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 zatif 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 realty 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............