Searching for the stolen diamond brooch, the police heard of a man attempting to sell a pawn ticket for exactly that, and they thought they had hit the jackpot. Why, the man was even living under a fake name and was planning to leave the country — all the evidence they needed to conclude he was a criminal ready to flee.

When the pawnbroker was located, however, Gilchrist’s maid was firm: The brooch he held was not the one belonging to her mistress. Not remotely daunted, the police, as Fox so neatly summarizes, followed the (il)logical syllogism: “All murders are committed by undesirables; Oscar Slater is an undesirable; therefore, Oscar Slater committed the Gilchrist murder.”

And so in 1909, after a brief and farcically prejudiced trial, Slater was convicted. It would be about another two years before Conan Doyle’s interest in the case was piqued, and he published “The Case of Oscar Slater,” detailing some of its more egregious elements: the lack of any evidence that Slater knew of Gilchrist and her jewels; the police’s assertion that the murder weapon was the hammer Slater had bought to make repairs to his flat, yet without any evidence that this bloody tool had stained his clothes when he carried it away again; the claim that the reason the jewels were untouched was that Slater, a stranger, did not know where they were. (Conan Doyle dryly pointed out that this was the case for “practically every man in Scotland.”)

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It was to be another three years before police files revealed that the man on the stairs, who had suspiciously vanished from all testimony, had early on been identified as both a member of Gilchrist’s family and someone with considerable connections to those in power. Instead of Slater being exonerated, the policeman who brought this to light was hounded out of his job, and the Establishment went to work once more on what it did best, covering up.

All of this is developed with brio by Fox. She is excellent in linking the 19th-century creation of policing and detection with the development of both detective fiction and the science of forensics — ballistics, fingerprints, toxicology and serology — as well as the quasi science of “criminal anthropology” as espoused by the Italian criminologist Cesare Lombroso.

She also illuminates the development of what she calls the “reconstructive” sciences at which the Victorians excelled, fields like geology, archaeology, paleontology and evolutionary biology, where gathered evidence is used to create “a narrative of things past … through the close reading, painstaking analysis and rigorous chronological ordering of what could be discerned in the present.” This is of course what detectives in theory, and Sherlock Holmes in fiction, did so well.