Page:Popular Science Monthly Volume 57.djvu/367

Rh make the method intelligible. The sound photographed in each case is the crack of an electric spark, which is illuminated and photographed by the light of a second spark, occurring a brief instant later. In front of a large lens (a telescope objective, for example) two brass balls are mounted, between which the 'sound spark.' as I shall call it, passes. The instant the spark jumps across the gap, a spherical wave of condensed air starts out, which, when it reaches our ear, gives the sensation of a snap. The object is to photograph this wave before it gets beyond the limits of the lens. The camera is mounted in front of the lens and focussedfocused [sic] on the



brass balls, which appear in line in the picture so that the sound spark is always hidden by the front one. The spark, on jumping between the balls, charges a Leyden jar, which instantly discharges itself between two wires placed behind the lens, producing the illuminating spark. This second spark can be made to lag behind the first just long enough to catch the sound wave when it is but a few inches in diameter, notwithstanding the fact that the spherical wave is expanding at the rate of eleven hundred feet a second. The photographs show in every case the circle of the lens filled tip with the light of the illuminating