THE Washington Opera will open the Terrace Theater productions of its current season on Wednesday night when it presents Handel's "Semele," followed on Thursday night by Rossini's "Il Barbiere di Siviglia." There will be six performances of "Semele," and 11 of the "Barber."

Handel's "Semele" is rightly called the first great, full-length English opera.But those who went to hear it at Covent Garden in 1744, when it was new, were in real danger of being mugged by toughs who were hired by Handel's rivals for the express purpose of keeping people from hearing the new music. The night that King George II went to hear his favorite composer's newest opera, the thugs were arrested and kept away from the theater. But as soon as the royal theatergoers had paid their single call, the attacks began again and went on so regularly that it became news if a "Semele" night passed without some roughhousing.

When Handel wrote "Semele," he planned deliberately to present it during the Lenten season when, in the puritanical England of that day, opera performances were forbidden. Since oratorios were regularly substituted for opera during Lent, Handel carefully labeled his new music, "Semele, After the Manner of an Oratorio."

But no one was fooled, neither Handel's friends nor his enemies -- who were outraged that their rival had so successfully outfoxed the official rules. Hence their hiring of the very kind of low-life characters that had been elevated to the stage of Lincoln's Inn Fields in 1728 in "The Beggar's Opera," that remarkable work which eventually spelled the end of Handel's composing Italian operas and turned him permanently to oratorio.

Why is the Washington Opera presenting "Semele?" Because Handel's operas -- like Haydn's -- are among the few neglected operatic areas remaining in the world, and because at their best they have few rivals for unending melodic beauty. They have been left largely unperformed for many years, chiefly because of production problems common to most baroque operas: elements of the supernatural, including gods and goddesses who should, properly presented, descend from and rise to clouds via fancy machines; and because so many of Handel's operas call for that non-existent voice, the castrato. Handel operas languished almost unheard until the years immediately after World War I when German musicians, led by the late Oskar Hagen, began to bring them back to life, often on college campuses.

At the Kennedy Center, Handel opera was an early arrival: A revival of "Ariodante" appeared on the second night that the Center was open -- a performance still vivid in the memories of those fortunate enough to attend, and one that set standards for any Handel opera to follow. The Terrace Theater is an ideal size and ambience for all baroque opera and "Semele" is a superb beginning for what seems likely to become a popular feature there.

Rossini's most popular opera, "The Barber of Seville," has been so abused by singers, conductors, stage directors and editors that the prospect of hearing the work as it is revealed in a recently published definitive edition is alluring.

John Maucerti, the company's music director and conductor of the "Barber" performances, points out that a definitive edition of the "Barber" was not published until 1969! Mauceri finds it even more amazing that there have been very few productions in those 11 years that have taken advantage of the authoritative work of musicologist Alberto Zedda, who edited the score.

The Washington Opera is probably right in believing that the great Rossini score will take on a whole new sound in the intimate proportions of the Terrace -- which is very close in size to the Teatro Argentina in Rome where the "Barber" was first performed. Terrace Theater audiences will hear Rossini's original orchestration, which has neither trombones nor timpani, but includes a sistrum and guitar. The sistrum is an indefinite-pitch instrument that emphasizes the Spanish locale in which the opera plays. They will also hear a harpischord in the recitatives, which Rossini said he would prefer instead of the piano which he played while conducting the first performance of the opera.

Most important, they will hear the vocal lines sung much more nearly as Rossini wrote them than the way they are usually sung today. The role of Rosina was originally sung by a deep mezzo-soprano, Geltrude Righetti-Giorgi -- a very different sound from the high-pitched, lighter sound of the customary coloratura soprano of today. The use of the correct voice also makes unnecessary the many transpositions that have to be adopted when the higher voice is used. And it places Rossini's melodic lines in their correct relationship to the other voices of the cast. It is also reassuring to hear that stage director Lou Galterio does not plan to lard the witty glow of Rossini's score with stage directions that emphasize the hammiest elements in acting.

There are hundreds of reasons why "The Barber of Seville" retains its place as one of the great half-dozen comedy operas in the world. The coming productions may make those reasons newly clear.