Electron microscope

An electron microscope was a microscope used to analyse samples of matter, often biological or chemical matter. (PROSE: Placebo Effect)

The Fourth Doctor once borrowed an electron microscope from the virology laboratory of a 20th century British hospital in order to study a rock hand, unveiling its biological nature. (TV: The Hand of Fear)

The Eighth Doctor had at least two electron microscopes in his TARDIS, along with a holographic magnifier, dimensional revisualiser and an antique brass microscope. (PROSE: The Dying Days, The Janus Conjunction)