After two years in the lab-years of deep anxiety-the most important Rembrandt lands cape in America is once again on view at the National Gallery of Art.

"The Mill" is not the same picture most viewers will remember. It has been cleaned. And its look has changed.

The painting has losts its "golden glow." a thick discolored coating of dirty yellow varnish. Its melancholy sky, once brown, is now white and blue and gray. An old man with a cane, invisible before, can now be discerned in its regions of deep shadows. So can once-concealed stonework, cows, a flock of sheep, shrubs and roadside flowers.

For the first time in decades, and perhaps in centuries, "The Mill" can be seen.

But should it have been cleaned"

Before its conservation, many thoughtful people would have answered no. They knew "The Mill" was dirty, but they loved it as it was -its sense of mystery and mood of sublime sadness.

The 19th-century romantics revered the yellowed painting J.M.W. Turner, the great painter, wrote approvingly, in 1808, of "its inestimable gloom." More recently John Walker, who preceded Carter Brown as the Gallery's director, thought cleaning was so risky he would not have it done

Many shared his fears. But having seen the picture, mine have been assuaged. What might have been a tragedy, has proved, instead, a triumph. Despite its newly lightened sky, its newly detailed shadows, the picture has lost nothing of its mystery or grandeur. The mystique of "The Mill" has not been damaged, but enanced.

And for the first time in many years its Rembrandt attribution, long questioned by the scholars, has begun to seem secure.

"The Mill", though neither signed nor dated, has been accepted as a Rembrandt, and a splendid one at that, since the early 18th century, when it was recorded in the French collection of the Duc d'Orleans. It was later sold to England where, despite its varnish, it changed English art.

First praised for its subject (its picturesque stone walls, its fidelity to Nature), it later was admired for its romantic gloom. Sir Joshua Reynolds studied it as a guide to landscape painting and John Constable judged it "sufficient to form an epoch in the art." When, in 1911, Lord Landsdowne sold "The Mill" to P.A.B. Widener, who later gave it to the Gallery, the British public raised an outcry. They felt a treasure had been lost.

Many 20th-century scholars, however, regarded it with skepticism. They did not trust its colors and could not see its brush strokes. Many recent catalogues of Rembrandt's works excluded the picture.

"As a result of the present cleaning, however," writes Charles Parkhurst, the Gallery's assistant director and chief curator, "the Gallery believes that stylistic objections to the attribution of "The Mill" have been virtually eliminated."

If it is not by Rembrandt, it is by someone just as good."I can't see how you can have a 17th-century Dutch landscape that powerful, that freely painted, that is not a Rembrandt," says Gallery curator Arthur K. Wheelock. "It has to be him."

"The Mill" is now surrounded by an entirely appropriate 17th-century Dutch ebony wood frame. It hangs in Gallery 47-where its beauty overhelms the other fine Dutch pictures on view, by Cuyp, Hobbema, and van Ruisdael

Heretofore "The Mill" has been dated circa 1650. Wheelock, who compares its drama, its small figures, and its general composition to those of "The Three Trees," a Rembrandt etching of 1643, feels "The Mill" was painted in 1645.

A "conservtion controversy" flared briefly at the Gallery when "The Mill" was in the lab there. Another Gallery picture, "The Gerbier Family," by Rubens, had lost much of its beauty in the conservation process.

Though it had been cleaned elsewhere, similar anxieties existed about "The Mill".

They seem to have been groundless. Through painting conservation stopped briefly at the Gallery, conservator Kay Silberfeld and here collegues were soon cleared of any taint of professional malpractice. Underneath its varnish, "The Mill" proved to be in remarkably good conditions. If, in the process of relining, cleaning and revarnishing, there was dmage done the picture, it cannot be seen.

"It was a very straightforward cleaning job," said Wheelcock, 'The Mill' was overcoming its mystique." CAPTION: Picture 1, Rembrandt's restored "The Mill"; photo by Gerald Martineau; Picture 2, Rembrandt's "The Three Trees"