Hi @luglimbe - Thanks for sharing your microscope image! If you're interested in measuring objects, you may want to consider using a calibration slide-- a calibration slide is simply a glass slide with a very small ruler printed on it. Sometimes it is possible to place your sample directly on the calibration slide, but you can also just take a picture of the ruler with your microscope (as long as you are using the same lens and setup for the calibration slide photo and your sample this should work reasonably well) and compare your images to that photo to find the measurement.

Below is an image I took of some beach sand with a calibration slide-- each of the lines represents 1/100 of a millimeter.

You might also find this post from @partsandcrafts helpful: Testing And Calibrating Your Microscope

A few more calibration methods are discussed here:

Microscope Calibration Overlays from @MaggPi

Use of a 5 Bill for Microscope Calibration from @amirberAgain