"Charles Hawthorne: The Late Watercolors," at the National Museum of American Art, is an awkward exhibition. Despite its repetitions and its ample size, it feels like half a show.

Its hastily made landscapes are poignant for one reason. They do not look at all like Hawthorne's handsome, heavy portraits. Observers who know that academic work--there cannot be a lot of them; he's been dead for half a century, and he was never famous--may find these little studies surprising and refreshing. But other viewers will wonder, why are these things here?

Most are little more than sketches, done, or so it seems, in 10 or 20 minutes with casual bravura and a skipping, dripping brush. There are 64 on view, all pretty much alike. Not one is a knock-out. This is merely so-so art.

Had the National Museum shown these works in context, say, beside Hawthorne's oils, they would seem more impressive. For they show Hawthorne at his life's end discovering new freedom, moving toward abstraction, and demonstrating, boldly, the methods that he taught.

Hawthorne was, by all accounts, an extraordinary teacher, patient and compassionate and beloved by his students (among them Edwin Dickinson). In 1899, after studying and working with William Merritt Chase, Hawthorne moved to Provincetown, Mass., and established the Cape Cod School of Art. Between 1899 and 1930, when he died, perhaps 1,000 students enrolled in his summer school. If Provincetown today is known as an art colony, a good part of the credit goes to him.

The paintings he sold (there are none in the show) were either robust formal portraits commissioned by the wealthy or pictures of the locals--the Yankee ladies and Portugese fisherfolk--he knew on Cape Cod. Formal portraiture, however, was not what he taught.

He particularly enjoyed teaching out of doors. He urged his pupils to paint boldly, to master what he called "the mechanics of putting one spot of color next to another--the fundamental thing."

"It is beautifully simple, painting--all we have to do is to get the color notes in their proper relation," he would tell his students. "The juxtaposition of spots of color is the only way."

Also: "The weight and value of a work of art depends wholly on its big simplicity--we begin and end with the careful study of the great spots in relation one to another . . . Consider the big spot of the earth and the big spot of the sky . . ."

Hawthorne taught that "a good watercolor is a happy accident." He urged his students to experiment. He'd say, "use more color, play with it, yell at me with color," and "let a watercolor get away from you. If you can't get it back, try another," and "never mind if they are sloppy. Get out and slop around . . ." In making the late pictures here he practiced what he preached.

He mostly ignored detail, he slopped around a lot, he happily accepted bleeds and drips and splashes. In "Cattle on the Ranch" (1928), the cattle of the title are red and maroon spots. One desert landscape here, titled "Mesquite Tree," looks, a bit mysteriously, like a picture of the sea. Most of these watercolors are summarized so loosely that one is almost shocked by the red sunlight on the tree trunks and the strong black shadows of his "Two Mesquite Trees" (1928).

Though born in Lodi, Ill., Hawthorne, whose father was a sea captain in Maine, spent most of his life on the Atlantic coast. His portraits have great presence. He taught with verve and love. But these landscapes are minor things. Their freedom, their abandon, is their only real virtue. This half-considered Hawthorne show does him no great service. It closes Sept. 5.