It was now possible to make "panchromatic" plates, which would photograph the entire spectrum without difficulty, and such plates were placed on the market in 1906.
In recent years enormous progress has been made owing to the discovery of new classes of dye which confer extremely high sensitivity through the visible spectrum and far out into the infra red. (See fig. 3.) Film.—The weight and fragility of the glass support led to attempts to replace it by lighter and flexible substitutes. In the film roll system of photography was invented by W. H. Walker, the emulsion being coated on paper, and a machine for continuously coating photographic paper in long rolls was patented by George Eastman in 1885. The sensitized paper was loaded into a roll holder which could be attached to the back of a camera in the same position as a plate holder. At first, the paper was waxed to reduce the printing time, but later a stripping film was used, this consisting of a temporary paper base coated with soluble gelatine, which in turn was coated with a gelatine emul sion.
This stripping film was used in the first portable roll film camera marketed in 1888—a box type camera with two rolls, a supply spool and the wind-up spool. The pictures were circular, about two inches in diameter, and the operator had merely to point the camera and press the button. When the film was used up, the whole camera was returned to the manufacturing company, who developed and printed the pictures and reloaded the camera.
Eastman made experiments to produce a flexible film from collodion, but the ether-alcohol solutions would give only films which were too thin to be of practical use. Finally the use of wood alcohol as a solvent for nitrocellulose was discovered, and a flexible film base was marketed in rolls in 1889. In the same year, J. Carbutt introduced emulsion coated on flexible sheets of plastic nitrocellulose compound cut from blocks. The new flexible film was applied at once by Edison to the photography of motion pictures.
In 1891, daylight loading film was made. The film was wound on a wooden core inside a light-tight box and black cloth leaders were attached to the ends of the film. Later, the film was wound
inside a protective sheet of black paper leaving sufficient over length to permit loading of the camera without fogging the sensi tive film. This was further modified in 1903, when a non-curling film was produced by coating the back of the film with plain gelatine.
In 1913, roll film packing was further modified by spooling the film in a thin and translucent red paper, the film being pro tected from light by the use inside this of "carbon" paper similar to that used for duplicating on a typewriter. After exposures are made, the photographer can write on the red paper with a stylus through an opening in the back of the camera. The pressure of the stylus removes some of the opaque black carbon from the sheet, so that on exposure to light the film is exposed along the tracks of the writing, and the image develops with the negative and serves for its identification.
In 1895, positive film was introduced for the printing of motion pictures, the film used previously being of the negative variety only. In 1909 slow burning motion picture film was first made by the substitution of cellulose acetate for cellulose nitrate as the plastic used in the preparation of the base.
Another method of packing films was invented in 1903 and was introduced on the market under the name of "film packs." These are now used mostly in cameras of the type originally de signed for employment with dry plates.
They have the considerable advantage of not requiring a special roll film back. During recent years flat cut films have taken the place of plates to a considerable extent especially in professional photography.
In 1917, X-ray films coated on both sides with emulsion were introduced. Owing to the transparency of the material to the X-rays, this enables double the density to be obtained, and the use of this film is now general throughout the world, it having almost entirely replaced plates in radiographic work.