Lecture notes on Micrometry

Micrometry involves measurement of minute or microscopic objects using microscope.The micrometer consists of two components namely a stage micrometer and an ocular micrometer 

Stage Micrometer

This is a glass slide that can be mounted on the stage of the microscope. It

is similar to a microscope slide and measures 7.5 cm x 2.5 cm. It has a linear scale of length 1 mm engraved on it. This 1 mm distance is precisely divided into 100 equal divisions. Therefore each small division is equal to 0.01 mm (10 um). Every 5th and 10th divisions are demarkated by longer lines.

Ocular Micrometer

Ocular micrometer is a glass disc with a diameter of 21 mm. While in use, it

is placed on top of the field diaphragm of the eye piece. At the centre of this disc

is engraved an arbitrary linear scale with 100 equal divisions. Every 10th division

is demarkated by longer lines.

Calibration

The magnification of a microscope is calibrated using the stage micrometer

and ocular micrometer. This is done by determining the value of each arbitrary

division of the ocular micrometer in terms of millimeters or microns.For calibration, stage micrometer is mounted on the stage of the microscope and focussed until the graduations become visible. Then, the ocular micrometer is introduced on top of the field diaphragm of the eye piece. Once

the stage and ocular micrometer scales become visible, both are aligned parallel

to each other. The stage micrometer is then moved and positioned so that its

initial division (or any other division near to the initial division) coincides with

the initial division (or any other division near to the initial division) of the ocular

micrometer. The number of divisions on both the scales is counted upto a farthest

point where the two scale divisions again coincide. The value of one ocular

micrometer is calculated as follows:

Value of one ocular micrometer division=

Number of divisions on the stage micrometer /Number of divisions on the ocular micrometer X 100

The calibration is done both under the low power and high power of the microscope separately. When calibration is over, the stage micrometer is replaced with the slide containing the object to be measured. Upon focussing, the object

can be seen superimposed on the ocular micrometer scale. The dimensions of

the object are recorded in terms of the ocular micrometer divisions. Since the

value of each division on the ocular micrometer is known, the actual dimensions of the object can be derived easily.