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THE MEASUREMENT OF BACTERIA. 



By R. Greig Smith, M.Sc, Macleay Bacteriologist to the 

 Society. 



Bacteria are generally measured by means of the micrometer 

 eyepiece, which contains a scale graduated into divisions. The 

 values of these divisions are actually determined for the various 

 objectives used by the microscopist by focussing the scale of a 

 stage-micrometer and noting the number of ocular divisions 

 included in a certain number of —y millimetre divisions of the 

 stage-micrometer scale. The value of a single division is then 

 calculated, and is thus a known constant for the objective with a 

 certain tube length. 



In measuring bacteria it is usual to employ the micrometer 

 eyepiece and the y 1 ^- oil-immersion objective, with a tube length 

 advised by the maker of the objective. On no account should 

 the value of a micrometer division be assumed or accepted without 

 personal confirmation. For instance, the values of the divisions 

 with a Leitz ^ oil-immersion, micrometer eyepiece ii. and tube 

 length 170 mm., by actual determination was found to be equal 

 to 1 - 5 fj.; according to Leitz's price list, it is 1 -8 ^. 



With a micrometer eyepiece the unit of measurement of which 

 equals say T5 /x, the measurement of bacteria is uncertain unless 

 the boundaries of the organisms coincide with the divisional lines. 

 Fractions of the unit (1-5 /n) necessitate an estimation, and it is 

 here that the uncertainty occurs, for the eye cannot divide a 

 small space into 10 or 15 equal parts. Errors of measurement 

 frequently happen. As far as the length is concerned, this is of 

 little consequence, because on a film the bacteria are found in 



