A device for quickly and accurately measuring the population density of a suspension of microorganisms, permitting the preparation of yeast suspensions of known density to within 1 per cent error, was constructed with two Visitron photoelectric cells, a single light of high intensity and a good Wheatstone bridge for balancing the currents from the two photoelectric cells. A large Pyrex milk culture tube holding the suspension is placed in the path of one beam of light coming through a small longitudinal slit and thence to one photocell; a second similar slit directs another beam of light upon the second photocell, thus causing dissimilar currents to flow, the ratio of whose magnitudes may be measured by the bridge resistances. A relation between these currents and the relative light intensities is shown, and the one significant unmeasurable variable (the characteristic constant of a photocell) is practically eliminated by the use of a method of ratios.

After careful standardization of technique the apparatus proved more accurate than other methods available for the purpose indicated. In rapid use its accuracy may be put safely at 1 per cent for measuring the densities of cultures of approximately the same age and composed of cells having comparable optical characteristics.

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