266 GENERAL MICROBIOLOGY 



heat (e.g., level wooden surface over a steam radiator). 

 Do not allow the smears to become too hot, as this causes 

 them to check, making satisfactory staining impossible. 



6. As soon as dry, place the slides in a staining jar 

 containing xylol for a short time to remove the fat. 



7. Remove the slide from the xylol, absorb the surplus 

 xylol about the edges with filter paper and allow it to 

 dry. 



8. Fix the film to the slide by immersing in 95% alcohol. 



9. Stain immediately by flooding the smears with Loef- 

 fler's methylen blue for two or three minutes. 



10. Decolorize to a light blue in 95% alcohol. 



11. In counting, use the oil immersion objective. Place 

 the draw tube at some convenient mark so that an even 

 number of fields of the microscope covers one square centi- 

 meter. 



To do this, determine the radius of the -microscopic 

 field in millimeters with the stage micrometer and calculate 

 its area by the formula irR 2 . (7r = 3.1416.) 



Then if z = the area of the smear in square millimeters 

 and if 0.01 c.c. of milk is used, 



y the factor necessary to transform the number of bacteria 

 found in one field of the microscope into terms of bacteria 

 per cubic centimeter. 



TO simplify the calculation, place the draw tube so that 

 y consists of as many ciphers as possible. Convenient 

 factors will be obtained if the length of R be 0.101 mm. 

 or 0.08 mm. 



Let z thousand equal the number of fields of the micro- 

 scope in one square centimeter. Since 0.01 c.c. of milk was 

 taken then each bacterium seen in one field represents lOOXz 

 thousand or z hundred thousand bacteria per cubic centi- 

 meter. 



