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plate, shutters, air, slides, bottom and dark-slides

VENTILATION (Fr., Ven tilation du laboratoire ; Ger., Dunkel simmer-ventilation) Adequate ventilation of the developing chamber is essential to health. Perhaps the simplest means of securing it is by the pro vision of light-traps (which see) at the to and bottom of the door, or in any other suitable places. Unless, however, these communicate with the outer air, the ventilation obtained will scarcely suffice. It is as important to secure egress for the foul air as to admit fresh, a fact which is often overlooked, and outlets should invariably be placed at the top. It is not at all uncommon to find small dark-rooms merely provided with ventilation apertures near the floor, in which case it is next to impossible for the vitiated and heated air in the upper part of the room to escape. A light-trapped cowl in the roof forms a very efficient outlet, but proper attention then requires to be paid to the pro vision of openings for the admission of fresh air at or near the bottom of the room ; or the cowl will merely serve to direct a stream of fresh air downwards, and will cause an unpleasant draught. It is often found necessary to use an external electric fan, either to drive in the fresh air or to draw out the foul, but care must be taken not to raise dust. Whenever dust is found to enter by ventilation openings, muslin stretched over a light frame should be inter posed before the aperture.

OR BACK (Fr., Chrism's, Chassis negati f ; Ger., Kassette) A light-tight case to hold the sensitive plate or film, always furnished with a shutter or shutters, and made to fit closely at the back of the camera, from which it may be withdrawn at will. There are several kinds of dark-slides. That commonly used with studio cameras has a hinged door at the back for the insertion of the plate, and some of the larger and better class studio slides have roller shutters instead of those that draw out. Field camera dark slides are generally hinged in the middle, and open like a book, taking two plates with an opaque cardboard or metal separator between of the way. Many hand cameras have solid

double slides, known as the American pattern, with pull-out shutters of ebonite or aluminium. These are not always perfectly light-tight, especially when they get worn. An improved pattern is shown at B. The shutters do not pull right out, and particulars of the exposure may be written on them. To insert a plate, a lever at the bottom is pressed downward (see illus tration B), and the plate then drops into position, the lever returning and securing it. Metal and cardboard dark-slides are also made. There are many special kinds of slides or adapters, to take film-packs, plates in daylight-loading envelopes, etc. Roller slides (which see) are intended for use with roll-films, and are furnished with a winding key and spools.

In process work, the dark-slides are essentially different from those used in ordinary photography. The plate is generally held by means of adjust able bars, the bottom one being placed in a notch corresponding to the size of the plate, and the top one sliding down to rest on top of the plate (see illustration C). The metal catches to pre vent the plate from falling outwards are of silver when the slide is used for wet-plate work. Sometimes the bottom bar is formed into a trough, to catch the silver drainings, and in an American dark-slide known as the Benster plate-holder a glass trough was let into the wooden bar. For half-tone work the dark slides are provided with an additional pair of them. These A are known as " double book form " dark-slides. The shutters are usually cut across and hinged, so that they will fold over the back of the slide when drawn, and be out bars to hold the ruled screen, and in some forms of holder these bars are adjustable so as to give more or less separation of the screen from the plate. Most of these process dark-slides have roller shutters.

Used for changing and developing plates. (See " Developing Tent.")