Scheele's experiments on silver chloride
1777 (Chemische Abhandlung von der Luft und dem Feuer)
Carl Wilhelm Scheele showed that light blackens silver chloride by turning it into metallic silver, and that violet light does it fastest.
How it began
Johann Heinrich Schulze had shown that light, not heat, darkens silver salts. The Swedish chemist Carl Wilhelm Scheele took the work further and published his results in 1777, in a book mostly about air and fire.
How it works
Scheele spread silver chloride on paper and placed it in a spectrum cast by a prism. It blackened fastest in the violet and hardly at all in the red. He also showed that the black deposit was metallic silver: ammonia dissolved the unchanged chloride but left the blackened part behind, a first hint at how a picture might be fixed. See silver halide chemistry.
Why it mattered
He explained what the dark trace actually was, and showed that the colour of the light matters. That is why early plates were blind to red and why darkrooms could use red light safely.
Limits and what replaced it
Scheele was not trying to make pictures. Turning his chemistry into images, and fixing them, took another sixty years.
Today
Every silver-based film and paper still works on the reaction Scheele described: light frees metallic silver from a silver salt.
Sources
- Wikipedia: Carl Wilhelm Scheele ↗
- Eder, ch. XI–XIII (pp. 83–99)