book we find the sketch of a flying bird to be introduced in some comedy; on another we read a suggestion for bringing snow from the mountains in summer, and scattering it on the Piazza at festivals. In later years he was appointed ducal engineer, and careful notes on the canals of Lombardy and fortifications of the Castello are to be found among his works. Certain mysterious circular engravings, designed by his hand and bearing the inscription, Accademia Leonardi Vinci, have been taken as evidence that the great master founded an Academy of Arts and Sciences at Milan; but the term was probably applied to those informal gatherings of scholars and artists which were held in the Castello, in the Duke's presence, and which Leonardo's friend, the mathematician Luca Pacioli, describes as "laudable and scientific duels." The great lasting influence which he exerted on the school of Milan is well-known, and it was at Lodovico Sforza's especial request that the artist wrote his famous Treatise on Painting.
These varied occupations left Leonardo little time for painting. Yet, during these busy years at the most brilliant court of Italy, he executed some of his most important works. The pictures which he painted for the Emperor Maximilian and the King of Hungary, and the portraits of the Moro's mistresses, Cecilia Gallerani and Lucrezia Crivelli, have perished, but one great altar-piece of this period is still in existence. This is the "Vierge aux Rochers," which the master painted about 1490, for the Church of S. Francesco of Milan, but which he asked the Duke's leave to keep, since the friars refused to pay him more than twenty-five florins, while another patron
