There's no tune-ness like show tune-ness. "Show tune" is in fact too frivolous a term for the best works to come out of the American musical theater during its halcyon, salad and palmiest days. It is but one more scandal for a Himalayan pile that we neglect this heritage as much as we do.

In a day and age when Ethel Merman -- for whom Gershwin, Porter and Berlin wrote -- decides to grovel through a disco album, one might be safe in assuming there is absolutely no hope and that Carter may as well reach for the little red phone.

but then along will come a boon and a bequest like "Song by Song," seven one-hour programs devoted to the very bon mots of American lyricists. It premieres tonight at 8 on Channel 26 and other public TV stations with a tribute to Alan Jay Lerner. Wouldn't it be loverly? It is.

Around 50 Lerner songs are cited, some only in snippets, and though the three hired singers do all right by most of them, it is the evening's guest star who does the knocking-for-loops. Lena Horne materializes just before midpoint to sing "I'm Glad I'm Not Young Any More" (from "Gigi") and "I've Grown Accustomed to His (Her) Face" from "My Fair Lady."

She also joins a rollicking trio for "What Do the Simple Folk Do?" from "Camelot."

There is little choice but to be at a loss for words in appreciating Lena Horne. Host-producer Ned Sherrin, from his quippy little perch at stage right, calls her "incandescent," which puts it mildly. Lena, Lena, Lena, Lena, Lena Lena Lena Lena Lena. In a word -- ohhhh, Lena.

She is royalty itself.

The hour is worth watching just for her, but it certainly has other pleasures, as well as a respect and affection for the honored work that comes across as more passionate and genuine than PBS's recent and recklessly overpraised "Musical Comedy Tonight," which might have been more aptly titled, "That's Entertainment, But We're Trying to Keep It a Secret."

Unwisely, funders have decided to turn that self-indulgent celebrity lark into a series. Better the Sherrin productions be extended beyond the planned seven.

Sherrin, as narrator, will stoop too low for a laugh, but his commentary is bright and lithe with no gratuitous showbiz shmoozing. Among the oddities culled for the Lerner show are a song from the flop musical "Lolita, My Love," written with composer John Barry, and a number from an unproduced musical film version of Huckleberry Finn."

The producing public TV station, Channel 13 in New York, appears to want to hide the fact that these programs were taped in London. One giveaway, beyond the accents of most in the cast, is that the British title, "Wedding Bells," is used to identify the MGM movie "Royal Wedding." represented with the song "How Could You Believe Me When I Said I Loved You, When You Know I've Been a Liar All My Life."

There is also a trace of what sounds suspiciously like artificial laughter. Congress should withhold all funds from public television until a truth-in- packaging guarantee is extracted. We get enough phoniness from the commercial side of broadcasting.

Though the tone of the program is definitely and infectiously festive, there is a hovering melancholy. The American musical theater has seen better days but few worse ones. Millicent Martin might be speaking for Broadway itself when she sings, appropriately off-key, Lerner's lines, "What did I have that I don't have? Where is the charm that I've run dry of? Wouldn't I be the late, great me, if I knew how? Oh, what did I have I don't have now?"

Song by Song" answers that question eloquently.