an interest in lensmaking. In the 1670s, he started to explore microbial life with his microscope.

Using single-lensed microscopes of his own design and make, Van Leeuwenhoek was the first to observe and to experiment with microbes, which he originally referred to as dierkens, diertgens or diertjes. He was the first to relatively determine their size. Most of the "animalcules" are now referred to as unicellular organisms,