What advice, provided by Sir John Herschel, was key to refining both Daguerreotype stabilization and the Calotype process?

Answer

Using sodium thiosulfate (hypo).

Sir John Herschel suggested the use of sodium thiosulfate (hypo) to stabilize the image, which refined Daguerre’s initial salt water stabilization and was adopted by Talbot.

What advice, provided by Sir John Herschel, was key to refining both Daguerreotype stabilization and the Calotype process?
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