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."