When Michelangelo began to work again in the Sistine Chapel, the wall above the altar was adorned with three great sacred subjects by the hand of Pietro Perugino. In the central fresco of the Assumption Perugino introduced a portrait of Sixtus IV. kneeling in adoration before the ascending Madonna. The side panels were devoted to the Nativity and the finding of Moses. In what condition Michelangelo found these frescoes before the painting of the Last Judgment we do not know. Vasari says that he caused the wall to be rebuilt with well-baked carefully selected bricks, and sloped inwards so that the top projected half a cubit from the bottom. This was intended to secure the picture from dust. Vasari also relates that Sebastiano del Piombo, acting on his own responsibility, prepared this wall with a ground for oil-colours, hoping to be employed by Michelangelo, but that the latter had it removed, preferring the orthodox method of fresco-painting. The story, as it stands, is not very probable; yet we may perhaps conjecture that, before deciding on the system to be adopted for his great work, Buonarroti thought fit to make experiments in several surfaces. The painters of that period, as is proved by Sebastiano's practice, by Lionardo da Vinci's unfortunate innovations at Florence, and by the experiments of Raffaello's pupils in the hall of Constantine, not unfrequently invented methods for mural decoration which should afford the glow and richness of oil-colouring. Michelangelo may even have proposed at one time to intrust a large portion of his fresco to Sebastiano's executive skill, and afterwards have found the same difficulties in collaboration which reduced him to the necessity of painting the Sistine vault in solitude. Be that as it may, when the doors of the chapel once closed behind the master, we hear nothing whatsoever about his doings till they opened again on Christmas Day in 1541. The reticence of Michelangelo regarding his own works is one of the most trying things about him. It is true indeed that his correspondence between 1534 and 1541 almost entirely fails; still, had it been abundant, we should probably have possessed but dry and laconic references to matters connected with the business of his art.