"Mazeroux," said the Prefect, "you will have heard, by this time, of your
comrade Vérot'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 Préfet. 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 Maître 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 Café du Pont-Neuf. Here they learned that
Inspector Vérot, 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 café 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