CBS: ON THE AIR," the network's 9 1/2-hour, seven-night celebration of its 50th birthday - starting with a two-hour blast at 9 tonight on Channel 9 - is a fabulous eulogy to class in television.

CBS had the class to hire stage producer Alexander H. Cohen to oversee the fiesta, and tonight's show includes excerpts from such imposing old Sunday shows as Ed Sullivan's, Alfred Hitchcock's, Jack Benny's, "See It Now" and "Small World." Even "Lassie" looks classy compared to "The Incredible Hulk."

Simultaneously, the party constitutes a last hurrah for network dignity and pride in presentation, both dead. Marching on the grave tonight are 122 "member of the CBS family" - from Eve Arden to Eric Sevareid - for a seven-minute round of canned applause. Mary Tyler Moore and Walter Cronkite are co-hosts and the week includes sparkling new material: on Wednesday, Dick Van Dyke will dance with animated Peanuts people, a surefire chamber.

But during production, at a Hollywood press conference, Cohen noted sadly that the great drama and variety of TV's Golden Age are "things we no longer have because of the strict competition between the networks." Oscar Katz, CBS executive in charge of the project, grumped, "I disagree. I think this age is much more golden. You have no idea how many 'Playhouse 90s' were bad."

The worst "Playhouse 90" was better, however, than the [WORD ILLEGIBLE] "Wonder Woman." Katz, alias, prevailed; the lengthier clips tonight are promos for bummers like "Kojak" that are still on the air.

At times, the producers' editorial judgement is baffling; at others, infuriating. The Smother Brothers show is acclaimed as controversial fare, though CBS repeatedly and stubbornly suppressed it topicality (writer Steve Martin can be glimpsed during a clip). Qualms aside, "On the Air" is divertingly presumptuous, grand and gala.

And it says, "Class is dead; long live schlock."