of the data. At length he spoke, without, however, removing his eyes from the red embers—
"This theory of yours, Jervis, does great credit to your ingenuity. We may disregard the improbability, seeing that the alternative theories are almost equally improbable, and the fact that emerges, and that gratifies me more than I can tell you, is that you are gifted with enough scientific imagination to construct a possible train of events. Indeed, the improbability—combined, of course, with possibility—really adds to the achievement, for the dullest mind can perceive the obvious—as, for instance, the importance of a finger-print. You have really done a great thing, and I congratulate you; for you have emancipated yourself, at least to some extent, from the great finger-print obsession, which has possessed the legal mind ever since Galton published his epoch-making monograph. In that work I remember he states that a finger-print affords evidence requiring no corroboration—a most dangerous and misleading statement which has been fastened upon eagerly by the police, who have naturally been delighted at obtaining a sort of magic touchstone by which they are saved the labour of investigation. But there is no such thing as a single fact that 'affords evidence requiring no corroboration.' As well might one expect to make a syllogism with a single premise."
"I suppose they would hardly go so far as that," I said, laughing.
"No," he admitted. "But the kind of syllogism that they do make is this—
"'The crime was committed by the person who made this finger-print.
"'But John Smith is the person who made the finger-print.
"'Therefore the crime was committed by John Smith.'"
"Well, that is a perfectly good syllogism, isn't it?" I asked.
"Perfectly," he replied. "But, you see, it begs the whole question, which is, 'Was the crime committed by the person who made this finger-print?' That is where the corroboration is required."
"That practically leaves the case to be investigated without reference to the finger-print, which thus becomes of no importance."
"Not at all," rejoined Thorndyke; "the finger-print is a most valuable clue as long as its evidential value is not exaggerated. Take our present case, for instance. Without the thumb-print, the robbery might have been committed by anybody; there is no clue whatever. But the existence of the thumb-print narrows the inquiry down to Reuben or some person having access to his finger-prints."
"Yes, I see. Then you consider my theory of John Hornby as the perpetrator of the robbery as quite a tenable one?"
"Quite," replied Thorndyke. "I have entertained it from the first; and the new facts that you have gathered increase its probability. You remember I said that four hypotheses were possible: that the robbery was committed either by Reuben, by Walter, by John Hornby, or by some other person. Now, putting aside the 'some other person' for consideration only if the first three hypotheses fail, we have left, Reuben, Walter, and John. But if we leave the thumb-print out of the question, the probabilities evidently point to John Hornby, since he, admittedly, had access to the diamonds, whereas there is nothing to show that the others had. The thumb-print, however, transfers the suspicion to Reuben; but yet, as your theory makes evident, it does not completely clear John Hornby. As the case stands, the balance of probabilities may be stated thus: John Hornby undoubtedly had access to the diamonds, and therefore might have stolen them. But if the thumb-mark was made after he closed the safe and before he opened it again, some other person must have had access to them, and was probably the thief.
"The thumb-mark is that of Reuben Hornby, a fact that establishes a prima facie probability that he stole the diamonds. But there is no evidence that he had access to them, and if he had not, he could not have made the thumb-mark in the manner and at the time stated.
"But John Hornby may have had access to the previously-made thumb-mark of Reuben, and may possibly have obtained it; in which case he is almost certainly the thief.
"As to Walter Hornby, he may have had the means of obtaining Reuben's thumb-mark; but there is no evidence that he had access either to the diamonds or to Mr. Hornby's memorandum block. The prima facie probabilities in his case, therefore, are very slight."
"The actual points at issue, then," I said, "are, whether Reuben had any means of opening the safe, and whether Mr. Hornby ever did actually have the opportunity of obtaining Reuben's thumb-mark in blood on his memorandum block."
"Yes," replied Thorndyke. "Those are the points—with some others—and they are likely to remain unsettled. Reuben's rooms have been searched by the police, who failed to find any skeleton or duplicate keys; but this proves nothing, as he would probably have made away with them when he heard of the thumb-mark being found. As to the other matter, I have asked Reuben, and he has no recollection of ever having made a thumb-mark in blood. So there the matter rests."
"And what about Mr. Hornby's liability for the diamonds?"
"I think we may dismiss that," answered Thorndyke. "He had undertaken no liability and there was no negligence. He would not be liable at law."
After my colleague retired, which he did quite early, I sat for a long time pondering upon this singular case in which I found myself involved. And the more I thought about it the more puzzled I became. If Thorndyke had no more satisfactory explanation to offer than that which he had given me this evening, the defence was hopeless, for the court was not likely to accept his estimate of the evidential value of finger-prints. Yet he had given Reuben something like a positive assurance that there would be an adequate defence, and had expressed his own positive conviction of the accused man's innocence. But Thorndyke was not a man to reach such a conviction through merely sentimental considerations. The inevitable conclusion was that he had something up his sleeve—that he had gained possession of some facts that had escaped my observation; and when I had reached this point I knocked out my pipe and betook myself to bed.
The Prisoner
On the following morning, as I emerged from my room, I met Polton coming up with a tray (our bedrooms were on the attic floor above the laboratory and workshop), and I accordingly followed him into my friend's chamber.
"I shan't go out to-day," said Thorndyke, "though I shall come down presently. It is very inconvenient, but one must accept the inevitable. I have had a knock on the head, and, although I feel none the worse, I must take the proper precautions—rest and a low diet—until I see that no results are going to follow. You can attend to the scalp wound and send round the necessary letters, can't you?"
I expressed my willingness to do all that was required and applauded my friend's self-control and good sense; indeed, I could not help contrasting the conduct of this busy, indefatigable man, cheerfully resigning himself to most distasteful inaction, with the fussy behaviour of the ordinary patient who, with nothing of importance to do, can hardly be prevailed upon to rest, no matter how urgent the necessity. Accordingly, I breakfasted alone, and spent the morning in writing and despatching letters to the various persons who were expecting visits from my colleague.
Shortly after lunch (a very spare one, by the way, for Polton appeared to include me in the scheme of reduced diet) my expectant ear caught the tinkle of a hansom approaching down Crown Office Row.
"Here comes your fair companion," said Thorndyke, whom I had acquainted with my arrangements, "Tell Hornby, from me, to keep up his courage, and, for yourself, bear my warning in mind. I should be sorry indeed if you ever
