or some
other striking and characteristic portion of the ridge-pattern.

"This system of marking with reference numbers," said Mr. Singleton, "is
better than your method of squares, because the numbers are only placed
at points which are important for comparison, whereas your squares or
the intersections of the lines fall arbitrarily on important or
unimportant points according to chance. Besides, we can't let you mark
our original, you know, though, of course, we can give you a photograph,
which will do as well."

"I was going to ask you to let me take a photograph presently," said
Thorndyke.

"Certainly," replied Mr. Singleton, "if you would rather have one of
your own taking. I know you don't care to take anything on trust. And
now I must get on with my work, if you will excuse me. Inspector Johnson
will give you any assistance you may require."

"And see that I don't pocket the original," added Thorndyke, with a
smile at the inspector who had shown us in.

"Oh, I'll see to that," said the latter, grinning; and, as Mr. Singleton
returned to his table, Thorndyke unlocked the microscope case and drew
forth the instrument.

"What, are you going to put it under the microscope?" exclaimed Mr.
Singleton, looking round with a broad smile.

"Must do something for my fee, you know," replied Thorndyke, as he set
up the microscope and screwed on two extra objectives to the triple
nose-piece. "You observe that there is no deception," he added to the
inspector, as he took the paper from Mr. Singleton's table and placed it
between two slips of glass.

"I'm watching you, sir," replied the officer, with a chuckle; and he did
watch, with close attention and great interest, while Thorndyke laid the
glass slips on the microscope stage and proceeded to focus.

I also watched, and was a good deal exercised in my mind by my
colleague's proceedings. After a preliminary glance with the six-inch
glass, he swung round the nose-piece to the half-inch objective and
slipped in a more powerful eye-piece, and with this power he examined
the blood-stains carefully, and then moved the thumb-print into the
field of vision. After looking at this for some time with deep
attention, he drew from the case a tiny spirit lamp which was evidently
filled with an alcoholic solution of some sodium salt, for when he lit
it I recognised the characteristic yellow sodium flame. Then he replaced
one of the objectives by a spectroscopic attachment, and ha