The diatoms in the image below are from a diatom test slide. Such slides are made to allow the microscopist to test the resolution of microscope objectives. Only the very best optics are able to resolve the individual lines and dots in the diatom shells.

This image was obtained using a low aperture objective and darkfield illumination.
It was not intended to demonstrate the resolving power of the lens but rather the wonderful interference colours created by the fine structure of the diatoms.