For 15 YEARS, that grassy knoll in Dallas has haunted much of the discussion of the assassination of President Kennedy. It was from that knoll, some theories hold, that an accomplice of Lee Harvey Oswald attempted to kill the president. There was a lot of speculation to this effect even before the recent reports on new scientific tests were presented to the House assassination committee. The new tests will require the careful attention of the committee, despite its desire to complete its work by the end of the year.

The committee's job is not easy. Unless it is done well, the temptation will be great-for those who believe Lee Harvey Oswald was part of a larger operation-to seize upon these new tests as hard evidence that such an operation did exist. On the other hand, the temptation will be just as great-for those who believe the Warren Commission's one-assassin theory-to dismiss the tests as meaningless because they raise more questions than they answer.

It is important to remember that what has been presented to the committee is not new evidence, but rather the results of test . The evidence has always been there-a recording of the radio transmissions made from a police motorcycle during the critical five minutes. What is new is the electronic equipment that has apparently convinced a group of specialists that noises on the recording were made by four bullets, one fired from the grassy knoll.

The first task of the committee is to establish or dispute the reliability of those tests. It needs to demonstrate, for example, how the experts distinguish between shots and automobile backfires or between rifle shots and pistol shots. It must also show how they determine from the noises on a recording the location of three different points-the place from which shots were fired, the place at which the bullets came to rest, and the place from which the radio transmissions were made. Above all, the committee must ask other electronic experts what they think of the capability of the new equipment and how much credence they put in the results that have been reached on it.

If the committee is persuaded that the tests are reliable and that a fourth shot was fired from the grassy knoll, other questions immediately arise. Why is it that in 15 years of investigation no credible evidence has been produced that anyone saw a person with a rifle on the knoll? Many people in Dallas that day thought the shots came from the knoll and looked toward it after the shots were fired, but no one whose story has checked out has claimed to have seen a person with a rifle there.

Even if the committee establishes that a fourth shot was fired at the president, that-standing alone-is not conclusive evidence that a conspiracy existed. But given the strange twists and turns of this tragic piece of history, everything needs to be weighed. We are beyond being surprised by any new tests or new evidence.There are incompletely answered questions all over the landscape, although the House committee's final report should complete many of the answers. But their existence, after all these years, counsels against making definitive judgments on these new test results. The committee must look upon what it has recently learned with objectivity and must not hestate to take whatever time and additional tests are necessary to answer the new questions that have appeared.