346 



NATURE 



[August 9. 1900 



The bending of the wave-front in the opposite direction 

 is shown in Fig. 12, where the same prism is filled with 

 hydrogen gas, in which sound travels faster than in air. 



In the next figure we have a very interesting case, 

 though, owing to the experimental difficulties, the photo- 

 graphs are not quite as satisfactory as some of the others. 

 It represents the transformation of a spherical into a 

 plane wave by passage through a double convex lens. 



Fig. 13. 



The construction of the cylindrical lens of exceedingly 

 thin collodion was a matter of great difficulty. The flat, 

 circular ends were made of thin mica as free from striae 

 as possible, that the passage of the wave through the 

 lens could be followed. On these discs the collodion 

 film was wound, the whole forming a hollow drum, which 



was then filled with carbonic acid. The sound-wave, 

 started at the principal fociis of this lens, is seen to be 

 ■quite flat after its emergence (Fig. 13). 



We will next take up some cases of difl"raction, begm- 

 ning with the well-known 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 secondary disturbance can act as a new 

 Centre in its turn. 



In Fig. 14 we have the wave starting above a plate 

 with a narrow slit in it. This slit is' seen to be the centre 

 of a secondary hemicylindrical wave which moves down 

 .precisely as if the spark were located at the slit. After 



NO 1606. VOL. 62] 



proceeding a short distance this secondary wave en- 

 counters a second slit, and the same thinor happens as 

 before, the little slice that gets through spreading out 

 into a complete wave, while the intercepted portion 

 bounces back and forth between the plates. 



Fig. 15 shows the very limited extent to which sound 

 shadows are formed. The wave is intercepted by a small 

 glass plate. Just below the plate in No. 3 of the series 

 a gap in the wave is found, which constitutes a shadow. 



UD 



But presently, by diffi-action, the wave curls in, closing up 

 the gap and obliterating the shadow entirely. In the 

 last one of the series it is interesting to note how the 

 diffracted waves have their centres at the edges of the 



obstacle, the edges acting as secondary sources, as m 

 the case of the diffraction of light. 



The passage of a wave through a diffraction gratmg is 

 shown in Fig. 16. The grating is made of strips of glass 



