Microscope

A microscope was a technology known on Earth and other planets, used to analyse matter or biological specimens. The specimens would be placed on a small piece of glass called a "slide", and then this slide would be positioned beneath the microscope's viewing lens. (PROSE: Plant Life)

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 one to see "small creatures" called "germs". (TV: The Massacre)

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

In the 1970s, the Third Doctor used a microscope to search for a cure to the Silurian virus. (TV: Doctor Who and the Silurians)

While at the Wholeweal community, 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 the discovery of the common genetic origin of the two species and the Alzarians. (TV: Full Circle)

TARDIS
The First Doctor had a microscope aboard his TARDIS. When he landed on Ashtallah, he brought it to Brenna and Sharlan to help them better study and observe a sample of Barbara Wright's blood. (AUDIO: The Invention of Death)

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

The Thirteenth Doctor's TARDIS control console had a built-in microscope, which she used to analyse substances, determine their component ingredients, and then calculate proportions. (COMIC: A New Beginning)

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