Such backing sheets may be used again and again for an indefinite number of times, if kept in contact with parchment paper when not in use. If after repeated usage the gelatine becomes less sticky, a little glycerine is applied and the excess blotted off. Of late years photographers have preferred to buy plates ready backed.
239. Photographic Plates. The sheets of glass used in the manufacture of photographic plates' are divided, according to thickness, into three classes, viz— Extra thin glass - 417 AI in.
Thin glass . . to in.
Ordinary glass . . Above in.
As the lower limit for the internal dimensions of negative holders and plate carriers is the same as the nominal dimensions of the glass (with a tolerance of i per cent at the most) the nominal dimensions may be considered as the maximum dimensions of the sensitive plates. The cutting tolerances, at the most equal to i per cent, can be allowed only within the nominal dimensions. 2 In order that the plates may be effectively held in the plate holder, the dimensions of the image (measured between grooves or angle blocks) must be 5 per cent less than nominal.
Photographic plates are usually sold in boxes of a dozen. Sizes from whole plate (8 x 6-1 in.) upwards are supplied in boxes of six. Finally, very small or thin plates are supplied, when required, in packets of eighteen, which is the normal capacity of certain types of changing box. In the interior of the boxes the plates are packed face to face 2 in groups of 2, 4, or 6, wrapped in red or black paper, and the various packets are themselves wrapped in black paper.
240. In addition to trade particulars (trade mark, type of plate, etc.), the label on the box always has on it the emulsion batch number, so that the maker, in case of (§ 430), can refer to the works register to find full particulars of the emulsion in question, together with the results of the tests made of the batch. The batch number is, moreover, of much greater value to the user than is generally believed. Two suc cessive batches of the same formula cannot be absolutely identical in spite of all the care which is exercised, and a change in number indicates a variation in one or more of the characteristic properties of the material (slight variation in sensitivity and, for panchromatic emulsions, variations in the relation of sensitivity to the different colours ; variation in speed of develop ment, which may be considerable, and is the chief variant from a practical point of view).
Photographic plates maintain all their qualities for a period of time much greater than a year, and it is advisable, when the quantity required can be estimated, to take a sufficiently large stock of plates with the same emulsion number to last for from three to six months. In this way one can be sure that all the plates used will behave similarly.
241. Photographic Film. Films are supplied in long strips for use in hand cameras for amateurs (§ 183), in automatic cameras for aerial photography (in lengths ranging from loo exposures of 7 X 91 in., up to 500 exposures of 7 X 5 in.), and for cinematography (Chapter L). Films are also supplied cut to various sizes, either in the form of film packs or in boxes of 12 or 24, to be used in place of plates for ordinary photography or for radiography. Roll films and the films for film packs which are coated on celluloid , in. in thickness, and cut films which are coated on celluloid Th in. in thickness, are all coated on the non-emulsified side with a layer of plain gelatine (about o.25 grm. per square decimetre), to counterbalance the curling tendency of the emulsion without this provi sion the films would tend to curl up during handling, 2 owing to the swelling of the gelatine in the solutions.
Films, and particularly those coated on very thin supports, allow prints to be taken either direct or reversed as regards right and left. This is of great service in the case of pigment processes (carbon, transfer in greasy ink, etc.), which usually give reversed prints.
242. Strips of film of great length, for aerial photography or cinematography, which are developed either by continuous processing machinery or after having been wound on to a frame or drum, cannot be coated on the back with gelatine owing to the risk of adhesion or staining. The film, which is a very bad conduc tor of electricity, is liable to become electrically charged if rubbed against another bad conductor, and thus to cause discharges (visible in darkness) which are registered on the emulsion as dark marks of foliage pattern.' These static markings are produced only when the atmosphere is dry and particularly in cold weather, or when the camera is suddenly chilled. Many means have been suggested for increasing the conductivity of the non-emulsion surface of the film (anti static films).