Hosler, Burkhart and Neuberger [1] found that the number of nuclei deduced from electron microscope examinations of a collodion film exposed on the stage of an Aitken counter gave concentrations up to five times as large as those observed by the normal visual method. They attributed the discrepancy to sedimentation of nuclei during the interval between sampling and expansion and to evaporation of drops before they reached the stage. These effects have been examined by comparing the visual counts with the number of craters on magnesium oxide coated surfaces exposed on the stage of the counter. It appears that sedimentation leads to losses of only 10 to 15 percent if the delay time is 2 seconds, while the loss by evaporation would amount to only a few percent of the nucleus population if the standard instructions for the use of the counter are followed. The high counts observed by Hosier et al with the electron microscope are attributed to nuclei falling on the stage before and during the cleaning process which precedes admission of the sample.

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