If, for example, a particular stage of the splash is photographed when the timing sphere falls just four feet to the gap, then by raising its releasing-lever about two-fifths of an inch, the laws of falling bodies tell us that we shall postpone the flash by just one-thousandth of a second, and the next photograph will accordingly reveal a stage just so much later. [Illustration: PLATE I Arrangement of apparatus for photographing splashes. LABORATORY E is the electrical machine. J J are the Leyden jars whose inner coats are connected to the sparking knobs S S. L is the lever for releasing the timing sphere T. C is the catapult. I is the light strip of iron held down by the electro-magnet M. DARK ROOM D is the drop resting on the smoked watch-glass W. M is the electro-magnet holding down the lever against the action of the catapult, by means of the thin strip of iron I. C is the camera directed towards the liquid L into which the drop will fall. S is the spark-gap between magnesium terminals connected to the outer coats of the Leyden jars. R is the concave mirror.] It ought still to be mentioned that to make the utmost use of the illuminating power of the spark, it is necessary to place close behind it a little concave mirror (R), by means of which a compact beam of rays, which would otherwise have been wasted, is directed to the required spot. By this addition we imitate, in miniature, the search-light of a man-of-war. * * * * * As with all experimental devices, the precision attainable with this arrangement is limited by several circumstances. In the first place, the demagnetization of the iron cores of the electro-magnets, when the current is cut off, is not truly instantaneous, and the time required depends on the strength of the magnetizing current and on the temperature of the iron, which in turn will depend on the length of time for which the current has been running. This variation would be of no importance if the two magnets were exactly alike and the springs of exactly equal strength, conditions which can be nearly but not perfectly fulfilled. [Illustration: PLATE II Photographs taken to test the accuracy of the "timing." 1 2 3 4 5]