Robert Hooke's microscope, or more precisely his refinements to the microscope, led to his discovery of the cell, the building block of all life. His findings were published in "Micrographia" in 1665.
Hooke invented ways of controlling the height and angle of microscopes as well as mechanisms of illumination. Variations in light allowed Hooke to see new detail, using multiple sources of illumination. Hooke's technical efforts created magnifications of 50x, enabling insight to a world not yet known in the 1600s. Adept at technical drawing, Hooke also had the gift of illustrating his discoveries.
Hooke's discovery was of plant cells, the cell walls of cork tissue, to be exact. He found similar material in wood and a variety of plants. Not only did he make this discovery but he also was responsible for describing these entities as "cells." In 1678, upon request of the Royal Society of London, Hooke peer reviewed his colleague Leeuwenhoek's work, research that confirmed the existence of what came to be known as bacteria and protozoa. Hooke was also the first to examine fossils using a microscope. His comparison of fossil remains to living organisms first established the scientific connection between the two, predating Darwin by two and a half centuries in the idea that species had lived and ceased to exist throughout the Earth's history.