As Charles surmised, Rupert's confinement was, at first, very vigorous. All the liberty that he enjoyed was an occasional walk in the castle garden; all his entertainment an occasional dinner with the Governor. Graf Kuffstein, himself a convert from Lutheranism, was commissioned by the Emperor to urge his desires on the young prisoner. "And very busy he was to get the prince to change his religion." At first he urged him to visit some Jesuits, but this Rupert refused to do unless he might also go elsewhere. Then Graf Kuffstein offered to bring the Jesuits to the Prince, but Rupert would only receive their visits on condition that other people might visit him also.[18] To the promise of liberal rewards if he would but serve in the Imperial army, the boy proved equally impervious; and though deprived of all society he found interests and occupations for himself. His artistic talents stood him in good stead, and he devoted himself much to drawing and etching. At {43} this period also he perfected an instrument for drawing in perspective, which had been conceived, but never rendered practical, by Albert Durer. This instrument was in use in England after the Restoration of 1660. Military exercises Rupert also used, as far as his condition would permit. He was allowed to practise with "a screwed gun," and, after some time, he obtained leave "to ride the great horse," and to play at tennis. Naturally, constant efforts were made to procure his release. In July 1640 Lord Craven wrote to Secretary Windebank on the subject: "Mr. Webb has informed me that His Majesty has imposed upon you the putting him in mind of pressing on the Spanish Ambassador the delivery of Prince Rupert. I know you will, of yourself, be willing enough to perform that charitable action, however, the relation I have to that generous prince is such that I should fail of my duty if I did not entreat your vigilance in it."[19] King Charles sent Ambassadors extraordinary, not only to the Emperor, but also to Spain, whose intercession he entreated. The Cardinal Infant promised to plead, at least, for Rupert's better treatment, and King Charles next turned to France. France, then at war with the Empire, held prisoner Prince Casimir of Poland who, it seemed to Charles, might be a fit exchange for his nephew. Through Leicester he urged Prince Casimir's detention until Rupert's liberty were promised. But the scheme failed; Rupert, it was answered, was "esteemed an active prince,"[20] and would not be released, so long as danger threatened the Empire. So early had he acquired a warlike reputation.