“I’ll send the dog first,” said M. Paul.  “Here, Caesar! Cherche!”

Once more the eager animal sprang forward, following slowly along the row of trees where the trail was confused, and then, at the corner, dashing ahead swiftly, only to stop again after a few yards and stand scratching uneasily at a closed door.

“That settles it,” said Coquenil.  “He has brought us to the alleyway door.  Am I right?”

“Yes,” nodded Gritz.

“The door that leads to Number Seven?”

“Yes.”

“Open it,” and, while the agitated proprietor searched for his pass key, the detective spoke to Tignol:  “I want impressions of these footprints, the best you can take.  Use glycerin with plaster of Paris for the molds.  Take this one and these two and this and this.  Understand?”

“Perfectly.”

“Leave Caesar here while you go for what you need.  Down, Caesar! Garde!”

The dog growled and went on guard forthwith.

“Now, we’ll have a look inside.”

The alleyway door stood open and, using his lantern with the utmost care, Coquenil went first, mounting the stairs slowly, followed by Gritz.  At the top they came to a narrow landing and a closed door.

“This opens directly into Number Seven?” asked the detective.

“Yes.”

“Is it usually locked or unlocked?”

“IT is always locked.”

“Well, it’s unlocked now,” observed Coquenil, trying the knob.  Then, flashing his lantern forward, he threw the door wide open.  The room was empty.

“Let me turn up the electrics,” said the proprietor, and he did so, showing furnishings like those in Number Six except that here the prevailing tint was pale blue while there it was pale yellow.

“I see nothing wrong,” remarked M. Paul, glancing about sharply.  “Do you?”

“Nothing.”

“Except that this door into the corridor is bolted.  It didn’t bolt itself, did it?”

“No,” sighed the other.

Coquenil thought a moment, then he produced the pistol found in the courtyard and examined it with extreme care, then he unlocked the corridor door and looked out.  The policeman was still on guard before Number Six.

“I shall want to go in there shortly,” said the detective.  The policeman saluted wearily.

“Excuse me,” ventured M. Gritz, “have you still much to do?”

“Yes,” said the other dryly.

“It’s nearly four and—­I suppose you are used to this sort of thing, but I’m knocked out, I—­I’d like to go to bed.”

“By all means, my dear sir.  I shall get on all right now if—­oh, they tell me you make wonderful Turkish coffee here.  Do you suppose I could have some?”

“Of course you can.  I’ll send it at once.”

“You’ll earn my lasting gratitude.”

Gritz hesitated a moment and then, with an apprehensive look in his beady eyes, he said:  “So you’re going in there?” and he jerked his fat thumb toward the wall separating them from Number Six.