"Paper in Prints," the current exhibition at the National Gallery of Art, teaches and demands. It is not an entertainment - seeing it is work.

Paper is its subject. Its medieval woodcuts, 15th-century engravings, its Rembrandts, Blakes and Durers, are here to teach us something about absorbancies and textures, washes, bleaching, dyes. It takes at least an hour of thought and careful scrutiny before one starts to see the subtle lessons being taught.

Walter H. Annenberg, for one, ought to see this show. Though he later changed his mind, Annenberg last year said that he would give $40 million for an art communications center to the Metropolitan Museum of Art. Saying he was much impressed by Kenneth Clark's TV shows, Annenberg suggested mass-producing video tapes, slides, films and recordings, to teach the masses art.

No electronic museum-of-the-future could reproduce this show.

Andrew Robison's catalog is a pamphlet without pictures. Pictures might be pleasing, but in the context of this show they would teach us nothing. Included in Robison's exhibition are four small wood engravings by England's Thomas Bewick, all pulled from the same block. In photographic reproduction they all would look the same, but they're not the same, they're different. The only way that one can see the differences between them is to look with care at the works of art themselves.

Bewick's 1797 engraving is called "The Runaway Cart." In each print the same frightened horse runs through the same village beneath the same tall trees. What changes is the paper on which the print was made. Wet ink applied to smooth fine-textured paper tends to remain on the surface. Softer, pebbly paper lets the ink soak in. Each print is slightly different. In one the horse's coat seems dull, in another it seems wet with sweat. It is the paper, not the artist, that makes that wheel seem to whirl and gives the coat its sheen.

Bewick in his long career used perhaps 100 different kinds of paper. Rembrandt, too, was an explorer. Included in the show are a pair of Rembrandt's etchings, both of the same seated nude, both pulled from the same metal plate, perhaps on the same day. One print is on paper, the other is on vellum. In the paper print, writes Robinson, Rembrandt's tiny "cross-hatched lines remain distinct rather than coalescing into dark forms. The fact that they are held distinct by the paper allows Rembrandt to crispen and clarify the lines of the image and to suffuse the impression with light and more light, light reaching into every corner." When he used hard-surfaced vellum, Rembrandt let his ink lines smear. Where once the viewer read a body in bright light, here the nude is clothed in deep obscuring shadows.

Robinson is the Gallery's curator of graphic arts. "The exhibition focuses on the esthetic role of paper in fine prints," he writes. "Every printed image has three major aspects: the image" (say a horse or a madonna, a still life or a tree), "the support on which it is printed" (pebbly paper, smooth paper, white paper or buff), "and the printmaking medium or process." The interactions of these variables are the subject of his show.

He asks us to consider how paper is produced and of what materials, linen, cotton, wood, Japanese bark or straw."White paper, he reminds us, is less often a dead white than some subtle shade of buff or gray. He also makes us see the flaws of restoration. Collectors, all too often, wash and bleach old prints in misguided efforts to iron or to clean them. He shows us, by comparison, how such cleansing tends to drain and somethings kill the subtle printed image it was designed to save.

Robinson's show is not about images or iconography, but it leads us, through the technical, to a richer understanding of the variables that interact in all works of art.

It is accompanied, at the Gallery, by another exhibition whose object is pure pleasure. "Prints of Paris: The 1890s" is full of song and light and color. To wander through this sweet display is to visit French music halls, cafes, whorehouses and race tracks. Among the printmakers represented are Edgar Degas, Pierre Bonnard, Henri de Toulouse-Lautrec, Vuillard, Mary Cassatt and Jacques Villion. There are no better guides to Paris in the spring. Both shows will close on July 4.