In 1503 Dürer's frail constitution yielded to an attack of illness. A drawing of Christ crowned with thorns, now in the British Museum, bears his inscription: "I drew this face in my sickness, 1503." In the same year he executed a copper-plate engraving of a skull emblazoned on an escutcheon, which is crowned by a winged helmet, and supported by a weird woman, over whose shoulder a satyr's face is peering. A contemporary copper-plate shows the Virgin nursing the Infant Jesus. The painting of this same subject, bearing the date of 1503, is now in the Vienna Belvedere, portraying an unlovely German mother and a very earthly baby. The celebrated "Green Passion" was executed in 1504, and is a series of twelve drawings on green paper, illustrating the sufferings of Christ. Some critics prefer this set, for delicacy and power, to either of the three engraved Passions. The theory is advanced that these exquisite drawings were made for the Emperor, or some other magnate, who wished to possess a unique copy. The Green Passion is now in the Vienna Albertina, the great collection of drawings made by the Archduke Albert of Sachsen-Teschen, which includes 160 of Dürer's sketches, designs, travel-notes, studies of costume and architecture, &c. Over 600 authentic sketches and drawings by Dürer are now preserved in Europe, and are of great interest as showing the freedom and firmness of the great master's first conceptions, and the gradual evolution of his ultimate ideas. They are drawn on papers of various colors and different preparations, with pen, pencil, crayon, charcoal, silver point, tempera, or water-colors. Some are highly finished, and others are only rapid jottings or bare outlines. The richest of the ancient collections was that of Hans Imhoff of Nuremberg, who married Pirkheimer's daughter Felicitas, and in due time added his father-in-law's Dürer-drawings to his own collection. His son Willibald further enriched the family art-treasures by many of the master's drawings which he bought from Andreas Dürer, and by inheriting the pictures of Barbara Pirkheimer. He solemnly enjoined in his will that this great collection should never be alienated, but should descend through the Imhoff family as an honored possession. His widow, however, speedily offered to sell the entire series to the Emperor Rudolph, and it was soon broken up and dispersed. The Earl of Arundel secured a great number of Dürer's drawings here, and carried them to England. In 1637 Arundel bought a large folio containing nearly 200 of these sketches, which was bequeathed to the British Museum in 1753 by Sir Hans Sloane. The museum has now one of the best existing collections of these works, some of which are of rare interest and value, especially the highly finished water-colors and pen-drawings.