The servant bowed and left them. A short wait and Mr. Oliver entered.
"I am sorry to disturb you at such a time, Mr. Oliver," said the scientist, "but if you can give me just a little information I think perhaps we may get a full light on this unfortunate affair."
Mr. Oliver bowed.
"First, let me ask you to confirm what I may say is my knowledge that your daughter, Eleanor, knew this man. I will ask, too, that you do not mention his name now."
He scribbled hastily on a piece of paper and handed it to Mr. Oliver. An expression of deep surprise came into the latter's face and he shook his head.
"I can answer that question positively," he said. "She does not know him. She had never been abroad and he has never been in this country until now."
The Thinking Machine arose with something nearly akin to agitation in his face, and his slender fingers worked nervously.
"What?" he demand abruptly. "What?" Then, after a pause: "I beg your pardon, sir. It startled me a little. But are you sure?"
"Perfectly sure," replied Mr. Oliver firmly. "They could not have met in any way."
For a long time The Thinking Machine stood squinting aggressively at his host with bewilderment plainly apparent in his manner. Hatch looked on with absorbed interest. Something had gone wrong; a cog had slipped; the wheels of logic had been thrown out of gear.
"I have made a mistake, Mr. Oliver," said The Thinking Machine at last. "I am sorry to have disturbed you."
Mr. Oliver bowed courteously and they were ushered out.
"What is it?" asked Hatch anxiously as they once more took their seats in the cab.
The Thinking Machine shook his head in frank annoyance.
"What happened?" Hatch insisted.
"I've made a mistake," was the petulant response. "I'm going home and start all over again. It may be that I shall send for you later."
Hatch accepted that as a dismissal and went his way wonderingly. That evening The Thinking Machine called him to the 'phone.
"Mr. Hatch?"
"Yes."
"Did Miss Oliver have any sisters?"
"Yes, one. Her name is Florence. There's something about her in the afternoon papers in connection with the murder story."
"How old is she?"
"I don't know-twenty-two or three."
"Ah!" came a long, aspirated sigh of relief over the wire. "Run by and bring Detective Mallory up to my place."
"All right. But what was the matter?"
"I was a fool, that's all. Good bye."
Detective Mallory was still delighted with himself when Hatch entered his office.
"What particular line is your friend Van Dusen working?" he asked a little curiously.
The reporter shrugged his shoulders.
"He asked me to come by and bring you up," he replied. "He has evidently reached some conclusion."
"If it's anything that doesn't count Knight in it's all wind," he said loftily. For once in his life he was confident that he could deliver a blow which would obliterate any theory but his own. In this mood, therefore, he went with Hatch. They found The Thinking Machine pacing back and forth across his small laboratory with his slender hands clasped behind his back. Hatch noted that the perplexed wrinkles had gone.
"In adding up a column of figures," began the scientist abruptly as he sat down, "the oversight of even so trivial a unit as one will make a glaring error in the result. You, Mr. Mallory, have overlooked a figure one, therefore your conclusion is wrong. In my first consideration of this affair I also overlooked a figure one and my conclusion toppled over just at the moment when it seemed to be corroborated. So I had to start over; I found the one."
"But this thing against Knight is conclusive," said the detective explosively.
"Except for the figure one," added the scientist.
Detective Mallory snorted politely.
"Now here is the logic of the thing," resumed The Thinking Machine. "It will show how I overlooked the figure one-that is a vital fact-and how I found it."
He dropped back into the reflective attitude which was so familiar to his hearers, squint eyes turned upward and with his fingers pressed tip to tip. For several minutes he was silent while Detective Mallory vented his impatience by chewing his moustache.
"In the beginning," began The Thinking Machine at last, "we have a girl, pretty, young and wealthy in a box at the opera with her parents and her fiance. It would seem, at first glance, to be as safe a place as her home would be, yet she is murdered mysteriously. A stiletto is thrust into her heart. We will assume that her death occurred in the box; that the knife thrust came while she was in a dead faint. This temporary unconsciousness would account for the fact that she did not scream, as the heart would have been pierced by a sudden thrust before consciousness of pain was awakened.
"Now the three persons who were with her. There seemed no reason to suspect either the father or mother, so we come to Sylvester Knight, her intended husband. There is always to be found a motive, either real or imaginary, for a man to kill his sweetheart. In this case Knight had the opportunity, but not the exclusive opportunity. Therefore, an unlimited field of speculation was opened up."
Detective Mallory raised his hand impressively and started to say something, then thought better of it.
"After Mr. Knight's arrest," The Thinking Machine continued, "your investigation, Mr. Mallory, drew a net about him. That's what you wanted to say, I believe. There was the stiletto, the other end of the cane and the alleged threats. I admit all these things. On this statement of the case it looked black for Mr. Knight."
"That's what," remarked the detective.
"Now a stiletto naturally suggests Italy. The blade with which Miss Oliver was killed bore an Italian manufacturer's mark. I presume you noticed it?"
"Oh, that!" exclaimed the detective.
"Means nothing conclusively," added The Thinking Machine. "I agree with you. Still it was a suggestion. Then I saw the thing that did mean something. This was the fact that the handle of the stiletto was not of the same wood as the part of the cane you found in Mr. Knight's room. This difference is so slight that you would hardly notice it even now, but it was there and showed a possible clue leading away from Mr. Knight."
Detective Mallory could not readily place his tongue on words to fittingly express his disgust, so he remained silent.
"When I considered what manner of man Mr. Knight is and the singular nature of the crime," resumed the scientist, "I had no hesitancy in assuring Mr. Hatch that you had the wrong man. After we first saw you we examined the opera box. It was on the left of the theatre and separated from the next box by a latticed partition. It was against this partition that Miss Oliver was leaning.
"Remember, I saw the box after I examined the stiletto and while I was seeking a method by which another person might have stabbed her without entering the box. I found it. By using a stiletto without a guard it would have been perfectly possible for a person in the next box to have killed her by thrusting the blade through the lattice partition. That is exactly what happened."
Detective Mallory arose with a mouth full of words. They tumbled out in incoherent surprise and protest, then he sat down again. The Thinking Machine was still staring upward.
"I then took steps to learn who was in the adjoining box at the time of her death," he continued quietly. "The manager of the theatre told me it was occupied by Mr. and Mrs. Franklin Dupree, and their guest an Italian nobleman. Italian nobleman! Italian stiletto! You see the connection?