Coquenil's examination of the pistol showed that it was a weapon of good make and that only a single shot had been fired from it; also that this shot had been fired within a few hours. Which, with the evidence of the seamstress and the dog, gave a strong probability that the instrument of the crime had been found. If the ball in the body corresponded with balls still in the pistol, this probability would become a practical certainty. And yet, the detective knit his brows. Suppose it was established beyond a doubt that this pistol killed the billiard player, there still remained the question how the shooting was accomplished. The murderer was in Number Seven, he could not and did not go into the corridor, for the corridor door was locked. But the billiard player was in Number Six, he was shot in Number Six, and he died in Number Six. How were these two facts to be reconciled? The seamstress's testimony alone might be put aside but not the dog's testimony. The murderer certainly remained in Number Seven.

Holding this conviction, the detective entered the room of the tragedy and turned up the lights, all of them, so that he might see whatever was to be seen. He walked back and forth examining the carpet, examining the walls, examining the furniture, but paying little heed to the body. He went to the open window and looked out, he went to the yellow sofa and sat down, finally he shut off the lights and withdrew softly, closing the door behind him. It was just as the commissary had said with the exception of one thing.

When he returned to Number Seven, M. Paul found that Gritz had kept his promise and sent him a pot of fragrant Turkish coffee, steaming hot, and a box of the choicest Egyptian cigarettes. Ah, that was kind! This was something like it! And, piling up cushions in the sofa corner, Coquenil settled back comfortably to think and dream. This was the time he loved best, these precious silent hours when the city slept and his mind became most active—this was the time when chiefly he received those flashes of inspiration or intuition that had so often and so wonderfully guided him.

For half an hour or so the detective smoked continuously and sipped the powdered delight of Stamboul, his gaze moving about the room in friendly scrutiny as if he would, by patience and good nature, persuade the walls or, chairs to give up their secret. Presently he took off his glasses and, leaning farther back against the cushions, closed his eyes in pleasant meditation. Or was it a brief snatch of sleep? Whichever it was, a discreet knock at the corridor door shortly ended it, and Papa Tignol entered to say that he had finished the footprint molds.

M. Paul roused himself with an effort and, sitting up, his elbow resting against the sofa back, motioned his associate to a chair.

"By the way," he asked, "what do you think of that?" He pointed to a Japanese print in a black frame that hung near the massive sideboard.

"Why," stammered Tignol, "I—I don't think anything of it."

"A rather interesting picture," smiled the other. "I've been studying it."

"A purple sea, a blue moon, and a red fish—it looks crazy to me," muttered the old agent.

Coquenil laughed at this candid judgment. "All the same, it has a bearing on our investigations."

"Diable!"

M. Paul reached for his glasses, rubbed them deliberately and put them on. "Papa Tignol," he said seriously, "I have come to a conclusion about this crime, but I haven't verified it. I am now going to give myself an intellectual treat."

"Wha-at?"

"I am going to prove practically whether my mind has grown rusty in the last two years."

"I wish you'd say things so a plain man can understand 'em," grumbled the other.

"You understand that we are in private room Number Seven, don't you? On the other side of that wall is private room Number Six where a man has just been shot. We know that, don't we? But the man who shot him was in this room, the little hair-brushing old maid saw the pistol thrown from this window, the dog found footprints coming from this room, the murderer went out through that door into the alleyway and then into the street. He couldn't have gone into the corridor because the door was locked on the outside."

"He might have gone into the corridor and locked the door after him," objected Tignol.

Coquenil shook his head. "He could have locked the door after him on the outside, not on the inside; but when we came in here, it was locked on the inside. No, sir, that door to the corridor has not been used this evening. The murderer bolted it on the inside when he entered from the alleyway and it wasn't unbolted until I unbolted it myself."

"Then how, in Heaven's name——"

"Exactly! How could a man in this room kill a man in the next room? That is the problem I have been working at for an hour. And I believe I have solved it. Listen. Between these rooms is a solid wooden partition with no door in it—no passageway of any kind. Yet the man in there is dead, we're sure of that. The pistol was here, the bullet went there—somehow. How did it go there? Think."

The detective paused and looked fixedly at the wall near the heavy sideboard. Tignol, half fascinated, stared at the same spot, and then, as a new idea took form in his brain, he blurted out: "You mean it went through the wall?"

"Is there any other way?"

The old man laid a perplexed forefinger along his illuminated nose. "But there is no hole—through the wall," he muttered.

"There is either a hole or a miracle. And between the two, I conclude that there is a hole which we haven't found yet."

"It might be back of that sideboard," ventured the other doubtfully.

But M. Paul disagreed. "No man as clever as this fellow would have moved a heavy piece covered with plates and glasses. Besides, if the sideboard had been moved, there would be marks on the floor and there are none. Now you understand why I'm interested in that Japanese print."

Tignol sprang to his feet, then checked himself with a half-ashamed smile.

"You're mocking me, you've looked behind the picture."

Coquenil shook his head solemnly. "On my honor, I have not been near the picture, I know nothing about the picture, but unless there is some flaw in my reasoning——"

"I'll give my tongue to the cats to eat!" burst out the other, "if ever I saw a man lie on a sofa and blow blue circles in the air and spin pretty theories about what is back of a picture when——"

"When what?"

"When all he had to do for proof was to reach over and—and lift the darn thing off its nail."

Coquenil smiled. "I've thought of that," he drawled, "but I like the suspense. Half the charm of life is in suspense, Papa Tignol. However, you have a practical mind, so go ahead, lift it off."

The old man did not wait for a second bidding, he stepped forward quickly and took down the picture.

"Tonnere de Dieu!" he cried. "It's true! There are two holes."

Sure enough, against the white wall stood out not one but two black holes about an inch in diameter and something less than three inches apart. Around the left hole, which was close to the sideboard, were black dots sprinkled over the painted woodwork like grains of pepper.

"Powder marks!" muttered Coquenil, examining the hole. "He fired at close range as Martinez looked into this room from the other side. Poor chap! That's how he was shot in the eye." And producing a magnifying glass, the detective made a long and careful examination of the holes while Papa Tignol watched him with unqualified disgust.

"Asses! Idiots! That's what we are," muttered the old man. "For half an hour we were in that room, Gibelin and I, and we never found those holes."

"They were covered by the sofa hangings."

"I know, we shook those hangings, we pressed against them, we did everything but look behind them. See here, did you look behind them?"

"No, but I saw something on the floor that gave me an idea."

"Ah, what was that?"

"Some yellowish dust. I picked up a little of it. There." He unfolded a paper and showed a few grains of coarse brownish powder. "You see there are only board partitions between these rooms, the boards are about an inch thick, so a sharp auger would make the holes quickly. But there would be dust and chips."

"Of course."

"Well, this is some of the dust. The woman probably threw the chips out of the window."

"The woman?"

Coquenil nodded. "She helped Martinez while he bored the holes."

Tignol listened in amazement. "You think Martinez bored those holes? The man who was murdered?"

"Undoubtedly. The spirals from the auger blade inside the holes show plainly that the boring was done from Number Six toward Number Seven. Take the glass and see for yourself."

Tignol took the glass and studied the hole. Then he turned, shaking his head. "You're a fine detective, M. Paul, but I was a carpenter for six years before I went on the force and I know more about auger holes than you do. I say you can't be sure which side of the wall this hole was bored from. You talk about spirals, but there's no sense in that. They're the same either way. You might tell by the chipping, but this is hard wood covered with thick enamel, so there's apt to be no chipping. Anyhow, there's none here. We'll see on the other side."

"All right, we'll see," consented Coquenil, and they went around into Number Six.

The old man drew back the sofa hangings and exposed two holes exactly like the others—in fact, the same holes. "You see," he went on, "the edges are clean, without a sign of chipping. There is no more reason to say that these holes were bored this side than from that."

M. Paul made no reply, but going to the sofa he knelt down by it, and using his glass, proceeded to go over its surface with infinite care.

"Turn up all the lights," he said. "That's better," and he continued his search. "Ah!" he cried presently. "You think there is no reason to say the holes were bored from this side. I'll give you a reason. Take this piece of white paper and make me prints of his boot heels." He pointed to the body. "Take the whole heel carefully, then the other one, get the nail marks, everything. That's right. Now cut out the prints. Good! Now look here. Kneel down. Take the glass. There on the yellow satin, by the tail of that silver bird. Do you see? Now compare the heel prints."

Papa Tignol knelt down as directed and examined the sofa seat, which was covered with a piece of Chinese embroidery.

"Sapristi! You're a magician!" he cried in great excitement.

"No," replied Coquenil, "it's perfectly simple. These holes in the wall are five feet above the floor. And I'm enough of a carpenter, Papa Tignol," he smiled, "to know that a man cannot work an auger at that height without standing on something. And here was the very thing for him to stand on, a sofa just in place. So, if Martinez bored these holes, he stood on this sofa to do it, and, in that case, the marks of his heels must have remained on the delicate satin. And here they are."

"Yes, here they are, nails and all," admitted Tignol admiringly. "I'm an old fool, but—but——"

"Well?"

"Tell me why Martinez did it."

Coquenil's face darkened. "Ah, that's the question. We'll know that when we talk to the woman."

The old man leaned forward eagerly: "Why do you think the woman helped him?"

"Somebody helped him or the chips would still be there, somebody held back those hangings while he worked the auger, and somebody carried the auger away."

Tignol pondered this, a moment, then, his face brightening: "Hah! I see! The sofa hangings were held back when the shot came, then they fell into place and covered the holes?"

"That's it," replied the detective absently.

"And the man in Number Seven, the murderer, lifted that picture from its nail before shooting and then put it back on the nail after shooting?"

"Yes, yes," agreed M. Paul. Already he was far away on a new line of thought, while the other was still grappling with his first surprise.

"Then this murderer must have known that the billiard player was going to bore these holes," went on Papa Tignol half to himself. "He must have been waiting in Number Seven, he must have stood there with his pistol ready while the holes were coming through, he must have let Martinez finish one hole and then bore the other, he must have kept Number Seven dark so they couldn't see him——"

"A good point, that," approved Coquenil, paying attention. "He certainly kept Number Seven dark."

"And he probably looked into Number Six through the first hole while Martinez was boring the second. I suppose you can tell which of the two holes was bored first?" chuckled Tignol.

M. Paul started, paused in a flash of thought, and then, with sudden eagerness: "I see, that's it!"

"What's it?" gasped the other.

"He bored this hole first," said Coquenil rapidly, "it's the right-hand one when you're in this room, the left-hand one when you're in Number Seven. As you say, the murderer looked through the first hole while he waited for the second to be bored; so, naturally, he fired through the hole where his eye was. That was his first great mistake."

Tignol screwed up his face in perplexity. "What difference does it make which hole the man fired through so long as he shot straight and got away?"

"What difference? Just this difference, that, by firing through the left-hand hole, he has given us precious evidence, against him."

"How?"

"Come back into the other room and I'll show you." And, when they had returned to Number Seven, he continued: "Take the pistol. Pretend you are the murderer. You've been waiting your moment, holding your breath on one side of the wall while the auger grinds through from the other. The first hole is finished. You see the point of the auger as it comes through the second, now the wood breaks and a length of turning steel shoves toward you. You grip your pistol and look through the left-hand hole, you see the woman holding back the curtains, you see Martinez draw out the auger from the right-hand hole and lay it down. Now he leans forward, pressing his face to the completed eyeholes, you see the whites of his eyes, not three inches away. Quick! Pistol up! Ready to fire! No, no, through the left-hand hole where he fired."

"SacrÉ matin!" muttered Tignol, "it's awkward aiming through this left-hand hole."

"Ah!" said the detective. "Why is it awkward?"

"Because it's too near the sideboard. I can't get my eye there to sight along the pistol barrel."

"You mean your right eye?"

"Of course."

"Could you get your left eye there?"

"Yes, but if I aimed with my left eye I'd have to fire with my left hand and I couldn't hit a cow that way."

Coquenil looked at Tignol steadily. "You could if you were a left-handed man."

"You mean to say—" The other stared.

"I mean to say that this man, at a critical moment, fired through that awkward hole near the sideboard when he might just as well have fired through the other hole away from the sideboard. Which shows that it was an easy and natural thing for him to do, consequently——"

"Consequently," exulted the old man, "we've got to look for a left-handed murderer, is that it?"

"What do you think?" smiled the detective.

Papa Tignol paused, and then, bobbing his head in comical seriousness: "I think, if I were this man, I'd sooner have the devil after me than Paul Coquenil."