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.

The men of science had long before analyzed the sunlight.  They had broken it up into the rays of different color that together make the white light we see.  Any boy can do it with a prism, and in the band or spectrum of red, yellow, green, blue, and violet that then appears, he has before him the cipher that holds the key to the secrets of the universe if we but knew how to read it aright; for the sunlight is the physical source of all life and of all power.  The different colors represent rays with different wave-lengths; that is, they vibrate with different speed and do different work.  The red vibrate only half as fast as the violet, at the other end of the spectrum, and, roughly speaking, they are the heat carriers.  The blue and violet are cold by comparison.  They are the force carriers.  They have power to cause chemical changes, hence are known as the chemical or actinic rays.  It is these the photographer shuts out of his dark room, where he intrenches himself behind a ruby-colored window.  The chemical ray cannot pass that; if it did it would spoil his plate.

This much was known, and it had been suggested more than once that the “disinfecting” qualities of the sunlight might be due to the chemical rays killing germs.  Finsen, experimenting with earthworms, earwigs, and butterflies, in a box covered with glass of the different colors of the spectrum, noted first that the bugs that naturally burrowed in darkness became uneasy in the blue light.  As fast as they were able, they got out of it and crawled into the red, where they lay quiet and apparently content.  When the glass covers were changed they wandered about until they found the red light again.  The earwigs were the smartest.  They developed an intelligent grasp of the situation, and soon learned to make straight for the red room.  The butterflies, on the other hand, liked the red light only to sleep in.  It was made clear by many such experiments that the chemical rays, and they only, had power to stimulate, to “stir life.”  Finsen called it that himself.  In the language of the children, he was getting “warm.”