Often, it seems that art historians are happy to work like old-fashioned zoologists, preoccupied with classifying every sign of life they see.

"Procession: The Art of Norman Lewis," at the Pennsylvania Academy of the Fine Arts, is an attempt to complicate and enrich the story of American art by finding a previously unoccupied niche for an interesting artist. Norman Lewis (1909-79), the exhibition tells us, was an African American abstract expressionist who, though he was active in the New York art scene, interacting with artists, teaching, and showing in a well-known gallery, was overlooked because of his race.

The exhibition invited us to catch up with him, and it is worth doing so because he was an interesting artist, a master of color, and he bore serious, committed witness to the tumultuous events of his time, from the 1930s to the 1970s. Whether it was racial bias or uneasiness with the range of his work that has kept him out of the history books thus far, his work certainly deserves a look.

Abstract expressionism is, notoriously, the movement that, coupled with American military and economic hegemony after World War II, made New York the center of world art. It was an art of big gestures and big egos, egged on by powerful critics with radical and doctrinaire visions about what art must be.

It is not surprising that Lewis would be an outlier in such an era, regardless of his race. He was more an explorer than an oracle. And even though he occasionally took on cosmic subjects, his work was rooted in visions of society and of the natural world.

Like many African American artists, he won his first exposure through New Deal programs that encouraged printmaking and the documentation of everyday life. By the early 1940s, when he did a series of lively, humorous, and pointed scenes of urban life, his colors seemed to break free of narrative and create a drama of their own. Meeting Place (aka Shopping) from 1941, is a funny, almost cartoonish depiction of several women, and one terrified boy, rummaging through merchandise on the last day of the sale. But the painting has one arresting passage, where the white of a clerk's arm seems to merge into the white lips of one of the African American shoppers.

By the time he painted Title Unknown (Potato Eaters) in 1945, its green, red, brown, and black are not associated with any of the details of the scene of a family around a table. The colors structure the canvas and establish its emotional tone; the people are barely sketched in.

From then on, Lewis' people became less specific. Often they were little more than stick figures. But they never disappeared. Indeed, they merged with color to constitute visions of society, with all its celebrations, rituals, conflicts, and barbarity.

In a video being shown in the exhibition, Lewis says he avoided showing people who were suffering because a painting can't do anything about their plight. If you want to do something effective, he added, you can go join a picket line.

It seems no coincidence that Lewis' most persistent theme is of people in lines, acting together. They might be participating in a carnival or a dance or a religious ritual. They might be a multicolored crowd, marching on Washington to demand civil rights. They can even be terrifying figures in the night, clad as Klansmen all in white, spreading violence and terror.

The exhibition makes a strong distinction between bright paintings done in the 1950s and early 1960s that depict celebration and that possibly reflect Lewis' Caribbean roots, and darker ones done later that involve conflict and protest. When you look at the works, though, it is difficult to make a real distinction. Title Unknown (March on Washington), from 1965, with its bright orange and azure sky and colorful people in harmonious commotion, is one of the most joyous works in the exhibition.

Throughout his life, Lewis disclaimed any interest in making political paintings. That's not surprising. This was the era in which artists were not even supposed to be making pictures. It was all supposed to be nothing but paint on a flat surface.

In fact, there were artists at the time who were committed both to abstraction and to social issues. In Philadelphia, for example, Sam Maitin was for decades our unofficial civic artist. But those who produced such work were often considered marginal in the art world of the time.

Lewis' processional works, which give the show its name, are far from all he did. There is also a body of work that deals with music, another devoted to nature, and quite a few that explore the spiritual qualities of color and clouds. The show classifies Spasms (1964) as a nature painting. It is certainly a sky painting, and a look into the cosmos and the soul.

Speaking of the cosmic, PAFA has revived its program of small Morris Gallery showcase exhibitions with "Mia Rosenthal: Paper Lens." Rosenthal (born 1977), a Philadelphia artist, has made meticulous drawings of imagery produced by such instruments as the Hubble Space Telescope and the Large Hadron Collider in Switzerland in response to the museum's challenge "to make the invisible visible."

In contrast to the late 18th- and early 19th-century scientifically oriented artists to be seen in the Art Museum's "American Still Life" show, Rosenthal works with images that already exist. She alters them only to the extent that the act of drawing inevitably does.

So why bother to make the drawings if the photographic images are available?

For one thing, she makes us look.

And as she notes, even the NASA photographs from which she works have been color-enhanced to make their content easier to see. That is, itself, a kind of artistic decision. This beautiful little show reminds us that the universe is out there. But what we see is an act of human imagination.

Art: ART EXHIBITION

Procession: The Art of Norman Lewis

Through April 3 at the Pennsylvania Academy of the Fine Arts, 118-128 N. Broad St.

Hours: 10 a.m.-5 p.m. Tuesday, Thursday and Friday; 10 a.m.-9 p.m. Wednesday; 11 a.m.-5 p.m. Saturday and Sunday

Admission: $15; students and 60 and over, $12; 13-18 $8; 12 and under: free. Free admission on Sundays for this exhibition.

Information: www.pafa.org or 215-972-7600.EndText