The methods of loading the various slides and changing boxes differ to such an extent on the cameras of various makers that a detailed description cannot be given. The instructions sent out with each camera should be followed.
179. Single Metal Slides. Although double dark slides with curtain shutters are used (in France) with some hand cameras (§ 156), it is far more usual to employ single metal slides, made of stamped sheet steel or nickel, with a metal pull-out shutter, such slides being cheaper, lighter, and more compact.' Among the advantages of metal slides over changing boxes are the avoidance of any risk of jamming and the possibility of taking a selection. of plates of various kinds or for various pur poses." Also, as the camera is loaded with only the one plate, it is less heavy and cumbersome when arranging and taking the photograph.
On the other hand, the changing of one plate for another is less rapid with slides than with a changing box for plates or films, and finally fog may be produced as the result of wear of the velvet plush fitted to both the slide and the camera in order to ensure light-tightness after the slide-shutter has been withdrawn, especially if the shutter is replaced not squarely but by inserting one corner first.' It is usually possible by means of carriers to use in these slides plates of size smaller than the Cut films and paper can also be used by placing them in holders, of which various patterns exist (book-form, sheath, or stretcher) or, in some cases, by the use of a metal sheet covered with a suitable adhesive.
This review of holders for plates would not be complete without a reference to the special slides or adapters intended for loading in day light by the use of envelopes of stout opaque paper each containing one plate. The envelope is opened when the shutter of the adapter is drawn and closed when it is pushed back. In England this envelope system is widely used by Press photographers.
Changing Boxes for Plates. In all chang ing boxes for plates the plates must first be placed each in a sheath or metal frame. The plates are usually slid into grooves formed by the turned-over edges of three of the sides.
The one pattern of the old type of changing box without mechanism which has survived is that known as the bag changer. In its simplest
form it is closed by a shutter and attachable tc the back of the camera. It is loaded with sheaths, each fitted with a small tab or handle To the upper end of the box is fitted a bag of soft leather or opaque fabric, flexible enough to allow of the tab of the sheath containing the plate that has just been exposed being grasped from outside. The sheath is then lifted up intc the bag and transferred to the back of the pile against the slight resistance offered by the springs, which keep the pile of sheaths con stantly pressed against the rebate at the front of the box. In current patterns the box is provided with a lever which raises each sheath in turn up into the bag.
Nearly all the changing boxes now in use are based, with many variations, on that invented about 1890 by E. Hanau. Its working will be seen from Fig. 132. The box consists of a chamber of wood or metal in which there slides a drawer within which are 6, 12, or 18 (the number varies according to the size).
When the first plate in the pile (position A in the figure) has been exposed, the drawer is pulled out (B). The first plate (top, exposed plate) is held back by stops until the drawer has been pulled right out, when it falls to the bottom or is pressed there by springs When the drawer is returned (D) this plate passes under the pile and a fresh plate comes into position for It will thus be seen that the working of the box depends on the drawer being pulled right out. It is usually necessary to pull it out witt a smart movement, to assist the fall of the plate but, of course, not so violently as to drag out the drawer. If the drawer is pushed back before it has been completely opened, the half-extrudee sheath is likely to be bent and the plate broken thus causing the box to jam. Jamming ma3 also result when the sheaths are not put in according to the instructions (in some boxes the open sides of the sheaths must be towards the handle of the drawer, while in others they must be away from it), or when the sheaths have been bent. If excessive resistance is felt when opening or closing the drawer, the movement should be reversed somewhat sharply, and a second at tempt then made to change the plate. Force should never be used.