But to Rupert poverty was no new thing, and he now enjoyed, for the first time since his captivity in Austria, leisure to devote himself to art, philosophy and science. In these years he first studied the art of engraving, in which he was afterwards so famous. He is popularly supposed to have invented the process of engraving by Mezzotint, the idea of which he is said to have conceived from watching a soldier clean a rusty gun. But the process was, as a matter of fact, communicated to him by a German soldier, Ludwig von Siegen. In 1642 von Siegen had completed his invention, and had sent a portrait, produced by his new process, to the Landgrave of Hesse, with the announcement that he had discovered "a new and singular invention of a kind never hitherto beheld." In 1658 he met Rupert in Vienna, and, finding in him a kindred spirit, disclosed his secret. They agreed only to reveal the process to an appreciative few, and it is probable that, but for Rupert's interest in it, the invention would have died with the inventor.[25] To the Prince belongs the credit of introducing it into England. "This afternoon Prince Rupert shewed me, with his own hands, the new {292} way of engraving," says Evelyn in his diary, March 16, 1661.[26] And in his "Sculptura" he says, after describing the process, "Nor may I without ingratitude conceal that illustrious name which did communicate it to me, nor the obligation which the curious have to that heroic person who was pleased to impart it to the world."[27] Rupert himself worked hard at his engravings, assisted by the artist, Le Vaillant; and Evelyn refers with enthusiasm to "what Prince Rupert's own hands have contributed to the dignity of that art, performing things in graving comparable to the greatest masters, such a spirit and address appears in all he touches, especially in the Mezzotinto."[28] While at Mainz, Rupert developed other inventions, among them the curious glass bubbles known as "Rupert's Drops," which will withstand the hardest blows, but crumble into atoms if the taper end is broken off. He also prepared to write his biography. This he intended as a vindication against all the calumnies which had been associated with his name. But long before the vindication was compiled the need for it had vanished. The Restoration of 1660 changed Rupert's fortunes as it changed those of his Stuart cousins. He found himself "in great esteem"[29] with the whole English nation, and he therefore abandoned the idea of writing his history. All that remains of the projected biography are a few fragments relating to his childhood and early career. [1] Strickland's Elizabeth Stuart, p. 218; also Green's Princesses, VI. 38-41. [2] Green's Princesses, Vol. VI. 55-58. [3] Thurloe, VI. p. 803, 24 Feb. 1658. [4] Bromley Letters, pp. 285-288. Elizabeth to Rupert, March 4, 1658. [5] Ibid. p. 289. [6] Bromley, pp. 287-288. [7] Bromley Letters, pp. 189, 295, Elizabeth to Rupert.