More of the prominences were now visible. Their redness brightened. The corona did not vanish (there was no atmosphere to scatter the prominence light and wash out dimmer glows), but it seemed less important now. The stars were still out and would stay out, Lucky knew, even when Mercury's sun was full in the sky, but who could pay attention to them now?

Lucky ran forward eagerly in the steady stride which he could maintain for hours without feeling unduly tired. Under the circumstances, he felt he could have maintained such a stride even under Earth's gravity.

And then, with no warning, no premonitory glow in the sky, no hint from any atmosphere, there was the Sun!

Rather, there was a hairline that was the Sun. It was an unbearable line of light edging a notch of broken rock on the horizon, as though some celestial painter had outlined the gray stone in brilliant white.

Lucky looked backward. Across the uneven ground that lay behind him there were the splotches of prominence-red. But now, just at his feet, there was a wash of white, catching crystal formations in glinting highlights.

He moved onward again, and the line of light became first a small splotch and then a larger one.

The boundary of the Sun was clearly visible, lifting a bit above the horizon in its center, curving gently down on each side. The curve was awesomely flat to one whose eyes were accustomed to the curvature of Earth's Sun.

Nor did the Sun's blaze drown out the prominences which crawled along its edge like flaming red snake-hair. The prominences were all over the Sun, of course, but only at the edge could they be seen. On the Sun's face, they were lost amid the glare below.

And over all was the corona.

Lucky marveled, even as he watched, at the manner in which the inso-suit had been adapted to its purpose.

A glance at the edge of Mercury's Sun would have been blinding to unprotected eyes, blinding forever. The visible light was bad enough in its intensity, but it was the hard ultra-violet, unfiltered by atmosphere, that would have meant death to vision... and to life itself, eventually.

Yet the glass of the inso-suit's face-plate was so arranged molecularly as to grow less transparent in direct proportion to the brightness of the light that fell upon it. Only a small fraction of a percent of the Solar blaze penetrated the plate, and he could stare at the Sun without danger, almost without discomfort. Yet at the same time, the light of the corona and the stars come through undiminished.

The inso-suit protected him in other ways. It was impregnated with lead and bismuth, not enough so as to raise its weight unduly, but enough to block out ultraviolet and x-radiation from the Sun. The suit carried a positive charge to deflect most of the cosmic rays to one side. Mercury's magnetic field was weak, but Mercury was close to the Sun and the cosmic ray density was large. Still, cosmic rays are composed of positively-charged protons, and like charges repel like.

And, of course, the suit protected him against the heat, not only by its insulating composition but by its mirrorlike reflecting surface, a pseudo-liquid molecular layer that could be activated by a touch on the controls.

In fact, Lucky reflected, when the advantages of the inso-suit were considered, it seemed a pity that it was not standard protection under all conditions. Unfortunately, he realized, its structural weakness, as a result of lacking metal in real quantity, made it impractical for use except where protection against heat and radiation were paramount considerations.

Lucky was a mile into the Sun-side now and not conscious of undue heat.

This did not surprise him. To stay-at-homes who confined their knowledge of space to the sub-etheric thriller shows, the Sun-side of any airless planet was simply a solid mass of undeviating heat.

This was an oversimplification. It depended on how high the Sun was in the sky. From this point on Mercury, for instance, with only a portion of the Sun above the horizon, comparatively little heat reached the surface, and that little was spread over a lot of ground as the radiation struck almost horizontally.

The "weather" changed as one went deeper into the Sun-side and finally, when one reached that portion where the Sun was high in the sky, it was everything the sub-etherics said it was.

And besides, there were always the shadows. In the absence of air, light and heat traveled in a straight line. Neither could reach within the shadow except for small fractions which were reflected or radiated into it from neighboring sunlit portions. The shadows were therefore frosty cold and carbon black though the Sun was ever so hot and bright.

Lucky was growing more aware of these shadows. At first, after the upper line of Sun had appeared, the ground had been almost all shadow with only occasional patches of light. Now, as the Sun rose higher and higher, the light spread and coalesced until the shadows were distinct things hovering behind boulders and hills.

At one time Lucky deliberately plunged into the shadow of a rise of rock a hundred yards across, and it was as though for a long minute he were back on the dark-side. The heat of the Sun, which he had scarcely noticed while it beat upon him, became evident by its decrease in the shadow. All around the shadow the ground glimmered brightly in sunlight, but within the shadow his suit-light was necessary to guide his steps.

He could not help noticing the difference in the surfaces that were in the shadow from those in the light. For on the Sun-side, at least, Mercury did have a kind of atmosphere. Not one in the Earthly sense, no nitrogen, oxygen, carbon dioxide, or water vapor, nothing like that. On the Sun-side, however, mercury would boil in places. Sulfur would be liquid and so would a number of volatile compounds. Traces of the vapor of such substances would cling to Mercury's superheated surface. These vapors froze out in the shadows.

This was brought forcibly to Lucky's mind when his insulated fingers brushed over the dark surface of one outcropping and came away smeared with a frozen hoar of mercury, glittering in his suit-light. It changed quickly into clinging liquid droplets as he emerged into the Sun and then, more slowly, evaporated away.

Slowly, the Sun seemed to be getting hotter. That did not worry Lucky. Even if it grew uncomfortably hot, he could always dodge into a shadow to cool off when necessary.

Short-wave radiation was perhaps a more important consideration. Lucky doubted even that was serious in this short-term exposure. Workers on Mercury had a horror of radiation, because they were continually exposed to small amounts. Lucky recalled Mindes's emphasis on the fact that the saboteur he had seen had remained standing in the Sun. It was natural that Min-des should be disturbed at that. When exposure was chronic any lengthening of the time of exposure was foolish. In Lucky's own case, however, exposure would be short-term-he hoped.

He ran across patches of blackish ground that stood out somberly against Mercury's more general reddish gray. The reddish gray was familiar enough. It resembled the soil of Mars, a mixture of silicates with the addition of iron oxide, which gave it that ruddy tinge.

The black was more puzzling. Wherever it was, the ground was definitely hotter, since black absorbed more of the Sun's heat.

He bent as he ran and found the black areas crumbly rather than gritty. Some of it came up on the palm of his gauntlet. He looked at it. It might be graphite, it might be iron or copper sulfide. It might be any of a number of things, but he would have bet on its being some variety of impure iron sulfide.

He paused in the shadow of a rock, finally, and took stock. In an hour and a half, he estimated he had traveled some fifteen miles, judging from the fact that the Sun was just about entirely above the horizon now. (At the moment, he was more interested in sipping sparingly at the suit's supply of liquid nutrient mixture than in estimating distance, however.)

Somewhere to the left of him were cables of Mindes's Project Light. Somewhere to the right of him were others. Their exact location did not matter. They covered hundreds of square miles, and to wander aimlessly among them in search of a saboteur would have been foolish.

Mindes had tried it, hit or miss, and had failed. If the object or objects he had seen had indeed been the saboteur, there might have been a warning from inside the Dome. Mindes had made no secret of the fact that he was heading out to Sun-side.

Lucky had, however. There would be no warning, he hoped.

And he had a form of help Mindes had not had. He flipped his small ergometer out of the pouch he had placed it in. He held it before him in cupped palm, his suit-light playing full on it.

Once activated, its red signal-patch blazed with incredible fury when held out in the sunlight. Lucky smiled tightly and adjusted it. There was short-wave radiation from the Sun.

The flame died.

Patiently, then, Lucky stepped out into the sunlight and scanned the horizon in every direction. Where, if anywhere, was there a source of atomic power other than the Sun? He found an indication of the Dome, of course, but the light due to that region increased as he dipped the ergometer downward. The Dome's power plant was nearly a mile underground, and a twenty degree downward dip was required for maximum power where he stood.

He turned slowly, the ergometer held gingerly between the two forefingers of each hand in order that the opaque material of the suit should not block off the telltale radiation. Around a second time and a third.

It seemed to him that in one particular direction there had been the briefest of flashes-scarcely enough to see against the sunlight, really. Perhaps no more than the product of wishful thinking.

He tried again.

No mistake now!

Lucky sighted along the direction in which that glow had appeared and moved in that direction. He did not conceal from himself the fact that he might only be tracking down a patch of radioactive ore.

He caught his first glimpse of one of Mindes's cables nearly a mile farther on.

It was not a single cable at all, rather a web of cables, lying half buried in the ground. He followed it some hundreds of yards and came upon a square metal plate, about four feet on a side and polished to a high gloss. It reflected the stars as though it were a clear pool of water.

No doubt, thought Lucky, if he placed himself in the proper position he would find himself staring into the reflection of the Sun. He became aware that the plate was changing its angle of elevation, becoming less horizontal, more vertical. He looked away to see if it were shifting in such a way as to catch the Sun.

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!