“What did you do when Warlock arrived?” questioned Barth, a bit suspiciously. “You had to leave the laboratory open then, didn’t you?”
“No,” replied the professor. “I had not opened the laboratory before his arrival. You see” — he waved his hand toward the door that led into the rear hall — “there is a lock also on that door, as well as a bolt. I have the key to open it. It is only when we are inside that I keep all the bolts closed.”
Barth nodded. He could see the closed bolt on the far door. The door to the office was also bolted.
Lessep had shot the bolt of the door through which they had entered.
“This is the key,” remarked Lessep, producing the object. “But it will be necessary, I now fear, to have a new lock placed on that one door.”
“Why?” asked Barth.
“Because,” replied the professor, “there is another key which I do not have.”
“Who has it?”
“Miles Crofton.”
BARTH’S eyes blinked through the pince-nez. The commissioner was about to make a comment, when Lessep saved him the trouble.
“Ah, yes,” affirmed the professor, in a troubled tone. “I have thought of what you wish to say. Crofton — that ungrateful assistant— still has access here. I was thinking about it, all this very afternoon. How he might even be here, watching me!”
As he spoke, the professor looked warily about. His eyes strained as though seeking view of an invisible object. Findlay Warlock nodded seriously.
“I had the same impression,” he declared. “Alone here, during the past few minutes. That of unseen eyes—”
“It’s ridiculous,” Marryat Darring snorted the interruption. “Come, come! This is carrying a hoax too far. What about to-night’s experiment? Are we going through with it, professor?”
“Certainly,” replied Lessep, in a crackly tone. “If you so wish. It was at one o’clock this afternoon that I came into this laboratory, to work until five. Hardly had I been here before the telephone rang in my office. It was my good friend here, Mr. Warlock.
“I told him that I would do the experiment as wished. I worked on then until five o’clock. Then I went out and locked the laboratory behind me. After having dinner at the restaurant, I came back. But I did not again open the laboratory until Mr. Warlock arrived.”
“The experiment is what interests us,” declared Darring, impatiently. “The commissioner wants you to perform it with Detective Cardona as the subject.”
“So I understand” — Lessep paused warily, to look at Cardona — “but it is very dangerous. I must tell you that beforehand. It is a risk—”
“I’m ready for it,” interrupted Cardona.
“And we’re ready to watch it,” added Darring.
“We must wait for Cranston,” remarked Barth. “He should be here shortly.”
“But that’s no reason why the professor can’t get started,” interposed Darring. “How about hooking up those motors, getting the machinery ready?”
“Proceed, professor,” ordered Barth, with a pompous nod.
Professor Lessep bowed. Methodically donning a white robe, he drew the glass cabinet from the far wall. Then he rolled up one electric motor and began to make attachments. It was while he was plugging in a cord that his face became troubled.
Leaning close to the motor, Lessep uttered a sudden exclamation. He blinked nervously as he looked up at the others; then made a sweeping motion with his hand, to indicate the screw-end of a bolt that projected from the machine.
“It is gone!” cried Lessep. “The connecting lever! The integral part of this machine. Some one has removed it without my knowledge!”
“How long ago?” quizzed Barth, sharply.
“Since after five o’clock,” replied Lessep, seriously. “I remember; I made one examination of this motor before I left for my dinner. He has been here!” Lessep’s voice rose shrilly. “Crofton has been here!”
“When?” demanded Barth.
“How could I know?” asked Lessep. “But I think — maybe — that I could guess. It must have been before one o’clock that he entered, with that key which he has.”
“Why before one?”
“So that he could have listened while I heard word from Mr. Warlock. Then, after I had gone, at five o’clock—”
“He wanted to block the experiment,” interposed Barth. “How much did you say over the telephone, professor, when you talked with Warlock?”
“A great deal. Too much, I fear.”
“A fine stall!” The sarcastic comment came from Darring. “I predicted this, commissioner. I told you, last night, that the professor would find some excuse. He has hidden the lever himself, I believe.”
“Look for it if you wish!” exclaimed Lessep. “Here; in the little office. Anywhere about.”
“All right,” agreed Darting. “What does the lever look like?”
“I shall show you.” Lessep turned and led the way into the adjoining office.
It was a little room, plainly furnished. A filing cabinet in one corner; a desk in another. A single window, metal-shuttered, like those in the laboratory. These arrangements were revealed when Lessep pressed a light switch just inside the door.
The room was illuminated by the action, from a single light in the ceiling. Lessep went to the filing cabinet.
There he turned on a hanging light. Under the increased glare, the professor opened a drawer; he found a photograph of the motor that was in the laboratory. He pointed out the lever in question.
Darring examined the picture.
“You have a better photograph,” remarked Warlock. “Where is it, professor?”
“I must have left it in the desk,” replied Lessep. “Let me see.” He went to the desk and turned on another light. “Ah, yes. Here it is.”
He brought the photograph to Darring, who had handed the first picture to Barth. Cardona was also looking on, when an exclamation came from Warlock, who had gone over to the desk.
“Here’s a note for you, professor,” announced Warlock. “Addressed in your name, with the word ‘important’ added. It hasn’t been opened.”
He handed the envelope to Lessep. Photographs were temporarily forgotten while the professor fumbled excitedly in opening the message. Lessep unfolded a sheet of paper. His lips began to quiver. The note nearly fell from his shaking hands.
Commissioner Barth seized the paper. He read aloud:
“To Professor Melrose Lessep:
“The missing lever is proof of my ability to thwart your actions. This letter is my warning. If you attempt to repeat your experiment in devisualization, you shall die. — THE UNSEEN KILLER.”
“It’s typewritten,” remarked Barth. “With the signature in red. This is a result of that newspaper story, Cardona. Those reporters termed Miles Crofton the ‘Unseen Killer’ and he has adopted the title. This constitutes a threat.”
“All the more reason,” put in Darring, “why you should proceed with the experiment, commissioner.”
“I agree,” declared Barth. “Come, professor. Let us search for that lever. Where would Crofton have hidden it? Here or in the laboratory?”
“He might have taken it with him,” replied Lessep, in a quavering voice. “You may search if you wish, but I feel—”
“Crofton may still be here,” suggested Warlock, in a troubled tone, as further words failed Lessep.
“Remember, the man is unseen. A search might bring serious consequences.”
“Still a hoax,” scoffed Darring. “What about a search, commissioner?”