THE DETECTION OF PASTEURIZED MILK 163 
except in the thickest places. The leucocytes are usually sur- 
rounded by a ‘‘dark halo’’, They are small,—not more than 
7.5 microns in diameter. If the sample is examined soon after 
pasteurization or has been kept under conditions which pre- 
vent the growth of bacteria, the bacteria will not stain at all 
or at least variably or indistinctly. 
In making this examination the slide is ordinarily first ex- 
amined under the low power, and the light, uneven distribu- 
tion of the background and the deeper stained leucocytes are 
noted if the milk has been properly pasteurized. In a raw 
milk the background is quite blue in the thicker places and 
the unstained leucocytes appear as clear areas in it. 
The slide would ordinarily next be examined under an 
oil immersion objective. A few leucocytes would be studied 
to determine their staining reaction and the condition of the 
nuclear fragments. The eyepiece should be provided with 
a micrometer scale and some of the leucocytes measured. The 
condition of the bacteria will be noted without effort at the 
same time. 
THE Use oF OTHER STAINS 
A number of the anilin dyes have been tried out in this test, 
as for example safranin, thionine, methylene green, dahlia, 
gentian violet and methylene blue, but none of them appear 
to possess advantages over methylene blue. 
