"What do you do?" says the dinner partner on my left. He's asking the man on my right, though, not me. Then he asks it again, this time skipping the woman beginning her gazpacho on his other side. She and I have bright, fixed smiles; in another second my gums will probably bleed. The other nonperson is also in her 40s, and I can tell this has happened to her (it turns out she's a designer) as often as to me. But this time I hear my voice break through the masculine exchange of job data. "Glad you asked," I shrill. "I'm a topless nuclear physicist."

Perhaps because I'm what's classified as "late bloomer," my - as they say - flowering is still so tentative that I'm thorny, impossible to please. Ignore me, like that dinner partner, and I'll bristle at what I see as an implicit assumption that a cute little old middle-aged blond like me spends her days bouncing through Bonwit's.But ask me, "What do you do?" and I'll probably resent, just as strenuously as my unemployed friends, the insinuation that you have to be salaried to be "interesting." My nominee for the Awesome Mental Health Award is the 40-ish woman who, I'm told, boomed at a cocktail party, "What do I do about what?"

Even middle-aged women who have been working for years report variations on the same problem. One I know happens to be a tenured professor, but when the stranger next to her at a dinner party sized her up, his idea of an appropriate conversational opener was: "It's nice to meet you; what does your husband do?"

It's not just the nervous grip on a career that can cause ambivalence in a woman of my vintage. Mine, after all, is the generation that has been besieged by rule changes, carpet-bombed for 25 years by contradictory messages. When our children were growing up, we were told that if we didn't have to work, our place was home raising them. That was the consensus of the "experts," few of whom troubled over such frivolities as inclination, temperament, training, to say nothing of what contributions we might have made to a society that saw us as a monolith of Moms.

If, as I did, you took all the '50s good mothering advice too seriously, you could get the impression that a child deprived of its mother's full-time focus might climb towers 20 years later and start shooting up the citizenry. The headlines would say, MOTHER CHOSE WORK, KID GOES BERSERK; the story would give your name, address, and alphabetize your inadequacies.

Doctrine started changing about 10 years ago when studies began indicating that children who are well cared for by outsiders fare just as well as those with stay-home mothers, that kids turn out best when mothers work or don't work according to a woman's own sense of herself. Not that the all-time final pronouncement is in, but it's hard cheese for those of us who went by the book when books had the opposite hypothesis.

What's hard to take, now that we have ripened by several decades, is being told today by so many of the experts that - for our mental well-being - we ought to be employed. But those experts aren't so quick to provide updated data telling middle-aged women how - if we took the old stay-home edicts as gospel - we might come up with job experience for this toughest of labor markets. And it must be just as galling not to want or need to work and feel you ought to apologize.

Generations of poor women, particularly minority, didn't have the comparative luxury of waiting for experts to tell them what's good for the family that happens to be hungry. But for an emerging group of middle-class women, survival has become an issue in the past few years, too. Yesterday's well-rounded nest egg doesn't go far toward supporting today's widow or the wife of an ill or unemployed husband (perhaps the very guy whose favorite long-playing record used to be, "No wife of mine is ever going to work!").

And whatever the reasons behind the huge rise in midlife divorce statistices - no fault, clear fault, or double fault - an army of women who never needed money before and have never worked (at least not in decades) is part of the numerical fallout.

The last time I made employment agency rounds was in the early '50s. I was 20 then - a just-married art major dropout given to informing personnel directors, "I want to do some thing cre-A-tive!" They rolled their eyeballs, and I took a sales job at Manhattan's Lord & Taylor, where it was soon clear that management had tabbed me for big things. Why else would they have notified our department that in the event of nuclear attack, I had been chosen to guard the Young People's Accessories' cash register?

Despite the honor, however, no missiles fell on Young People's Accessories that season, so I moved on to doing my best to impart lyricism to Montgomery Ward catalogue copy. But art would have to wait. I perceived a higher calling and bought a maternity wardrobe, just as all my friends were doing. It was time, we said, to get ready for motherhood.

Nobody forced me into that timetable, of course; there were women - even in those narrowly prescribed days - who were independent enough not to see life as an immutable schedule. Yet most of us played it pretty close to that well-ordered '50s routine, and today we're called empty nester. A lot of us love it. The vacated nest can hatch new beginnings - an opportunity for untried or favorite causes, creativity, school, delicious solitude. For some, it means time for the tennis court and the happy courting of old husbands - one's own and, in some cases, other people's.

Nevertheless, there are alarming empty-nest studies indicating that when our birds go off, some of us go off our birds; habit and the customs of a culture and a lifetime leave us picking through the straw, wondering what to do with the rest of our lives.

A college acquaintance quit law school to get married 23 years ago when she came down with a case of premature pregnancy, the first of four children. She and her husband agreed that her "place was at home." That is, until he decided their place was in divorce court; she didn't agree to That , but it was final and no-fault two years ago. I don't know their financial arrangements, but last time I glimpsed her, she was setting out lemon Jell-Os from the service side of a cafeteria looking as acerb as her wares.

Self-image can be the worst problem of all: egos in need of resuscitation, if not intensive care. If some of us were directors of personnel, we'd be the first to stamp ARE YOU KIDDING? on our own job applications. I once did an article about a state program that had a remarkable success record in helping the hard-core, longterm unemployed to find work."We've been extremely successful in placing men who haven't worked for years," a program pyschologist said, "recovered alcoholics, former psychiatric patients - you name the problem. But the people with the least self-confidence are women who have been longtime homemakers; they simply don't think of what they've done as having any value."

So it's encouraging, considering the rigors of late entry, that a number of women have managed a delayed launching into work that is exciting to them, sustaining, intellectually rewarding (though rarely financially so). Many of them say it took heart-pounding bravado until they learned a couple of trade tricks and some vocabulary. But given a chance to stick around till you get a grip on the patios, who can't learn to say "point of sale" or "show me the bottom line!"? Getting one's middle-aged metartarsus in the doors is, of course, the tricky part, but I know women who delayed careers till their offspring were grown - or nearly so - and managed to make the late entry. And to see some of them in action today, you'd think they are born holding briefcases.

Not so long ago, a sweet-face 63 year-old woman (42 years a wife, mother of four) applied for her first Social Security card. "Do you mean you've never worked before in you whole life?" said the incredulous young clerk. "The hell I didn't!" she exploded. "I just never got paid for it." She is obviously attempting her job debut later than most, and even if she's lucky enough to find work she likes, she paobably has some bad times ahead. It happens to most late bloomers, those days of despairing that we'll never "catch up." I know all about such days, when the putting together of just one respectable sentence thumbs its its prose at me, and I rage, rage against the dying of the brain cells. What, I scream at myself, have you been doing all these years?

"Ah, well," says the temporizer within (an indulgent and much too practiced voice), "better late . . ."

(From "Starting in the Middle" by Judith Wax. Copyright 1979 by Judith Wax. Published by Holt Rinehart & Winston.) Distributed by the Los Angeles Times Syndicate.