e walked now to the center of the laboratory, where stood a huge dial of white crystal, ranked with many levers and switches, all capped with the same material.
"Behold!" he said, throwing over one.
Instantly there came again that peculiar low humming that had so puzzled him a few minutes before—and the entire room, its engines, its attendants, Cor himself, leapt into invisibility. Only Kendrick remained, facing the faintly visible crystal dial.
Then he saw a switch move, as though automatically. But no, for the dwarf's hand was on it now. Visibility had returned. The vibration ceased.
"That is the central control," said Cor. "Our city and all its inhabitants become invisible when that switch is thrown. Only the dial remains, for the guidance of the operator, and even that cannot be seen at a distance of more than fifty feet. But now behold!"
He raised his hand, touched a watch-like device strapped to his wrist—and was instantly invisible. But the laboratory and every machine and person in it remained in plain view. Nor was there any vibration now.
he next moment, having touched that curious little device again, Cor reappeared.
"That is the local control," he said. "Every one of our inhabitants, except those under discipline, has one of these little mechanisms. It enables us to make ourselves invisible at will. A convenience at times, you must admit."