For very rapid work the choice is limited to one kind of shutter—that is, the focal-plane, which in effect is a curtain with an adjustable slit which is placed directly in front of the plate. Great speed with this shutter is in part secured by increasing the tension of the spring, which acts as its motive power, but more particularly by decreasing the width of the slit. Assuming, therefore, that it takes one second for the slit to pass from top to bottom of a plate four inches high, and that the slit is one inch in width, it follows that each portion of the plate is exposed to the light for a quarter of a second. Decreasing the width of the slit one half, proportionally reduces the time of the exposure, and by this means, in connection with an increase in the speed with which the curtain is moved, an exposure of ¹⁄₁₀₀₀ of a second is possible. In addition to possessing the advantage of great speed, this shutter also passes a higher percentage of light than a lens shutter even when the actual time of the so-called exposure is the same. This is due to the fact that the lens opening is in no way affected, it being the same throughout the exposure. With a lens shutter, on the contrary, the full value of the opening is given for only a fractional part of the exposure, the parts of the shutter more or less filling the opening during the rest of the time. With a focal-plane shutter, therefore, one may do rapid work under conditions where a lens shutter could not be successfully employed; time exposures, however, can not be made with the focal-plane shutter, and for all-around work the camera should be fitted with both a lens and a focal-plane shutter. The reflecting camera, as before stated, is fitted with a focal-plane shutter, and, as described, it is released by pressing the lever, which raises the mirror. Lens shutters, however, are released by a pneumatic bulb, or in some cases by a thread or string. When the exposure is to be made from a distance as much as one hundred feet of tubing may be employed. With any length of over twenty-five feet an extra large bulb is required. The ordinary tubing sold by photographers will not be found so well adapted to long-distance work as a less elastic kind, which does not so readily yield to pressure and transmits a larger portion of the force applied when squeezing the bulb. _The Tripod._—A stout two-length tripod is to be preferred to one of the slender multifolding type, in which stability is sacrificed to weight and size. The legs, except the inner sides of the upper section into which the lower section slides, and brass work should be painted bark color in order to make them as inconspicuous as possible. For use in the water a metal tripod will prove more serviceable than one of wood.