"Let's get away in the flier," he said.

"Can't," Grim said. "Those lenses you see on the instrument board are the controls. No one knows how to operate them except the Mercutians. Our people managed to capture a few, but couldn't do a thing with them."

Hilary stared at the motionless flier with interest. "What are those round glass disks stretched along the hull in a double row?" he asked. "They look like burning glasses."

"That's just what they are," said Grim sadly. "The top row are sun-lenses, that throw a terrible ray for a distance of two to three hundred feet. Melts everything in its path—men trees, rocks even. You saw one in action in the sun-tube with which poor old Peabody was cut in half. The lower row of lenses on the flier are search beams."

"Search beams?" Hilary echoed inquiringly.

"Yes. They act like X-rays, more powerful though, and with the further property of rendering everything they touch transparently crystal for depths of ten to fifteen feet. Lead is the only element they can not penetrate. Another secret our scientists can not fathom, so they talk learnedly about the stream of rays polarizing the structure of matter along a uniaxis."

"Can't those lenses be duplicated, and turned as weapons against the Mercutians?"

"No. They are made of a peculiar vitreous material native to Mercury."

"And no one has found out the principle on which they work?"