But while prosecuting the work of the painter, he did not neglect the art of the engraver, and in 1511, brought out in complete form his great book of woodcuts in folio, and began to develop that marvellous art of etching which is indissolubly connected with his name. Among the products of the etcher's needle which attest his activity in this direction are those masterpieces which have for centuries been at once the delight and the puzzle of artistic minds: the "Melancholia," "The Knight and the Devil," and "St. Jerome in his Cell." The most reasonable explanation of these weird fancies is that they were intended to represent in allegorical style the three temperaments--the melancholic, the sanguine, and the phlegmatic. The Diet of Augsburg, which was convened in 1518, gave Dürer a passing opportunity to depict the lineaments of the Emperor Maximilian, who gave him several sittings, and who manifested great interest in the painter. The death of the emperor in the following year, the outbreak of an epidemic in Nuremberg, together with the coronation of Charles V. at Aix-la-Chapelle, led Dürer to undertake a journey to the Low Countries, in which he was accompanied by his faithful wife. He was present at the coronation and was one of the distinguished civilians whose appearance added dignity to the occasion. His diary, in which he recounts his experiences upon this journey, and which is accompanied by a multitude of wayside sketches, is still preserved, and contains, besides the dry entries of his current expenditures, most entertaining allusions to the distinguished people whom he met, and who received him with the utmost cordiality. Intermingled with these narrative details are outbursts of feeling, which are provoked by passing political and ecclesiastical events, in which he took a profound interest, though he never appears to have committed himself with positive openness to the party of reform. His sympathies are, however, clearly shown by his writings, as well as by his works of art, to have been with the Reformers, and he lived on terms of intimacy with Erasmus and Melancthon, of both of whom we have portraits from his hand. Dürer returned from the Netherlands in 1521, about the middle of July, and the remaining years of his life were spent in the prosecution of the art of the engraver, in painting, and in the effort to elucidate the sciences of perspective, geometry, and fortification, upon all of which he has left treatises.