- To measure the dimensions of microorganisms with the help of a microscope.
Microorganisms are microscopic objects that are visible only with the help of a microscope. Sometimes it is necessary to measure its dimensions (length breadth and diameter) for its identification process. But, determination of the size of a microorganism is not an easy process. Micrometry refers to the measurement of dimensions of the desired microorganisms under a microscope which uses two micro-scales known as ‘micrometers’. At first, the diameter of the microscopic field must be established with the help these micrometers namely ocular micrometer and stage micrometer. Ocular micrometer with microscopic graduations etched on their surfaces is a circular glass disc that fits into the circular shelf inside the eyepiece of the microscope. It has 100 equally spaced divisions marked as 0 to 10. Depending on the objective being used, the distance between these graduations will vary that determines the size of the field. The other micrometer, stage micrometer is clipped to the stage of the microscope. In the centre of the stage micrometer a known 1mm distance is etched into100 equally spaced divisions making each division equals 0.01 mm or 10 µm.
With respect to the required objective, the graduations on the ocular microscope are calibrated against the standard graduations on the stage micrometer. Hence the graduations on both micrometers gets superimposed each other.
- Rotate the ocular lens.
- Determine the number of ocular divisions per known distance on the stage micrometer.
Figure 1. Calibration and use of the ocular micrometer (Adapted from Lab manual of Microbiology, Cappucino)
Calibration factor for One division on ocular micrometer (in mm) = (Known distance between two lines on stage micrometer) /(Number of divisions on ocular micrometer)
After calibration, the ocular micrometer measures the size of various microbes including its length, breadth, and diameter. First count the number of spaces occupied by the organism. Then multiply this number by the calculated calibration factor. This value indicates the length of the organism. It can also be used to find out organism’s breadth and diameter.
Helps to determine the sizes of different microorganisms including bacteria, protozoa and yeast.