(1) Since the discovery and rapid development of dry-plate photography, many attempts have been made to dispense with the use of glass as a support for the film, and so far with considerable success. One of the most promising inventions of the kind is that of Thiebaut, a photographer, of Paris. It consists in the preparation of a ge'atino-bromide of silver film paper, from which the film can be detached in a dry state after exposure and development. The advantages arc that the sensitive coating is regular, and its thickness is uniform throughout the entire surface; it can be exposed for a luminous impression in any kind of slide, and it can be developed and fixed as easily as a negative on glass. Further, the negative dries quite flat on blotting-paper, and the film being without grain, the negatives are as fine and as transparent as those upon glass. The negatives can be printed from either face, which is of advantage in connection with phototypy and photo-engraving. The manufacture is carried out in the following manner: -
A gelatinised sheet of paper is damped with cold water, and when evenly saturated it is placed on a glass, to which it is attached by means of bands of paper pasted partially on the glass and partially on the edges of the sheet; in this state it is allowed to dry, and is stretched quite flat. The dry sheet is then coated with a solution of ordinary collodion, containing 1-2 per cent. of guncotton (l 1/2 per cent. gives very good results), and 1 1/2-2 1/2 per cent. of castor oil (2 per cent. gives very good results); this coating is allowed to dry.
The glass with the prepared paper upwards is levelled, and then it is coated in a room from which all but red rays of light are excluded, with a tepid emulsion of silver bromide to the extent of about 1 millimeter thick, and after leaving it in this position until the gelatine has set, say about 5 minutes, with the film paper still attached, it is placed upright in a drying room, where it should remain about 12 hours exposed to a temperature of 62°-66° F. (17°-20°C.).
The film paper is detached from the glass ready for exposure, development, and fixing in the usual manner; for the purpose of developing, iron oxalate or pyrogallic acid answers equally well. For the purpose of fixing, a mixture by weight of 100 water, 15 soda hyposulphite, and 6 powdered alum, produces excellent results. After being allowed to dry, the film is peeled off the paper by hand, and can be immediately used for producing negatives.
(2)Warnerke gives the details of a discovery he has made respecting the action of pyrogallic acid on gelatino-bromide. This discovery consists in the fact that a gelatine plate submitted to pyrogallic acid becomes insoluble in those parts acted upon by light, exactly in the same way as gelatine acted upon by chrome salts, the insolubility being in proportion to the amount of light and the thickness of the gelatine. This property Warnerke proposes to utilise in various ways. The drawback in the ordinary gelatine process being that, unless the exposure is very accurately timed, there is considerable danger of over-exposure, and intensification being very difficult, pictures by the gelatine process are often inferior to those by collodion. By the new process he is, however, able not only to intensify, but also to overcome the d awbacks arising from over-exposure. The latter he effects by using the emulsion on paper. He has found that no matter how much the paper is over-exposed, the picture, provided the developer is restrained sufficiently, is not injured, while in the case of the emulsion on glass, there is not only halation of the image, but a reversal also. The transfer of the image from paper on to the glass is very easy.
The paper is immersed in water, and placed in contact with a glass plate. The superfluous moisture being removed by a squeegee, the paper may then be stripped off, leaving the gelatine on the glass. Hot water is then applied, which dissolves all the gelatine not acted on by light, and the image is left upon the glass in relief. Intensification is effected by mixing with the emulsion a non-actinic colouring matter, and which is not affected by silver. Aniline colours answer the purpose, and in this way special emulsion for special purposes can be prepared. This method of preparation would be especially suitable for magic - lantern slides. Warnerke claims that by his discovery relief can be obtained far more easily than by the ordinary bichromatised gelatine, and therefore it is especially suitable for the Woodbury type process. By mixing emery powder with the emulsion, it is rendered fit for engraving purposes, and by a combination with vitrified colours the image can be burnt in, and being so adapted for enamels. By using a suitable emulsion, however, so little gelatine can be employed as to obviate all difficulty in carbonising. The process can also be adapted for collotype printing.
The sensitive paper can be used in the camera in lengths, wound on rollers.