Now, one of the essentials in my profession is the ability to put the finger on the small mistakes a criminal makes when he endeavors to cover up his tracks. I suppose nine cases out of ten are solved in this way, and more often than not the thing left undone, unthought of, is the very one, you would imagine, which the criminal would have thought of first. I fancy the reason lies in the fact that the criminal does not believe he will be suspected. I said nothing to my chief about my visit to Gray's Inn last night.