"It's like looking over the artist's shoulder and getting closer and closer," says National Gallery curator E.A. Carmean of his superb little show, "Picasso: The Saltimbanques."

The exhibition, which opened yesterday at the Gallery's East Wing, becomes a fascinating detective story employing art history and modern technology to explain the evolution and meaning of one of the Gallery's greatest 20th-century paintings: Picasso's first large-scale masterwork, "The Family of Saltimbanques."

Last summer, in the process of an X-ray examination of the painting, Carmean and Gallery conservator Ann Hoenigswald made an astonishing discovery: Under this painting were two earlier paintings -- the first, a circus family; the second, a pair of acrobats.

The discovery suddenly clarified the chronology in an important but previously murky area of Picasso's stylistic evolution: the transition between the end of his Blue Period in 1904 and the early Rose Period in 1905. It now becomes clear that the artist had worked upon, painted out and reworked the same canvas for more than a year. Scholars may never know whether he was too poor to buy more canvas and paint, or whether the earlier versions didn't satisfy him.

But by studying the changes he made, they can see a distinct shift from the highly personalized expressions of hopelessness and despair in the earliest Blue Period works, to the less tragic, more formal depictions of human melancholy and isolation seen in "Family of Saltimbanques." "This painting embraces the very moment when Picasso the storyteller became Picasso the modern artist," says Carmean. The purpose of this show of 38 objects is to explore that "moment."

It begins by examining the artistic precedents for the use of Saltimbanques -- vagabond performers descended from the 17th-century commedia dell'arte theater -- in a few examples by earlier artists, including Watteau, Daumier and Manet. Picasso had used the subject as early as 1900 while still in Barcelona, and his illustration of "Pierrot Celebrating the New Year," made for a carnival handbill, is on view here.

In 1904, Picasso, his mistress Fernande Olivier and his friends Max Jacob and Apollinaire spent many evenings at a Montmartre circus. At that time, he began to produce several drawings and watercolors now known to be related to the paintings under the "Family of Saltimbanques." Many are here, including one gouache from the Cone Collection in Baltimore which shows a circus family happily involved in practicing their craft. A young girl balances on a ball as a harlequin watches and a woman hoists a baby playfully into the air. Previously thought to be a study for an uncompleted work, the X-rays clearly show this drawing to be a study for the first painting Picasso made on this giant canvas.

As the show moves on, other drawings and small paintings introduce the cast of characters who will ultimately become the "Family of Saltimbanques" -- El Tio Pepe, the rotund jester in the red suit and pointed cap, and the two young acrobats who are the only two figues on the canvas in the second stage.

The X-rays also revealed that even the final painting underwent changes. Its initial form resembled a gouache in the Pushkin Museum in the Soviet Union, reproduced here. It shows four male figures and a young girl -- all isolated from one another -- standing in an empty landscape.This is no doubt how the canvas looked just before Picasso finally finished it off by transforming the faces into portraits of himself and his friends, made other adjustments, and added Fernande as the female figure at the right.

At this point, Carmean suggests that Picasso saw within the painting a metaphor for an incident in his own life, and altered it accordingly. Fernande had brought home a 10-year-old girl from an orphanage and then willfully returned her. In the final painting, Picasso, dressed as harlequin, passively holds one hand behind his back, but reaches with it toward the child, who holds him by the other hand. While the jester and acrobats (Picasso's friends) look coldly in her direction, the woman at right -- Fernande -- sits isolated from the others. One hand is held at her shoulder in a classic pose of vanity, while the other -- disposed as if holding a child -- remains empty. The story adds a final emotional impact to a work that was already profoundly moving.

The show concludes with some later works on the saltimbanque theme, including one version of the great "Three Musicians," and some technical notes and X-radiographs -- all carefully detailed in the catalogue.

This is one of a series of projected small National Gallery exhibitions intended to heighten appreciation of individual works of art, which is what museums were once all about. In substituting brains for blockbusters, the Gallery is to be congratulated on leaving its audiences with something considerably more meaningful than sore feet.