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“Come at once! The game is afoot!”

What was “elementary” to the legendary detective Sherlock Holmes was not so obvious to his friend Dr. John Watson, or to the (frequently mistaken) police. In the new “Sherlock Holmes” exhibition at the St. Louis Science Center, however, visitors can learn how Holmes came to his conclusions and solve a mystery themselves.

Holmes’ creator, Sir Arthur Conan Doyle, was a physician. He had a keen interest in new medical techniques, including methods of examination, which he passed on to his fictional detective and to his readers.

“Sherlock Holmes” at the science center begins with scene-setting, bringing visitors into late 19th century London with movie props and period artifacts. We meet some influences on Conan Doyle and get to know the great detective. Moving on, we encounter a crime scene, and we’re invited to figure out just what happened there.

Geoffrey Curley, creator of the exhibition, was in St. Louis this week to oversee the installation. It originated in St. Paul, Minn., where he lives, but its content came from many sources, from the Museum of London to Warner Bros. Studios.

Visitors can learn more about the new technologies developed in that age of fast-moving scientific change. The scientific tools developed in the 19th century, from microscopes to fingerprints to the telegraph, were coming of age in the mid-1880s, Curley says, when Conan Doyle wrote his first Sherlock Holmes story, “A Study in Scarlet.”

Conan Doyle worked with the Scots medical lecturer Joseph Bell a decade earlier and gave Bell’s methods of diagnosis through observation to Holmes.

“Bell’s innovation was to observe the patient instead of the symptoms,” Curley says. At his lectures, patients would come in, and Bell would describe their lives before talking to them. “It was radical at that point.”

Holmes’ rooms at 221B Baker Street have been meticulously re-created from Conan Doyle’s descriptions of them, from his violin to his laboratory. A selection of the disguises for which the detective was known is featured.

From there, it’s only a few steps to a crime scene and a new Sherlock Holmes mystery that astute visitors can solve. Approved by the Conan Doyle Estate, it was written by Conan Doyle biographer Daniel Stashower with suitably Holmesian surprises.

Would-be detectives are given notebooks to record their observations and preserve evidence. They study the crime scene carefully, read the police reports, evaluate them and then test the evidence to see if the official conclusions match their discoveries.

A blood splatter over the mantelpiece has a bullet hole next to it; how was it made? Were those marks in the sand caused by dragging a body or by people walking over it multiple times? Test your hypotheses in the crime lab, then register your opinion by punching a hole in a page of the notebook. If you’ve guessed right, you’ll find a message from the master at the end of that section. Warning: It deliberately wasn’t made too easy.

From there, the exhibit switches to more modern times, with videos featuring real-life forensic scientists.

“I used my new house in St. Paul as a crime scene,” says Curley, as blood spurted and dripped on walls in a video. “The neighbors wonder about me.” (He had to repair and repaint his walls.)

The exhibit features posters, props and costumes from a variety of Holmesian adaptations from Warner Bros., CBS and the BBC, from the Robert Downey Jr. films and “Elementary,” along with toys, games and books inspired by Holmes.

It’s not just about entertainment, though. “The end goal,” Curley says, “is to enable people to use these techniques to evaluate the world around them.”

What The International Exhibition of Sherlock Holmes • When 9:30 a.m.-4 p.m. Monday-Saturday; 11 a.m.-4 p.m. Sunday; through Jan. 4 • Where St. Louis Science Center, 5050 Oakland Avenue • How much Adults $17.50; children and seniors $15.50 • More info 314-289-4400; slsc.org

Sarah Bryan Miller is the classical music critic for the St. Louis Post-Dispatch; she has also written on a variety of other topics.