This is part 4 of 6. The first three are here: Part 1 Part 2 Part 3 In the 1720s the German professor Johann Heinrich Schulze (1687–1744) showed that certain silver salts, most notably silver chloride and silver nitrate, darken in the presence of light, not heat as some scholars had previously believed (the process … Continue reading Fjordman: A History of Optics, part 4