stared down at the broken high tension line, and thought of what would have happened had it touched him in its spasm.

And then, suddenly, his blue eyes went wide with stark amazement.

"Mark!" he gasped.

"What's wrong, professor? What is it?"

"That glass on the floor! Look at it!"

The other stared uncomprehendingly.

"The waves, Mark! See the waves!"

A startled exclamation burst from the younger man's lips. He dropped to his knees. Scrutinized the puddle of glass.

But the scientist pulled him erect again.

"My instruments!" he ordered in a voice that trembled. "Quick! I must make tests—"

For half an hour he worked. And when at last he straightened, complete confidence gleamed deep in his eyes.

"Is it—"

Professor Duchard nodded.

"It is. That was the secret, my boy. The secret we sought but could not find. The time mirror is merely a special glass which has been subjected to a terrific electrical discharge, then silvered. That piece on the floor is worthless, of course; too many elements were uncontrolled.

"But knowing the formula as we do; knowing exactly what we are searching for and how to prepare it, I would stake my reputation that we can duplicate the mirror Adrian Vance sent to Elaine."

Mark's eyes were gleaming. His jaw hard.

"Then do it!" he commanded.

"But what good would it do? We cannot bring Elaine back—"

"Maybe not." The other's tanned face was grim. "But we can send me back to where she is."

"Send you back!"

"Yes." A pause. "You see, I've been thinking about the things you've told me, Professor Duchard. About time travel, and how it works.

"You say we can't save Elaine. Well, that's probably true. Maybe she's got to die in France, back in the days before the revolution."

A tremor of emotion passed over him as he said it. He swallowed hard. Then:

"But if she must die, she can at least die easily. Cleanly. Quickly, with a knife through her heart. She doesn't have to go the way Adrian Vance wants her to—tortured by a bunch of drunken scum, then cut to pieces without a chance to fight back."

There was pain in the professor's face, too, when he answered.

"I wish it were as easy as that, Mark."

Mark's voice was fierce.

"What's wrong with it? What's to stop me?"

The other sighed. Brushed back white hair with a sweep of one frail hand.

"You cannot change history, my boy," he said sadly. "A study of cosmology would show you that such things are immutable. You can go backward or forward through time and participate in them, but you cannot change them."

"How do you know? Who's traveled through time and then come back to say we can't change events?"