Outside, at the other end of the passageway, was one of his men, waiting in the shadows as Del Mar came up. For a moment they talked. "I'll be there, at sunrise," agreed Del Mar, as the man left and he reentered the secret passage.
While he was conferring, at the library window appeared a face. It was Professor Arnold's. Cautiously he opened the window and listened. Then he entered.
First he went over to the door and set a chair under the knob. Next he drew an electric pocket bull's-eye and flashed it about the room. He glanced about and finally went over to Del Mar's desk where he examined a batch of letters, his back to the secret panel.
Arnold was running rapidly through the papers on the desk, as he flashed his electric bull's-eye on them, when the panel in the wall opened slowly and Del Mar stepped into the room noiselessly. To his surprise he saw a round spot of light from an electric flashlight focussed on his desk. Some one was there! He drew a gun.
Arnold started suddenly. He heard the cocking of a revolver. But he did not look around. He merely thought an instant, quicker than lightning, then pulled out a spool of black thread with one hand, while with the other he switched off the light, and dived down on his stomach on the floor in the shadow.
"Who's that?" demanded Del Mar. "Confound it! I should have fired at sight."
The room was so dark now that it was impossible to see Arnold. Del Mar gazed intently. Suddenly Arnold's electric torch glowed forth in a spot across the room.
Del Mar blazed at it, firing every chamber of his revolver, then switched on the lights.
No one was in the room. But the door was open. Del Mar gazed about, vexed, then ran to the open door.
For a second or two he peered out in rage, finally turning back into the empty room. On the mantlepiece lay the torch of the intruder. It was one in which the connection is made by a ring falling on a piece of metal. The ring had been left up by Arnold. Connection had been made as he was leaving the room by pulling the thread which he had fastened to the ring. Del Mar followed the thread as it led around the room to the doorway.