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How did Johann Heinrich Schulze show that light rather than heat darkened silver salts?
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History of photography
Child with Striped Dress (Younger Raab Child), autochrome photograph by Alfred Stieglitz, 1907; in the Art Institute of Chicago.
In 1727, the German professor of anatomy Johann Heinrich Schulze demonstrated that the darkening of silver salts was caused by light rather than heat by using sunlight to record words onto the substance.12
While it had been known since the 16th century that these compounds darkened, Schulze’s experiment provided the proof that light was the active agent. Although he successfully used this method to create temporary markings, he did not attempt to preserve the images permanently. His discovery, when combined with the camera obscura, established the fundamental technological basis for the eventual invention of photography.12