Justice Lewis F. Powell Jr. sits second from the left when the Supreme Court takes the bench; seating follows seniority.

But if justices were arrayed by philosophy, Powell would sit exactly in the middle.

In an extraordinary illustration of how delicately the justices are now divided, Powell so far this term has been the court's decisive figure, its swing man and majority-maker.

* In 50 cases in which Powell has participated, he has been in the minority only once. That compares with six and four times respectively for the two other centrist justices, Byron R. White and Harry A. Blackmun, and 15 and 12 times for Justice Thurgood Marshall on the left and Justice William H. Rehnquist on the right.

* Nine times this year the court has split 5 to 4; in eight of those cases Powell made the difference. The ninth was his only time in the minority.

* In 12 cases this year Powell, who has been ill, did not take part. Five of those, including two important cases last week, ended in 4-to-4 ties.

That record illustrates again the significance of each justice's presence on a sharply divided court where five of the nine members are at least 76 years old.

If President Reagan has an opportunity to replace Powell, 77, William J. Brennan, 78, Marshall, 76, or Blackmun, 76, the court could be turned decisively to the right.

Of the 5-to-4 cases this year, two involved criminal law, an area where the court has been especially split. In one case, Powell gave Chief Justice Warren E. Burger a majority in a case involving improper conduct by a prosecutor. Conservative justices Rehnquist and Sandra Day O'Connor, along with White -- a conservative on most criminal issues -- also joined Burger.

In another case involving police interrogation, Powell gave his vote to Blackmun, who was joined by liberals Marshall and Brennan and moderate John Paul Stevens.

Powell was away from the court for 10 weeks following surgery for prostate cancer in January. He returned last Monday.

In his absence, the court heard oral argument in 56 cases. It has since dealt with 13 of those. Three it decided unanimously, and two by 6-to-2 votes. But three others it ordered reargued next month, apparently because the justices were closely divided without Powell.

On the remaining five, the court tied. Two of these five, controversial cases involving a Christmas nativity scene in a public park and the right of teachers to discuss homosexuality in school, were disposed of last week.

By court rules, a tie vote affirms the decision of the appeals court that heard the case, but is not binding in other appellate circuits. In most instances, tie votes thus have no more weight than if the high court had never taken the case.

The five ties this term are the most since 1970. Records dating back to 1927 show that the court has not had more than eight tie votes in one term, a record that was set in the 1940 term and might well be eclipsed during this term.

Powell has the option of voting in any or all of the remaining 43 cases on which he missed oral arguments. He can read the lawyers' briefs, listen to a tape of the oral argument and vote. He can do nothing at all, in fact, and simply vote. There is no law or rule on the matter, only a tradition that, if justices are ill for an extended time and do not hear oral argument or participate in the conference after argument, they generally stay out of a case.

Powell appears to have decided to follow that tradition, rather than selectively participate in a few cases where there are close votes. His colleagues apparently have not pressed him to take part in any cases.

One court observer suggested it was possible that some of the remaining justices, taking advantage of Powell's absence, may have decided after oral argument that some of the cases never should have been granted a hearing. It takes six votes to dismiss a case without opinion. A 4-to-4 tie, however, often accomplishes the same result.

If Powell stays on his present course, 43 more cases will be decided by an eight-member court. It is possible more will end up as ties.

Those cases include:

* Lowe v. Securities and Exchange Commission, which involves the SEC's right to censor certain financial newsletters;

* Devine v. NAACP Legal Defense Fund, which asks whether advocacy groups can be excluded from the Combined Federal Campaign;

* Brockett v. Spokane Arcades, involving whether materials inciting "lust" can be banned as obscene.

But Powell heard arguments and is expected to vote in a series of cases that involve church-state relations and promise to be among the most controversial of the term. Those cases question the constitutionality of a "moment of silence" in public schools, government aid to parochial schools and sabbath closing laws.

Most observers expect that Powell, a former president of the Richmond, Va., school board, will be the deciding vote in some of those cases, especially those involving aid to parochial schools.

Powell's recovery from recent surgery has been slow and difficult, sparking rumors he will retire at the end of this term. But he has picked clerks for the next term and has indicated he does not, at this time, have any intention of resigning.