Chief Steckel eyed him strangely, perhaps a trifle incredulous in his expression. "You really mean then, Mortenson," he said softly and in a baffled tone, "that this vanishing killer-bandit can rob and murder and hide behind a curtain of invisibility beyond the reach of the law?" "That gives us exactly his reason for taking my apparatus," said the scientist, "and obviously for what he is doing now, hiding somewhere in the Fifth Dimension, safe from human apprehension!" "Good Lord, man!" Steckel exploded suddenly. "Does that mean we'll never have a chance to grab him?" Professor Mortenson smiled shrewdly. "You'll never lay hands on him, Steckel," he said flatly. "Unless you give me your aid in carrying out a plan I have already devised. Even then it may be impossible, but we can try. Meanwhile this criminal is going to go about his way killing and robbing at will and may, at this very moment, be in this room!" Steckel's face paled and his eyes flashed. Mechanically he glanced around as though searching for some imaginary eavesdropper. But Mortenson quickly placed him at ease. "He could not hear a thing from this world if he is hiding in the other," he said. "Sound is similar to light in that respect. There are sounds too high or too low in pitch to be heard by our limited auditory capacities. The desperado would have to be right here and visible to hear what we say. He may watch every move I make now, for he warned me that I would be a dead man, the first attempt I made to follow him into the Fifth Dimension." Steckel's eyes continued to flash apprehensively. "Why, the devil might have designs on _my_ life," he said. "He might pop up at any moment, kill me for the good of the underworld, and vanish again! Or he might kill you if he thought you could duplicate the apparatus!" Mortenson scrutinized the chief thoughtfully. "He doesn't, chief," he said seriously. "At least that is my impression. I told him I had no plans of the instrument; that I could not duplicate them. Had he believed otherwise, I'd have gotten his bullet instead of a crack on the skull." "But you can duplicate them, Mortenson?" the chief asked eagerly. Mortenson laughed shrewdly. "Of course," he replied. "I have a complete set of plans. I merely lied to save my neck!" "You intend to use them, then?" Steckel inquired, leaning forward. The scientist nodded. Without hesitation he explained his scheme. Steckel listened intently, his eyes narrowed. "So you see," Mortenson finally concluded, "we can make it mighty hot for the killer if I get the support of your department. It will be an exceptionally dangerous adventure, but worth...."