Daylight Changing.—During the last twenty-five years we have tried many ingenious mechanical contrivances which professed to replace the exposed plates in the dark slide with new ones ready for fresh exposures. Some of them, after much coaxing and shaking before they would work at all, ended by replacing the original plate in the slide to receive a fresh and quite superfluous image upon it. The others fogged one or both the plates in transit. The modern day light envelope system, however, seems to accomplish the reloading process with a fair degree of safety and certainty, and is not at all difficult to master. Large numbers of these envelopes may be carried in the pocket. Users of films may avail themselves of the improved film pack or rolls of cartridge film. But, as these cannot usually be developed until the entire package is exhausted, they are more properly considered in the section on Hand Cameras.
The Tripod.—The height of the eye averages about 5 ft. 6 in., and the stand at full length ought therefore to be capable of raising the camera base-board to five feet from the ground with the legs spread out. The form which is most portable is not always the most rigid, and the fact that pictures frequently have to be taken while a strong gale is blowing must be borne in mind. An elastic belt drawn round the legs will help to prevent vibration under such circumstances. Wooden stands are, on the whole, most re liable and the sliding pattern is most easily adjusted to various heights. It is important that the screw adjustments should not be removable, or they will drop out and be lost. The new metal tubular tripods of brass and aluminium are very convenient—closing to about fifteen inches to fit a small leather case—and in practice some of these will prove sufficiently rigid for small cameras. The metal heads should be covered with a layer of felt or thin leather.
Spirit Levels.—Even when the view comprises merely open landscape or groups it is most desirable that the focussing screen and dark slide should be accurately vertical. For ordinary purposes a watch hung by its chain beside the camera will serve the purposes of a plummet in gauging the upright position, but a proper level is part of the equip ment of every serious worker. The small round levels are good enough for hand cameras ; for the field camera we prefer the mason's level, which has both horizontal and vertical bubbles.
Instantaneous Shutters.—Fast plates and large diaphragms are the order of the day, and the range of work will be circumscribed indeed if exposures have always to be given by means of the lens cap. Such exposures must be hap
hazard if they are timed with a watch, and are of less period than, say, two or three seconds ; whereas the average exposure required for landscape in good light is a small fraction of a second. The forms of shutter available may be divided into four classes according to their position, and the manner in which they work. (I) The focal plane close to the plate. (2) In front of the lens. (3) Behind the lens. (4) The dia phragm shutter between the components of the doublet, Focal-plane Shutters.—The focal plane shutter is attached to the camera immediately in front of the focussing screen and dark slide. Some ingenuity is required in fixing it on field cameras as it must form part of the swing back. Theoretically it is the very best kind of shutter for photo graphing very rapidly moving objects. Its construction has been brought to the very highest perfection, and for very short exposures it works with much less loss of light than others. In principle it consists of a roller blind with an adjustable slit which is caused to travel with great speed across the surface of the plate. For exposures varying between as and sec. no shutter can compare for efficiency with the focal plane. In practice the operator will more frequently give A' to as full exposure of the plate is the first consideration. At such moderate speeds (and still more, of course, with sec. or r sec.) focal plane shutters are not recommended, owing to their tendency to jerk and cause vibration in the camera.
Shutter behind the Lens.—The Thornton Pickard type of roller-blind shutter is still often supplied with field cameras fixed on the sliding front, and the lens flange screwed on to it. Though sometimes regarded as old-fashioned this is a very reliable piece of apparatus, very durable, and in practice we have found the speeds from to sec. fairly accurate. It is not, however, noiseless ; and no provision is made for lower speeds such as sec.
Shutter in front of Lens.—The shutter just mentioned is often employed as a movable cap to be placed on the lens just before exposure. The blind will not always exclude bright light for any length of time, and care should be taken not to open the slide until the last moment. There are also one or two good pneumatic shutters made to rise and fall in a manner giving fuller exposure to the foreground than to the sky—often a great desideratum in landscape work : but the range of speed is necessarily limited.