By the way, "wet plate" or "collidion process" photography using glass plates to create negatives was invented in 1851. Brady and others used it to photograph Civil War scenes, people (especially soldiers, who wanted photos for their families), and so on. The advantage was that several "positives" could be made from one glass negative. The process only started going out of fashion in fields like astronomy in the 1980s and even now has some advantages over digital cameras.