He struck a match and looked beneath the couch; there was nothing there.  He ransacked about the studio for a few minutes, and then summoned his servant.

“Was there a stranger here at any time during the last two weeks?” he asked; “any person whom you did not know?”

Alphonse shook his head decidedly.

“There was no one, monsieur.  I am certain of that.”

“And my friends—­”

“On such occasions as monsieur’s friends called while he was out, I was in the studio as long as they remained.”

“Yes, of course.  When did you sweep under this couch?”

“About three weeks ago, monsieur,” was the hesitating reply.

“No less than that?”

“No less, monsieur.”

Jack was satisfied.  There was no room for suspicion, he told himself.  The man’s word was to be relied upon.  But by what agency, then, had the canvas disappeared?  How could a thief break into the studio without leaving some trace of his visit, in the shape of a broken window or a forced lock?  There had been plenty of opportunities, it is true—­nights when Alphonse had been at home and Jack in town.

“Has monsieur lost something?”

“Yes, a large painting has been stolen,” Jack replied.

He went to the door and examined the lock from the outside, by the aid of matches, though with no hope of finding anything.  But a surprising and ominous discovery rewarded him at once.  In and around the key-hole, sticking to it, were some minute fragments of wax.

“By Jove, I have it!” cried Jack.  “Here is the clew!  Look, Alphonse!  The scoundrel, whoever he was, took an impression in wax on his first visit.  He had a key made from it, came back later at night, and stole the picture.  It was a cunning piece of work.”

“Monsieur is right,” said Alphonse.  “A thief has robbed him.  You suspect nobody?”

“Not a soul,” replied Jack.

Though the shreds of wax showed how the studio had been entered, he was no nearer the solution of the mystery than before.  He excepted the few trustworthy friends—­only three or four—­who knew that he had the duplicate Rembrandt.

“And even in Paris there were not many who knew that I painted the thing,” he thought.  “I painted it at the Hotel Netherlands, and when Von Whele went home and left it on my hands, I locked the canvas up in an old chest.  No, I can’t suspect any of my friends, past or present.  But then who—­By Jove!  I have overlooked one point!  The man who stole the picture knew just where it was kept, and he went straight to it.  Otherwise he would have rummaged the studio, and disarranged things badly before he found what he wanted.”

A light flashed on Jack—­a light of inspiration, of certainty and conviction.  He remembered the visit of M. Felix Marchand, that he had commented on the painting, and had seen it restored to its place in the portfolio.  Beyond doubt the mysterious Frenchman was the thief.  Armed with his craftily-won knowledge, provided with a duplicate key to the studio, he had easily and safely accomplished his purpose.  At what hour, and on what night, it was impossible to say.  Probably a day or two after his first visit in the guise of a buyer.