Like many people, Lt. G. James Roby's knowledge of hypnotism stemmed from childhood memories of itinerant showmen. He recalls that his parents warned him against volunteering to be hypnotized by such crowd-pleasers who, they told him, had the power to control minds.

Roby left Uniontown, Pa., 19 years ago to join the Montgomery County Police Department. Today he is one of the police force's two trained hypnotists, rapidly becoming an expert in forensic hypnosis and somewhat of an itenerant himself.

"Law enforcement is by tradition resistant to change, but I've always considered myself open-minded," said Roby, of Olney, who in the two years since his first course in the art of inducing trances has used his skills in 45 investigations in Montgomery, Baltimore, Charles, Anne Arundel, St. Mary's and Howard counties. Nine of those cases subsequently were closed by arrests, five of them direct results of having information obtained by hypnosis.

"After 19 years as a police officer, you get to sense that people know something or should have seen something, but they can't focus their attention on it," Roby, 40, who is director of the Crimes Against Persons Division. "With hypnosis we get them around their prejudices, misconceptions and fears so they can recall what they saw."

Hypnosis has been used increasingly since the mid-1970s throughout the nation in investigations to help witnesses or victims of violent crimes recall details of a grisly event that trauma has blotted from memory. Critics call it gimmickry and question the validity of information obtained by hypnosis.

An important case contributing to the acceptance of hypnotically obtained evidence was Harding vs. the State of Maryland in 1968, in which a girl who was shot and raped could recall the crime only under hypnosis. The trial court admitted that evidence and the Maryland Court of Special Appeals affirmed the resulting conviction. Maryland, Virginia and the District of Columbia have no laws restricting the use of hypnosis, although until recently Virginia forbade its use except by physicians, dentists or clinical psychologists.

Roby's Crimes Against Person's Division is ideally suited to the use of hypnosis in its investigations of murder, rape, robbery, assaults, kidnapping and extortion. Speaking to a criminology class at the University of Maryland this month, Roby described a case in which a woman was beaten severely and could recall nothing of her assailant.

Under hypnosis, she screamed as she remembered a boot coming down on her face. Roby spoke to her in a quiet, monotonal voice and the woman began to describe the boot, the leg, torso and finally she was able to say what the face of her assailant looked like. Based on that information, investigators identified a suspect, obtained a search warrant, retrieved a boot and matched it to a footprint at the scene of the crime. The case is still under investigation.

Roby became entranced by hypnotism two years ago at a seminar on the latest developments in police science. He convinced the department to send him and Sgt. Tom Lechner to a three-day course at the Law Enforcement Hypnosis Institute in Los Angeles run by Martin Reiser, the Los Angeles Police Department's staff psychologist and a pioneer in forensic hypnosis techniques.

Since then Roby has completed courses at the Ethical Hypnosis Training Center in South Orange, N.J., the Iowa Institute of Medical, Therapeutic and Forensic Hypnosis in Des Moines and Reiser's advanced course. He teaches a three-day seminar at the Investigative Hypnosis Institute in Towson, Md., several times yearly and wants to become president of the International Association for Forensic Hypnosis, a year-old organization he helped found, which now has 150 members.

He has had little success, however, in getting funding to train other officers in his department, and recognizes that there still is widespread prejudice against hypnotism.

"The courts intellectually acknowledge the fact that hypnotism can be beneficial, but because of a personal bias, they have a hard time accepting it," he said.

Roby takes some good-natured ribbing from his colleagues at police headquarters off Shady Grove Road -- "don't look at his eyes or he'll hypnotize you and take over the department" -- but the record shows that in more than 80 percent of cases in which Roby or Lechner used hypnotism, the information obtained was important in furthering the investigation.