As already mentioned, the size of
structures is important in biological sciences.

I've reworked the diagram to give the
idea of the circular field of view when observing a specimen under the
microscope.

Diagram of the microscope field of view of onion
cells using a relatively high magnification.

The more cells you can clearly see and count in a row
across the diameter of the field of view, the more accurate is your
estimate cell size.

A better image to estimate the size of an individual cell.

Suppose the field of view of the above cells was
~0.20 mm, what would the average width of the cell be?

In order to make accurate
measurements of cell size you need to be able to **calibrate the
microscope**.

**Both the eyepiece and the
field of view of the microscope stage need an accurate scale **
that can be focussed as well as the image of the specimen being
examined under the microscope.

(i) **The graticule**

A graticule is a thin piece
of glass/plastic onto which an accurate scale has been draw.

The graticule is positioned
into the eyepiece of the microscope.

(ii) **The stage micrometer**

A stage micrometer is a
microscope slide on which an accurate scale has been etched.

The stage micrometer is
placed onto the microscope stage.

**The microscope procedure using
the graticule and micrometer**

You place a stage micrometer
on the stage of the microscope.

You line up one of the scale
divisions of the eyepiece graticule with specific point on the
stage micrometer.

You count the number of
divisions on the eyepiece graticule that correspond with a
specific measurement on stage micrometer.

You calculate the distance in
micrometers of one division on the eyepiece graticule.

**
**

**Comparing the eyepiece graticule and stage micrometer scales**

Diagram to show the positioning of the eyepiece graticule and stage
micrometer scales in a microscope

**You use the stage
micrometer scale to calibrate the eyepiece graticule scale.**

On the above diagram I've
drawn two thin vertical lines to match up the scales of the
eyepiece graticule and stage micrometer.

The stage micrometer is
marked in 50
µm divisions.

The eyepiece graticule is marked as 100 arbitrary units (a.u.).

From the two vertical lines we can now calibrate the arbitrary
graticule scale.

As you can see from the diagram: 64 - 35 = 29 a.u.
≡
50
µm

Therefore **each a.u. on the graticule scale** = 50/29 **
= 1.72 **
** µm**

In this case the field of view is about 200
µm (0.20 mm)

Once the eyepiece graticule
is calibrated, the stage micrometer can be removed from the
stage and replaced with a specimen microscope slide for
examination.

**
**

**How to use the eyepiece graticule scale to measure cell size.**

Diagram showing the eyepiece
graticule superimposed on the microscope specimen image.

From above we have a calibration
of
each a.u. on the graticule scale = 1.72 µm

Looking at the last diagram, the middle seven cells are measure from
7 to 90 a.u. on the graticule scale.

Therefore the average cell width is (90 - 7) / 7 = 11.87 a.u.

Using the conversion factor from above: average cell width =
11.86 x 1.72 = 6.96 = __
__**20****
****
****µm**
(2 sf)

You can also pick out an individual cell and measure its size using
the calibrated eyepiece graticule scale.

e.g. the 4th cell from the left: width = 43 - 30 = 13 a.u. on
the graticule scale.

Therefore using the calibration factor: width of cell = 13 x
1.72 = **22
**
**µm**
(2 sf)

Some other calculations based on the same cell diagram from above

(i) The
diameter of the nucleus

**
**The
nucleus is about 3 a.u. wide on the eyepiece graticule.

This equates to 3 x 1.72
= **5****.1
**
**µm**
(2 sf)

**OR** you can make a **less accurate** visual
estimate from the image.

On average the diameter of the nucleus is about a 1/4 of the
diameter of the cell.

It looks as if the cell diameter equals about 4
diameters of the cell.

The average width of the cells was measured to be 20
µm

Therefore the average
diameter of the nucleus is 20/4 =
5**
**
**µm**
(1 sf)

(ii) The
**area of a
cell**

Most cells are roughly a square or rectangle in shape.

Suppose one of the cells in the diagram is 20
µm wide and 22
µm in length.

Area = length x width =
20 x 22 = **400
****
µm**^{2}