"I did not meet the men personally. My friend took me there and showed me the picture."

"This same expert to whom you referred?"

"Yes."

"And yet you feel that his judgment was unbiased?"

"I do," Mrs. Dillon maintained loyally, "but I did not depend entirely upon his opinion. I am a very good judge of pictures myself."

"Has it occurred to you that possibly you did not receive the same painting which you purchased? I understand that sometimes art thieves prey upon innocent buyers by showing them a genuine picture and then delivering into their hands only a cheap copy."

"I am too shrewd to be so easily duped," Mrs. Dillon retorted. "I don't mind telling you that I protected myself against just such trickery."

"How?"

"When I viewed the picture and satisfied myself as to its quality, I marked the back of the canvas with a tiny symbol. In that way you see, another painting could not be subst.i.tuted, for the marking would be absent."

"The symbol might be duplicated."

"No, I would instantly detect the difference."

Penny sat lost in thought for a moment. She now understood the significance of the strange marking on the back of the Rembrandt which had puzzled Amy and herself. Was it possible that the Coulter girl had been mistaken in the quality of the painting?

"Mrs. Dillon," she said after a long silence, "you confidently believe that your painting is the same one which was stolen from the Gage Galleries?"

"All I know is that my picture is a genuine Rembrandt. I did not learn that a picture had been stolen from the museum until after I had made my purchase. I do not know even now that I have this same painting."

"In the event that it is the same, you wish to return it to the museum?"

Mrs. Dillon glared at Penny in frank dislike. She had been fairly trapped and knew it.

"Of course," she replied coldly. "I hope you do not think I would intentionally keep stolen property?"

"I thought you would see it that way," Penny declared, smiling. "And with your cooperation, the police should be able to capture the real culprits."

"What do you want me to do?"

"When will you see this agent with whom you dealt?"

"He is coming either today or tomorrow for the second payment."

"I don't need to advise you to refuse to give him any more money. But I wish you would try to learn from him the names of the original dealers who handled the picture."

"I'll try to find out."

"And another thing, Mrs. Dillon. You must notify the Gage Galleries immediately that you have the Rembrandt."

The woman made no response.

"You will do that?" Penny asked.

"Yes," Mrs. Dillon answered harshly.

"I'll see you again tomorrow," Penny said, arising to depart. "Until then you have my promise that I will not talk with the police."

"I have nothing to fear from them," Mrs. Dillon announced proudly.

"Not if you show a willingness to cooperate," Penny agreed. "When you think the matter over, I believe you will decide to reveal the name of your friend--the agent who negotiated the sale."

She waited an instant, hoping that Mrs. Dillon would reconsider. When the woman did not speak, she turned and walked from the living room, letting herself out the front door.

Emerging upon the street, Penny's first thought was to find a good hiding place where she could wait to view Mrs. Dillon's expected caller.

"I may have a tedious time of it," she reflected, "but if I learn the ident.i.ty of the agent with whom she dealt it will be worth all the trouble."

A half block away she noticed a large truck parked along the curbing.

The vehicle had been abandoned, a cracked-up front wheel giving mute evidence that it had been in an accident. The truck was of the closed cab type and it dawned upon Penny that if she could get inside, she would have a perfect observation post.

Luckily the cab of the truck had not been locked and she slipped into the driver's seat, slamming the door shut.

An hour pa.s.sed. The job of watching Mrs. Dillon's house became irksome. No one had called except a peddler and a delivery boy from a laundry.

Penny tried to pa.s.s the time by examining the many gadgets with which the great truck was equipped. She imagined that it might be loads of fun to drive such a powerful machine.

Suddenly her attention was arrested by an automobile which with a shrill screeching of brakes came to a halt in front of the Dillon residence. A well-dressed middle-aged man, carrying a black leather brief case, got out of the car.

Penny was sure she had never seen him before. She observed him closely as he emerged from his automobile. He crossed the street with a quick, energetic stride as if he knew just where he was going and what he intended doing after he arrived. She saw him standing patiently at Mrs. Dillon's door, waiting for a servant to answer his ring.

Was the man the agent Mrs. Dillon had mentioned? The rogue who had sold the fake painting to the gullible woman? He certainly did not look like a crook, Penny thought, nor did he act like one. Just one more reason, she decided, why she must take nothing for granted. She produced a notebook and pencil from her purse and made a careful notation of the stranger's automobile license number as well as its make and model.

For perhaps forty-five minutes the man remained inside the house. When he crossed the street to his car he skipped along with an agility surprising in a man of his years. He smiled broadly as if his mission, whatever it may have been, was successful. Scarcely had he driven away when another automobile swung into the same parking s.p.a.ce.

From her place of advantage, Penny fixed her attention on the newcomer, but before she could see his face, she was startled by a gruff voice, almost in her ear:

"Hey there! Come down out of that!"

A roughly dressed truck driver stood on the running board, gesturing angrily. "What do you think this truck is?" he demanded. "A free park seat?"

Penny hastily climbed out of the cab, making an offhand apology for her presence.

"Okay Miss," the truck driver said, "seein' as you're a gal. But if you had been a man, I would have taken a fall out of ya. It's a crime that a man can't go for help without having some strange sister cuddle down in his cab."

The trucker's loud, gruff voice had attracted the attention of the man in the parked automobile. He stepped from his car and came toward the couple.

"What's the idea of abusing a helpless young girl?" he asked.

Penny recognized the voice, and resisted an impulse to turn her head.

She knew that the newcomer was Hanley Cron. He had come to call upon Mrs. Dillon. That was plain. She must not let him discover that she was watching the house. Quickly, before either of the men were aware of her intention, she darted behind the truck and fled down the street.