It's only in your 20s and in your 70s and 80s that you do your greatest work." The quotation is by Orson Welles, and artist Isabel Bishop has it framed and hung in her paint-splattered studio overlooking New York City's Union Square.

The diminutive, white-haired Miss Bishop enters her 73rd year with a triumph—her first major retrospective—at Manhattan's Whitney Museum of American Art. It is an honor many critics and fellow artists feel is long overdue.

A painstaking artist who often spends a full year on a single painting, Isabel Bishop has never been prolific, but her limited output of old master-like works has consistently been snapped up by U.S. museums and dozens of private collectors. The current exhibition—182 drawings, prints and paintings—represents almost half a century of the artist's work. It also represents as many years observing life in Union Square, whose flouncy shopgirls, merchants and bums are the principal subjects of her work. Watching the scene pass beneath her fourth-floor studio, she muses, "Oh, I've seen so much from this window. Communists, socialists. You know, both Eisenhower and Kennedy spoke here. Not long ago I watched young men burn their draft cards on the bandstand platform."

Union Square is now decaying and seedy. Yet Miss Bishop finds life there still as fascinating as when she arrived from Detroit at age 16, intent on a career as an illustrator. With money from a relative she attended the Art Students League, where, she remembers, "the air was inflamed with excitement over art. Things no one in Detroit had ever heard of.

"My first studio was across the square," she recalls. "It cost $20 a month and I took baths standing in the sink and pouring water warmed on the potbelly stove over me." Soon she was sketching the girls and hoboes who lounged in the square. "I guess it was because they were available," she laughs. "The bums would knock on my door and say, 'Well, if you need me I'll be under the horse's tail of the George Washington statue.' "

Bishop's early Rubenesque nudes, drawn so delicately they barely emerge from the canvas, now rank among her finest works. The young girls look out with faces of haunting wistfulness. "I never used a professional model," says Bishop proudly. "Everything they do is fallacious. They even brush their teeth in a false way."

There were desperate moments in her seven decades. At 24, after a long love affair ended, she made three serious attempts at suicide. "Once I jumped into the Hudson River in the middle of the night—but my body just wouldn't die. It began to swim." From then on, she recalls, "my belief in painting was like a religious conversion. It has held ever since. Fortunately, I married a man who fancied me because of that commitment." Her husband was Harold G. Wolff, a noted neurologist, who died in 1962. "Once, desperate over a painting, I called him at his hospital. He dropped everything and came to the studio. It was an outrageous thing to do," she smiles, "but it showed he cared as much about my work as about himself."

Her own commitment has never wavered, nor has her zest for painting. Seven days a week, by subway, the artist commutes from her Riverdale home to her studio, sketchbook in hand. "Now I want to examine the modern nude," she says. "Female bodies have changed with social ideas. Now they're wiry, supple, with no hips. I want to try to capture that quality on canvas."