When I arrived at Thorndyke's chambers on the following morning, I found my friend already hard at work. Breakfast was laid at one end of the table, while at the other stood a microscope of the pattern used for examining plate-cultures of micro-organisms, on the wide stage of which was one of the cards bearing six thumb-prints in blood. A condenser threw a bright spot of light on the card, which Thorndyke had been examining when I knocked, as I gathered from the position of the chair, which he now pushed back against the wall.  
"I see you have commenced work on our problem," I remarked as, in response to a double ring of the electric bell, Polton entered with the materials for our repast.
 
"Yes," answered Thorndyke. "I have opened the campaign, supported, as usual, by my trusty chief-of-staff; eh! Polton?"
 
The little man, whose intellectual, refined countenance and dignified bearing seemed oddly out of character with the tea-tray that he carried, smiled proudly, and, with a glance of affectionate admiration at my friend, replied—
 
"Yes, sir. We haven't been letting the grass grow under our feet. There's a beautiful negative washing upstairs and a bromide enlargement too, which will be mounted and dried by the time you have finished your breakfast."
 
"A wonderful man that, Jervis," my friend observed as his assistant retired. "Looks like a rural dean or a chancery judge, and was obviously intended by Nature to be a professor of physics. As an actual fact he was first a watchmaker, then a maker of optical instruments, and now he is mechanical factotum to a medical jurist. He is my right-hand, is Polton; takes an idea before you have time to utter it—but you will make his more intimate acquaintance by-and-by."
 
"Where did you pick him up?" I asked.
 
"He was an in-patient at the hospital when I first met him, miserably ill and broken, a victim of poverty and undeserved misfortune. I gave him one or two little jobs, and when I found what class of man he was I took him permanently into my service. He is perfectly devoted to me, and his gratitude is as boundless as it is uncalled for."
 
"What are the photographs he was referring to?" I asked.
 
"He is making an enlarged facsimile of one of the thumb-prints on bromide paper and a negative of the same size in case we want the print repeated."
 
"You evidently have some expectation of being able to help poor Hornby," said I, "though I cannot imagine how you propose to go to work. To me his case seems as hopeless a one as it is possible to conceive. One doesn't like to condemn him, but yet his innocence seems almost unthinkable."
 
"It does certainly look like a hopeless case," Thorndyke agreed, "and I see no way out of it at present. But I make it a rule, in all cases, to proceed on the strictly classical lines of inductive inquiry—collect facts, make hypotheses, test them and seek for verification. And I always endeavour to keep a perfectly open mind.
 
"Now, in the present case, assuming, as we must, that the robbery has actually taken place, there are four conceivable hypotheses: (1) that the robbery was committed by Reuben Hornby; (2) that it was committed by Walter Hornby; (3) that it was committed by John Hornby, or (4) that it was committed by some other person or persons.
 
"The last hypothesis I propose to disregard for the present and confine myself to the examination of the other three."
 
"You don't think it possible that Mr. Hornby could have stolen the diamonds out of his own safe?" I exclaimed.
 
"I incline at present to no one theory of the matter," replied Thorndyke. "I merely state the hypotheses. John Hornby had access to the diamonds, therefore it is possible that he stole them."
 
"But surely he was responsible to the owners."
 
"Not in the absence of gross negligence, which the owners would have difficulty in proving. You see, he was what is called a gratuitous bailee, and in such a case no responsibility for loss lies with the bailee unless there has been gross negligence."
 
"But the thumb-mark, my dear fellow!" I exclaimed. "How can you possibly get over that?"
 
"I don't know that I can," answered Thorndyke calmly; "but I see you are taking the same view as the police, who persist in regarding a finger-print as a kind of magical touchstone, a final proof, beyond which inquiry need not go. Now, this is an entire mistake. A finger-print is merely a fact—a very important and significant one, I admit—but still a fact, which, like any other fact, requires to be weighed and measured with reference to its evidential value."
 
"And what do you propose to do first?"
 
"I shall first satisfy myself that the suspected thumb-print is identical in character with that of Reuben Hornby—of which, however, I have very little doubt, for the finger-print experts may fairly be trusted in their own speciality."
 
"And then?"
 
"I shall collect fresh facts, in which I look to you for assistance, and, if we have finished breakfast, I may as well induct you into your new duties."
 
He rose and rang the bell, and then, fetching from the office four small, paper-covered notebooks, laid them before me on the table.
 
"One of these books," said he, "we will devote to data concerning Reuben Hornby. You will find out anything you can—anything, mind, no matter how trivial or apparently irrelevant—in any way connected with him and enter it in this book." He wrote on the cover "Reuben Hornby" and passed the book to me. "In this second book you will, in like manner, enter anything that you can learn about Walter Hornby, and, in the third book, data concerning John Hornby. As to the fourth book, you will keep that for stray facts connected with the case but not coming under either of the other headings. And now let us look at the product of Polton's industry."
 
He took from his assistant's hand a photograph ten inches long by eight broad, done on glazed bromide paper and mounted flatly on stiff card. It showed a greatly magnified facsimile of one of the thumb-prints, in which all the minute details, such as the orifices of the sweat glands and trifling irregularities in the ridges, which, in the original, could be seen only with the aid of a lens, were plainly visible to the naked eye. Moreover, the entire print was covered by a network of fine black lines, by which it was divided into a multitude of small squares, each square being distinguished by a number.
 
"Excellent, Polton," said Thorndyke approvingly; "a most admirable enlargement. You see, Jervis, we have photographed the thumb-print in contact with a numbered micrometer divided into square twelfths of an inch. The magnification is eight diameters, so that the squares are here each two-thirds of an inch in diameter. I have a number of these micrometers of different scales, and I find them invaluable in examining cheques, doubtful signatures and such like. I see you have packed up the camera and the microscope, Polton; have you put in the micrometer?"
 
"Yes, sir," replied Polton, "and the six-inch objective and the low-power eye-piece. Everything is in the case; and I have put 'special rapid' plates into the dark-slides in case the light should be bad."
 
"Then we will go forth and beard the Scotland Yard lions in their den," said Thorndyke, putting on his hat and gloves.
 
"But surely," said I, "you are not going to drag that great microscope to Scotland Yard, when you only want eight diameters. Haven't you a dissecting microscope or some other portable instrument?"
 
"We have a most delightful instrument of the dissecting type, of Polton's own make—he shall show it to you. But I may have need of a more powerful instrument—and here let me give you a word of warning: whatever you may see me do, make no comments before the officials. We are seeking information, not giving it, you understand."
 
At this moment the little brass knocker on the inner door—the outer oak being open—uttered a timid and apologetic rat-tat.
 
"Who the deuce can that be?" muttered Thorndyke, replacing the microscope on the table. He strode across to the door and opened it somewhat brusquely, but immediately whisked his hat off, and I then perceived a lady standing on the threshold.
 
"Dr. Thorndyke?" she inquired, and as my colleague bowed, she continued, "I ought to have written to ask for an appointment but the matter is rather urgent—it concerns Mr. Reuben Hornby and I only learned from him this morning that he had consulted you."
 
"Pray come in," said Thorndyke. "Dr. Jervis and I were just setting out for Scotland Yard on this very business. Let me present you to my colleague, who is working up the case with me."
 
Our visitor, a tall handsome girl of twenty or thereabouts, returned my bow and remarked with perfect self-possession, "My name is Gibson—Miss Juliet Gibson. My business is of a very simple character and need not detain you many minutes."
 
She seated herself in the chair that Thorndyke placed for her, and continued in a brisk and business-like manner—
 
"I must tell you who I am in order to explain my visit to you. For the last six years I have lived with Mr. and Mrs. Hornby, although I am no relation to them. I first came to the house as a sort of companion to Mrs. Hornby, though, as I was only fifteen at the time, I need hardly say that my duties were not very onerous; in fact, I think Mrs. Hornby took me because I was an orphan without the proper means of getting a livelihood, and she had no children of her own.
 
"Three years ago I came into a little fortune which rendered me independent; but I had been so happy with my kind friends that I asked to be allowed to remain with them, and there I have been ever since in the position of an adopted daughter. Naturally, I have seen a great deal of their nephews, who spend a good part of their time at the house, and I need not tell you that the horrible charge against Reuben has fallen upon us like a thunderbolt. Now, what I have come to say to you is this: I do not believe that Reuben stole those diamonds. It is entirely out of character with all my previous experience of him. I am convinced that he is innocent, and I am prepared to back my opinion."
 
"In what way?" asked Thorndyke.
 
"By supplying the sinews of war," replied Miss Gibson. "I understand that legal advice and assistance involves considerable expense."
 
"I am afraid you are quite correctly informed," said Thorndyke.
 
"Well, Reuben's pecuniary resources are, I am sure, quite small, so it is necessary for his friends to support him, and I want you to promise me that nothing shall be left undone that might help to prove his innocence if I make myself responsible for any costs that he is unable to meet. I should prefer, of course, not to appear in the matter, if it could be avoided."
 
"Your friendship is of an eminently practical kind, Miss Gibson," said my colleague, with a smile. "As a matter of fact, the costs are no affair of mine. If the occasion arose for the exercise of your generosity you would have to approach Mr. Reuben's solicitor through the medium of your guardian, Mr. Hornby, and with the consent of the accused. But I do not suppose the occasion will arise, although I am very glad you called, as you may be able to give us valuable assistance in other ways. For example, you might answer one or two apparently impertinent questions."
 
"I should not consider any question impertinent that you considered necessary to ask," our visitor replied.
 
"Then," said Thorndyke, "I will venture to inquire if any special relations exist between you and Mr. Reuben."
 
"You look for the inevitable motive in a woman," said Miss Gibson, laughing and flushing a little. "No, there have been no tender passages between Reuben and me. We are merely old and intimate friends; in fact, there is what I may call a tendency in another direction—Walter Hornby."
 
"Do you mean that you are engaged to Mr. Walter?"
 
"Oh, no," she replied; "but he has asked me to marry him—he has asked me, in fact, more than once; and I really believe that he has a sincere attachment to me."
 
She made this latter statement with an odd air, as though the thing asserted were curious and rather incredible, and the tone was evidently noticed by Thorndyke as well as me for he rejoined—
 
"Of course he has. Why not?"
 
"Well, you see," replied Miss Gibson, "I have some six hundred a year of my own and should not be considered a bad match for a young man like Walter, who has neither property nor expectations, and one naturally takes that into account. But still, as I have said, I believe he is quite sincere in his professions and not merely attracted by my money."
 
"I do not find your opinion at all incredible," said Thorndyke, with a smile, "even if Mr. Walter were quite a mercenary young man—which, I take it, he is not."
 
Miss Gibson flushed very prettily as she replied—
 
"Oh, pray do not trouble to pay me compliments; I assure you I am by no means insensible of my merits. But with regard to Walter Hornby, I should be sorry to apply the term 'mercenary' to him, and yet—well, I have never met a young man who showed a stronger appreciation of the value of money. He means to succeed in life and I have no doubt he will."
 
"And do I understand that you refused him?"
 
"Yes. My feelings towards him are quite friendly, but not of such a nature as to allow me to contemplate marrying him."
 
"And now, to return for a moment to Mr. Reuben. You have known him for some years?"
 
"I have known him intimately for six years," replied Miss Gibson.
 
"And what sort of character do you give him?"
 
"Speaking from my own observation of him," she replied, "I can say that I have never known him to tell an untruth or do a dishonourable deed. As to theft, it is merely ridiculous. His habits have always been inexpensive and frugal, he is unambitious to a fault, and in respect to the 'main chance' his indifference is as conspicuous as Walter's keenness. He is a generous man, too, although careful and industrious."
 
"Thank you, Miss Gibson," said Thorndyke. "We shall apply to you for further information as the case progresses. I am sure that you will help us if you can, and that you can help us if you will, with your clear head and your admirable frankness. If you will leave us your card, Dr. Jervis and I will keep you informed of our prospects and ask for your assistance whenever we need it."
 
After our fair visitor had departed, Thorndyke stood for a minute or more gazing dreamily into the fire. Then, with a quick glance at his watch, he resumed his hat and, catching up the microscope, handed the camera case to me and made for the door.
 
"How the time goes!" he exclaimed, as we descended the stairs; "but it hasn't been wasted, Jervis, hey?"
 
"No, I suppose not," I answered tentatively.
 
"You suppose not!" he replied. "Why here is as pretty a little problem as you could desire—what would be called in the jargon of the novels, a psychological problem—and it is your business to work it out, too."
 
"You mean as to Miss Gibson's relations with these two young men?"
 
Thorndyke nodded.
 
"Is it any concern of ours?" I asked.
 
"Certainly it is," he replied. "Everything is a concern of ours at this preliminary stage. We are groping about for a clue and must let nothing pass unscrutinised."
 
"Well, then, to begin with, she is not wildly infatuated with Walter Hornby, I should say."
 
"No," agreed Thorndyke, laughing softly; "we may take it that the canny Walter has not inspired a grand passion."
 
"Then," I resumed, "if I were a suitor for Miss Gibson's hand, I think I would sooner stand in Reuben's shoes than in Walter's."
 
"There again I am with you," said Thorndyke. "Go on."
 
"Well," I continued, "our fair visitor conveyed to me the impression that her evident admiration of Reuben's character was tempered by something that she had heard from a third party. That expression of hers, 'speaking from my own observation,' seemed to imply that her observations of him were not in entire agreement with somebody else's."
 
"Good man!" exclaimed Thorndyke, slapping me on the back, to the undissembled surprise of a policeman whom we were passing; "that is what I had hoped for in you—the capacity to perceive the essential underneath the obvious. Yes; somebody has been saying something about our client, and the thing that we have to find out is, what is it that has been said and who has been saying it. We shall have to make a pretext for another interview with Miss Gibson."
 
"By the way, why didn't you ask her what she meant?" I asked foolishly.
 
Thorndyke grinned in my face. "Why didn't you?" he retorted.
 
"No," I rejoined, "I suppose it is not politic to appear too discerning. Let me carry the microscope for a time; it is making your arm ache, I see."
 
"Thanks," said he, handing the case to me and rubbing his fingers; "it is rather ponderous."
 
"I can't make out what you want with this great instrument," I said. "A common pocket lens would do all that you require. Besides, a six-inch objective will not magnify more than two or three diameters."
 
"Two, with the draw-tube closed," replied Thorndyke, "and the low-power eye-piece brings it up to four. Polton made them both for me for examining cheques, bank-notes and other large objects. But you will understand when you see me use the instrument, and remember, you are to make no comments."
 
We had by this time arrived at the entrance to Scotland Yard, and were passing up the narrow thoroughfare, when we encountered a uniformed official who halted and saluted my colleague.
 
"Ah, I thought we should see you here before long, doctor," said he genially. "I heard this morning that you have this thumb-print case in hand."
 
"Yes," replied Thorndyke; "I am going to see what can be done for the defence."
 
"Well," said the officer as he ushered us into the building, "you've given us a good many surprises, but you'll give us a bigger one if you can make anything of this. It's a foregone conclusion, I should say."
 
"My dear fellow," said Thorndyke, "there is no such thing. You mean that there is a prima facie case against the accused."
 
"Put it that way if you like," replied the officer, with a sly smile, "but I think you will find this about the hardest nut you ever tried your teeth on—and they're pretty strong teeth too, I'll say that. You had better come into Mr. Singleton's office," and he conducted us along a corridor and into a large, barely-furnished room, where we found a sedate-looking gentleman seated at a large writing table.
 
"How-d'ye-do, doctor?" said the latter, rising and holding out his hand. "I can guess what you've come for. Want to see that thumb-print, eh?"
 
"Quite right," answered Thorndyke, and then, having introduced me, he continued: "We were partners in the last game, but we are on opposite sides of the board this time."
 
"Yes," agreed Mr. Singleton; "and we are going to give you check-mate."
 
He unlocked a drawer and drew forth a small portfolio, from which he extracted a piece of paper which he laid on the table. It appeared to be a sheet torn from a perforated memorandum block, and bore the pencilled inscription: "Handed in by Reuben at 7.3 p.m., 9.3.01. J. H." At one end was a dark, glossy blood-stain, made by the falling of a good-sized drop, and this was smeared slightly, apparently by a finger or thumb having been pressed on it. Near to it were two or three smaller smears and a remarkably distinct and clean print of a thumb.
 
Thorndyke gazed intently at the paper for a minute or two, scrutinising the thumb-print and the smears in turn, but making no remark, while Mr. Singleton watched his impassive face with expectant curiosity.
 
"Not much difficulty in identifying that mark," the official at length observed.
 
"No," agreed Thorndyke; "it is an excellent impression and a very distinctive pattern, even without the scar."
 
"Yes," rejoined Mr. Singleton; "the scar makes it absolutely conclusive. You have a print with you, I suppose?"
 
"Yes," replied Thorndyke, and he drew from a wide flap-pocket the enlarged photograph, at the sight of which Mr. Singleton's face broadened into a smile.
 
"You don't want to put on spectacles to look at that," he remarked; "not that you gain anything by so much enlargement; three diameters is ample for studying the ridge-patterns. I see you have divided it up into numbered squares—not a bad plan; but ours—or rather Galton's, for we borrowed the method from him—is better for this purpose."
 
He drew from the portfolio a half-plate photograph of the thumb-print which appeared magnified to about four inches in length. The print was marked by a number of figures written minutely with a fine-pointed pen, each figure being placed on an "island," a loop, a bifurcation or some other striking and characteristic portion of the ridge-pattern.
 
"This system of marking with reference numbers," said Mr. Singleton, "is better than your method of squares, because the numbers are only placed at points which are important for comparison, whereas your squares or the intersections of the lines fall arbitrarily on important or unimportant points according to chance. Besides, we can't let you mark our original, you know, though, of course, we can give you a photograph, which will do as well."
 
"I was going to ask you to let me take a photograph presently," said Thorndyke.
 
"Certainly," replied Mr. Singleton, "if you would rather have one of your own taking. I know you don't care to take anything on trust. And now I must get on with my work, if you will excuse me. Inspector Johnson will give you any assistance you may require."
 
"And see that I don't pocket the original," added Thorndyke, with a smile at the inspector who had shown us in.
 
"Oh, I'll see to that," said the latter, grinning; and, as Mr. Singleton returned to his table, Thorndyke unlocked the microscope case and drew forth the instrument.
 
"What, are you going to put it under the microscope?" exclaimed Mr. Singleton, looking round with a broad smile.
 
"Must do something for my fee, you know," replied Thorndyke, as he set up the microscope and screwed on two extra objectives to the triple nose-piece.
 
"You observe that there is no deception," he added to the inspector, as he took the paper from Mr. Singleton's table and placed it between two slips of glass.
 
"I'm watching you, sir," replied the officer, with a chuckle; and he did watch, with close attention and great interest, while Thorndyke laid the glass slips on the microscope stage and proceeded to focus.
 
I also watched, and was a good deal exercised in my mind by my colleague's proceedings. After a preliminary glance with the six-inch glass, he swung round the nose-piece to the half-inch objective and slipped in a more powerful eye-piece, and with this power he examined the blood-stains carefully, and then moved the thumb-print into the field of vision. After looking at this for some time with deep attention, he drew from the case a tiny spirit lamp which was evidently filled with an alcoholic solution of some sodium salt, for when he lit it I recognised the characteristic yellow sodium flame. Then he replaced one of the objectives by a spectroscopic attachment, and having placed the little lamp close to the microscope mirror, adjusted the spectroscope. Evidently my friend was fixing the position of the "D" line (or sodium line) in the spectrum.
 
Having completed the adjustments, he now examined afresh the blood-smears and the thumb-print, both by transmitted and reflected light, and I observed him hurriedly draw one or two diagrams in his notebook. Then he replaced the spectroscope and lamp in the case and brought forth the micrometer—a slip of rather thin glass about three inches by one and a half—which he laid over the thumb-print in the place of the upper plate of glass.
 
Having secured it in position by the clips, he moved it about, comparing its appearance with that of the lines on the large photograph, which he held in his hand. After a considerable amount of adjustment and readjustment, he appeared to be satisfied, for he remarked to me—
 
"I think I have got the lines in the same position as they are on our print, so, with Inspector Johnson's assistance, we will take a photograph which we can examine at our leisure."
 
He extracted the camera—a quarter-plate instrument—from its case and opened it. Then, having swung the microscope on its stand into a horizontal position, he produced from the camera case a slab of mahogany with three brass feet, on which he placed the camera, and which brought the latter to a level with the eye-piece of the microscope.
 
The front of the camera was fitted with a short sleeve of thin black leather, and into this the eye-piece end of the microscope was now passed, the sleeve being secured round the barrel of the microscope by a stout indiarubber band, thus producing a completely light-tight connection.
 
Everything was now ready for taking the photograph. The light from the window having been concentrated on the thumb-print by means of a condenser, Thorndyke proceeded to focus the image on the ground-glass screen with extreme care and then, slipping a small leather cap over the objective, introduced the dark slide and drew out the shutter.
 
"I will ask you to sit down and remain quite still while I make the exposure," he said to me and the inspector. "A very little vibration is enough to destroy the sharpness of the image."
 
We seated ourselves accordingly, and Thorndyke then removed the cap, standing motionless, watch in hand, while he exposed the first plate.
 
"We may as well take a second, in case this should not turn out quite perfect," he said, as he replaced the cap and closed the shutter.
 
He reversed the dark slide and made another exposure in the same way, and then, having removed the micrometer and replaced it by a slip of plain glass, he made two more exposures.
 
"There are two plates left," he remarked, as he drew out the second dark slide. "I think I will take a record of the blood-stain on them."
 
He accordingly made two more exposures—one of the larger blood-stain and one of the smaller smears.
 
"There," said he, with an air of satisfaction, as he proceeded to pack up what the inspector described as his "box of tricks." "I think we have all the data that we can squeeze out of Scotland Yard, and I am very much obliged to you, Mr. Singleton, for giving so many facilities to your natural enemy, the counsel for the defence."
 
"Not our natural enemies, doctor," protested Mr. Singleton. "We work for a conviction, of course, but we don't throw obstacles in the way of the defence. You know that perfectly well."
 
"Of course I do, my dear sir," replied Thorndyke, shaking the official by the hand. "Haven't I benefited by your help a score of times? But I am greatly obliged all the same. Good-bye."
 
"Good-bye, doctor. I wish you luck, though I fear you will find it 'no go' this time."
 
"We shall see," replied Thorndyke, and with a friendly wave of the hand to the inspector he caught up the two cases and led the way out of the building.