For two or three years after getting his degree he taught in the medical school as demonstrator, eking out his scant income by tutoring students in anatomy. His sure hand and clear decision in any situation marked him as a practitioner of power, and he had thoughts once of devoting himself to the most delicate of all surgery,—that of the eye. He was even then groping for his life-work, without knowing it, for it was always light, light—the source or avenue or effect of it—that held him. And presently his work found him. It has been said that Finsen was a sick man. A mysterious malady[15] with dropsical symptoms clutched him from the earliest days with ever tightening grip, and all his manhood’s life he was a great but silent sufferer. Perhaps it was that; perhaps it was the bleak North in which his young years had been set that turned him to the light as the source of life and healing. He said it himself: “It was because I needed it so much, I longed for it so.” Probably it was both. Add to them his unique power of turning the things of every day life to account in his scientific research, and one begins to understand at once his success and his speedy popularity. He dealt with the humble things of life, and got to the heart of things on that road. And the people comprehended; the wise men fell in behind him—sometimes a long way behind. [Illustration: DR. NIELS FINSEN] In the yard of Regentsen there grows a famous old linden tree. Standing at his window one day and watching its young leaf sprout, Finsen saw a cat sunning itself on the pavement. The shadow of the house was just behind it and presently crept up on pussy who got up, stretched herself, and moved into the sunlight. In a little while the shadow overtook her there, and pussy moved once more. Finsen watched the shadow rout her out again and again. It was clear that the cat liked the sunlight. A few days later he stood upon a bridge and saw a little squad of insects sporting on the water. They drifted down happily with the stream till they came within the shadow of the bridge, when they at once began to work their way up a piece to get a fresh start for a sunlight sail. Finsen knew just how they felt. His own room looked north and was sunless; his work never prospered as it did when he sat with a friend whose room was on the south side, where the sun came in. It was warm and pleasant; but was that all? Was it only the warmth that made the birds break into song when the sun came out on a cloudy day, made the insects hum joyously and man himself walk with a more springy step? The housekeeper who “sunned” the bed-clothes and looked with suspicion on a dark room had something else in mind; the sun “disinfected” the bedding. Finsen wanted to know what it was in the sunlight that had this power, and how we could borrow it and turn it to use.