NATURE OF SOUXD. 



279 



solid matter of the walls, and from this to the air within. 

 When sound thus passes obliquely from one medium to 

 another, the rays are bent. Tills bending of a sound 

 ray is called refraction of sound. 



439. A Sound Focus. Ordinarily, sound rays are 

 divergent. The sound is therefore continually diminishing 

 in intensity. By means of their refrangibility, they may 

 be made convergent. If the divergent rays strike the side 

 of a sack shaped like a double convex lens, made of two 

 films of collodion, or very thin India rubber, and filled 

 with carbonic acid gas (C0 2 ), their divergence will be di- 

 minished ; they may thus be made parallel, or even con- 

 vergent, after passing through the sack. At the point 

 where these rays converge their total energy will be con- 

 centrated, and the intensity of the sound be thus increased. 

 The point where the refracted rays intersect is called the 

 focus of the lens. The laws of refracted sound are the 

 same as those of refracted light, to be studied further on. 



(a.) If a watch be hung near such a refractor, its ticking may be 

 heard by placing the ear at the focus on the other side of the sack ; 

 when the sack is re- 

 moved, the ticking is 

 no longer audible. A 

 few trials will enable 

 the experimenter to 

 determine the proper 

 positions for the watch, 

 the lens and the ear. 

 The refraction directs 

 to the ear all the en- 

 ergy exerted upon the 

 anterior surface of the 



FIG. 219. 



sack. This energy is sufficient to excite the sensation of hearing. 

 A little reflection will show that when the sack is removed, the 

 energy exerted upon the smaller surface of the tympanum at the 



