This century has seen a number of figures from the world of entertainment win deserved reputations as artists. George Gershwin comes immediately to mind, but he still worked in a traditional orchestral idiom. More recently such unabashedly popular musicians as the Beatles and Bob Dylan have been taken seriously - sometimes so seriously that they themselves become embarrassed. What's new is that the pop people have by now created their own artistic traditions and that their traditions have begun to merge, in some still vague and elusive sense, with the mainstream of high art.

Today, there are a number of supposedly "pop" performers who are in no reasonable way distinguishable from "artists". They may make money, but they're not enslaved by it. They may lack a traditionally crafted technique, but they work out their own means to their own artistic ends. They may move in a glamorous world far removed from a starving, Bohemian stereotype, but they still make art rather that fluffy entertainment. Joni Mitchell, who will be at the Forest Hills Tennis Stadium Saturday night, is such an artist - as serious and experimental as they come.

Miss Mitchell is also in the news these days because of her unusual, touching collaboration with another popular artist seemingly far different from her in background and achievement - Charles Mingus, the great jazz bass player and bandleader who died early this year of the muscular disease that killed Lou Gehrig. The result, a record called "Mingus," is not a total success. But the very route by which a 35-year-old, one-time folksinger and folk-rock princess from Canada by way of Los Angeles came first to jazz itself and then to a leading jazz experimentalist of the 1960's - and something of a black cultural polemicist, to boot - is an unusual tale. And it helps give some hints as to the evolution of Miss Mitchell's own artistry.

"When Charles became ill, it fell to his wife, and his friends to try to find some projects to stimulate him, to keep him active and interested," she explained a few weeks ago in Los Angeles in her slightly stilted, almost Biblical manner of speaking. "Charles had called a friend one night and said, 'Come over here, I want to talk to you about God.' The friend felt ill-equipped to talk about God, so he stopped in a bookstore and purchased a copy of T.S. Eliot's 'Four Quartets.' From that came an idea to Charles that he wanted to do a piece of music with a very formal, educated voice reading the Quartets, or parts of them. My job was to translate what he conceived as the formality of Eliot's language into the vernacular. His analogy was the Baptist Church, where you have somebody reading a Biblical text and someone else translating that down into the jargon. So, I bought the book, and then I called back and said I couldn't do it - there was no way I could take it and distill it down. It seemed like a sacrilege. I thought I'd rather do it with the Bible. So, we scrapped that project.

"Later I heard again he wanted to see me. He had written six melodies for me to set words to. By then Charles was paralyzed. He sang them into a tape recorder, and a piano-player friend of his fleshed them out into a voiced piece of music. The tape presented to me was a piano with a metronome. It nearly drove me crazy, this tape. I took it out to the beach at Malibu and the second day I woke up in bed with my foot ticking back and forth under the covers, to the rhythm of this metronome."

Miss Mitchell had several meetings with Mr. Mingus in which he suggested specific themes for songs, and then she set about writing texts and music. Last summer, she began to think she should go into the recording studio right away if Mr. Mingus were to have any creative say in the record. The recording sessions, hastily conceived and prepared, went poorly. Mr. Mingus resisted electronic instruments and preferred a traditional jazz arranger. "At that point, I realized that in order to continue with the project, I couldn't just be a cog in someone else's scheme. I had to continue the way I always had, in an intuitive exploration."

The final result, to this taste, isn't always successful, clouding the energetic directness of jazz with a certain self-consciousness. But her music to come may well blossom into something extraordinary as she pursues this style.

"This record is a logical extension of what I've been doing, in that my direction obviously magnetized Charles to me and me to Charles. I would never have written melodies that moved like that myself. But having been disciplined now to learn those intricacies, that will definitely influence my future music."

Nowhere in this narration, it should be stressed, do commercial questions intrude. Miss Mitchell's records sell roughly the same, between 500,000 and 1 million copies each, which is certainly respectable but hardly overwhelming in the pop world.

"In my timid moments, I worry about not having hits," she said. "In strong moments, I feel that I can't be all that unique, and if I have a craving to create something that pleasures me because it's unusual, then there must be a certain percentage of people who also have a craving to hear something fresh. But at the same time, I can't resist it, I have no choice. I want to create something that is beyond vogue, that is classic. There's the long dollar and the short dollar. If I were to become a 'hit-making' artist, I would come to a peak more rapidly and I would decline more rapidly. The steadiness of my career has been the success of my career."

Miss Mitchell's evolution has been a steady one, too, both in here poetic themes and in her music. She describes her central concerns as follows: "The search for love, which is an evasive search. Being drawn by that evasive thing into meeting with individuals. Finding out that they are either incompatible or inflict damage of some kind. An examination of the reason why, and the anger that accompanies the sense of failure - an analysis of the crime. The pursuit of love in the romantic sense, which leads inevitably to cynicism. The avoidance of cynicism for the development of an ironic point of view. I still do fall into illusion myself from time to time - romantic illusion is a veil for me, and this foolish veil sweeps me away."

Some critics find all this a disturbing, narcissistic sort of self-preoccupation. It is a complaint that Miss Mitchell can understand, but she sticks to her guns: "The only way to justify being a public confessor is that as personal as these experiences are, they're common to us all. Nietzsche says in 'Zarathustra,' ' The poet is the vainest of the vain.' But at the bottom he says, 'I see of the poet a new breed, born of penitents of the spirit. These poets write in their own blood. Of them comes the superman.'"

Miss Mitchell's shift in musical style from folk to jazz began in earnest five years ago with "Court and Spark," her most successful album in commercial terms. It is a fascinating shift, a move from a style predicated on a nostalgia for the land into a world of urban energy and darkness. And on a more purely technical level, it involves the cultivation of an exact and improvisatory sense of rhythm and pitch that most folksingers don't even dream of.

Based on the criteria of either classical music of jazz, however, Miss Mitchell remains a naïf. In the eyes of some, that might invalidate her out of hand. In her own eyes and in those of her admirers, it merely defines her artistry as being of a different kind.

Many of her collaborators came to appreciate her music as something remarkably individual. John Guerin, a drummer and former collaborator and romantic involvement, remarks that "she's been able to keep growing within her technical limits. Her technique is perfectly valid for what she's doing." Or as Tom Scott, a saxophonist and another former collaborator puts it, "She took whatever she had and made music."

Miss Mitchell admits to difficulties in communicating with classical and jazz musicians in the recording studio. "There were moments when I felt handicapped that I couldn't express myself within the number and letter system of musical talk. I would be forced to deal in metaphors that would bewilder the players or that they would think were precious. That put me in the position of having a 'what-she-means-is' guy on the session, and generally he wasn't equipped to speak for me. It would come out safer than I had intended."

Painting is at least as important to her as music, and she falls easily into graphic metaphors to describe her music. "It's the way I view the world," she said. "Musicians are more audio-oriented, but I'm drawn first to color juxtapositions. I have much more color available in my voice than I did a few years ago. I have this painterly ability now with this line that comes out of my throat. It can break into a shading, or it can be a very strong, straight line. With the 'Mingus' album, you have to listen in an almost painterly way. If you begin to think of each of the players as brush-strokes, you can see the seed of the idea.

"Let's say a guitarist in the studio lays down four tracks," she continued. "Some of them have magic moments, but they also have clunkers and warm-up chords. So, what I have is four tracks that need weaving. I edit each of those tracks individually, and then I run them all together. This is the way I've chosen to compose, through technology, through tape. It's audio composition - the elimination of things that do not work by erasure, all by ear. And by a graphic system which runs inside, behind my eyes. I see a graphic indication of what I'm hearing. I see where it's tangled in a graphically interesting way and where it's tangled in a confusing way.

"One day I was thinking about all that and it suddenly occurred to me that I was a musician," she laughed. "Before, I'd always felt I was primarily a painter, working in audio graphics."

If Miss Mitchell's credentials as an artist are sometimes challenged because of her technical limitations, they are also questioned because of her very membership in the social world of pop stardom.

"I remember David Geffen [the founder and former president of her record company, Asylum] looking at an album cover I'd painted and joking, 'Not bad, for an acid casuality,'" Miss Mitchell recalled. "Historically, artists have always tampered with their consciousness to produce passageways into that space from which inspiration flows - to tap into the river. I think all drugs have a function. But everything is double-edged. If you abuse any one of those potions and it becomes the norm, it no longer has a positive effect.

"I'm not afraid of glamour. I've presented a certain amount of opulence of taste in my work and my lifestyle. And I am very self-interested. But sometimes I can't stand it when the conversation turns to me too much - it's like too much chocolate cake. I do receive preferential treatment in the public. I must accept it, because this is reality for me now. People say, 'Oh, they're living in a dream world.' It's not a dream. You're an illusion-maker, so you're dealing with other people's illusions."

Right now, with houses near Vancouver and in Los Angeles's Bel Air district and a loft-studio in Manhattan's SoHo, Miss Mitchell could keep busy just leading her life, quite apart from her music.

"I try to prevent myself as a discipline from too much future-fantasy - the fantasy of growing old with someone. I am in fact growing old in a social way with men. I have loved, but not under the same roof. We still share affection for one another, and this suffices. The idea of being a single old woman without a family is no longer frightening to me. I'm subject to mood, and I prefer the option of being on my own if these moods should strike. I know nothing except to indulge myself in them, because of them comes my creative power.

"I don't worry so much any more about imperfections in my work. Imperfections are not a problem when you turn in a project. They are only signposts to the next project. But the most creative thing of all is life. All the arts are shorthand manifestations of life. To me, my first act of art is how I live my life."