The sensitive silver salts in the emulsion Of the plate or film have undergone an invisible change when exposed to the action Of the rays of light — some of the particles have been affected by the light, while others remain as they were. It will be found when this exposed plate, or film, is placed in a solution composed Of some oxidizing agent, called a developer, that a Change takes place in the color of these light affected particles and they are thus rendered visible to the eye, and may also be built up to form an image varying in density according to the amount of the light action on the sensitive emulsion.