Coloured pictures, or any coloured object, can only be photographed successfully by the help of a screen or interceptor, which gives the true tone values of the colours. In addition, the emulsion with which the plate is coated must be specially sensitive to red and orange. Such plates (termed chromatic, isochromatic, or ortho-chromatic, or colour-correct) may be had of all dealers in photographic materials, those of Edwards being particularly good. These plates must be developed only in a dull ruby light. Pyro-soda is the most suitable developer. The screen may be fixed either before or behind the lens, and may either be made by staining a sheet of gelatine in a weak solution of picric acid, or purchased ready for use. Generally, the screen should be a very pale lemon yellow, but the more the two colours named above predominate, the deeper should be the tint.