COIMUR SCREEN PLATES AND FILMS 884. Various Commercial Forms. The Auto chrome plate was the first practical application of a sensitive emulsion coated on an irregular tricolour mosaic for the direct photography of colours by permanent additive synthesis. Since 1933 the Autochrome has ceased to be supplied in the form of plates, but only in two varieties of film : Filincolor stiff film cut in sizes, always used with the lens fitted with a compensating filter chosen according to the character of the light illuminating the subject, and Lumicolor roll-film or film-packs with a much faster emul sion, the gelatine backing of which forms the compensating colour filter for photography in natural light.
Agfacolor plates and fiat films H. Christen sen, 1912), similar in structure to the Auto chrome, are used with a compensating filter, while the more rapid Agfacolor Ultra roll-film permits photography in natural light without a compensating filter.
The Dufaycolor film (1934) with a screen of regular geometrical pattern results from very important successive improvements by many English technicians to the Dioptichrome plate (L. Dufay, 19o9). This film is also available in two varieties : 1).2, fiat films and sub-standard cinematograph films for use with a filter, and D.r, roll-films, film packs, and per forated 35 mm. film in short lengths for minia ture cameras, for use in natural light without a compensating filter.' Important advances have recently been made in the speed of several of these sensitive emul sions and others are announced as imminent at the time of writing this chapter. We will there fore refrain from giving 'precise information, which is likely soon to be obsolete, and we cannot insist too strongly on users following the instructions of the various makers as regards exposure and the various