Then she takes the instrument and moves to the far wall, the girl following, and stands before a blank space in the wall. She places her hand on the stone, leaning a little against it, her free hand on her chest.

Isaac’s eyes widen.

A carved wood door appears, as if out of a mist, forming itself right within the stone. The door opens, and the woman and girl go through it, and before Isaac can fumble his way across the room they disappear and the door disappears and Isaac is left standing facing a solid blank stone wall.

Except . . .

As Isaac stares at the wall, he sees a carving in the stone. He lifts one finger and traces it, looping and winding.

The eternity knot.

He blinks and tries to breathe. What kind of magic is this? Is this what he needs to find here?

The watch in his fist tick-tick-ticks. Panic tightens his lungs. He’s afraid he’ll run out of time before he understands his purpose here.

He turns back to the table and the documents scattered there and leans over them.

Mathematics. Lots of complicated equations. Diagrams, measurements, calculations. Geometry. This is very sophisticated stuff, all written out in a fine neat hand. The woman had been working these equations.

The book brought by the girl still lies on the table. Isaac sets down the watch, opening the skull so that he can see the watch face, and reaches for the book.

He can touch it, feel it, pick it up. Aha.

Isaac makes a connection. He must be a ghost only when he holds the watch, for as long as he holds the watch. When he doesn’t have hold of it, when he isn’t touching it, he becomes visible. He can interact with the past.

Well, he’s alone, so what harm can he do?

The paper of the book is thick and rough with long embedded fibers. There are words, and Isaac recognizes Latin. He can read at least some of the words.

Closed. Expelled, he reads. And Iudaeum, which means “Jew.”

Isaac’s stomach drops as he realizes. Expelled from this place in ancient Greece. Just like he was, from Prague. Just like his friends were. He takes a step back and has to pull himself together. Try to focus.

If only he’d paid a little more attention when his father rattled on about ancient history, he’d understand where he was and what to look for.

He moves away from the table, wondering what to do next. He glances at the watch that he left lying open on the table—if he reads it right, he has about twenty minutes left before the hand reaches the top of the hour.

The sun is just at the brink of setting, the last rays piercing a low window, and Isaac has to shield his eyes and turn away. The sun’s rays strike a cubby on the far wall.