Handel, who was a great man for innovations, had one of his best inspirations back in the spring of 1733. As he was planning performances of "Deborah" and "Esther," he decided that between the acts of the two oratorios he would do something he had never done before: play an organ concerto.

Handel was a great organist. While studying in Italy in his early 20s, he got into a contest with Domenico Scarlatti. In the end, the Italian was called the better harpsichordist but Handel wasa acclaimed the superior organist.

It is probable that in all Europe, only Sebastian Bach surpassed him as an organist. So there was plenty of reason for Handel to give his adoring audiences something of the virtuosity that automatically went with his name and music, and which they were used to from the famous opera singers he employed.

Sir John Hawkins, a contemporary fan of the composer's, wrote about his organ playing this way: "Few but his intimate friends were sensible that on this instrument he had scarce his equal in the world; and he could not but be conscious that he possessed a style of performing on it that at least had the charm of novelty to recommend it." Hawkins also noted that Handel "may be said to be the inventor of the concerto."

By Feb. 19, 1736, those who went to Londons's Covent Garden to hear Handel's newest oratorio, "Alexander's Feast, or the Power of Music," could take for granted that between its two acts they would again hear the great man cutting loose not only with brilliant playing, full of dynamic contrasts and melodic inspiration, but also filled with the special excitement of improvisational genius - and Handel was, by every account, a stunning, imaginative master of on-the-spot invention.

Handel's organ concertos soon became polular, not as concert pieces, but invariably as the interludes or entractes brightening up the time between the acts of the oratorios.

This afternoon, in the Kennedy Center, between the acts of "Alexander's Feast," Kurt Rapf, an Austrian musician of renown, who carries the weighty title of "Music Director for the City of Vienna," will sit in for Handel to play the Organ Concerto in B Flat, No. 4 of Opus 4.

If Rapf is a bear for historical accuracy, his feet will not touch the pedals during the concerto, since English pipe organs in Handel's time, with the single, unfortunate exception of one in St. Paul's Cathedral, lacked a pedal board. Indeed, some of Handel's acquaintances sneered at the pedals as "the gridiron." The attractions in Handel's keyboard concertos lay in their vivid melodic ideas, the interplay between the organ tone and that of the accompanying instruments, which were always clearly delineated, and ultimately, the dazzle of the improvisation.

From publications of Handel's time, we know fairly certainly which concertos were played at the first performances and revivals of some oratorios. We also know that for at least th premiere of "Alexander's Feast." Handel did something very unusual: He switched from using an organ concerto to write one in the older form of the concerto grosso. The result is a brilliant work in C Major, with oboes, bassoons and strings.

What makes this particular concerto the more unusual is that it is also likely that at the same premiere, still another concerto, "for harp, lute and other instruments," was interpolated in the midst of Act One. Obviously Handel did not regard his oratorios as Holy Writ never to be altered or presented differently from one year to the next. On the contrary, he considered them, without exception, as dramatic pieces into and out of which he felt to add or remove sections as he chose.

Today's performance of "Alexander's Feast," which is based on one of John Dryden's odes in praise of St. Cecilia, will include and organ concerto, though not the concerto grosso. Its three solo roles, those of Thais, Alexander the Great and Timotheus, will be sung by Maralin Niska, Richard Lewis and Justin Diaz, with Stephen Simon conducting.

Did someone say "Thais?" Is that the famous courtesan of Alexandria, whose house is set afire by the monk Athanael at the end of the second act of Massenet's opera? Yep. Same girl.Only now, thanks to some fancy Dryden footwork, she is Alexander's girl friend. You see, Dryden decided to shift the action back, via Plutarch, to the time of a great feast celebrating Alexander's conquest of Persepolis.

Do not fear, however, gentle reader. Incendiarism triumphs in Handel just as much as in Massenet. Only this time both Thais and Alexander set fire, and not merely to a single house, but to all of Persepolis! For further details, visit the Kennedy Center this afternoon at 2.