EDGAR DEGAS' obsession with the corps de ballet of the Paris Opera was a magnificent one. The National Gallery's exhibit "Degas: The Dancers" gives us an orchestra seat.

The show marks the 150th anniversary of Degas' birth. Among the almost 60 works here are about a dozen oils, two sculptures and a number of pastels and related drawings.

We share the good seats with the abonn,es -- the gentlemen-subscribers to the Paris Opera, the upper crust of Paris society who not only sat close, but went backstage! They can be seen here in black capes, their faces obscured under top hats -- dark presences watching. The dancers, their bodices cut low, their skirts short, are alluring on stage -- fair, fragile dreams in pastel. But how clumsy they are backstage, leaning over to adjust a ballet slipper or pull on tights.

A subscriber to the Opera, Degas was watching them all. At first, his treatments were particular. The bassoonist central to "The Orchestra of the Opera" was his good friend Desir,e Dihau.

The dance master in "The Dance Class" was Jules Perrot. And in this class, each girl possesses a personality -- unlike the anonymous dancers in Degas' later works. Sitting on a piano, a dancer scratches her back. Others sit awkwardly or fuss with their tutus.

Degas wrote in his notebook in 1878, "Study from every perspective a figure or an object, no matter what." Of course, as can be seen in five studies on view here, he chose a dancer for this exercise.

His model for the studies was a young ballet student, Belgian-born Marie van Goethem; out of them grew his greatest sculpture, "The Little Fourteen-Year-Old Dancer." Both girl and woman, gangly and lovely, she was one of les rats of the Opera, skittering to and from homes in Montmartre, daughters of concierges and cleaning women. She sticks out her tummy and turns up her nose; her eyes barely open under her long bangs. But her feet are set in such a lovely turn-out.

Degas -- he dressed her in a gauze skirt! -- broke the chains of classical sculpture with the little dancer, and just in time to exhibit his wax model at the Paris impressionist exhibition in 1881. (The bronze model on display here was posthumously cast.)

In another innovation, he used rectangular canvas more suitable for landscapes to compose variations on the theme of the dance rehearsal. But a ballet studio is long and shallow -- and the shape emphasizes the ballerinas like figurines in shadow boxes.

The National Gallery's "Four Dancers" -- and a number of related pastels and drawings -- show Degas' movement, late in life, to the abstract. The dancers are fluid form and glowing light.

Why, of all Degas' subjects -- Paris scenes and portraits of friends and family -- did the ballet consume him?

"They call me the painter of dancers," Degas said. "They don't realize that for me the dancer is a pretext for painting pretty materials and rendering movements."

DEGAS: THE DANCERS -- In the National Gallery of Art's East Building through March 10, 1985.