208 



THE REFRACTION OF SOUND. 



time upon the removal of the lens and was immediately audible again 

 when the lens was replaced between the watch and the ear." — (Foggen- 

 dorff^s Aimalen, 1853, Ixxxv, 381, translated and republished in the Phil. 

 Mag. February, 1853, v, 75.) 

 The accompanying Fig. 1, representing a vertical section of the gas 



12 3 4 



Fig. 1. — Carbonic-acid lens. 



lens through its center, will serve to give a more definite idea of the 

 action it exercises on the sound-waves passing through it. For any 

 small area, the wave-frout, at some distance from its origiu, may be con- 

 sidered as practically a plane surface, and 1, 2, 3, &c., (Fig. 1,) may rep- 

 resent the successive positions of a single advancing wave-front. On 

 entering the convex surface of the carbonic-acid lens at its central 

 point a, the wave-face is at once retarded, and successive annuli of the 

 wave passing the surface at increasing degrees of obliquity, the Ibrm of 

 the wave front becomes concave, as shown at 3 and 4, advancing concen- 

 trically according to the law of normal impacts, with a uniform though 

 retarded velocity, as shown at 4, 5, G, 7, &c. On emerging first from 

 the outer margin of the reversed convex surfiice h, the wave-front is 

 accelerated in passing into the common air, and meeting the boundary 

 of the same obliquely becomes still more concave, as shown at 8, 9, 10, 

 &c. Advancing concentrically, its impulses converge with uniform 

 velocity, but increasing energy, toward a focal point, /. 



It is obvious that if this convex envelope were tilled with hydrogen, 

 the action would be just reversed, as shown in Fig. 2. The wave of 

 sound, on entering the convex surface c, would be accelerated (com- 

 mencing at the middle) so as to acquire a continuously convex front, as 

 shown at 5, 6, &c. Passing through the second surface, d, and being 

 retarded in a reverse order, the wave-front would advance with an in- 

 creased convexity, as seen at 8, 9, 10, «&;c., giving a general divergence 

 of the sound-rays, the focus being negative. 



