TEMPORARY ADDITIVE SYNTHESIS 878. Triple Projection. We have already dis cussed (§ 867) the principle of this method of synthesis by addition of lights. In working the process one is led to employ for synthesis filters which are somewhat different from those used for analysis. Our eyes are sufficiently imperfect not to distinguish between two identical hues, but differing qualitatively and quantitatively in composition, so that it is not necessary to have filters of a rigorously determined degree of absorption. The only conditions requiring to be fulfilled are that the total of the three trans mitted beams, from equal intensities of incident beams, should give pure white, and that, under the same conditions, their sums, two by two, should form a pure yellow, a greenish-blue, and an intense pink.
The employment of three separate lanterns would involve great difficulties in adjustment. Use has been made of three lanterns superposed or juxtaposed and illuminated either by three separate light-sources or by one single light source. Two of the three lenses must be fitted with horizontal and vertical sliding movements, so as to allow the three images to be superposed on the screen. The three transparencies are usually assembled once and for all in a frame fitted with register screws, so as to reduce the operations of adjustment when projecting. Such arrangements have been made at various times for still projection (F. E. Ives, 1889; L. Vidal, 1892 ; W. de Abney, 1904, etc.).
This same method of synthesis has been employed in tricolour cinematograph projection, the elementary images being projected either in rapid succession (H. Isensee, 1897, following a suggestion by C. Cros in 1869), or, simul taneously (B. jumcaux and W. N. L. Davidson, 1903 ; L Gaumont, 1909, etc.).
879. Chromoscopes. Instruments with semi transparent or coloured mirrors, similar to those described for simultaneous selection by one lens (§ 877) can be used for viewing simultaneously in register the three transparencies, each in contact with an appropriate colour filter. One of the simplest of the many chromoscopes sug gested or made is the one described by L. Ducos du Hauron (Fig. 217). Three mirrors, of which two are semi-transparent, are fixed at 45° to the line of sight ; three transparencies on a single plate are in the focal plane common to the three convergent systems, and have their images thrown back to infinity in coincidence. As the beams are parallel on reflection there is no fear of double outlines by reflection from the two surfaces of each mirror.
In connection with chromoscope synthesis, reference may be made to a method of synthesis by intermingled images, patented in 1899 by J. Szczepanik, and of which several variants have since been tried. The diaphragm of a photographic lens is divided into three parallel bands covered respectively with the three tri colour selection filters. By placing in front of the photographic plate, parallel to its plane and at a suitably adjusted distance, a grating of opaque equal and equidistant lines, arranged parallel to the bands in the diaphragm, the sharp image is divided into continuous bands, each formed entirely by one of the coloured lights transmitted by the filters, the negative thus obtained forming a medley of elements taken from the three selection negatives. By
placing a transparency printed from this corn posite negative in the same position and illumin ating it by transparence by a light-source and condenser, an image is projected on the screen which, when viewed at some distance, reproduces in its colours the object photographed.' 880. Lenticular Colour Photography. A film coated with a reversible panchromatic emulsion that would have been obtained through a line three-colour screen.
After development and inversion of the image, the latter can be projected in colour on a screen, by means of an optical system composed of a lens and of a filter with three coloured strips, such that this projection filter and the camera filter are seen from each point of the E (Fig. 218) is embossed under heat on the free surface of the support S, by passing under pres sure between a smooth cylinder and a cylinder engraved with continuous grooves, at the rate of about 20 per millimetre. By this means there are formed corrugations each constituting a cylindrical lens, of which the focal length must be about equal to the thickness of the base. In front of the lens 0 is mounted a filter F with three coloured segments B, 6, 1?, respec tively blue, green, and red (shown separately, seen from the front, at a) of which the strips are parallel to the cylindrical lenses. The camera lens 0 is focussed on the emulsion sur face of the film, and must have a relative aper ture equal to that of the diopters formed by the embossing. Each of the corrugations thus gives on the sensitive emulsion an image of the tri colour filter (the combination of these images is shown diagrammatically at c) of a length equal to that of the corrugations, and in which the illumination of each line is proportional to the intensity of the considered primary colour diffused by the corresponding point of the sub ject. In this manner there are recorded three intermingled selection images identical with those film within the same angle. All the rays issuing from a band B of the image are directed by the corresponding corrugation to the band B of the filter. In the same way, the light that has passed a G or R band of the image is directed respec tively to the (x or R band of the filter. The image projected on the screen reconstructs the form, luminosities and colours of the subject by narrow continuous bands whose structure is not apparent unless the screen is viewed from very close quarters.
Difficulties have occurred in copying such films, but they now appear to have been over come, which will enable the process to be used on an industrial scale hitherto impossible, the pro cess having been used only in amateur cinemato graphy and in photography in miniature sizes.