436 



BACTERIOLOGY. 



dish, the diameter of which must not exceed 9.5 cm., is placed in this 

 collar and is then wedged, so as to be immovable. A somewhat simi- 

 lar counter (Park or Jeffer) ruled on paper can be obtained at very 

 little expense, and is to be preferred to the glass plate of Laf ar. 



a b 



FIG. 58. a Lafar's counter; b Jeffers modification. 



It is sometimes desirable to have an approximate idea of the 

 number of colonies on a Petri dish, when their number is so great as 

 to render the preceding- method impracticable. In such cases Buch- 

 ner, Neisser and others determine the average number of colonies 

 present in the field of a microscope. A low power objective (No. 3 

 with No. 1 ocular) should be used for this purpose. The diameter of 

 the microscopic field must be known. This can be ascertained by 

 means of a stage micrometer (p. 127) or in a manner similar to that 

 employed in the measurement of objects (p. 128). The field is pro- 

 jected on the table and the diameter measured. This divided by the 

 magnifying power employed will give the diameter of the actual field 

 under the microscope. Since circles are to each other as the squares 

 of their radii, the area of the large circle or Petri dish can be easily 

 obtained. Thus, if the radius of the microscopic field is .65 mm.; that 

 of the Petri dish 46 mm.; and the average number of colonies in the 

 microscopic field is 76; then, 



r- : R 2 :. 76 : x 

 .4225 : 2116 :: 76 : x x = 380,000 colonies. 



