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Double-Coated Stock 643
the ratio of the light reflected to one film was determined by the distance the prism was set above or below the center of the lens. In 3 the face of the prism 26 was provided with linear reflecting surfaces, as in 7. In 5 the surface of the prism 33 has recesses 36, 36 ground into its face, the light being reflected from those parts situated over the recesses, and the other part of the light passing through the cemented surfaces 37, 37, as shown in 6. Other forms of two-lens cameras are shown in 7 , 8 and 9. In the first case the lenses are vertically juxtaposed and in 8 the components are separated and mirrors used for dividing the beam ; whilst in 9 prisms are used.
Abandoning the use of prisms as costly Brewster patented21 a lightsplitting device, which though invented for portraiture, was applicable for cinematography. The same inventor also patented22 a special type of negative film. This consisted of two transparent supports 4, 6, Fig. 179, with the color-sensitive emulsions 5, 7 with a spacer 8 and a separator between the two films, specially sensitized for red and green respectively, and they might be on the outside as in 1 or facing as in 2. The two films and spacer might be attached in any desired manner as by lacing through the holes 19, 19. The separator might be integral with the spacer and should be sufficiently opaque to prevent the passage of any light. The function of the spacer was to maintain a separation between the films equal to the thickness of a positive film. After the exposure the two films were separated and treated as usual, then reassembled. The positive film, coated on both sides, was placed between the two negatives and printed. The two films might be inseparably connected along one edge as at 19b in 2 and 6, in which case they might be held open during development, etc. The type of camera for this film is shown in 8, and the passage of the light is sufficiently clear. The surface 23 of the prism 15, was preferably provided with alternating clear bands 25 and silvered ones 24, as shown in 9 and cemented with Canada balsam.
Modifications were also patented23 and this requires but little enough comment. In Fig. 180 is shown the film in section in the upper diagram, with a type of optical system in which the film lies on the plane of the optic axis, the two images being obtained by the split and reflected rays. It was suggested that the original film might be converted into a positive by a reversal method and further negatives made from this. Also that in consequence of the non-parallelism of the condenser rays in projection and the thickness of the base the two images would not coincide and there would be fringing on the screen. Hence the inventor preferred "to have the two images exactly superposed or congruent at the center, but one progressively larger than the other to compensate, as it were, for the angularity of the rays, non-axial or extra-axial, used in projection." Consequently he proposed to have a lens in the camera of substantially the same focal length as that of the projecting system. This would obviously mean