"We're not optically invisible," answered Kane amusedly. "So far as I know, that's a physical impossibility. Actually, those Germans saw us, but they didn't notice us. Ever catch yourself looking right at something and not seeing it, because it was too familiar or because you were thinking about something else? That's the effect the field has. Anything in the middle of it hides behind a psychic block in the mind of whoever looks at it. That's why it works on hearing, too, and even on touch. It's not perfect; if you set off a magnesium flare in front of somebody, or punched him in the nose, he'd notice something was up--but hardly before. When you get acquainted with the effect it makes you feel like a ghost. Back in that cafe, I had to shout in your ear till I deafened myself before I could make you hear." * * * * * They glided down the current, and the lights and voices around the bridge receded rapidly. As Manning bent to his oar, his imagination was busy with the first item of twenty-first century technology which went completely beyond his twentieth-century knowledge. In Germany he had seen the evidences of a mighty and advanced civilization, but everything had been the logical perfection of inventions already known.... Kane seemed to read his thoughts. "Working like we do, we can't compete with the Germans in things that call for a lot of resources and equipment. They have all the big weapons--the rockets and tanks and atomic bombs. For anything to be useful to us, it has to be something that can be invented and built in a cellar. So we've had to open up brand-new lines of development--and in fields like psychoelectronics we're miles ahead of the Germans, because they didn't have to.... Better pull over. We don't want to get rammed," he interrupted himself. A blinding eye was bearing down on them across the water. In its stark glare Manning felt nakedly visible again. But they veered sharply toward the bank, and the launch went past in a swish of foam, still scanning river and shore ahead. "Where we going?" Dugan asked practically. "We're about there," answered Kane. "Easy now." He pointed to where a jumble of ruins projected like a pier into the stream, the ripples lapping and gurgling in the spaces between the great piled fragments. "In there--the only space big enough for the boat. Better duck." Their craft slid with scant clearance into an opening like the mouth of a cave. Kane produced a flashlight, and they saw that a timbered tunnel ran back into the bank at right angles to the entrance.