CRASH COURSE in Modern Masters

When 200 realist painters protested outside New York's Whitney Museum of American Art recently, the event underlined both a reversal in museum policy toward contemporary art and continued public perplexity.

Forty-five years ago, it was abstract painters, dubbed the ''Irascibles'' in a famous Life magazine photograph, who protested their exclusion from exhibitions at the Metropolitan Museum of Art. Now it seems that museums favor avant-garde art and, according to the protesters, exclude traditional painting.

Certainly museums have enshrined the works of modernist masters. Nevertheless, an ongoing feeling of dismay characterizes public response to modern art.

Ever since the Armory show in 1913 when European innovators like Picasso and Matisse first received broad attention in this country, modern art has caused confusion. Back then, critics ridiculed modernists as ''lunatics,'' calling the room of Cubist paintings a ''Chamber of Horrors.'' Although these works are now icons of Art History 101, public skepticism persists.

To understand modern art requires a backward glance at the forces that produced it. For about a century, roughly 1880 to 1970, modernism was the prevailing philosophy of art. Modernism is an attitude more than a movement - a gung-ho zest to cultivate the new and disparage the old.

This rejection of the past was an about-face after centuries of reverence for tradition. It resulted from profound changes due to industrialization and urbanization. Within the space of one century, life probably changed more than in all preceding centuries. Describing the reaction to a burst of inventions - from radium to radio - before World War I, the novelist Ford Madox Ford said, ''It was - truly - like an opening world.'' Artists' response to the scientific advances was overwhelmingly positive. They, too, wanted to explore new horizons and develop art for the new age.

The demise of the patronage system reinforced the break with the past. The state, church, or aristocracy no longer commissioned art to glorify the status quo. Avant-garde artists created for themselves, experimenting with new forms and subjects. The camera assumed the task of documenting appearances.

The upshot for art was that, instead of representing the visual world objectively, art became subjective. The novelist and art critic mile Zola defended the new art, saying it portrayed ''nature seen through a temperament.''

Color no longer described a scene accurately. Matisse painted a green stripe down his wife's face to energize a composition, saying, ''Above all, I did not create a woman, I made a picture.'' Cubism threw out perspective and the illusion of three-dimensional reality.

Here's where the difficulty arose for the audience. Matisse dreamed of creating, he said, ''an art comfortable as an armchair after a hard day's work.'' Yet his simplified figures occasioned outrage.

With the demise of the perspective system practiced since the Renaissance, the picture was no longer like a window into ''deep'' space. The viewer responded solely to the surface of the painting, an arrangement of line, shape, and color. The spectator, according to the late formalist critic Clement Greenberg, felt a sense of loss because ''Pictorial space has lost its inside and become all outside.'' We no longer escape into a fantasy scene through art. We also no longer have an easy way to judge the worth of a painting, that is, by whether it creates an illusion of reality.

Appreciating modern art is not an instant transaction, but the reward for careful examination is new insight into the world.

Trust your instincts

''People should experience art the way they experience coming across an unknown object in the woods,'' says Leslie Hendrix, an artist and lecturer who leads tours of contemporary art galleries at Parsons School of Design in New York. ''You don't worry about whether or not you are having the 'right' reaction. You experience it for your own curiosity's sake.''

People are often intimidated by innovative art because ''they don't 'get' it,'' Ms. Hendrix adds, ''and they think they're ignorant.'' She urges novices to rely on their instincts. ''Maybe they dislike an art work because it is powerful, although not beautiful in the classical sense. Some of the strongest, most haunting pieces are the most disturbing.''

Cynthia Nachmani, school programs coordinator in the Museum of Modern Art's department of education, has developed a curriculum to overcome resistance to modern art. ''We try to get people to look longer and harder,'' she says, defining art as anything that ''makes you feel and think about something in a way you didn't before.'' Ms. Nachmani believes many outmoded notions about art - that it must be beautiful, uplifting, or carefully crafted - actually hinder appreciation of modern art.

The Museum of Modern Art's Visual Thinking Curriculum is based on questioning strategies. It does not impart information but asks spectators to answer the classic journalist's queries - who, what, when, where, and why - applied to a work of art.

The first step, according to Carlos Barbot, education assistant and lecturer, is to ''trust your intuition.'' The second is to support each opinion with evidence derived from the art object. ''I try to get people to think about ideas rather than just an image. Why did the artist choose a particular technique? Once they've had an interaction with art, they'll come away with something more meaningful.''

Gallery tour

A tour of the Museum of Modern Art's permanent collection constitutes a ramble through the history of modern art. One of the best-loved paintings is Vincent van Gogh's ''The Starry Night'' (1889). Van Gogh contrasts the chaotic, churning night sky with a tranquil village below. His technique of thick, swirling lines energizes the painting, conveying not the scene's actual appearance but the dynamic vitality of nature. A cypress tree and church spire connect the human and natural worlds.

Henri Matisse's ''The Dance'' (1909) is another work exploding with energy. Matisse used simple shapes and few colors to create an image of joy. Inspired by fishermen dancing on the beach, Matisse created a composition where form and color are expressive. He described his work as ''an art of equilibrium,'' and here the forces of tension and suspension are balanced. The taut figure at left is in counterpoise to the others, who seem to float. ''I have worked for years,'' Matisse said, ''in order that people might say, 'It seems so easy to do!'''

The idea behind Marcel Duchamp's ''Bicycle Wheel'' (replica of 1913 original) is more important than the object itself. This combination of an ordinary wheel and stool challenges the concept of what constitutes art. Duchamp wished to go beyond ''retinal'' art that appeals to the eye. His work, which he called ''brain-fact,'' poses questions rather than providing answers.

Constantin Brancusi pared ''Bird in Space'' (1919) down to a pure arc of form seeming to soar off its pedestal. Brancusi reduced the subject to its essence. ''Flight, what bliss,'' he said, evoking, not describing, his subject.

People often react negatively to unfamiliar art, but they should realize, as Nachmani says, that ''liking a work of art is irrelevant, but understanding is important.''