He found an invisible form of light by leaving a thermometer past a rainbow's red edge

In 1800, astronomer William Herschel split sunlight with a prism and measured the temperature at each colour. On a hunch, he placed a thermometer just past the red end, where no visible light fell — and it registered the highest temperature of all. He had discovered infrared radiation, the first form of light ever found beyond the visible spectrum.

— William Herschel, Investigation of the Powers of the Prismatic Colours to Heat and Illuminate Objects — Philosophical Transactions of the Royal Society, 1800