OF ALL THE arts, film has the strongest power to mirror reality on the one hand and to stimulate dreams on the other. Some films do both, of course. The filmmaker will start with a real situation but take it a crucial step beyond; and by pushing mundane desires, hopes or fears to extremes, uncover something new about the human condition.

This is the case with two of the most striking features screened at the 16th New York Film Festival, which runs through Oct. 8 at Lincoln Center. The two pictures - Bertrand Blier's "Get Out Your Handkerchiefs" and Francois Truffaut's "The Green Room" - stand at opposite poles: The Blier is wildly funny, while the Traffant to gloomy about to the point of morbility. Yet there are points of correspondence that bring them close together.

Both pictures begin with seemingly plausible people, caught in common predictaments. But both do the [WORD ILLEGIBLE] mile - to see what happens when the limitations of reality are replaced by the freedom of imagination. Traffan has said that he made "The Green Room" specifically to ask, "What would happen if, oblivious to the passage of time, we stayed attacked to the dead by ties as strong as those that link us to the living?"

"Get Out Your Handkerchiefs" paves by implications, a similarly external question: "what would happen if the loving husband of a childless, [WORD ILLEGIBLE] woman decided to purchase his wife's happiness at any conceivable cost?"

It is not only this basic strategy that the films share. Both Blier, who at 30 has had one big previous success ("Going Places," 1973) and is still a relative newcomer, and Truffaut, who is seven years older and one of the world's most highly respected directors, exhibit a peculiarly French sensibility that binds them to the humanistic tradition of Jean Renoir - indeed, "Get Out Your Handkerchiefs" reminds one strongly of Renoir's masterful "Boudu Saved From Drowning" in its comic daring and irreverence. Like Renoir, Blier and Truffaut seem able to view their characters whole, warts and all, with a subtimely balanced mixture of detachment and sympathy. This is a rare [WORD ILLEGIBLE] in any art, and precious enough to make both these movies eminantly worth seeing, even though only one of them - the Blier - is likely to attain box-office popularity.

In "Get Out Your Handkerchiefs," the distraught husband, Raoul, convinced that his wife, Folsage need some strong new attentions to pull her out of her chronic temper, accents another young man, Stephen, a total stranger, in a restaurant and practically begs him to sleep with solange. Stephase is initially aghan; but after some intricate comic parrying, he begins to feel sympathetic to Raoul's plight (besides, he's immensely attracted to Solange) and falls in with the "experiment."

The two men develop a bizarre comradeship, becoming almost more interested to each other and their common dilemma (Solange's apathy) than they are in Solange herself. In addition, they share a love for the music of Mozart. One night at 3 a.m., while they are playing some Mozart on the hi-fi at top volume, an irate, middle-aged neighbor pound on their door to complain about the noise, and they hustle him, too, into bed with Solange (to no greater avail).

Later on, at a summer camp where Stephane is a counselor, and where Stephane and Raoul are taking turns as Solange's mate, Solange shows the first signs of shaking off her lassitude and it has nothing to do with either of them. She's suddenly disarmed by one of the boys at the camp, a lonely but intellectually precocious 13-year-old who's constantly being taunted by the others.

It would be inconsiderate to reveal any more of the plot, which is full of brisk asides, such as the scene in which Raoul and Stephane speculate on what might have been if Mozart could have been kept alive with antibiotics. Suffice it to say that Blier piles outrage upon comic outrage, but so deftly and slyly that the whole delicate imbroglio never edges over into burlesque. In its ebuillient amorality, the picture's is worthy of its Mozart (it also has some superb original music by Georges Delerue).

It's only afterward that you realize how hard Blier has made you think about men behaving like boys, and boys behaving like men, and the difference between maternal affection and erotic desire, and why love drives everybody crazy. In reviewing "Going Places," The Washington Post's Gary Arnold declared that Blier's earlier hit was the first picture from Europe since Godard's "Breathless" to demonstrate such a "surprising mixture of originality, audacity and pertinence." "Get Out Your Handkerchiefs" is more than ample confirmation of the promise. Incidentally, Blier wrote the script of the new opus with actors Gerard Depardieu and Patrict Dewaere, the incorrigibles of "Going Places," specifically in mind as Raoul and Stephane, and they are marvelously supported by a cast that includes Carol Laure as Solange and Michel Serrault as the neighbor.

As far out as Blier's conception is, Truffaut's "The Green Room" departs from a still more-challenging premise. Like a number of Truffaut's earlier pictures ("The Wild Child," "The Story of Adele H.," "The Man Who Loved Women"), this one is about an obsession - in this case, an obsession with death, or more accurately, with the dead. Julien, the hero of the story (which is partly based on writings and also the life story of henry James), is a man who has dedicated his existence to cultivating the memory of his dead wife and friends.

Julien lived through World War I, while most of his friends perished in the trenches; he's never overcome the guilt. His young wife dies a few months after their wedding. He writes for a moribound weekly journal, and his editor calls him "a virtuoso with obituaries." After hours, he sits among his wife's portraits and effects in a locked, candlelit room, speaking to her as if she were alive. When the room is destroyed by fire, he commissions a mannikin in his wife's likeness, only to have it brutally dismembered in his horror at the result.

Then he conceives the idea of restoring a decaying chapel as his own private shrine "to my Dead." In the meantime, however, he has become involved with a woman who shares his obsession though not quite to his own fanatic degree. As an adolescent, she had worshiped him from afar, because of his resemblance to her father. She too nurses a sorrow for a lost lover, a political martyr whose mistress she had been for some years.

There is a tenuous point of balance in the film when were are led to think that her newly awakened passion for Julien might extricate him from his sepulchral submersion. But it turns out, in a coindence too patently contrived for the purpose, that her lover, the politician, was the one man Julien despised, a man who had once been an intimate friend but who had, Julien thinks, betrayed him utterly and polluted his taste for life.

Despite the sensitively understated performances by Truffaut himself as Julien and Nathalie Baye as the woman, as well as the affecting, luminouis pallor of the color photography by Nestor Almendros and the poetic astringency of Truffaut's direction, the "movie never quite connects emotionally with the heart of its subject. There's something so comatose about the characters and the rhythm of the film that one is held at arm's length, engaged, perhaps, with the intellectual issues but not with the currents of feeling Truffaut clearly wanted to generate.

If it is a failure it's at least an intelligent and honest one. At a minimum, the picture provokes disturbing reflections on the responsibility of the living on the dead. Truffaut confronts something we all get to ask ourselves sooner or later. How do we pay adequate respect to those we held dear, without infringing on our own need to live? There are few film-makers who would care to grapple with such enigmas, and fewer still who could bring Truffaut's compassion and gentility to the task.

On second thought, both films examine precisely the same problem - it's only the tone and perspective that separate them.Both Julien, in "The Green Room," and Raol, in "Get Out Your Handkerchiefs," are trying to resuscitate their wives: with respect to the ensuing psychological development, it makes little difference that one is dead and one is not. Julien fails by cutting himself from life, and the end is tragic. Raoul fails by leaning over so far towards arrested adolescence that Solange is driven to refuge with the real thing, and here the consequences are prodigiously amusing. Thus Blier and Truffaut take off from the same sport and land miles apart; but in the final analysis, the movies have as many affinities as differences.