Slim was already turning dials. Tubes were lighting up. The set was humming. Pretty soon he pointed to a screen, and I damn near lost my breath. There on a screen about twelve by eighteen inches, big enough so there wasn't any mistake, I saw myself on the night of July Fourth, just as I bought one ticket for the roller coaster. I guess my eyes stuck out a foot, for Slim was looking at me with that kind of sad smile. "Roller coasters," he said gently. "Got enough, Doc?" I gulped. "Plenty. Cut it off, please." In the screen I saw the blonde just behind me, and I didn't want Slim to see her put her arms around me when the roller coaster went over the dip. [Illustration: It was hard to make out the faces.] Slim smiled and snapped a bunch of switches. The lights in the tubes went out. "Think what this will mean in criminal prosecutions, to be able to follow a man in the past. Present-day testimony will be archaic. The courts won't have to take anybody's word for anything; they can follow a man and watch him in the past." "Judge Monday wouldn't admit that kind of evidence," I pointed out. "Naturally not. It will take twenty-five years to get this kind of evidence admitted in court. In the meantime, we'll have to go easy. But we can make millions, just by bluffing. When we _know_ that a man was playing poker in Jones's basement until six o'clock Sunday morning, then we can bluff and put it over. Just so we don't tangle with a real tough guy the first time. For instance--sh! Somebody's at the door." Slim ran to the door while I ran for my pants. I ducked back into the other room and got them on. I heard the voice. It was a man's voice, and I had heard it before--just recently. I peeked out. Yes, it was Tom Ellingbery. I stayed quiet. "A pot-bellied little guy just served divorce papers on me," he said harshly. "I got off the train and came here. A friend of mine sent me; I want your services." "Yes," said Slim. "Here's a hundred-dollar bill," Tom Ellingbery said. "Start shadowing my wife; get something on her. I'll give you five thousand to get something--ten if it's necessary," he said with a slight leer. Slim gravely picked up the C note. "We don't do business that way," he said; "but if your wife has been misbehaving we'll find it out." Ellingbery was a big man with a sharp go-getter look about him. He stared hard at Slim and Slim stared back. Ellingbery's expression didn't show anything; then he left. * * * * * Slim locked the door after Ellingbery, and I took off my pants and set up the ironing-board on the desk. Slim went back to adjust the dials on his machine. "This gadget is a sort of super-sensitive radar," he said as it warmed up. "I can tune it to your brain-waves and pick you up anywhere within forty miles or three months." A purple indicator began to wink. "It proves I've got brains, anyway," I pointed out.