That this power, like any other, had its perils, and that nature, if not man, was awake to them, he proved by some simple experiments with sunburn.  He showed that the tan which boys so covet was the defence the skin puts forth against the blue ray.  The inflammation of sunburn is succeeded by the brown pigmentation that henceforth stands guard like the photographer’s ruby window, protecting the deeper layers of the skin.  The black skin of the negro was no longer a mystery.  It is his protection against the fierce sunlight of the tropics and the injurious effect of its chemical ray.

Searching the libraries in Copenhagen for the records of earlier explorers in his field, and finding little enough there, Finsen came across the report of an American army surgeon on a smallpox epidemic in the South in the thirties of the last century.  There were so many sick in the fort that, every available room being filled, they had to put some of the patients into the bomb-proof, to great inconvenience all round, as it was entirely dark there.  The doctor noted incidentally that, as if to make up for it, the underground patients got well sooner and escaped pitting.  To him it was a curious incident, nothing more.  Upon Dr. Finsen, sitting there with the seventy-five-year-old report from over the sea in his hand, it burst with a flood of light:  the patients got well without scarring because they were in the dark.  Red light or darkness, it was all the same.  The point was that the chemical rays that could cause sunburn on men climbing glaciers, and had power to irritate the sick skin, were barred out.  Within a month he jolted the medical world by announcing that smallpox patients treated under red light would recover readily and without disfigurement.

The learned scoffed.  There were some of them who had read of the practice in the Middle Ages of smothering smallpox patients in red blankets, giving them red wine to drink and hanging the room with scarlet.  Finsen had not heard of it, and was much interested.  Evidently they had been groping toward the truth.  How they came upon the idea is not the only mystery of that strange day, for they knew nothing of actinic rays or sunlight analyzed.  But Finsen calmly invited the test, which was speedy in coming.

They had smallpox in Bergen, Norway, and there the matter was put to the proof with entire success; later in Sweden and in Copenhagen.  The patients who were kept under the red light recovered rapidly, though some of them were unvaccinated children, and bad cases.  In no instance was the most dangerous stage of the disease, the festering stage, reached; the temperature did not rise again, and they all came out unscarred.

Finsen pointed out that where other methods of treatment such as painting the face with iodine or lunar caustic, or covering it with a mask or with fat, had met with any success in the past, the same principle was involved of protecting the skin from the light, though the practitioner did not know it.  He was doing the thing they did in the middle ages, and calling them quacks.