128 MICROSCOPIC TKSTS OF MILK. 



smears are dried in air without fixation by heat, the preparation is stained 

 with the Jenner blood stain for two minutes, keeping the stain in con- 

 stant motion. The excess of stain is washed off in water, and the prep- 

 aration is dried in air. By this blood-staining method, the pus and blood 

 cells are stained perfectly, and the ordinary microorganisms take the blue 

 stain well. 



"The stained specimens are examined with a one-twelfth Leitx. objec- 

 tive and a No. 3 eye-piece. The character of the bacteria is noted, and 

 the average number of pus cells per field is counted. This average num- 

 ber is multiplied by 4,400, since there are about 4,400 fields to a square 

 centimeter, as estimated by the stage micrometer. This result is approxi- 

 mately the number of pus cells per cubic centimeter of milk." 



When a sample of milk showed over 100,000 cells per cc., 

 or showed "streptococci and the ordinary pus producing organ- 

 isms," an examination of the herd was ordered. Stewart 

 gave figures showing that inspections of condemned herds by 

 veterinarians, revealed diseased animals in those herds. 



The quantitative features of the test have not been found 

 accurate. The average percentage variations between dupli- 

 cate determinations have been found to be 112% in a series of 

 -23 determinations made by Russell and Hoffmann (23, 24). 

 The present writer and associates (32) also noted its very unsat- 

 isfactory behavior. The numerical results are always very 

 much below the true number present and bear no constant 

 ratio to the true number. 



The test very rarely indicates the presence of streptococci, 

 even though streptococci are proven by other methods to be 

 very common in the milk. The presence of insoluble detritis, 

 blood, etc., is indicated by the test. 



Slack's test. Slack (26) modified the Stewart method by 

 using tubes of a larger bore containing 2 cc., and smeared the 

 sediment over 4 square centimeters. He further lowered the 

 standard to 50 cells or over per one-twelfth oil immersion field, 

 as warranting the condemnation of milk. When streptococci 

 are found in the sediment, he condemns for them alone if these 

 three conditions are fulfilled : 



1. Microscopic examination of sediment shows streptococci, 

 diplococci, or cocci. 



