While the telescope magnified the sky and widened the universe, in the same century lenses were turned the other way — toward the very small — and an entirely different frontier opened. In 1665 the English scientist Robert Hooke published the observations made with his own compound microscope in the magnificent book "Micrographia": the never-before-seen details of a flea, a fly's eye, a plant tissue, drawn in large engravings.

When Hooke looked at a thin slice of cork, he saw the small, empty, regular chambers that composed it; likening them to the cells of monks in a monastery, he called them "cells." The word stuck; but Hooke did not yet know that these chambers were the basic unit of life — he had seen only their empty walls.

A few years later in the Netherlands, Antonie van Leeuwenhoek, a cloth merchant of Delft, made tiny but extraordinarily powerful single-lens microscopes with his own hands. He looked at a drop of pond water, at dental plaque, at rainwater, and could not believe his eyes: they were full of countless tiny creatures moving, swimming, twisting within. He called them "animalcules." For the first time a human being was seeing bacteria and single-celled organisms.

The consequences of these two observations took centuries to unfold. The concept of the cell led, in the nineteenth century, to the theory that all living things are made of cells; Leeuwenhoek's "animalcules" opened the road to Pasteur's germ theory and to modern microbiology. Making the invisible visible was one of science's most fertile moves: it showed that reality extends far beyond the boundary drawn by our senses.