Ocular Micrometer and Stage Micrometer

Ocular micrometer and stage micrometer is a straightforward and accurate technique for measuring elements observed using a microscope. The ocular micrometer scale is calibrated by comparing it to a calibrated stage micrometer.

What is ocular micrometer?

One of the most important physical characteristics used in the identification and characterization of an organism is size. Only a ocular micrometer calibration done can determine the exact size of a microorganism. An ocular micrometer (Fig. 1-1) is a glass disc with a series of evenly spaced lines inscribed on it. 

The ocular micrometer is placed in one of the microscope’s eyepieces, but the distance between the etched lines is determined by the objective lens used to view the specimen. Ocular micrometer calibration must be done with a stage meter in order to determine the precise distance between its lines (Fig. 1-2). The inscribed lines on a stage micrometer are separated by exactly 0.01 mm (or 10 m).

These are superimposed in order to ocular micrometer calibration for a specific objective lens, and the number of ocular graduations per stage micrometer graduation is determined. Figure 1-3 shows that six ocular micrometer graduations fit between two stage micrometer graduations; thus, one ocular micrometer space equals 10 m/6 or 1.66m. 

The number of graduations occupied by an organism (Fig. 1-4) is counted and multiplied by the distance between graduations to determine its dimensions. For example, if an organism took up seven graduations of space, its dimension would be 7 x 1.66 or 11.6m.

ocular micrometer and stage micrometer

Ocular micrometer diagram and stage micrometer diagram

Ocular micrometer requirements

  1. Light microscope
  2. Ocular and stage 
  3. Cell preparation

Ocular micrometer procedure

Ocular Micrometer

1. Apparatus is placed on the circular shelves inside the eye piece. In such a way that the graduations sketched on the ocular is visible when an observation is made using the microscope.

2. Place the stage meter on the stage of a microscope and focus the graduation using low power objectives the graduation on stage micrometer are space 0.01mm (10mm ) apart.

3. Superimposed the two scales and accord the number of ocular division coinciding exactly with the number of divisions of stage micrometer.

4. Ocular micrometer calibration factors are the least count of the micrometer is calculated as follows.  If 13 ocular divisions coincides with two division (2x10mm = 20mm) of stage micrometer then one ocular division = 20mm /13 divisions =1.54mm.

5. Now remove the slide having cell preparation and a low power magnification. Position the cell using absorbed in such a way that the ocular micrometer is able to measure the diameter of cell or the length/diameter of the cell component in arbitrary unit.

Ocular micrometer calculation

If the diameter is occupying 5 division of ocular the diameter of the cell will be 5 division x 1.54mm = 4.5mm. Similarly for high power objective ocular micrometer calibration has to be done again following the same procedure and then cell diameter can be measured focusing the cell in high magnification.

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