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.” 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.