Warm-tone photographs (P.O.P. prints, red or sepia-toned bromides) are nearly always falsified by copying with non-colour-sensitive emulsions, the darker tones tending to merge into the deepest shadows. All difficulty will be avoided by using an orthochromatic emulsion with a deep yellow filter, or, preferably, a pan chromatic emulsion with a medium yellow filter. These same means are suitable for copying black tone prints on chamois or "antique" tinted paper.
739. Photography of Originals on an Opaque Base. The grain of the paper can generally be subdued by greatly-diffused illumination, slight departure from sharp focus, and somewhat ample exposure.
The texture of the paper is emphasized chiefly by the reflection of light on the convexity of each grain. Thus any means of suppressing these reflections results in the disappearance of nearly all traces of the structure of the paper, at the same time scratches and local abrasions due to rubbing are reduced or suppressed, and the contrast of the image is increased.
The most perfect means of eliminating all reflections consists in lighting the original by polarized light and by suitably orientating a polarizing screen fitted in front of the lens (§122a).
If a vertical camera is available, a good method consists in copying the original im mersed under a few millimetres of water in an ordinary photographic dish with a flat bottom (A. L. Donnadieu, 1883), care being of course taken to avoid all movements or currents of air liable to ripple the surface of the water.' Immersion also improves the reproduction of all photographs on matt or semi-matt papers; it is indeed well-known that such papers have a more extended scale of tones when wet than when dry. This method is especially necessary when the print is to be made on matt paper, as otherwise there will be a cumulative loss of details and of modelling.
In the absence of a vertical copying camera, it may be possible to immerse the print in a vertical glass tank with plane walls, but it is simpler to soak the print in water containing about To per cent of glycerine and to apply it by its face to a sheet ot flawless glass, just as is done when enamelling prints, or against the gela tine surface of a plate cleared of silver bromide, the pressure of the squeegee then causing the grain of the paper to penetrate the swollen The glycerine prevents drying and the resultant risk of sticking to the glass. After
the copy has been made, the print is well washed, dried, and mounted again if necessary.
The use of infra-red, which is reflected in very different proportions by metallic silver and by its amalgam, has permitted very contrasty copies to be made of Daguerreotypes, which gave only very poor results with an ordinary emulsion (B. Svenonius, 1934).
740. Lighting of Originals to be Copied by Transmitted Light. In the copying of trans parent on an enlarged or reduced scale the requisite uniform illumination is afforded by a diffusing surface such as an un creased sheet of white paper, the front surface of which is illuminated according to the rules already stated (j§ 295), a sheet of thin opal glass, or by a series of sheets of ground glass illumin ated from behind (§ 761), by means, for instance, of tubular electric lamps parallel with each other, or, if the surface to be illuminated is not great, by a single mercury tube of M-shape.
In all cases where the photographer is satisfied with daylight, in spite of its fluctuations, the entire apparatus (camera and easel) is directed towards the sky, or at least towards a window, outside which a mirror or diffusing screen of adequate size supplies the necessary illumination.
Unless a triple-body camera specially adapted for copying is used (§ 153), it is at least necessary to cut out the major part of the light reflected on the front surface of the original illuminated by transmitted light ; otherwise the copy will be badly fogged or covered with reflections. It is sufficient to have a tunnel of black fabric enclosing four sliding rods (or four cords) passing from the adapter frame (serving as the object carrier when copying a transparency) to the front of the camera ; complete light-tightness is not necessary.
741. Expert Photographic Examination of Documents. The expert photographic examina tions of documents may be classed in two groups— (a) The comparison of handwritings or of type written matter.