By 1668, he was grinding lenses for his own simple microscopes and looking at every tiny thing he could find. Those two things — his boundless curiosity and the fact that he kept improving his lenses — were critical to his discoveries.
Van Leeuwenhoek's discoveries were documented in letters he wrote to Henry Oldenburg, secretary of the Royal Society of London, between 1673 and Van Leeuwenhoek's death in 1723. The letters made him famous, and the Royal Society made him a fellow in 1680.
Over the course of his lifetime, van Leeuwenhoek made at least 500 microscopes. The few that survive are little more than powerful magnifying glasses. However, he developed his own technology for making them, and he never revealed the secrets of their power and brightness.