Time a in Bottle : THE FERMATA, <i> By Nicholson Baker (Random House: $21; 320 pp.)</i>

Poor Achilles. He never could catch that tortoise, not because he was slow or the tortoise speedy but because Zeno wouldn’t let him. In the famous paradox, the tortoise gets a 10-meter start, say, and by the time Achilles runs the 10 meters, the tortoise has crept 10 centimeters; when Achilles goes the 10 centimeters, the tortoise has done a millimeter, and so on. But the rules are stacked. Achilles doesn’t catch up in the first stage, lasting a few seconds, but he would do it in the next stage if it lasted even one second instead of only a few hundredths of one. Zeno arranges tinier and tinier distances in tinier and tinier intervals that rapidly approach stasis.

Nicholson Baker is becoming his own Zeno, and approaching an equivalent stasis. His is a clever, a comic and a genial spirit. In “The Mezzanine” and “Room Temperature” he devised an enticing micro-literature. The first--a lunchtime walk in a department store--and the second--a morning spent holding his baby--explored whole worlds of tactile and temporal associations through two kinds of dailiness. The reader suspends belief, and perhaps the author did too; and both are rewarded. It is as if by musing on the nail in the general’s horseshoe and squinting across it toward the horizon, Baker had evoked an entire war.

You might think that Baker’s command of sensation’s nuances, and his ability to perch far-fetched yet oddly reasonable structures atop them, would work particularly well with the highly sensory, not to say sensational, subject of sex. Yet his last book, “Vox,” sagged despite some brilliant variations in its story of a man and woman having phone sex. “The Fermata,” narrated by a man who possesses the power to stop time and uses it to undress women, is a greater letdown. Although it has brio and speculative ingenuity, Baker works them with a mildly ill feverishness.

Arno Stryne is a permanent temp in a succession of Boston offices. It’s not that he’s dumb, quite the contrary, but his energies go into his peculiar gift. When he was a child he brought a transformer to school and found that by switching it on he could switch time off for everyone but himself. He used the interval to take off his clothes, go up to his much-loved teacher, investigate her bodily parts and dab her navel with blue chalk.

He has used his developing talent--he steadily simplifies the machinery for switching off time until simply snapping his fingers will do it--to strip hundreds of women, fondle some of them and play ludicrously elaborate sexual tricks with a few. He draws the line at actual intercourse because that would be rape. He usually rearranges whatever he has disarranged so that when he starts time up again, his target, though sometimes hot and bothered, has no sense that anything has been done to her. He argues that he is doing good as well as having fun because he pays tribute to female body parts, even if they sag or are oddly shaped and thus unused to admiration. He has his code though he admits it has a few holes.

“I would condemn in the strongest terms anyone else who did what I have done,” he tells us. “But the thing is, I did it, I did it, and I know myself. I know that I mean no harm, I mean well.” An outrageous argument, to say the least; especially as we read of the various extremes that he goes to.

Arno stops time in a subway, attaches a vibrating device to a woman passenger and then, alternately restarting time so the device can work and stopping it each time the woman tries to investigate what is happening, brings her to orgasm. He achieves a similar effect on a beach by writing a pornographic story on his portable typewriter, stopping time to slip it into the sand beside a sunbathing woman, restarting, watching her arousal as she reads it and then, in a complex series of temporal starts and stops, hiding in her house for a twin round of masturbation of whose simultaneity only he is aware.

The arrangements for what he punningly calls “chronanism” become ever more elaborate. Placed inside a magnetic resonance chamber, he practices it on a woman doctor who is measuring the carpal tunnel damage to his wrist caused by his temp typing and his single-handed sex life. Spotting an attractive fellow driver on the Massachusetts Turnpike he stops time and traffic, hikes an hour to the nearest town, buys a tape recorder, tapes another pornographic story, gets into the woman’s car and slips the cassette into her player to replace her Suzanne Vega. He restarts everything, and follows her car, fantasizing a further adventure of even more absurd complication. Instead, she tosses the cassette out the window.

Finally, thank goodness, a comic chink (after all that work) opens in the cabinet of this consciously comic, preemptively discriminating and suffocatingly creepy Caligari. Does Baker perceive him as such? It is unclear. True, “The Fermata”--the musical term for a note held beyond its prescribed value--tells Arno’s story largely in retrospect with a framing-device that offers the possibility of some chastening perspective. At the start, after a time-stopping peek at a fellow-worker, Arno falls in love with her. He writes out his past adventures, which become most of the book. Then he confesses to her before taking things farther. It is to be a real relationship instead of Arno’s time-gimmick--except that she quickly warms to the gimmick and finds a way to join in.

“The Fermata” is frequently funny; sometimes in the Monty Pythonesque elaboration of Arno’s stratagems, sometimes in the microscopic parsing of his sensations. It is not enough to counteract the chill. Never mind Catherine MacKinnon; the book is enough to blow the fuses on Kant and his categorical imperative. (Treat others as ends and never as means.) Arno’s account may satirize today’s voyeuristic culture; the trouble is, he acts too comfortable in the telling, whether in the mode of skillful erotica, crude pornographic parody or loopy confessional. What is stickier, beneath the apparent comfort there is a hint of unease as if this kind of outrageousness were not quite in Baker’s line of belief but an experiment gone faintly whiffy.

To see the world in a drop of water, you need to choose your drop. In some drops all you can see is your own eye. “The Mezzanine” stretched its net across the tide of daily trivia whose small hopes and despairs constitute the wetlands of our lives. “Room Temperature” did something similar with the mingled awe and tedium of parenthood. The sexual feelings that Baker explores in “Vox” and “The Fermata” arise less from real sex than from his characters’ masturbatory fantasies. His microscope reveals the color-dots of a printed picture, not the mysterious forms of a living cell.