As he did so he passed a Mexican attired in brilliant native costume.  At a sign from Del Mar he paused and received a small package which Del Mar slipped to him, then passed on as though nothing had happened.  The keen eyes of the gray friar, however, had caught the little action and he quietly slipped out after the Mexican bolero.

Just then the domino girl hurried into the conservatory.  “What’s doing?” she asked eagerly.

“Keep close to me,” whispered Del Mar, as she nodded and they left the conservatory, not apparently together.

Up-stairs, away from the gayety of the ballroom, the bolero made his way until he came to Elaine’s room, dimly lighted.  With a quick glance about, he entered cautiously, closed the door, and approached a closet which he opened.  There was a safe built into the wall.

As he stooped over, the man unwrapped the package Del Mar had handed him and took out a curious little instrument.  Inside was a dry battery and a most peculiar instrument, something like a little flat telephone transmitter, yet attached by wires to ear-pieces that fitted over the head after the manner of those of a wireless detector.

He adjusted the head-piece and held the flat instrument against the safe, close to the combination which he began to turn slowly.  It was a burglar’s microphone, used for picking combination locks.  As the combination turned, a slight sound was made when the proper number came opposite the working point.  Imperceptible ordinarily to even the most sensitive ear, to an ear trained it was comparatively easy to recognize the fall of the tumblers over this microphone.

As he worked, the door behind him opened softly and the gray friar entered, closing it and moving noiselessly over back of the shelter of a big mahogany high-boy, around which he could watch.

At last the safe was opened.  Rapidly the man went through its contents.  “Confound it!” he muttered.  “She didn’t put it here—­ anyhow.”

The bolero started to close the safe when he heard a noise in the room and looked cautiously back of him.  Del Mar himself, followed by the domino girl, entered.

“I’ve opened it,” whispered the emissary stepping out of the closet and meeting them, “but I can’t find the—­”

“Hands up—­all of you!”

They turned in time to see the gray friar’s gun yawning at them.  Most politely he lined them up.  Still holding his gun ready, he lifted up the mask of the domino girl.

“So—­it’s you,” he grunted.

He was about to lift the mask of the Mexican, when the bolero leaped at him.  Del Mar piled in.  But sounds down-stairs alarmed them and the emissary, released, fled quickly with the girl.  The gray friar, however, kept his hold on Mephistopheles, as if he had been wrestling with a veritable devil.

. . . . . . .

Down in the hall, I had again met my domino girl, a few minutes after I had resigned Elaine to another of her numerous admirers.