Casually, like well-bred amateurs, the ten musicians adjusted lights and music racks, then began to play Handel's Concerto Grosso in G Major. There was nothing casual, nothing amateurish about their playing. They were, in fact, a hardworking group of Philadelphia professionals, called the American Society of the Ancient Instruments, and last week they were giving their 17th annual festival in the University of Pennsylvania's little cream-brick Museum auditorium.
The Society of the Ancient Instruments, whose recordings have long been treasured by chamber-music addicts, is a nonprofit labor of love. It is the creation of a little grey-maned, Dutch-born music teacher named...