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)

In 1999, Grace Holloway used a microscope to study a sample of the Eighth Doctor's blood. (TV: Doctor Who)

In the 1970s, at the Wholeweal, Professor Clifford Jones and the Third Doctor employed a microscope to study the slime from the giant maggots, in order to find a cure for the "green death" they were causing. (TV: The Green Death)

In 2070, in the Moonbase, the Second Doctor discovered that the Cybermen had infected the human scientists with a large neurotropic virus named Neurotrope X, 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)

TARDIS
In her room on the Doctor's TARDIS, Nyssa had got a microscope, used p.e. to study enzymes. (TV: Terminus)

Other types
Electron microscopes were used by the Fourth Doctor (TV: The Hand of Fear) and other individuals. (TV: Four to Doomsday)