In 1889 the world flocked to Paris to witness the marvels of the onrushing future: a gallery of 16,000 gleaming and whirring machines in a city illuminated by electric light and surmounted by the great wonder of the modern world, the newly erected Eiffel Tower. But on the warm summer evening of July 26, a sensational murder was committed that would grab headlines and make legal history. Toussaint-Augustin Gouffé, a wealthy businessman with bohemian tastes, slipped away from his friends for a secret rendezvous from which he never returned. It would eventually be revealed that he had been strangled in a rented apartment, apparently by a noose made with the silk cord of his mistress's dressing gown.
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Little Demon in the City of Light

By Steven Levingston
(Doubleday, 333 pages, $26.95)
But his mistress, Gabrielle Bompard, had not acted alone. Gouffé had been lured to his death by her partner, a smooth-talking, vicious con man named Michel Eyraud. When the case finally came to trial, Gabrielle would claim that, although she was physically present at the fatal moment, she was an unwitting accomplice: Eyraud had put her into a mesmeric trance while he strangled his victim.
This was the first time that mesmerism, or hypnosis, was used as a defense in a court of law. If Eyraud had complete control of Gabrielle's mind, how could she be guilty? But can hypnosis really force the innocent to acquiesce in violent and criminal acts? These questions continue to haunt the modern imagination, through fictional creations from Svengali to the Manchurian Candidate and sensational trials such as Charles Manson's and Patty Hearst's .
The "affaire Gouffé" has been exhumed several times in recent years, most notably by the Oxford University Prof. Ruth Harris, whose work Steven Levingston credits as his original inspiration for his own study, "Little Demon in the City of Light." But Mr. Levingston, who is nonfiction book editor of the Washington Post and knows a good story when he sees one, has given it a richly enjoyable telling. Its lurid and improbable plot twists are expertly transposed into a breathless true-crime thriller set against a sumptuous evocation of the boulevards, nightclubs and boudoirs of Belle Époque Paris.
The case was, to begin with, a complete mystery: Gouffé vanished without trace while Gabrielle and Eyraud fled to Canada incognito, passing themselves off as father and daughter. The threads were gathered by the inspired detective work of police inspectors from the Paris Sûreté and the pioneering forensics of Dr. Alexandre Lacassagne, who identified Gouffé from the fragmentary clues offered by a hideously decayed corpse discovered outside a village near Lyon several months after the murder.
To a public equally fascinated by the recent Jack the Ripper murders in London, such astonishing feats of detection seemed to blur fact and fiction. The public's appetite was stoked by a sensationalist press "in frantic competition to outdo one another in scandal, murder and pathos," as Mr. Levingston writes. The crime was not only imagined and analyzed but physically re-created. A waxwork museum made a tableau of the apartment complete with chaise longue and red silk sash; a replica of the trunk in which the body was spirited away was placed on display in the Paris morgue, where it attracted 20,000 visitors; the police staged a re-enactment that gathered a huge crowd at the scene of the crime.
The investigators knew that progress depended on keeping press and public engaged. As the chief of the Sûreté, Marie-François Goron, observed, "the more the newspapers occupy themselves with this case, the more information will come to us."
After months of fevered speculation, Gabrielle calmly returned to Paris from New York, on the arm of a French businessman who had no idea that she was wanted for murder. When placed under arrest, she denied any knowledge of the crime, but under intensive questioning she spun a web of false confessions before eventually settling on a version in which she had watched, paralyzed by fear, as Eyraud strangled Gouffé with his bare hands and threatened her with death if she betrayed him.
Gabrielle presented herself as a victim in the affair and seemed scarcely interested in the accusations against her, consumed instead by her sudden celebrity and preoccupied by her choice of outfits for the daily appearances before the scrum of journalists and public. To them, she was a fascinating femme fatale, a "little demon" of amoral wickedness; but to some psychiatrists her case appeared more complex.
It emerged that, when she was a young girl, her family doctor had discovered that she was exceptionally susceptible to hypnosis, and she was practiced at performing under the influence. Medical commentators began to diagnose her as a "hysteric," subject to mental dissociation during which she might be unconscious of her own actions.
Michel Eyraud was finally arrested in Havana after scamming his way across Canada, the U.S. and Mexico and was shipped back to Paris in a cage. He and Gabrielle stood trial together, though their versions of the murder conflicted wildly.
The views of the expert witnesses were also opposed. The neurologists of the dominant Paris school, led by Freud's mentor Jean-Martin Charcot, maintained that hypnotized subjects could be made to carry out trivial acts by the power of suggestion but not acts that required them to override their free will or moral compass. But for Jules Liégeois, a specialist in hypnotic crime at the University of Nancy, there were no such limits: An impressionable subject could be transformed into an automaton entirely under the hypnotist's control, no more conscious or guilty than a knife or gun.
The trial would set an important legal precedent—both defendants were found guilty, with Eryraud sentenced to death and Gabrielle to 20 years in prison—but to most of the public it was simply gripping drama: Le Figaro pronounced the trial more fascinating than anything to be seen in the Paris theaters. Mr. Levingston vividly conveys the courtroom drama and the aftermath of the trial to round out his fine portrait of a bright city in the grip of a dark scandal.
Mr. Jay is the author of "A Visionary Madness" and "The Atmosphere of Heaven."