Ever hear of ambrotypes? Probably not, to the dismay of James Ambrose Cutting, who invented them. An ambrotype is a small (4 by 21/2 inches) portrait made by exposing a glass plate in a camera. Much the rage around 1854, the ambrotype was supplanted by the easier-to-produce tintype and faded from public view. But photographer Allan Janus does not believe such obscurity is deserved. He does old-fashioned ambrotype portraits, for which subjects must sit motionless for 20 seconds or so. They pose outdoors on a sunny day, wait while Janus dashes into the darkroom, then pose again. Results are chancy but charming. "Ambrotypes seem to swim in the glass," says Janus. "They're a little more special and rare than the usual old-fashioned portrait. I use 1854 methods, with a trifling technical change here and there. For instance, cyanide used to be used as a fixer. At that much authenticity, I draw the line."

Ambrotypes. $75 for the results of one session: at least two different portraits, often three if things go well. No more than two people to a portrait. Kathleen Ewing Gallery, 3243 P St. NW. 342- 8101. Photographing takes place at the gallery.