THESE ARE old women coming down the staircase. They are dumpy, their hair is dyed, they don't exactly keep time with the music. They are not very secure, and, for that matter, neither is the staircase they are descending. It is ratty. But it doesn't make any difference. The staircase is on the stage of a theatre that is about to become a parking lot, and the women-well, the women don't have much farther to go before they die.

These women were once Follies girls, the most beautiful and dreamed-about girls in America, stars of a musical entertainment that as much as anything defined the popular culture of this country during the period between the two wars. Follies, the new Harold Prince musical trying out in Boston on its way to New York, is about what has happened to these women since their golden moment and, more importantly, what has happened to the American dreams they symbolized for a generation.

The action of this show takes place during one evening, the night a group of Follies girls are reunited for a party thirty years after their theatre closed its doors. The occasion is the demolition of their building itself, although the glamorous home of their show has already in fact been gutted by the ravages of age. Age has not improved these women's lives either, and Follies is largely the many little stories its characters have to tell.

It is a measure of this show's brilliance (and its brilliance is often mind-hoggling) that it uses a modern musical form, rather than the old-fashioned one that the Follies helped create, to get at its concerns. As in his Company of last year, producer-director Prince has thrown out the time-honored musical convention of using songs to advance a simple-minded script in favor of letting the music add new levels of meaning to a sophisticated libretto (by James Goldman). In this way, the central plot idea of Follies becomes merely one more ingredient of the show rather than its raison d'etre.

This central "story" (and it cannot really be called that) involves two women. Phyllis and Sally, who had been best friends as showgirls, and had married Ben and Buddy, two stage-door johnnies before Pearl Harbor. The two couples have not seen each other in thirty years and, now that they meet, it is only to discover that they have all pursued the wrong goals and, worse, chosen the wrong mates.

This story is not told straight, however, developing slowly over the first act, almost incidentally to what is really happening on stage. For what is really going on is a party-a half-hearted celebration of faded performers-and, most of the time, it is the many other guests, singly or in groups, who are stage center.

Stephen Sondheim's score uses old conventions of songwriting as well as new ones, and it is in his music and lyrics that Follies puts across its extraordinarily upsetting point of view. Almost cruelly, we watch old performers of yesteryear relive their greatest moments on the stage, singing melodies that sound as ancient and scratched as our parents' old 78 records, dancing steps proclaiming a kind of spirit that has long since passed from their lives as well as our own. The world of the dead Follies and the reality of the present intermingle constantly in Sondheim's work. No sooner does a performer do her old soft shoe than the tin-pan-alley trumpet fades into a somber and often dissonant piece of music Sondheim has written to capture the mood of disintegration that hangs over the ongoing celebration.

The tension between the two very different eras represented on stage is more painful to watch than can possibly be described here, and the creators of the musical have found an amazing number of ways to exploit it. Besides the score, there is the device of having the characters shadowed throughout the show by their former selves, wearing the glamorous old costumes and white-faced make-up. There are two bands, a rich Follies orchestra in the pit and a downbeat jazz combo for the party on stage. Choreographer and co-director Michael Bennett has blended the dance steps of two generations, and Jonathan Tunick's orchestrations switch constantly from the Busby Berkeley sound to that of Mahler. Most important of all, the cast is filled with show business old-timers (prominent among them are Alexis Smith, Dorothy Collins, Gene Nelson Yvonne De Carlo, Ethel Shutta, Mary McCarty, Fifi D'Orsay and Ethel Barrymore Coh), all of whom are at once wonderful and sad.

PERHAPS no place do all these elements join together in such force as in a number in the first act that stopped the show dead for several minutes on opening night. All the old chorines have gotten together to do their old kickline number ("Mirror, mirror on the wall, who's that girl I see?/... That girl is me"), and, as they recreate it, we see the same number being performed in mirror image and full costume by the old girls as they were three decades earlier in the rear of the stage. The constrast between the two images of these characters and their times (as well as the unexpected double meaning of the trivial lyrics they are singing) produce an effect that is not nostalgic, as one might guess, but harsh and pathetic.

In the second act comes a more sustained triumph. As the four central characters (or eight, since each character is seen as two people) reach a bottleneck in trying to resolve their pasts and their futures, the lights come up, the set changes, and the grim shattered playhouse is transformed into the gaudy palace of yesteryear. Follies becomes an old Follies show itself. The follies of the characters are now expressed in terms of the old prewar musical until finally the follies and the Follies merge into one surrealistic nightmare. The play within the play (titled "Loveland") is as depressing as anything I've seen in the recent months (and that says a hell of a lot). It is roughly a combination of No, No Nanette! and Satyricon. The costumes are out-of-this-world in their vulgar beauty, to the point of becoming eerie; female chorines turn out to be men; a sprightly tap-dance number turns out to express one character's perception of how he hates himself. And on and on.

It is in this final segment that form and content become indistinguishable in Follies. It is then that it becomes clear that this musical play is not only about the failures of its characters and the death of a popular art form, but about the close relationship between the two. It is the cheap values that the Follies helped glamorize that has led Follies' characters to throw away their lives-values that belong to an America that is passing away, just like these old entertainers and their theatre. But it is a past that haunts these people and their society in its gaudiness and promises of dreams unfulfilled.

AT THIS point, six weeks before its Broadway opening, Follies is not yet in perfect shape, although it clearly will be soon. A good fifteen minutes of repetition in the first act can be pruned without much difficulty and a few awkward speeches in which the characters point too obviously at their failures during the first part of Act II can be eliminated. Suffice it to say, on the other hand, that Sondheim has composed his richest score (and also his most difficult), that Prince and Bennett have not missed out on any opportunities in their complex staging, and that Boris Aronson has designed a set that in itself evokes the substance of the show it houses.

It is easy to avoid Follies on the grounds that it is, after all, a Broadway musical-and, given what Broadway musicals have come to mean, such a bias is understandable. But that is precisely why you should see it, for Follies is a musical about the death of the musical and everything musicals represented for the people who saw and enjoyed them when such entertainment flourished in this country. If nothing else, Follies will make clear to you exactly why such a strange kind of theatre was such an important part of the American consciousness for so long. In the playbill for this show, the setting is described as "a party on the stage of this theatre tonight." They are not kidding, and there is no getting around the fact that a large part of the chilling fascination of Follies is that its creators' are in essence presenting their own funeral.