"That's what Doctor Le Bon calls it. You see, the visible spectrum (or all the light we can see) is only about one per cent. of all the vibrant energy emitted by the sun or any other luminous body. Beyond that visible spectrum lie, at one end the ultraviolet rays, and at the other the infra-red. I have here a lighted lantern enclosed in an opaque box, which cuts off all the visible rays, but permits the other ninety-nine per cent, to pass through. The flame inside is now casting rays of black light through the opaque sides, black, because they are invisible; light, because they will illuminate certain objects.
"I want you to witness an experiment. You recall the celebrated interference experiment of Fresnel, in which light added to light produced darkness? Well, I shall show you how darkness added to darkness may give birth to light. It is Le Bon's discovery. Now come into my dark room, and I'll show it to you."
At the farther end of the laboratory he opened a door which led into a small dark room. Entering this, and closing the laboratory door, he opened one into another dark room beyond, carrying the dark lantern. They both entered the inner dark room, which was ventilated through a circuitous light-proof pipe. The room was absolutely black; but Astro, well used to the place, feeling his way with his hands, set the lantern on a table.
"Upon a shelf here," he said, "is a Chinese image of Buddha, which some weeks ago I coated with phosphorescent sulfid of calcium. By this time all its luminosity is gone, and it is absolutely invisible. But now I shall direct the invisible rays of black light from this lantern upon it. Watch!"
