 Guidobaldo became a staunch friend of Galileo and helped him again in 1592, when he had to apply to the chair of mathematics at the University of Padua, due to the hatred and machinations of Giovanni de' Medici, a son of Cosimo I de' Medici, against Galileo.Notwithstanding their friendship, Guidobaldo was a critic of Galileo's principle of the isochronicity of the pendulum, a major discovery which Guidobaldo thought was impossible.Guidobaldo wrote an influential book about perspective, titled "Perspectivae Libri VI", published at Pesaro in 1600.