Now this is just really useful or productive, but it coughtmy imagination in the erly dark this morning:

Given what is now known about various photographic processes: How early could some form of photography been done? The Daguerreotype process was published in 1839 of course, but years of experimentation in many processes had gone before. If someone already KNEW that it could be done, how early could it have been done?

Mu guess is that Dags could have been made at least thirty years earlier. Of course polished silver and elemental Mercury were known in antiquity. For that matter, red glass was available way back too, so Becquerel development was possible le in ancient times too. I think the choke point was the isolation of elemental Iodine in 1800 or so.

Now, what about some of the salted paper processes? What about collodion on glass? When could Silver Nitrate come into use?

Anybody?