. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. 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 ;—(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 o
. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. 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 ;—(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 sur
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