IN THE summer of 1750, George Frederick Handel was feeling what he called "the uncertainty of human life." His artistic twin, Johann Sebastian Bach, born in the same year as he, 1685, had died on July 28. Late in the summer, Handel traveled to the German city of Halle where he was born. It was his last visit to the place; and on the way, while going through Holland, he was in an accident in which, he later wrote to his friend, Georg Philipp Telemann, he was "terribly hurt."

Being 65 that year, Handel had decided to make his will, a document dated June 1. He was full of honors. He also was rich. Early that year he withdrew 8,000 pounds from his bank account to pay for several paintings, including a large Rembrandt.

By then, Handel had written over 40 operas, 18 oratorios and a vast array of cantatas, anthems, music dramas, and in the purely instrumental realm, such great instrumental works as the organ concertos and concerti grossi. But he was not through composing. One more oratorio was still to come from the author of "Messiah," "Samson" and "Israel in Egypt." (How is it possible that no one has come up with a special gala performance of this last great work, in the light of recent history?)

Recovering from his accident, Handel began, on Jan. 21, 1975, to compose "Jephtha." Derived from an Old Testament story found in the book of Judges, it was to contain some of the most profound music Handel ever wrote. For 23 days he worked on the score, and then, on Feb. 13, he had to stop.

A page of the score on which he was then working has this note: "Got so far as this on Wednesday, Feb. 13, 1751, unable to continue because of the weakening of the sight of my left eye." However, 10 days later, on his 66th birthday, Handel was able to return to work on "Jephtha."

That it eventually took him until Aug. 30 to finish the oratorio is not a sign that he was working more slowly than his customary pace, but a reminder that he was again interrupted, for about three months, by a further dimming of his eyesight, a condition that grew worse in spite of various treatments, until, by early 1853, Handel was almost totally blind.

On the last page of the score, Handel wrote, "Aetatis 66," meaning, simply, "age 66." He had almost eight years of life left, in which he made minor revisions in and additions to various works. Far more important, he continued to rehearse, play and conduct many of his works right up to the last months of his life.

As for "Jephtha," which being performed this afternoon at 3 in the Kennedy Center, as the final work in this year's Handel Festival, it shows no signs whatsoever of being the last large score to come from this singular genius. Its story is about Jephtha, who foolishly promises Jehovah that if he is given the victory over the oppressing Ammonites, he will make a sacrifice to the Lord of "what, or whoe'er shall first salute mine eyes," upon his return.

You guessed it: Jephtha is victorious, and the first "thing" he sees when he returns home is his daughter, Iphis, coming out to hail him "glorious conqueror." The situation is almost precisely parallel to that between Agamemmon and his daughter, Iphigenia, in Greek mythology. In spite of the unforgiveable promise, Jephtha persists in his intention of sacrificing his daughter, until, in a foreshadowing of Christian charity, an angel appears and says Iphis need not die, she must simply be dedicated "to God, in pure and virgin state forever." While it avoids the necessity of human sacrifice, the deal is not just what Iphis's fiance, Hamor, would have chosen, but everyone agrees to the new deal.

Meanwhile, into the music Handel poured much of his finest writing: The character of Jephtha is richly outlined through recitatives and arias which culminate in the great scene, "Deeper and deeper still," and the famous "Waft her, angels, through the skies." The chorus that closes the second act, "How dark, O Lord, are thy decrees!" is called by biographer Paul Henry Lang, "Handel's greatest choral piece, one that is unparalleled in the entire choral literature." When you think of "Messiah," the Passion settings by Bach, or any other great choral music, the plane on which Handel was operating while writing this, his final masterpiece, becomes clear.