In some boxes the drawer carries with it only one plate, putting it back under the pile. In others, the exposure must be made when the drawer is out, the sharp image being formed on the plate held at the bottom of the box. The bulk of the camera is thereby reduced by the thickness of the changing box, but the camera is not so well balanced at the instant of making the exposure. In order to reduce cost and bulk, some changing boxes are formed of two distinct parts, the loader and the cartridge. The car tridge is the drawer of the box, and it is possible to use several such cartridges, 1 each loaded with a different kind of plate if desired.
More than any other part of the camera, the changing boxes must be kept perfectly clean. On the one hand, the fall of the plates to the bottom of the box often produces fine splinters of glass, while the rubbing of the sheaths (when of lacquered metal) gives rise to dust consisting of varnish. All these particles are set in motion each time the drawer is worked, its action being something similar to that of a suction and ram ming pump.
181. A changing box must have some device for showing the number of plates exposed or the number of plates still to be exposed. In the early patterns of changing boxes each sheath was stamped with a depression that was convex towards the plate and acted as a spring. On the concave surface of this depression a numbered label was pasted. This number was visible from outside through a red window. As these numbers occasionally acted on the sensitive emulsion of the plates, this arrangement has been generally abandoned in changing boxes, and has been replaced by a counter, advancing one number each time a plate is When a very large number of plates have to be exposed before they can be developed, the identification of the negatives is much simplified if a number is automatically marked on each negative. In place of numbers pierced in the edges of each sheath, the same result can be obtained by marking one edge with a certain number of notches or perforations, another edge being marked to show the particular changing box or cartridge.
It often happens that after exposing all the plates in a box, neglect to read the counter may result in some of the plates being exposed a second time. Such double exposures are entirely avoided in boxes in which the last sheath is too thick to pass from the drawer.
It is usually possible to secure this same advan tage by riveting to the underside of a sheath a plate of aluminium about in. thick, and of exactly the same width and length as the sheath.
Some changing boxes are fitted with an auto matic lock, preventing the removal of the box from the camera (or the cartridge from the loader) until the shutter has been closed.
182. Roll-film Cameras. It is to George East man (1889) that the invention of the roll-film holder and of the appropriate packing of the film is due. The forms originated by him differ little from those in use at the present day.
Two spools,' one empty (B) and the other (B') with film wound on it (Fig. 133), are placed on either side of the camera proper, their end disc pieces being fitted to pivots, one of which can be turned from the outside, whilst both pairs act as axes of rotation. After taking off the detachable cover CC, the spools are placed in position on their pivots, noting that the two spools must be placed inversely to each other, the groove in one of the discs of the spool placed in B having to fit the pin of a winding-key operated from outside the camera. Four to six inches of the black or red paper wound on the spool are unwound and the taper end in serted in the slot of the empty spool at B. A few turns of the key are given to wind the paper tightly round spool B, and then the back CC is replaced. Some fifteen half-turns of the key are then given, the paper passing from B' to B, and moving without friction over the rollers As soon as the film, carried by the paper, reaches the field of the lens, a warning sign appears in the red window V. and is soon followed by the figure 1, showing that the film is in position for the first exposure. After making the exposure the key is turned until the figure 2 appears, and so After taking the last photograph (the spools are usually for 6, 8, io, or 12 exposures, according to the size of the pictures), the key is given about fifteen half-turns, and the cover is then taken off.
The winding of the paper is then continued, pressing the fingers very gently on the spool to prevent the paper from uncoiling when it is no longer kept under tension by the springs which brake the supply spool. The spool is then removed and sealed with the gummed strip attached to the end of the black or red paper.