The Temple clock had announced in soft and confidential tones that it was a quarter to seven, in which statement it was stoutly supported by its colleague on our mantelpiece, and still there was no sign of Thorndyke.  It was really a little strange, for he was the soul of punctuality, and moreover, his engagements were of such a kind as rendered punctuality possible.  I was burning with impatience to impart my news to him, and this fact, together with the ghostly proceedings of Polton, worked me up to a state of nervous tension that rendered either rest or thought equally impossible.  I looked out of the window at the lamp below, glaring redly through the fog, and then, opening the door, went out on to the landing to listen.

At this moment Polton made a silent appearance on the stairs leading from the laboratory, giving me quite a start; and I was about to retire into the room when my ear caught the tinkle of a hansom approaching from Paper Buildings.

The vehicle drew nearer, and at length stopped opposite the house, on which Polton slid down the stairs with the agility of a harlequin.  A few moments later I heard his voice ascending from the hall ­

“I do hope, sir, you’re not much hurt?”

I ran down the stairs and met Thorndyke coming up slowly with his right hand on Polton’s shoulder.  His clothes were muddy, his left arm was in a sling, and a black handkerchief under his hat evidently concealed a bandage.

“I am not really hurt at all,” Thorndyke replied cheerily, “though very disreputable to look at.  Just came a cropper in the mud, Jervis,” he added, as he noted my dismayed expression.  “Dinner and a clothes-brush are what I chiefly need.”  Nevertheless, he looked very pale and shaken when he came into the light on the landing, and he sank into his easy-chair in the limp manner of a man either very weak or very fatigued.

“How did it happen?” I asked when Polton had crept away on tip-toe to make ready for dinner.

Thorndyke looked round to make sure that his henchman had departed, and said ­

“A queer affair, Jervis; a very odd affair indeed.  I was coming up from the Borough, picking my way mighty carefully across the road on account of the greasy, slippery mud, and had just reached the foot of London Bridge when I heard a heavy lorry coming down the slope a good deal too fast, considering that it was impossible to see more than a dozen yards ahead, and I stopped on the kerb to see it safely past.  Just as the horses emerged from the fog, a man came up behind and lurched violently against me and, strangely enough, at the same moment passed his foot in front of mine.  Of course I went sprawling into the road right in front of the lorry.  The horses came stamping and sliding straight on to me, and, before I could wriggle out of the way, the hoof of one of them smashed in my hat ­that was a new one that I came home in ­and half-stunned me.  Then the near wheel struck my head, making a dirty little scalp wound, and pinned down my sleeve so that I couldn’t pull away my arm, which is consequently barked all the way down.  It was a mighty near thing, Jervis; another inch or two and I should have been rolled out as flat as a starfish.”

“What became of the man?” I asked, wishing I could have had a brief interview with him.

“Lost to sight though to memory dear:  he was off like a lamplighter.  An alcoholic apple-woman picked me up and escorted me back to the hospital.  It must have been a touching spectacle,” he added, with a dry smile at the recollection.

“And I suppose they kept you there for a time to recover?”

“Yes; I went into dry dock in the O. P. room, and then old Langdale insisted on my lying down for an hour or so in case any symptoms of concussion should appear.  But I was only a trifle shaken and confused.  Still, it was a queer affair.”

“You mean the man pushing you down in that way?”

“Yes; I can’t make out how his foot got in front of mine.”

“You don’t think it was intentional, surely?” I said.

“No, of course not,” he replied, but without much conviction, as it seemed to me; and I was about to pursue the matter when Polton reappeared, and my friend abruptly changed the subject.

After dinner I recounted my conversation with Walter Hornby, watching my colleague’s face with some eagerness to see what effect this new information would produce on him.  The result was, on the whole, disappointing.  He was interested, keenly interested, but showed no symptoms of excitement.

“So John Hornby has been plunging in mines, eh?” he said, when I had finished.  “He ought to know better at his age.  Did you learn how long he had been in difficulties?”

“No.  But it can hardly have been quite sudden and unforeseen.”

“I should think not,” Thorndyke agreed.  “A sudden slump often proves disastrous to the regular Stock Exchange gambler who is paying differences on large quantities of unpaid-for stock.  But it looks as if Hornby had actually bought and paid for these mines, treating them as investments rather than speculations, in which case the depreciation would not have affected him in the same way.  It would be interesting to know for certain.”

“It might have a considerable bearing on the present case, might it not?”

“Undoubtedly,” said Thorndyke.  “It might bear on the case in more ways than one.  But you have some special point in your mind, I think.”

“Yes.  I was thinking that if these embarrassments had been growing up gradually for some time, they might have already assumed an acute form at the time of the robbery.”

“That is well considered,” said my colleague.  “But what is the special bearing on the case supposing it was so?”

“On the supposition,” I replied, “that Mr. Hornby was in actual pecuniary difficulties at the date of the robbery, it seems to me possible to construct a hypothesis as to the identity of the robber.”

“I should like to hear that hypothesis stated,” said Thorndyke, rousing himself and regarding me with lively interest.

“It is a highly improbable one,” I began with some natural shyness at the idea of airing my wits before this master of inductive method; “in fact, it is almost fantastic.”

“Never mind that,” said he.  “A sound thinker gives equal consideration to the probable and the improbable.”

Thus encouraged, I proceeded to set forth the theory of the crime as it had occurred to me on my way home in the fog, and I was gratified to observe the close attention with which Thorndyke listened, and his little nods of approval at each point that I made.

When I had finished, he remained silent for some time, looking thoughtfully into the fire and evidently considering how my theory and the new facts on which it was based would fit in with the rest 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.