The development of the microscope closely paralleled the growth of the telescope, for a microscope is actually a telescope designed to work at very short distances. The principles of both instruments are the same. Galileo made the first microscope, an adaptation of one of his telescopes. At the beginning, this use of lenses to look at tiny objects was an amusing hobby.

A Dutchman, Anton von Leeuwenhoek made a hobby of grinding lenses. He worked for years, gradually perfecting his technique and eventually learned to make lenses of extreme curvature that were only eight of an inch in diameter. He mounted these tiny polished bits of glass in gold frames of his own make. Once he had mounted them, Leeuwenhoek proceeded to look into a world that had been far too small for the naked eye to see. He examined the pores of his skin, the wings of insects, the teeming life in the drop of water.
He discovered how pure water becomes infested with microbes when allowed to stand in open air. In 1674, the microscope focused his attention downward into a world he never knew existed, a world that contained the source of many diseases that plagued his body.