Charles Hawthorne, who helped form an artists' colony in Provincetown, Massachusetts, in the early 1900s, is best known for his portraits of rugged New England fishermen. But late in life he began dabbling in watercolors -- painting landscapes as though the details had run together for him over time.

Sixty-four of those watercolors, many never exhibited before, are on display at the National Museum of American Art.

Unfortunately, the show doesn't include any of the earlier portraits; they would have relieved the sameness of the landscapes and given contrast, as the watercolors are clearly a departure toward the abstract. To get the gist of them, you stand against the wall across the gallery, a full 15 feet: Then, you can discern a shape -- a tree, maybe a cottage.

The details of these landscapes are obscure, but the mood, expressed through color, never is. Hawthorne has 50 colors for daylight -- a gleaming gold summer afternoon, a purpling nightfall. How soothing they are. The placid hues of his Provincetown paintings -- pastel blues, greens and pinks -- contrast with the livelier colors of scenes in Mexico and Spain -- oranges, magentas, browns and deeper blues.

But the risk of ambiguity is the risk of saying nothing at all. The bold stroke of "Toward Evening, Mexico" works very well, with its indistinct mesas, dark-blue sky, a feeling of stars coming out. But the sparse, sand-colored "On the Ranch, Mexico" could as easily have been named, "On the Beach, Anywhere." CHARLES HAWTHORNE: THE LATE WATERCOLORS -- At the National Museum of American Art, Eighth and G streets NW, through September 5.