Abstract
If a wire loop is thrust down into a suspension of bacteria in a test tube, and viewed by looking down into the mouth of the tube, the depth at which the loop disappears will be determined by the opacity of the supervening suspension. If, however, a second suspension of the same organism containing half as many bacteria per cubic centimeter is similarly examined, or if an equal amount of the diluent is added to the original suspension, and the “depth of disappearance” again measured it will be found to be less than twice as great as in the original suspension. In other words, the observed depths of disappearance are not in proportion to the bacterial concentrations or the corresponding volumes.
This discrepancy is due to the presence in each reading of a constant which is apparently related to the size and opacity of the individual organisms. It is found that this constant may be eliminated, thus bringing the opacity observations into inverse ratio with the corresponding bacterial concentrations, and a corrected reading (the observed reading minus the constant) for any suspension may be obtained by making two readings at different dilutions of the suspension, and substituting the observed values in the following equation:
in which
A = the corrected reading for the first volume of the suspension = (a - constant)
a = first observed reading.
vol a = first volume of the suspension on which reading a is made.
b = second observed reading.
vol b = second volume of the suspension (diluted), on which reading b is made.
A concrete example will illustrate the method. In 4 C.C. (vol a) of a given suspension the loop disappears 1.2 cm. below the meniscus (reading a).
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