"Art '78" at the D.C. Armory, with 135 dealers from 15 countries, spans the income spectrum. For the rich, there's a Max Ernst-designed cage bed and screen, just like Nelson Rockefeller's, for $44,000. For those of more modest means, there are posters galore by big names and little knowns for as little as $2.

Art fairs - this one continues today, Sunday (11 a.m. to 9 p.m.) and Monday (11 a.m. to 6 p.m.) - are organized chiefly to give dealers a chance to supply each other with stock. Art lovers are welcome to browse and buy, but what you see at a fair is what sells, not a survey of the best contemporary art, (though obviously the larger the overlap, the better the fair).

Thus, there is a Calder and Miro print in every booth, and an Agam and Appel in every other booth. There are also curiosities, like the small, take-apart wearable sculptures by Berrocal, and small cloth nudes stuffed into jars.

"Schlock Alley," where art-as-interior decoration" still flourishes, has conveniently - for those who know the difference - been more concentrated in one area, and is thus more easily avoided.

Print publishers still dominate, as is often the case, because they are eager to distribute their editions, and dealers are eager to stock up. Those present at "Art '78," however, include most of the world's best, and, for collectors of contemporary prints, the fair has endless possibilities.

The Savile Row of the print stalls is straight down the center aisle, opposite the main entrance. On the left is Transworld Art, where earlier this week, American Indian artist Fritz Scholder was busily signing copies of a poster reproducing his latest lithograph, "Indian With Pistol," on sale for $1,000. The poster, which carries the same image, signed or unsigned, is $40.

Will Barnet is slated to be signing posters at this booth on Sunday. (Israeli artist Yaacov Agam was also on view early this week, beside his new movable print called "Hidden Images," possibly the first graphic ever made with ball bearings.)

Proceeding down the center aisle, on the right, are other publishers (Poligrafa from Barcelona and newcomer von Loeper from Hamburg), and some fine and reasonably priced surrealist etchings by a Czech artist named Janak are a standout.

Petersburg Press is here, featuring classic Americans like Dine, Stella, Oldenburg and Johns; Jacobson of London, next door, is showing fresh work by English contemporaries Richard Smith and Ivor Abrahams.

Around the corner, Landfall Press of Chicago has some new drypoints by Pat Steir, and David Hockney's Picasso-inspired portfolio, "The Blue Guitar." Brooke Alexander has several fine realist prints, including New York City as seen by Philip Pearlstein and Richard Haas.

One important dealer not located in this area is Maeght Editeurs of Paris, publishers of fine portfolios by Miro and others, and notable to budget-minded collectors for the annual subscriptions they offer at $60, which bring five posters, and five issues of "Derriere la Miroir," their high-quality, highly illustrated publication.

Another graphics gallery of interest is Merlin of Hamburg, which has several prints by artists who illustrate books published by this company. Prices are low, including the very funny posters by Arno Waldschmidt at $2.

The biggest improvement in this fair, by far the best of the three to date, is the increase in the number of dealers showing unique works. Foremost among them is former artist-furriere Jacques Kaplan, who deals privately in New York and has put together a show of paintings and drawings from the past 150 years amusingly titled "The Girl Next Door."

It begins with a 19th-century painting of a Turkish concubine and ends with a gouache by Kaplan's friend, the late Richard Lindner. Beside it is a touching handwritten sign which says: "Our very dear friend died on the 16th of April, 1978. We will always miss him." The work is not for sale.

Elsewhere throughout the fair, however, a brisk business in Lindner is going on, in part because of his recent death, and in part because the day before he died he was informed that he would, at last, have a retrospective at the Museum of Modern Art upon completion of the new building. "I think that satisfied him," says Kaplan, "and he just went to sleep."

Maeght has a recent Lindner portfolio for sale, and Transworld has a lithograph called "Washington Holiday" which sells for $900 in the limited, signed edition of 175. For $25 the same image is available in a poster, said to have been published from the same stone. If so, it is a bargain.

Holly Solomon from SoHo, who, with photo-realist dealer Louis Meisel is one of the few good contemporary galleries from New York, is featuring pattern-painting, the closest thing to an art "trend" visible at the fair.

Robert Kushner is shown here both in the Holly Solomon booth and at Lunn's where Washington artists Sam Gilliam, Jacob Kainen, Kevin MacDonald and Michael Clark also are on view. Barabara Kohl from Milwaukee, showing in the Judith Posner stand, is another delightful manifestation of the "pattern" genre, and Washington's own Jerry Clapsaddle shines in the Protetch-McIntosh stand.

The Delson-Richter Galleries from Old Jaffa, Israel, also have original work, of high quality, including some charming nude figures fashioned from old stockings and cotton, other sculptures and very funny take-offs on the art world by cartoonist-illustrator Bischof.

Most exotic and least interesting of the many foreign entries are the "Russian Images" brought to Pittsburgh by an American dealer who will sell them (if anyone will buy them) exclusively. The Bulgarian exhibit is a considerable improvement, though the art is dreary for the most part.The Polish tapestries from Cepelia have always been beautiful, but the recent designs - or at least those on display here - cannot approach those of earlier years.

The "Art Fair" from Gaithersburg, incidentally, has woodblock prints by mainland Chinese artists which at least match, if they do not surpass the aforementioned political category.

Photography is represented chiefly by three Washington dealers - Lunn, Sander and Ewing - with private dealer Kathleen Ewing providing the freshest work by several young and very talented artists.

Elias Felluss, who runs the fair, announced earlier this season that he might move the fair to Texas, at least in alternate years. Was he still considering such a move? "That's a heavy question," he said, just before the opening. "The Armory roof leaks, there's no downtown location large enough for us, and we get no backing from the city. We asked Mayor Washington just to cut an opening ribbon for us and he never even answered the letter."

(A call to the mayor's office brought the response that they could neither immediately confirm nor deny this.)

The fair opened to 2,500 area art lovers without the mayor. Despite a downpour, business was thriving by Thursday afternoon. But suddenly Felluss' problems were made clear: "Attention everyone," said a voice over the loudspeaker, "the roof is leaking. Please be sure your art is safe." A man with a mop came by to eradicate a puddle that had accumulated only inches from an expensive new portfolio of prints.

If the sun comes out today, there will be a sky-sculpture performance over RFK Stadium by aerobat Steve Poleski at 2:30. It can be watched from a small cafe set up in front of the Armory. Whether the sun comes out or not, video artist William Wegman will perform at the fair tonight at 7:30 and Sunday at 7:00, and Joan Jonas will appear at 3:30 Sunday.

Admission to all of the above is $3. A very helpful catalog is also available at $3.