In Chapter 1 we treated lenses as if they were perfect. Sadly, they are not, and it is precisely for this reason that Antonie van Leeuwenhoek did so much better with his simple microscope than Robert Hooke — or anyone else — did with the compound microscopes of the 17th and 18th centuries. If you cannot correct the defects of your lenses, it is better not to have too many of them. As late as the early 19th century, the great botanist Robert Brown was using a simple microscope to make discoveries — the cell nucleus, Brownian motion — that had eluded the apparently more sophisticated compound microscope. In the 70 years that followed, the microscopes available to biologists improved immensely as the imperfections inherent in simple lenses were finally corrected. Nevertheless, these aberrations, though cured, are still highly relevant to today’s practical biological microscopist.