“Thank you,” said Thorndyke, taking it from her hand just as Mrs. Hornby was reaching out to intercept it.  He cut the thread and drew from its wrappings a little book bound in red cloth, with the word “Thumbograph” stamped upon the cover, and was beginning to inspect it when Mrs. Hornby rose and stood beside him.

“That,” said she, as she opened the book at the first page, “is the thumb-mark of a Miss Colley.  She is no connection of ours.  You see it is a little smeared—­she said Reuben jogged her elbow, but I don’t think he did; at any rate he assured me he did not, and, you know—­”

“Ah!  Here is one we are looking for,” interrupted Thorndyke, who had been turning the leaves of the book regardless of Mrs. Hornby’s rambling comments; “a very good impression, too, considering the rather rough method of producing it.”

He reached out for the reading lens that hung from its nail above the mantelpiece, and I could tell by the eagerness with which he peered through it at the thumb-print that he was looking for something.  A moment later I felt sure that he had found that something which he had sought, for, though he replaced the lens upon its nail with a quiet and composed air and made no remark, there was a sparkle of the eye and a scarcely perceptible flush of suppressed excitement and triumph which I had begun to recognise beneath the impassive mask that he presented to the world.

“I shall ask you to leave this little book with me, Mrs. Hornby,” he said, breaking in upon that lady’s inconsequent babblings, “and, as I may possibly put it in evidence, it would be a wise precaution for you and Miss Gibson to sign your names—­as small as possible—­on the page which bears Mr. Reuben’s thumb-mark.  That will anticipate any suggestion that the book has been tampered with after leaving your hands.”

“It would be a great impertinence for anyone to make any such suggestion,” Mrs. Hornby began; but on Thorndyke’s placing his fountain pen in her hand, she wrote her signature in the place indicated and handed the pen to Miss Gibson, who signed underneath.

“And now,” said Thorndyke, “we will take an enlarged photograph of this page with the thumb-mark; not that it is necessary that it should be done now, as you are leaving the book in my possession; but the photograph will be wanted, and as my man is expecting us and has the apparatus ready, we may as well despatch the business at once.”

To this both the ladies readily agreed (being, in fact, devoured by curiosity with regard to my colleague’s premises), and we accordingly proceeded to invade the set of rooms on the floor above, over which the ingenious Polton was accustomed to reign in solitary grandeur.

It was my first visit to these mysterious regions, and I looked about me with as much curiosity as did the two ladies.  The first room that we entered was apparently the workshop, for it contained a small woodworker’s bench, a lathe, a bench for metal work and a number of mechanical appliances which I was not then able to examine; but I noticed that the entire place presented to the eye a most unworkmanlike neatness, a circumstance that did not escape Thorndyke’s observation, for his face relaxed into a grim smile as his eye travelled over the bare benches and the clean-swept floor.