The first flexible film was coated on to a plate glass table, about 5o ft. long, by a moving hopper. The collodion was dried and the emulsion was then coated and dried before the complete film was stripped from the plate glass. Film base, as it is called, is now made by a continuous process. The machine consists of a slowly-revolving large cylindrical drum with a highly-polished nickel surface. A layer of collodion overflows from a trough on to the drum, and is dried by a current of warm air. Before the revolution of the drum is completed the film is stripped off and carried by an endless band to the spooling machine. In some plants the cylindrical drum is replaced by an endless band of polished aluminium or nickel.
The most serious objection to is its inflammability, particularly as it is very difficult, if not impossible, to control a celluloid fire, in which combustion can continue in absence of air. Attempts have long been made to replace it by a less inflammable substitute. The best results have been obtained with cellulose acetate ; unfortunately, no compound will play the part of plasticiser for this compound as camphor does for nitro-cellulose. Acetate film base, in proportion as it loses its solvents by evaporation, loses some of its original mechanical properties, particularly its flexibility.
Whatever be the base employed, it must, before coating with emulsion, be coated on the side which is to receive the emulsion with a substratum' designed to secure adherence of the gelatine to the support during the various manipulations_ This generally consists of gelatine dissolved in a solvent of the base, e.g. acetic acid.
For coating the emulsion, the base passes round a large cylindrical drum, cooled internally to allow the gelatine to set ; a kind of tank formed by silver rollers, rolling by pressure against the film, is kept full of emulsion, which adheres to the film in a quantity determined by the temperature of the emulsion and the speed of the machine. The film, covered with wet emulsion, is hung in festoons from rods, distributed in a continuous manner on an over head track, and thus travels a sufficiently long distance in the drying room, which is traversed by a current of warm air. The dry film is wound on to reels containing about 400 ft., and, after examination, is passed to the automatic cutting up machines.
230. Papers. The properties required of papers which are to be used as supports for emulsions vary according to the use to which the (paper) negative will be put. Paper nega tives may be used for printing by transmitted light, for direct reading, as in the case of self recording instruments, or for reproduction only by reflected light, e.g. in the primitive cameras
used by itinerant photographers and in some semi-automatic " while-you-wait " portrait cab inets introduced in In the first case the paper should be homo geneous in structure, to avoid, as far as possible, the structure appearing on the prints made from it ; to avoid too long a time being taken in the printing of each print, a thin unloaded paper is chosen, having sufficient mechanical strength, particularly in the large sizes, not to tear under the stress of the weight of the water absorbed by the paper and by the gelatine. As the emulsion is coated directly on to the substance of the paper without any intermediate layer, the material of the paper should be quite free from all impurities capable of affecting the emulsion or the image in the course of development. Metallic dust should be rigidly excluded.
In the second case, the emulsion is separated from the paper " raw base," as it is called, by a semi opaque coating made by spreading, with brushes, a mixture of pure barium sulphate suspended in a small quantity of gelatine solution. ; after drying, the paper is By virtue of this greater latitude may be exercised in the choice of paper, particularly as the perfect homogeneity demanded of a paper used as a transparency is not of import ante ; in these circumstances either thin or thick (card substance) papers are used.3 In either case resin sizing must be protected against the action of alkaline baths, used for many operations, by adding a colloid such as gelatine or casein to the substance of the paper.
To coat the emulsion the paper passes under an ebonite cylinder, adjusted either to a trough containing emulsion kept at a constant level, or to a second cylinder half immersed in the emulsion. It then passes round part of a cooled drum and finally is hung in festoons for drying, in the same manner as that already described for films.
231. Halation. If an attempt is made to register an isolated luminous point on a photo graphic plate, or, if a plate is exposed to light under an opaque screen pierced by a small hole, the image of the luminous point will be found to be surrounded by an aureole, which is limited internally by a sharply defined circle, while externally it fades gradual ly into the background. This aureole is termed a halo, and effects of this kind are comprised in the term halation. In Fig. a represents a luminous point registered without halo, and b the halo obtained on an ordinary plate having a thickness of inch (each part of the illustration is a facsimile of the original negative).