In the foyer of the Coolidge Auditorium this week, in a casual show of its lavish holdings, the Library of Congress has a display of old music manuscripts--among them, Franz Joseph Haydn's "Surprise" Symphony in the composer's handwriting. It is open to the slow movement (the one with the "surprise" in it), and it shows Haydn's first thoughts on how it should go, neatly crossed out and followed by a revised version of the best-known passage in all his music.

But the real surprise, last night, happened inside the auditorium, where more than two hours of Haydn's music were performed in the proper style. This sort of thing is beginning to happen more often, but it is still rare. It was a perfect way to usher in Haydn's 250th birthday, which is observed today.

The first surprise was the orchestra, the Ensemble for Early Music's Grande Bande Chamber Orchestra, which ranged in size from 10 to 15 players, including music director Frederick Renz, who conducted part of the program from the harpsichord. All of the orchestra stood up while playing except for the harpsichord, cello, bassoon and double bass players. That's how it was done in Haydn's time, when symphonies were shorter.

The instruments were authentic and tuned to 18th-century pitch: violins with the exquisite, soft-edged tone of gut strings; Baroque oboes (which look something like recorders) and natural horns with no valves, a marvelously fresh, open sound and some fiendish technical challenges. The balances were a revelation; two horns playing against six violins are not at all like two horns in a modern orchestra.

The performances were almost completely without vibrato, which began to be used like pancake makeup in the 19th century. The effect of this was most striking in the vocal music--two arias exquisitely styled by countertenor Peter Becker and a cantata ("Qual dubbio omai") that evoked some brilliant coloratura work from soprano Sally Sanford. To hear two voices of this caliber singing in a style untouched by the mannerisms of Romantic opera, a style almost reminiscent of folk music, was first a shock and then a delight.

The most familiar music on the program, the "Trauer" Symphony, was beautifully interpreted and a totally fresh experience in the readjusted balances and scaled-down dynamics of this ensemble. As a soloist, Renz showed a very fluent technique and particularly a mastery of the little tricks of phrasing that are needed (without the piano's dynamic range) to make this instrument expressive.

Early in the concerto, the harpsichord tended to be overbalanced by the orchestra, but that was adjusted smoothly before the end of the first movement. Otherwise, except for an occasional vocal tone that was less than pear-shaped, the concert seemed to have few technical problems. In overall impact, it was a revelation and a delight.