Micrometers: Subject-Matter, Types and Exercise | Botany

In this article we will discuss about:- 1. Subject-Matter of Micrometers 2. Types of Micrometers 3. Exercise.

Subject-Matter of Micrometers:

Micrometry is the science in which we have some measurement of the dimensions of an object being ob­served under the microscope. The method employs some special types of measuring devices which are so oriented that these can well be attached to or put into the microscope and observed. The object to be mea­sured is calibrated against these scales.

Once we are observing an object under a micro­scope by the objective 10X and the eye piece 10X we say that the image that we are able to perceive is 100 times of the object.

We get the magnified view no doubt and also that it is perfect coordination of the di­mensions, but to find out the absolute size of the object will need precision and that is achieved through the application of micrometers.

Types of Micrometers:

i. Stage Micrometer:

As the name indicates it is for the measurement on the stage of the microscope where an object is to be kept. This micrometer is of slides size and has a mount of very finely graduated scale (Fig. 8A). The scale mea­sures only 1 millimetre and has a least count of 0.01 mm, i.e., 1 mm region is divided into 100 divisions. As 1 mm has 1000µ, so one division of stage micrometer will be equivalent to 10µ.

ii. Ocular Meter:

This is a micrometer which is used inside the eye piece. The upper eye lens is unscrewed and the ocular meter is put into the tube of eye piece, and the eye lens is again replaced. There are usually 50 or 100 divi­sions in the ocular meter which are engraved on the glass.

Exercise of Micrometers:

To measure the dimensions of common microorganisms by calibration and standardization of microscope with the help of stage and ocular mi­crometers.

Requirements:

Microscope, stage micrometer, ocu­lar meter, slide of microorganisms (e.g., bacteria or fungus) to be measured.

Procedure:

To work out the measurements per ocular divisions the stage micrometer is kept under low power of the microscope and is observed through the eye piece having ocular meter. Suppose, we have 10X objective and 5X eye piece in the microscope with a tube of 170 mm length.

At this magnification the number of ocular divisions coinciding the stage micrometer are observed and thence calculated for microns per ocular divisions, e.g., let us take that 6 ocular divisions coincide with 8 divisions of stage micrometer (Fig. 9).

Therefore, 6 ocular divisions = 8 stage micrometer divisions, or 6 ocular divisions = 0.08 mm (since 1 division = 0.01mm).

Thus, the microscope is caliberated for different combinations of eye pieces and objective lenses and is kept for record. It is to note hence that this caliberation will be just only of the tried lenses on a particular microscope.

In this way take three readings, and the mean value of these readings will be the actual value of one pan of ocular meter.

Record your data as in the following table:

Result:

The mean value (…………… µ) of all the three readings of the above table is the actual value or cali­brated value of one part of the ocular meter under this microscope.

Measurement of Spore or Any Other Object:

Requirements:

Microscope, calibrated ocular meter, permanent slide of spore/bacteria/or of any other object to be measured.

Procedure:

Insert the ocular meter in the eye piece of your calibrated microscope by unscrewing its upper eye lens. Now replace the eye lens again in its original position. Keep the spore or bacterial slide (or any other object) to be measured, on the stage of the microscope, and observe through the eye piece containing ocular meter.

The object is measured in the particular magni­fication by ocular divisions and then is changed into microns by multiplying ocular divisions with cali­brated value of one ocular division in that particular magnification.

Suppose, the width of the spore (or the object to be measured) is observed to be equal to 5 divisions of ocular meter (Fig. 10) so the width of this spore (or the object to be measured) in microns will be:

5 × 13.3µ (or any other calibrated value of one part of the ocular meter) = 66.5µ.

Take at least four readings in this manner, and tabulate your observations in the form of following table:

Result:

The mean value (……….. µ) is the actual mea­surement of the length (or width or diameter) of the given spore (or the object).