Otto entered and handed a visiting card to the criminologist. It bore the engraved name - Clifford Gage.

Doctor Zerndorff studied it impatiently.

"Tell him I am busy, Otto."

"He says it is very important, sir," said the attendant, in German.

Doctor Zerndorff glanced at the card again. It was then that he noted something peculiar. Across the face of the card lay a light gray shadow, extending diagonally from one corner to the other.

Doctor Zerndorff moved the card beneath the light, but the shadow did not change. Then it faded away.

Zerndorff blinked. He stared at the white wall of the laboratory, to see if his vision was failing him.

Satisfied that his eyes had not deceived him, he smiled.

"Bring him into the reception room, Otto," he said.

Doctor Zerndorff concluded his experiments for the evening. He took off his working coat and put on his dressing gown.

Thus, informally attired, he went into the reception room to meet his visitor. He took a chair opposite Clifford Gage, who accepted a cigar.

"Doctor Zerndorff," remarked Gage, "I have come to discuss important matters. A few nights ago a man - a friend of mine - who called himself Henry Arnaud - escaped arrest for a murder which he did not commit. "The real criminal has now confessed his crime and is dead. I refer to Killer Bryan."

Zerndorff nodded.

"This Mr. Arnaud," continued Gage, in a quiet voice, "wished to discuss certain matters with you.

Inasmuch as his whereabouts are now unknown, I have come in his stead."

Zerndorff smiled and bowed.

"I have learned," said Gage, "that you are a man of great capacity, unrestricted by the usual limitations that surround those who are connected with police departments."

"I thank you, Mr. Gage."

There was a peculiar tone in Doctor Zerndorff's voice. The words did not reveal his actual thoughts. The tone was different from Zerndorff's customary speech. It might have betokened keen interest; or it might have been tinged with irony.

Clifford Gage was momentarily thoughtful, then he became definitely confidential.

"Doctor Zerndorff," he said, "I have discovered new and important angles that concern the bombings which you have been investigating!

"It is my usual procedure to keep my findings to myself. In this instance, there are reasons why I have chosen to confide in you. I have come to offer you my full cooperation!"

The criminologist raised his eyebrows. He was interested now. He suspected the ident.i.ty of this stranger.

Intuitively, he linked him with the mysterious man known as The Shadow. He had every reason to suppose that Clifford Gage was The Shadow himself.

"This interests me," he said. "It is most interesting, yes. I shall promise to you this: whatever you may say to me, no one else shall hear!"

Clifford Gage reached forward and the men shook hands. It was an action of mutual understanding.

The piercing gaze of the visitor met the stern glance of the German professor. Each man knew that he had found a coworker worthy of himself.

"Let me ask you, doctor," said Gage, settling back in his chair. "What is your full theory regarding these explosions? Is it exactly as stated in the newspapers?"

Doctor Zerndorff nodded.

"It is quite plain," he replied. "Plain to me, yes. It is the work of those two men - Sforza and Pecherkin - who have been in this country so long, now, waiting until they could do as they have done in Russia and in Italy."

"Then you believe that the bombs were placed in vital spots about New York, simply to create terrorism - as an aftermath of the unsuccessful May Day activities."

"That is just so!"

"Do you link any other crimes with the bombings?" Gage questioned. "No."

"What of the murder of Perry Warfield? Why was he killed? Why was Matthew Stokes murdered?"

Doctor Zerndorff shrugged his shoulders despairingly.

"There are many such killings in New York," he said. "It is incredible to me, Mr. Gage, that you should speak of them. Simply because they have happened soon after those big explosions is not a reason why we should connect them!"

Clifford Gage rose from his chair and walked slowly across the floor. He reached the window and stood there.

Doctor Zerndorff noted the slimness of his tall form and could not help comparing it with the curtain by the window. He could almost fancy the man within the folds of that hanging drapery.

Then his eyes dropped to the floor and he became interested in the long, weird shadow that stretched across the floor, almost a perfect silhouette of the standing man.

"The men who placed the bombs have been arrested," Gage was speaking in a thoughtful, faraway voice.

"They have virtually confessed. But not to any connection with either Sforza or Pecherkin.

"They have spoken of a mysterious master, who has controlled them all, as individuals. Who is this man, whom they know and yet do not know? Is he one - or many?"

"He is two," answered Zerndorff.

"Two?" questioned Gage.

"Yes! Two, yes! Sforza and Pecherkin!" Zerndorff raised his right hand and extended two fingers. "Those are two men; very bad men, yes. I have known of them before. They have worked in the dark, as the masters of those who have put the bombs where they would explode. That is the answer, yes!"

"Before Perry Warfield died" - Gage was leaning against the wall as he spoke - "in fact, when Warfield was dying, he spoke of a man whom he called The Master.

"He referred to him even more specifically. He called him The Black Master. It was The Black Master whom he feared.

"Did you know that, doctor?"

The criminologist shook his head. His chin was on his hand, his elbow on the arm of his chair. He was listening with intense interest. Clifford Gage was speaking in a voice that carried a most convincing tone.

"Perry Warfield was murdered by Killer Bryan," continued Gage. "He was shot at the very moment set for him to die. That is proof that Killer Bryan was an agent of The Black Master.

"Before he died, Perry Warfield revealed that another man was marked for death. He named Matthew Stokes. That man was killed the same night!

"Again the murderer was Killer Bryan!"

Doctor Zerndorff nodded slowly after the speaker paused. He was impressed by the words that Clifford Gage had uttered. His keen mind was working on these same problems. He looked at Gage again, as though he understood that the man had more to tell.

"Perry Warfield named another intended victim," said Gage quietly. "A third man was to die at Killer Bryan's hand! Fortunately, that murder has been forestalled; and you were responsible for it, Doctor Zerndorff!

"You were forced to end the life of Killer Bryan. By so doing, you saved the life of Hubert Banks, a millionaire who was to have been Bryan's third victim!"

"This is wonderful, yes!" exclaimed Doctor Zerndorff. "I have not supposed it for one minute. The detective - Herr Cardona - has told me that it was a killing of the - what is it that you call them - the gangsters, yes?

"I believe now that you have found some more important reason. Is there other proof, yes?"

"When Killer Bryan lay dying," said Gage quietly, "he confessed that he had murdered Warfield and Stokes. It is an unwritten law of gangland that gunmen remain silent until death. Why did Bryan confess?"

"I do not know."

"I shall tell you why. It was because someone showed him - this!"

Clifford Gage strode across the room. He extended his arm and opened his hand before Doctor Zerndorff's surprised eyes.

In the visitor's palm lay an oval-shaped disk of thin metal, painted black. Gage placed it in Zerndorff's hand. The criminologist examined it with eager interest.

"What is this?" he asked.

"It is the token of The Black Master!" said Gage. "It was given by Perry Warfield to - to a friend - before he died!"

Doctor Zerndorff studied the disk for several minutes. Then he looked up and returned the token.

"Keep this, yes," said Zerndorff. "It may be of important use. You have found something very valuable.

My brain is thinking, yes - thinking much." He tapped his forehead. "Sit down, my friend, sit down, while I discover something!"

He went to the telephone. Clifford Gage heard him call police headquarters. Doctor Zerndorff was connected with Detective Joe Cardona.

"Ah!" he said. "Listen, my friend and tell me this. Did you make one search of that man who died in that hospital? The man that was called the Killer Bryan?

"Ah, you did, yes? And did you find a piece of metal that was flat and like the shape of an egg? Yes, it was all black. Ah, you did? Will you bring it to me tomorrow? It is important, yes."

Doctor Zerndorff hung up the receiver and beamed as he turned to his visitor. His eyes sparkled with the delight of the scientist who has made a new discovery.

He placed his hands together and clasped them, as though congratulating the man who had led him to this trial. He sat down and spoke thoughtfully.

"Tomorrow," he said, "I, too, shall have that token to study. You must keep the one that you have.Together we shall work. It is a mystery, yes.

"You may be right that it has connection with these bombings. And yet" - his voice was doubtful - "you may also be wrong! Unless you have other things to tell me, yes?"

Clifford Gage smiled at the subtle inquiry.

"We're getting there now, Doctor Zerndorff," he replied. "You are right. I have more to tell. I have proof to offer.

"I have been to see Hubert Banks - the man whose life was threatened. He recognized me as an old friend whom he had not seen for fifteen years. He is melancholy, because four men have betrayed him; and those four have all died suddenly. Perry Warfield is one!"

"Ah, yes? That is so? And who are the others?"

"One was named Houston; another Pennypacker; the third was a man named Houghton."

"I have never heard those names."

"That is strange," said Gage, with a smile.

"Strange, yes? Why?"

Gage puffed on his cigar. It had gone out. He relighted it before he replied. Then he looked steadily at Doctor Zerndorff as though he knew that his words would bring amazement to the criminologist.

"Those three men died suddenly, the same day," he said. "One was killed by an explosion in Wall Street.

The second perished when a bomb went off in Grand Central Station. The third died in the subway at Columbus Circle!"

Gage was not disappointed. Utter bewilderment came over Doctor Zerndorff. His lips parted and he tried to speak. But he was too confused to utter a single word. Gage came to his rescue.

"So of four men," he said, "only one was spared that day - and that man was Perry Warfield! Each died alone.

"But there is something else that I must tell you. Perry Warfield did not come to New York that day. He was at home, ill. But he was here the next morning - and he went to the office of Barr Childs, in the Financial Building."

"Where the unexploded bomb was found, yes?"

"Exactly. Once again, he escaped death. So Killer Bryan settled him!

"Now, Doctor Zerndorff, you understand why I consider The Black Master - whoever he may be - to be the man behind these crimes."

Doctor Zerndorff nodded. Then he looked sharply at Clifford Gage. An expression of doubt pa.s.sed over his features, as though he suspected that this amazing visitor had discovered too much in so short a time.

Gage detected the look and smiled.

"I agree with you, Doctor Zerndorff," he said, as though reading the other man's thoughts. "Two things are quite possible. First, that my findings are too unreal to be true; that I have told you all this merely to confuse you. "Second, that I have spoken the absolute truth, but that I, too, am an agent of The Black Master - or, perhaps, the man himself.

"But both of those theories are incorrect. I am just what I claim to be - a man who through careful observation and good fortune has been able to uncover the machinations of a most desperate criminal - The Black Master!"

Doctor Zerndorff watched Gage carefully. He seemed to be weighing the man's words. He nodded slowly.

"I believe you," he said.

"But there is still one question in your mind," added Gage.

Again Doctor Zerndorff nodded. He was amazed at the man's perceptiveness.

"I can tell you that question!" said Gage. "You are wondering why I have come to you. Since I have done so well alone, why should I confide in you? That is what you are thinking, is it not?"