Fundamentals of Microbiology (2+2)

Micrometer and Principle of Micrometry

            Microscopic objects can be measured with the help of compound microscope. The measurements are expressed in micrometers and are made using micrometer. It involves an ocular (eye-piece) micrometer and a stage micrometer. The ocular micrometer is a transparent circular glass disc etched with 100 equal divisions which is placed in the body of the eye-piece (between field lens and eye-lens). The stage micrometer is a glass slide on which a scale of known intervals is marked. The scale has 100 graduations equal to 1 mm.

            The first step in micrometry is the calibration of ocular micrometer with stage micrometer. Calibration is done for each objective of the microscope separately. When the scale of the stage micrometer is focused, the ocular micrometer is also seen in the superimposed condition. While focusing the stage the ocular (eye-piece) is rotated and positioned in such a way that the zero line of stage micrometer coincides with the zero line of the ocular micrometer. The number of divisions of stage micrometer that corresponds to a full scale on the ocular micrometer is first recorded. Since the measurement of each division of stage micrometer is known, the ocular micrometer divisions are appropriately converted to micrometers. Once the ocular micrometer is calibrated, dimensions of objects can be measured.