. Laboratory work in bacteriology. Bacteriology. 436 BACTERIOLOGY. dish, the diameter of which must not exceed 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 Fig. 58. a—Lafar's counter; *—Jeffer's modification. It is sometimes desirable to have an approximate idea of the number of colonies on a Petri dish, when their number is so g'reat as to render the preceding method impracticable- In such cases Buch- ner, Nei
. Laboratory work in bacteriology. Bacteriology. 436 BACTERIOLOGY. dish, the diameter of which must not exceed 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 Fig. 58. a—Lafar's counter; *—Jeffer's modification. It is sometimes desirable to have an approximate idea of the number of colonies on a Petri dish, when their number is so g'reat 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, t' : R' :: 76 : X .4225 : 2116 :: 76 : x x = 380,000 Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Novy, Frederick George, 1864- . Ann Arbor [Mich. ] G. Wahr
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