Its works of art are delicate, colorful and lovely, yet "Brazilian Indian Feather Art," at the National Museum of Natural History, is a frustrating exhibit. It feeds the eye but not the mind. Its armbands and tiaras, bracelets, masks and lip plugs, bandoliers and earrings, are texts in complex languages that none of us can read.

Sunny yellows, inky blacks, reds and pale greens, iridescent blues--their colors are most beautiful. The forms of these adornments are beautiful as well. Some look like radial sunbursts, others like fresh flowers. But their beauty was not the sole reason for their making.

These Brazilian "feather ornaments," the catalogue reminds us, "cannot be regarded as merely decorative features. They are veritable codes, transmitting non-verbal information about sex, age, clan affiliation, social position, ceremonial and political office . . . . This information also refers to the historical past and mythical origins of the people involved." But the current exhibition leaves its objects mute, their complex codes unbroken.

The catalogue, but not the show, offers rare, intriguing hints. The Kayapo' people, we are told, believe their body ornaments--their necklaces and lip plugs and the white king vulture down that they glue to their hair--make their wearers human. "The great krokrokti diadem," the catalogue continues, "has profound symbolic meaning." It may represent an eye, the feathers being lashes, or symbolize the sun, the feathers being rays. "But more than anything else the diadem represents the circular form of the village: The blue central feathers symbolize the plaza . . . while the row of red feathers represents the periphery, the houses, the domestic world of women; the white down tied to the tips of the feathers being the forest . . . the forest that encircles the world." But almost all the 300 other objects shown remain unexplained.

One yearns to see them move or worn, not frozen as they are in their plastic cases.

Whether using small feathers or large ones, trimmed feathers or whole ones, the people represented--the Bororo, the Tukano, the Kayabi, the Mundrukyu, the Oiampi, the Canela, and more than 30 other tribes--seem extraordinary artists. The feathers that they use come from wild birds, from herons, macaws, ducks, crows and hawks and toucans, or from caged birds raised primarily for plumage.

There is one feather here that is partly purple, partly yellow, partly green and orange, and yet it was not dyed. The catalogue explains that "the Tukano Indians practice 'tapirage': They tear out the green feathers of the parrot and on its skin smear the milky secretion of a small frog. When the new feathers grow they are yellow orange."

The show suggests a fading art. Old skills have been forgotten, and a number of the rarer Brazilian jungle birds have been hunted to extinction. The admiration that we feel before these glowing objects is soured by our ignorance. These emblems, badges, crowns contain histories and legends, but the keys that might unlock them also have been lost.

A number of Brazilian museums and collectors lent the works on view. The show was organized to make the recent visit here of Brazilian President Joa o Baptista de Oliveira Figueiredo. The show will continue through May.