268 PRINCIPLES AND PRACTICE OF MILK HYGIENE 



five to ten minutes ; but excessive heat must be avoided 

 or the dry films may crack and peel from the slides in later 

 handling. 



" After drying the slides are to be dipped in xylol 

 (gasoline may be used) for one minute, then drained 

 and the slides dried. They are then immersed in 90 per 

 cent, grain or denatured alcohol for one minute or more 

 and then transferred to a fresh aqueous solution of methy- 

 lene blue. Old or unfiltered stains are to be avoided, as 

 they may contain troublesome precipitates. The slides 

 remain in this solution from five seconds to one minute 

 or longer, depending upon the effect desired, and are then 

 rinsed in water to remove the surplus stain, and decolor- 

 ized in alcohol. The decolorization takes several seconds 

 to a minute, during which time the slides must be under 

 observation in order that the decolorization may not pro- 

 ceed too far before they are removed from the alcohol. 

 When properly decolorized the general background of 

 the film should show a faint blue tint. Poorly stained 

 slides may be decolorized and restained as many times as 

 necessary without any apparent injury. After drying, 

 the slides may be examined at once, or they may be filed 

 away and preserved for further reference." 



Standardization of the Microscope. " The micro- 

 scope to be used must be adjusted in such a way that 

 each field of the microscope covers a certain known frac- 

 tion of the total square centimetre's area. This pro- 

 cedure is simple with the proper materials at hand. The 

 microscope should have a 1.9 mm. (Vi2 inch) oil immer- 

 sion objective, and an ocular giving approximately the 

 field desired, and should preferably be fitted with a 

 mechanical stage. To standardize the microscope, place 

 upon the stage a stage micrometer, and by the selection 



