Microscope

A microscope was a technology known on Earth and other planets, used to analyse matter or biological specimens.

Earth
In Paris, in 1572, the First Doctor revealed to Charles Preslin that "a man in Germany" was "working on optics, trying to make a machine, which" enabled to see "small creatures" called "germs". (TV: The Massacre of St Bartholomew's Eve)

The Fourth Doctor 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)

In 2070, in the Moonbase, the Second Doctor discovered that the Cybermen had infected the human scientists with a large neurotropic virus named Neurotrope, thanks to a microscope in the medical unit of the base. (TV: The Moonbase)

Other planets
On the planet Alzarius in the E-Space, microscopes were available. The Fourth Doctor used one to compare specimens from the Marshmen and the Alzarian spider. This led to discover the common genetic origin of the two species and the Alzarians. (TV: Full Circle)