<P> In black - and - white photographic film, there is usually one layer of silver halide crystals . When the exposed silver halide grains are developed, the silver halide crystals are converted to metallic silver, which blocks light and appears as the black part of the film negative . Color film has at least three sensitive layers, incorporating different combinations of sensitizing dyes . Typically the blue - sensitive layer is on top, followed by a yellow filter layer to stop any remaining blue light from affecting the layers below . Next comes a green - and - blue sensitive layer, and a red - and - blue sensitive layer, which record the green and red images respectively . During development, the exposed silver halide crystals are converted to metallic silver, just as with black - and - white film . But in a color film, the by - products of the development reaction simultaneously combine with chemicals known as color couplers that are included either in the film itself or in the developer solution to form colored dyes . Because the by - products are created in direct proportion to the amount of exposure and development, the dye clouds formed are also in proportion to the exposure and development . Following development, the silver is converted back to silver halide crystals in the bleach step . It is removed from the film during the process of fixing the image on the film with a solution of ammonium thiosulfate or sodium thiosulfate (hypo or fixer). Fixing leaves behind only the formed color dyes, which combine to make up the colored visible image . Later color films, like Kodacolor II, have as many as 12 emulsion layers, with upwards of 20 different chemicals in each layer . </P> <P> The earliest practical photographic process, the daguerreotype, introduced in 1839, did not use film . The light - sensitive chemicals were formed on the surface of a silver - plated copper sheet . The calotype process produced paper negatives . Beginning in the 1850s, thin glass plates coated with photographic emulsion became the standard material for use in the camera . Although fragile and relatively heavy, the glass used for photographic plates was of better optical quality than early transparent plastics and was, at first, less expensive . Glass plates continued to be used long after the introduction of film, and were used for astrophotography and electron micrography until the early 2000s, when they were supplanted by digital recording methods . Ilford continues to manufacture glass plates for special scientific applications . </P> <P> The first flexible photographic roll film was sold by George Eastman in 1885, but this original "film" was actually a coating on a paper base . As part of the processing, the image - bearing layer was stripped from the paper and attached to a sheet of hardened clear gelatin . The first transparent plastic roll film followed in 1889 . It was made from highly flammable nitrocellulose ("celluloid"), now usually called "nitrate film". </P> <P> Although cellulose acetate or "safety film" had been introduced by Kodak in 1908, at first it found only a few special applications as an alternative to the hazardous nitrate film, which had the advantages of being considerably tougher, slightly more transparent, and cheaper . The changeover was completed for X-ray films in 1933, but although safety film was always used for 16 mm and 8 mm home movies, nitrate film remained standard for theatrical 35 mm films until it was finally discontinued in 1951 . </P>

Who introduced the flexible film to the market