In response to the summons, Polton made his appearance from some lair unknown to me, but presumably the laboratory, and, having received his instructions, retired, and presently returned carrying a box, which he laid on the table.  From this receptacle Thorndyke drew forth a bright copper plate mounted on a slab of hard wood, a small printer’s roller, a tube of finger-print ink, and a number of cards with very white and rather glazed surfaces.

“Now, Mr. Hornby,” said he, “your hands, I see, are beyond criticism as to cleanliness, but we will, nevertheless, give the thumb a final polish.”

Accordingly he proceeded to brush the bulb of the thumb with a well-soaked badger-hair nail-brush, and, having rinsed it in water, dried it with a silk handkerchief, and gave it a final rub on a piece of chamois leather.  The thumb having been thus prepared, he squeezed out a drop of the thick ink on to the copper plate and spread it out with the roller, testing the condition of the film from time to time by touching the plate with the tip of his finger and taking an impression on one of the cards.

When the ink had been rolled out to the requisite thinness, he took Reuben’s hand and pressed the thumb lightly but firmly on to the inked plate; then, transferring the thumb to one of the cards, which he directed me to hold steady on the table, he repeated the pressure, when there was left on the card a beautifully sharp and clear impression of the bulb of the thumb, the tiny papillary ridges being shown with microscopic distinctness, and even the mouths of the sweat glands, which appeared as rows of little white dots on the black lines of the ridges.  This manoeuvre was repeated a dozen times on two of the cards, each of which thus received six impressions.  Thorndyke then took one or two rolled prints, i.e. prints produced by rolling the thumb first on the inked slab and then on the card, by which means a much larger portion of the surface of the thumb was displayed in a single print.

“And now,” said Thorndyke, “that we may be furnished with all the necessary means of comparison, we will take an impression in blood.”

The thumb was accordingly cleansed and dried afresh, when Thorndyke, having pricked his own thumb with a needle, squeezed out a good-sized drop of blood on to a card.

“There,” said he, with a smile, as he spread the drop out with the needle into a little shallow pool, “it is not every lawyer who is willing to shed his blood in the interests of his client.”

He proceeded to make a dozen prints as before on two cards, writing a number with his pencil opposite each print as he made it.

“We are now,” said he, as he finally cleansed his client’s thumb, “furnished with the material for a preliminary investigation, and if you will now give me your address, Mr. Hornby, we may consider our business concluded for the present.  I must apologise to you, Mr. Lawley, for having detained you so long with these experiments.”