As the shadow of cowboy hats bobbed in the dilating spotlight, a gangly, long-legged fiddler swayed to and fro and the Johnson Mt. Boys began whooping it up.
So went the rest of the nearly sold-out performance of two of the country's finest bluegrass bands last Saturday night - the Johnson Mt. Boys, and Seldom Scene. Their enthusiastic reception in the 1,132-seat Hancock Hall may well indicate a revival of interest in this Appalachian Mountain/Kentucky/ Southern ''Alabam' '' music.
Grounded in the traditional Flatt and Scruggs genre, the Johnson Mt. Boys performed the usual wailing, oh-so-blue bluegrass songs. There were other old favorites, too.
Seldom Scene, a group that has played and recorded with such pop stars as Linda Ronstadt and Emmy Lou Harris, offered an entirely different sound. Loose and progressive, its style has won it the title of best bluegrass band in the land. ''The House of the Rising Sun'' was easily rendered, and other tunes took them to new limits. Yet, occasionally the the group lagged in energy and perhaps rode its name a bit too assuredly.
On Sunday, Dec. 4, another fine bluegrass show will bring five of the best bluegrass musicians in the county to Hancock Hall. This special New England performance of ''The Bluegrass Album'' Band is a show bluegrass enthusiasts won't want to miss.