Improvements in Determining Total Counts of Bacteria

SYNOPSIS An interferometric method of measuring the thickness of counting chambers is described. It is simple enough to be used every time a chamber is assembled. Accurate measurement of chamber depth removes a large systematic error from total counts as ordinarily calculated. Experiments on various other details of counting technique lead to recommendations conducive to still further improvement. The standard error of a count under the best conditions need be no more than 50 p.c. greater than the error due to random sampling.