To celebrate the 400th birthday of the Spanish painter Diego de Silva y Velázquez, the Frick Collection, is showing six Velázquez paintings. All the works are from New York museum collections: three from the Met, two from the Hispanic Society of America and one from the Frick itself.

The last time New Yorkers were treated to such a wealth of Velázquez paintings was ten years ago, when the Met mounted its definitive show. At that time there were 40 Velázquez canvases on display, almost half of his known output.

Those paintings came from everywhere -- the Prado, London, Vienna, Edinburgh, the Escorial and America's premier showcases -- the Boston Museum of Fine Arts, the Cleveland Museum, the Dallas Museum, the Kimball in Fort Worth, the National Gallery in Washington, D.C., and the Ringling Museum in Sarasota, Fla.

But this writer looked in vain for the Frick or the Hispanic Society paintings. Why weren't they included then?

The Hispanic Society had not released a painting on loan since 1910, and is now making its first exception. As for the Frick, its founding statute requires that no art object purchased by Frick personally would leave his museum for exhibition elsewhere. The Fraga Philip -- so called because it was painted after the battle of Fraga in 1644 -- falls under this stricture, which is partly the reason the Frick has mounted this show.

The six canvases, all portraits from the last decade of Velázquez's life, represent the painter in his most mature and accomplished phase. The canvases shimmer with the elegance of his brush, the mature judgment of mind and eye, and the lessons so aptly acquired during two long trips to Italy and the advice of that other painter-diplomat resident in Madrid, Peter Paul Rubens.

At the age of 12, Diego's Portuguese parents recognized the artistic talents of their son, and apprenticed him to one of Sevilla's top painters, Francesco Pacheco. Here he learned the humanist theories of the day, read Dürer and Vesalius on human anatomy, Euclid on geometry and studied other seminal thinkers on arithmetic and architecture. He also earned the support and patronage of the Duke of Alvarez, Don Gaspar Guzman, who brought him to the attention of the Spanish king, Philip IV. An appointment as court painter quickly followed, strongly contested by the other painters at court who considered him nothing but an upstart.

Rubens met Velázquez while on an ambassadorial mission from the Netherlands to Spain and was much impressed. Rubens urged the king to send his new court painter to Italy for further study. Leaving in 1628, Diego studied the great Venetian masters, especially Tintoretto, and the architecture and sculpture of Rome.

From the very beginning of his time in court, Velázquez painted portraits of the king, depicting him first as a golden-haired, pleasure-loving youth; then as a ruler in a decaying state riven by corruption and greed; and finally as a man bemoaning his wasted life. The Fraga Philip at the Frick is that last formal portrait Velázquez was to execute. The tired, elaborately sketched head looks out from a lace-covered shoulderlet. A weak right hand grasps the commander's baton while the gray sleeves of the court costume mirror the paleness of his face.

A second Velázquez visit to Italy in the late 1640s led him to paint his color-grinder and servant, Juan de Pareja. This painting gained the acclaim of Roman artists. "All else," painters in Rome were quoted, "was merely painting, the Pareja was truth rendered in brilliant color." The Pareja is the famous work that the Met acquired at a cost of $5 million in 1971, the highest price ever paid at the time by a museum for a painting. The painting shows the sitter in firm control, looking out at the beholder with a gaze that is regal, even defiant. So popular was the painting that a number of copies were made by the artist himself, including one that currently hangs at the Hispanic Society.

The Metropolitan's portrait of Maria Teresa, Infanta of Spain is one of Velázquez's greatest compositions. The brilliant picture shows a frightened young girl, whose image would be hawked around Europe's princely horde in order to attract a husband. She eventually married the Sun King of France, Louis XIV, with whom she had a miserable existence. The face here is carefully and sensitively modeled, but the wig, covered with white feathers and bows in the shape of butterflies, lends a frivolous air to the somber face.

From the Hispanic Society comes another small painting (just over 20 by 16 inches) of a young girl thought to have been one of Velázquez's grandchildren, but not securely attributed. It demonstrates the great sensitivity and mastery Velázquez brought to his finest work. His pigments and colors bring out the highlights of the girl's skin, the seriousness of the gently penetrating eyes and a serenity unlikely in so young a person.