a stamp identical in character with the fingertip, 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 fingerprint 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 fingerprint, 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 fingerprints. 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 fingerprint. Of this fingerprint 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 fingerprint, 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 fingerprint having actual ridges and furrows identical in character with the ridges and furrows of the fingertip. 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 fingerprint 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 fingerprint 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 fingerprint, with which an undetectable forgery could be executed.”
“You have asserted that the counterfeit fingerprint 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,
