Measurements under the microscope

As part of practical 1 you need to draw a couple of Zebrafish embryos and add a scale bar indicating the size of the embryo. In order to do this you will have to be able to measure the embryos under the microscope. You might find this a little tricky so I have written this step-by-step guide to help you out....

To make any measurements with your microscope you will need to calibrate your eyepiece micrometer using a stage micrometer. 

So, first of all, what are these?

Eyepiece micrometer : This is a fine scale that fits inside the eyepiece lens.
                                      Its units are arbitrary.
Stage micrometer : This is a scale fitted on to a coverslip. The size of the divisions are known.

Your aim is to find out how big one division is on the eyepiece micrometer at the magnification you are using. You will find this out by comparing the eyepiece micrometer with the known scale on the stage micrometer. This is called calibrating your eyepiece micrometer.

Steps for calibrating your eyepiece micrometer:

1. Align the eyepiece micrometer (arbitrary scale) with the stage micrometer (known scale).

2. Count how many divisions on the eyepiece micrometer correspond to a set number of stage micrometer divisions. 
TIP: if you forget which scale is which, rotate your eyepiece a little - the eyepiece micrometer is the one that rotates

So in this example for every 10 stage micrometer divisions 

there are 21 eyepiece micrometer divisions.

3. Calculate how big one eyepiece division is.

This is where you need to know how long every small division on your stage micrometer is.  Check the size of your micrometer - If your micrometer is 1 mm long in total, then every set of 10 small divisions is actually 0.1 mm long (the 1 mm is divided up into 10 equal parts, each of which is 0.1 mm long), and so every small division is 0.01 mm long.

So, if 21 eyepiece micrometer divisions correspond to 0.1 mm on the stage micrometer

then 1 eyepiece micrometer division must correspond to 0.1 / 21 = 0.0048 mm (2sf) 

Now you can measure your sample:

Once you have gone through all of the above steps, you can measure your specimen using the eyepiece micrometer, and convert the number of eyepiece micrometer divisions you have counted to an actual length:

So, if I am looking at some cheek cells, 

I can see that the length of one cheek cell is 16 units long on the eyepiece micrometer. 

I now know that one eyepiece micrometer division is 0.0048 mm long, 

so this cell must be 16 x 0.0048 = 0.0768 mm long.

I know this can sound quite confusing at first... read through this post several times and try out the calculations yourself. 

:)

Your Life107 teaching team