On this day in 1609, Galileo carried a tube of leather and glass up the Campanile in Venice and let the city's lawmakers look through it at ships still hours from harbour. The mechanism is kitchen-table simple. The lens at the far end is fat in the middle, so it bends all the light falling on it into a small bright picture inside the tube — a funnel for light. The lens at your eye is thin in the middle, and it spreads that little picture back out again until it fills your view. Nothing is added. The same light arrives, just laid out wider, and the sail eight times nearer than your bare eye could manage.

What I like is the grinding. He did not buy that glass; he ground and polished it himself, and the early attempts were poor — bubbles, wrong curves, the muddy halo you get when the surface is not quite true. He kept going until three or four lenses in a batch were good enough to keep. I remember that rhythm from my own bench, though my failures involved rather more spilled solvent and no lawmakers watching.

The portrait below shows him bust-length in a black gown, the gold lettering behind his shoulder. A man who checked, and then checked again, and then invited other people to look for themselves.