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The positive reproduction, by contact with another sensitive ribbon of these three negative images, gives an accurately superposable black and white reproduction of the images of the elementary colours of the polychromatic object.
An exact polychromatic effect of a scene may be obtained if, prior to placing these black images one above the other, they are coloured respectively blue, yellow and red with well selected dyes, or else lit up by transparency, fasces of luminous rays which have passed through the positive by means of screens coloured with the same colours as those used for dyeing being directed on to them.
This principle has been applied to the several processes which we are about to describe. We will point out the drawbacks attaching to each one of them and their practical possibilities.
i . The Process of Successive Images.
This process, which is based on the phenomenon of the persistence of images on the retina, makes use of a panchromatic film upon which each polychrome image is represented by three successive elementary images obtained by means of three selective screens, one green, one orange and one violet.
These three images must be obtained in a space of time practically equivalent to that normally devoted to obtaining a single image in black and white. Thus the film must be turned at about three times the normal speed, or at the rate of 50 images per second. Upon development, a negative three times the standard length is obtained. This is printed as a positive in the usual way, by contact, with a positive emulsion ribbon. By colouring each of these elementary images in blue, yellow and red, with suitably selected dyes, a succession of images of respectively the same colours is obtained on the positive ribbon.
By turning this ribbon at three times the normal speed and projecting it on the screen, the persistence of the retinal impression will allow the three elementary images to be superposed in our eyes, producing the illusion of the reconstituted polychrom
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