She paused at the door and listened again. She could hear voices but this time it was impossible to catch even a word.

Penny moved on to the next door. She gently turned the k.n.o.b. The door was locked. So were all the others along the corridor until she came to the last one.

To Penny's surprise, it opened. Cautiously, she peeped inside. The room appeared to be empty. She entered.

It was only a small office, empty of furniture. A few papers were scattered over the bare floor, but upon examination Penny found them of no significance. It was clear that if she were to learn anything of value, she must find a means of entering the room where Hanley Cron, the ex-museum worker and the others were talking.

An inside door opened into an adjoining room. Penny was elated to find it unlocked. But her satisfaction was of short duration, for the next office likewise was empty and devoid of any clues.

By placing her ear against the north wall, she was able to hear the three men talking. It was provoking to be so close and yet unable to learn what they were saying. She felt convinced that if only she could hear their conversation, a great many puzzling matters might be cleared up.

Presently, Penny heard a door slam. She peeped out into the hallway in time to see Cron, Hoges and another man disappearing down the stairway.

"The coast is clear now!" she thought. "If I can just find some way to enter that room while they're away!"

She made another tour of the hall, trying the door. As she had antic.i.p.ated it was locked.

Returning to the room she had just left, she went to the window and looked out. A wide ledge of stone extended along the wall of the building, connecting the windows. At best it offered a dangerous footing. Yet Penny was tempted to try to reach the adjoining room by means of it, for there was no other way to gain admittance.

She raised the window and looked down. Her courage nearly failed her.

While the ledge was wide, it meant a long fall and instant death should she become dizzy and lose her balance.

"I can do it--easy," Penny told herself grimly.

Climbing out on the ledge, she clutched an overhanging telephone wire for support and cautiously eased herself along, an inch at a time. She kept her gaze ahead, resisting the temptation to glance toward the deserted street.

She reached the next window which was open an inch at the bottom. The gap provided a finger-hold and enabled her to raise the window. With a sigh of intense relief, she dropped lightly to the floor.

She found herself in a large, studio room, well illuminated by two sky lights. Obviously, several artists had been working there, for the place was cluttered with easels, palettes, and discarded paintings. A number of pictures of uniform size stood in a little pile, face downward.

Curiously, Penny lifted one to gaze at it.

"The stolen Rembrandt!" she gasped.

Then she knew better. It was only a copy, identical with the one she had viewed at Mrs. Dillon's home.

She lifted the other pictures and looked at them. They were all the same.

"So this is where Mrs. Dillon's fake came from!" she thought. "The men who rented this place apparently are manufacturing Rembrandts in wholesale quant.i.ties!"

At the other side of the room she noticed a picture which was only half finished, and beside it a canvas covered easel. She crossed over to lift the protecting cloth.

Still another Rembrandt was revealed.

"Just a copy," Penny told herself, and started to replace the canvas.

Then she looked at the picture again. It did not look exactly like the others. The detail was the same, yet this painting seemed to have a depth and quality which the others lacked. Penny wondered if it could be the original Rembrandt, the priceless painting which had been stolen from the Gage Galleries.

"I believe it is!" she decided.

As Penny stood gazing at the picture, she was dismayed to hear footsteps in the hallway. Frantically, she looked about for a hiding place.

It was too late to escape through the window. The only refuge available was a clothes closet.

Penny darted inside and softly shut the door. Scarcely had she secreted herself when three men entered the room. Peering out through the keyhole, she distinguished Cron, Hoges, and the man in gray whom she had once followed to the Franklyn Street address. Apparently, the men had returned for something they had forgotten. Hanley Cron searched in a table drawer.

"Say, who left that window open?" he demanded unexpectedly.

"I didn't," Hoges said.

"You can't blame me for it," the other man growled. "Probably you opened it yourself."

"I did not," Cron retorted. He crossed the room and slammed down the window. "Be careful about things like that. If we're not more cautious we'll have the cops on us."

"If you ask me, I think it's about time we blow," Hoges commented.

"This town is getting pretty hot for us."

"Maybe you're right," Cron muttered. "I had a disagreeable hour with that simple minded Mrs. Dillon. She's still afraid to notify the police, but that Nichols girl has been talking with her, and she may make us trouble."

"Christopher Nichols has been a.s.signed to the jewel case too," Hoges added. "He's no sloth when it comes to action!"

"Our game has just about played out," Cron agreed. "But I have one more good customer lined up. I told him to come here at one-thirty to see the picture."

"Maybe we could pull this last job," Hoges agreed. "Does he know much about painting?"

"Very little. We ought to nip him for three thousand at least."

Hoges glanced at his watch.

"If your customer is coming at one-thirty we'd better get the stage set."

"All right," Cron nodded. "Let's clean up the joint."

Uncovering the genuine Rembrandt, he took one of the copies, and deftly inserted it in the picture frame behind the original painting, but in such a manner that only the back of the canvas was visible. When the frame was replaced only a person with keen eyesight could detect the trickery.

"We'll pull the usual gag about identifying the picture with a signature or a symbol," Cron muttered. "That always goes big."

By this time Penny had seen enough to understand how Mrs. Dillon and other gullible customers had been duped. They had been shown the original stolen Rembrandt, but when invited to place an identifying mark on the back of the canvas to insure that they received the same picture, actually signed the fake copy. It was then a simple matter to remove the two paintings from the frame and send the customer the worthless one which bore his mark.

"Cron and his confederates have worked a fairly safe racket too," Penny thought. "Even if a customer learns he has been cheated, he's afraid to go to the police for fear he'll expose himself as a person willing to buy stolen property!"

She was not greatly surprised to learn that Cron was a party to the dishonest scheme, notwithstanding that Mrs. Dillon had denied the art critic was the mysterious agent who had visited her. Now Penny knew that the woman had not spoken the truth. Doubtlessly, she had feared to accuse Cron, lest he in turn expose her to the police.

A knock sounded on the door. Cron and his confederates froze into tense att.i.tudes, then relaxed.

"It must be our customer," Cron whispered. "Open the door."

As it swung back, Max Lynch stepped into the room. He smiled blandly.