PHOTOGRAPHY OF SOUND WAVES. 



3d 



See the third and fourth pictures of the series. The bowl of the 

 crater shrinks to a point half-way between the surface of the mirror 

 and its center of curvature, and represents that portion of the light 

 which comes to a focus, while the sides of the cone run in under the 

 collapsing bowl, and eventually cross. (No. 6 of the series.) From now 

 on the portion which has come to a focus diverges, uniting with the 

 sides of the cone, the whole passing out of the mirror in the form of 

 a horseshoe. 



We will now consider a case of refraction, and show the slower 



A WAV?: FROM AN Kl.l.Il'TK AL MlRI 



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A Wave Stj 



riT'TSIIlE THE FlI 



:>f the Lens. 



Fig. 7. A Case of Refraction. 



velocity of the sound wave in carbonic acid. A narrow glass tank, 

 covered with an exceedingly thin film of collodion, was filled with the 

 heavy gas and placed under the brass balls. When the sound wave 

 strikes the collodion surface, it breaks up into two components, one re- 

 flected back into the air, the other transmitted down through the car- 

 bonic acid. An examination of the series shows that the reflected 

 wave in air has moved farther from the collodion film than the trans- 

 mitted wave, which, as a matter of fact, has been flattened out into 



