We have long used diatom slides as nice test samples and resolution targets for brightfield microscopy. The diatoms have silica shells with regularly spaced small holes in them. The hole spacing varies from species to species and ranges from a few hundred nanometers to a little over a micron, making them perfect for showing the affect of NA on microscope resolution. For a long time, we have gotten slides from Klaus Kemp, who is apparently the last producer of diatom slides.
The New York Times is now hosting a very nice short movie on Klaus’s work, called The Diatomist. It’s worth checking out if you’ve ever used a diatom slide or wondered what went into making them.