When he looked back he was amazed. The clear square was no longer clear. Instead, it was a dull black, so dull that not all the light of Mercury's Sun seemed to be able to brighten it.

Then, as he watched, that dullness trembled, broke, and fragmented.

It was bright again.

He watched it through three more cycles as the angle of elevation made it more and more vertical. First, incredible reflection; then, complete dullness. During the dullness, Lucky realized, light would be absorbed; during the glossiness, it would be reflected. The alternation in phase might be perfectly regular, or there might be a deliberate, irregular pattern. He could not linger to find out and, if he did, it was doubtful whether his knowledge of hyperoptics would be enough to enable him to understand the purpose of it all.

Presumably hundreds or even thousands of such squares, all connected by a network of cables and all powered from an atomic micropile inside the Dome, were absorbing and reflecting light in a set way at different angles to the Sun. Presumably, this, in some way, could force light energy through hyperspace in a controlled manner.

And, presumably, torn cables and smashed plates prevented the over-all pattern from being properly completed.

Lucky tried his ergometer again. It was much brighter now, and again he followed in the indicated direction.

Brighter, brighter! Whatever it was he was following, it was something that was changing its position. The source of gamma rays was not a fixed point on Mercury's surface.

And that meant it was not merely an outcropping of radioactive ore. It was something portable, and to Lucky that meant it was man or something belonging to man.

Lucky saw the figure first as a moving speck, black against the fire-lit ground. The sight came after a long spell in the open Sun, at a time when he had been about to find himself a shadow in which to let the slowly accumulating heat drain away.

Instead, he accelerated his pace now. He estimated the temperature outside his suit to be at not quite the boiling point of water. Inside, fortunately, it was considerably lower.

He thought grimly: If the Sun were overhead and not at the horizon, even these suits would be of no use.

The figure paid no attention to him. It continued on its own path, its gait showing it far from as expert in handling low gravity as was Lucky. Indeed its motion might almost be described as lumbering. Yet it managed to devour space. It covered the ground.

It wore no inso-suit. Even at long distance, the surface exposed to Lucky's gaze was obviously one of metal.

Lucky paused briefly in the shade of a rock but forced himself into the open again before there was time for much cooling.

The figure seemed unbothered by the heat. At least, in the time Lucky watched him he made no move to enter shadows, though, he passed within a few feet of some.

Lucky nodded thoughtfully. It all fit well. He sped on. The heat was beginning to feel like something he could touch and squeeze. But it would only be a few moments now.

He had abandoned his low-slung lope now. Every bit of his muscular power was being put into giant strides of up to fifteen feet each.

He shouted, "You! You there! Turn around!" He said it peremptorily, with all the authority he could produce, hoping that the other could receive Ms radio signal and that he would not be reduced to sign language.

Slowly the figure turned, and Ducky's nostrils flared in a kind of cold satisfaction. So far, at least, it was as he thought, for the figure was no man-nothing human at all!