>> allen positions a model in front of his camera obscura and faces to it the sun. >> the whole setup has to be calculated very, very carefully before you can start the exposure. >> a medieval artist would need to ensure that any image projected on a sheet could be permanently fixed. the sheet would be soaked with photographic chemicals. today, this means using silver salts. >> both silver nitrate and silver sulfate have been around in europe from at least the 13th century. and it's very easy to make. >> a sheet is soaked in silver sulfate solution. dried. and then mounted in the camera.