Measuring Techniques
D3
Part III:
Measuring the Size of a Specimen
Using a Stage Micrometer and an
Ocular Micrometer
Preface:
Before beginning this module, you should
be familiar with the parts, use, and care of
the compound microscope.
 If you are unsure about the microscope’s
operation, please review the Use of the
Microscope module.
 Mishandling of the microscope can lead to
damage of the instrument.

Module Requirements:

To complete this module, the following equipment will be
provided to you by an SLC assistant.
-microscope with ocular micrometer
-prepared microscope slide
-stage micrometer
-worksheet
If materials are missing or you are having problems getting
started, please ask one of the SLC staff for help.
Notes
The Stage and Ocular Micrometer Technique is
the most precise method used for measuring
microscopic specimens here at the Science
Learning Center.
An ocular micrometer consists of an ocular lens
with equal space gradations that, unlike the stage
micrometer, have no units.
Look into the microscope at your workstation to
make sure you see the ocular micrometer. If you
don’t see the micrometer, contact a Student
Assistant.
Notes
If you have completed module D1, you should be
familiar with the stage micrometer. The modules
here at the Science Learning Center use a 2mm
micrometer. In lab situations though, always
check the label for the calibration.
Position the stage micrometer on the microscope’s
stage and clearly focus the scale using the 10X
objective lens.
Note: If you are having trouble finding the scale at 10X, switch to a lower power,
such as 4X, to center scale. Then select the 10X objective and use the fine
adjustment to focus.
Notes
Though stage micrometers are very accurate, we
cannot place both a specimen on a slide and the
micrometer under the microscope at the same
time.
To solve this dilemma, we can make use of the
ocular scale. It’s important to remember that the
ocular micrometer divisions have no units, and
that each division will change with each
magnification. Therefore, you must calibrate the
ocular scale before you can use it.
Notes
To calibrate the ocular scale, use the stage
micrometer. Your calibration will determine how
many millimeters are represented by one division
on the ocular scale, or one ocular division.
If you find that one ocular division equals 0.3mm,
then you know two divisions equal 0.6mm and so
forth.
Therefore, if you find a specimen covers three
ocular divisions, you will know that it measures
0.9mm in length.
Calibration
With the stage micrometer in focus, rotate the
eyepiece until the ocular micrometer aligns with
the stage micrometer.
This example shows that
one ocular division (od)
equals 0.1mm.
Example
Using the ocular division from our last slide, we can
measure the size of this animal. Based on what you have
learned, can you figure out its length?
Example
This animal occupies 3.4 ocular divisions. We
found that 1 od is equal to 0.1 mm. So the size of
this animal is 0.34mm.
The simple equation can be written as:
(# of ocular divisions) x (# of mm per division)
= diameter of cell in mm
Notes
The last slide gave a quick example. Do
not take for granted that all ocular
micrometers are calibrated the same.
Always calibrate your instrument before
making your measurements.
Example
Suppose your microscope had an ocular lens
where one division equaled 0.02mm. You
found that your specimen covers 3 ocular
divisions. What is the length of your
specimen?
(3) x (0.02mm) = 0.06mm
Try it!
Follow the procedure with the practice
slides provided. Remember it’s good
practice to calibrate your microscope for
each specimen.
Module Completed!

Please return to the main desk to obtain a
post test from your friendly Science
Learning Center personnel.