In 1800 William Herschel, the astronomer who had found Uranus, split sunlight with a prism and laid blackened thermometers in the colours to see which one carried the most heat. As the story is usually told, he also set thermometers just past the red end, where there was no colour at all, to show what the room was doing on its own. Those came out warmest of all. Something invisible was arriving there, behaving like light in every way except being seen, and he called it calorific rays; we call it infrared. I like that the discovery sat in the place he had set aside to mean nothing. A control is a quiet promise that the dark part of an experiment stays dark, and now and then the dark part declines.