As Thorndyke took his place in the box I looked at him with a sense of unreasonable surprise, feeling that I had never before fully realised what manner of man my friend was as to his externals. I had often noted the quiet strength of his face, its infinite intelligence, its attractiveness and magnetism; but I had never before appreciated what now impressed me most: that Thorndyke was actually the handsomest man I had ever seen. He was dressed simply, his appearance unaided by the flowing gown or awe-inspiring wig, and yet his presence dominated the court. Even the judge, despite his scarlet robe and trappings of office, looked commonplace by comparison, while the jurymen, who turned to look at him, seemed like beings of an inferior order. It was not alone the distinction of the tall figure, erect and dignified, nor the power and massive composure of his face, but the actual symmetry and comeliness of the face itself that now arrested my attention; a comeliness that made it akin rather to some classic mask, wrought in the ivory-toned marble of Pentelicus, than to the eager faces that move around us in the hurry and bustle of a life at once strenuous and trivial.  
"You are attached to the medical school at St. Margaret's Hospital, I believe, Dr. Thorndyke?" said Anstey.
 
"Yes. I am the lecturer on Medical Jurisprudence and Toxicology."
 
"Have you had much experience of medico-legal inquiries?"
 
"A great deal. I am engaged exclusively in medico-legal work."
 
"You heard the evidence relating to the two drops of blood found in the safe?"
 
"I did."
 
"What is your opinion as to the condition of that blood?"
 
"I should say there is no doubt that it had been artificially treated—probably by defibrination."
 
"Can you suggest any explanation of the condition of that blood?"
 
"I can."
 
"Is your explanation connected with any peculiarities in the thumb-print on the paper that was found in the safe?"
 
"It is."
 
"Have you given any attention to the subject of finger-prints?"
 
"Yes. A great deal of attention."
 
"Be good enough to examine that paper" (here the usher handed to Thorndyke the memorandum slip). "Have you seen it before?"
 
"Yes. I saw it at Scotland Yard."
 
"Did you examine it thoroughly?"
 
"Very thoroughly. The police officials gave me every facility and, with their permission, I took several photographs of it."
 
"There is a mark on that paper resembling the print of a human thumb?"
 
"There is."
 
"You have heard two expert witnesses swear that that mark was made by the left thumb of the prisoner, Reuben Hornby?"
 
"I have."
 
"Do you agree to that statement?"
 
"I do not."
 
"In your opinion, was the mark upon that paper made by the thumb of the prisoner?"
 
"No. I am convinced that it was not made by the thumb of Reuben Hornby."
 
"Do you think that it was made by the thumb of some other person?"
 
"No. I am of opinion that it was not made by a human thumb at all."
 
At this statement the judge paused for a moment, pen in hand, and stared at Thorndyke with his mouth slightly open, while the two experts looked at one another with raised eyebrows.
 
"By what means do you consider that the mark was produced?"
 
"By means of a stamp, either of indiarubber or, more probably, of chromicized gelatine."
 
Here Polton, who had been, by degrees, rising to an erect posture, smote his thigh a resounding thwack and chuckled aloud, a proceeding that caused all eyes, including those of the judge, to be turned on him.
 
"If that noise is repeated," said the judge, with a stony stare at the horrified offender—who had shrunk into the very smallest space that I have ever seen a human being occupy—"I shall cause the person who made it to be removed from the court."
 
"I understand, then," pursued Anstey, "that you consider the thumb-print, which has been sworn to as the prisoner's, to be a forgery?"
 
"Yes. It is a forgery."
 
"But is it possible to forge a thumb-print or a finger-print?"
 
"It is not only possible, but quite easy to do."
 
"As easy as to forge a signature, for instance?"
 
"Much more so, and infinitely more secure. A signature, being written with a pen, requires that the forgery should also be written with a pen, a process demanding very special skill and, after all, never resulting in an absolute facsimile. But a finger-print is a stamped impression—the finger-tip being the stamp; and it is only necessary to obtain a stamp identical in character with the finger-tip, in order to produce an impression which is an absolute facsimile, in every respect, of the original, and totally indistinguishable from it."
 
"Would there be no means at all of detecting the difference between a forged finger-print and the genuine original?"
 
"None whatever; for the reason that there would be no difference to detect."
 
"But you have stated, quite positively, that the thumb-print on this paper is a forgery. Now, if the forged print is indistinguishable from the original, how are you able to be certain that this particular print is a forgery?"
 
"I was speaking of what is possible with due care, but, obviously, a forger might, through inadvertence, fail to produce an absolute facsimile and then detection would be possible. That is what has happened in the present case. The forged print is not an absolute facsimile of the true print. There is a slight discrepancy. But, in addition to this, the paper bears intrinsic evidence that the thumb-print on it is a forgery."
 
"We will consider that evidence presently, Dr. Thorndyke. To return to the possibility of forging a finger-print, can you explain to us, without being too technical, by what methods it would be possible to produce such a stamp as you have referred to?"
 
"There are two principal methods that suggest themselves to me. The first, which is rather crude though easy to carry out, consists in taking an actual cast of the end of the finger. A mould would be made by pressing the finger into some plastic material, such as fine modelling clay or hot sealing wax, and then, by pouring a warm solution of gelatine into the mould, and allowing it to cool and solidify, a cast would be produced which would yield very perfect finger-prints. But this method would, as a rule, be useless for the purpose of the forger, as it could not, ordinarily, be carried out without the knowledge of the victim; though in the case of dead bodies and persons asleep or unconscious or under an anaesthetic, it could be practised with success, and would offer the advantage of requiring practically no technical skill or knowledge and no special appliances. The second method, which is much more efficient, and is the one, I have no doubt, that has been used in the present instance, requires more knowledge and skill.
 
"In the first place it is necessary to obtain possession of, or access to, a genuine finger-print. Of this finger-print a photograph is taken, or rather, a photographic negative, which for this purpose requires to be taken on a reversed plate, and the negative is put into a special printing frame, with a plate of gelatine which has been treated with potassium bichromate, and the frame is exposed to light.
 
"Now gelatine treated in this way—chromicized gelatine, as it is called—has a very peculiar property. Ordinary gelatine, as is well known, is easily dissolved in hot water, and chromicized gelatine is also soluble in hot water as long as it is not exposed to light; but on being exposed to light, it undergoes a change and is no longer capable of being dissolved in hot water. Now the plate of chromicized gelatine under the negative is protected from the light by the opaque parts of the negative, whereas the light passes freely through the transparent parts; but the transparent parts of the negative correspond to the black marks on the finger-print, and these correspond to the ridges on the finger. Hence it follows that the gelatine plate is acted upon by light only on the parts corresponding to the ridges; and in these parts the gelatine is rendered insoluble, while all the rest of the gelatine is soluble. The gelatine plate, which is cemented to a thin plate of metal for support, is now carefully washed with hot water, by which the soluble part of the gelatine is dissolved away leaving the insoluble part (corresponding to the ridges) standing up from the surface. Thus there is produced a facsimile in relief of the finger-print having actual ridges and furrows identical in character with the ridges and furrows of the finger-tip. If an inked roller is passed over this relief, or if the relief is pressed lightly on an inked slab, and then pressed on a sheet of paper, a finger-print will be produced which will be absolutely identical with the original, even to the little white spots which mark the orifices of the sweat glands. It will be impossible to discover any difference between the real finger-print and the counterfeit because, in fact, no difference exists."
 
"But surely the process you have described is a very difficult and intricate one?"
 
"Not at all; it is very little more difficult than ordinary carbon printing, which is practised successfully by numbers of amateurs. Moreover, such a relief as I have described—which is practically nothing more than an ordinary process block—could be produced by any photo-engraver. The process that I have described is, in all essentials, that which is used in the reproduction of pen-and-ink drawings, and any of the hundreds of workmen who are employed in that industry could make a relief-block of a finger-print, with which an undetectable forgery could be executed."
 
"You have asserted that the counterfeit finger-print could not be distinguished from the original. Are you prepared to furnish proof that this is the case?"
 
"Yes. I am prepared to execute a counterfeit of the prisoner's thumb-print in the presence of the Court."
 
"And do you say that such a counterfeit would be indistinguishable from the original, even by the experts?"
 
"I do."
 
Anstey turned towards the judge. "Would your lordship give your permission for a demonstration such as the witness proposes?"
 
"Certainly," replied the judge. "The evidence is highly material. How do you propose that the comparison should be made?" he added, addressing Thorndyke.
 
"I have brought, for the purpose, my lord," answered Thorndyke, "some sheets of paper, each of which is ruled into twenty numbered squares. I propose to make on ten of the squares counterfeits of the prisoner's thumb-mark, and to fill the remaining ten with real thumb-marks. I propose that the experts should then examine the paper and tell the Court which are the real thumb-prints and which are the false."
 
"That seems a fair and efficient test," said his lordship. "Have you any objection to offer, Sir Hector?"
 
Sir Hector Trumpler hastily consulted with the two experts, who were sitting in the attorney's bench, and then replied, without much enthusiasm—
 
"We have no objection to offer, my lord."
 
"Then, in that case, I shall direct the expert witnesses to withdraw from the court while the prints are being made."
 
In obedience to the judge's order, Mr. Singleton and his colleague rose and left the court with evident reluctance, while Thorndyke took from a small portfolio three sheets of paper which he handed up to the judge.
 
"If your lordship," said he, "will make marks in ten of the squares on two of these sheets, one can be given to the jury and one retained by your lordship to check the third sheet when the prints are made on it."
 
"That is an excellent plan," said the judge; "and, as the information is for myself and the jury, it would be better if you came up and performed the actual stamping on my table in the presence of the foreman of the jury and the counsel for the prosecution and defence."
 
In accordance with the judge's direction Thorndyke stepped up on the dais, and Anstey, as he rose to follow, leaned over towards me.
 
"You and Polton had better go up too," said he: "Thorndyke will want your assistance, and you may as well see the fun. I will explain to his lordship."
 
He ascended the stairs leading to the dais and addressed a few words to the judge, who glanced in our direction and nodded, whereupon we both gleefully followed our counsel, Polton carrying the box and beaming with delight.
 
The judge's table was provided with a shallow drawer which pulled out at the side and which accommodated the box comfortably, leaving the small table-top free for the papers. When the lid of the box was raised, there were displayed a copper inking-slab, a small roller and the twenty-four "pawns" which had so puzzled Polton, and on which he now gazed with a twinkle of amusement and triumph.
 
"Are those all stamps?" inquired the judge, glancing curiously at the array of turned-wood handles.
 
"They are all stamps, my lord," replied Thorndyke, "and each is taken from a different impression of the prisoner's thumb."
 
"But why so many?" asked the judge.
 
"I have multiplied them," answered Thorndyke, as he squeezed out a drop of finger-print ink on to the slab and proceeded to roll it out into a thin film, "to avoid the tell-tale uniformity of a single stamp. And I may say," he added, "that it is highly important that the experts should not be informed that more than one stamp has been used."
 
"Yes, I see that," said the judge. "You understand that, Sir Hector," he added, addressing the counsel, who bowed stiffly, clearly regarding the entire proceeding with extreme disfavour.
 
Thorndyke now inked one of the stamps and handed it to the judge, who examined it curiously and then pressed it on a piece of waste paper, on which there immediately appeared a very distinct impression of a human thumb.
 
"Marvellous!" he exclaimed. "Most ingenious! Too ingenious!" He chuckled softly and added, as he handed the stamp and the paper to the foreman of the jury: "It is well, Dr. Thorndyke, that you are on the side of law and order, for I am afraid that, if you were on the other side, you would be one too many for the police. Now, if you are ready, we will proceed. Will you, please, stamp an impression in square number three."
 
Thorndyke drew a stamp from its compartment, inked it on the slab, and pressed it neatly on the square indicated, leaving there a sharp, clear thumb-print.
 
The process was repeated on nine other squares, a different stamp being used for each impression. The judge then marked the ten corresponding squares of the other two sheets of paper, and having checked them, directed the foreman to exhibit the sheet bearing the false thumb-prints to the jury, together with the marked sheet which they were to retain, to enable them to check the statements of the expert witnesses. When this was done, the prisoner was brought from the dock and stood beside the table. The judge looked with a curious and not unkindly interest at the handsome, manly fellow who stood charged with a crime so sordid and out of character with his appearance, and I felt, as I noted the look, that Reuben would, at least, be tried fairly on the evidence, without prejudice or even with some prepossession in his favour.
 
With the remaining part of the operation Thorndyke proceeded carefully and deliberately. The inking-slab was rolled afresh for each impression, and, after each, the thumb was cleansed with petrol and thoroughly dried; and when the process was completed and the prisoner led back to the dock, the twenty squares on the paper were occupied by twenty thumb-prints, which, to my eye, at any rate, were identical in character.
 
The judge sat for near upon a minute poring over this singular document with an expression half-way between a frown and a smile. At length, when we had all returned to our places, he directed the usher to bring in the witnesses.
 
I was amused to observe the change that had come over the experts in the short interval. The confident smile, the triumphant air of laying down a trump card, had vanished, and the expression of both was one of anxiety, not unmixed with apprehension. As Mr. Singleton advanced hesitatingly to the table, I recalled the words that he had uttered in his room at Scotland Yard; evidently his scheme of the game that was to end in an easy checkmate, had not included the move that had just been made.
 
"Mr. Singleton," said the judge, "here is a paper on which there are twenty thumb-prints. Ten of them are genuine prints of the prisoner's left thumb and ten are forgeries. Please examine them and note down in writing which are the true prints and which are the forgeries. When you have made your notes the paper will be handed to Mr. Nash."
 
"Is there any objection to my using the photograph that I have with me for comparison, my lord?" asked Mr. Singleton.
 
"I think not," replied the judge. "What do you say, Mr. Anstey?"
 
"No objection whatever, my lord," answered Anstey.
 
Mr. Singleton accordingly drew from his pocket an enlarged photograph of the thumb-print and a magnifying glass, with the aid of which he explored the bewildering array of prints on the paper before him; and as he proceeded I remarked with satisfaction that his expression became more and more dubious and worried. From time to time he made an entry on a memorandum slip beside him, and, as the entries accumulated, his frown grew deeper and his aspect more puzzled and gloomy.
 
At length he sat up, and taking the memorandum slip in his hand, addressed the judge.
 
"I have finished my examination, my lord."
 
"Very well. Mr. Nash, will you kindly examine the paper and write down the results of your examination?"
 
"Oh! I wish they would make haste," whispered Juliet. "Do you think they will be able to tell the real from the false thumb-prints?"
 
"I can't say," I replied; "but we shall soon know. They looked all alike to me."
 
Mr. Nash made his examination with exasperating deliberateness, and preserved throughout an air of stolid attention; but at length he, too, completed his notes and handed the paper back to the usher.
 
"Now, Mr. Singleton," said the judge, "let us hear your conclusions. You have been sworn."
 
Mr. Singleton stepped into the witness-box, and, laying his notes on the ledge, faced the judge.
 
"Have you examined the paper that was handed to you?" asked Sir Hector Trumpler.
 
"I have."
 
"What did you see on the paper?"
 
"I saw twenty thumb-prints, of which some were evident forgeries, some were evidently genuine, and some were doubtful."
 
"Taking the thumb-prints seriatim, what have you noted about them?"
 
Mr. Singleton examined his notes and replied—"The thumb-print on square one is evidently a forgery, as is also number two, though it is a passable imitation. Three and four are genuine; five is an obvious forgery. Six is a genuine thumb-print; seven is a forgery, though a good one; eight is genuine; nine is, I think, a forgery, though it is a remarkably good imitation. Ten and eleven are genuine thumb-marks; twelve and thirteen are forgeries; but as to fourteen I am very doubtful, though I am inclined to regard it as a forgery. Fifteen is genuine, and I think sixteen is also; but I will not swear to it. Seventeen is certainly genuine Eighteen and nineteen I am rather doubtful about, but I am disposed to consider them both forgeries. Twenty is certainly a genuine thumb-print."
 
As Mr. Singleton's evidence proceeded, a look of surprise began to make its appearance on the judge's face, while the jury glanced from the witness to the notes before them and from their notes to one another in undisguised astonishment.
 
As to Sir Hector Trumpler, that luminary of British jurisprudence was evidently completely fogged; for, as statement followed statement, he pursed up his lips and his broad, red face became overshadowed by an expression of utter bewilderment.
 
For a few seconds he stared blankly at his witness and then dropped on to his seat with a thump that shook the court.
 
"You have no doubt," said Anstey, "as to the correctness of your conclusions? For instance, you are quite sure that the prints one and two are forgeries?"
 
"I have no doubt."
 
"You swear that those two prints are forgeries?"
 
Mr. Singleton hesitated for a moment. He had been watching the judge and the jury and had apparently misinterpreted their surprise, assuming it to be due to his own remarkable powers of discrimination; and his confidence had revived accordingly.
 
"Yes," he answered; "I swear that they are forgeries."
 
Anstey sat down, and Mr. Singleton, having passed his notes up to the judge, retired from the box, giving place to his colleague.
 
Mr. Nash, who had listened with manifest satisfaction to the evidence, stepped into the box with all his original confidence restored. His selection of the true and the false thumb-prints was practically identical with that of Mr. Singleton, and his knowledge of this fact led him to state his conclusions with an air that was authoritative and even dogmatic.
 
"I am quite satisfied of the correctness of my statements," he said, in reply to Anstey's question, "and I am prepared to swear, and do swear, that those thumb-prints which I have stated to be forgeries, are forgeries, and that their detection presents no difficulty to an observer who has an expert acquaintance with finger-prints."
 
"There is one question that I should like to ask," said the judge, when the expert had left the box and Thorndyke had re-entered it to continue his evidence. "The conclusions of the expert witnesses—manifestly bona fide conclusions, arrived at by individual judgement, without collusion or comparison of results—are practically identical. They are virtually in complete agreement. Now, the strange thing is this: their conclusions are wrong in every instance" (here I nearly laughed aloud, for, as I glanced at the two experts, the expression of smug satisfaction on their countenances changed with lightning rapidity to a ludicrous spasm of consternation); "not sometimes wrong and sometimes right, as would have been the case if they had made mere guesses, but wrong every time. When they are quite certain, they are quite wrong; and when they are doubtful, they incline to the wrong conclusion. This is a very strange coincidence, Dr. Thorndyke. Can you explain it?"
 
Thorndyke's face, which throughout the proceedings had been as expressionless as that of a wooden figurehead, now relaxed into a dry smile.
 
"I think I can, my lord," he replied. "The object of a forger in executing a forgery is to produce deception on those who shall examine the forgery."
 
"Ah!" said the judge; and his face relaxed into a dry smile, while the jury broke out into unconcealed grins.
 
"It was evident to me," continued Thorndyke, "that the experts would be unable to distinguish the real from the forged thumb-prints, and, that being so, that they would look for some collateral evidence to guide them. I, therefore, supplied that collateral evidence. Now, if ten prints are taken, without special precautions, from a single finger, it will probably happen that no two of them are exactly alike; for the finger being a rounded object of which only a small part touches the paper, the impressions produced will show little variations according to the part of the finger by which the print is made. But a stamp such as I have used has a flat surface like that of a printer's type, and, like a type, it always prints the same impression. It does not reproduce the finger-tip, but a particular print of the finger, and so, if ten prints are made with a single stamp, each print will be a mechanical repetition of the other nine. Thus, on a sheet bearing twenty finger-prints, of which ten were forgeries made with a single stamp, it would be easy to pick out the ten forged prints by the fact that they would all be mechanical repetitions of one another; while the genuine prints could be distinguished by the fact of their presenting trifling variations in the position of the finger.
 
"Anticipating this line of reasoning, I was careful to make each print with a different stamp and each stamp was made from a different thumb-print, and I further selected thumb-prints which varied as widely as possible when I made the stamps. Moreover, when I made the real thumb-prints, I was careful to put the thumb down in the same position each time as far as I was able; and so it happened that, on the sheet submitted to the experts, the real thumb-prints were nearly all alike, while the forgeries presented considerable variations. The instances in which the witnesses were quite certain were those in which I succeeded in making the genuine prints repeat one another, and the doubtful cases were those in which I partially failed."
 
"Thank you, that is quite clear," said the judge, with a smile of deep content, such as is apt to appear on the judicial countenance when an expert witness is knocked off his pedestal. "We may now proceed, Mr. Anstey."
 
"You have told us," resumed Anstey, "and have submitted proofs, that it is possible to forge a thumb-print so that detection is impossible. You have also stated that the thumb-print on the paper found in Mr. Hornby's safe is a forgery. Do you mean that it may be a forgery, or that it actually is one?"
 
"I mean that it actually is a forgery.&............