SUMMARY: Total counts were made of bacterial spore suspensions using counting slides with chamber depths of 0·02 mm. and of 0·10 mm. Sources of error were analysed. In addition to an inherent sampling error, significant variation between counts on replicate slides may result from: (i) variation in the depth of different counting chambers, (ii) variation in the fit of cover-glasses with counting chambers. The use of 0·10 mm. rather than 0·02 mm. depth slides results in a lower estimate of the total count of a spore suspension. The former slides give more reproducible counts; evidence for their greater accuracy has been obtained by a comparison of ‘percentage viability’ of spores determined by counting and by slide-culture techniques.


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