I have been pleased to catch the big exhibition of Treasures of the Hispanic Society at the Prado, having long loved its rather gloomy headquarters on 155th. Street.   Inevitably in a big, celebratory exhibition, it shows what the Society owns to better effect, beginning with antiquities, including an amazing bronze lamp, a Visigothic belt buckle and Umayyad pyxis.   Wonderful pieces of medieval metalwork, all acquired unprovenanced from Lionel Harris, a London antique dealer, in 1906.   Huntington collected medieval books and manuscripts fanatically as well, from a German dealer called Karl Hiersemann.   Then the great paintings for which the collection is best known – El Grecos, Zurbarán, Ecce Homo by Luis de Morales, two wonderful Velázquez portraits, and Goya’s Duchess of Alva. Upstairs is a collection of portraits of grand Spaniards of his day.   An exhibition particularly, and unexpectedly, rich in sculpture and decorative arts.