He gave much attention to perspective, which was then in a very evil plight by reason of many errors that were made therein; and in this he spent much time, until he found by himself a method whereby it might become true and perfect--namely, that of tracing it with the ground-plan and profile and by means of intersecting lines, which was something truly most ingenious and useful to the art of design. In this he took so great delight that he drew with his own hand the Piazza di S. Giovanni, with all the compartments of black and white marble wherewith that church was incrusted, which he foreshortened with singular grace; and he drew, likewise, the building of the Misericordia, with the shops of the Wafer-Makers and the Volta de' Pecori, and the column of S. Zanobi on the other side. This work, bringing him praise from craftsmen and from all who had judgment in that art, encouraged him so greatly that it was not long before he put his hand to another and drew the Palace, the Piazza, and the Loggia of the Signori, together with the roof of the Pisani and all the buildings that are seen round that Piazza; and these works were the means of arousing the minds of the other craftsmen, who afterwards devoted themselves to this with great zeal. He taught it, in particular, to the painter Masaccio, then a youth and much his friend, who did him credit in this art that Filippo showed him, as it is apparent from the buildings in his works. Nor did he refrain from teaching it even to those who worked in tarsia, which is the art of inlaying coloured woods; and he stimulated them so greatly that he was the source of a good style and of many useful changes that were made in that craft, and of many excellent works wrought both then and afterwards, which have brought fame and profit to Florence for many years. [Illustration: THE CRUCIFIXION (_After_ Filippo Brunelleschi. _Florence: S. Maria Novella_) _Alinari_]