Attempts are often made to prevent these pinholes from forming by dusting the face of the plate before its insertion into the dark slide or sheath. These precautions, however, often make matters worse, especially if a hair brush or a velvet pad, which has been left lying about the room and is consequently full of dust, is used for the dusting.
The most effective method for preventing pinholes is to clean the dark-room, the plate holders, and the changing-box very frequently, preferably with a vacuum cleaner, and also to follow closely the instructions given above for the loading of the plates. Where it is not possible to get rid of dust in this way, then a brush should be used, but it should be kept in a smooth-walled box when not in use and should be frequently cleaned by shaking and washing in alcohol.' In any case, replace the slide of the holder very gently, holding the dark slide with the plate underneath, so that any dust on it is able to fall away.
285. Unloading the Dark Slides. If it is not possible to proceed with development as the plates are removed from the dark slides they must be replaced in their original Plates and films should be packed face to face, and the packets should be firmly pressed down so as to prevent the emulsion faces from rubbing one against the other. Remember that the use of all common papers, with or without writing or printing, is liable to cause various defects (§ 200). The boxes should be sealed with a label stating explicitly that it contains exposed plates. 2 When unloading dark-slides or the sheaths of a changing-box, avoid any contact with the emulsified surface and any rubbing of the glass edges against the rear partition as this might scratch the black varnish 286. Identification of Negatives. In all cases where a large number of negatives are dealt with, or where it is required to determine the effect of certain alterations in treatment on a number of negatives of the same subject, it is necessary to adopt a scheme of identification which will permit of no mistake.
The simplest method is to write a number with a soft pencil 4 (No. i or B) in one of the corners of the plate, taking care not to encroach on the picture part of the plate. If the inscrip
tion can be made during loading (where there are only a few negatives to identify) the pencil mark protects the emulsion from the action of light, so that after development the number appears light on a dark ground, even after the original pencil mark has been removed. If the inscription cannot be made until after the exposure, the number corresponding to the customer's order (in portrait studios and busi nesses catering for amateurs) or similar in criptions should be made with a semi-hard pencil (No. 2 or F), so as to scratch the gelatine.
Various arrangements have been devised which will give each negative a permanent or provisional number more or less automatically, viz., (x) in a portrait studio, a number plate or a slate, with the number corresponding to the customer's order written upon it in chalk, can be placed in such a position that the image will appear in one of the corners of the negative ; (2) one of the fixed holding pieces in each dark. slide may be replaced by a metal plate perfor ated with the required figures ; (3) the rebate of the sheath of a changing box may be notched or perforated on one of its longer sides, and, if necessary, a further mark, denoting the particu lar changing box, may be perforated in the shorter side.
In the developing and printing firms of any size, 1 the small length of film which extends beyond the last exposure is often used to record photographically the number of the order. The numbered receipt, made of very thin paper, is inserted, together with the end of the film, into a specially constructed printer, in which they are pressed tightly together by a shutter which automatically turns on the light used for the exposure as it is brought into position. The number may also be impressed on the film in greasy ink by means of a numbering machine. Finally, the method usually employed in cine matography could be employed, the films and order form being perforated! with the required number by means of a machine similar to that used in banks for indicating the amount of a cheque.