PHOTOGRAPHY OF SOUND WAVES. 



357 



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 focussed on the 



Kinetoscope Film of Explosion. 



brass balls, which appear in line in the picture so that the sound qjark 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, not- 

 withstanding 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 



