eye. Moreover, the entire print was covered by a network of fine black
lines, by which it was divided into a multitude of small squares, each
square being distinguished by a number.

"Excellent, Polton," said Thorndyke approvingly; "a most admirable
enlargement. You see, Jervis, we have photographed the thumb-print in
contact with a numbered micrometer divided into square twelfths of an
inch. The magnification is eight diameters, so that the squares are here
each two-thirds of an inch in diameter. I have a number of these
micrometers of different scales, and I find them invaluable in examining
cheques, doubtful signatures and such like. I see you have packed up the
camera and the microscope, Polton; have you put in the micrometer?"

"Yes, sir," replied Polton, "and the six-inch objective and the
low-power eye-piece. Everything is in the case; and I have put 'special
rapid' plates into the dark-slides in case the light should be bad."

"Then we will go forth and beard the Scotland Yard lions in their den,"
said Thorndyke, putting on his hat and gloves.

"But surely," said I, "you are not going to drag that great microscope
to Scotland Yard, when you only want eight diameters. Haven't you a
dissecting microscope or some other portable instrument?"

"We have a most delightful instrument of the dissecting type, of
Polton's own make--he shall show it to you. But I may have need of a
more powerful instrument--and here let me give you a word of warning:
whatever you may see me do, make no comments before the officials. We
are seeking information, not giving it, you understand."