"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!" * * * * *