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 uinder 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 aniliu 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. 



