Danish Dr. Finsen awarded 1903 Nobel Prize for medicine
“In a flash” Niels Finsen had an answer before he asked the question. He had spent years learning the details that made his discovery possible.
Niels Finsen was awarded a Nobel prize in 1903 for the use of blue light to relieve severe acne and for the exclusion of blue light to prevent scarring from smallpox.
+ Finsen's discovery in an old pamphlet
"Finsen was at this point in his researches when, one day at the medical library in Copenhagen, he came upon a pamphlet published in 1832 [about 50 years prior] by Dr. Pictou of New Orleans. In the pamphlet there was incidental mention of the fact that, during a certain smallpox epidemic, some soldiers confined in dark dungeons had suffered the disease and recovered without suppuration or scarring. No attempt was made at explanation. But the mere fact was sufficient for Finsen, who, in a flash of insight, seized upon a truth that had lain here for years, understood by no one. The soldiers had recovered without scarring simply because, being in the dark cells, they were protected against the irritating actinic rays, the same blue rays that disturb the earthworms so." ...
"Within a month after the question had suggested itself Finsen offered the world his red-light treatment…And the curious part of it is that at this time Finsen had never seen a case of smallpox, and based his conclusions entirely on theoretical grounds." Google books, Mclure’s Magazine Vol XX November 1902 to April 1903 Page 361
https://books.google.com/books?id=KQgv47if93EC Search for "Pictou"
+ Preparation for Finsen's Discovery
In the 1850s, raging debates focused on whether red light or blue light is "better". Among many experiments with colored light, Finsen studied the effects of different colors of light on earthworms. In a box with two filters, the worms moved away from the blue light to remain in the red light. He recognized that the skin was sensitive to different colors of light. This basic experiment helped prepare him for making his discovery.