152 Prof. B. W. Wood on Photography of Sound- Waves 



by diffraction, and the secondary wavelets reflected from 

 corrugated surfaces, interesting in connexion with the dif- 

 fraction-oratino;. 



Rk. 6. 



Various cases of refraction are also shown, the only novel 

 one being the transformation of a spherical into a plane wave 

 by a carbonic-acid lens. The construction of the cylindrical 

 lens, of exceedingly thin collodion, a matter of considerable 

 difficulty, was successfully accomplished, the circular flat 

 ends of very thin mica, free from strise, enabling the passage 

 of the wave through the lens to be followed (fig. 7). The 



Fw. 7. 



other cases of refraction have already been described, as well 

 as the very beautiful instance of the formation of a train of 

 waves, or musical note, by the reflexion of a single pulse from 

 a steep flight of steps (fig. 8). 



