156 UNIVERSITY OF WISCONSIN STUDIES 
and at'sufficiently high speed to throw down most of the leu- 
cocytes without injuring them, e. g., 2,000 r. p. m. for ten min- 
utes. 
Spreading of sediment.— When the tubes are taken out 
of the centrifuge, the fat layer at the top of the tube is 
loosened, the milk and stain mixture poured out, the cork re- 
moved, and the sediment on it carefully spread on a glass 
slide, as one would make a blood smear. It must always be 
berne in mind, however, that water cannot be used to spread 
the smear, as this violates the principles involved and in- 
variably causes the cell nuclei to take up the stain, even in 
raw milk. 
Microscopical examination of smears.—The smears arc 
first examined with the low power (16 mm. lens). In the 
raw milks the background, or the entire microscopic field, 
is stained blue, the depth of the stain depending upon the 
thickness of the film: in this blue background appear numerous 
clear areas. The smaller of these are fat globules. The larger 
ones may be large fat globules or clusters of the same, but 
usually they are the leucocytes of the milk, Occasionally 
there are the deeply stained mononuclear leucocytes. The 
general impression one gets from these preparations is a blue 
field, which tends to be uniform, and in which there are a 
number of holes or clear places. : 
The smears of the pasteurized milk, on the other hand, do 
not have the background as deeply stained as do the smears 
from the raw milk, although in thick portions it may be quite 
blue. The nuclei of the leucocytes here are always stained and 
their volor is deeper than that of the background. The area 
immediately surrounding the cells usually takes the stain 
deeply and shades off into the color of the background, forming 
what I have called a ‘‘dark halo’’. 
Because of the lighter color of the field, the fat globules 
do not stand out as they do in the raw milk. The most 
prominent objects here are the leucocytes with their darkly 
stained nuelei. 
The cells are noticeably smaller in the pasteurized than 
they are in the raw milk. 
