Since "The Corn is Green" was chosen for revival by the Elizabeth Theatre Group as a vehicle for Cicely Tyson, there is some irony to the fact that Tyson is precisely what is holding the show back in the Opera House, where it opened last night.

Emlyn Williams' drama may look a bit rickety in places, some four decades after it first triumphed with Ethel Barrymore at the helm. But it remains a surprisingly ennobling piece of stagecraft on the whole, and--who knows?--its fervent belief in the value of education and hard work may even have become fashionable again. But if you expect to experience the full measure of the play, you will have to tuck away several serious reservations that keep popping up in the course of the evening's three acts.

As Miss Moffat, a headstrong English schoolmarm who has vowed to educate the youthful miners in a small Welsh village out of their grinding poverty, Tyson has set herself a considerable challenge. Tradition, sociology and a fair amount of evidence in the script would seem to indicate that Miss Moffat is a role for a white actress. It is not probable that a black woman, especially such an unconventional one as Miss Moffat must necessarily be, would turn up in Wales under the particular circumstances of Williams' play. Still, it is not impossible and what are actresses for, if not to make us look at the world with fresh eyes?

The problem is that Tyson never makes the casting seem likely. Authority and principle, the qualities she projects so effortlessly on the movie or TV screen are, curiously enough, just what is lacking here. She is a handsome woman with a fine-featured severity, but the firm resolve she suggests in moments of repose tends to evaporate when she flies into battle. Miss Moffat is nothing if not a woman dedicated to her mission, a tireless champion of the world's accumulated knowledge, who gets others to adopt her ideals through the sheer force of her will. Surrounded, as she is, by an adept supporting cast, Tyson never seems of particularly exceptional stature.

Then there's the matter of the English accent that she has adopted for the role, but as yet has failed to master. Since the heart of Miss Moffat's credo is the notion that her Welsh-spouting charges must learn to do justice to the King's English if they ever hope to rise in the world, it is distressing that the teacher herself is, all too frequently, unintelligible. Tyson's delivery, which has a charming musicality in its natural state, assumes a rather predictable bird-like monotony when imprisoned in an English accent. Vocally, the actress seems to be operating at half-power. What dominance she achieves is managed largely through the dark flashing of her eyes and a defiant tilt of the chin.

Tyson has proved her mettle under other circumstances, and it is possible that with more performances under the belt of the pinch-waisted fashions she wears so elegantly, she may rise to the challenge of the play. Right now, however, it is the play that is carrying her along. Essentially, it's a rags to riches story--only the rags and riches are those of the mind. Left an inheritance by an uncle, Miss Moffat strides into the remote village of Glansarnom, announces her intentions of opening a school, inducts a silly local spinster and a religious zealot onto her teaching staff, and promptly defies the stuffy squire who owns most of the town and the townsfolk in it.

In short order, a rum-soaked orphan named Morgan Evans, whose sooty exterior hides the sensitive soul of a writer, becomes her prize pupil. Not without a goodly number of mishaps along the way, the play chronicles his education, often under duress, and his eventual admission to Oxford University. Although Williams wrote an unforgettable character in the person of Miss Moffat, whose efficient and sometimes officious manner sometimes blinds her to the simple demands of the human heart, he also took care to write his supporting characters. As matters stand, they are what animate this revival and give it humor and humanity.

Peter Gallagher certainly gives a deft performance as Morgan, a young man coming out of the mines into the light of knowledge. He has a strapping outdoors presence, but at the same time he is like a child in toyland. The portrayal is subtly balanced between eagerness and reticence, grace and awkwardness. And when he receives notice from the breathless postmistress that he has passed his entrance exams, you will hear from the audience an audible gasp of satisfaction.

Marge Redmond is wonderfully robust as Miss Moffat's housekeeper, who used to pick pockets until she got religion and put her fingers to work in the kitchen. Frank Hamilton and Elizabeth Seal are convincing as fellow teachers, even if Seal sometimes threatens to dislocate her jaw by smiling so broadly. And Gil Rogers' squire is a proper embodiment of harrumph.

But the unadulterated delight of this revival is Mia Dillon, who plays the housekeeper's shiftless daughter, a creature who'd rather chew on sweets than the intricacies of Greek. Dillon has concocted a cockney version of Baby Doll with considerable originality and flawless timing. Dim-witted, petulant, self-centered--one of her few enthusiasms is for winding her hair into ringlets, although she complains her fingers get awfully tired--the girl is really without redemption in this sober-sided world. As Dillon plays her, she is irresistible.

If he has yet to get from his star the star performance this revival calls for, director Vivian Matalon has staged the rest of "The Corn is Green" with ease and the same appreciation for the little moments of life that he demonstrated in "Morning's At Seven." William Ritman's set is a handsome example of a cluttered, but comfortable countryhouse, and Theoni V. Aldredge's costumes are astutely tailored to the characters' personalities.

With so much right--and right in place--Tyson's deficiencies tend to get thrown into high relief. The role is simply not being filled to its potential. A black Miss Moffat is not the issue. Tyson's Miss Moffat is.

THE CORN IS GREEN. By Emlyn Williams. Directed by Vivian Matalon; sets, William Ritman, costumes, Theoni Aldredge; lighting, Richard Nelson; With Cicely Tyson, Peter Gallagher, Mia Dillon, Elizabeth Seal, Gil Rogers, Frank Hamilton, Myvanwy Jenn, John Eames. At the Kennedy Center Opera House through July 31. CAPTION: Pictures 1 and 2, Cicely Tyson as Miss Moffat and Peter Gallagher as Morgan Evans in drama at the Kennedy Center; Cicely Tyson in "The Corn Is Green". Photos Copyright (c) 1983, Martha Swope