But the thumb-mark? Well, it was possible (though unlikely)—still possible—that the mark might have been made accidentally on some previous occasion and forgotten by Reuben, or even unnoticed. Mr. Hornby had seen the "Thumbograph," in fact his own mark was in it, and so would have had his attention directed to the importance of finger-prints in identification. He might have kept the marked paper for future use, and, on the occasion of the robbery, pencilled a dated inscription on it, and slipped it into the safe as a sure means of diverting suspicion. All this was improbable in the highest degree, but then so was every other explanation of the crime; and as to the unspeakable baseness of the deed, what action is too base for a gambler in difficulties?
I was so much excited and elated by my own ingenuity in having formed an intelligible and practicable theory of the crime, that I was now impatient to reach home that I might impart my news to Thorndyke and see how they affected him. But as I approached the centre of the town the fog grew so dense that all my attention was needed to enable me to thread my way safely through the traffic; while the strange, deceptive aspect that it lent to familiar objects and the obliteration of landmarks made my progress so slow that it was already past six o'clock when I felt my way down Middle Temple Lane and crept through Crown Office Row towards my colleague's chambers.
On the doorstep I found Polton peering with anxious face into the blank expanse of yellow vapour.
"The Doctor's late, sir," said he. "Detained by the fog, I expect. It must be pretty thick in the Borough."
(I may mention that, to Polton, Thorndyke was The Doctor. Other inferior creatures there were, indeed, to whom the title of "doctor" in a way, appertained; but they were of no account in Polton's eyes. Surnames were good enough for them.)
"Yes, it must be," I replied, "judging by the condition of the Strand."
I entered and ascended the stairs, glad enough of the prospect of a warm and well-lighted room after my comfortless groping in the murky streets, and Polton, with a final glance up and down the walk reluctantly followed.
"You would like some tea, sir, I expect?" said he, as he let me in (though I had a key of my own now).
I thought I should, and he accordingly set about the preparations in his deft methodical way, but with an air of abstraction that was unusual with him.
"The Doctor said he should be home by five," he remarked, as he laid the tea-pot on the tray.