What is the earliest principle of camera imaging? How do you develop photos?

"Preparation of photosensitive substance silver iodide on copper plate: put the silver-plated copper plate on the upper part of the container, with the silver-plated surface facing down, and put a plate under the container where iodine is put. The sublimated iodine vapor reacts with silver to form a photosensitive substance-silver iodide on the silver surface of copper plate. When the silver-plated copper plate exposed for 5-40 minutes is placed on hot water silver, the exposed part absorbs mercury due to mercury vapor fumigation, the exposed image is enhanced, the exposed latent image is gradually developed by mercury vapor, and the scene' appears' into a visible image. Then immerse the copper plate in the ocean wave solution, remove the unexposed photosensitive substance silver iodide, and' fix' the displayed image to obtain a permanent image. "

The negative film taken by silver plate photography is a positive image, that is, the negative film is a silver plate photo. It is recommended to refer to the history of photography and photography in Daguerre.