Discovery of infrared light
On February 11th, 1800, using thermometers and a prism, William Herschel noticed that the spectrum of light produced "radiant heat" beyond that of visible light (below the red). He named them "calorific-rays" and later came to be known as infra-red, a kind of "if I may be permitted the expression, invisible light"
Source: Cosmos: Possible Worlds
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Full disclosure, I may occasionally borrow a sentence from Will Durant's Story of Civilization. I absolutely love that collection!