“There are certain actions of light which may be called negative actions. By this name I mean to indicate a series of new actions, without pretending to enter into any theoretical considerations. In acting upon an iodised plate, common light produces successively different states, which are made manifest by exposing the plate, when taken out of the camera, to the vapours of mercury. If the plate has remained in the camera but a short time, we shall see only the first traces of an image. The parts most strongly affected by the light condense the mercurial vapour only in small quantities. If the light acts during a longer time, the image will appear with more details and more distinctly, the mercurial vapours being condensed more abundantly. By increasing the time of action, the image gains in details, but the parts which have been affected with a strong light condense the mercurial vapours in smaller quantities.