Mr. Woodthorpe under lock and key the first thing to-morrow morning. But how did he do it, do you think?"
"He might have taken duplicate finger-prints and kept one form; the prisoners would not know there was anything wrong; but he did not in this case. He must have contrived to take a photograph of the form before sending it in—it would take a skilful photographer only a minute or two with a suitable hand-camera placed on a table at the proper distance from the wall; and I have ascertained that he is a skilful photographer. You will probably find the apparatus, and the stamps too, when you search his rooms."
"Well, well. You do give us some surprises, doctor. But I must be off now to see about this warrant. Good-night, sir, and many thanks for your help."
When the superintendent had gone we sat for a while looking at one another in silence. At length Thorndyke spoke. "Here is a case, Jervis," he said, "which, simple as it is, teaches a most invaluable lesson—a lesson which you should take well to heart. It is this: The evidential value of any fact is an unknown quantity until the fact has been examined. That seems a self-evident truth, but like many other self-evident truths, it is constantly overlooked in practice. Take this present case. When I left Caldwell's
