The golden age of microscopy lasted roughly from the mid-nineteenth century to the first decades of the twentieth century. It was a time of major breakthroughs in microscope technology, which in turn resulted in dramatic progress in medical research and treatment. The result: countless lives saved and untold amounts of suffering relieved.

One crucial discovery made during this period concerned the importance of cells in human development and health. Scientists had been aware of cells for centuries. With the aid of improved microscopes, however, they discovered that cells contain basic building blocks of life. This realization, called cell theory, became the underlying basis of much of all medical research in the years since.

Another major breakthrough was the germ theory. Many had previously suggested the possibility that germs caused some diseases. However, not until researchers had better microscopes could this...