Ozier Muhammad/The New York Times

Were you watching closely when the man in the blue blazer brushed by Henry R. Schlesinger, turned and started up the stairs in Grand Central Terminal?

“If that didn’t go well, I’d be sitting in prison,” Mr. Schlesinger said, fingering the rolled-up sheet of paper the other man had slipped him as they passed.

He left out another important if: If he and the other guy were real spies.

As it was, the paper that changed hands was a list of places in New York that figured in famous espionage cases. It is a long list.

“There are more spies in New York City than any other city,” said the other man, H. Keith Melton, an intelligence historian and the author of “The Ultimate Spy: Inside the Secret World of Espionage” (DK Publishing, updated 2009). “They could be the person next to you on the subway, or standing on the corner.”

Here is a helpful hint: The real spy is probably not the one muttering “Bond, James Bond.” But the real spy may not be as good with high-tech gadgets as with martinis that are shaken, not stirred.

Mr. Melton said that Anna Chapman, one of 10 sleeper agents returned to Russia in a spy swap in 2010, had a reputation for heavy-duty partying but seemed flummoxed by her laptop as she sat in a Starbucks on Eighth Avenue. “The last seat by the window,” he said. The hard part, for her, was establishing a wireless connection with a Russian handler hovering nearby.

“She was never sure he was getting the signal,” she said. The F.B.I. released a photograph showing Ms. Chapman “standing up and holding her computer to get a better signal,” he said. “Terrible tradecraft.”

Mr. Melton says he is a graduate of the United States Naval Academy and describes himself as “a historical consultant within the U.S. intelligence community.” He says he has been a guest lecturer at the Federal Bureau of Investigation, the National Security Agency and the Joint Military Intelligence College. He also says he is “an ongoing adviser to other U.S. intelligence agencies.”

And he has amassed a collection of some 10,000 espionage artifacts. Some of them will go on display starting Friday at Discovery Times Square, at 226 West 44th Street, in an exhibition that includes items from the Central Intelligence Agency and the F.B.I. Mr. Schlesinger, a freelance writer, worked with him on a forthcoming book listing dozens of cloak-and-dagger sites.

There are many, judging by everything they pointed out on a walk that began in Times Square — and they turned around long before the United Nations, which Mr. Melton called “the largest concentration of espionage in the world.”

The subway station at Seventh Avenue and 42nd Street? It is a historic landmark for the trench-coat set, Mr. Melton said. It is where a German spy, Erich Gimpel, was captured in 1944. Gimpel and William C. Colepaugh, an American traitor, arrived in Manhattan as would-be saboteurs. Their mission was to hang out in bars, eavesdrop on American servicemen and transmit what they learned to Germany.

Mr. Melton said they went to Rockefeller Center, they bought stylish clothes and they went to see “National Velvet” with Elizabeth Taylor and Mickey Rooney.

“They decided life in America is better than life in Germany,” he said. “It was a problem the Germans had here. It’s a Catch-22.” Colepaugh switched sides again, identifying Gimpel to the F.B.I.

He and Mr. Schlesinger strolled east on 42nd Street, passing an office building where the F.B.I. used one-way mirrors in a medicine cabinet to catch a German spy ring in 1941. Mr. Melton pointed to the Grand Hyatt New York, once the Commodore Hotel. He said that in Room 1400, Alger Hiss confronted Whittaker Chambers at a special subcommittee of the House Un-American Activities Committee investigating Soviet espionage.

And then he and Mr. Schlesinger ducked into Grand Central, where he said the C.I.A. had demonstrated the “brush-pass,” just as they did.

It works even if someone is being followed, Mr. Melton said, because there is usually a “gap of 10 seconds when surveillance can’t see you” — the surveillance agent has to stay a bit behind, and you turn a corner. He said the C.I.A. first demonstrated it for a Czech diplomat who would not meet with agents in Prague.

Of the items in his collection, Mr. Melton was clearly proud of a rock that figured in the Anna Chapman case. It was the hiding place for $60,000 in cash for her spy ring. Mr. Melton said the F.B.I. confiscated the money but never disclosed the exact location of the rock, except to say that it was in Wurtsboro, N.Y., about 80 miles from Midtown Manhattan.

“That’s all we knew,” Mr. Melton said. “Our goal was to find the rock. Using Google Earth, we put ourselves in the mind of the intelligence officer, and with the available information, we found the rock. I went back and met with the F.B.I. and they said, ‘What did you do today?’ I said, ‘I got the rock.’ They said, ‘Our rock?’ I said, ‘Not anymore.’”