I could not help recalling to my memory the anecdote told by Forkel of Sebastian Bach’s visit to the great Prussian monarch in the company of his son William Friedmann. “At this time,” says the biographer, “ the King had every evening a private concert, in which he himself generally performed some concertos for the flute. One evening, just as he was getting his flute ready, and his musicians were assembled, an officer brought him the list of the strangers who had arrived. With his flute in his hand he ran over the list, but immediately turned to the assembled musicians, and said, with a kind of agitation, ‘ Gentlemen, old Bach is come.’ The flute was now laid aside, and old Bach, who had alighted at his son's lodgings, was immediately summoned to the palace. [...]

But what is more important than this is that the King gave up his concert for this evening, and invited Bach, then already called ‘ the old Bach,’ to try his fortepianos, made by Silbermann, which stood in several rooms of the palace. The musicians went with him from room to room, and Bach was invited everywhere to try and to play unpremeditated compositions. After he had gone on for some time, he asked the King to give him a subject for a fugue, in order to execute it immediately without any pre­paration. The King admired the learned manner in which his subject was thus executed extempore ; and, probably to see how far such an art can be carried, expressed a wish to hear a fugue with six obbligato parts. But as it is not every subject that is fit for such full harmony, Bach chose one himself, and immediately executed it, to the astonishment of all present, in the same magnificent and learned manner as he had done that of the King. His Majesty desired also to hear his performance on the organ. The next day, therefore, Bach was taken to all the organs in Potsdam, as he had before been to Silbermann’s fortepianos. After his return to Leipzig, he composed the subject which he had received from the King in three and six parts, added several artificial passages in strict canon to it, and had it engraved, under the title of ‘ Musikalisches Opfer ’ (Musical Offering), and dedicated it to the inventor.”