By day, Isabel Fine is a mild-mannered librarian in the cataloguing section of the National Library of Medicine. By night -- and when nobody's looking -- she's learning how to juggle. "I'm the type who'll think of trying anything but usually doesn't go ahead with it," Fine, 55, said the other day. "But a few months ago, when I saw this juggling course in a catalogue, I just wanted to see if I could do it. Things don't always have to be subconscious. Sometimes they can just come suddenly and consciously." Fine, in sweatsuit and sneakers, was one of a score of people who showed up one recent Sunday night for a juggling class at Open University. In a mirror-paneled studio on upper Connecticut Avenue, she and her fellow students grappled with bean bags, tossing them in the air and plopping them on the floor as the instructor, Nancy Lynner, shouted commands. "Keep your hands down," Lynner, a professional mime, called out over a noise that resembled the snapping of wet towels, but was really beanbags hitting polished wood. "Also, try breathing. It's easy to forget to breathe when you're learning any new movement like this." In the group, aside from Fine, was a diminutive man in his 50s with a long gray mane and shirt pockets stuffed with cigars. "I think these are getting in your way," Lynner said, removing them. "I'd just as soon not be in the paper," he said, smiling shyly and retiring to a corner to commune with his bean bags. Some, though, were happy to talk. "I just needed to do something that made no sense whatsoever," said Pat Colwell, 33, a computer software marketer and partime college student who brought along her 10- year-old daughter, Cassi, to watch. "I've got a heavy semester going at school, with all these stastitics and economics courses, and I thought, 'Yeah, juggling: it's absolutely useless.' " Cassi divided her attention between her mother's attempts at juggling and a small rubber ball, which she continually bounced on the floor. "I want to learn, too," she said longingly. Fred Firestone, 29, and Steve Smith, 30, said they had dared each other to learn and decided the time had come. "I think it's probably a good thing to do at parties to pick up chicks," suggested Firestone, a teacher. "I just thought it'd be fun," said Smith, a typesetter. And Deborah Sherwood, a 30-ish Washington housewife, said she wanted to make a fantasy come true. "I have this vision of myself, walking down the aisle of a supermarket and juggling oranges in the air," she said dreamily. In the hour-long class, the first of two sessions for which participants paid $20 (a price that included three beanbags plus instruction booklet), Lyner guided her students through juggling theory -- "It's a spiritually involving thing but it's not a religion" -- first having them practice tossing and aiming with one bag, then progressing to two and finally three. "The art of juggling is a little bit like learning to ride a bicycle," said Lynner, who herself picked it up six years ago, mastered the juggling of four objects and started teaching the course in 1977. "It was a real struggle for me to learn. Some people tried to tell me how verbally, but I was getting all mixed up. It took me two weeks of working on it every day." That would give comfort to Isabel Fine, who by the hour's end -- with the likes of Steve Smith already juggling up a storm -- had just begun to get the knack of tossing her bags in a straight line.

"I'm the uncoordinated type," Fine said the day after class. "But I've been practicing. I can't very well practice here in the office, but I did this morning at home and I7ll do it again tonight before going to bed. "

Fine said she has told neither family nor friends of her recent exploit.

I want it to be a surprise.I'd love to be able to show off. If you've got it, i say flaunt it." DON'T DROP THE BAG For information on the juggling class, a perennial at Open University, call 966-9606.