On a busy Sunday 40 years ago, a million people would come here, making this 3 1/2-mile stretch of beach, boardwalk and amusement park temporarily the sixth largest city in the nation.

They called Coney Island the Playground of the World and nobody snickered, but the world was already passing it by. For Coney Island, it has been a long slide, not as stomach-wrenching or thrill-packed as the rides in the amusement park, but just as inevitably heading in the same direction - down.

Only a small section of the amusement area still operates. Weeds grow where steeplechase park used to pack them into watch women's skirts blown aloft by air jets. Signs have long since lost letters, paint peels, games, bars and theaters along Surf Avenue are shuttered.

In the era of theme parks, of Walt Disney and all his creations, Coney Island is an ugly ducking. If it had a theme, it would be seediness, or as a Chamber of Commerce spokesman called it, "honky-tonk." The 300-foot high parachute jump proudly acquired from the 1939 World's Fair is the tallest derelict. If it can still be seen from 30 miles at sea, as Coney Island used to boast, the sailors see only its wire cables flapping in the wind, an old wreck whose last owner tried to make it a sort of aviary - the tallest bird feeding station in the world.

People with money to go elsewhere haven't flocked to Coney Island for years. On a summer Sunday, much of the wide, fine-grained beach that you once had to reach early in the day to capture a good spot is empty. Robert Moses once remarked that people packed the beach so closely each one took up less space than a coffin. Grave diggers could plant a lot of coffins in the sand today without disturbing a soul.

Coney Island was a fashionable playground when it was difficult to reach and there were no airplanes to take New Yorkers to Florida, the Caribbean, or California for their vacations. In 1920, the five-cent-fare subway was extended to Coney Island and the resort gave up its fashionable character and was jammed by people of all incomes. The playground of the world picked up a new nickname - "The Empire of the Nickel."

Charles Lindbergh rode the Cyclone, the fastest of what were four Coney Island roller coasters adn Coney Island quoted him as saying he considered "the Cyclone a greater experience than flying an airplane at top speed."

On a recent Sunday, a couple in black leather and dyed hair (his: white, hers: blue and red) rode the Cyclone's front car again and again, holding their fists aloft.

"Our clientele wouldn't like the theme," said Matthew Kennedy, executive secretary of the Coney Island Chamber of Commerce. "Some only come to ride one ride and stay on it half a day. It's the nature of the beast."

"This place used to be beautiful," said a woman who has worked 47 years at Coney Island. "Now, it's detriorated and so is its clientele."

When Coney Island was beautiful, Eddie Cantor, Jimmy Durante and Irving Berlin were singing waiters. Jim Jeffries fought Gentleman Jim Corbett for the heaveweight championship. Cary Grant walked around an stilts with a sandwich board advertising the rides and Al Capone was a bouncer. Angelo Siciliano worked on his muscles before changing his name to Charles Atlas.

Long before it became the Playground of the World, however, Coney Island had a history of sharp dealing and con men. One story even has it that the Canarsie Indians pulled the first fast one when they sold it to the Dutch in 1649. The Dutch had to buy it again from the Nyacks who really owned it.

The Dutch named it for its large rabbit population; Coney is a corruption of the Dutch word for rabbit.

In the late 19th century, one writer described Coney Island as a place for men who dug clams by day and cut throats by night. It was infamous for its three-card monte games.

Three-card monte has returned, along with a dice game operated on makeshift tables along the gaint pier.The pier is also now lined with crabbers who turn chicken into crabs. Traps are baited with pieces of raw chicken fresh from their supermarket wrappers.

If you forget to bring your chicken, you couldn't catch crabs, but if you forget to bring your radio, you won't miss a thing. Some of the largest "portables" going spend weekends here and you can sit or lie down within the sound pattern of the station of your choice.

The portables are only drowned out if they are carried too close to one of the awesomely loud "disco" rides like the Himalaya. For your 75 cents, the Himalaya not only gives you a dizzying trip forward and backward over its undulating track, but keeps you from hearing your own (or anyone else's) screams with a solid wall of music played by the ride's deejay.

On a good weekend, a small crowd of John Travolta look-alikes can be found showing off their steps on the Surf Avenue pavement next to the disco bumper cars.

Coney Island once had six imitators in the New York area, but most of them have been replaced by high-rise apartments or shopping centers.

Coney Island honky-tonk ambience is what the new, successful amusement parks are not. They thrive on clean-shaven employes for whom "have a nice day" is a constant refrain.

At Coney Island, close shaving is an elective and genial good wishes are nobody's stock in trade.

Riders on the Spook-a-Rama get their first fright from signs warning them not to touch the bushes passing by them: "danger, poison ivy," the signs say. The Wonder Wheel, a 58-year-old, 150-foot high monster that gives anyone who thought he was boarding a tame ferris wheel a nasty surprise, carries two dogs around and around. The dogs are a tradition of the Garms family, which built the wonder wheel.

Joseph Heller grew up at Coney Island and resented the smiling politicians who toured the beach and boardwalk looking for votes. How many would walk through Coney Island smiling if they had to live there? he once wondered.

Coney Island today depresses people who knew it long ago as children, not only because it is sleazy, but also to see the peeling paint if no people are blocking your view.

But for all the seediness, Kennedy of the Chamber of Commerce said that this summer will be Coney Island's best since 1970. With a good Labor Day weekend, Coney Island will have attracted 16 million visitors, Kennedy said.

The reason for this year's upturn is the reverse of the reason for Coney Island's long decline, Kennedy speculated. People don't have as much money as in years past and they aren't going as far afield for their recreation.

The fare is now 50 cents, but the Playground of the World and Empire of the Nickel, which among its other achievements made the hot dog famous, is still only a subway ride away.