James Somers in The New Yorker:
It was by accident that Antoni van Leeuwenhoek, a Dutch cloth merchant, first saw a living cell. He’d begun making magnifying lenses at home, perhaps to better judge the quality of his cloth. One day, out of curiosity, he held one up to a drop of lake water. He saw that the drop was teeming with numberless tiny animals. These animalcules, as he called them, were everywhere he looked—in the stuff between his teeth, in soil, in food gone bad. A decade earlier, in 1665, an Englishman named Robert Hooke had examined cork through a lens; he’d found structures that he called “cells,” and the name had stuck. Van Leeuwenhoek seemed to see an even more striking view: his cells moved with apparent purpose. No one believed him when he told people what he’d discovered, and he had to ask local bigwigs—the town priest, a notary, a lawyer—to peer through his lenses and attest to what they saw.
Van Leeuwenhoek’s best optics were capable of more than two hundred times magnification. That was enough to see an object a millionth the size of a grain of sand. Even so, the cells appeared minuscule. He surmised that they were “furnished with instruments for motion”—tiny limbs that must “consist, in part, of blood-vessels which convey nourishment into them, and of sinews which move them.” But he doubted that science would ever advance enough to reveal the inner structure of anything that small.
More here. (Note: A must read for cell-biology-aficionados)