Some of the most delightful new works of architecture these days are illusions.

One of the most astonishing of them, no doubt, is the cutway interior of a domed Beaux Atrs palazzo, Boston's Back Bay. You see it all the rising above humdrum buildings in way from Boylston Street and the Massachusetts Turnpike.

Or look at those ornate, turn-of-the-century facades of a six-story office building at the corner of Green and Prince Streets in New York's newly chic SoHo (South of Houston Street).

Then look again. The side of that great facade is painted, like the Bostom domes on the back of the Architecture Center, on apreviously blank and ugly firewall. It is painted with such consummate realism that it deceives the eye - tromple I'oeill, as the French and art historians put it.

The artist of these wall paintings is Richard Haas, whose reward for deceiving us is the 1978 Medal for outstanding art and craftmanship from the American Institute of Architectects.

The art of deceptivity realistic painting goes back to the ancients. The fifth-century B.V. Greek artist Zeuxis, it is said, printed a garland of grapes on an open-air theater with such realism that birds flew up and started to peck at them to be outdone his rival Parrhasios painted a picture of a curtain.

"All right, already," Zeuxis said. Pull the curtain and let's see what you have painted!" When he realized his mistake, Zeuxis complimented Parrhasios. "I only fooled some birds," he is reported to have said. "But you, my friend, fooled a great artist!" (Or words to that effect.)

"Architectural ilusionism," as Haas prefers to call architectural trompe I'oeil, has been around ever since.It was used most frequently in the Renaissance when even the richest princes, popes and potentates ran out of money for marble, niches, colonades, ornaments and statuary. Again, the painting was often so skillfully done that you might call it "double-take architecture."

(Renaissance clients probably also did double-takes when they got the painters' bills. Some work would probably have been cheaper in marble.).

What seems new about the "architectural illusionism" revival in the past few years is that some of it seems painted tongue in cheek. Haas, who has done 10 large walls and is headed for Munich, Germany, to do his biggest yet, likes to surprise his viewers with unexpectedly open windows, a complacent cat, a person where you least expect him.

On dirty old walls in New York's Little Italy, Haas has painted 19th-century storefronts - a hat shop, a bakery, a lawyers' office - with such charm that one expects Mary Poppins to descend at the moments, fold her umbrella, try a bonnet, select a murffin and seek counsel.

A trompe I'oeil painted a by Fabio Rieti in Paris, opposite the Pompidou arts and cultural center, shows a charming townhouse with a woman peeking out from behind Parrhasios like curtains. I tried not to stare at the motionless apparition, until I realized that she would hardly reproach me. The whole "townhouse," in fact, disguises a concrete mechnical plant servicing the Pompidou.

Most of Haas' works is sponsored, or partly sponsored, by City Walls, Inc., a private organization devoted to brighten our cities woth well-chosen art works.

Headed by Doris C. Freedman, who has long been active in New York City's cultural activities, City Walls came out of the social unheavals of the 1960s when art, too, took to the street. The organization has assisted the completion of 50 murals in eight American cities.

At first most of these wall paintings tended to make either social statements in the tradition of Diego Rivera and Jose Clemente Orozco, or assert the ethnic character of the community in which they were painted. Some reflected currently fashionable art styles such as Pop or Surrealism. There were many bland and banal "supergraphics." A few walls showed humor.

The growing trend of "architectural illusionism" undoubtedly reflects a disillusion with today's architectural reality, a nostalgia for more human and humane surroundings. It is part of the current surge for the preservation and "recycling" of old buildings and has all the makings of developing into an effective tool of good urban design. Illusion always has been used to make cities the stage for the human drama.

Haas, who will receive his medal at the annual AIA convention at Dallas in May, was born 42 years ago in Spring Green, Wis. His father worked for Frank Lloyd Wright and kindled his interest in architecture.

Haas is a painter and printmaker, however, whose work has been shown in gallaries and museums throughout the country, including last year, the Corcoran Gallery of Art.