What was my idea in this nonsense?
It was this: When I was a youth, I knew an old Frenchman who had been a prison-keeper for thirty years, and he told me that there was one thing about a person which never changed, from the cradle to the grave--the lines in the ball of the thumb; and he said that these lines were never exactly alike in the thumbs of any two human beings.

In these days, we photograph the new criminal, and hang his picture in the Rogues' Gallery for future reference; but that Frenchman, in his day, used to take a print of the ball of a new prisoner's thumb and put that away for future reference.

He always said that pictures were no good--future disguises could make them useless; 'The thumb's the only sure thing,' said he; 'you can't disguise that.' And he used to prove his theory, too, on my friends and acquaintances; it always succeeded.
I went on telling fortunes.

Every night I shut myself in, all alone, and studied the day's thumb-prints with a magnifying-glass.

Imagine the devouring eagerness with which I pored over those mazy red spirals, with that document by my side which bore the right-hand thumb-and-finger-marks of that unknown murderer, printed with the dearest blood--to me--that was ever shed on this earth! And many and many a time I had to repeat the same old disappointed remark, 'will they NEVER correspond!' But my reward came at last.