., which is left in the centrifuge tube. 

 Shake well, place a small drop on a counting chamber, and count 

 several hundred squares. Estimate the average number of leukocytes 

 per square and from it calculate the number of leukocytes present in 

 1 cubic centimeter of milk. Ward recommends leaving only c.c. 

 in the centrifuge tube. 



Thoma-Zeiss Counting Chamber. This instrument, which is used 

 in counting red and white blood corpuscles in blood, milk, pus, urine, 

 and other fluid media, is constructed as follows : On the centre of a 

 strong slide a small round glass plate of exact known thickness is 

 mounted; a second glass plate, also of known thickness, with a larger 

 circular opening in the centre is so mounted on the heavy slide that 

 it is outside of the small round glass plate in the centre. The outer 

 plate is exactly -^ mm. higher than the small round inner plate, 

 which is ruled in such a manner that 1 square millimeter has been 

 divided into 400 equal squares. In other words, each one of the small 

 ruled squares is equal to T ^ 7 of one square millimeter. In using the 

 counting chamber a small drop of fluid containing the corpuscles (in 

 the present case milk sediment) is placed on top of the centre of 

 the inner round (ruled) glass plate. A clean, rather thick cover- 

 glass is then placed over the drop. This must be done very carefully 

 in an inclined manner, to prevent air-bubbles from entering between 

 the glass and the drop of fluid. The cover-glass, after being in place, 

 is now (at the margin where it rests on the outer plate) pressed down 

 with a tissue needle, glass rod, or lead pencil. Some of the fluid 

 which is between the cover-glass and the small, central, ruled glass plate 

 will run into the moat formed between the round inner and the outer 

 plate. The whole slide is now lifted to the stage of the microscope and 

 is focussed with a ^ or ^ inch objective. 1 A certain number of leuko- 

 cytes, easily recognizable by their nuclei and more or less granular 

 protoplasm are now seen. The number of leukocytes in 200 squares 

 is counted. Every space seen through the microscope has a base of 

 3-J-Q- square millimeter and its height is -^ mm., which is the distance 

 of the cover-glass resting on the outer plate from the inner ruled, 



1 As the specimen is unstained the iris diaphragm must be closed so that the field is only 

 dimly lighted. 



