183. A very ingenious device is used for identifying the negatives as they are being taken (autographic film and cameras, H. J. Gaisman, 1913). Instead of a single opaque paper wrapping for the film, two papers are used, one a " carbon " paper, as employed for inanifolding, and the other a thin red paper. Neither of these is completely opaque alone. The back of the camera is provided, a little beyond the edge of the picture, with a narrow slot, which is kept covered by a spring flap. Following an exposure, the flap is raised and the desired note is made with a metal stylus, attached to the camera. An ordinary lead pencil, not too finely pointed, may be used instead of the stylus, but never a copying pencil, as the colour of the latter may come off on the sensitive surface of the next turn of film. No mark appears on the surface of the red paper, but the coating on the underlying carbon paper is removed where the stylus has pressed on it. The slight play between the two papers enables the light, diffused by the red paper, to act on the film through the inscription thus stencilled on the carbon paper, the camera being pointed to the sky (but not to the sun) for this purpose with the flap open. The flap is then closed and secured.
184. Various improvements in the construc tion of film cameras have been suggested, and some of them have been applied, without, how ever, coming into general use. To ensure the flatness of the film, which is especially nec essary with lenses of large film cam eras are made in which the film is pressed against a sheet of glass by a plate, the pressure of which is relaxed during the time that the exposed ft m is being changed To permit focussing on a ground-glass screen a thing impossible with the usual cameras, the film-holding part of the camera has been made separate from the rest, or attached to it by a hinge, a shutter being fitted to the film-holder part to allow of detachment. To simplify the changing of the exposed section it has been suggested that the turning of the key should be replaced by a pull on a cord instantly wound back by a spring, or by a threaded rod pro truding from a tube like the mechanism of an Archimedean drill, and causing the rotation of the take-up spool. Various devices have been evolved for using a camera with spools of size smaller than the normal one (junction pieces for the pivots and metal masks restricting the length of the exposed film).
Reference must lastly be made to the common use of adapters for dark slides for plates, inter changeable with the ordinary back, or attachable to the film camera after the ordinary back has been removed. Some adapters bring the sensi tive surface of the plate into the plane normally occupied by the sensitive surface of the film, and thus permit of the usual focussing scales being employed. With most adapters, however, a correction of the focussing is necessary unless the camera is fitted with a second focussing scale, engraved in a different colour or clearly marked " Plates."
185. The necessity to use bare film in minia ture cameras fitted with large aperture lenses in order to avoid defective flatness due to the sliding of the film on the paper (§ 182, foot-note) has led, in order to facilitate the winding of the film and the working of a counter, to the adop tion of 35 mm. tine film with two marginal rows of perforations, leaving between them a usable space of 25 mm. width.
This film is usually supplied for 36 exposures 24 X 36 mm. (the length of 36 mm. is equal to six times the pitch of the perforations) in a special charger in lengths of 150 cm., of which about 6 in. are sacrified at the free end of the strip as the leader, suitably tapered for attachment in the camera in daylight. To avoid losing a similar length at the other end the exposed film is not wound on to a second spool, its rigidity being taken advantage of to push it by means of two toothed wheels, engaging with the perforations and actuating the film counter, into a receptacle from which, after all the exposures have been made and before opening the camera, it is returned to the charger by rotation of a milled knob taking with it the core of the charger. To make this return possible the interior end of the film strip must not be detached from the core, as would happen if after the 36th exposure an attempt were made to overcome the resistance then felt to unwinding. If this accident should occur the film cannot be returned to the charger until the camera has been opened in a dark room.
186. Film-packs. The cut-film packing form ing a changing box for the films it contains still retains the name of " film pack " under which it was first issued in 1903 by the Rochester Optical Co.
Each of the 12 films P (Fig. 134) is attached (on one of its short sides) by an adhesive strip A to a long band B of enamelled opaque paper ending in a tabL. The 12 films (only two are shown in Fig. 135) are placed in a pile in a cardboard box (Fig. 135) with a fixed interior partition, which is extended as a kind of metal gutter, and with arectangular cut-out with in which the film is exposed. The tabs of the films are led round the gutter and project from the cas ing. They are covered with a band (of the same paper) which forms the safety cover of the pack. Between the fixed partition and the pack of films there is a plate of thin metal. It is held away from the partition by springs and presses the safety cover against the cut-out and the films against the safety cover. Strips of felt press the tabs together at the point where they emerge from the casing, and another strip of felt presses the curved portion of the paper bands against the convex surface of the gutter.