"As one wag said," Eugene C. Hosmer jokes to a caller," all we need now is an infestation of the two-year locusts in January, and a seven-year drought in February!"

This twitting reference to Philadelphia's image problems is being made in an unlikely place: the city's Convention and Visitors Bureau. But Hosmer, the bureau director whose job it is to sell the City of Brotherly Love to thousands of tourists and conventiongoers, tells it with a laugh. "We haven't lost our sense of humor," he says.

Indeed, a sense of humor may comfort Philadelphians looking back on a less-than-perfect Bicentennial year. After the fanfare and festivities had died down, the nation's birthday party didn't bring in as many tourists as anticipated.

Then, starting in late July, a mysterious ailment struck a total of 248 persons attending, or connected with, a convention here of the Pennsylvania American Legion. Twenty-nine deaths were attributed to the respiratory illness, and "Legion Fever" precipitated a scare that eventually led to the closing of the Bellevue-Stratford, the "grande dame" of Philadelphia's hotels, where the Legionnaires stayed. (Medical investigators failed to draw any connection between the cause of the mysterious outbreak and the Bellevue, but occupancy had plummeted.)

Some 6.8 million tourists visited Philadelphia in the Bicentennial year, Hosmer says, down from the bureau's projection of 9 million. Hosmer blames part of the falloff on estimates published earlier in the year that ran as high as 40 million. He believes the figure was unrealistic and tourists may have been frightened off by fears of overcrowding. Conventions, he says, were way up overall.

But "Legion fever" definitely took a toll. "We lost 34 conventions, we lost 160 board meetings, new product displays. We lost 300 dinners and galas," Hosmer says, blaming it on "the stigma of Legionnaire illness and the mystique of the unknown," as well as continuing publicity about cases - and non-cases - of the ailment.

(Last week a newly-discovered bacterium was identified as the cause of "Legion fever," according to the Center for Disease Control in Atlanta. But scientists still do not know where the bacterium came from or how it was spread.)

At first, "most all the hotels suffered to some extent," the bureau director explains, "but (tourists) have begun to trickle back and other hotels retained their solidarity." In some places, he says, the stigma still is felt. "The Horn and Hardart, a 100-year-old restaurant across the street from the Bellevue, has suffered a 41 per cent drop in business," he notes sadly.

After Legion fever erupted, Hosmer and his associates began a vigorous campaign to allay fears of illness and attract more visitors to the city. They contacted planners to each of 258 conventions remaining in the year, conducted sales blitzes in New York, Chicago and Washington, set up travel shows and brought travel agents to visit the city.

This year the visitors bureau is pushing a package called "The Great American Experience," a three-day, two-night sojourn designed to lure all those who didn't make it to Philadelphia in 1976 to come and see a city spruced up for the Bicentennial. "The party goes on," the tour literature promises.

The package includes three days and two nights lodging at one of 11 hotels, two dinners at any of those hotels or at seven separate restaurants, choice of a Gray Line tour or horse-drawn carriage ride about the city, plus free or reduced admission at a number of museums and tourist attractions. Prices range from $76 to $88 per person (double occupancy) to $98 to $124 (single occupancy).

Philadelphia is also casting an eye at Atlantic City, N.J., whose November referendum to allow casino gambling makes it a tougher competitor for convention business. Hosmer says his bureau is talking with its Atlantic City counterpart about tying in tourist - "not conventioneers" - activities of the two cities.

The effect of Legion fever on the city's image may be waning but has yet to wear off completely. And the outbreak came on the heels of another image the city has long had to endure. "For years, Philadelphia has been the brunt of nightclub jokes. It has lived with the W.C. Fields syndrome," Hosmer says. But he feels that the picture of Philadelphia as an overly conservative place is fading, too, and he doesn't hold the barbs against Fields.

In Hosmer's office hangs a framed black and white glossy photo of Fields with the wry inscription: "Philadelphia causes my heart to palpitate with excitement."

"Business is fine," says the young woman selling Gray Line tour tickets at the Sheraton Hotel. She says people aren't worried about Legion fever. It is a raw, windy day during the week after Christmas. A light snowfall the night before has frosted the city, but the streets are animated - if not bustling - with pedestrians.

The Gray Line Bicentennial Tour of Historic Philadelphia departs at 1 p.m., and there are three of us waiting at the bus at the appointed time. Our driver and guide, Melvin Blakeslee, bounds up to the bus, jovially asking us to wait for other tourists who "must be coming." Ten others straggle in and the Blakeslee pulls the big bus away for a three-hour blitz of Philly's historical high points.

His patter mixes demographic facts ("Philadelphia used to get be used wo square miles, 2,000 people, folks, it now is the nation's fourth largest city with just under 2 million people") with pride in the modern sculptures springing up in front of new buildings downtown ("1 per cent of the cost of public building must go into art to beautify that building"). We zip past the Franklin Institute, which houses a new exhibit of printing technology through the centuries, past City Hall with a statue of William Penn atop its tower, past the huge sculpture of a clothespin, its spring forming a 76, in Penn Center.

On Board Street, we pass the red brick Union League building - it dates from the Civil War and is now a bachelors club. Up comes the Bellevue-Stratford, shades pulled in its ornate windows. "It's for sale, folks, now's your chance," Blakeslee quips.

We shoot down narrow, colonial streets toward the area known as Society Hill. Our guide points out "busybodies," little colonial-era mirrors perched outside second and third-story windows that enable inhabitants to see who is callong on their neighbors. We pass by Elfreth's Alley, the oldest continuously occupied street in the United States. We stop at the Christ Church, and sit in the pew where George and Martha Washington worshipped.

We drive through South Philly, originally settled by Swedes, then Dutch, then English, and today, predominantly Polish and Italian families. The tour whips us by the port area and Penns Landing, there's a stop at Betsy Ross' house, a tour through Independence Hall and a visit to the Liberty Bell in its wood and glass pavilion built especially for the Bicentennial.

The guide provides a motherlode of small details about the city's history and how colonials lived. But the tour moves at a dizzying pace and makes one want to return for more leisurely exploring.

Society Hill is one neighborhood that lends itself to meandering. Its colonial rowhouses and gardens are being carefully restored, with attention to preserving each building's historical integrity. Little plaques show what type of fire insurance each home carried. And shops offer a blending of the old and the new.

But Philadelphia isn't all memories of the Declaration of Indepence and the birth of the republic. It's home for vast numbers of descendants of 19th-century immigrants, to whom tribute is paid at the Balch Institute, a few blocks from the Liberty Bell. A display brings together folk art, dolls, knicknacks, banners made by area high school students of a variety of ethnic groups, tables showing origins of words and tables listing immigrant inventors who contributed to American technology.

From Balch Institute it's a short walk to Chestnut Street, a pedestrian mall-busway. There, a different Philadelphia emerges: one of shoppers and strollers, modern department stores, cut-rate clothing shops and record stores. Many of the record merchants have put loudspeakers above their store doors, and rock and soul music blare into the street.

Turning into Broad Street brings a welcome silence. Office buildings, the Union League building and, reigning over the block, the Bellevue-Stratford. Affixed to the corner of the ornate building is a simple green and white sign: "For Sale. Bellevue-Stratford. Philadelphia's Finest Hotel." Below it is a telephone number that would-be buyers can call.

A barber shop in the basement "will remain open until further notice," another sign reads. In a front office of the hotel, an airline is selling tickets. Inside the Bellevue's elaborate marble and-gilt lobby, now dark, two security guards try to keep the curious out. But people still come to gaze at the 72-year-old landmark where Presidents, VIPs, such figures as Buffalo Bill Cody, Charles Lindbergh, Enrico Caruso - and the Legionnaires - came to visit.

"The Hunt Room over there was one of the city's most popular bars," whispers one passer-by to two companions. Another trio comes in for a look. They shake their heads over the prospect of the Bellevue being torn down so a new hotel can go up in its place.

One of the security guards commiserates a bit, then gently asks everyone to move on.