"Louis Comfort Tiffany at the Metropolitan Museum of Art," which was organized by Alice Cooney Frelinghuysen, is an exhibition drawn mainly from the museum's own collection. As a result, it reflects the eccentric history of collecting and gift-making. Is that a problem? In this case, it is a strength -- because it edges us away from conventional ways of thinking. Tiffany (1848-1933) is much too closely identified with his ponderous lamps. The Met owns very few of them. And so the curators can devote their energy to enriching our understanding of his stained-glass windows and to studying less familiar aspects of his work. What jewelry Tiffany designed is lovely, for example, though not for the reasons one might expect. Instead of working with flashy, expensive rocks, he preferred to use semiprecious stones -- and his wits. In one hair ornament, he depicted two dragonflies that have alighted among dandelion balls. Above all, the curators can emphasize his vases. Made of blown glass, they seem to be fashioned of light. Their glass never appears too hard; their shapes never tyrannize the space. Many have a mysterious inner glow. Glass has rarely seemed so alive.