It is well known that when he [Mendelssohn] revived Bach's 'Passion Music,' and conducted the first performance of that immortal work after it had been dormant for about a century. He found, stepping to the conductor's desk, that a score similar in binding and thickness, but of another work, had been brought by mistake. He conducted this amazingly complicated work by heart, turning leaf after leaf of the book he had before him, in order not to create any feeling of uneasiness on the part of the executants.