Appendix 1: Diatoms

Diatoms are unicellular algae that were often used in the past to assess the resolution of microscopes. The reason is that their cell wall exhibits periodic structures, which can only be resolved if the resolution of the microscope is finer than the period of the structures, which is in the micrometer range and varies across diatom species.

The figure below shows a few of the diatom species that you might encounter in the test slides at your disposal. An excellent website on diatom, where the period of the striae for many different species is catalogued, is https://diatoms.org.

Figure 5: The unmarked diatoms, from left to right, are: Pinnularia nobilis, Gyrosigma attenuatum, Cocconeis placentula and Navicula oblonga (pictures credit Stefano Barone). The name of the other 7 diatoms is indicated in the figure, together with the period of their striae (adapted from Fundamentals of light microscopy and electron imaging by Douglas B. Murphy).