Mary Malone was constructing a mirror. Not out of vanity, for she had little of that, but because she wanted to test an idea she had. She wanted to try and catch Shadows, and without the instruments in her laboratory she had to improvise with the materials at hand.

Mulefa technology had little use for metal. They did extraordinary things with stone and wood and cord and shell and horn, but what metals they had were hammered from native nuggets of copper and other metals that they found in the sand of the river, and they were never used for toolmaking. They were ornamental. Mulefa couples, for example, on entering marriage, would exchange strips of bright copper, which were bent around the base of one of their horns with much the same meaning as a wedding ring.

So they were fascinated by the Swiss Army knife that was Mary's most valuable possession.

Atal, the zalif who was her particular friend, exclaimed with astonishment one day when Mary unfolded the knife and showed her all the parts, and explained as well as she could, with her limited language, what they were for. One attachment was a miniature magnifying glass, with which she began to burn a design onto a dry branch, and it was that which set her thinking about Shadows.

They were fishing at the time, but the river was low and the fish must have been elsewhere, so they let the net lie across the water and sat on the grassy bank and talked, until Mary saw the dry branch, which had a smooth white surface. She burned the design, a simple daisy, into the wood, and delighted Atal; but as the thin line of smoke wafted up from the spot where the focused sunlight touched the wood, Mary thought: If this became fossilized, and a scientist in ten million years found it, they could still find Shadows around it, because I've worked on it.

She drifted into a sun-doped reverie until Atal asked:

What are you dreaming?

Mary tried to explain about her work, her research, the laboratory, the discovery of shadow particles, the fantastical revelation that they were conscious, and found the whole tale gripping her again, so that she longed to be back among her equipment.

She didn't expect Atal to follow her explanation, partly because of her own imperfect command of their language, but partly because the mulefa seemed so practical, so strongly rooted in the physical everyday world, and much of what she was saying was mathematical; but Atal surprised her by saying, Yes - we know what you mean - we call it... and then she used a word that sounded like their word for light.

Mary said, Light?

Atal said, Not light, but... and said the word more slowly for Mary to catch, explaining: like the light on water when it makes small ripples, at sunset, and the light comes off in bright flakes, we call it that, but it is a make-like.

Make-like was their term for metaphor, Mary had discovered.

So she said, It is not really light, but you see it and it looks like that light on water at sunset?

Atal said, Yes. All the mulefa have this. You have, too. That is how we knew you were like us and not like the grazers, who don't have it. Even though you look so bizarre and horrible, you are like us, because you have  - and again came that word that Mary couldn't hear quite clearly enough to say: something like sraf, or sarf, accompanied by a leftward flick of the trunk.

Mary was excited. She had to keep herself calm enough to find the right words.

What do you know about it? Where does it come from?

From us, and from the oil, was Atal's reply, and Mary knew she meant the oil in the great seedpod wheels.

From you?

When we have grown up. But without the trees it would just vanish again. With the wheels and the oil, it stays among us.

When we have grown up... Again Mary had to keep herself from becoming incoherent. One of the things she'd begun to suspect about Shadows was that children and adults reacted to them differently, or attracted different kinds of Shadow activity. Hadn't Lyra said that the scientists in her world had discovered something like that about Dust, which was their name for Shadows? Here it was again.

And it was connected to what the Shadows had said to her on the computer screen just before she'd left her own world: whatever it was, this question, it had to do with the great change in human history symbolized in the story of Adam and Eve; with the Temptation, the Fall, Original Sin. In his investigations among fossil skulls, her colleague Oliver Payne had discovered that around thirty thousand years ago a great increase had taken place in the number of shadow particles associated with human remains. Something had happened then, some development in evolution, to make the human brain an ideal channel for amplifying their effects.

She said to Atal:

How long have there been mulefa?

And Atal said:

Thirty-three thousand years.

She was able to read Mary's expressions by this time, or the most obvious of them at least, and she laughed at the way Mary's jaw dropped. The mulefa 's laughter was free and joyful and so infectious that Mary usually had to join in, but now she remained serious and astounded and said:

How can you know so exactly? Do you have a history of all those years?

Oh yes, said Atal. Ever since we have had the sraf, we have had memory and wakefulness. Before that, we remembered nothing.

What happened to give you the sraf?

We discovered how to use the wheels. One day a creature with no name discovered a seedpod and began to play, and as she played she -

She?

She, yes. She had no name before then. She saw a snake coiling itself through the hole in a seedpod, and the snake said...

The snake spoke to her?

No, no! It is a make-like. The story tells that the snake said, "What do you know? What do you remember? What do you see ahead?" And she said, "Nothing, nothing, nothing." So the snake said, "Put your foot through the hole in the seedpod where I was playing, and you will become wise." So she put a foot in where the snake had been. And the oil entered her blood and helped her see more clearly than before, and the first thing she saw was the sraf. It was so strange and pleasant that she wanted to share it at once with her kindred. So she and her mate took the seedpods, and they discovered that they knew who they were, they knew they were mulefa and not grazers. They gave each other names. They named themselves mulefa. They named the seed tree, and all the creatures and plants.

Because they were different, said Mary.

Yes, they were. And so were their children, because as more seedpods fell, they showed their children how to use them. And when the children were old enough to ride the wheels, they began to generate the sraf as well, and the sraf came back with the oil and stayed with them. So they saw that they had to plant more seedpod trees for the sake of the oil, but the pods were so hard that they seldom germinated. So the first mulefa saw what they must do to help the trees, which was to ride on the wheels and break them, so mulefa and seedpod trees have always lived together.

Mary directly understood about a quarter of what Atal was saying, but by questioning and guessing she found out the rest quite accurately; and her own command of the language was increasing all the time. The more she learned, though, the more difficult it became, as each new thing she found out suggested half a dozen questions, each leading in a different direction.

But she pulled her mind after the subject of sraf, because that was the biggest; and that was why she thought about the mirror.

It was the comparison of sraf to the sparkles on water that suggested it. Reflected light like the glare off the sea was polarized; it might be that the shadow particles, when they behaved like waves as light did, were capable of being polarized, too.

I can't see sraf as you can, she said, but I would like to make a mirror out of the sap lacquer, because I think that might help me see it.

Atal was excited by this idea, and they hauled in their net at once and began to gather what Mary needed. As a token of good luck there were three fine fish in the net.

The sap lacquer was a product of another and much smaller tree, which the mulefa cultivated for that purpose. By boiling the sap and dissolving it in the alcohol they made from distilled fruit juice, the mulefa made a substance like milk in consistency, and delicate amber in color, which they used as a varnish. They would put up to twenty coats on a base of wood or shell, letting each one cure under wet cloth before applying the next, and gradually build up a surface of great hardness and brilliance. They would usually make it opaque with various oxides, but sometimes they left it transparent, and that was what had interested Mary: because the clear amber-colored lacquer had the same curious property as the mineral known as Iceland spar. It split light rays in two, so that when you looked through it you saw double.

She wasn't sure what she wanted to do, except that she knew that if she fooled around for long enough, without fretting, or nagging herself, she'd find out. She remembered quoting the words of the poet Keats to Lyra, and Lyra's understanding at once that that was her own state of mind when she read the alethiometer - that was what Mary had to find now.

So she began by finding herself a more or less flat piece of a wood like pine, and grinding at the surface with a piece of sandstone (no metal: no planes) until it was as flat as she could make it. That was the method the mulefa used, and it worked well enough, with time and effort.

Then she visited the lacquer grove with Atal, having carefully explained what she was intending, and asked permission to take some sap. The mulefa were happy to let her, but too busy to be concerned. With Atal's help she drew off some of the sticky, resinous sap, and then came the long process of boiling, dissolving, boiling again, until the varnish was ready to use.

The mulefa used pads of a cottony fiber from another plant to apply it, and following the instructions of a craftsman, she laboriously painted her mirror over and over again, seeing hardly any difference each time as the layer of lacquer was so thin, but letting it cure unhurriedly and finding gradually that the thickness was building up. She painted on over forty coats - she lost count - but by the time her lacquer had run out, the surface was at least five millimeters thick.

After the final layer came the polishing: a whole day of rubbing the surface gently, in smooth circular movements, until her arms ached and her head was throbbing and she could bear the labor no more.

Then she slept.

Next morning the group went to work in a coppice of what they called knot wood, making sure the shoots were growing as they had been set, tightening the interweaving so that the grown sticks would be properly shaped. They valued Mary's help for this task, as she on her own could squeeze into narrower gaps than the mulefa, and, with her double hands, work in tighter spaces.

It was only when that work was done, and they had returned to the settlement, that Mary could begin to experiment, or rather to play, since she still didn't have a clear idea of what she was doing.

First she tried using the lacquer sheet simply as a mirror, but for lack of a silvered back, all she could see was a doubled reflection faintly in the wood.

Then she thought that what she really needed was the lacquer without the wood, but she quailed at the idea of making another sheet; how could she make it flat without a backing anyway?

The idea came of simply cutting the wood away to leave the lacquer. That would take time, too, but at least she had the Swiss Army knife. And she began, splitting it very delicately from the edge, taking the greatest of care not to scratch the lacquer from behind, but eventually removing most of the pine and leaving a mess of torn and splintered wood stuck immovably to the pane of clear, hard varnish.

She wondered what would happen if she soaked it in water. Did the lacquer soften if it got wet? No, said her master in the craft, it will remain hard forever; but why not do it like this? And he showed her a liquid kept in a stone bowl, which would eat through any wood in only a few hours. It looked and smelled to Mary like an acid.

That would hurt the lacquer hardly at all, he said, and she could repair any damage easily enough. He was intrigued by her project and helped her to swab the acid delicately onto the wood, telling her how they made it by grinding and dissolving and distilling a mineral they found at the edge of some shallow lakes she had not yet visited. Gradually the wood softened and came free, and Mary was left with the single sheet of clear brown-yellow lacquer, about the size of a page from a paperback book.

She polished the reverse as highly as the top, until both were as flat and smooth as the finest mirror.

And when she looked through it...

Nothing in particular. It was perfectly clear, but it showed her a double image, the right one quite close to the left and about fifteen degrees upward.

She wondered what would happen if she looked through two pieces, one on top of the other.

So she took the Swiss Army knife again and tried to score a line across the sheet so she could cut it in two. By working and reworking, and by keeping the knife sharp on a smooth stone, she managed to score a line deep enough for her to risk snapping the sheet. She laid a thin stick under the score line and pushed sharply down on the lacquer, as she'd seen a glazier cutting glass, and it worked: now she had two sheets.

She put them together and looked through. The amber color was denser, and like a photographic filter it emphasized some colors and held back others, giving a slightly different cast to the landscape. The curious thing was that the doubleness had disappeared, and everything was single again; but there was no sign of Shadows.

She moved the two pieces apart, watching how the appearance of things changed as she did so. When they were about a hand span apart, a curious thing happened: the amber coloring disappeared, and everything seemed its normal color, but brighter and more vivid.

At that point Atal came along to see what she was doing.

Can you see sraf now? she said.

No, but I can see other things, Mary said, and tried to show her.

Atal was interested, but politely, not with the sense of discovery that was animating Mary, and presently the zalif tired of looking through the small pieces of lacquer and settled down on the grass to maintain her wheels and claws. Sometimes the mulefa would groom each other's claws, out of pure sociability, and once or twice Atal had invited Mary to attend to hers. Mary, in turn, let Atal tidy her hair, enjoying how the soft trunk lifted it and let it fall, stroking and massaging her scalp.

She sensed that Atal wanted this now, so she put down the two pieces of lacquer and ran her hands over the astonishing smoothness of Atal's claws, that surface smoother and slicker than Teflon that rested on the lower rim of the central hole and served as a bearing when the wheel turned. The contours matched exactly, of course, and as Mary ran her hands around the inside of the wheel, she could feel no difference in texture: it was as if the mulefa and the seedpod really were one creature, which by a miracle could disassemble itself and put itself together again.

Atal was soothed, and so was Mary, by this contact. Her friend was young and unmarried, and there were no young males in this group, so she would have to marry a zalif from outside; but contact wasn't easy, and sometimes Mary thought that Atal was anxious about her future. So she didn't begrudge the time she spent with her, and now she was happy to clean the wheel holes of all the dust and grime that accumulated there, and smooth the fragrant oil gently over her friend's claws while Atal's trunk lifted and straightened her hair.

When Atal had had enough, she set herself on the wheels again and moved away to help with the evening meal. Mary turned back to her lacquer, and almost at once she made her discovery.

She held the two plates a hand span apart so that they showed that clear, bright image she'd seen before, but something had happened.

As she looked through, she saw a swarm of golden sparkles surrounding the form of Atal. They were only visible through one small part of the lacquer, and then Mary realized why: at that point she had touched the surface of it with her oily fingers.

Atal! she called. Quick! Come back!

Atal turned and wheeled back.

Let me take a little oil, Mary said, just enough to put on the lacquer.

Atal willingly let her run her fingers around the wheel holes again, and watched curiously as Mary coated one of the pieces with a film of the clear, sweet substance.

Then she pressed the plates together and moved them around to spread the oil evenly, and held them a hand span apart once more.

And when she looked through, everything was changed. She could see Shadows. If she'd been in the Jordan College Retiring Room when Lord Asriel had projected the photograms he'd made with the special emulsion, she would have recognized the effect. Everywhere she looked she could see gold, just as Atal had described it: sparkles of light, floating and drifting and sometimes moving in a current of purpose. Among it all was the world she could see with the naked eye, the grass, the river, the trees; but wherever she saw a conscious being, one of the mulefa, the light was thicker and more full of movement. It didn't obscure their shapes in any way; if anything it made them clearer.

I didn't know it was beautiful, Mary said to Atal.

Why, of course it is, her friend replied. It is strange to think that you couldn't see it. Look at the little one...

She indicated one of the small children playing in the long grass, leaping clumsily after grasshoppers, suddenly stopping to examine a leaf, falling over, scrambling up again to rush and tell his mother something, being distracted again by a piece of stick, trying to pick it up, finding ants on his trunk and hooting with agitation. There was a golden haze around him, as there was around the shelters, the fishing nets, the evening fire: stronger than theirs, though not by much. But unlike theirs it was full of little swirling currents of intention that eddied and broke off and drifted about, to disappear as new ones were born.

Around his mother, on the other hand, the golden sparkles were much stronger, and the currents they moved in were more settled and powerful. She was preparing food, spreading flour on a flat stone, making the thin bread like chapatis or tortillas, watching her child at the same time; and the Shadows, or the sraf, or the Dust, that bathed her looked like the very image of responsibility and wise care.

So at last you can see, said Atal. Well, now you must come with me.

Mary looked at her friend in puzzlement. Atal's tone was strange: it was as if she were saying, Finally you're ready; we've been waiting; now things must change.

And others were appearing, from over the brow of the hill, from out of their shelters, from along the river: members of the group, but strangers, too, mulefa who were new to her, and who looked curiously toward where she was standing. The sound of their wheels on the hard-packed earth was low and steady.

Where must I go? Mary said. Why are they all coming here?

Don't worry, said Atal, come with me, we shall not hurt you.

It seemed to have been long planned, this meeting, for they all knew where to go and what to expect. There was a low mound at the edge of the village that was regular in shape and packed with hard earth, with ramps at each end, and the crowd - fifty or so at least, Mary estimated - was moving toward it. The smoke of the cooking fires hung in the evening air, and the setting sun spread its own kind of hazy gold over everything. Mary was aware of the smell of roasting corn, and the warm smell of the mulefa themselves - part oil, part warm flesh, a sweet horselike smell.

Atal urged her toward the mound.

Mary said, What is happening? Tell me!

No, no... Not me. Sattamax will speak...

Mary didn't know the name Sattamax, and the zalif whom Atal indicated was a stranger to her. He was older than anyone she'd seen so far: at the base of his trunk was a scatter of white hairs, and he moved stiffly, as if he had arthritis. The others all moved with care around him, and when Mary stole a glance through the lacquer glass, she saw why: the old zalif 's Shadow cloud was so rich and complex that Mary herself felt respect, even though she knew so little of what it meant.

When Sattamax was ready to speak, the rest of the crowd fell silent. Mary stood close to the mound, with Atal nearby for reassurance; but she sensed all their eyes on her and felt as if she were a new girl at school.

Sattamax began to speak. His voice was deep, the tones rich and varied, the gestures of his trunk low and graceful.

We have all come together to greet the stranger Mary. Those of us who know her have reason to be grateful for her activities since she arrived among us. We have waited until she had some command of our language. With the help of many of us, but especially the zalif Atal, the stranger Mary can now understand us.

But there was another thing she had to understand, and that was sraf. She knew about it, but she could not see it as we can, until she made an instrument to look through.

And now she has succeeded, she is ready to learn more about what she must do to help us.

Mary, come here and join me.

She felt dizzy, self-conscious, bemused, but she did as she had to and stepped up beside the old zalif. She thought she had better speak, so she began:

You have all made me feel I am a friend. You are kind and hospitable. I came from a world where life is very different, but some of us are aware of sraf, as you are, and I'm grateful for your help in making this glass, through which I can see it. If there is any way in which I can help you, I will be glad to do it.

She spoke more awkwardly than she did with Atal, and she was afraid she hadn't made her meaning clear. It was hard to know where to face when you had to gesture as well as speak, but they seemed to understand.

Sattamax said, It is good to hear you speak. We hope you will be able to help us. If not, I cannot see how we will survive. The tualapi will kill us all. There are more of them than there ever were, and their numbers are increasing every year. Something has gone wrong with the world. For most of the thirty-three thousand years that there have been mulefa, we have taken care of the earth. Everything balanced. The trees prospered, the grazers were healthy, and even if once in a while the tualapi came, our numbers and theirs remained constant.

But three hundred years ago the trees began to sicken. We watched them anxiously and tended them with care and still we found them producing fewer seedpods, and dropping their leaves out of season, and some of them died outright, which had never been known. All our memory could not find a cause for this.

To be sure, the process was slow, but so is the rhythm of our lives. We did not know that until you came. We have seen butterflies and birds, but they have no sraf. You do, strange as you seem; but you are swift and immediate, like birds, like butterflies. You realize there is a need for something to help you see sraf and instantly, out of the materials we have known for thousands of years, you put together an instrument to do so. Beside us, you think and act with the speed of a bird. That is how it seems, which is how we know that our rhythm seems slow to you.

But that fact is our hope. You can see things that we cannot, you can see connections and possibilities and alternatives that are invisible to us, just as sraf was invisible to you. And while we cannot see a way to survive, we hope that you may. We hope that you will go swiftly to the cause of the trees' sickness and find a cure; we hope you will invent a means of dealing with the tualapi, who are so numerous and so powerful.

And we hope you can do so soon, or we shall all die.

There was a murmur of agreement and approval from the crowd. They were all looking at Mary, and she felt more than ever like the new pupil at a school where they had high expectations of her. She also felt a strange flattery: the idea of herself as swift and darting and birdlike was new and pleasant, because she had always thought of herself as dogged and plodding. But along with that came the feeling that they'd got it terribly wrong, if they saw her like that; they didn't understand at all; she couldn't possibly fulfill this desperate hope of theirs.

But equally, she must. They were waiting.

Sattamax, she said, mulefa, you put your trust in me and I shall do my best. You have been kind and your life is good and beautiful and I will try very hard to help you, and now I have seen sraf, I know what it is that I am doing. Thank you for trusting me.

They nodded and murmured and stroked her with their trunks as she stepped down. She was daunted by what she had agreed to do.

At that very moment in the world of Citt¨¤gazze, the assassin-priest Father Gomez was making his way up a rough track in the mountains between the twisted trunks of olive trees. The evening light slanted through the silvery leaves and the air was full of the noise of crickets and cicadas.

Ahead of him he could see a little farmhouse sheltered among vines, where a goat bleated and a spring trickled down through the gray rocks. There was an old man attending to some task beside the house, and an old woman leading the goat toward a stool and a bucket.

In the village some way behind, they had told him that the woman he was following had passed this way, and that she'd talked of going up into the mountains; perhaps this old couple had seen her. At least there might be cheese and olives to buy, and springwater to drink. Father Gomez was quite used to living frugally, and there was plenty of time.