It is easy to feel pity for the painter William H. Johnson (1901-1970). His poverty was lifelong, his happiness was brief. The critics of his day denied him recognition. The policemen of his home town, Florence, S.C., tossed him into jail for setting up his easel on the public street. He knew misery and pain, bigotry, injustice. A mind-decaying sickness killed him in the end.

Syphilis destroyed him. In 1946, he was picked up on the street, wandering deleriously, and taken to the hospital. He did not paint again. He stayed there until he died.

His pictures are not bitter. They have a fiery integrity. They are bright and energetic. A number of the best of them are included in "William H. Johnson: The Scandinavian Years," which opened yesterday at the Smithsonian Institution's National Museum of American Art.

There is a strangeness to the shifts we see in Johnson's style. It is generally assumed that a primitive, if gifted, if given the right schooling, can eventually acquire high sophistication and academic polish. Johnson moved the other way. He was an academic early. Though he called himself a "primitive," he came to primitivism late. The pictures on display show his style in transition.

Johnson was illegitimate. His father, whom he never knew, was a white South Carolinian. His mother was part black, part Sioux Indian. Johnson was so poor he could not afford pencils. He might have been a field hand had a teacher not discovered him drawing in the dirt with a sharpened stick.

With his heart set on painting, Johnson traveled to New York when he was 17. He worked there on the docks, and in a Chinese restaurant, until, three years later, he had saved enough to begin a five-year course at the National Academy of Design. His intelligence, his talent, soon became apparent. He won the Academy's coveted $100 Cannon Prize for his work in painting from the nude in 1924 and, again, in 1926. He won the $100 Hallgarten Prize in 1925. His teacher, Charles W. Hawthorne, raised the $1,000 with which Johnson went to Paris in 1926.

Europe set him free. There was no Jim Crow in Paris, and the modern art he saw there--by van Gogh, Cezanne, Soutine--drove him far beyond the cautious academicism of his New York years. He spent two years in the south of France and, in 1928, had a one-man show in Nice. In 1929 he met Holcha Krake, a Danish weaver and ceramicist, 15 years his senior. They fell deeply in love.

In November 1929, Johnson returned to New York and spent the winter there in a fifth-floor Harlem loft without heat or electricity. The paintings that he made there won $400 and the Harmon Foundation's gold medal for "Distinguished Achievement Among Negroes."

He took the cash and went to Denmark, to Kerteminde, a little fishing village on the Island of Fyn. He married Holcha Krake as soon as he arrived. It says something of their marriage--and something of the life that blacks lived in this country in the 1930s--that his years in Kerteminde were the happiest of his life.

He loved the sea, the fishermen, the trips he took to Norway (there he met Edvard Munch), the fjords, the midnight sun. These are the chief subjects of his present show.

Genteel academicism has been banished from his painting. His colors have grown bright and free, his brushwork bold and fluid. He is using now the space-carving paint strokes he has learned from van Gogh. Thumb-sized brushstrokes strong as bricks surround the orange sun of his "Sun Setting, Denmark" of 1930-1932. The form-distorting rhythms one sees in his self-portraits and his landscapes seem borrowed from Soutine. His mountains and his flowering apple trees, his fishing boats and fjords, seem to writhe with pleasure, to sway and stretch and dance.

Though he had no money -- "I am just next door to being a beggar," he wrote in 1932 -- he did not seem to mind. He found his life idyllic. He shared a studio with his wife, and they often showed together. It was only when the two of them returned to New York, fleeing from the coming war, that the happiness they'd known began to fall apart.

Johnson once again was living in the Harlem slums. He was poor, black and unlucky. His pictures were not selling. There was a fire in his loft. He was working as a laborer in the Navy Yard, and his wife was ill. His painting had changed utterly. It was as if he had decided to become a folk artist. Black life was his subject now. He painted black Christs and black soldiers, black prisoners, black angels. His colors had grown harsher. His forms had become "primitive," poignant in their awkwardness, strongly outlined, angular. Holcha died of cancer in 1943. Johnson never was the same.

He tried returning home, to his mother in South Carolina. He did not stay there long. His mind was deteriorating. In 1946, he somehow found the funds to return to Denmark and to Norway. Arrested as a vagrant there, he was sent back to New York and hospitalized for life.

At his death in 1970, the pictures he had left were turned over to the Harmon Foundation. When it went out of existence in 1967, it gave more than 1,100 Johnsons to the National Museum of American Art. It was from that gift that this show was drawn. Of the 46 works of art on view, 22 were included in the full-scale retrospective the museum gave to Johnson in 1971. "William H. Johnson: The Scandinavian Years" coincides with the national promotion "Scandinavia Today." The show closes Nov. 28.