Diatom
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 Ergonom 500 microscope (same resolution capability as Ergonom 4000).
To see the images at full resolution, just click on them.
(Note: Magnifications given relate to a picture size of DIN A4).
Amphipleura pellucida: 6308x Holes cannot be seen clearly at this magnification
Amphipleura pellucida: 8705x Holes can now be seen
Diatom Triceratium favus (2020x)