The Daguerreotype is a negative image, but one that will appear as a positive under the correct lighting conditions. It is supported on a copper plate, one side of which is covered by a highly polished layer of silver which has been made sensitive to light by exposing it to the vapour of iodine crystals (to form silver iodide). After exposure the plate is developed by exposing the surface to the vapour of heated mercury and then fixed by soaking in a solution of common salt.