The benefits to medical science of the electron microscope have been numerous. For instance, it allowed researchers for the first time to view viruses directly, instead of merely suspecting their existence and guessing about their structure and behavior. Science historian Nicolas Rasmussen writes, "Only the electron micro-scope could provide convincing evidence that viruses were distinct entities present in infected tissue, or purified intact therefrom. . . . The electron micrograph [photograph] of a virus was its official, definitive portrait."45 By studying viruses from such portraits, researchers could begin formulating medicines to combat a wide range of serious viral diseases, including polio and influenza.

The electron microscope has been crucial to another major area of biomedical research. Researchers have used electron microscopy (in conjunction with related techniques such as X-ray diffraction) to observe DNA, the chemical instruction code that controls human heredity. As researchers...