Arago was an influential scientist and powerful advocate for others. Biot and he conducted research across four countries, to fix the exact length of the metre. Humboldt was the first to contact him, after his imprisonment by pirates and other nautical misadventures. He took over Malus’ research after his death, championed his close friend Fresnel’s theories of light transmission, and (eyesight failing) suggested experiments on the velocity of light to Fizeau and Foucault. He corresponded on wave theory and other matters with Young, and got official backing for Daguerre’s photographic work. Gay-Lussac and Delacroix were close friends, Le Verrier a student.