|Plant Physiology (Biology 327) - Dr. Stephen G. Saupe; College of St. Benedict/ St. John's University; Biology Department; Collegeville, MN 56321; (320) 363 - 2782; (320) 363 - 3202, fax; email@example.com|
Calibrating an Ocular Micrometer
precise and simple method for measuring objects in the microscope is to use an
ocular micrometer. In essence, this
instrument is a ruler etched in the ocular of a microscope. It consists of a series of ruled marks (Fig. 1
**not inserted**). For each
objective of the microscope, the increment between each mark must be determined. Once calibrated for a given objective, the calibration need
not be repeated unless the objective is switched.
An object is measured by determining its size in ocular micrometer units. These units are then converted into actual units by multiplying by the calibration factor.
Ocular micrometers are typically calibrated with a stage micrometer. This is a standardized glass slide with an accurately ruled scale. A typical stage micrometer is depicted in Fig. 2 (***not inserted***). The method for calibration follows:
the stage micrometer on the microscope stage and focus on the marks with low
the ocular micrometer on the stage micrometer so that the left edge of the
ocular micrometer ("O") lines up with the left edge of the stage
scan the two micrometers to locate a place where a line from each micrometer
is perfectly superimposed.
the scale provided with the stage micrometer, determine the distance from
the left edge of the ocular micrometer ("O") to the superimposed
this distance by the total number of ocular micrometer units represented by
this distance to yield the calibration factor (mm omu-1
this value and keep it with the microscope.
step 1 - 6 with each of the other objectives.
determine the actual size of an object, measure its size in ocular
micrometer units. Then,
multiply by the calibration factor (mm omu-1
Rather than recording ocular micrometer units when taking data and then multiplying by the calibration factor, many microscopists prepare a table listing ocular micrometer units and length for each objective and then enter their data in actual units.
01/07/2009 � Copyright by SG