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.”