layers before the film is exposed, and they are removed or destroyed in part in the processing.
In order to be able to process the layers so that the dye images forming the final color picture are in their proper places, advantage is taken of special color sensitizing by itself or in relationship to the spectral transmission of dyes which might be in the layers. Some proposals have been made, particularly by Gaspar,48 the Eastman Kodak Company,37 and Kodak, Ltd., and Schinzel,78 to use an infrared-sensitive layer which can be exposed by light which passes through the other layers, whether they are dyed or not, without affecting them.
In making photographs on multilayer color film, it has been proposed that the sound track be printed in an infrared-sensitized layer, separate from those carrying the color images.78 In another case, it is proposed to include an interlayer of fogged emulsion which is developed to aid in confining the reversal exposures to their proper layers.77 If this interlayer is made sensitive to the infrared, a negative sound track toned in gold can be introduced into the film without interference from the interlayer. In color films carrying sound records of dyes, it is necessary to select dyes having appropriate absorption in the infrared to which the photocells of sound reproducers respond.67 Most dyes have high infrared transmission, and in the infrared region to which the commonly used caesium oxide cell responds, the sound modulation would be low and the noise level high. It is suggested in such cases that a photocell with response at shorter wavelengths 52 be used. It should clearly be preferable to use an infrared- absorbing dye or other image in the sound track and a caesium oxide cell.
The publications dealing with general infrared photography listed at the end of Chapter I should be consulted. Articles treating of the penetration of the atmosphere by infrared are listed at the end of Chapter XV. They are of particular interest in connection with photographic survey.
1. ABNEY W. DE W., "On the Photographic Method of Mapping the Least Refrangible End of the Solar Spectrum," Phot. J., 1881, 5, (new ser.), 95-104.
2. ADAMS W. S., and DUNHAM T., JR., "Absorption Bands in the Infra- Red Spectrum of Venus," Pub. Astron. Soc. Pacific, 1932-33, 44-45, 243-5.