There are also gelatine-bromide papers with a removable sensitive coating which, after the print is finished (preferably after drying), can be transferred to any other support, such as paper with a metallized surface, wood, ivory, opal glass, etc. Treatment with tepid water dissolves an under-coating of soft gelatine tem porarily holding the emulsion to the paper. The instructions sent out with these papers must he followed.
498. Photographic papers can be obtained either in sheets of various sizes, which are sold in packets of 12 or in boxes of roc. (large-sized. sheets are supplied rolled in a cylindrical card board case), or on reels of various sizes in lengths of ro, 25, 50, or Ioc. yards (lo, 25, 50, or Ioo metres).
Cut sheets of sensitive paper sent out in packets or in boxes are sometimes protected by an interior wrapping of paper waterproofed with paraffin wax, contact of which with the sensi tive surface can cause spots owing to the greasy material deposited on the emulsion preventing the action of the baths. In the case of such packing it is necessary to turn over the sheet of which the sensitive side would touch the wax paper_ It is well to place sheets of paper required for immediate use in a perfectly light-tight cardboard box, fitted with a closely-fitting lid, so saving the nuisance of frequent re-wrapping. All details concerning the nature of the paper should be written on the box or lid, so as to avoid any risk of confusion.
In cases where it is necessary to use large sheets of paper for making small-sized prints, the best method of cutting up the paper with the least possible waste should be found by a few trial lay-outs. Unused paper should be handled only with clean and dry hands, avoiding contact with the sensitive surface as much as possible. When cutting-up paper, two sheets should be placed with the emulsion sides face to face, and the two cut at the same time.
499. Positive Plates. Although various at tempts have been made to introduce positive plates coated with a print-out emulsion or with an emulsion comparable with that of gaslight development papers, only two types of positive emulsions on glass are commonly met with in commerce. These consist of a slow gelatino bromide emulsion for obtaining pictures of black tone, and a chloro-bromide emulsion of very fine grain and very much slower than the preceding one, giving a very varied range of warm tones. With the latter plates, tones ranging from warm black to vermilion may he obtained by direct development, according to the methods adopted.
In the case of transparencies which are to be used for window decoration, the positive plates are coated with a matt emulsion, thus obviating binding up with ground The range of tones obtainable on a trans parency plate is very much greater than that obtainable on paper under the best conditions. In fact, in a transparency, it is possible to differ entiate between black and a grey which trans mits -th-th of the light transmitted by plain glass. With a paper viewed by reflected light it becomes impossible to differentiate between black and a grey reflecting of the light reflected by plain paper.
soo. Positive Films. Positive cinematograph films are coated with a black-tone emulsion which is faster than that of positive plates, so as to allow of very rapid production in machine printing. The contrasty negative films intended for reproduction work may be successfully used for positive printing. They can be used advantageously for lantern slides made for exchange between slide makers and for circu lating collections, the films being placed between glass for exhibition ; or, if sent between glasses, the latter can be replaced without damaging the slide itself in the case of breakage during transit.
501. Colour of Image and of Base. It has been shown that there is an almost unlimited choice of colours of the image and the tint of the base of photographic papers. Although a slightly-tinted base sometimes helps to suggest certain effects (§ 21), and variations in the colour and tone of the pictures themselves are useful in relieving the monotony of a collection, such means should only be used with discretion and in moderation. Engravings (lithographs, etch ings, etc.) and original drawings, for the execu tion or printing of which the artist has a large selection of crayons or inks of the most varied colours at his disposal, are often finished in a black or warm black tone on white or slightly tinted papers rather than in bright colours (excepting, of course, originals in colour). Incidentally, many cinematograph films would be all the better if they did not contain such a complete assortment of all available colours. Still more should startling effects be avoided, such as would be given by the use of green for a portrait, or deep-red for a seascape or land scape. Finally, the colour of the picture and the tint of the base should be in perfect harmony. Thus, a blue-black image would not suit a cream or chamois base, whereas a warm black picture would go well with a base of that colour.