To be seen in a light microscope an object must be larger than the wavelength of light. Viruses are smaller than this - apart from a couple of recently discovered giants, called mimivirus and megavirus, and a few that are really long but very thin. So almost all viruses are too tiny to be seen with a light microscope.
What the scientists who study viruses - virologists - needed was an instrument that used something smaller than the wavelength of light. That instrument was the electron microscope, invented in 1931 by physicist Ernst Ruska.
The simplest version is the transmission electron microscope, which sends electrons through a thin slice or film of material. Denser parts of the specimen absorb the electrons. So those that pass through show its shape and structure, when stopped on a photographic plate.
Solid objects instead of slices could be imaged with the scanning electron microscope, which was invented soon afterwards. In this instrument, the electrons do not pass through the specimen but are absorbed all over its surface. This gives off various forms of energy, such as light and electrons, which are then captured and used to build stunning 3-D images.
Other instruments for studying viruses have been developed since the 1930s, but the electron microscope - which let scientists see viruses for the first time - remains a mainstay of virus research and medicine.