Last Friday, Sandra O'Connor joined The Brethren. And an elite sisterhood. Last Friday, Sandra O'Connor became the 102nd Supreme Court Justice. And the first woman justice. As all the speeches about barrier-breaking, history-making, inner-circle-integrating end, she is settling down to two of the hardest roles in the country: Supreme Court Justice and First Woman.

In some ways, this woman who won her robe with the unanimous consent of the Senate and the goodwill of the people, with the approval of conservatives and the best wishes of the women's rights activitists, faces the issues shared by any woman who has ever been the first, the exception, the only, the other.

How do you deal with the extra burdens? How do you live with the attention and the expectations? With the demands of conscience and history?

She wants to be remembered as a good justice, but she will be judged, in large part, as a woman justice. Her opinions will be scrutinized for signs of her sex; her behavior will be analyzed for clues of her kind. Like every other first woman, she will be visible and vulnerable, the one justice in the photograph whom everyone can name.

She will be criticized if she doesn't "think like a male justice" and criticized if she does. Someone will surely want her to prove that a woman on the bench makes a difference, and someone else will want her to prove that women on the bench are no different.

It is, as Margaret Hennig, dean of the Graduate School of Business Management at Simmons College (and who has studied first women in business) describes it, "a godawful burden. She knows she is going to have to prove, prove, prove." To begin with, having run the gauntlet of confirmation hearings. O'Connor now will have to deal with a new set of all-male colleagues in the give-and-take of the Supreme Court chambers.

As Hennig says, "She will be walking into a male world, and she'll have to establish herself. Most women who successfully join a male task group try to establish themselves at the level of technical competence. And while they do that, they begin to build one-on-one relationships so that when they are in group settings, they have some alliances. That's real hard when you are on the Supreme Court and have to perform right away."

If the public expects first women to be either wonderful or terrible, says Hennig, that woman is inclined to respond by being safe and moderate.

But Sandra O'Connor is not Every First Woman. Nor is the Supreme Court every job. She comes to a post with lifetime tenure, and she comes to it with strengths. At 51, she has the maturity, the manners and mentors of success. Like any woman lawyer or judge of her age, she must by now have become accustomed to being the first, the only.

Like Frances Perkins, the first woman Cabinet member, appointed in 1933 by FDR, O'Connor has consciously chosen the risky rewards of a pioneer. As secretary of labor Perkins put it, "I had been taught long ago by my grandmother that if anybody opens a door, one should always go through."

The first woman justice didn't have to leave much behind as she went through the doors. Unlike others before her, she hasn't had to choose between love and work. Unlike others before her, she hasn't had to choose between being a woman and a success. She has been bolstered by both the support of family and the women filling up the ranks in the law behind her.

With all these strengths, she may hold the one quality that separates the First Woman who makes a difference: the quality of personal integrity. The successful First Woman has the ability to put aside all the ambivalent expectations of the world, the ability to put to rest all the over-attentive claims of co-workers, and be true to her own beliefs.

That, it seems to me, is a pretty good qualification for any justice -- the 102nd or the first.