Laboratory Investigation:

The Micrometer Eyepiece



A microscope can be used not only to see very small things but also to measure them.  Things seen in microscopes are so small that centimeters or even millimeters are too big.  As a result, micrometers (or microns) are used.    A micrometer, also written Ám, is one thousandth of a millimeter - it's 10-6m. 

For this, a micrometer eyepiece is used in place of the standard eyepiece of the microscope.  This has a series of numbered lines inside of it which make it look like a ruler (see image to the right, click on it to see a bigger version). 

The images below show what the eyepiece looks like (with its protective box) and where to put it on the microscope.




Method - How to use it:

  1. After placing the special eyepiece, it is necessary to calibrate the microscope.  To do this, a calibration slide must be used.  This is a glass slide with one one-hundredth of a millimeter, 0.01mm, engraved on to its top surface (see photo to the right).  Use care when handling this little piece of glass - it costs a lot of money to replace!  Since a hundredth of a millimeter is very small and difficult to see, a circle is drawn around it.  This slide allows us to find out how big things are as we look at them through the microscope at different powers of magnification.  Put the slide on the stage as shown in the photo.  Be sure that the top of the slide (the surface with the microscopic lines engraved on it) is pointing up. 

  1. Set the microscope to low power and focus on the lines engraved on the surface of the calibration slide.   You should see the following: 

  1. The number of lines must be counted.  As shown in the figure above, the eyepiece lines have numbers on them whereas the calibration slide's lines do not.  The total number of eyepiece lines (which will be called X) are from line 21 to 59.  That's a total of 38 lines.  The number of calibration slide lines (which will be called Y) show a total of 10 lines (note that the little lines mark off half spaces and that the first line is not counted because is shows the zero mark). 

  2. Calculate how much each line of the eyepiece measures.  In other words find out what distance is shown between each line of the eyepiece.  To do so, use this equation: 

If we plug in the numbers from the example in step 3, we get 10/38 x 10Ám = 2.63 Ám.  That means that as you are looking through the microscope at low power, the space between each line on the micrometer eyepiece measures 2.63Ám. Try it on the cell to the right.  The cell goes from line 22 to line 29 on the eyepiece so it measures about 7 lines across. 

If a cell is 7 lines wide when observed on low power, that means that in reality, it measures 7 x 2.63Ám = 18.4Ám.  Since we know that cells are supposed to be between 10 and 30Ám, it makes sense that the human cheek cell in the photo above measures just over 18Ám.   

  1. After calibrating for low power observations, medium power should be calibrated.  Switch to the medium power objective lens and focus once again on the lines etched into the top of the calibration slide.  Use the same technique as steps 3 and 4 counting the lines and calculating the measurement between two lines on the eyepiece.  Try with this example (click to see what is viewed in the microscope): 

  • What is the value for Y? (click on the ??? to see the answer)

  • What is the value for X?

  • Using the calculation, what is the value for the measurment between any two lines on the eyepiece? 

  • Now look at the cell below and measure its size in Ám.  It is a bit difficult to see where its cell membrane ends because it is stuck together with other cells.  What do you get?  Size = Is this close to what we got for the size of a cell in low power?  It should be!

  1. Do the same for high power.  Here is what the calibration slide looks like:

If you follow the same steps, you should get a value of 0.26Ám for the space between the lines on the eyepiece.  Question:  How many lines would an 18Ám-wide cell take up on the eyepiece as you look at it through high power?  If you work it out, you should get about 70 lines. 

  1. Now it is possible to use the microscope to observe and measure things.  Take the calibration slide out and carefully put it back in its protective box so that it does not get damaged.  Now place the slide of what you would like to look at on the stage. 

Note that if you change microscopes, the calibration process must be done again for each of the objective lenses that you are using.  Why?  Because the magnification is different on different microscopes.  To save time, only calibrate the eyepiece for the objective lenses that you will be using. 


(Optional) Using a calculation to get other powers:

(If you don't want to do this part, go down to the bottom of the page where there are more examples.)

Another time-saving method is to calibrate one power (low power, for example) and then calculate the proportions rather than measuring the other two.  Note that you do not need to know this for an exam.   

Look at our example above:  for 40x (low power), the calibration was 2.63Ám.  For 160 power (medium power), the calibration was 1.05Ám and for 400 power, it was 0.26Ám. A proportion can be set up: 

Notice that each time we zoomed in with ojective lens, it increased the value of the proportion by 10 fold.  So, if you have the number for low power (2.63Ám for 40x), find out what the magnification power is for the other objective lenses and use the formula below to calculate the calibration for medium power:


In our examle this means:

or for medium power to high power...




More examples:

An onion skin at medium power.  How big are the cells?

A tomato cell's nucleus seen at high power.  How big is the nucleus?

An onion skin at high power.  How big is a typical nucleus? 

A banana cell at high power.  How big are the starch grains?