DIRECT METHODS 129 



method only in the method of smear examination: these are 

 made on a cover glass and examined whilst wet. 



In some of the comparative experimental work reported by 

 Conn and discussed on page 123, a series of bacterial counts 

 was made by Breed and this was supplemented in a further 

 series by the inclusion of Brew, a co-worker with Breed. These 

 experimenters made microscopical counts on the samples plated 

 by other observers, and Conn ^^ considered that when the 

 groups of organisms only were counted, the count agreed some- 

 what closely with the plate count. When raw market milk 

 was examined, the variations found were generally not greater 

 than the differences between the plate counts in various labora- 

 tories, but for high-grade raw milk and pasteurised products it 

 is comparatively useless. The details of Breed's process are 

 as follows: 0.01 c.cm. of milk, from a well-shaken sample, is 

 measured out by means of an accurately calibrated special 

 pipette and deposited on a glass slide on which an area of 1 

 square centimetre has been previously marked out. The drop is 

 evenly smeared over this area with a stiff needle and gently 

 dried at about 50° C. The slide is then placed in a Coplin 

 staining jar containing xylol or gasoline to remove the fat, and, 

 after drjdng, fixed in alcohol (70 to 95 per cent). Immediately 

 afterwards the smear is stained with 1 per cent aqueous methy- 

 lene blue and, finally decolourised to a light blue in 95 per cent 

 alcohol. The microscopical examination is made with a ts 

 inch oil immersion objective. In order to find the factor for 

 converting the number of organisms per field into organisms 

 per cubic centimetre the diameter of the field is determined with 

 a stage micrometer. The factor is then calculated from the 

 formula: 



where y is the factor sought, x, the area of the smear in square 

 millimetres, and R the radius of the field. 



In practice it is convenient to pull out the draw tube until 



