What crucial discovery did Louis-Jacques-Mandé Daguerre make regarding the faint image on his treated plates?

Answer

It could be revealed to full visibility using mercury fumes (development).

Daguerre discovered that a faint, latent image formed on a silver-coated copper plate could be brought to full visibility using mercury fumes, a technique known as development.

What crucial discovery did Louis-Jacques-Mandé Daguerre make regarding the faint image on his treated plates?
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