“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 fingerprint.
“ ‘But John Smith is the person who made the fingerprint.
“ ‘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 fingerprint?’ That is where the corroboration is required.”
“That practically leaves the case to be investigated without reference to the fingerprint, which thus becomes of no importance.”
“Not at all,” rejoined Thorndyke; “the fingerprint is a most valuable clue as long as its evidential value is not exaggerated. Take our present case, for instance. Without the thumbprint, the robbery might have been committed by anybody; there is no clue whatever. But the existence of the thumbprint narrows the inquiry down to Reuben or some person having access to his fingerprints.”
“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 thumbprint 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 thumbprint, 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 thumbmark 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 thumbmark 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 thumbmark in the manner and at the time stated.
“But John Hornby may have had access to the previously-made thumbmark 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 thumbmark; 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 thumbmark 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 thumbmark being found. As to the other matter, I have asked Reuben, and he has no recollection of ever having made a thumbmark 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 fingerprints. 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.
IX
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 today,” 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
