Micrometry

A note on micrometry, the technique of using microscopes to measure objects.

1. SYED MUHAMMAD KHAN ( BS HONS. ZOOLOGY)

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MICROMETRY
Micrometry is the process of measuring microscopic objects via an instrument called
a micrometer (which is placed inside a microscope). One of the important steps in
the identification of organisms is to measure their size. Although it is very useful but
it cannot always be done easily, especially in the case of living specimens. For
instance, a microscopist cannot follow and measure the fast-moving microbes, such
as flagellates, ciliates, rotifers, etc. Furthermore, unskilled investigators cannot
identify and measure a microorganism at the same time.
Types of Micrometers
Micrometers are of two types – ocular and stage micrometers, their details are as
follows:
1. Stage Micrometer
It is a microscope slide with a finely divided scale marked on the surface. The scale
is of a known true length and is used for calibration of optical systems with eyepiece
divisions. This is particularly important when alternating between objectives (i.e.
changing magnifications) on one microscope or when using the same eyepiece on
different microscopes. The scale measures only 1 mm and has a least count of 0.01
mm, i.e. 1 mm region is divided into 100 divisions. As 1 mm has 1000 µm, one
division of stage micrometer is equivalent to 10 µm.

Figure: Stage micrometer.

2. SYED MUHAMMAD KHAN ( BS HONS. ZOOLOGY)

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2. Ocular Micrometer
It is a glass disc on which a series of uniformly spaced lines have been inscribed.
The distance between the etched lines depends upon the objective lens
(magnification) being used. It must be calibrated via a stage micrometer to know the
distance between its divisions on any given magnification. It is placed in the
eyepiece of the microscope, the upper eye lens is unscrewed and the ocular meter is
put into the tube of the eyepiece, and the eye lens is again replaced in its original
position. There are usually 50 or 100 divisions in the ocular meter which are
engraved on the glass.

Figure: A view of the stage micrometer through the eyepiece (ocular micrometer).
Conversion Factor
The conversion factor corresponds to the value with which the reading of the ocular
divisions (of the reticule) must be multiplied to get the final answer. It can be
calculated as follows:
 To work out the measurements per ocular divisions the stage micrometer is kept
under low power of microscope and is observed through the eyepiece having an
ocular meter. Suppose that a 10X objective and a 5X eyepiece (500X
magnification) have been fitted in a microscope.
 At this magnification the number of ocular divisions coinciding the stage
micrometer is observed and thence calculated for microns per ocular divisions,

3. SYED MUHAMMAD KHAN ( BS HONS. ZOOLOGY)

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e.g., let us take that 10 ocular divisions coincide with one division of the stage
micrometer.

Figure: Align the ocular and stage micrometer calibrations like this.
 Therefore, 10 ocular divisions = 1 stage micrometer divisions, or 10 ocular
divisions = 0.01 mm (since 1 stage division = 0.01 mm). The value of one ocular
division can be calculated by following formula:

 In the case mentioned above it will be: (1/10) x 10 = 1 µm.
 In this way, the microscope is calibrated for different combinations of eyepieces
and objective lenses and is kept for record. It is to note that this calibration will be
just only of the tried lenses on this particular microscope.
 The conversion to other magnifications is accomplished by factoring in the
difference in magnification, i.e. the product of the value of ocular division and the
magnification is constant.
 In the example, since the value of one ocular division was 1 µm at a
magnification of 500X, the calibration would be 2 µm at 250X, 2.5 µm at 200X,
and so on.
Making the Measurements
When the microscope is calibrated, then the object or organism to be measured is
kept on the sage of the microscope and is observed through the eyepiece with

4. SYED MUHAMMAD KHAN ( BS HONS. ZOOLOGY)

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






Figures: Various cells being measured by an ocular micrometer, under a
microscope. The number of divisions must be multiplied with the conversion factor to
get the final answer.