Washington Squares: folk at the Rat; Picasso at Fogg; songs of city life; Spanish masters

In honor of the recent visit of King Juan Carlos of Spain to Harvard's commencement, the Fogg Art Museum has mounted an exhibition of ''Spanish Masters of Modern Art.'' Drawn from the museum's rich collection of Spanish art, it includes works by such pivotal figures of the modern period as Picasso, Juan Gris, and Miro.

The show provides an occasion to become reacquainted with these artists, as well as to explore the Spanish character. It's a sensibility that ranges in hot and cool extremes, most of which are encapsulated in the brilliant twists and turns of Picasso's still surprising development. In fact, the exhibit is valuable simply for the opportunity to trace the diversity of this master, beginning with the melancholy realism of the blue period through full-fledged Cubism and, later, Surrealism. Classic examples from the blue period include a delicate yet monumental ''Mother and Child'' of 1901. Despite their extensive exposure, such Cubist works as ''Man With a Pipe'' of 1911, or the ''Still Life'' of 1912, remain intriguing puzzles of spatial manipulation. Picasso's extraordinary fluid line and unfailing sense of composition are constant throughout, in such contrasting periods as the elegant artist-and-model etchings of the early '30s and the surreal, Angst-ridden lithograph ''Le Homard'' of 1949 .

In addition, there are fine examples of Gris's pristine Cubism, and for those who like that sort of thing, a typical Dali, complete with metamorphic forms and quasi-pornographic women. There's also a roomful of Miros, whose surreal amoebalike forms have always seemed overrated to this viewer. More interesting are the postwar artists, especially Antonio Saura, whose ''Katy'' of 1959, a fervid abstract portrait of a woman, recalls the Abstract Expressionism of de Kooning and Kline, yet is startlingly contemporary in its stark emotionality and crude execution. One is struck by the authenticity of Saura's spirit and the undeniable keenness of his passion.