"It must be capable of working," Tate told himself querulously. "The Martians knew--they wouldn't have tried to stop us if--Wait a minute." He paced back and forth, biting his lip. Syme watched him with catlike eyes, clenching and unclenching his great fists. Tate paused. "I think I have it," he said slowly. "I haven't enough power to hetrodyne the whole screen, although that's theoretically possible. But there must be weaker portions of the field--doors--set to open on the impact of a beam like this one. But I've only got power enough for two more tries. Jones, where would you put an entrance, if you'd built Kal-Jmar?" Syme's eyes widened, and he stared around slowly. "A thousand years ago?" he muttered. "Two thousand? These hills were raised in five hundred. We can't go by topography. "In front of one of the main arteries, then. But there are dozens, no one larger than the other. Did they have dozens of doors?" "Maybe," said Tate. He pointed to the right, where the fairy towers of Kal-Jmar swept aside to leave a broad avenue. "It's the nearest--as good as any other." They walked over to it in silence, and in silence Tate set up his equipment once more. He shifted it from side to side, squinting, until he had it lined up exactly on the center of the avenue. Then he took a long breath, and closed the switch again. The switch came up. Syme stared with fierce eagerness, eyes ablaze. For a moment there was nothing, and then-- Tate clutched the big man's arm. "Look!" he breathed. Where the ray from Tate's machine had impinged, a faintly-glowing spot of violet radiance! As they watched it widened, dilating into a perfect circle of violet, enclosing nothingness. The door was opening. "It worked," Tate said softly. "It worked!" Syme shook off his grip impatiently, put his hand to the gun in the holster of his suit. Tate was still watching, fascinated. "Look," he said again. "The color is changing slightly, falling down the spectrum. I think that's a warning signal. When it reaches red, the door will close." He moved toward the widening door, like a sleepwalker. "Wait," Syme said hoarsely. "You forgot the machine." Tate turned, said, "Oh yes," and walked back. Then he saw the gun in Syme's hand. His jaw dropped slightly, but he didn't say anything. He just stood there, looking dumbly from the gun to Syme's dark face. Syme shot him carefully in the chest. He dropped like a rag doll, but Syme's aim had been bad. He wasn't dead yet. He rolled his eyes up, like a child. His lips moved. In spite of himself, Syme bent forward to listen. "_You'll be_--_sorry_," Tate said, and died. Air was sighing out through the widening hole in the screen. Syme straightened and smiled tolerantly. For a moment, he had been unreasonably afraid of what Tate was about to say. Some detail he had forgotten, perhaps, something that would trap him now that Tate, the man who knew the answers, was dead. But--he'd be sorry! For what? Another dead fool?