"Eye does that," Felix explained again. "The electric eye and X-ray. Eye turns on the current that starts the X-ray going. Quite a convenience. If your would-be visitors carry hard things like guns or knives, you see them and need not admit them unless you want to. "We are seeing ourselves now," he chuckled, "as we have never been, but as we shall be. Come inside." The skeletons vanished. The next door opened. "In five minutes the 'eye' will have made us a cup of cocoa." Felix sat down. "It's really very simple," he went on after a moment. "The electric eye, or photo-electric cell, is a vacuum tube treated chemically on the inside. A peep hole admits light. When light strikes the chemicals it starts a small electric discharge. This electric discharge, when stepped up, will start any piece of mechanism you may wish it to. "It works as well when I cut off the light as when I turn it on. So, when I pass before a light in the wall that plays on one electric eye, it causes the door to open. Another closes the door, and so forth. "Just now an 'eye' turned on the current under a pan of milk. When the milk is hot and rises in the pan, a second eye slides the pan aside and adds the cocoa and sugar. So we have steaming cocoa with no trouble at all. "Impractical?" He threw back his head and laughed. "Yes, but it's lots of fun. "But the eye is revolutionizing the world, for all that!" he added, handing Johnny his cocoa. "I told you we fixed up a rig for sorting a carload of beans a day. That is done by thousands of electric eyes. Pineapples are sorted the same way. In school rooms an eye watches the light. When it gets too dark the eye throws on the lighting switch. The eye umpires bowling matches and would umpire a baseball game, call a ball a ball, a strike a strike, and never be wrong. And that certainly would be something! "Guess that's enough for tonight. I'll get that phone." He hurried away. It was not enough, not half enough for Johnny. He wanted to ask if the eye had helped him see what he had seen that afternoon, if the eye could have anything to do with the whispers at dawn. He wanted to ask a hundred questions. But Felix was gone. When Johnny mounted to his room, he found the telephone in its place on a stand by his bed, but Felix was nowhere to be seen. CHAPTER XVIII THE TRAP IS SPRUNG As a rule, Johnny was a heavy sleeper. All the strange doings of the past few days must have gotten on his nerves, for next morning, more than an hour before dawn, he found himself lying in bed wide awake, thinking. The ceiling of his room, he noticed, had dropped again during the night. This neither surprised nor disturbed him. In fact, in this strange house had the attraction of gravity been reversed and had he found his bed resting on the ceiling instead of the floor, he would not have been greatly surprised.