use of that mirror now!"

With that final sally, he backed away and out the door, the Magnum in his hand still grim and unwavering as he covered Mark and the old scientist.

Curtly:

"I wouldn't come out too soon if I were you."

The door slammed shut.

Mark started forward. But the professor caught his arm.

"It is useless," the savant said. "To follow him would bring death and would avail nothing, my boy. He has won."

Like men in a daze, then, they stared into each other's eyes. They saw only dull hopelessness. The last spark was gone out.

Slowly, Mark walked over to the corner where stood the shattered mirror. Looked blankly down at its fragments. Bending, he picked up a splinter. Inspected it idly.

The next instant he whirled about.

"Professor Duchard!" he rapped. "How did this devil's looking-glass work?"

The scientist looked up dispiritedly, shrugged.

"I could not make you understand. It is a complicated matter of space-time theory—"

The other strode back to him. Gripped his shoulder.

"I don't care about the details. Just try to give me a simplified version of the principle."

Professor Duchard gazed into the younger man's eyes. Caught the fierce light within them—the gleam of spirit that marks those who will not be downed for long, no matter what the odds. The ray of struggle that only death could take away.

For a long moment, then, the old man sat buried in thought. At last he looked up again. Broke the silence.

"Have you ever seen the physical experiment in which a wave of sound is used to break a glass?"

"No. But I've heard of it. I know what you're talking about."

"Very well, then. Imagine, if you can, that the barrier between space and time is that glass. It is apparently impenetrable."

"I see." Elaine's fiance nodded eagerly.

"Then try to conceive of a terrific wave of energy being concentrated against it, just as the sound wave is concentrated on the glass. But this time, the wave must be so manipulated as to strike the barrier as a pebble strikes and breaks a window. Otherwise it would be too weak to break through. Or, if it was strong enough, it would break down the entire space-time relationship."

Again Mark nodded, this time more slowly.

"You mean that the wave of energy really must be like a sword, stabbing one small hole through the barrier?"

"Exactly." A pause. "The time mirror represented just such a hole through the barrier. What appeared to us to be waves in the glass actually were frozen ripples in the space-time continuum—just as if you had dropped a stone in water, and the hole and ripples had frozen."

"Then when you looked into the mirror—"

"Your mind went out through that gap in the barrier. Ordinarily, of course, you would not even know that this was happening. But if your mind was concentrated on something in the past or future—as Elaine's was upon the picture of her ancestor—, you were automatically hurtled through time to that period."

The younger man frowned.

"Then why didn't my mind go, too, when Elaine's did? We both were looking into the mirror."

"But from different angles," the professor reminded him. "Remember, the actual break in the continuum was relatively small. Elaine, seated before the mirror, must have been directly in front of the gap, so she was sucked through. You, on the other hand—"

"Yes. I was standing up. Off center. So I didn't go." Mark nodded. "I see."

"And now," said the scientist, "the mirror is broken. Our last chance of saving Elaine is gone."

"No!"

"What?" The professor peered up at the other incredulously. "What do you mean, Mark?"

Brown eyes narrowed with excitement, Elaine's sweetheart held out the splinter of glass he had picked up. He shook it in front of the savant's face.

"Professor, every piece of glass that went to make up that mirror is laying over there on the floor."

"I am sorry, my boy." The elder man frowned. "I do not understand."

"Professor, if you break your glasses, all you have to do to get a new pair is to take the pieces to an optician. He'll figure the formula of the lens from the fragments and make you a new set."

"You mean—"

"I mean that we can put the pieces of that mirror together as if it was a jigsaw puzzle. From it, you can figure out some kind of a formula. Then, by experimenting, you can find what kind of energy bolt it takes to blast through the barrier!"

Something of the man's intensity, his enthusiasm, communicated itself to the professor. His blue eyes came alight.

"It is conceivable!" he declared. "Not likely. But conceivable." He gripped the fragment of glass which Mark held. "Yes! We shall try it! If it works, we can—"

He stopped short. His face fell.

"We can what?" he finished. "Another time mirror will not help us bring Elaine back—"

His companion interrupted fiercely:

"How do we know? There's always a chance we'll think of something, isn't there? And it's a cinch we won't accomplish anything just sitting here."

"But—"

"The least we can do is try!"

They worked like madmen in the hours that followed, heedless of the wedding guests who came and went from the house in bewildered knots. Unmindful of gashed fingers, Mark fitted the slivers of mirror together, while Professor Duchard tested and analyzed and figured at his side.

And then—

"I have it!" shouted the savant triumphantly. "I have the formula!"

"Then we can construct another mirror?"

Some of the old man's elation dropped away. He shook his head.

"Not yet. We know only the effect we want. But how to achieve it—" He shrugged.

Experiments. More experiments. Hours of experiments, with Mark and the professor hovering over an electric crucible bubbling with molten glass.

Hours of failure.

At last the old scientist straightened, his face haggard with weariness.

"It is no use," he said sadly. "I have exhausted my knowledge, and to no avail."

He turned away, shoulders sagging. Stumbled toward the door.

The next instant Mark's voice rose in a scream.

"Look out!"

Instinctively, without so much as a backward glance, the old man lunged forward. Even as he did so, he felt something jerk at his ankle. His leg came out from under him. He pitched to the floor.

Crash!

The crucible was falling, jerked from its place atop the lab bench! The electric cable which supplied its current was twisted about the professor's ankle, somehow unconsciously caught by his foot as he worked.

Molten glass burst out of the pot in a white-hot wave. Slopped over the composition floor in a steaming river. Engulfed table legs and radiator pipes alike.

And then, like a writhing snake, the high tension line from which the crucible cable stemmed was whipping down, torn loose by the jar of the professor's leap!

Down it came! Struck the floor once. Lashed against the glass-engulfed radiator pipes, bare wires flashing.

A ball of purple fire exploded at the contact point, while the cable jerked and twisted like a living thing. The laboratory was suddenly permeated with ozone's peculiar odor.

"Look out!" cried Mark again.

But already Professor Duchard had jerked his foot free of the crucible line. He shrank back under the long bench, away from the writhing cable.

An instant later the current went dead. The crackling ball of purple fire evaporated into thin air.

Mark sprang across the room to where the scientist lay. He pulled him to his feet.

"Are you all right, professor? Are you hurt?"

"Yes, yes, my boy. It was a narrow escape, but your warning saved me. I am all right."

The savant leaned against the bench, trying to still the reflexive trembling of his body. His face was pale. He ran his tongue over lips suddenly gone dry as he