PHOTOGBAPHY OF SOUND WAVES. 



363 



of a noise, was reproduced by reflecting the crack of the spark from a 

 little flight of steps. In the first picture the wave is seen half way 

 between its origin and the reflecting surface. In the second it has 

 struck the top stair, which is giving off its echo, the first wave of our 

 artificially constructed musical tone. In the third we find the original 

 wave at the sixth step, with a well-developed train of five waves rising 

 from the flight. The following three pictures show the further de- 

 velopment of the wave train. The height of each step was about a 

 ■quarter of an inch; consequently the distance between the waves was 

 half an inch. This would correspond to a note about three octaves 

 above the highest ever used in music. 



Fig y. The Reflection Inside the Hollow Sphere. 



While experimenting with the complete circular mirror, which, it 

 will be remembered, gave the most complicated forms, it occurred to me 

 that a very vivid idea of how these curious wave surfaces are produced 

 could be obtained by preparing a complete series in proper order on a 

 kinetoscope film, and then projecting them in succession on the screen. 

 The experimental difficulties were, however, too great to make it seem 

 worth while to attempt to obtain a series of pictures of the actual waves, 

 it being very difficult to accurately regulate the time interval between 

 the two sparks. The easier method of making a large number of 

 geometrical constructions, and then photographing them in succession 



