he hypotheses. John Hornby had access to the
diamonds, therefore it is possible that he stole them."

"But surely he was responsible to the owners."

"Not in the absence of gross negligence, which the owners would have
difficulty in proving. You see, he was what is called a gratuitous
bailee, and in such a case no responsibility for loss lies with the
bailee unless there has been gross negligence."

"But the thumb-mark, my dear fellow!" I exclaimed. "How can you possibly
get over that?"

"I don't know that I can," answered Thorndyke calmly; "but I see you are
taking the same view as the police, who persist in regarding a
finger-print as a kind of magical touchstone, a final proof, beyond
which inquiry need not go. Now, this is an entire mistake. A
finger-print is merely a fact--a very important and significant one, I
admit--but still a fact, which, like any other fact, requires to be
weighed and measured with reference to its evidential value."

"And what do you propose to do first?"

"I shall first satisfy myself that the suspected thumb-print is
identical in character with that of Reuben Hornby--of which, however,
I have very little doubt, for the finger-print experts may fairly be
trusted in their own speciality."

"And then?"

"I shall collect fresh facts, in which I look to you for assistance,
and, if we have finished breakfast, I may as well induct you into your
new duties."

He rose and rang the bell, and then, fetching from the office four
small, paper-covered notebooks, laid them before me on the table.

"One of these books," said he, "we will devote to data concerning Reuben
Hornby. You will find out anything you can--anything, mind, no matter
how trivial or apparently irrelevant--in any way connected with him and
enter it in this book." He wrote on the cover "Reuben Hornby" and passed
the book to me. "In this second book you will, in like manner, enter
anything that you can learn about Walter Hornby, and, in the third book,
data concerning John Hornby. As to the fourth book, you will keep that
for stray facts connected with the case but not coming under either of
the other headings. And now let us look at the product of Polton's
industry."

He took from his assistant's hand a photograph ten inches long by eight
broad, done on glazed bromide paper and mounted flatly on stiff card. It
showed a greatly magnified _facsimile_ of one of the thumb-prints, in
which all the minute details, such as the