FILMS OF DRIED MILK 



15 



of ground or etched glass on the longer 

 edges of the slide about yi inch in 

 width allows lead pencil labeling. The 

 margins may be ground with an emery 

 wheel, or they may be etched with hy- 

 droflouric acid. The cost of these home 

 made slides ought to not to exceed 2 to 

 3 cents each, whereas the similar slides 

 listed by supply houses cost much more 

 than this. A special guide plate (size 

 2 by 4^ inches) marked off with 

 16 square centimeter areas is also need- 

 ed. This can be obtained from regular 

 supply houses.* Only one of these is 

 needed as it used is as a guide plate un- 

 derneath the slides on which the milk 

 preparations are made. 



Preparation of films of dried milk. 



After a thorough shaking of the 

 sample, 0.01 cc. of milk or cream shall 

 be deposited upon a clean glass slide 

 by means of the pipette above de- 

 scribed. Spread the drop of milk 

 uniformly over an area of one square 

 centimeter by means of a clean, stiff 

 needle. This may be most con- 

 veniently done by placing the slide 

 upon the guide plate just described, or 

 upon any other form of guide plate of 

 glass or paper which is ruled in square 

 centimeter areas. The marks showing 

 through the glass serve as guides. 

 After spreading, the preparation shall 

 be dried in a warm place upon a level 

 surface protected from dust. In order 

 to prevent noticeable growth, this 

 drying must be accomplished within 

 five to ten minutes; but excessive heat 

 must be avoided or the dry films 

 may crack and peel from the slide in 

 later handling. 



After drying, the slides are to be 

 dipped in xylol, or any other suitable 

 fat solvent, for a sufficient time to 

 remove the fat (at least one minute), 



*Listed by the Will Corporation, Rochester, N. Y. 



then drained and again dried. After 

 this, the slides are to be immersed in 

 90 per cent grain or denatured alcohol 

 for one or more minutes, and then 

 transferred to a fresh aqueous or 

 carbolic acid solution of methylene 

 blue (about 1 per cent, exact strength 

 unimportant) that has previously been 

 tested and found to stain the bacteria 

 satisfactorily in milk preparations. 

 Some methylene blue now on the 

 market in powder form is very un- 

 satisfactory in that solutions will dis- 

 solve the milk films, or will wash them 

 with an even blue color in which the 

 bacteria fail to show distinctly. Old 

 or unfiltered stains are to be avoided 

 as they may contain troublesome 

 precipitates. 



The slides are to be left in the stain 

 until overstained. They are then to 

 be rinsed in water and decolorized in 

 alcohol. The decolorization takes from 

 several seconds to a minute or more, 

 during which time the slide should be 

 under observation, in order that the 

 decolorization may not proceed too 

 , far. When properly decolorized the 

 background of the film should show a 

 faint blue tint. Poorly stained slides 

 may be decolorized and restained with- 

 out apparent injury. After drying, the 

 slides may be examined at once, or 

 they may be preserved indefinitely. 



Standardization of the microscope. 



The microscope used must be so 

 adjusted that each field covers a cer- 

 tain known fraction of the area of a 

 square centimeter. This adjustment 

 is simple if a micrometer slide, ruled in 

 hundredths of a millimeter, is at hand 

 (sometimes called a stage micrometer 

 as it is used under the objective on the 

 stage of the microscope). The mi- 

 croscope should have a 1.9 mm. (1/12 

 inch) oil immersion lens, and an ocular 



