How to Calculate an Ocular Micrometer

Many microscopes incorporate a scale called an ocular micrometer into one of their eyepieces, which allows for measurements of objects being viewed. Scientists and technicians use object dimensions in many applications, including the identification of parasite eggs and the characterization of blood cells. Technicians can easily make calculations of object size after measuring an object against the dimensions of an ocular micrometer, which are calibrated using a stage micrometer, a microscope slide with its own surface scale visible when viewed through the microscope.

Things You'll Need

• Microscope
• Ocular micrometer
• Microscope slides
• Stage micrometer slide

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Instructions

• 1

  Look through your microscope's eyepieces and determine whether there is an ocular micrometer in place. Ocular micrometers appear as a scale of parallel black lines similar to lines on a ruler, often with numbers indicating sequential measures of ten lines.

• 2

  Calibrate the ocular micrometer if this has not been done previously. Place a stage micrometer slide on the stage and view it through the eyepieces, making sure that both eyepieces are focused. By rotating the eyepiece containing the ocular micrometer and moving the stage micrometer slide, align the two micrometers.

  The stage micrometer has divisions of known dimensions; use these dimensions to determine the ocular micrometer dimensions for the objective, or microscope lens, directly over the stage. For example, if each stage micrometer division using a certain objective can be aligned with ten ocular micrometer divisions, then each ocular micrometer division is one-tenth of the known stage micrometer division length. In the example, if each stage micrometer division measures 100 microns, then each ocular micrometer division using the objective measures 10 microns.

  After calibrating the ocular micrometer for one objective, repeat the procedure for the other microscope objectives for greatest accuracy. Alternatively, the calibration of the other objectives can be calculated from the measured objective calibration; however, this method can lead to error because of variations in exact magnification. For example, a division measuring 10 microns under the 10 times magnification objective would be calculated to measure one micron under the 100 times magnification objective.

• 3

  Place a slide on the microscope stage. Align the ocular micrometer with the surface of the object on the slide to be measured by rotating the eyepiece containing the micrometer and moving the microscope slide with the object until the micrometer is aligned with the surface. Count the number of micrometer divisions aligned along the surface.

• 4

  Calculate the surface length by multiplying the number of measured micrometer divisions by the conversion factor determined through ocular micrometer calibration in step one. For example, if each division is one micrometer and the surface measured aligns with 10 divisions, then the surface measurement is 10 micrometers.