 It has been widely speculated that they made use of the camera obscura, but the extent of their use by artists at this period remains a matter of fierce contention, recently revived by the Hockney–Falco thesis.German philosopher Johann Sturm published an illustrated article about the construction of a portable camera obscura box with a 45° mirror and an oiled paper screen in the first volume of the proceedings of the Collegium Curiosum, "Collegium Experimentale, sive Curiosum" (1676).Johann Zahn's "Oculus Artificialis Teledioptricus Sive Telescopium", published in 1685, contains many descriptions, diagrams, illustrations and sketches of both the camera obscura and the magic lantern.