Some thoughts on 'realism' in fiction for teen-agers; It All Began With Jane Eyre, by Sheila Greenwald. Boston: Atlantic -- Little, Brown & Co. $7.95; A Place Apart, by Paula Fox. New York: Farrar, Straus & Giroux. $9.95

Sometimes the mirror that art holds up to nature seems shadowy and untrustworthy. Its distorted images trouble the reader who turned to art's mirror in the first place for revelation, comfort, or healing. On bright days, of course, it's enough to laugh and remember that art's images are always illusory. On darker days, though, the heart searches for serious answers; lighthearted dismissals of art's promise to help us see more clearly may seem to cheat us out of something we could treasure very deeply.

Intuitively, we prize literature that pays homage at least to some sense of the real. But it is not always easy to determine what qualifies as literary realism. Such a consideration becomes even more difficult when children or young adults are the ones confronting a literary work. It is hard to imagine recommending pure documentary realism in all cases to a child. And yet saccharine tales not only bore readers, but seem to deny the substance and value of literature and the other arts.

Right now, a large number of the American novelists writing for children and young adults are attempting to represent serious social problems with frankness and clarity. This seems a worthy goal. How, if not by confronting social and cultural problems directly, can we hope to find much-needed cures?

And yet, as one critic put it, the young people in these novels are "the unhappiest, most upset, distressed, suspicious, alienated, introspective generation the world has ever known." Moreover, "most of the time they are smothered in their own unhappiness and for fairly ordinary reasons."

In many cases, it isn't clear whether this trend reflects a change in the experience of American adolescents or rather just mirrors the feelings of a generation of adult novelists who happen at this moment to be writing for youngsters.

While the value of forthrightness seems clear, some criteria are needed to help adolescents judge what kind or measure of realism is valid or satisfying in their fiction. Two exceptional new novels help sort out these questions.

In Sheila Greenwald's satiric comedy, "It All Began With Jane Eyre," 12 -year-old Franny Dillman runs into trouble because of her susceptibility to fiction. Munching her way through potato chips and Victorian novels by flashlight on the floor of her closet, Franny lets her imagination run away with her, spreading an embarrassing tale that patterns the dean of her school after a character in "Jane Eyre."

Hoping to set Franny on a better track and keep her out of trouble, Mrs. Dillman buys her some modern teen novels, recommended as "true to life." If she'd taken the time to read these books, Mrs. Dillman would probably have preferred "Jane Eyre." Comparing herself to the heroines of the new fiction, Franny decides her own life is totally dull -- no affairs, no divorces, no passionate impulses -- that is, until she looks closer and begins to imagine scandalous intrigue in her own home, with disastrous consequences.

Greenwald's hilarious account of the dangers of certain kinds of teen reading delivers a strong dose of literary criticism between the lines: The boundary between "real life" and the invented world of the novels Franny reads is hazy, and art is probably never more influential than when the feelings and motives of adolescents are involved. With a lighthearted touch and an understanding that adolescent problems do not have to be catastrophes, Greenwald tests the limits of literary realism.

And yet "It All Began With Jane Eyre" is really just a spoof. To use its revelations as a critique of all juvenile realism would be unfair to strong, serious authors for this age group, whose characters confront serious problems and who can thus hope to offer serious answers or at least some measure of comfort and consolation.

Another fine novel that touches (less intentionally) on these problems is "A Place Apart," by Paula Fox, a winner of the Newbury Medal for children's literature.

Fox, who also writes excellent novels for adults, possesses a distinctive kind of poise and balanced control over dialogue and imagery. These set "A Place Apart" well above most juvenile fiction. An exceptional stylist, Fox resists shielding young readers from a troubled world. While there are some generous adults on the sidelines, her young heroine, Vicky Flint, is required to face sad, difficult problems on her own.

Soon after the death of her father, sensitive young Vicky moves from Boston to a small New England town, where her mother hopes their expenses will be minimal. Vicky's mother, though sympathetic, is somewhat aloof, and her uncle, whose wisdom provides the most comforting, interesting moments in the novel, can take only limited time to help them get established in their new home.

Paula Fox's ability to record the thoughts of her young protagonist as she makes tenuous new friendships is exceptional. The author does not present a prettier reality than she believes in, but out of the richness of her novel's characters she offers young readers the kind of value missing from the novels Sheila Greenwald satirized. Fox's careful depiction of a point of reference in the adult world against which Vicky can measure her own life represents one extremely important difference between strong fiction for young readers and the stuff Franny Dillman consumes. Another difference: Vicky's problems are grave, but not exaggerated.

While "A place Apart" provides an outstanding example of serious realistic fiction for a juvenile audience, the book leaves some emotional loose ends still untied at its conclusion, and the events leading up to the final scenes are grim. The book may seen troubling and depressing to some readers, though it will not exploit their emotions.

"A Place Apart" both promises and delivers the serious probing and soul-searching that realism at its best can provide. Its honest depiction of a chilly winter of one adolescent's life represents a strong alternative to the teen novels Greenwald comically deplores.