This is a photo I have taken with a microscope (100x objective). What are these circles? When I rotate the eyepiece or the objective they stay still. Are they particles on the mirror? In this case what kind of particles?
As suggested in comments these circles are airy disks around dust particles on the camera lens. After wiping the lens few of them were left and the remaining ones moved.
You can find out more about airy disks on Wikipedia