Jerome Kern, the composer who opened a new world for popular song and brought new dimensions to the American musical theater, appears on the first commemorative of 1985, coming out Wednesday.

The stamp marks the 100th anniversary of the birth of the man called "King of the American Musical Stage," but is actually being released four days earlier. Kern was born Jan. 27 in New York City, where the stamp is being issued. The 27th is a Sunday, a day when first days of issue are rarely scheduled.

The vertical commemorative, the eighth in the Performing Arts Series, is the first of the new issues to bear the 22-cent first-class rate that goes into effect Feb. 17.

Kern, who died in 1945, composed more than 1,000 songs and 108 theatrical scores. One of his works, "Show Boat," is generally considered America's best folk operetta.

His first classic stage tune was "They Didn't Believe Me" in 1914, a new and revolutionary song for its times that sold an astronomical 2 million copies in sheet music. Then came "Till the Clouds Roll By," "Look for the Silver Lining," "Who" and "Lovely to Look At."

Also "The Night Was Made for Love," "She Didn't Say Yes," "I've Told Every Little Star," "The Song Is You," "Smoke Gets in Your Eyes," "All the Things You Are," "The Way You Look Tonight" and "The Last Time I Saw Paris," a melody written in one day in an emotional response to World War II.

These songs, with their simplicity and flow, give the impression of being knocked out with ease. But few popular composers have worked so painstakingly, spending hours and hours at the piano on a single modulation.

This is reflected in the new stamp. The lower third of this vignette, superimposed on a head-and-shoulder portrait of Kern, shows the composer at the piano making notations on a music score. The name appears above in black type, and the series designation in red at the bottom. Tucked away in the lower right corner is the denomination, "22," without a "c" or cent sign under the new format adopted by the Postal Service.

The stamp has been produced by gravure in yellow, red, blue and black. On the selvage of each post office pane of 50 stamps is a four-digit plate number preceded by the letter A. This letter has preceded plate numbers of stamps produced since 1979 by a private company, the American Bank Note Co., under contract with the Postal Service.

The commemorative was designed by Jim Sharpe of Westport, Conn., who also designed the previous seven issues of the series. These paid tribute to George M. Cohan, Jimmie Rodgers, Will Rogers, W.C. Fields, the three Barrymores, Ethel, John and Lionel, Douglas Fairbanks and John McCormack.

Kern was a collector of stamps, coins, silver goblets and drinking vessels, and rare books -- manuscripts and first editions -- and could well afford these hobbies, for his songs had made him a millionaire. He loved parlor games, baseball and betting on the horses. He was a devoted member of the Thanatopsis Literary and Inside Straight Club, along with George S. Kaufman, Heywood Broun and Alexander Woollcott, and the largest financial contributor to the club's sole activity, playing poker at regular sessions.

Collectors of first-day-of-issue cancellations have the customary 30-day grace period for sending in their orders -- orders must be postmarked no later than Feb. 22 -- and the usual alternative ways of ordering.

Collectors affixing stamps: Those acquiring the new issue themselves and affixing it on envelopes, which must bear return addresses, should send their first-day covers to Customer-Affixed Envelopes, Jerome Kern Stamp, Postmaster, New York, N.Y. 10001-9991. No remittance is necessary.

Postal Service affixing stamps: Collectors perferring complete processing by the Postal Service should send their envelopes, which must be addressed, to Jerome Kern Stamp, Postmaster, New York, N.Y. 10001-9992. The cost is 22 cents for each stamp to be affixed on a cover. Personal checks are accepted, cash is not welcomed, payment by postage stamps is rejected.