This loss of time has been reduced by devices for the mechanical changing of the filters and of the plates. For instance, the filters may be held in contact with the plates in a repeating black sliding vertically and dropping after each exposure under its own weight or by springs, so as to bring the next plate into position. The actuating mechanism is controlled by the shutter or by the shutter release. In other patterns, the filters are placed on the edges of a disc and move automatically in front or behind the lens each time a plate is changed. The plates have also been placed on the three faces of a prism with equilateral section and turning on itself after each exposure. Numerous cameras built on these lines have given excellent results, without, however, affording the certainty attain able with an instrument taking the three pictures simultaneously.
876. Simultaneity will obviously be ensured if three linked cameras are used, with three identical lenses released at the same moment, but the images thus obtained (in conditions comparable with those of stereoscopic photo graphy) are usually not identical, and cannot therefore be superposed, unless the nearest planes of the subject are at a considerable distance. 1 Various devices have been adopted to decrease these stereoscopic differences between the three images. The lenses have been brought close together, their centres occupying the points of a triangle with three mirrors at 45 on each optical axis reflecting the sharp images on to three of the lateral walls of the camera ; a colli mating lens has been placed in front of the three lenses (G. Lippniann, 1886), substituting for comparatively close objects a distant virtual image ; reflectors have been placed in front of the lenses to substitute for the three real viewing points either one single virtual viewing point or three virtual viewing points aligned in depth and thus occasioning only negligible differences between the images.
The beam of light issuing from one lens can be split into three geometrically identical beams by means of transparent reflectors placed one behind the other at to the common axis (platinum-coated mirrors, colour filters acting as reflectors for the rays complementary to those transmitted, opaque perforated mirrors, opaque mirrors formed of two sectors opposed at the summit and rotating at high speed round an axis perpendicular to their plane and passing through their centres). Finally, the beam can be divided into three beams which are not geometrically identical, but of which the olifter CrICCS are practically negligible, by means of annular reflectors inclined at 45 to the optical axis (and which, owing to the obliquity are limited to elliptic contours), the images being received on the back and two lateral walls of the camera.
The ingenuity of inventors has been given free rein in this domain, and innumerable combina tions of mirrors and prisms have been patented, and sometimes even used, without always recording advances on their predecessors. In describe them, even in brief, would take us too far.
To reconcile a sufficient depth of field with the need for the ultra large aperture lenses required in order to reduce exposures to the minimmn, cameras for taking the three pictures simul taneously are generally built for very small formats, the negatives being subsequently en larged. It should be noted that the use of dividing systems involving an appreciable thick ness of glass (for instance, a cube of glass formed by the assembly of prisms of which the surfaces are partially silvered) would introduce inad missible aberrations in a lens which was not specially computed for such an addition. This drawback is avoided by using pellicular reflec tors, which are, however, very fragile.
877. Simultaneous Selection on Superposed Emulsions. Numerous attempts have been made to permit simultaneous tricolour sAection with out special apparatus on three superposed sensi tive emulsions, carried on independent supports (tolvfolimit chrom)dialytique of Ducos Hauron, 1897) with interposition of coloured filters (which can possibly be formed in the emulsion layers or on the supports by means of dyes, the colour of which is discharged during; normal treatment). After many failures, gener ally due to lack of sharpness of the images recorded on the second and third sensitive layers, encouraging results have been obtained by using for the first two films very thin, almost trans parent emulsions coated on very thin supports. Very satisfactory results have been achieved in two-colour cinematography, the selection being effected on two films unrolled with their sensi tive surfaces in contact (Multicolor and similar processes).
A far more daring idea of L. D. Mannes and L. Godowsky (1923) has given rise, after long and delicate elaboration, to the Kodachrome process (1935) in which the selection is done on three thin layers of emulsion successively coated on the same support with layers of dyed gela tine interposed. A series of very complicated operations, carried out in the factory for the user, produces by inversion of the triple nega tive a positive in colour formed by the super position of three pigment images (§ 883).