More worthy of record is the fact that the King gave up his concert for that evening and invited Bach, already known as “Old Bach,” to try the Silbermann pianofortes(104) which stood in various parts of the Palace.(105) Accompanied from room to room by the King and the musicians, Bach tried the instruments and improvised upon them before his illustrious companion. After some time he asked the King to give him a subject for a Fugue, that he might treat it extempore. The King did so, and expressed his astonishment at Bach’s profound skill in developing it. Anxious to see to what lengths the art could be carried, the King desired Bach to improvise a six-part Fugue. But as every subject is not suitable for polyphonic treatment, Bach himself chose a theme and, to the astonishment of all who were present, developed it with the skill and distinction he had shown in treating the King’s subject. His Majesty expressed a wish to hear him on the Organ also. Accordingly, next day, Bach inspected all the Organs in Potsdam,(106) as the evening before he had tried the Silbermann pianofortes. On his return to Leipzig he developed the King’s theme in three and six parts, added Canones diversi upon it, engraved the whole under the title _Musikalisches Opfer_ and dedicated it to the royal author of the theme.(107)
His visit to Potsdam was Bach’s last journey. The indefatigable diligence he had shown all his life, and particularly in his younger years, when successive days and nights were given to study, seriously affected his eye-sight. The weakness grew with age and became very distressing in character. On the advice of friends who placed great confidence in the skill of a London oculist lately come to Leipzig,(108) Bach submitted to an operation, which twice failed. He lost his sight completely in consequence, and his hitherto vigorous constitution was undermined by the drugs administered to him. He sank gradually for full half a year, and expired on the evening of July 30, 1760, in the sixty-sixth year of his age.(109) Ten days before his death(110) he was suddenly able to see again and to bear the light. A few hours later he was seized by an apoplexy and inflammatory fever, and notwithstanding all possible medical aid, his weakened frame succumbed to the attack.