628 LABORATORY EXERCISES 



LESSON 25 



Plate Counter. — The most accurate method of counting the colonies on each of 

 the plates is by means of the counting disk. These disks consist of a piece of paper, 

 upon which is printed a dead black disk, subdivided by concentric circles and radii 

 painted in white. In Jeffer's counter each subdivision has an area of i sq. cm.: in 

 Pake's counter, radii divide the circle into sixteen equal sectors, and counting is 

 facilitated by equidistant concentric circles. (For disks see Eyre, p. 322.) 



(a) In the final counting of each plate, place the Petri dish over the counting 

 disk, and center it, if possible, making its periphery coincide with one or other of the 

 concentric circles. 



{h) By means of a hand lens count the colonies appearing in each sector in turn. 

 Make a note of the number present in each. 



(c) If the colonies present are fewer than 500 the entire plate should be counted. 

 If, however, they exceed this number, enumerate one-half, or one-quarter of the 

 plate, or count a sector here and there, and from these figures estimate the number 

 of colonies present on the entire plate. 



Jeffers' counting plate^ (Fig. 224) consists of concentric zones which are divided 

 into small sections, each having an area of i sq. cm. To determine the position of 

 the circles marked 10, 20, the position of the circles marked 10, 20, 40, 60, 100 and 

 140 in the diagram, whose areas equal 10, 20, 40, 60, 100 and 140 sq. cm. respectively, 

 the formula, wr"^ = area, was used. In order to show the application of the formula, 

 the radius of the circle whose area is equal to 10 sq. cm., will be found from the formula 

 as follows: 



TT = 3.I416. 



irr- = 10 or r^ = 10 -i- tt. 

 10 - ^ 3-1416 = 3.18309 or r-. 

 \/3-i8309 = 1.78-f or r. 



1.78 + cm. = the radius of a circle whose area is 10 sq. cm. Dividing the circle 

 into ten equal sectors, each sector has an area equal to i sq. cm. By the same 

 method we find the radius of a circle whose area equals 20 sq. cm. thus making each 

 of the ten spaces between circles 10 and 20 and bounded laterally by the ten radii 

 equal to i sq. cm. We next construct a circle whose area equals 40 sq. cm. and divide 

 each sector as far as circle 20, making twenty equal areas between circles 20 and 40, 

 each equal to i sq. cm. In like manner we construct circles 60, 100 and 140 divid- 

 ing the sectors in the zone lying between circles 60 and 140 to produce areas equal 

 to I sq. cm. each. If a plate whose area is greater than 140 sq. cm. is used, a circle 

 whose area is 180 sq. cm. can be drawn and the radiating lines extended out to the 

 circle (Fig. 224). 



The Petri dish can be centered upon this apparatus by the circles and the area 

 read from the line its edges approach. To facilitate the reading of the area of the 

 plate the circles 80 and 120, whose areas are equal to 80 and 120 sq. cm., respectively, 



1 Jeffers, H. W. : An Apparatus to Facilitate the Counting of Colonies of 

 Bacteria on Circular Plates. Journ. Applied Micros., I: 53-54, March, 1898. 



