286 Prof. K. W. Wood. Photography of Sound-iuaves, and the 



geometry I shall show presently ; and by means of the animatograph, 

 which Mr. Paul has most kindly placed at our disposal, we can actually 

 see the wave going through its gymnastics. 



Fig. 6. 



The principle of Huygens, that any small portion of a wave-front 

 can be considered as the centre of a secondary disturbance, and that a 

 small portion of this new disturbance can in turn be regarded as a new 

 centre, can be shown by the soimd-waves, as well as the obliteration of 

 the shadow by diffraction, and the secondary wavelets reflected from 

 corrugated surfaces, interesting in connection with the diffraction 

 grating. 



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 accom- 

 plished, the circular flat ends of very thin mica, free from striae, 

 enabling the passage of the wave through the lens to be followed 

 (fig. 7), The other cases of refraction have already been described, as 



Fig. 7. 



