126 THE ENUMERATION OF BACTERIA IN MILK 



Counting Lens. The lens recommended by the Committee 

 in 1914 is more fully defined. It is known as Engraver's lens 

 No. 146, Bausch & Lomb catalogue. It is designated as S^X, 

 its magnification being 2| diameters. Persons who are near- 

 sighted should wear their ordinary glasses while using this lens. 

 Farsighted persons should use the lens without their glasses. 



Direct Methods. The direct methods of enumeration of 

 bacteria in milk are of comparatively recent development; in 

 these the milk or, centrifugalised sediment is smeared over a 

 slide, and, after suitable staining, examined under a high- 

 power objective and the bacteria counted. The direct method 

 as modified by Slack * is as follows. Two c.cms. of the sample, 

 after thorough shaking, are inserted into special tubes with 

 rubber stoppers at each end, and centrifugalised for ten minutes 

 at 2500 revolutions per minute in a special apparatus. This 

 apparatus is a modification of the one used by Stewart of Phil- 

 adelphia for leucocyte estimation, and consists of an aluminiimi 

 disc and cover 10 inches in diameter and f inch in depth, fitted 

 to hold twenty tubes arranged radially. This apparatus is 

 manufactured by the International Instrument Co., of Cam- 

 bridge, Mass., and can be used with the usual electrical cen- 

 trifuge. After centrifugalising, the tubes are carefully removed, 

 and, to obtain the sediment with the least disturbance, the tube 

 is held with the cream end downwards, whilst the cream layer 

 is removed by means of a platinum loop. The milk is then 

 carefully poured out without permitting air bubbles to ascend 

 the tube, and finally, with the tube in the same position, the 

 other stopper is removed and the sediment is smeared on a glass 

 slide with the aid of a drop of sterile water. An area of 459 

 cms. is a convenient one and squares of this size may be marked 

 off on a strip of glass with a blue grease pencil. The smear is 

 dried, fixed by heat, and stained with methylene blue. The 

 specimen is then examined under a -^ inch oil immersion lens 

 and the organisms counted. Each coccus, bacillus, diplococcus, 

 or chain represents a colony on the 1-10,000 plate of the same 

 sample when grown on agar for twenty-four hours at 37° C. 



