Laboratory Investigation:The Micrometer Eyepiece 
(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 timesaving 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 2.63µm. For 160 power (medium power), the calibration was 1.05µm and for 400 power, it was 0.26µm. 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 (2.63µm 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... or...

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? 
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