“Go, then,” replied M. Segmuller, “and good luck to you!”

Good luck!  Yes, indeed, Lecoq looked for it.  If up to the present moment he had taken his successive defeats good-humoredly, it was because he believed that he had a talisman in his pocket which was bound to insure ultimate victory.

“I shall be very stupid if I can’t discover the owner of such a valuable jewel,” he soliloquized, referring to the diamond earring.  “And when I find the owner I shall at the same time discover our mysterious prisoner’s identity.”

The first step to be taken was to ascertain whom the earring had been bought from.  It would naturally be a tedious process to go from jeweler to jeweler and ask:  “Do you know this jewel, was it set by you, and if so whom did you sell it to?” But fortunately Lecoq was acquainted with a man whose knowledge of the trade might at once throw light on the matter.  This individual was an old Hollander, named Van Numen, who as a connoisseur in precious stones, was probably without his rival in Paris.  He was employed by the Prefecture of Police as an expert in all such matters.  He was considered rich.  Despite his shabby appearance, he was rightly considered rich, and, in point of fact, he was indeed far more wealthy than people generally supposed.  Diamonds were his especial passion, and he always had several in his pocket, in a little box which he would pull out and open at least a dozen times an hour, just as a snuff-taker continually produces his snuffbox.

This worthy man greeted Lecoq very affably.  He put on his glasses, examined the jewel with a grimace of satisfaction, and, in the tone of an oracle, remarked:  “That stone is worth eight thousand francs, and it was set by Doisty, in the Rue de la Paix.”

Twenty minutes later Lecoq entered this well-known jeweler’s establishment.  Van Numen had not been mistaken.  Doisty immediately recognized the earring, which had, indeed, come from his shop.  But whom had he sold it to?  He could not recollect, for it had passed out of his hands three or four years before.

“Wait a moment though,” said he, “I will just ask my wife, who has a wonderful memory.”

Madame Doisty truly deserved this eulogium.  A single glance at the jewel enabled her to say that she had seen this earring before, and that the pair had been purchased from them by the Marchioness d’Arlange.

“You must recollect,” she added, turning to her husband, “that the Marchioness only gave us nine thousand francs on account, and that we had all the trouble in the world to make her pay the balance.”

Her husband did remember this circumstance; and in recording his recollection, he exchanged a significant glance with his wife.

“Now,” said the detective, “I should like to have this marchioness’s address.”

“She lives in the Faubourg St. Germain,” replied Madame Doisty, “near the Esplanade des Invalides.”