Historically, the diatom Amphipleura pellucida has often been used to test the resolution of microscopes. The holes have a diameter of just 180nm which is a challenge for all light microscopes to see. Here are some examples imaged through the microscope.
To see the images at full resolution, just click on them.
(Note: Magnifications given relate to a picture size of DIN A4).
Diatom Triceratium favus (2020x)