The Prefect hesitated.  He was bound to reflect how greatly to Don Luis Perenna’s interest it was that none of the Mornington heirs should be discovered, or at least be able to come between him and the millions of the inheritance.  Was it safe to attribute to a noble sentiment of gratitude, to a lofty conception of friendship and duty, that strange longing to protect Hippolyte Fauville against the death that threatened him?

For some seconds M. Desmalions watched that resolute face, those intelligent eyes, at once innocent and satirical, grave and smiling, eyes through which you could certainly not penetrate their owner’s baffling individuality, but which nevertheless looked at you with an expression of absolute frankness and sincerity.  Then he called his secretary: 

“Has any one come from the detective office?”

“Yes, Monsieur le Prefet; Sergeant Mazeroux is here.”

“Please have him shown in.”

And, turning to Perenna: 

“Sergeant Mazeroux is one of our smartest detectives.  I used to employ him together with that poor Verot when I wanted any one more than ordinarily active and sharp.  He will be of great use to you.”

* * * * *

Sergeant Mazeroux entered.  He was a short, lean, wiry man, whose drooping moustache, heavy eyelids, watery eyes and long, lank hair gave him a most doleful appearance.

“Mazeroux,” said the Prefect, “you will have heard, by this time, of your comrade Verot’s death and of the horrible circumstances attending it.  We must now avenge him and prevent further crimes.  This gentleman, who knows the case from end to end, will explain all that is necessary.  You will work with him and report to me to-morrow morning.”

This meant giving a free hand to Don Luis Perenna and relying on his power of initiative and his perspicacity.  Don Luis bowed: 

“I thank you, Monsieur le Prefet.  I hope that you will have no reason to regret the trust which you are good enough to place in me.”

And, taking leave of M. Desmalions and Maitre Lepertuis, he went out with Sergeant Mazeroux.

As soon as they were outside, he told Mazeroux what he knew.  The detective seemed much impressed by his companion’s professional gifts and quite ready to be guided by his views.

They decided first to go to the Cafe du Pont-Neuf.  Here they learned that Inspector Verot, who was a regular customer of the place, had written a long letter there that morning.  And the waiter remembered that a man at the next table, who had entered the cafe at almost the same time as the inspector, had also asked for writing-paper and called twice for yellow envelopes.

“That’s it,” said Mazeroux to Don Luis.  “As you suspected, one letter has been substituted for the other.”

The description given by the waiter was pretty explicit:  a tall man, with a slight stoop, wearing a reddish-brown beard cut into a point, a tortoise-shell eyeglass with a black silk ribbon, and an ebony walking-stick with a handle shaped like a swan’s head.