somewhere, as the slit and the image must cross each other. In this case the image will be horizontal, also it will undoubtedly be made in one-hundredth of a second, but there may be a time error of anything less than a second in the relation of the image of the moving bar to other moving objects in the scene : a kind of consideration which must not be forgotten should a focal plane photograph ever be offered as legal evidence, say in the case of a railway collision. If the moving bar or pole were vertical, and moving across the scene in one second as suggested, the image of the pole would obviously be a diagonal line on the plate, as the exposure of the pole would commence at one corner of the plate, and finish at the opposite corner. The whole exposure would be one second, although the exposure of any assignable point would be one-hundredth of a second. The focal plane shutter is at its best in photographing active scenes in which no figure is very near the camera, but before such photographs can be used as records in the matter of time, the data of the shutter would have to be known and corrections made. Rapidly moving objects shown large on the plate must of necessity be much distorted when a focal plane shutter is used.
The case may be summarised by saying that ordinarily no true representation of moving objects, or accurate statement as to speed of exposure, is possible when a focal plane shutter is used ; exposure by a shutter at or near the position of the diaphragm being essential for true representation and accurate timing.
The speed of all portable shutters as attached to the usual cameras is, from the nature of their mechanical construction, a little variable; indeed, no very portable shutter has been invented which will constantly give very accurately timed ex-posures. At the same time it must be remembered that for ordinary photographic work there is no need of great accuracy in timing the shorter exposures.
As far back as 1866 Mr. J. W. Swan introduced a system of speed control, which is good in principle, the control being by the drop of a brass ball on a silk cord, the shutter opening at the time the ball is released and closing when the ball reaches such a point as to give the cord a slight tug.
A similar principal (the law of falling bodies) can be applied, although with less accuracy, to determining exposures with a drop shutter, and on the next page we give the table of Herr Holetschek.