. The principles of bacteriology: a practical manual for students and physicians. nor twelve, take the mean of the results and multiplythis by the number of areas containing colonies. Thelatter procedure obtains, of course, only when all theareas are of the same size. By this method, however,the results vary so much in different counts of the sameplate that they cannot be considered as more than roughapproximations. Note.—Prepare a plate; calculate the number ofcolonies upon it by this latter method. Now repeat 600 BACTERIOLOGY. the calculation, making the average from another setof squares. N
. The principles of bacteriology: a practical manual for students and physicians. nor twelve, take the mean of the results and multiplythis by the number of areas containing colonies. Thelatter procedure obtains, of course, only when all theareas are of the same size. By this method, however,the results vary so much in different counts of the sameplate that they cannot be considered as more than roughapproximations. Note.—Prepare a plate; calculate the number ofcolonies upon it by this latter method. Now repeat 600 BACTERIOLOGY. the calculation, making the average from another setof squares. Now actually count the entire number ofcolonies on the plate. Compare the results. For facilitating the counting of colonies several veryconvenient devices exist. Wolffhugels Counting-apparatus.—This appa-ratus (Fig. 104) consists of a flat wooden stand, thecentre of which is cut out in such a way that either ablack or white glass plate may be placed in it. Theseform a background upon which the colonies may moreeasily be seen when the plate to be counted is placed Fig. Wolffhugels apparatus for counting colonies. upon it. When the gelatin plate containing the colonieshas been placed upon this background of glass it iscovered by a transparent glass plate which swings on ahinge. This plate, which is ruled in square centimetresand subdivisions, when in position is just above thecolonies, without touching them. The gelatin plate ismoved about until it rests under the centre of the areaoccupied by the ruled lines. The number of colonies BACTERIOLOGICAL STUDY OF WATER. 601 in each square centimetre is then counted, and the sum-total of the colonies in all these areas gives the numberof colonies on the plate; or, as has already been indi-cated, if the number of colonies be very great, a meanmay be taken of the number in several (six or eight)squares; this is to be multiplied by the total numberof squares occupied by the gelatin. The result is anapproximation of the total numb
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