354 POPULAR SCIENCE MONTHLY. 



THE PHOTOGRAPHY OF SOUND WAVES. 



By Professor R. W. WOOD, 



UNIVERSITY OF WISCONSIN. 



ANY one who has stood near a large naval gun during its discharge, 

 will, I think, be prepared to admit that the sound of the 

 explosion affects not only the ears, hut the whole body as well, 

 which experiences something not unlike a sudden blow. This blow, 

 or concussion, as it is generally termed, is merely the impact of the 

 wave of compressed air, spreading out in all directions around the gun. 

 In the case of ordinary sounds, the compression of the air in the wave is 

 so slight that only the delicate auditory nerves respond to the impact, 

 hence we naturally conclude that sounds are perceived only by the 

 ear. WJien dealing with sounds of very great intensity, this notion 

 must be somewhat modified, for they certainly can be felt as well as 

 heard. In some extreme cases, in fact, the sensation of feeling may be 

 stronger than that of hearing, as in the case of which I shall speak 

 presently. Is it also possible that we can perceive sound through the 

 medium of any other sense organ, say the eye? 'To see a noise' certainly 

 sounds like an absurdity; yet under certain conditions, sound waves in 

 air can be made as distinctly visible as the ripples on a pond surround- 

 ing the splash of a stone. That they are not seen under ordinary con- 

 ditions does not justify us in assuming them to be invisible. We all 

 know that the currents of hot air rising from a stove, while not usually 

 conspicuous, can be made visible by properly regulating the illumination, 

 as by looking along the surface of the stove towards a window. The 

 hot air is visible because in its optical properties it is different from the 

 cold air surrounding it. The rays of light, passing through the un- 

 equally heated portions of the air, are bent in different directions, 

 causing a distortion of objects seen through the heated currents. What 

 we see, strictly speaking, is not the hot air itself, but a wavering and 

 swimming of the objects seen through it. Yet I think we are justified 

 in saying that the eye perceives the hot air. 



Now sound waves in air, which are merely regions where the air is 

 somewhat compressed, differ in their optical properties from the un- 

 compressed portions, just as the hot air differs from the cold. As the 

 pictures illustrating this article testify, they may be seen and photo- 

 graphed under proper conditions of illumination as readily as solid ob- 

 jects. We must remember, in the first place, that a sound wave travels 

 with a velocity something greater than a thousand feet a second, rather 



