Joseph Priestley's microscope, 1767.
© Science Museum / Science & Society Picture Library
This compound microscope belonged to the 18th century English chemist, Joseph Priestley (1733-1804). It is made up of two short-focus convex lenses, the objective and the eyepiece, which are mounted at opposite ends of a tube. It was built in 1767 by Benjamin Martin, a London based amateur scientist. Priestley discovered various gaseous elements and compounds and, in an experiment in 1774, obtained a gas which he named 'dephlogisticated air'. This was in fact oxygen, although it was not named thus until the French chemist Lavoisier (1743-1794) repeated Priestley's experiment.