He also worked on the velocity of light which he believed was finite and was pleased to hear of Römer's experiments which gave an approximate velocity for light determined by observing Jupiter's moons.
Siya rin ay nagtrabaho sa bilis ng ilaw na siya sumampalataya ay may wakas at noon ay nalulugod sa marinig ng Römer ng mga eksperimento na ibinigay ng isang wastong bilis para sa ilaw tinutukoy ng observing Jupiter's Buwan.