"Je vous demande pardon, mon ami. I wished to see if the door was really bolted on this side also."

"Well," I said, glancing at the door which communicated with the room we had just left, "it is bolted." Poirot nodded. He still seemed to be thinking.

"And, anyway," I continued, "what does it matter? The case is over. I wish you'd had more chance of distinguishing yourself. But it was the kind of case that even a stiff-backed idiot like that inspector couldn't go wrong over."

Poirot shook his head.

"The case is not over, my friend. It will not be over until we find out who stole the pearls."

"But the maid did!"

"Why do you say that?"

"Why," I stammered, "they were found - actually in her mattress."

"Ta, ta, ta!" said Poirot impatiently. "Those were not the pearls."

"What?"

"Imitation, mon ami."

The statement took my breath away. Poirot was smiling placidly.

"The good inspector obviously knows nothing of jewels. But presently there will be a fine hullabaloo!"

"Come!" I cried, dragging at his arm.

"Where?"

"We must tell the Opalsens at once."

"I think not."

"But that poor woman -"

"Eh bien; that poor woman, as you call her, will have a much better night believing the jewels to be safe."

"But the thief may escape with them!"

"As usual, my friend, you speak without reflection. How do you know that the pearls Mrs Opalsen locked up so carefully tonight were not the false ones, and that the real robbery did not take place at a much earlier date?"

"Oh!" I said bewildered.

"Exactly," said Poirot, beaming. "We start again."

He led the way out of the room, paused a moment as though considering, and then walked down to the end of the corridor, stopping outside the small den where the chambermaids and valets of the respective floors congregated. Our particular chambermaid appeared to be holding a small court there, and to be retailing her late experiences to an appreciative audience. She stopped in the middle of a sentence. Poirot bowed with his usual politeness.

"Excuse that I derange you, but I shall be obliged if you will unlock for me the door of Mr Opalsen's room."

The woman rose willingly, and we accompanied her down the passage again. Mr Opalsen's room was on the other side of the corridor, its door facing that of his wife's room. The chambermaid unlocked it with her pass-key, and we entered.

As she was about to depart Poirot detained her.

"One moment; have you ever seen among the effects of Mr Opalsen a card like this?" He held out a plain white card, rather highly glazed and uncommon in appearance. The maid took it and scrutinized it carefully.

"No, sir, I can't say I have. But, anyway, the valet has most to do with the gentlemen's rooms."

"I see. Thank you."

Poirot took back the card. The woman departed. Poirot appeared to reflect a little. Then he gave a short, sharp nod of the head.