If light reaches a dry plate by any other way than through the lens when the plate is exposed in the camera, the result is fog; that is, the sensitiveness of the plate is destroyed, and development produces black patches of greater or less intensity according to the amount of light that has accidentally fallen on the plate. This fog may be due to defective slides, to cracks in the camera, to leakages of outside light into the dark room, or to an unsafe lamp. In a score of other ways, all of which may be classed under careless or faulty handling of the plates during their journey from the maker's box to the developing dish, light may reach the sensitive plate and cause fog.